A XXXXX AS t Si 'v£_ i si, 1 si a y Wm W» W* M* W + flrXtttraut^ % \\J .:. H :;:H;:; ? ■ -O'i ii .= : /< ;" ■■■•i'i;:,00n;.- I® It*"!' I ~*l *>£: THOMAS H. HUXLEY'S WORKS. Collected Essays. Vol. i. Method and Results. " 2. Darwiniana. " 3. Science and Education. " 4. Science and Hebrew Tradition. " 5. Science and Christian Tradition. " 6. Hume. " 7. Man's Place in Nature. " 8. Discourses, Biologicaland Geological. " 9. Evolution and Ethics, and Other Essays. nmo. Cloth, $1.25 per volume. The Crayfish : An Introduction to the Study of Zoology. With 82 Illustrations. i2mo. Cloth, $1.75. Manual of the Anatomy of Vertebrated Animals. Illustrated. i2mo. Cloth, $2.50. Manual of the Anatomy of Invertebrated Animals. Illustrated, nmo. Cloth, $2.50. Physiography : An Introduction to the Study of Nature. With Illustrations and Colored Plates, nrao. Cloth, $2. 50. New York : D. APPLETON & CO., 72 Fifth Avenue. A MANUAL OF THE ANATOMY OF INVERTEBRATED ANIMALS. BY THOMAS H. HUXLEY, LL. D., F. R. S. NEW YORK: D. APPLETON AND COMPANY, 72 FIFTH AVENUE. 1901. PBEFACE. The present volume on the Anatomy of Invert ebrated Animals fulfills an undertaking to produce a treatise on comparative anatomy for students, into which I entered two-and-twenty years ago. A considerable installment of the work, relating wholly to the Invertebrate, appeared in the Medical Times and Gazette for the years 1856 and 1857, under the title of " Lectures on General Natural History.'1 But a variety of circumstances having con- spired, about that time, to compel me to direct my atten- tion more particularly to the Vertebrata, I was led to in- terrupt the publication of the " Lectures " and to com- plete the Vertebrate half of the proposed work first. This appeared in 1871, as a " Manual of the Anatomy of Verte- brated Animals." A period of incapacity for any serious toil prevented me from attempting, before 1874, to grapple with the im- mense mass of new and important information respecting the structure, and especially the development, of Inverte- brated animals, which the activity of a host of investiga- tors has accumulated of late vears. That my progress has been slow will not surprise any one who is acquainted with the growth of the literature of animal morphology, or with the expenditure of time involved in the attempt to verify for one's self even the cardinal facts of that science ; but I have endeavored, in 4 PREFACE. the last chapter, to supply the most important recent ad- ditions to our knowledge, respecting the groups treated of in those which have long been printed. "When I commenced this work, it was my intention to continue the plan adopted in the " Manual of the Anatomy of Vertebrated Animals," of giving a summary account of what appeared to me to be ascertained morphological facts, without referring to my sources of information. I soon found, however, that it would be inconvenient to carry out this scheme consistently ; and some of my pages are, I am afraid, somewhat burdened with notes and ref- erences. I am the more careful to mention this circumstance as, had it been my purpose to give any adequate Bibliography, the conspicuous absence of the titles of many important books and memoirs might appear unaccountable and in- deed blameworthy. My object, in writing the book, has been to make it useful to those who wish to become acquainted with the broad outlines of what is at present known of the morphol- ogy of the Invertebrata / though I have not avoided the incidental mention of facts connected with their physiol- ogy and their distribution. On the other hand, I have ab- stained from discussing questions of aetiology, not because I underestimate their importance, or am insensible to the interest of the great problem of Evolution ; but because, to my mind, the growing tendency to mix up setiological speculations with morphological generalizations will, if unchecked, throw Biology into confusion. For the student, that which is essential is a knowledge of the facts of morphology ; and he should recollect that generalizations are empty formulas, unless there is some- thing in his personal experience which gives reality and substance to the terms of the propositions in which these generalizations are expressed. PREFACE. 5 The dissection of a single representative of each of the principal divisions of the Invertebrata will give the student a more real acquaintance with their comparative anatomy than any amount of reading of this, or any other book. And I have endeavored to facilitate practical study by supplying a somewhat full description of individual forms, in the case of the more complicated types. That the power of repeating a " Classification of Ani- mals," with all the appropriate definitions, has anything to do with genuine knowledge is one of the commonest and most mischievous delusions of both students and their examiners. The real business of the learner is to gain a true and vivid conception of the characteristics of what may be termed the natural orders of animals. The mode of ar- rangement, or classification, of these into larger groups is a matter of altogether secondary importance. As such, I have relegated this subject to a subordinate place in the last chapter ; and I have thought it unnecessary, either to discuss the systems proposed by others, or to give reasons for passing over, in silence, my own former attempts in this direction. Of the manifold imperfections in the execution of the task which I have set myself, few will be more sensible than I am ; but I trust that the book, such as it is, may be of use to the beginner. Those who desire to pursue the study of the Inverte- Irata further will do well to consult the excellent treatises of Yon Siebold,1 Gegenbaur,2 and Clans ; 3 and the elabo- 1 " Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere," 1848. One of the best books on the subject ever written, and still indispensable. 2 " Grundzuge der vergleichenden Anatomie," 1870 ; and " Gnmdriss der vergleichenden Anatomie," 1874. 3 " Grundziige der Zoologie." 3tte Auflage, 1876. 6 PREFACE. rate works of Milne-Edwards1 and Bronn,2 in which a very full Bibliography will be met with. Dr. Rolleston's valuable " Types of Animal Life," and the "Elementary Instruction in Practical Biology," by myself and Dr. Martin, will prove useful adjuncts to the appliances of the practical worker. 1 " Legons sur la Physiologie et l'Anatomie comparee de l'Homme et des Animaux." Tomes i.-xii. (incomplete). 2 " Die Klassen und Ordnungen des Tkierreichs." Bde. i.-vi. (incomplete). London, June, 1877. CONTENTS. ?AGE Preface, 3 Introduction : The General Principles of Biology, . 9 Chap. I. — The Distinctive Characters of Animals, . . . .44 II. — The Protozoa, . . ' 73 III. — The Porifera and the Cgxenterata, 102 * IV. — The Tcrbellaria, the Rotifera, the Trematoda, and the Cestoidea, 157 V. — The Hirudinea, the Oligoch^eta, the Polych^eta, the Gephyrea, 189 YI. — The Arthropoda, 219 VII. — The Air-breathing Arthropoda, 320 VIII. — The Polyzoa, the Brachiopoda, and the Mollusca, . . 389 IX. — The Echinodermata, . 466 X. — The Tunicata or Ascidioida, 510 XI. — The Peripatidea, the Myzostomata, the Enteropneusta, THE CH.ETOGNATHA, THE NeMATOIDEA, THE PHYSEMARIA, THE ACANTHOCEPHALA, AND THE DlCYEMIDA, . . . 534 XT!. — The Taxonomy of Inyertebrated Animals, .... 561 Index, 589 THE ANATOMY OF INVEKTEBKATED ANIMALS. INTRODUCTION. I. — THE GEXEKAL PEIXCIPLES OF BIOLOGY. The biological sciences are those which deal with the phenomena manifested by living matter; and though it is customary and convenient to group apart such of these phe- nomena as are termed mental, aud such of them as are ex- hibited by men in society, under the heads of Psychology and Sociology, yet it must be allowed that no natural boun- dary separates the subject-matter of the latter sciences from that of Biology. Psychology is inseparably linked with Physiology ; and the phases of social life exhibited by ani- mals other than man, which sometimes curiously foreshadow human policy, fall strictly within the province of the biolo- gist. On the other hand, the hiological sciences are sharply marked off from the abiological, or those which treat of the phenomena manifested by not-living matter, in so far as the properties of living matter distinguish it absolutely from all other kinds of things, and as the present state of knowledge furnishes us with no link between the living and the not- livino*. These distinctive properties of living matter are — 1. Its chemical composition — containing, as it invariably does, one or more forms of a complex compound of carbon, hydrogen, oxygen, and nitrogen, the so-called protein (which has never yet been obtained except as a product of living bodies) united with a large proportion of water, and forming 10 THE ANATOMY OF IXVERTEBRATED ANIMALS. the chief constituent of a substance which, in its primary un- modified state, is known as protoplasm. 2. Its universal disintegration and waste by oxidation/ and its concomitant reintegration by the intussusception of neic matter. A process of waste resulting from the decomposition of the molecules of the protoplasm, in virtue of which they break up into more highly-oxidated products, which cease to form any part of the living body, is a constant concomitant of life. There is reason to believe that carbonic acid is al- ways one of these waste products, while the others contain the remainder of the carbon, the nitrogen, the hydrogen, and the other elements which may enter into the composition of the protoplasm. The new matter taken in to make good this constant loss is either a ready-formed protoplasmic material, supplied by some other living being, or it consists of the elements of protoplasm, united together in simpler combinations, which consequently have to be built up into protoplasm by the agency of the living matter itself. In either case, the addi- tion of molecules to those which already existed takes place, not at the surface of the living mass, but by interposition between the existing molecules of the latter. If the processes of disintegration and of reconstruction which characterize life balance one another, the size of the mass of living matter remains stationary, while, if the reconstructive process is the more rapid, the living body grows. But the increase of size wThich constitutes growth is the result of a process of molec- ular intussusception, and therefore differs altogether from the process of growth by accretion, which may be observed in crystals and is effected purely by the external addition of new matter — so that, in the well-known aphorism of Linnaeus,1 the word "grow," as applied to stones, signifies a totally dif- ferent process from what is called " growth " in plants and animals. 3. Its tendency to undergo cyclical changes. In the ordinary course of Nature, all living matter proceeds from preexisting living matter, a portion of the latter being detached and acquiring an independent existence. The new form takes on the characters of that from which it arose ; ex- hibits the same power of propagating itself by means of an offshoot ; and, sooner or later, like its predecessor, ceases to 1 " Lapides crescunt: vegetdbilia crescunt et vivunt: animalia crescunt, vi- vunt et sentiunt." CHARACTERS OF LIVING MATTER. H live, and is resolved into more highly-oxidated compounds of its elements. Thus an individual living body is not only constantly changing its substance, but its size and form are undergoing continual modifications, the end of which is the death and decay of that individual ; the continuation of the kind being secured by the detachment of portions which tend to run through the same cycle of forms as the parent. No forms of matter which are either not living, or have not been derived from living matter, exhibit these three properties, nor any approach to the remarkable phenomena defined under the sec- ond and third heads. But, in addition to these distinctive characters, living matter has some other peculiarities, the chief of which are the dependence of all its activities upon moisture and upon heat, within a limited range of tempera- ture, together with the fact that it usually possesses a certain structure, or organization. As has been said, a large proportion of water enters into the composition of all living matter ; a certain amount of dry- ing arrests vital activity, and the complete abstraction of this water is absolutely incompatible with either actual or poten- tial life. But many of the simpler forms of life may undergo desiccation to such an extent as to arrest their vital manifes- tations and convert them into the semblance of not-living matter, and yet remain potentially alive ; that is to say, on being duly moistened they return to life again. And this revivification may take place after months, or even years, of arrested life. The properties of living matter are intimately related to temperature. Not only does exposure to heat sufficient to decompose protein matter destroy life, by demolishing the molecular structure upon which life depends ; but all vital activity, all phenomena of nutritive growth, movement, and reproduction, are possible only between certain limits of tem- perature. As the temperature approaches these limits the manifestations of life vanish, though they may be recovered by return to the normal conditions ; but, if it pass far beyond these limits, death takes place. This much is clear ; but it is not easy to say exactly what the limits of temperature are, as they appear to vary in part with the kind of living matter, and in part with the con- ditions of moisture which obtain along with the temperature. The conditions of life are so complex in the higher organisms, that the experimental investigation of this question can be 12 THE ANATOMY OF INVERTEBRATED ANIMALS. satisfactorily attempted only in the lowest and simplest forms. It appears that, in the dry state, these are able to bear far greater extremes both of heat and cold than in the moist condition. Thus Pasteur found that the spores of fungi, when dry, could be exposed without destruction to a tem- perature of 120°-125° C. (248°-257° Fahr.), while the same spores, when moist, were all killed by exposure to 100° C. (212° Fahr.). On the other hand, Cagniard de la Tour found that dry yeast might be exposed to the extremely low tem- perature of solid carbonic acid ( — 60° Cor —76° Fahr.) with- out being killed. In the moist state he found that it might be frozen and cooled to —5° C. (23° Fahr.), but that it was killed by lower temperatures. However, it is very desirable that these experiments should be repeated, for Conn's careful observations on Bacteria show that, though they fall into a state of torpidity, and, like yeast, lose all their powers of ex- citing fermentation at, or near, the freezing-point of water, they are not killed by exposure for five hours to a tempera- ture below —10° C. (14° Fahr.), and, for some time, sinking to —18° C. (— 0°.4Fahr.). Specimens of Spirillum volutans, which had been cooled to this extent, began to move about some little time after the ice containing them thawed. But Cohn remarks that Euglenoe, which were frozen along with them, were all killed and disorganized, and that the same fate had befallen the higher Infusoria and Motif era, with the ex- ception of some encysted Vorticellce, in which the rhythmical movements of the contractile vesicle showed that life was preserved. Thus it would appear that the resistance of living matter to cold depends greatly on the special form of that matter, and that the limit of the Euglena, simple organism as it is, is much higher than that of the Bacterium. Considerations of this kind throw some light upon the apparently anomalous conditions under which many of the lower plants, such as Protococcus and the Diatomacea?, and some of the lower animals, such as the Madiolaria, are ob- served to flourish. Protococcus has been found not only on the snows of great heights in temperate latitudes, but cover- ing extensive areas of ice and snow in the Arctic regions, where it must be exposed to extremely low temperatures — in the latter case for many months together ; while the Arctic and Antarctic seas swarm with Diatomacece and Madiolaria. It is on the Diatomacece, as Hooker has well shown, that all surface-life in these regions ultimately depends ; and their enor- RESISTANCE TO HEAT AND COLD. 13 mous multitudes prove that their rate of multiplication is ade- quate to meet the demands made upon them, and is not seri- ously impeded by the low temperature of the waters, never much above the freezing-point, in which they habitually live. The maximum limit of heat which living matter can resist is no less variable than its minimum limit. Kiihne found that marine Amoebce were killed when the temperature reached 35° C. (95° Fahr.), while this was not the case with fresh-water Amoebce, which survived a heat of 5°, or even 10°, C. higher. Actinophrys Eichhornii was not killed until the temperature rose to 44° or 45° C. Didymium serpula is killed at 35° C. ; while another Myxomycete, JEthalium septicum, succumbs only at 40° C. * Colin (" Untersuchungen iiber Bacterien," Beitrage zur Biologie der Pflanzen, Heft 2, 1872) has given the results of a series of experiments conducted with the view of ascertain- ing the temperature at which Bacteria are destroyed when living in a fluid of definite chemical composition, and free from all such complications as must arise from the inequalities of physical condition when solid particles other than the Bac- teria coexist with them. The fluid employed contained 0.1 gramme potassium phosphate, 0.1 gr. crystallized magnesium sulphate, 0.1 gr. tribasic calcium phosphate, and 0.2 gr. am- monium tartrate, dissolved in 20 cubic centimetres of distilled water. If to a certain quantity of this " normal fluid " a small proportion of water containing Bacteria was added, the mul- tiplication of the Bacteria went on with rapidity, whether the mouth of the containing flask was open or hermetically closed. Hermetically-sealed flasks, containing portions of the normal fluid infected with Bacteria, were submerged in water heated to various temperatures, the flask being carefully shaken, with- out being raised out of the water, during its submergence. The result was. that in those flasks which were thus sub- jected, for an hour,' to a heat of 60°-62° C. (140°-143° Fahr.), the Bacteria underwent no development, and the fluid re- mained perfectly clear. On the other hand, in similar experi- ments in which the flasks were heated only to 40° or 50° C. (104°-122° Fahr.), the fluid became turbid, in consequence of the multiplication of the Bacteria, in the course of from two to three days. I am in the habit of demonstrating annually, that Pasteur's solution and hay-infusion, after five minutes' boiling in a flask properly stopped with cotton-wool, remain perfectly clear of living organisms, however long they may be kept. The same 14 THE ANATOMY OF INVERTEBRATED ANIMALS. holds good for a solution analogous to Cohn's, but in which all the saline ingredients are ammonia salts ; ' and in which Bacteria nourish luxuriantly. Prof. Tyndall's large series of experiments give the same results for fluids of the most diverse composition. The cases of milk and some other fluids in which Bacteria are said to appear, after they have been heated above the boiling-point, require renewed investigation. Both in Ktihne's and in Cohn's experiments, which last have lately been confirmed and extended by Dr. Roberts, of Man- chester, it was noted that long exposure to a lower temper- ature than that which brings about immediate destruction of life produces the same effect as short exposure to the latter temperature. Thus, though all the Bacteria were killed, with certainty, in the normal fluid, by short exposure to temper- atures at or above 60° C. (140° Fahr.), Cohn observed that, when a flask containing infected normal fluid was heated to 50°-52° C. (122°-125° Fahr.) for only an hour, the conse- quent multiplication of the Bacteria was manifested much earlier than in one which had been exposed for two hours to the same temperature. It appears to be very generally held that the simpler vege- table organisms are deprived of life at temperatures as high as 60° C. (140° Fahr.) ; but it is affirmed by competent ob- servers that Algce have been found living in hot springs at much higher temperatures, namely, from 168° to 208° Fahr., for which latter surprising fact we have the high authority of Descloiseaux. It is no explanation of these phenomena, but only another mode of stating them, to say that these organ- isms have become " accustomed " to such temperatures. If this degree of heat were absolutely incompatible with the activity of living matter, the plants could no more resist it than they could become " accustomed " to be being made red- hot. Habit may modify subsidiary, but cannot affect funda- mental, conditions. . Recent investigations point to the conclusion that the im- mediate cause of the arrest of vitality, in the first place, and of its destruction, in the second, is the coagulation of certain substances in the protoplasm, and that the latter contains various coagulable matters, which solidify at different temper- atures. And it remains to be seen how far the death of any form of living matter, at a given temperature, depends on the i These were as pure as I could obtain them. It is possible the fluid may have contained an infinitesimal proportion of fixed mineral matter, RESISTANCE TO HEAT AND COLD. 15 destruction of its fundamental substance at that heat, and how far death is brought about by the coagulation of merely accessory compounds. It may be safely said of all those living things which are laro-e enough to enable us to trust the evidence of micro- scopes,1 that they are heterogeneous optically, and that their different parts, and especially the surface layer, as contrasted with the interior, differ physically and chemically ; while, in most living things, mere heterogeneity is exchanged for a definite structure, whereby the body is distinguished into visibly diverse parts, which possess different powers or func- tions. Living things which present this visible structure are said to be organized ; and so widely does organization obtain among living beings, that organized and living are not unfre- quently used as if they were terms of coextensive applicabil- ity. This, however, is not exactly accurate, if it be thereby implied that all living things have a visible organization, as there are numerous forms of living matter of which it cannot properly be said that they possess either a definite visible structure or permanently specialized organs : though doubt- less the simplest particle of living matter must possess a highly-complex molecular structure, which is far beyond the reach of vision. The broad distinctions which, as a matter of fact, exist between every known form of living substance and every other component of the material world, justify the separation of the biological sciences from all others. But it must not be supposed that the differences between living and not-living matter are such as to bear out the assumption that the forces at work in the one are different from those which are to be met with in the other. Considered apart from the phenomena of consciousness, the phenomena of life are all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. It may be convenient to use the terms " vitality " and " vital force " to denote the causes of certain great groups of natural opera- 1 In considering the question of the complication of molecular structure which even the smallest and simplest of living beings may possess, it is well to recollect that an organic particle tssoo of an inch in diameter, in which our best microscopes may be incompetent to reveal the slightest differentiation of parts, may be made up of 1,000,000 particles too^ooo of an inch in diameter, while the molecules of matter are probably much less than Toriooo of an inch in diameter. Hence in such a body there is ample scope for any amount of com- plexity of molecular structure. 2 16 THE ANATOMY OF LNYERTEBRATED ANIMALS. tions, as we employ the names of " electricity " and " electrical force " to denote others ; but it ceases to be proper to do so, if such a name implies the absurd assumption that either " elec- tricity " or "vitality" is an entity playing the part of an effi- cient cause of electrical or vital phenomena. A mass of living protoplasm is simply a molecular machine of great complexity, the total results of the working of which, or its vital phenom- ena, depend, on the one hand, upon its construction, and, on the other, upon the energy supplied to it ; and to speak of " vitality " as anything but the name of a series of operations is as if one should talk of the " horologity " of a clock. Living matter, or protoplasm and the products of its meta- morphosis, may be regarded under four aspects : (1.) It has a certain external and internal form, the la iter being more usually called structure ; (2.) It occupies a certain position in space and in time ; (3.) It is the subject of the operation of certain forces, in virtue of which it undergoes internal changes, modifies exter- nal objects, and is modified by them ; and — (4.) Its form, place, and powers, are the effects of certain causes. In correspondence with these four aspects of its subject, Biology is divisible into four chief subdivisions — I. Morphol- ogy; II. Distribution ; III. Physiology; IV. ^Etiology. I. Morphology. So far as living beings have a form and structure, they fall within the province of Anatomy and Histology, the latter being merely a name for that ultimate optical analysis of living structure which can be carried out only by the aid of the microscope. And, in so far as the form and structure of any living being- are not constant during the whole of its existence, but undergo a series of changes from the commencement of that existence to its end, living beings have a Development. The historv of development is an accuont of the anatomy of a liv- ing being at the successive periods of its existence, and of the manner in which one anatomical stage passes into the next. Finally, the systematic statement and generalization of the facts of Morphology, in such a manner as to arrange liv- ing beings in groups, according to their degrees of likeness, is Taxonomy. HISTOLOGY. 17 The study of Anatomy and Development has brought to light certain generalizations of wide applicability and great importance. 1. It has been said that the great majority of living beings present a very definite structure. Unassisted vision and or- dinary dissection suffice to separate the body of any of the higher animals, or plants, into fabrics of different sorts, which always present the same general arrangement in the same organism, but are combined in different ways in different organisms. The discrimination of these comparatively few fabrics, or tissues, of which organisms are composed, was the first step toward that ultimate analysis of visible structure which has become possible only by the recent perfection of microscopes and of methods of preparation. Histology, which embodies the results of this analysis, shows that every tissue of a plant is composed of more or less modified structural elements, each of which is termed a cell ; which cell, in its simplest condition, is merely a spheroidal mass of protoplasm, surrounded by a coat or sac — the cell- wall — which contains cellulose. In the various tissues, these cells may undergo innumerable modifications of form — the protoplasm may become differentiated into a nucleus with its nucleolus, a primordial utricle, and a cavity filled w7ith a wa- tery fluid, and the cell-wall may be variously altered in com- position or in structure, or may coalesce with others. But, however extensive these changes may be, the fact that the tissues are made up of morphologically distinct units — the cells — remains patent. And, if any doubt could exist on the subject, it would be removed by the study of development, which proves that every plant commences its existence as a simple cell, identical in its fundamental characters with the less modified of those cells of which the whole body is composed. But it is not necessary to the morphological unit of the plant that it should be always provided with a cell-wall. Cer- tain plants, such as Protococcus, spend longer or shorter peri- ods of their existence in the condition of a mere spheroid of protoplasm, devoid of any cellulose wall, while, at other times, the protoplasmic body becomes inclosed within a cell-wall, fab- ricated by its superficial layer. Therefore, just as the nucleus, the primordial utricle, and the central fluid, are no essential constituents of the morpho- logical unit of the plant, but represent results of its meta- morphosis, so the cell wall is equally unessential ; and either the term " cell " must acquire a merely technical significance 18 THE ANATOMY OF INVERTEB RATED ANIMALS. as the equivalent of morphological unit, or some new term must be invented to describe the latter. On the whole, it is probably least inconvenient to modify the sense of the word " cell." The histological analysis of animal tissues has led to sim- ilar results, and to difficulties of terminology of precisely the same character. In the higher animals, however, the modifi- cations which the cells undergo are so extensive that the fact that the tissues are, as in plants, resolvable into an aggrega- tion of morphological units, couid never have been established without the aid of the study of development, which proves that the animal, no less than the plant, commences its exist- ence as a simple cell, fundamentally identical with the less modified cells which are found in the tissues of the adult. Though the nucleus is very constant among animal cells, it is not universally present ; and, among the lowest forms of animal life, the protoplasmic mass which represents the mor- phological unit may be, as in the lowest plants, devoid of a nucleus. In the animal the cell-wall never has the character of a shut sac containing: cellulose : and it is not a little diffi- cult, in many cases, to say how much of the so-called " cell- wall" of the animal cell answers to the " primordial utricle' and how much to the proper " cellulose cell-wall " of the vege- table cell. But it is certain that in the animal, as in the plant, neither cell-wall nor nucleus is an essential constituent of the cell, inasmuch as bodies which are unquestionably the equivalents of cells — true morphological units — may be mere masses of protoplasm, devoid alike of cell-wall and nucleus. For the whole living world, then, it results : that the mor- phological unit — the primary and fundamental form of life — is merely an individual mass of protoplasm, in which no fur- ther structure is discernible ; that independent living forms may present but little advance on this structure; and that all the higher forms of life are aggregates of such morphological units or cells variously modified. Moreover, all that is at present known tends to the conclu- sion that, in the complex aggregates of such units of which ail the higher animals and plants consist, no cell has arisen otherwise than by becoming separated from the protoplasm of a preexisting cell ; whence the aphorism, " Omnis cellula e celluhi." It may further be added, as a general trutli applicable to nucleated cells, that the nucleus rarely undergoes any consid- erable modification, the structures characteristic of the tis- DEVELOPMENT. 19 sues being formed at the expense of the more superficial pro- toplasm of the cells ; and that, when nucleated cells divide, the division of the nucleus, as a rule, precedes that of the whole cell. 2. In the course of its development every cell proceeds, from a condition in which it closely resembles every other cell, through a series of stages of gradually-increasing diver- gence, until it reaches that condition in which it presents the characteristic features of the elements of a special tissue. The development of the cell is, therefore, a gradual progress from the general to the special state. The like holds good of the development of the body as a whole. However complicated one of the higher animals or plants may be, it begins its separate existence under the form of a nucleated cell. This, by division, becomes con- verted into an aggregate of nucleated cells — the parts of this aggregate, following different laws of growth and multiplica- tion, give rise to the rudiments of the organs ; and the parts of these rudiments again take on those modes of growth, mul- tiplication, and metamorphosis, which are needful to convert the rudiment into the perfect structure. The development of the organism as a whole, therefore, repeats in principle the development of the cell. It is a prog- ress from a general to a special form, resulting from the grad- ual differentiation of the primitively similar morphological units of which the body is composed. Moreover, when the stages of development of two animals are compared, the number of these stages which are similar to one another is, as a general rule, proportional to the close- ness of the resemblance of the adult forms ; whence it fol- lows that the more closely any two animals are allied in adult structure, the later are their embryonic conditions distinguish- able. And this general rule holds for plants no less than for animals. The broad principle, that the form in which the more com- plex living things commence their development is always the same, wyas first expressed by Harvey in his famous aphorism, " Omne vivum ex ovo" which wTas intended simply as a mor- phological generalization, and in no wise implied the rejection of spontaneous generation, as it is commonly supposed to do. Moreover, Harvey's study of the development of the chick led him to promulgate that theory of "epigenesis," in which the doctrine that development is a progress from the general to the special is implicitly contained. 20 THE ANATOMY OF IXVERTEBRATED AXLMALS. Caspar F. Wolff furnished further, and indeed conclusive, proof of the truth of the theory of epigenesis ; but, unfortu- nately, the authority of Haller and the speculations of Bonnet led science astray, and it was reserved for Von Baer to put the nature of the process of development in its true light, and to formulate it in his famous law. 3. Development, then, is a process of differentiation by which the primitively similar parts of the living body become more and more unlike one another. This process of differentiation may be effected in several wavs : (1.) The protoplasm of the germ may not undergo divi- sion and conversion into a cell aggregate ; but various parts of its outer and inner substance may be metamorphosed di- rectly into those physically and chemically different materials which constitute the body of the adult. This occurs in such animals as the Infusoria, and in such plants as the unicellular Algee and Fungi. . (2.) The germ may undergo division, and be converted into an aggregate of division masses, or blastomeres, which become cells, and give rise to the tissues by undergoing a metamorphosis of the same kind as that to which the whole body is subjected in the preceding case. The body, formed in either of these ways, may, as a whole, undergo metamorphosis by differentiation of its parts ; and this differentiation may take place without reference to any axis of symmetry, or it may have reference to such an axis. In the latter case, the parts of the body which become dis- tinguishable may correspond on the two sides of the axis (bi- lateral symmetry), or may correspond along several lines paral- lel with the axis (radial symmetry). The bilateral or radial symmetry of the body may be fur- ther complicated by its segmentation, or separation by divi- sions transverse to the axis, into parts, each of which corre- sponds with its predecessor or successor in the series. In the segmented body, the segments may or may not give rise to symmetrically or asymmetrically disposed processes, which are appendages, using that word in its most general sense. And the highest degree of complication of structure, in both animals and plants, is attained by the body when it be- comes divided into segments provided with appendages ; when the segments not only become very different from one another, but some coalesce and lose their primitive distinctness ; and DIFFERENTIATION OF STRUCTURE. 21 when the appendages and the segments into which they are subdivided similarly become differentiated and coalesce. It is in virtue of such processes that the flowers of plants, and the heads and limbs of the Arthropoda and of the Ver- tebrate^ among animals, attain their extraordinary diversity and complication of structure. A flower-bud is a segmented body or axis, with a certain number of whorls of appendages ; and the perfect flower is the result of the gradual differentia- tion and confluence of these primitively similar segments and their appendages. The head of an insect or of a crustacean is, in like manner, composed of a number of segments, each with its pair of appendages, which by differentiation and con- fluence are converted into the feelers and variously modified oral appendages of the adult. In some complex organisms, the process of differentiation by which they pass from the condition of aggregated embryo cells to the adult, can be traced back to the laws of growth of the two or more cells into which the embryo cell is divided, each of these cells giving rise to a particular portion of the adult organism. Thus the fertilized embryo cell in thearche- gonium of a fern divides into four cells, one of which gives rise to the rhizome of the young fern, another to its first root- let, while the other two are converted into a placenta-like mass which remains imbedded in the prothallus. The structure of the stem of Chara depends upon the dif- ferent properties of the cells, which are successively7 derived by transverse division from the apical cell. An intemodal cell, which elongates greatly, and does not divide, is suc- ceeded by a noded cell, which elongates but little, and becomes greatly subdivided ; this by another internodal cell, and so on in regular alternation. In the same way the structure of the stem, in all the higher plants, depends upon the laws which govern the manner of division and of metamorphosis of the apical cells, and of their continuation in the cambium layer. In all animals which consist of cell-aggregates, the cells of which the embryo is at first composed arrange themselves by the splitting, or by a process of invagination, of the blas- toderm into two layers, the epiblast and the hypoblast ^ be- tween which a third intermediate layer, the mesoblast, ap- pears ; and each layer gives rise to a definite group of organs in the adult. Thus, in the Vertebrata, the epiblast gives rise to the cerebro-spinal axis, and to the epidermis and its deriva- tives ; the hypoblast, to the epithelium of the alimentary 22 THE ANATOMY OF IXVERTEBRATED ANIMALS. canal and its derivatives ; and the mesoblast, to intermediate structures. The tendency of recent inquiry is to prove that the several layers of the germ evolve analogous organs in in- vertebrate animals, and to indicate the possibility of tracing the several germ-layers back to the blastomeres of the yelk, from the subdivision of which they proceed. It is conceivable that all the forms of life should have pre- sented about the same differentiation of structure, and should have differed from one another by superficial characters, each form passing by insensible gradations into those most like it. In this case Taxonomy, or the classification of morphological facts, would have had to confine itself to the formation of a serial arrangement, representing the serial gradation of these forms in Nature. It is conceivable, ao-ain, that living beings should have dif- fered as widely in structure as they actually do, but that the interval between any two extreme forms should have been filled up by an unbroken series of gradations ; in which case, again, classification could only affect the formation of series— the strict definition of groups wrould be as impossible as in the former case. As a matter of fact, living beings differ enormously, not onlv in differentiation of structure, but in the modes in which that differentiation is brought about ; and the intervals be- tween extreme forms are not filled up, in the existing world, by complete series of gradations. Hence it arises that living beings are, to a great extent, susceptible of classification into groups, the members of each group resembling one another, and differing from all the rest, by certain definite peculiarities. No two living beings are exactly alike, but it is a matter of observation that, among the endless diversities of living things, some constantly resemble one another so closely that it is impossible to draw any line of demarkation between them, while they differ only in such characters as are associated with sex. Such as thus closely resemble one another consti- tute a morpholor/ical species / while different morphological species are defined by constant characters which are not merely sexual. The comparison of these lowest groups, or morphological species, with one another, shows that more or fewer of them possess some character or characters in common — some feat- ure in which they resemble one another and differ from all other species — and the group or higher order thus formed is MORPHOLOGICAL GROUPS. 23 a genus. The generic groups thus constituted are susceptible of being arranged in a similar manner into groups of succes- sively higher order, which are known as families, order 's, classes, and the like. The method pursued in the classification of living forms is, in fact, exactly the same as that followed by the maker of an index in working out the heads indexed. In an alphabetical arrangement, the classification may be truly termed a mor- phological one, the object being to put into close relation all those leading words which resemble one another in the arrangement of their letters, that is, in their form, and to keep apart those which differ in structure. Headings which begin with the same word, but differ otherwise, might be compared to genera with their species ; the groups of words with the same first two syllables, to families ; those with identical first syllables, to orders ; and those with the same initial letter, to classes. But there is this difference between the index and the Taxonomic arrangement of living forms, that in the for- mer there is nothing but an arbitrary relation between the various classes, while in the latter the classes are similarly capable of coordination into larger and larger groups, until all are comprehended under the common definition of living- beings. The differences between " artificial " and " natural " clas- sifications are differences in degree, and not in kind. In each case the classification depends upon likeness ; but in an artifi- cial classification some prominent and easil3T-observed feature is taken as the mark of resemblance or dissemblance ; while, in a natural classification, the things classified are arranged ac- cording to the totality of their morphological resemblances, and the features which are taken as the marks of groups are those which have been ascertained by observation to be the indications of many likenesses or unlikenesses. And thus a natural classification is a great deal more than a mere index. It is a statement of the marks of similarity of organization ; of the kinds of structure which, as a matter of experience, are found universally associated together ; and, as such, it fur- nishes the whole foundation for those indications by which conclusions as to the nature of the whole of an animal are drawn from a knowledge of some part of it. When a paleontologist argues from the characters of a bone or a shell to the nature of the animal to which that bone or shell belonged, he is guided by the empirical morphologi- cal laws established by wide observation, that such a kind of 24 THE AX ATOMY OF IXVERTEBRATED ANIMALS. bone or shell is associated with such and such structural feat- ures in the rest of the body, and no others. And it is these empirical laws which are embodied and expressed in a natural classification. II. Distribution. Living beings occupy certain portions of the surface of the earth, inhabiting either the dry land, or the fresh or salt waters; or being competent to maintain their existence in either. In any given locality, it is found that these different media are inhabited by different kinds of living beings ; and that the same medium, at different heights in the air and at different depths in the water, has different living inhabitants. Moreover, the living populations of localities which differ . considerably in latitude, and hence in climate, always present considerable differences. But the converse proposition is not true — that is to say, localities which differ in longitude, even if they resemble one another in climate, often have very dis- similar Fawice and Florae. It has been discovered, by careful comparison of local fau- nae and florae, that certain areas of the earth's surface are inhabited by groups of animals and plants which are not found elsewhere, and which thus characterize each of these areas. Such areas are termed Provinces of Distribution. There is no parit}?- between these provinces in extent, nor in the phys- ical configuration of their boundaries ; and, in reference to existing conditions, nothing can appear to be more arbitrary and capricious than the distribution of living beings. The study of distribution is not confined to the present order of Nature ; but, by the help of geology, the naturalist is enabled to obtain clear, though too fragmentary, evidence of the characters of the faunae and florae of antecedent epochs. The re- mains of organisms which are contained in the stratified rocks prove that, in any given part of the earth's surface, the living population of earlier epochs was different from that which now exists in the locality ; and that, on the whole, the difference becomes greater the farther we go back in time. The organic remains which are found in the later Cainozoic deposits of any district are always closely allied to those now found in the province of distribution in which that locality is included ; while in the older Cainozoic the resemblance is less ; and in the Mesozoic, and the Palaeozoic strata, the fossils may be similar to creatures at present living in some other province, or may be altogether unlike any which now exist. DISTRIBUTION IN TIME. 25 In any given locality, the succession of living forms may appear to be interrupted by numerous breaks — the associated species in each fossiliferous bed being quite distinct from those above and those below them. But the tendency of all palaeontological investigation is to show that these breaks are only apparent, and arise from the incompleteness of the series of remains which happens to have been preserved in any given locality. As the area over which accurate geological investi- p-ations have been carried on extends, and as the fossiliferous rocks found in one locality fill up the gaps left in another, so do the abrupt demarkations between the faunae and florae of successive epochs disappear — a certain proportion of the gen- era and even of the species of every period, great or small, being found to be continued for a longer or shorter time into the next succeeding period. It is evident, in fact, that the changes in the living population of the globe which have taken place during its history have been effected, not by the sud- den replacement of one set of living beings by another, but by a process of slow and gradual introduction of new species, accompanied by the extinction of the older forms. It is a remarkable circumstance that, in all parts of the globe in which fossiliferous rocks have yet been examined, the successive terms of the series of living forms which have thus succeeded one another are analogous. The life of the Mesozoic epoch is everywhere characterized by the abundance of some groups of species of which no trace is to be found in either earlier or later formations ; and the like is true of the Palaeozoic epoch. Hence it follows, not only that there has been a succession of species, but that the general nature of that succession has been the same all over the globe ; and it is on this ground that fossils are so important to the geologist as marks of the relative age of rocks. The determination of the morphological relations of the species which have thus succeeded one another, is a problem of profound importance and difficulty, the solution of which, however, is already clearly indicated. For, in several cases, it is possible to show that, in the same geographical area, a form A, which existed during a certain geological epoch, has been replaced by another form B, at a later period; and that this form B has been replaced, still later, by a third form C. When these forms, A, B, and C, are compared together they are found to be organized upon the same plan, and to be verv similar even in most of the details of their structure; but B differs from A by a slight modification of some of its 26 THE ANATOMY OF INVERTEBRATED ANIMALS. parts, which modification is carried to a still greater extent inC. In other words, A, B, and C, differ from one another in the same fashion as the earlier and later stages of the em- bryo of the same animals differ ; and, in successive epochs, we have the group presenting that progressive specialization which characterizes the development of the individual. Clear evidence that this progressive specialization of structure has actually occurred has as yet been obtained in only a few cases (e. g., Equidoe, Crocodilia), and these are confined to the highest and most complicated forms of life ; while it is de- monstrable that, even as reckoned by geological time, the pro- cess must have been exceedingly slow. Among the lower and less complicated forms, the evidence of progressive modification, furnished by comparison of the oldest with the latest forms, is slight, or absent ; and some of these have certainly persisted, with very little change, from extremely ancient times to the present day. It is as important to recognize the fact that certain forms of life have thus persisted, as it is to admit that others have undergone progressive modification. It has been said that the successive terms in the series of living forms are analogous in all parts of the globe. But the species which constitute the corresponding or homotaxic terms in the series, in different localities, are not identical. And, though the imperfection of our knowledge at present pre- cludes positive assertion, there is every reason to believe that geographical provinces have existed throughout the period during which organic remains furnish us with evidence of the existence of life. The wide distribution of certain Palaeozoic forms does not militate against this view ; for the recent in- vestigations into the nature of the deep-sea fauna have shown that numerous Crustacea, Echinodermata, and other inver- tebrate animals, have as wide a distribution now as their ana- logues possessed in the Silurian epoch. III. Physiology. Thus far, living beings have been regarded merelj' as definite forms of matter, and biology has presented no con- siderations of a different order from those which meet the student of mineralogy. But living things are not only natural bodies, having a definite form and mode of structure, growth, and development. They are machines in action ; and, under FUNCTIONS AND ORGANS. 27 this aspect, the phenomena which they present have no par- allel in the mineral world. The actions of living matter are termed its functions ; and these functions, varied as they are, may be reduced to three categories. They are either — (1), functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand and those of assimilation on the other ; or (2), they are functions which subserve the process of reproduction, which is essentially the detachment of a part endowed with the pow- er of developing into an independent whole ; or (3), they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, becomes a source of molar motion. The first may be termed sustentative, the second generative, and the third correlative functions. Of these three classes *of functions the first two only can be said to be invariably present in living beings, all of which are nourished, grow, and multiply. But there are some forms of life, such as many Fungi, which are not known to possess any powers of changing their form ; in which the protoplasm exhibits no movements, and reacts upon no stimulus ; and in which any influence which the different parts of the body ex- ert upon one another must be transmitted indirectly from molecule to molecule of the common mass. In most of the lowest plants, however, and in all animals yet known, the body either constantl}7, or temporarily changes its form, either with or without the application of a special stimulus, and thereby modifies the relations of its parts to one another, and of the whole to surrounding bodies ; while, in all the higher animals, the different parts of the body are able to affect, aud be affected bv one another, by means of a special tissue, termed nerve. Molar motion is effected on a large scale by means of another special tissue, muscle ; and the organism is brought into relation with surrounding bodies by means of a third kind of special tissue — that of the sensory organs — by means of which the forces exerted by surrounding bodies are trans- muted into affections of nerve. In the lowest forms of life, the functions which have been enumerated are seen in their simplest forms, and they are ex- erted indifferently, or nearly so, by all parts of the proto- plasmic body ; and the like is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated cell, which constitutes the 28 THE ANATOMY OF IXVERTEBRATED ANIMALS. starting-point of their development. But the first process in that development is the division of the germ into a number of morphological units or blastomeres, which, eventually, give rise to cells ; and- as each of these possesses the same physio- logical functions as the germ itself, it follows that each mor- phological unit is also a physiological unit, and the multicellu- lar mass is strictly a compound organism, made up of a mul- titude of physiologically independent cells. The physiologi- cal activities manifested by the complex whole represent the sum, or rather the resultant, of the separate and independent physiological activities resident in each of the simpler con- stituents of that wThole. The morphological changes which the cells undergo in the course of the further development of the organism do not affect their individuality ; and, notwithstanding the modi- fication and confluence of its constituent cells, the adult or- ganism, however complex, is still an aggregate of morphologi- cal units. Nor is it less an aggregate of physiological units, each of which retains its fundamental independence, though that independence becomes restricted in various wTays. Each cell, or that element of a tissue wThich proceeds from the modification of a cell, must needs retain its sustentative functions so long as it grows or maintains a condition of equilibrium ; but the most completely metamorphosed cells show no trace of the generative function, and many exhibit no correlative functions. Contrariwise, those cells of the adult organism which are the unmetamorphosed derivatives of the germ exhibit all the primary functions, not only nourishing themselves and growing, but multiplying, aud frequently showing more or less marked movements. Organs are parts of the body which perform particular functions. In strictness, perhaps, it is not quite right to speak of organs of sustentation or generation, each of these functions being necessarily performed by the morphological unit which is nourished or reproduced. What are called the organs of these functions are the apparatuses by which cer- tain operations, subsidiary to sustentation and generation, are carried on. Thus, in the case of the sustentative functions, all those organs may be said to contribute to these functions which are concerned in bringing nutriment within the reach of the ulti- mate cells, or in removing waste matter from them ; while in the case of the generative function, all those organs contribute to the function which produce the cells from which germs are MUSCLE AND NERVE. 29 given off ; or help in the evacution, or fertilization, or develop- ment, of these germs. On the other hand, the correlative functions, so long as they are exerted by a simple undifferentiated morphological unit or cell, are of the simplest character, consisting of those modifications of position which can be effected by mere changes in the form or arrangement of the parts of the pro- toplasm, or of those prolongations of the protoplasm which are called pseudopodia or cilia. But, in the higher animals and plants, the movements of the organism and of its parts are brought about by the change of the form of certain tis- sues, the property of which is to shorten in one direction when exposed to certain stimuli. Such tissues are termed contractile ; and, in their most fully developed condition, muscular. The stimulus by which this contraction is natu- rally brought about is a molecular change, either in the sub- stance of the contractile tissue itself, or in some other part of the body ; in which latter case, the motion which is set up in that part of the body must be propagated to the contractile tissue through the intermediate substance of the body. In plants, there seems to be no question that parts which retain a hardly modified cellular structure may serve as channels for the transmission of this molecular motion ; whether the same is true of animals is not certain. But, in all the more com- plex animals, a peculiar fibrous tissue — nerve — serves as the agent by which contractile tissue is affected by changes oc- curring elsewhere, and by which contractions thus initiated are coordinated and brought into harmonious combination. While the sustentative functions in the higher forms of life are still, as in the lower, fundamentally dependent upon the powers inherent in all the physiological units which make up the body, the correlative functions are, in the former, deputed to two sets of specially modified units, which constitute the muscular and the nervous tissues. When the different forms of life are compared together as physiological machines, they are found to differ as machines of human construction do. In the lower forms, the mechan- ism, though perfectly well adapted to do the work fcr which it is required, is rough, simple, and weak ; w7hile, in the higher, it is finished, complicated, and powerful. Considered as machines, there is the same sort of difference between a polyp and a horse as there is between a distaff and a spin- ning-jenny. In the progress from the lower to the higher organism, there is a gradual differentiation of organs and of 30 THE ANATOMY OF INVERTEBRATED ANIMALS. functions. Each function is separated into many parts, which are severally intrusted to distinct organs. To use the strik- ing phrase of Milne-Edwards, in passing from low to high organisms, there is a division of physiological labor. And exactly the same process is observable in the development of any of the higher organisms ; so that, physiologically as well as morphologically, development is a progress from the gen- eral to the special. Thus far, the physiological activities of living matter have been considered in themselves, and without reference to any- thing that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it; and the study of the in- fluence of the " conditions of existence " thus determined constitutes a most important part of physiolog}7. The sustentative functions, for example, can only be ex- erted under certain conditions of temperature, pressure, and light, in certain media, and with supplies of particular kinds of nutritive matter ; the sufficiency of which supplies, again, is greatly influenced by the competition of other organisms, which, striving to satisfy the same needs, give rise to the passive " struggle for existence." The exercise of the correl- ative functions is influenced by similar conditions, and by the direct conflict with other organisms, which constitutes the ac- tive struggle for existence. And, finally, the generative func- tions are subject to extensive modifications, dependent partly upon what are commonly called external conditions, and part- ly upon wholly unknown agencies. In the lowest forms of life, the only mode of generation at present known is the division of the body into two or more parts, each of which then grows to the size and assumes the form of its parent, and repeats the process of multiplication. This method of multiplication by fission is properly called generation, because the parts which are separated are sev- erally competent to give rise to individual organisms of the same nature as that from which they arose. In many of the lowest organisms the process is modified so far that, instead of the parent dividing into two equal parts, only a small portion of its substance is detached, as a bud, which develops into the likeness of its parent. This is generation by gemmation. Generation by fission and by gemmation is not confined to the simplest forms of life, however. On the contrary, both modes of multiplication are AGAMOGENESIS. 31 common not only among plants, but among animals of con- siderable complexity. The multiplication of flowering plants by bulbs, that of annelids by fission, and that of polyps by budding, are well- known examples of these modes of reproduction. In all these cases, the bud or the segment consists of a multitude of more or less metamorphosed cells. But, in other in- stances, a single cell detached from a mass of such undiffer- entiated cells contained in the parental organism is the foun- dation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segment — whether the process is more akin to fission or to gemma- tion. In all these cases the development of the new being from the detached germ takes place without the influence of other living matter. Common as the process is in plants and in the lower animals, it becomes rare among the higher animals. In these, the reproduction of the whole organism from a part, in the w7ay indicated above, ceases. At most we find that the cells at the end of an amputated portion of the organism are capable of reproducing the lost part ; in the very highest animals, even this power vanishes in the adult ; and, in most parts of the body, though the undifferentiated cells are capable of multiplication, their progeny grow, not into wrhole organisms like that of which they form a part, but into ele- ments of the tissues. Throughout almost the whole series of living beings, how- ever, we find concurrently with the process of a gaino genesis, or asexual generation, another method of generation, in which the development of the germ into an organism resembling the parent depends on an influence exerted by living matter different from the germ. This is gamogenesis or sexual gen- eration. Looking at the facts broadly, and without reference to many exceptions in detail, it may be said that there is an inverse relation between agamogenetic and gamogenetic re- production. In the lowest organisms gamogenesis has not yet been observed, while in the highest agamogenesis is ab- sent. In many of the lower forms of life agamogenesis is the common and predominant mode of reproduction, while gamo- genesis is exceptional ; on the contrary, in many of the high- er, w^hile gamogenesis is the rule, agamogenesis takes place exceptionally. In its simplest condition, which is termed u conjugation ," sexual generation consists in the coalescence of two similar 3 32 THE ANATOMY OF IXVERTEBRATED ANIMALS. masses of protoplasmic matter, derived from different parts of the same organism, or from two organisms of the same species, and the single mass which results from the fusion develops into a new organism. In the majority of cases, however, there is a marked mor- phological difference between the two factors in the process, and then one is called the male, and the other the female, element. The female element is relatively large, and under- goes but little change of form. Jn all the higher plants and animals it is a nucleated cell, to which a greater or less amount of nutritive material, constituting a food-yelk, may be added. The male element, ou the other hand, is relatively small. It may be conveyed to the female element by an outgrowth of the wall of its cell, which is short in many Algce and Fungi, but becomes an immensely elongated tubular filament, in the case of the pollen-cell of flowering plants. But, more com- monly, the protoplasm of the male cell becomes converted into rods or filaments, which usually are in active vibratile movement, and sometimes are propelled by numerous cilia. Occasionally, however, as in many Nematoidea and Arthro- pocla, they are devoid of mobility. The manner in which the contents of the pollen-tube affect the embryo cell in flowering plants is unknown, as no perforation through, which the contents of the pollen-tube may pass, so as actually to mix with the substance of the em- bryo cell, has been discovered ; and there is the same diffi- culty with respect to the conjugative processes of some of the Cryptogamia. But in the great majority of plants, and in all animals, there can be no doubt that the substance of the male element actually mixes with that of the female, so that, in all these cases, the sexual process remains one of con- jugation; and impregnation is the physical admixture of pro- toplasmic matter derived from two sources, which may be either different parts of the same organism, or different organ- isms. The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions (blastorneres), which may remain united as a single cell-aggre- gate, or some or all of which may become separate organ- isms. A longer or shorter period of rest, in many cases, intervenes between the act of impregnation and the com- mencement of the process of division. As a general rule, the female cell, which directly receives GAMOGEXESIS. 33 the influence of the male, is that which undergoes division and eventual development into independent germs ; but there are some plants, such as the Floridece, in which this is not the case. In these, the protoplasmic body of the trichogyne, which unites with the spermatozooids, does not undergo division itself, but transmits some influence to adjacent cells, in virtue of which they become subdivided into independent germs or spores. There is still much obscurity respecting the reproductive processes of the Infusoria ; but, in the Vorticellidae, it would appear that conjugation merely determines a condition of the whole organism, which gives rise to the division of the endo- plast or so-called nucleus, by which germs are thrown off; and, if this be the case, the process would have some analogy to what takes place in the Floridece. On the other hand, the process of conjugation by which two distinct Dlporpce combine into that extraordinary double organism, the Diplozoon paradoazum, does not directly give rise to germs, but determines the development of the sexual organs in each of the conjugated individuals ; and the same process takes place in a large number of the Infusoria, if what are supposed to be male sexual elements in them are really such. The process of impregnation in the JFloridece is remark- ably interesting, from its bearing upon the changes which fecundation is known to produce upon parts of the parental organism other than the ovum, even in the highest animals and plants. The nature of the influence exerted by the male element upon the female is wholly unknown. No morphological dis- tinction can be drawn between those cells which are capable of reproducing the whole organism without impregnation and those which need it, as is obvious from what happens in insects, where eggs which ordinarily require impregnation, exceptionally, as in many moths, or regularly, as in the case of the drones among bees, develop without impregnation. Even in the higher animals, such as the fowl, the earlier stages of division of the germ may take place without im- pregnation. In fact, generation may be regarded as a particular case of cell-multiplication, and impregnation simply as one of the many conditions which may determine or affect that process. In the lowest organisms the simple protoplasmic mass divides, and each part retains all the physiological properties of the 34 THE ANATOMY OF IXVERTEBRATED ANIMALS. whole, and consequently constitutes a germ whence the whole body can be reproduced. In more advanced organisms each of the multitude of cells into which the embryo cell is converted at first, probably retains all, or nearly all, the physiological capabilities of the whole, and is capable of serving as a re- productive germ ; but, as division goes on, and many of the cells which result from division acquire special morphological and pliysiological properties, it seems not improbable that they, in proportion, lose their more general characters. In propor- tion, for example, as the tendency of a given cell to become a muscle-cell or a cartilage-cell is more marked and definite, it is readily conceivable that its primitive capacity to reproduce the whole organism should be reduced, though it might not be altogether abolished. If this view is well based, the power of reproducing the whole organism would be limited to those cells which had acquired no special tendencies, and conse- quently had retained all the powders of the primitive cell in which the organism commenced its existence. The more ex- tensively diffused such cells were, the more generally might multiplication by budding or fission take place ; the more lo- calized, the more limited would be the parts of the organism in which such a process would take place. And, even where such cells occurred, their development or non-development might be connected with conditions of nutrition. It depends on the nutriment supplied to the female larva of a bee wheth- er it shall become a neuter or a sexually perfect female ; and the sexual perfection of a large proportion of the internal parasites is similarly dependent upon their food, and perhaps on other conditions, such as the temperature of the medium in which they live. Thus the gradual disappearance of aga- mogenesis in the higher animals would be related with that increasing specialization of function which is their essential characteristic ; and, when it ceases to occur altogether, it may be supposed that no cells are left which retain unmodified the powers of the primitive embryo cell. The organism is like a society in which every one is so engrossed by his spe- cial business that he has neither time nor inclination to marry. Even the female elements in the highest organisms, little as they differ to all appearance from undifferentiated cells, and though they are directly derived from epithelial cells which have undergone very little modification from the condi- tion of blastomeres, are incapable of full development unless they are subjected to the influence of the male element, which may, as Caspar Wolff suggested, be compared to a kind of THE ALTERNATION OF GENERATIONS. 35 nutriment. But it is a living nutriment, in some respects comparable to that which would be supplied to an animal kept alive by transfusion, and its molecules transfer to the impregnated embryo cell all the special characters of the or- ganism to which it belonged. The tendency of the germ to reproduce the characters of its immediate parents, combined, in the case of sexual genera- tion, with the tendency to reproduce the characters of the male, is the source of the singular phenomena of hereditary transmission. No structural modification is so slight, and no functional peculiarity is so insignificant in either parent, that it may not make its appearance in the offspring. But the transmission of parental peculiarities depends greatly upon the manner in which they have been acquired. Such as have arisen naturally, and have been hereditar}- through many an- tecedent generations, tend to appear in the progeny with great force ; while artificial modifications — such, for example, as result from mutilation — are rarely, if ever, transmitted. Circumcision through innumerable ancestral generations does not appear to have reduced that rite to a mere formality, as it should have done if the abbreviated prepuce had become hereditary in the descendants of Abraham ; while modern lambs are born with long tails, notwithstanding the long-con- tinued practice of cutting those of every generation short. And it remains to be seen whether the supposed hereditary transmission of the habit of retrieving among dogs is really what it seems at first sight to be ; on the other side, Brown- Sequard's case of the transmission of artificially-induced epi- lepsy in Guinea-pigs is undoubtedly very weight}7. Although the germ always tends to reproduce, directly or indirectly, the organism from which it is derived, the result of its development differs somewhat from the parent. Usually the amount of variation is insignificant ; but it may be con- siderable, as in the so-called " sports ; " and such variations, whether useful or useless, may be transmitted with great te- nacity to the offspring of the subjects of them. In many plants and animals which multiply both asexually and sexually there is no definite relation between the aga- mogenetic and the gamogenetic phenomena. The organism may multiply asexually before, or after, or concurrently with, the occurrence of sexual generation. But in a great many of the lower organisms, both animal and vegetable, the organism (A) which results from the im- pregnated germ produces offspring only agamogenetically. 36 THE ANATOMY OF INVERTEBRATED ANIMALS. It thus gives rise to a series of independent organisms (B, B, B, . . .), which are more or less different from A, and which sooner or later acquire generative organs. From their impregnated germs A is reproduced. The process thus de- scribed is what has been termed the "alternation of genera- tions" under its simplest form — for example, as it is exhibited by the SailpoB. In more complicated cases the independent organisms which correspond with B may give rise agamo- genetically to others (BJ, and these to others (B2), and so on (e. g., Aphis). But, however long the series, a iinai term appears which develops sexual organs, and reproduces A. The " alternation of generations " is, therefore, in strictness, an alternation of asexual with sexual generation, in which the products of the one process differ from those of the other. The Hydrozoa offer a complete series of gradations be- tween those cases in which the term B is represented by a free, self-nourishing organism (e. g., Cyancea), through those in which it is free but unable to feed itself ( Calycophoridce), to those in which the sexual elements are developed in bodies which resemble free zooids, but are never detached, and are mere generative organs of the body on which they are devel- oped (Cordylophora). In the last case the " individual " is the total product of the development of the impregnated embryo, all the parts of which remain in material continuity with one another. The multiplication of mouths and stomachs in a Cordylophora no more makes it an aggregation of different individuals than the multiplication of segments and legs in a centipede con- verts that Arthropod into a compound animal. The Cordy- lophora is a differentiation of a whole into many parts, and the use of any terminology which implies that it results from the coalescence of many parts into a whole is to be depre- cated. In Cordylophora the generative organs are incapable of maintaining a separate existence ; but in nearly-allied Hydro- zoa the unquestionable homologues of these organs become free zooids, in many cases capable of feeding and growing, and developing the sexual elements only after they have un- dergone considerable changes of form. Morphologically, the swarm of Medusce thus set free from a Hydrozoon are as much organs of the latter as the multitudinous pinnules of a Comatula, with their genital glands, are organs of the Echi- noderm. Morphologically, therefore, the equivalent of the CAUSES OF THE PHENOMENA OF LIFE. 37 individual Comatula is the Hydrozoic stock plus all the Me- clusce which proceed from it. No doubt it sounds paradoxical to speak of a million of Aphides, for example, as parts of one morphological individ- ual ; but beyond the momentary shock of the paradox no harm is done. On the other hand, if the asexual Aphides are held to be individuals, it follows, as a logical consequence, not only that all the polyps on a Gordylophora tree are " feeding individuals," and all the genital sacs " generative individuals," while the stem must be a " stump individual," but that the eyes and legs of a lobster are "ocular" and " locomotive individuals." And this conception is not only somewhat more paradoxical than the other, but suggests a conception of the origin of the complexity of animal struct- ure which is wholly inconsistent with fact. IV. ^Etiology. Morphology, distribution, and physiology, investigate and determine the facts of biology. ^Etiology has for its object the ascertainment of the causes of these facts, and the ex- planation of biological phenomena, by showing that the}^ con- stitute particular cases of general physical laws. It is hardly needful to say that aetiology, as thus conceived, is in its in- fancy, and that the seething controversies, to which the attempt to found this branch of science made in the "Origin of Species " has given rise, cannot be dealt with in this place. At most, the general nature of the problems to be solved, and the course of inquiry needful for their solution, may be indi- cated. In any investigation into the causes of the phenomena of life, the first question which arises is, Whether we have any knowledge, and if so, what knowledge, of the origin of living matter ? In the case of all conspicuous and easily-studied organ- isms, it has been obvious, since the study of Nature began, that living beings arise by generation from living beings of a like kind ; but, before the latter part of the seventeenth cen- tury, learned and unlearned alike shared the conviction that this rule was not of universal application, and that multitudes of the smaller and more obscure organisms were produced by the fermentation of not-living, and especially of putrefying dead matter, by what was then termed generatio cequivoca or spontanea, and is now called abiogenesis. Redi showed 38 THE ANATOMY OF INVERTEBRATED ANIMALS. that the general belief was erroneous in a multitude of in- stances ; ISpallanzani added largely to the list ; while the in- vestigations of the scientific helminthologists of the present century have eliminated a further category of cases in which it was possible to doubt the applicability of the rule " omne vivam e vivo" to the more complex organisms which consti- tute the present fauna and flora of the earth. Even the most extravagant supporters of abiogenesis at the present day do not pretend that organisms of higher rank than the lowest Fungi and Protozoa are produced otherwise than by genera- tion from preexisting organisms. But it is pretended that Bacteria, Torulw, certain Fungi, and "Monads," are de- veloped under conditions which render it impossible that these organisms should have proceeded directly from living matter. The experimental evidence adduced in favor of this prop- osition is always of one kind, and the reasoning on which the conclusion that abiogenesis occurs is based may be stated in the following form : All living matter is killed by being heated to n degrees. The contents of a vessel, the entry of germs from without into which is prevented, have been heated to n degrees. Therefore, all living matter which may have existed there- in has been killed. But living Bacteria, etc., have appeared in these contents subsequently to their being heated. Therefore, they have been formed abiogenetically. No objection can be taken to the logical form of this rea- soning, but it is obvious that its applicability to any particu- lar case depends entirely upon the validity, in that case, of the first and second propositions. Suppose a fluid to be full of Bacteria in active motion, what evidence have we that they are killed when that fluid is heated to n degrees ? There is but one kind of conclusive evidence, namely, that from that time forth no living Bacteria make their appearance in the liquid, supposing it to be prop- erly protected from the intrusion of fresh Bacteria, The oniy other evidence, that, for example, which may be fur- nished by the cessation of the motion of the Bacteria, and such slight changes as our microscopes permit us to observe in their optical characters, is simply presumptive evidence of death, and no more conclusive than the stillness and paleness of a man in a swoon are proof that he is dead. And the caution is the more necessary in the case of Bacteria, since ABIOGENESIS. 39 many of them naturally pass a considerable part of their ex- istence in a condition in which they show no marks of life whatever save growth and multiplication. If indeed it could be proved that, in cases which are not open to doubt, living matter is always and invariably killed at precisely the same temperature, there might be some ground for the assumption that, in those which are obscure, death must take place under the same circumstances. But what are the facts? It has already been pointed out that, leaving Bacteria aside, the range of high temperatures be- tween the lowest, at which some living things are certainly killed, and the highest, at which others certainly live, is rather more than 100° Fahr., that is to say, between 104° Fahr. and 208° Fahr. It makes no sort of difference to the argument how living- being's have come to be able to bear such a tern- perature as the last mantioned ; the fact that they do so is sufficient to prove that, under certain conditions, such a tem- perature is not sufficient to destroy life.1 Thus it appears that there is no ground for the assumption that all living matter is killed at some given temperature be- tween 104° and 208° Fahr. No experimental evidence that a liquid may be heated to n degrees, and yet subsequently give rise to living organisms, is of the smallest value as proof that abiogenesis has taken place, and for two reasons : Firstly, there is no proof that organisms of the kind in question are dead, except their per- manent incapacity to grow and reproduce their kind ; and, secondly, since we know that conditions may largely modify the power of resistance of such organisms to heat, it is far more probable that such conditions existed in the experiment in question, than that the organisms were generated afresh out of dead matter. Not only is the kind of evidence adduced in favor of abiogenesis logically insufficient to furnish proof of its occur- rence, but it may be stated, as a well-based induction, that the more careful the investigator, and the more complete his mastery over the endless practical difficulties which surround experimentation on this subject, the more certain are his ex- periments to give a negative result ; while positive results are no less sure to crown the efforts of the clumsy and the careless. 1 Messrs. Dallinger and Drysdale have recently shown good grounds for believing that the germs of some Monads are not destroyed by exposure to a temperature of 260° Fahr. or even 300° Fahr. 40 THE ANATOMY OF IXVERTEBRATED ANIMALS. It is argued that a belief in abiogenesis is a necessary- corollary from the doctrine of Evolution. This may be true of the occurrence of abiogenesis at some time ; but if the present day, or any recorded epoch of geological time, be in question, the exact contrary holds good. If all living beings have been evolved from preexisting forms of life, it is enough that a single particle of living protoplasm should once have appeared on the globe, as the result of no matter what agency. In the eyes of a consistent evolutionist, any further indepen- dent formation of protoplasm would be sheer waste. The production of living matter since the time of its first appearance, only by way of biogenesis, implies that the spe- cific forms of the lower kinds of life have undergone but little change in the course of geological time, and this is said to be inconsistent with the doctrine of evolution. But, in the first place, the fact is not inconsistent with the doctrine of evolu- tion properly understood, that doctrine being perfectly con- sistent with either the progression, the retrogression, or the stationary condition, of any particular species for indefinite periods of time ; and, secondly, if it were, it would be so much the worse for the doctrine of evolution, inasmuch, as it is un- questionably true that certain, even highly-organized, forms of life have persisted without any sensible change for very long periods. The Terebratula psittacea of the present day, for example, is not distinguishable from that of the Cretaceous epoch, while the highly-organized Teleostean fish, Beryw, of the Chalk, differed only in minute specific characters from that which now lives. Is it seriously suggested that the ex- isting lerehratulm and JBeryces are not the lineal descendants of their Cretaceous ancestors, but that their modern repre- sentatives have been independently developed from primordial germs in the interval ? But if this is too fantastic a sugges- tion for grave consideration, why are we to believe that the Globif/erince of the present day are not lineally descended from the Cretaceous forms? And, if their unchanged genera- tions have succeeded one another for all the enormous time represented by the deposition of the Chalk and that of the Tertiary and Quaternary deposits, what difficulty is there in supposing that they may not have persisted unchanged for a greatly longer period ? The fact is, that at the present moment there is not a shadow of trustworthy direct evidence that abiogenesis does take place, or has taken place, within the period during which the existence of life on the globe is recorded. But it ORIGIX OF SPECIES. 41 need hardly be pointed out that the fact does not in the slightest degree interfere with any conclusion that may be arrived at, deductively, from other considerations that, at some time or other, abiogenesis must have taken place. If the hypothesis of evolution is true, living matter must have arisen from not-living matter ; for, by the hypothesis, the condition of the globe was at one time such that living matter could not have existed in it,1 life being entirely in- compatible with the gaseous state. But, living matter once originated, there is no necessity for another origination, since the hypothesis postulates the unlimited, though perhaps not indefinite, modifiability of such matter. Of the causes which have led to the origination of living matter, then, it may be said that we know absolutely nothing. But postulating the existence of living matter endowed with that power of hereditary transmission, and with that tendency to vary which is found in all such matter, Mr. Darwin has shown good reasons for believing that the interaction between living matter and surrounding conditions, which results in the survival of the fittest, is sufficient to account for the gradual evolution of plants and animals from their simplest to their most complicated forms, and for the known phe- nomena of Morphology, Physiology, and Distribution. Mr. Darwin has further endeavored to give a physical explanation of hereditary transmission by his hypothesis of Pangenesis ; while he seeks for the principal, if not the only cause of variation in the influence of changing condi- tions. It is on this point that the chief divergence exists among those who accept the doctrine of evolution in its general outlines. Three views may be taken of the causes of varia- tion : a. In virtue of its molecular structure, the organism may tend to vary. This variability may either be indefinite, or may be limited to certain directions by intrinsic conditions. In the former case, the result of the struggle for existence would be the survival of the fittest among an indefinite number of varieties ; in the latter case, it would be the survival of the fittest among a certain set of varieties, the 1 It makes no difference if we adopt Sir "W. Thomson's hypothesis, and suppose that the germs of living things have "been transported to our globe from some other, seeing that there is as much reason for supposing that all stellar and planetary components of the universe are or have been gaseous, as that the earth has passed through this stage. 42 THE ANATOMY OF IXVERTEBRATED ANIMALS. nature and number of which would be predetermined by the molecular structure of the organism. b. The organism may have no intrinsic tendency to vary, but variation may be brought about by the influence of con- ditions external to it. And in this case, also, the variability induced may be either indefinite or defined by intrinsic limi- tation. c. The two former cases may be combined, and variation may to some extent depend upon intrinsic, and to some ex- tent upon extrinsic, conditions. At present it can hardly be said that such evidence as would justify the positive adoption of any one of these views exists. If all living beings have come into existence by the gradual modification, through a long series of generations, of a pri- mordial living matter, the phenomena of embryonic develop- ment ought to be explicable as particular cases of the general law of hereditary transmission. On this view, a tadpole is first a fish, and then a tailed amphibian, provided with both gills and lungs, before it becomes a frog, because the frog was the last term in a series of modifications whereby some ancient fish became a urodele amphibian; and the urodele amphibian became an anurous amphibian. In fact, the de- velopment of the embryo is a recapitulation of the ancestral history of the species. If this be so, it follows that the development of any organism should furnish the key to its ancestral history ; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their develop- ment has given rise to a special branch of biological specula- tion, termed phytogeny. In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing mem- bers is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than an abbreviated and condensed sum- mary of ancestral conditions ; while this summary is often strangely modified by variation and adaptation to conditions ; and it must be confessed that, in most cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adapta- tion. The only perfectly safe foundation for the doctrine of evolu- tion lies in the historical, or rather archaeological, evidence PHYLOGENY. 4 o that particular organisms have arisen by the gradual modifi- cation of their predecessors, which is furnished by fossil remains. That evidence is daily increasing in amount and in weight ; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, deduced 'from the facts of embryology alone, may be satisfactorily tested. CHAPTER I. I. THE DISTINCTIVE CHARACTERS OF ANIMALS. The more complicated forms of the living things, the general characters of which have now been discussed, appear to be readily distinguishable into widely-separated groups, animals, and plants. The latter have no power of locomo- tion, and only rarely exhibit any distinct movement of their parts when these are irritated, mechanically or otherwise. They are devoid of any digestive cavity ; and the matters which serve as their nutriment are absorbed in the gaseous and fluid state. Ordinary animals, on the contrary, not only possess conspicuous locomotive activity, but their parts readily alter their form or position when irritated. Their nutriment, consisting of other animals or of plants, is taken in the solid form into a digestive cavity. But even without descending to the very lowest forms of animals and plants, wTe meet with facts which weaken the force of these apparently broad distinctions. Among animals, a coral or an oyster is as incapable of locomotion as an oak ; and a tape-wTorm feeds by imbibition and not by the ingestion of solid matter. On the other hand, the Sensitive-Plant and the Sundew exhibit movements on irritation, and the recent observatious of Mr. Darwin and others leave little doubt that the so-called " insectivorous plants " really digest and assimi- late the nutritive matters contained in the living animals which they catch and destroy. All the higher animals are dependent for the protein compounds which they contain upon other animals or upon plants. They are unable to man- ufacture protein out of simpler substances ; and, although positive proof is wanting that this incapacity extends to all animals, it may safely be assumed to exist in all those forms of animal life which take in solid nutriment, or which live parasitically on other animals or plants, in situations in which they are provided with abundant supplies of protein in a dissolved state. THE DISTINCTIVE CHARACTERS OF ANIMALS. 45 The great majority of the higher plants, on the contrary, • are able to manufacture protein when supplied with carbonic acid, ammoniacal salts, water, and sundry mineral phosphates and sulphates, obtaining the carbon which they require by the decomposition of the carbonic acid, the oxygen of which is disengaged. One essential factor in the performance of this remarkable chemical process is the chlorophyll which these plants contain, and another is the sun's light. Certain animals {Infusoria, Godenterata, Turbellaria) possess chlorophyll, but there is no evidence to show what part it plays in their economy. Some of the higher plants when parasitic, and a great group of the lower plants, the Fungi (which may be parasitic or not), are, however, devoid of chlorophyll, and are consequently totally unable to derive the carbon which they need from carbonic acid. Nevertheless they are sharply distinguished from animals, inasmuch as they are still, for the most part, manufacturers of protein. Thus such a Funo-us as Penicilliuni is able to fabricate all the con- stituents of its body out of ammonium tartrate, sulphate, and phosphate, dissolved in water {see supra, p. 14, note) ; and the yeast- plant nourishes and multiplies with exceeding rapid- ity in water containing sugar, ammonium tartrate, potassium phosphate, calcium phosphate, and magnesium sulphate. Nevertheless, the experiments of Mayer have shown that when peptones are substituted for the ammonium tartrate, the nutrition of the yeast-plant is favored instead of being impeded. So that it would seem that the yeast-plant is able to take in protein compounds and assimilate them, as if it were an animal ; and there can be no reasonable doubt that many parasitic Fungi, such as the JBotrytis Bassiana of the silk-worm caterpillar, the Empusa of the house-fly, and, very probably, the Peronospora of the potato-plant, directly as- similate the protein substances contained in the bodies of the plants and animals which they infest ; nor is it clear that these Fungi are able to maintain themselves upon less fully elaborated nutriment. Cellulose, amyloid, and saccharine compounds were former- ly supposed to be characteristically vegetable products ; but cellulose is found in the tests of Ascidians ; and amyloid and saccharine matters are of very wide, if not universal, occur- rence in animals. And on taking a comprehensive survey of the whole ani- mal and vegetable worlds, the test of locomotion breaks down as completely as does that of nutrition. For it is the rule 46 THE ANATOMY OF INVERTEBRATED ANIMALS. rather than the exception among the lowest plants, that at one stage or other of their existence they should be actively locomotive, their motor organs being usually cilia, altogether similar in character and function to the motor organs of the lowest animals. Moreover, the protoplasmic substance of the body in many of these plants exhibits rhythmically pulsating spaces or contractile vacuoles of the same nature as those characteristic of so many animals. No better illustration of the impossibility of drawing any sharply-defined distinction between animals and plants can be found than that which is supplied by the history of what are commonly termed '•Monads." The name of "Monad"1 has been commonly applied to minute free or fixed, rounded or oval bodies, provided with one or more long cilia (flagella), and usually provided with a nucleus and a contractile vacuole. Of such bodies, all of which would properly come under the old group of Monadi- dce, the history of a few has been completely worked out ; and the result is that, while some (e. g., Chlamydomoiias, zoospores of JPeronospora and Coleochaite) are locomotive conditions of indubitable plants, others (Hadiolaria, Nocti- luca) are embryonic conditions of as indubitable animals. Yet others (zoospores of Myxomycetes) are embryonic forms of organisms which appear to be as much animals as plants ; inasmuch as in one condition they take in solid nutriment, and in another have the special morphological, if not physio- logical peculiarities of plants; while, lastly, in the case of such monads as those recently so carefully studied by Messrs. Dallinger and Drysdale, the morphological characters of which are on the whole animal, while their mode of nutrition is un- known, it is impossible to say whether they should be regarded as animals or as plants. Thus, traced down to their lowest terms, the series of plant forms gradually lose more and more of their distinctive vegetable features, while the series of animal forms part with more and more of their distinctive animal characters, and the two series converge to a common term. The most character- istic morphological peculiarity of the plant is the investment of each of its component cells by a sac, the walls of which contain cellulose, or some closely analogous compound ; and - - 0. F. Miiller, " Historia Vermium," 1773. " Vermis ineonspicuus, sim- plicissimu:?, pellucidus, punctiformis," MORPHOLOGICAL DIFFERENTIATION. 47 the most characteristic physiological peculiarity of the plant is its power of manufacturing protein from chemical com- pounds of a less complex nature. The most characteristic morphological peculiarity of the animal is the absence of any such cellulose investment.1 The most characteristic physiological peculiarity of the animal is its want of power to manufacture protein out of simpler compounds. The great majority of living things are at once referable to one of the two categories thus defined ; but there are some in which the presence of one or other characteristic mark cannot be ascertained, and others which appear at different periods of their existence to belong to different categories. II. — THE MORPHOLOGICAL DIFFERENTIATION OF ANIMALS. The simplest form of animal life imaginable would be a protoplasmic body, devoid of motility, maintaining itself by the ingestion of such proteinaceous, fatty, amyloid, and min- eral matters as might be brought into contact with it by ex- ternal agencies ; and increasing by simple extension of its mass. But no animal of this degree of simplicity is' known to exist. The very humblest animals with which wre are ac- quainted exhibit contractility, and not only increase in size, but, as they grow, divide, and thus undeigo multiplication. In the simplest known animals — the Protozoa — the proto- plasmic substance of the body does not become differentiated into discrete nucleated masses or cells, which by their meta- morphosis give rise to the different tissues of which the adult body is composed. And, in the lowest of the Protozoa, the body has neither a constant form nor any further distinction of parts than a greater density of the peripheral, as com- pared with the central, part of the protoplasm. The first steps in complication are the appearance of one or more rhythmically contractile vacuoles, such as are found in some of the lower plants ; and the segregation of part of the in- 1 No analysis of the substance composing the cysts in which so many of the Protozoa inclose themselves temporarily has yet been made. But it is not im- probable that it may be analogous to cfvitin ; and, if so, it is worthy of remark that, though chitin is a nitrogenous body, it readily yields a substance appar- ently identical with cellulose when heated with the double hyposulphite of copper and ammonia. It is possible, therefore, that the difference between the chitinous investment of an animal and the cellulose investment of a plant may depend upon the proportion of nitrogenous matter which is present in each case in addition to the chitin, 4 48 THE ANATOMY OF INVERTEBRATED AXIAJALS. terior protoplasm as a rounded mass, the "endoplast" or " nucleus." Other Protozoa advance further and acquire permanent locomotive organs. These may be developed only on one part of the surface of the body, which may be modified into a special organ for their support. In some, a pedicle of attachment is formed, and the body may acquire a dense envelope (Infusoria), or secrete an internal skeleton of calcareous or silicious matter (Foraminifera, Madiolaria) , or fabricate such a skeleton by gluing together extraneous par- ticles (Foraminifera). A mouth and gullet, with an anal aperture, may be formed, and the permeable soft central portion of the protoplasm may be so limited as to give rise to a virtual alimentary tract be- tween these two apertures. The contractile vacuole may be developed into a complicated system of canals (Paramoeci- um), and the endoplast may take on more and more definite- ly the characters of a reproductive organ, that is, may be the focus of origin of germs capable of reproducing the individ- ual ( Vorticella). In fact, rudiments of all the chief system of organs of the higher animals, with the exception, more or less doubtful, of the nervous, are thus sketched out in the Protozoa, just as the organs of the higher plants are sketched out in Caulerpa. In the Metazoa, which constitute the rest of the animal kingdom, the animal, in its earliest condition, is a protoplas- mic mass with a nucleus — is, in short, a Protozoon. But it never acquires the morphological complexity of its adult state by the direct metamorphosis of the protoplasmic matter of this nucleated body — the ovum — into the different tissues. On the contrary, the first step in the development of all the Metazoa is the conversion of the single nucleated body into an aggregation of such bodies of smaller size — the Morula — by a process of division, which usually takes place with great regularity, the ovum dividing first into two segments, which then subdivide, giving rise to four, eight, sixteen, etc., portions, which are the so-called division masses or blasto- ?neres. A similar process takes place in sundry Protozoa and gives rise to a protozoic aggregate, which is strictly comparable to the Morula. But the members of the protozoic aggregate become separate, or at any rate independent existences. What distinguishes the metazoic aggregate is that, though its component blastomeres also retain a certain degree of physi- ological independence, they remain united into one morpho- MORPHOLOGICAL DIFFERENTIATION. 49 logical whole, and their several metamorphoses are so ordered and related to one another that they constitute members of a mutually dependent commonalty. The Metazoa are the only animals which fall under com- mon observation, and have therefore been known from the earliest times. All the higher languages possess general names equivalent to our beast, bird, reptile, fish, insect, and worm ; and this shows the very early perception of the fact ohat, notwithstanding the wonderful diversity of animal forms, they are modeled upon comparatively few great types. In the middle of the last century the founder of modern Taxonomy, Linnaeus, distinguished animals into Mammalia, Aves, Amphibia, Pisces, Insecta, and Vermes, that is to say, he converted common-sense into science by defining and giv- ing precision to the rough distinctions arrived at by ordinary observation. At the end of the century, Lamarck made a most impor- tant advance in general morphology, by pointing out that mammals, birds, reptiles, and fishes, are formed upon one type or common plan, the essential character of which is the pos- session of a spinal column, interposed between a cerebro-spi- nal and a visceral cavity ; and that in no other animals is the same plan of construction to be discerned. Hence he drew a broad distinction between the former and the latter, as the Vertebrata and the Invertebrata. But the advance of knowledge respecting the structure of invertebrated animals, due chiefly to Swammerdam, Trembley, Reaumur, Peyssonel, Goeze, Roesel, Ellis, Fabricius, O. F. Muller, Lyonet, Pallas, and Cuvier, speedily proved that the Invertebrata are not framed upon one fundamental plan, but upon several ; and, in 1795, Cuvier1 showed that, at fewest, three morphological types, as distinct from one another as they are from that of the vertebrated animals, are distinguishable among the In- vertebrata. These he named — I. Mollusques ; II. Insectes et Vers ; III. Zoophytes. In the " Regne animal " (1816), those terms are Latinized, Anlmalia Mollusea, Articulata, and JRa- diata. Thus, says Cuvier : " It will be found that there ex- ist four principal forms, four general plans, if it may thus be expressed, on which all animals appear to have been modeled ; and the ulterior divisions of which, under whatever title natu- ralists may have designated them, are merely slight modifica- tions, founded on the development or addition of certain parts. 1 Tableau elementaire de l'Histoire des Animaux. An vi. 50 THE ANATOMY OF INYERTEBRATED ANIMALS. These four common plans are those of the Vertebrata, the MoU lusca, the Articulately and the Radiata." For extent, variety, and exactness of knowledge, Cuvier was, beyond all comparison, the greatest anatomist who has ever lived ; but the absence of two conditions rendered it impossible that his survey of the animal kingdom should be exhaustive, grand and comprehensive as it was. Up to the time of Cuvier's death in 1832, microscopic in- vestigation was in its infancy, and hence the great majority of the lowest forms were either unknown or little understood ; and it was only in the third decade of the present century that Rathke, Dollinger, and Von Baer, commenced that won- derful series of exact researches into embryology which Von Baer organized into a special branch of morphology, develop- ing all its most important consequences and raising it to its proper position, as the criterion of morphological theories. Upon embryological grounds Von Baer arrived at the same conclusion as Cuvier, that there are four common plans of animal structure. In the course of the last half-century the activity of anat- omists and embryologists has been prodigious, and it may be reasonably doubted whether any form of animal life re- mains to be discovered which will not be found to accord with one or other of the common plans now known. But at the same time this increase of knowledge has abolished the broad lines of demarkation which formerly appeared to sepa- rate one common plan from another. Even the hiatus between the Vertebrata and the Inver- tebrate!, is partly, if not wholly, bridged over; and though among the Invertebrata there is no difficulty in distinguish- ing the more completely differentiated representatives of such types or common plans as those of the Arthropoda, the Annelida, the Mollusca, the Tnnicata, the Ecliinodermata, the Ccdenterata, and the Porifera, yet every year brings forth fresh evidence to the effect that, just as the plan of the plant is not absolutely distinct from that of the animal, so that of the Vertebrate has its points of community with that of certain of the Invertebrates ; that the Arthropod, the Mol- lusk, and the Echinoderm plans are united by that of the lower worms; and that the plan of the latter is separated by no very great differences from that of the Ccelenterate and that of the Sponge. Whatever speculative views may be held or rejected as to the origin of the diversities of animal form, the facts of anat- ANXULOSE DIFFERENTIATION. 51 omy and development compel the morphologist to regard the whole of the Metazoa as modifications of one actual or ideal primitive type, which is a sac with a double cellular wall, inclosing a central cavity and open at one end. This is what Haeckel terms a Gastrwa. The inner wall of the sac is the hypoblast {endoderm of the adult), the outer the epiblast {ectoderm). Between the two, in all but the very lowest Metazoa, a third layer, the mesoblast [mesoderm of the adult), makes its appearance. In the Porifera, the terminal aperture of the gastrasa becomes the egestive' opening of the adult animal, and the ingestive apertures are numerous secondary pore-like aper- tures formed by the separation of adjacent cells of the ec- toderm and endoderm. The body may become variously branched, a fibrous or spicular endoskeleton is usually de- veloped in the ectoderm, and no perivisceral cavity is de- veloped. There are no appendages for locomotion or pre- hension ; no nervous system nor sefnsory organs are known to exist; nor are there any circulatory, respiratory, renal, or generative organs. In the Coelenterata, the terminal aperture of the gastraea becomes the mouth, and, if pores perforate the body-walls, they do not subserve the ingestion of food. There is no sep- arate perivisceral cavity, but, in many, an enter occele or sys- tem of cavities, continuous with, but more or less separate from, the digestive cavity, extends through the body. Pre- hensile appendages, tentacula, are developed in great variety. A chitinous exoskeleton appears in some, a calcareous or chit- inous endoskeleton in others. There are no circulatorv, re- spiratory, or renal organs (though it is possible that certain cells in the Porpitm, e. g., may have a uropoietic function); but special genital organs make their appearance, as do a definitely-arranged nervous system and organs of sense. The lowest Tarbellaria are on nearly the same grade of organization as the lower Coelenterata, but the thick meso- derm is traversed by canals which constitute a icater-vascular system. In the adult state these canals open, on the one side, into the interstices of the mesodermal tissues, and, on the other, communicate with the exterior. Their analogy to the contractile vacuoles of the Infusoria on the one hand, and to the segmental organs of the Annelids on the other, lead me to think that they are formed by a splitting of the mesoblast, and that they thus represent that form of perivisceral cavity which I have termed a schizocoele. A nervous system, con- 52 THE ANATOMY OF IXYERTEBRATED ANIMALS. sisting of a single or double ganglion with two principal lon- gitudinal nerve-cords, is found in many ; and there may be eyes and auditory sacs. Upon this foundation a gradual complication of form is based, brought about by — 1. The elongation of the bilaterally symmetrical body and the formation of a chitinous exoskeleton. 2. The development of a secondary aperture near the an- terior end of the body, which becomes the permanent mouth. 3. The division of the mesoblast into successive segments (somites). 4. The development of two nervous ganglia in each somite. 5. The outgrowth of a pair of appendages from each so- mite, and their segmentation. 6. The gradual specialization of the somites into cephalic, thoracic and abdominal groups ; and that of their appendages into sense organs, jaws, locomotive limbs, and respiratory or- gans. 7. The conversion of the schizoccele into a spacious peri- visceral cavity containing blood ; the reduction of the water- vascular system, and the appearance of pseudo-haemal vessels ; and the replacement of these, in the higher forms, by a heart, arteries, and veins, which contain blood. 8. The conversion of the simple inner sac of the gastraea into a highly-complex alimentary canal, with special glandu- lar appendages, representing the liver and the kidneys. 9. A similar differentiation of the genital apparatus. 10. A gradual complication of the eye, w7hich, in its most perfect form, presents a series of crystal-clear conical rods, disposed perpendicularly to the transparent corneal region of the chitinous exoskeleton, and connected by their inner ends with the optic nerves of the prae-cesophageal ganglia. By such modifications as these the plan of the simple Turbellarian gradually passes into that of the highest Ar- thropod. Starting from the same point, if the mesoblast does not become distinctly segmented * if few, probably not more than three, pairs of ganglia are formed ; if there are no seg- mented appendages, but the chief locomotive organ is a mus- cular foot developed in the neural aspect of the body ; if, in the place of the chitinous exoskeleton, a shell is secreted by a specially modified part of the haemal wall termed the man- tle ; if the schizoccele is converted into a blood-cavity, which communicates with the exterior by an organ of Bojanus, which THE PLAN OF THE ECHINODERMS. 53 appears to represent the water-vascular system and the seg- mental organs ; and if, along with these changes, the aliment- ary, circulatory, respiratory, genital, and sensory organs take on special characters, we arrive at the complete Molluscan plan. From the Turbellarian to the Tunicate, or Ascidian, the passage is indicated, if not effected, by Palanoglossus, which, in its larval state, is comparable to an Appendicular ia with- out its caudal appendage. On the other hand, the large pharynx of the Tunicata and the circle of tentacula around the oral aperture, with the single ganglion, approximate them to the Polyzoa. In the perforation of the pharynx by lateral apertures, which communicate with the exterior, either di- rectly or by the intermediation of an atrial cavity, the Tuni- cata resemble only Balanoglossiis and the Vertebrata. The axial skeleton of the caudaJ appendage has no parallel except in the vertebrate notochord. In the structure of the heart and the regular reversal of the direction of its contractions, the Tunicata stand alone. The general presence of a test solidified by cellulose is a marked peculiarity, but in esti- mating its apparent singularity the existence of cellulose as a constituent of chitin must be remembered. Finally, the tadpole-like larvae of many Ascidians are comparable only to the Cercarice of Trematodes, on the one hand, and to ver- tebrate larval forms on the other. Yet another apparently very distinct type is met with in the extensive group of the Echinodermata. . In all the other Metazoa, except the Porifera and Ccelen- terata, the plan of the body is, obviously, bilaterally sym- metrical, the halves of the body on each side of a median ver- tical plane being similar. Any disturbance of this symmetrv, such as is found in some Arthropoda and in many Mollusca, arises from the predominant development of one half. But, in a Sea-urchin or Starfish, five or more similar sets of parts are disposed around a longitudinal axis, which has the mouth at one end and the anus at the other ; there is a radial sym- metry, as in a sea-anemone or a Ctenophoran. Nevertheless, close observation shows that, as is also the case in the Actinia or Ctenophoran, this radial symmetry is never perfect, and that the body is really bilaterally symmetrical in relation to a median plane which traverses the centre of length of one of the radiating met a meres. Another marked peculiarity of the Echinoderm type is 54 THE ANATOMY OF IXVERTEBRATED ANIMALS. the general, if not universal, presence of a system of " am- bulacra! vessels" consisting of a circular canal around the mouth, whence canals usually arise and follow the middle line of each of the ambulacral metameres. And, in the typical Echinoderm, these canals give off prolongations which enter certain diverticula of the body-wall, the pedicels or suckers. All Echinoderms have a calcareous endoskeleton. In the chapter allotted to these animals, it will be shown that they are modifications of the Turbellarian type, brought about by a singular series of changes undergone by the endo- derm and mesoderm of the larva or Echinopoedium. III. THE PHYSIOLOGICAL DIFFERENTIATION OF ANIMALS, AND THE MORPHOLOGICAL DIFFERENTIATION OF THEIR ORGANS. Regarded as machines for doing certain kinds of work, animals differ from one another in the extent to which this work is subdivided. Each subordinate group of actions or functions is allotted to a particular portion of the body, which thus becomes the organ of those functions ; and the extent to which this division of physiological labor is carried differs in degree within the limits of each common plan, and is the chief cause of the diversity in the working out of the common plan of a group exhibited by its members. Moreover, there are certain types which never attain the same degree of physi- ological differentiation as others do. Thus, some of the Protozoa attain a grade of physiological complexity as high as that which is reached by the lower Me- tazoa. And, notwithstanding the multiplicity of its parts, no Echinoderm is so highly differentiated a physiological ma- chine as is a snail. A mill with ten pairs of millstones need not be a more complicated machine than a mill wTith one pair ; but if a mill have two pairs of millstones, one for coarse and one for fine grinding, so arranged that the substance ground passes from one to the other, then it is a more complicated machine — a machine of higher order — than that with ten pairs of similar grindstones. In other words, it is not mere multiplication of organs which constitutes physiological differentiation ; but the multiplication of organs for different functions in the first place, and the degree in which they are coordinated, so as to work to a common end, in the second place. Thus, a lobster is a higher animal, from a physiological point of view, than a THE TEGUMENTARY SYSTEM. 55 Cyclops, not because it has more distinguishable organs, but because these organs are so modified as to perform a much greater variety of functions, while they are all coordinated toward the maintenance of the animal, by its wellrdeveloped nervous system and sense-organs. But it is impossible to say that, e. g., the Arthropoda, as a whole, are physiologically higher than the Mollusca, inasmuch as the simplest embodi- ments of the common plan of the Arthropoda are less differ- entiated physiologically than the great majority of Mollusks. I may now rapidly indicate the mode in which physiologi- cal differentiation is effected in the different groups of organs of the body among the Metazoa. Integumentary Organs. — In the lowest Metazoa, the integ- ument and the ectoderm are identical, but, so soon as a mes- oderm is developed, the layer of the mesoderm wmich is in contact with the octoderm becomes virtually part of the in- tegument, and in all the higher animals is distinguished as the dermis (enderon), while the ectodermal cells constitute the epidermis (eederoti). The connective tissue and muscles of the integument are exclusively developed in the enderon ; while, from the epidermis, all cuticular and cellular exoskele- tal parts, and all the integumentary glands, are developed. The latter are always involutions of the epidermis. The hard protective skeletons in all invertebrate Metazoa, except the Porifera, the Act i n ozoa, the Echinodermata, and the Tuni- cata, are cuticular structures, which may be variously impreg- nated with calcareous salts formed on the outer surface of the epidermic cells. In the Porifera, the calcareous or silicious deposit takes place within the ectoderm itself, and probably the same pro- cess occurs, to a greater or less extent, in the Actinozoa. In those Tunicata which possess a test, it appears to be a struct- ure sui generis, consisting of a gelatinous basis excreted by the ectoderm, in which cells detached from the ectoderm divide, multiply, and give rise to a deposit of cellulose. The test may take on the structure of cartilage or even of connec- tive tissue. In the Vertebrata alone do we find hard exo- skeletal parts formed by the cornification and cohesion of epi- dermic cells. In the Actinozoa and the Echinodermata. the hard skele- 7 ton is, in the main, though perhaps not wholly, the result of calcification of elements of the mesoderm. In some Mollusks portions of the mesoderm are converted into true cartilage, 56 THE ANATOMY OF INVERTEBRATED ANIMALS. while the enderon of the integument often becomes the seat of calcareous deposit. The endoskeleton and the dermal exo- skeleton of the Vertebrata are cellular (cartilage, notochord) or fibrous (connective tissue) modifications of the mesoderm, which may become calcified (bone, dentine). Recent investi- gations tend to show that the enamel of the teeth is derived from the ectoderm. The Alimentary Apparatus. — From the simple sac of the Hydra or aproctous Turbellarian, we pass to the tubular ali- mentary tract of the proctuchous Turbellaria. In the Hoti- fera and Polyzoa there is a marked distinction into buccal cavity, pharynx, oesophagus, stomach, and intestines ; while distinct salivary, hepatic, and renal glands, are found in the majority of the higher invertebrates, and, not unfrequently, glands secreting an odorous or colored fluid appear in the region of the termination of the alimentary canal. The oral and gastric regions are armed with cuticular teeth in many Invertebrata / but teeth formed by the calcifi- cation of papillary elevations of the enderon of the lining of the mouth are confined to the Vertebrata ; unless, as seems probable, the teeth of the Echinidea have a similar origin. The lining membrane of the oral cavity is capable of being everted, as a proboscis, in many Invertebrata. The margins of the mouth may be raised into folds, armed with cuticular plates. In the Vertebrata, the jaws are such folds, supported by endoskeletal cartilages, belonging to the system of the visceral arches, or by bones developed in and around them ; but, in the Arthropoda, what are usually termed jaws are modified limbs. The Blood and Circulatory Apparatus. — In the Coelen- terata, the somatic cavity, or enteroccele, is in free commu- nication with the digestive cavity, and not unfrequently communicates with the exterior by other apertures. The fluid which it contains represents blood ; it is moved by the con- tractions of the body, and generally by cilia developed on the endodermal lining of the enteroccele. In the Turbellaria, Trematoda, and Cestoidea. the lacunae of the mesoderm and the interstitial fluid of its tissues are the only representatives of a blood-vascular system. It is probable that these com- municate directly with the terminal ramifications of the water- vascular system. In the Motifera, a spacious perivisceral cavity separates the mesoderm into two layers, the splanch- THE BLOOD-SYSTEM. 57 nopleure, which forms the enderon of the alimentary canal, and the somatopleure, which constitutes the enderon of the integument. The terminations of the water- vessels open into this cavity. In Annelids, there is a similar perivisceral cavity communicating in the same way with the segmental organs ; but, in most, there is, in addition, a system of canals with contractile walls, which, in some, communicate freely with the perivisceral cavity, but, in the majority, are shut off from it. These canals are filled by a clear, usually non-corpuscu- lated fluid, which may be red or green, and constitute the pseud-hoemal system. The fluid which occupies the perivis- ceral cavity contains nucleated corpuscles, and has the characters of ordinary blood. It seems probable that the fluid of the pseud-haemal vessels, as it contains a substance resembling haemoglobin, represents a sort of respiratory blood. In the Arthropoda, no segmental organs or pseud-haemal vessels are known. In the lowest forms, the perivisceral cavity and the interstices of the tissues represent the whole blood-system, and colorless blood-cells float in their fluid con- tents. In the higher forms, a valvular heart, with arteries and capillaries, appears, but the venous system remains more or less lacunar. In the Mollusca, the same gradual differen- tiation of the blood-vascular system is observable. In very many, if not all, the blood-cavities communicate directly with the exterior by the " organs of Bojanus " — which resemble very simple segmental organs, and appear to be always asso- ciated with the renal apparatus. In the Vertebrata, Amphioxus has a system of blood-ves- sels, with contractile walls, and no distinct heart. In all the other Vertebrates there is a heart with at fewest three chambers (sinus venosus, atrium, ventricle), arteries, capil- laries, and veins, and a system of lymphatic vessels connected with the veins. The lymphatic fluid consists of a colorless plasma, with equally colorless nucleated corpuscles ; the blood- plasma contains, in addition, red corpuscles, which are nucle- ated in Ichthyopsida and Sauropsida, but have no nucleus in the Mammalia. The lymphatic vessels always communi- cate with the interstitial lacunae of the tissues, and in the lower Vertebrates are themselves, to a great extent, irregular sinuses. The venous system presents many large sinuses in the lower Vertebrates ; while, in the higher forms, these sinuses are for the most part replaced by definite vessels with muscular walls. But the " serous cavities " remain as vast 58 THE ANATOMY OF IX VERTEBRATED ANIMALS. lymphatic lacunae. Valves make their appearance in the lym- phatics and in the veins, and the heart becomes subdivided in such a manner as to bring about a more and more complete separation of the systemic circulatory apparatus from that which supplies the respiratory organs. The Respiratory System. — In the lower Metazoa respira- tion is effected by the general surface of the body. In the Annelids, processes of the integument, which are sometimes branched and usually are abundantly ciliated and supplied with pseud-haemal vessels, give rise to branch ice. Branchiae abundantly supplied with blood-vessels, but never ciliated, attain a great development in the Crustacea. The access of fresh water to them is secured by their attachment to some of the limbs ; and, in the higher Crustaceans, one of the ap- pendages, the second maxilla, serves as an accessory organ of respiration. Although especially adapted for aquatic res- piration, they are converted into air-breathing organs in the laud-crabs, being protected and kept moist in a large cham- ber formed by the carapace. In some mollusks (e. g., Pteropoda\ the delicate lining membrane of the pallial cavity serves as the respiratory organ ; but, in most, branched or laminated processes of the body give rise to distinct branchiae. The mantle becomes an accessory organ of respiration, being so modified as to direct, or to cause, the flow of currents of water over the branchiae contained in its cavity. In many adult urodele Amphibia (Perennibranchiatci), and in the embrj'onic condition of all Amphibia and of manv fishes, branchiae of a similar character, abundantly supplied with blood-vessels, are attached to more or fewer of the visceral arches. In all these cases the branchiae are external, and are de- veloped from the integument. In Crustaceans and Mollusks the blood with which they are supplied is returning to the heart ; while, in the Vertebrata mentioned, it is flowing from the heart ; and it will be observed that the gradual per- fectioning of the respiratory machinery consists, first, in the outgrowth of parts of the integument specially adapted to subserve the interchange between the gases contained in the blcod and those in the surrounding medium ; secondly, in the increase of the surface of the branchiae, so as to enable them to do their work more rapidly ; thirdly, in the development of accessory organs, by which the flow of water over the branchiae is rendered definite and constant, and may be in- THE RESPIRATORY SYSTEM. 59 creased or diminished in accordance with the needs of the economy. It is probable that the water-vascular system and the seg- mental organs of Turbellarians and Annelids, the cloacal tubes of the Gephyrea and of some Holothuridea, the ambu- lacral vesicles of the Echinoderms, and the large pharyngeal cavity of the Polyzoa, to a greater or less extent, subserve respiration, and constitute internal respiratory organs. In Myriapoda and Jnsecta, the tracheae, — tubes which open on the surface of the body and contain air, and are curiously similar in their distribution to the water-vessels of the worms — constitute a very complete internal aerial respira- tory apparatus. In Arachnida, tracheae may exist alone, or be accom- panied by folded pulmonary sacs, or the latter may exist alone, as in the Scorpion. In this case, these lungs are sup- plied by blood which is returning from the heart. In these animals, the flow of air into and out of the air- cavities is governed by the contractions of muscles of the body, disposed so as to alter its vertical and longitudinal dimensions. In the higher foims, the entrance and exit of air is regulated by valves, placed at the external openings (stigmata) of the trachea?, and provided with muscles, by which they can be shut. In the Enteropneusta and the Tunicata a new form of internal aquatic respiratory apparatus appears. The large pharynx is perforated by lateral apertures, which place its cavity in communication with the exterior ; and water, taken in by the mouth, is driven through' these branchial clefts and aerates the blood which circulates in their interspaces. The respiratory apparatus of Ampldoxm, of all adult fishes, and of the tadpoles of the higher anurous Amphibia, in a certain stage of their existence, is of an essentially simi- lar character. The accessory respiratory apparatus lor the maintenance and the regulation of the currents of water over the gills is furnished by the visceral arches and their mus- cles; and the respiratory blood flows from the heart. In Mollusks which live on land (Pidmogasteropoda), the lining wall of the mantle cavity becomes folded and highly vascular, and subserves the aeration of the venous blocd, which flows through it on its way to the heart. The lung is here a modification of the integument, and might be termed an external lung. The lungs of the air-breathing Vertebrata, on the contrary, are diverticula of the alimentary canal, pos- 60 THE ANATOMY OF IXVERTEBRATED ANIMALS. terior to the hindermost of the visceral arches. They receive their blood from the hindermost aortic arch. It therefore flows from the heart. The gradual improvement of these lungs as respiratory machines is effected, first, by the increase of the surface over which the venous blood brought to the lungs is distributed; secondly, by changes in the walls of the cavity in which the lungs are contained, by which that cavity gradually becomes shut off from the peritoneal cham- ber, and divided from it by a muscular partition. Concur- rently with these modifications, a series of alterations takes place in the accessory apparatus of respiration, whereby the machinery of inspiration, which, in the lower Vertebrata, is a buccal force-pump, which drives air into the lungs, in the same way as water is driven through the branchiae, is replaced by a thoracic suction-pump, which draws air into the lungs by dilatation of the walls of the closed cavity in which they are contained. Along with these changes, modifications of the heart take place, in virtue of which one-half of its total mechanical power becomes more and more exclusively ap- propriated to the task of driving the blood through the lungs. The term *' double circulation " applied to the course of the blood in the highest Vertebrata is, however, a misnomer. In the highest, as in the lowest, of these animals, the blood com- pletes but one circle, and the respiratory organ is in the course of the outward current. Many animals are truly amphibious, combining aquatic and aerial respiratory organs. Thus, among Mollusks, Ampullaria and Onchidum com- bine branchiae with pulmonary organs ; many Teleostean fishes have the lining membrane of the enlarged branchial chamber vascular and competent to subserve aerial respiration. And in the Ganoids and Teleostel the presence of an air-bladder, which is both functionally and morphologically of the same nature as a lung, is very common. But, in the majority of the Teleostei, the air-bladder is turned aside from its pulmo- nary function to subserve mechanical purposes, in affecting the specific gravity of the body. On the other hand, in the Ganoids and Dipnoi, the wdiole series of modifications by which the air-bladder passes into the lung are patent. In such lower Amphibia as Proteus and Menobranchus, bran- chial respiration is predominant, and the lungs are subsidi- ary ; but, in the higher, the lungs acquire greater importance, while the branchiae diminish, and eventually disappear. THE UROPOIETIC SYSTEM. 61 The Uropoietic System. — Uropoietic organs, distinct from the alimentary canal, are probably represented by the water- vascular system and segmental organs of the worms. The " organs of Bojanus " of Mollusks are sacs or tubes opening, on the one side, on the exterior of the body, and, on the other, into some part of the blood-vascular system. So far, as Gegenbaur has shown, they resemble the segmental organs of Annelids. In the majority of the Jfollusca, some part of the wall of the organ of Bojanus is in close relation with the venous system near the heart, and the nitrogenous waste of the body is here eliminated from the venous blood. In the Vertebrata, the? renal apparatus is constructed en the same principle. If for simplicity's sake we reduce a mammalian kidney to a ureter with a single uriniferous tubule, it cor- responds with an organ of Bojanus, so far as it contains a cavity communicating with the exterior at one end, and hav- ing a vascular plexus — the Malpighian body — in intimate contact with the opposite end. In the adult mammal there is no direct communication between the urinary duct and the blood-vascular system. But, inasmuch as recent researches have proved that the ureter is formed by subdivision of the Wolffian duct, and that the Wolffian duct is primitively a di- verticulum of the peritoneal cavity, and remains for a longer or shorter time (permanently, in some of the lower Verte- brata, as Myxine) in communication therewith ; and since it has further been shown that the peritoneal cavity communi- cates directly with the lymphatics, and therefore indirectly with the veins ; it follows that the vertebrate kidney is an extreme modification of an organ, the primitive type of which is to be found in the organ of Bojanus of the Mollusk, and in the segmental organ of the Annelid ; and, to go still lower, in the water-vascular system of the Turbellarian. And this, in its lowest form, is so similar to the more complex conditions of the contractile vacuole of a Protozoon, that it is hardly straining analogy too far to regard the latter as the primary form of uropoietic as well as of internal respiratory apparatus. The Nervous System. — In its essential nature, a nerve is a definite tract of living substance, through which the molec- ular changes which occur in any one part of the organism are conveyed to and affect some other part. Thus, if, in the simple protoplasmic body of a Protozoon, a stimulus applied to one part of the body wTere more readily transmitted to some other part, along a particular tract of the protoplasm, 62 THE ANATOMY OF INVERTEBRATED ANIMALS. that tract would be a virtual nerve, although it might have no optical or chemical characters which should enable us to distinguish it from the rest of the protoplasm. It is important to have this definition of nerve clearly before us in considering the question whether the lowest animals possess nerves or not. Assuredly nothing of the kind is discernible, by such means of investigation as we at present possess, in Protozoa or Porifera ; but any one who has attentively wTatched the ways of a Colpoda, or still more of a Vorticella, will probably hesitate to deny that they possess some aj3paratus by which external agencies give rise to localized and coordinated movements. And when we reflect that the essential elements of the highest nervous system — the fibrils into which the axis-fibres break up — are filaments of the extremest tenuity, devoid of any definite structural or other characters, and that the nervous system of animals only becomes conspicuous by the gathering to- gether of these filaments into nerve-fibres and nerves, it will be obvious that there are as strong morphological, as there are physiological, grounds for suspecting that a nervous sys- tem may exist very lowr dow^n in the animal scale, and possi- bly even in plants. The researches of Kleinenberg, which may be readily veri- fied, have shown that, in the common Hydra, the inner ends of the cells of the ectoderm are prolonged into delicate pro- cesses, which are eventually continued into very fine longi- tudinal filaments, forming a layer between the ectoderm and the endoderm. Kleinenberg terms these neuro-muscular elements, and thinks that they represent both nerve and muscle in their undifferentiated state. But it appears to me that while the assumed contractility of these fibres might account for the shortening of the body of the Polyp, they can have nothing to do with its lengthening. As the latter movements are at least as vigorous as the former, we are therefore obliged to assume sufficient contractility in the general constituents of the body to account for them. And if so, what ground is there for supposing that this contractility can be exerted by only one tissue wThen the body shortens ? To my mind, it is more probable that " Kleinenberg's fibres " are solely inter- n uncial in function, and therefore the primary form of nerve. The prolongations of the ectodermal cells have indeed a strangely close resemblance to those of the cells of the olfac- tory and other sense-organs in the Vertebrata * and it seems • THE NERVOUS SYSTEM. 63 probable that they are the channels by which impulses affect- ing any of the cells of the ectoderm are conveyed to other cells and excite their contraction. The researches of Eimer ' upon the nervous system of the Ctenophora are in perfect accordance with this view. The mesoderm is traversed in all directions by very fine fibrils, varying in diameter from 30^00 to T2|00 of an inch. These fibrils present numerous minute varicosities, and, at intervals, larger swellings which contain nuclei, each with a large and strongly refracting nucleolus. These fibrils take a straight course, branch dichotomouslv, and end in still finer filaments, which also divide, but become no smaller. They terminate partly in ganglionic cells, partly in muscular fibres, partly in the cells of the ectoderm and endoderm. Many of the nerve- fibrils take a longitudinal course beneath the centre of each series of paddles, and these are accompanied by ganglionic cells, which become particularly abundant toward the aboral end of each series. The eight bands meet in a central tract at the aboral pole of the body; but Eimer doubts the nervous nature of the cellular mass which lies beneath the lithocyst and supports the eye-spots. The nervous system of the Ctenophoran is, therefore, just such as would arise in ITjdra, if the development of a thick mesoderm gave rise to the separation and elongation of Kleinenberg's fibres, and if special bands of such fibres, developed in relation with the chief organs of locomotion, united in a central tract directly connected with the higher sensory organs. We have here, in short, virtual, though in- completely differentiated, brain and nerves. AH recent investigation tends more and more completely to establish the following conclusions : firstly, that the central ganglia of the nervous system in all animals are derived from the ectoderm; secondly, that all the nerves of the sensory organs terminate in cells of the ectoderm ; thirdly, that ail motor nerves end in the substance of the muscular fibres to which they are distributed. So that, in the highest animals, the nervous system is essentially similar to that of the lowest; the difference consisting, in part, in the proportional size of the nerve-centres, and, in part, in the gathering together of the internuncial filaments into bundles, having a definite arrangement, which are the nerves, in the ordinary anatomical sense of the term. 1 " Zoologiseln Studien auf Capri." Leipsic, 1873. 5 64 THE ANATOMY OF IXVERTEBRATED ANIMALS. And as respects the ectodermal cells which constitute the fundamental part of the organs of the special senses, it is becoming clear that the more perfect the sensory apparatus, the more completely do these sensigenous cells take on the form of delicate rods or filaments. Whether we consider the organs of the lateral ]ine in fishes and amphibia, the gusta- tory bulbs, the olfactory cells, the auditory cells, or the elements of the retina, this rule holds good. Every one of the organs of the higher senses makes its appearance in the animal series as a part of the ectoderm, the cells of which have undergone a slight modification. In the case of the eye, accessory structures, consisting of vari- ously-colored masses of pigment, which surround the visual cells, and of a transparent refracting cuticular or cellular structure which lies superficially to them — a rudimentary choroid and cornea — are next added. The highest form of compound Arthropod eye differs from this only in the differ- entiation of the layer of sensigenous cells into the crystalline cones and their appendages, and it has not been clearly made out that the simple eyes of most other Invertebrata have undergone any further change. But in Nautilus the nerve-cells and choroid line the walls of a deep cup open externally ; which, though its development has not been traced, may be safely assumed to result from the involution of the retinal ectoderm. It may be compared to an arthropod compound eye become concave instead of convex. In the higher Cephalopoda, the margins of the ocular pouch unite and give rise to a true cornea, which, however, frequently remains perforated, and a crystalline lens is de- veloped. In the higher Vertebrata the retina is still a modi- fied portion of the ectoderm. For, inasmuch as the anterior cerebral vesicle is formed by involution of the epiblast, and the optic vesicle is a diverticulum of the anterior cerebral vesicle, it necessarily follows that the outer wall of the optic vesicle is really part of the ectoderm, its inner fa, horizonial ; E and E, vertical sections of helicoid form. In E the chambers of each turn of the spiral overlap their predecessors and conceal them, as in the genus Nwnmulites. reaches the surface and gives off pseudopodia all over the body. Hence, while the hard parts of the Imperforata form a sort of exoskeleton, those of the Perforata have rather the nature of an endoskeleton. The simplest skeletons are spherical or flask-shaped, and single-chambered. #But complication arises by the addition of new chambers, which may form a linear series, or be coiled upon one another in various ways, or be irregularly aggre- gated. Moreover, the new chambers may overlap those al- ready formed in different degrees, and the interspaces between the walls of the chambers may be variously filled up by sec- ondary deposition until such large and apparently compli- cated bodies as the Nummulites are built up. The Foraminifera are almost all marine animals, living in the sea, from the surface to great depths, sometimes free, and" sometimes attached to other bodies. The investigations of Major Owen, confirmed and extend- ed by the recent work of H. M. S. Challenger, have proved that such forms as Globigerina, Pulvimdaria, and Orbidina, 6 80 THE ANATOMY OF IXVERTEBRATED ANIMALS. constantly occur at the surface of all temperate and tropical seas, and, together with the Radiolaria and the diatoma- ceous plants which accompany them, form an important in- gredient in the food of pelagic animals, such as the Salpce. It is no less certain that, at all depths down to 2,400 fath- oms or thereabouts, Globigerinoe in all stages of growth, and containing more or less protoplasmic matter, are found at the bottom mixed with the cases of the surface Diatoms and the skeletons of Radiolaria. The proportion of Globigerince, Orbulince, and Pulv miliaria?, in the deep-sea mud increases with the depth until, at depths beyond 1,000 fathoms, the sea-bottom is composed of a fine, chalky ooze made up of little more than the remains of these Foraminifera and their associated Diatoms and Radiolaria. It may be regarded as certain, therefore, that some of the chalky ooze arises from the precipitation to the bottom of the skeletons of dead Globigerince, Pidvinularice, and Orbulince, and it may be that the whole has this origin. On the other hand, it may be that a greater or smaller proportion of these Foraminifera really live at the bottom, as their congeners are known to do at less depths. It has been said that the condition of the surface-waters and sea-bottom which has just been described obtains in all temperate and hot seas ; or, speaking roughly, for 55° on either side of the equator. Toward the northern and south- ern limits of this zone the Foraminifera diminish, while Ra- diolaria remain and Diatomacece increase in proportion, so that, in the circumpolar areas north and south of 60° in each hemisphere, the surface-organisms are chiefly such as have silicious skeletons. In accordance with this condition of the surface-life, the ooze covering the sea-bottom in these regions is no longer calcareous but silicious, being composed of the cases of Diatoms and the skeletons of Radiolaria often largely mixed with ice, drifted mud, stones, gravel, and bowl- ders. If we suppose the globe to be uniformly covered with an ocean 1,000 fathoms deep, the solid land forming its bottom would be out of reach of rain, waves, and other agents of degradation, and no sedimentary deposits wTould be formed. But if Foraminifera and Diatoms, following the same laws of distribution as at present obtain, were introduced into this ocean, the fine rain of their silicious and calcareous hard parts would commence, and a circumpolar cap of silicious deposit would begin to make its appearance in the north and PROTOZOA AS ROCK-BUILDERS. 81 in the south ; while the intermediate zone would be covered with Globigerina ooze, containing a comparatively small pro- portion of silicious matter. The thickness of the calcareo- silicious and silicious beds thus formed would be limited only by time and the depth of the ocean. These strata, once ac- cumulated, would be liable to all those influences of percolat- ing moisture and subterranean heat which are known to suf- fice to convert silicious matters into opal, or quartzite, and calcareous matters into the various forms of limestone and marble. And such metamorphic agencies might more or less completely obliterate the traces of their primitive structure. But yet other changes might be effected. At the present day, in the Gulf of Mexico, off the Agulhas Bank and else- where, at no great depths (103 to 300 fathoms) the Fora- miniferal mud is undergoing a metamorphosis of another character. The chambers of the Foraminifera become filled by a green silicate of iron and alumina, which penetrates into even their finest tubuli, and takes exquisite and almost in- destructible casts of their interior. The calcareous matter is then dissolved away, and the casts are left, constituting a fine dark sand, which, when crushed, leaves a greenish mark, and is known as " green-sand." Moreover, the researches of the Challenger have shown that in great areas of the Atlantic and Pacific Oceans over which the sea has a depth exceeding 2,400 fathoms — areas in some cases of many thousand square miles in superficies — the bottom is covered not by Globigerina ooze, but by a fine red clay, which is also a silicate of iron and alumina. In this clay no remains of Globigerina or other calcareous organisms are found ; but, where these great depths gradually pass into shal- lower water, they make their appearance in a fragmentary condition — gradually becoming more and more perfect as the depth diminishes to 2,400 fathoms or thereabouts. Nevertheless the Globigerince and other Foraminifera abound at the surface over these areas as they do elsewhere, and their remains must be rained down upon it. Why they disappear, and what relation the red-clay mud has to them, is a problem not yet satisfactorily solved. It has been suggested that they are dissolved and that the red clay is merely the insoluble residue, left after the calcareous portion of their skeletons has disappeared. In this case the red clay, like the Globigerina ooze, the silicious mud, and the green-sand, will be an indirect product of living action. Metamorphic processes operating upon clay, however, may 82 THE AXATOMY OF IXVERTEBRATED ANIMALS. convert it into slate ; and thus, all the fundamental minerals of which rock-masses are composed may have formed part of living organisms, though no trace of their origin may be dis- cernible in them in their final state. Paleontology lends much support to the view that what is here suggested as a theoretically possible origin of much of the superficial crust of the globe may have been its actual origin. The nummulitic limestones of the Eocene epoch cover an enormous area of Central and Southern Europe, North Africa, West Asia, and India. And their chief mass is made up of the more or less metamorphosed remains of JForaminifera. The beds of chalk which underlie the nummulitic lime- stones, and occupy a still greater area, are essentially iden- tical with the Globigerina ooze, the species of Globigerina found in it being indistinguishable from those now livincr. The remains of i oraminifera have been detected in the lime- stones of all epochs as far as the Silurian, and Ehrenberg dis- covered that an old Silurian green-sand, near St. Petersburg, is composed of casts of Foraminifera just such as are now being formed in the Gulf of Mexico. And if the JEozoon C ana- dense be, as it appears to be, nothing but an incrusting form of Foraminifer, the existence of these oganisms is carried back to an epoch far beyond that at which any other evidence of life has yet been found. So that it is possible that, as Wy- ville Thomson has sug-gested, the enormously thick " azoic " slaty and other rocks, wmich constitute the Laurentian and Cambrian formations, may be to a great extent the metamor- phosed products of Foraminiferal life. Hence the words of Linnaeus may be literally true : " Petrefaeta non a calce, sed calx a petrefactis. Sic lapides ab animalibus, nee vice versa. Sic rupes saxei non primaevi, sed temporis filiae." And there may be no part of the common rocks, which enter into the earth's crust, which has not passed through a living organism at one time or another. II. THE EXDOPLASTICA. 1. The Radiolaeia. — Most species of the genus Actino- phrys or " sun-animalcule," which is common in ponds, are simply free-swimmino; mvxopods with stiffish pseudopodia, which radiate from all sides of the globular body. The sub- stance of the latter presents one or more " contractile spaces " THE RADIOLARIA. 83 or "vacuoles," which, rhythmically, become distended with water, and are then obliterated by the contraction of the sur- rounding protoplasm. But in the Actinophrys (or more properly Actinosphcerium) Eichornii (Fig. 4), the central part of the protoplasm is distinguished from the rest by con- taining a number of endoplasts. It thus leads to the Radiola- ria (Polycistina of Ehrenberg), the simplest forms of which Fig. 4.- Actinosphcerium Eichhornii (after Hertwig and Lesser, " Ueber Rhizopo- den." Schulze's Archiv, 1876). I.— The entire animal : c, c, contractile vacuoles. ♦•ff„«,1i .nh II.— Part of the periphery much magnified; a, a, a, pseudopodia with stiff axial tuD- stance : ?}. nuclei or endoplasts. . , ^,.„ IIL— A very young ActinosphCErium, with only two nuclei and two pseudopodia, much magnified. consist essentially of a myxopod provided with filamentous, radiating, and often anastomosing, pseudopodia. The centre of the body is occupied by a capsule filled with protoplasm ; $± THE AtfATOM? OF INVERTEBRATED ANIMALS. this sometimes contains only an oil-globule, at others cells, or nuclei, and crystalline bodies. In the layer of protoplasm Fig 5 —Sphcerozoum punctatum.—A, a mass of the natural size ; B. two of the ova! central sacs with trie colored vesicles and spicula which lie in the investing pro- toplasm, magnified. Fig. 6.— Sphcerozoum ovodimare (after Haeckel), magnified. from which the pseudopodia proceed, cellreform bodies of a bright-yellow color, which have been found to contain starch, are usually developed,1 and this layer also gives rise to a skele- ton of a horny, or, more usually, silicious character, which 1 Even after the death of the Radiolarian, these yellow cells are said by Cien- kowsky to thrive and multiply, and the possibility that they may be parasites must be borne in mind. THE RADIOLARIA. 85 may have the form of detached spicula, or of coarticulated rods, or of networks, or of plates of silicious matter, often of the most exquisite delicacy and beauty. Most of the Hadi- olaria are simple, solitary, and microscopical in size ; but some, such as Collosphcera and Sphcerozoum (Figs. 5 and 6), are formed of aggregates of such simple forms, and float, as visible gelatinous masses, at the surface of the sea, which is the habitation of the great majority of the Hadiolaria. The manner of multiplication and the development of the Hadiolaria have not yet been thoroughly worked out. Cien- kowsky, howTever, has observed, in Collosph&ra, that the protoplasm contained in the central capsule breaks up into numerous rounded masses. The several capsules which are associated together in the compound Radiolarian then be- come isolated, by the dissolution of the protoplasm which invested and connected them, and finally burst, giving exit to the rounded bodies ; which, while yet within the capsules, were observed to be in active motion. The germs (for such they appear to be) thus set free are 0.008 mm. long, ovate, and carrv two flagelliform cilia at their narrow ends ; so that they are " monads." Each has in its interior a crystalline rod and a few minute oil-globules. The further development of these mastigopods has not yet been traced ; but, if, as is probable, they pass into young Hadiolaria (which, according to Haeckel, possess no capsule, but resemble Arti/iosphce- ria), the Hadiolaria, as members of the Endoplastiea, would typify Protomonas among the JSIonera. Neither conjugation nor fission has been observed among the ordinary Hadio- laria, but both these processes take place in Actinosphm- rium y and, considering the resemblance of the young Hadio- laria to Actinosphcerium, it seems probable that conjugation and fission will yet be discovered among them. Aetinosphcerium has been observed to undergo multipli- cation, by division of its central substance into a certain number of spheroids, and every spheroid becomes inclosed in a silicious case. After a period of rest, a young Actinosphce- rium emerges from each of these cysts. The marine Hadiolaria all inhabit the superficial stratum of the sea, and must fabricate their skeletons at the expense of the infinitesimal!? small proportion of silex which is dis- solved in sea-water; but, when they die, these skeletons sink to the bottom, and there accumulate, together with the Fora- minifera, in warm and temperate regions ; and with the cases of the diatomaceous plants, which abound at the sur- 86 THE ANATOMY OF IXVERTEBRATED ANIMALS. face, along with the Radiolaria, all over the globe {see p. 80). The late investigations of Archer and others have demon- strated the existence of a considerable number of fresh-water Radiolaria. Extensive masses of tertiary rock, such as that which is found at Oran, and that which occurs at Bissex Hill, in Bar- badoes, are very largely made up of exquisitely preserved skeletons of Radiolaria. But, though there can be little doubt that Radiolaria abounded in the Cretaceous sea, none are found in the chalk, their silicious skeletons having prob- ably been dissolved and redeposited as flint. 2. The Peotoplasta. — The proper Amoebae have broad and ovate pseudopodia, and resemble Rrotamceba (p. 75) very closely; but they present an advance upon its structure, by exhibiting a distinct endoplast (nucleus) and a contractile vacuole. In Arcella, there are many such nuclei. They thus stand in somewhat the same relation to Rrotarnceba as Acti- nophrys does to Protogenes. Moreover, there are Amcebce in which the power of throw- ing out pseudopodia is confined to one region of the body ; and others, as Arcella, in which a shell is formed over the rest of the body. In other Amoeba?, as A. radiosa, the pseu- dopodia are few, narrow, and but little mobile. But the Amcebce present no such diversity of skeletal development as the Foraminifera do. They multiply by division, and in some cases — e. g., A. spthcerococeus of Haeckel — become en- cysted before they divide. Amcebce (the " proteus animalcules " of the older writers) are not uncommon, and sometimes are very abundant, in fresh waters ; they also occur in damp earth and in the sea, but there is much doubt whether many of them are to be regarded as independent organisms, or whether they are not rather stages in the development of other animals or even of plants, such ^Myxomyoetes. Leaving out the contractile vacuole, the resemblance of an Amoeba in its structure, man- ner of moving, and even of feeding, to a colorless corpuscle of the blood of one of the higher animals is particularly note- worthy.1 3. The Geegaeixid^: are very closely allied to the Amce- bce, but, in the cycle of forms through which they pass, they curiously resemble Myxastrum. In form they are spheroidal 1 Contractile vacuoles have been observed in the colorless blood-corpus- cles of Amphibia under certain conditions. THE GREGARIXID.E. 87 or elongated oval bodies, sometimes divided by constrictions into segments. Occasionally, one end of the body is pro- duced into a sort of rostrum, which may be armed with re- curved horny spines. In the ordinary Grregarinm, the body presents a denser cortical layer (ectosarc) and a more fluid inner substance (endosarc), in which last the endoplast (nucleus) is imbed- ded. The presence of contractility is manifested merely by slow changes of form, and nutrition appears to be effected by the imbibition of the fluid nutriment, prepared by the organs of the animals in which the Gregarince are parasitic. There is no contractile vacuole. The Gregarince have a peculiar mode of multiplication, sometimes preceded by a process which resembles conju- gation. A single Gregarina (or two which have become applied together) surrounds itself with a structureless cyst. Fig. l.—A. Gregarina of the earthworm (after LieberkuhrO: B. encvsted ; C, Z>, contents divided into pseud o-navicellae ; E, F, free pseudo-navicellae, G. H, free amcebiform contents of the latter. The nucleus disappears, and the protoplasm breaks up (in a manner very similar to that in which the protoplasm of a 88 THE ANATOMY OF INVERTEBRATED ANIMALS. sporangium of Mucor divides into spores) into small bodies, each of which acquires a spindle-shaped case, and is known as a pseudo-navicella. On the bursting of the cyst these bodies are set free, and, when placed in favorable circum- stances, the contained protoplasm escapes as a small active body like a Protamoeba. M. E. van Beneden has recently dis- covered a very large Gregarina ( G. giganted), which inhab- its the intestine of the lobster, and his careful investigation of its structure and development has yielded very interesting results. Gregarina gigantea attains a length of two-thirds of an inch. It is long and slender, and tapers at one extremity, while the other is obtuse, rounded, and separated by a slight constriction from the rest of the body, which is cylindroidal. The outer investment of the body is a thin structureless cu- ticle ; beneath this lies a thick cortical layer (ectosarc), dis- tinguished by its clearness and firmness from the semifluid central substance (endosarc), which contains many strongly- refracting granules. In the centre of the body, the ellipsoid " nucleus," with its " nucleolus," fills up the whole cavity of the cortical layer, and thus divides the medullary substance into two portions. The body of this Gregarina may present longitudinal striations, arising from elevations of the inner surface of the cortical layer, which fit into depressions of the medullary substance ; but these are inconstant. On the other hand, there are transverse striations which are constant, and which arise from a layer of what are apparently muscular fibrilla?, developed in a peripheral part of the cortical layer, immediately below the cuticle. The fibrillas themselves are formed of elongated corpuscles joined end to end. A trans- verse partition separates the cephalic enlargement from the body, and the layer of muscular fibres only extends into the posterior part of the enlargement. The embryos of Gregarina gigantea, when they leave their pseudo-navicella?, are minute masses of protoplasm simi- lar to Protamoebce, and like them devoid of nucleus and con- tractile vacuole. They soon cease to show any change of form, and acquire a globular shape, the peripheral region of the body at the same time becoming clear. Next, two long processes bud out from this body; one is actively mobile, the other still. The former, detaching itself, assumes the appear- ance and exhibits the motions of a minute thread-worm, whence M. van Beneden terms it a pseudo-jilaria. The en- largement at one end becomes apparent, the pseudo-filaria THE INFUSORIA. 89 passes into a quiescent state, and the " nucleolus " makes its appearance in its interior. Around this a clear layer is differ- entiated, giving rise to the " nucleus," and the pseudo-filaria passes into the condition of the adult Gregarina gigantea. 4. The Catallacta of Haeckel, represented by the genus Magosphcvra, are, in one stage, myxopcds with long pseudo- podia, which, broad and lobe-like at the base, break up into fine filaments at their ends, and may therefore be said to be intermediate between those of Protogenes and those of Prot- amoeba. The myxopod is provided with a distinct endoplast and a well-marked contractile space. When fully fed, it se- cretes a cyst and divides into a number of masses, each of which is converted into a conical body, with its base turned outward and its apex inward. These conical bodies are im- bedded in gelatinous matter, and thus cohere into a ball, from the centre of which they radiate. Each develops cilia around its base, and contains an endoplast and a contractile vacuole. After the complex globe thus formed has burst its envelope, it swims about for a while, like a Vblvox. The several cilia- ted animalcules feed by taking in solid particles through the disk. They then separate, and, finally, retracting their cilia, become myxopods such as those with which the series started. Magosphmra is thus very nearly an endoplastic repetition of the moneran Protomonas — the mastigopod being provided with many small cilia, instead of with a couple of large fla- gella. On the other hand, the Catallacta are closely allied to the next group, and, I am disposed to think, might well be included in it. 5. The Infusoria. — Excluding from the miscellaneous as- semblage of heterogeneous forms, which have passed under this name, the Desmidice, Diatomacece, Volvocinece, and Vibrionidce, which are true plants, on the one hand ; and the comparatively highly-organized Potifera, on the other ; there remain three assemblages of minute organisms, which may be conveniently comprehended under the general title of Infu- soria. These are — (a) the so-called '" Monads," or Infusoria flagellata ; (b) the Acinetce, or Infusoria tentaculifera ; and (c) the Infusoria ciliata. (a.) The Flagellata. — These are characterized by pos- sessing only one or two long, whip-like cilia, sometimes (when more than one are present) situated at the same end of the body, sometimes far apart. The body very generally exhib- its an endoplast and a contractile vacuole. There is no per- manently open oral aperture, but there is an oral region, into 90 THE ANATOMY OF IXVERTEBRATED AXIMALS. which the food is forced, and, passing into the endosarc, re- mains for some time surrounded by a globule of contempo- raneously ingested water — a so-called " food-vacuole." Prof. H. James Clark, who has recently carefully studied the Fia- gellata, points out that, in Bicosceca and Codonceea, a fixed monadiform body is inclosed within a structureless and trans- parent calyx. In Codosiga a similar transparent substance rises up round the base of the flagellum, like a collar. In Saljnngoeca the collar around the base of the flagellum is combined with a calycine investment for the whole animai. In Anthophysa, there are two motor organs — the one a stout and comparatively stiff flagellum, wThich moves by occasional jerks, and the other a very delicate cilium, which is in con- stant vibratory motion. The discrepancy between the -two kinds of locomotive organs attains its maximum in Aniso?iema, which presents interesting points of resemblance to Noctiluca. Multiplication by longitudinal fission was observed in Codosiga and Anthophysa, and probably occurs in the other genera. In Codosiga the flagellum is retracted before fission takes place, but the body does not become encysted ; in An- thophysa the body assumes a spheroidal form, and is sur- rounded by a structureless cyst, before division occurs. Conjugation has not been directly observed among most of the Infusoria JJagellata, nor do any of them exhibit a structure analogous to the endoplastule of the Ciliata. Messrs. Dallinger and Drysdale have recently worked out the life-history of several flagellate " Monads," which occur in putrefying infusions of fish. They show that these lla- gellata not only present various modes of agamic multiplica- tion by fission, preceded or not by encystment, but that they conjugate, and that the compound body which results (the equivalent of the zygospore in plants) becomes encysted. Sooner or later, the contents of the cyst become divided either into comparatively large or excessively minute bod- ies, which enlarge and gradually take on the form of the parent. The careful investigations of these authors lead them to conclude that, while the adult forms are destroyed at from 61°-80° C, the excessively minute sporules which have been mentioned, and which may have a diameter of less than 2 0 010 0 0 of an inch, may be heated to 148° C. without the destruction of their vitality. In Euglena viridis (which, however, may be a plant), THE FLAGELLATA. 91 Stein ' has observed a division of the " nucleus " to take place, whereby it becomes converted into separate masses, some of which acquire an ovate or fusiform shape, surrounding them- selves with a dense coat, while others become thin-walled sacs, full of minute granules, each of which is provided with a single cilium. The ultimate fate of these bodies has not been traced. A careful study of the singular genus Noctiluca led me, in 1855, to assign it a place among the Infusoria, and recent investigations have conclusively proved that it is one of the Flagellata. The spheroidal body of Nbctiluca miliaris (Fig. 8) is about one-eightieth of an inch in diameter, and, like a peach, presents a meridional groove, at one end of which the mouth is situated. A long and slender, transversely striated ten- tacle overhang's the mouth, on one side of which a hard- toothed ridge projects. Close to one end of this is a vibratile cilium. A funnel-shaped depression leads into a central mass of protoplasm, connected by fine radiating bands with a layer of the same substance which lines the cuticular enve- lope of the body. There is no contractile vacuole, but an oval endoplast lies in the central protoplasm. Bodies which are ingested are lodged in vacuoles of the latter until they are digested. According to the observations of Cienkowskv,2 if a Noc- tiluca be injured, the body bursts and collapses, but the pro- toplasmic and other contents, together with the tentacle, form an irregular mass, the periphery of which eventually becomes vacuolated, enlarges, and secretes a new investment. But even a small portion of the protoplasm of a mutilated Nbcti- luca is able to become a perfect animal. Under some condi- tions, the tentacle of a Nbctiluca may be retracted into the body, and, at all times of the year, spheroidal Noctiluca}, devoid of flagellum, tooth, or meridional groove, but other- wise normal, are to be found. These last are probably to be regarded as encysted forms. Multiplication may take place in at least two ways. Fission may occur in the spheroidal forms, as well as in those possessed of a tentacle ; it is in- augurated by the enlargement, constriction, and division, of the endoplast. This process takes place more especially in the latter part of the year. i lt Oiganisnras der Infusionstliiere," ii., 5fi. 2"Ueber Noctiluca miliaris." (Sckulze's " Archiv fur mikroskop. Anato- mie," 1872.) 92 THE ANATOMY OF IXVERTEBRATED ANIMALS. Another mode of a sexual multiplication, which has a sin- gular resemblance to the process of partial yelk division, Fig. S.—Noctiluca miliaris.—e, gastric vacuole ; g, radiating filaments ; /, anal aperture (?). occurs only in the spheroidal Koctilucm. The endoplast dis- appears, and the protoplasm, accumulating on the inner side of one region of the cuticle, divides first into two, then four, eight, sixteen, thirty -two, or more masses ; the division of the protoplasm being accompanied by the elevation of the cuticle into protuberances, which, at first, corresj^ond in number and dimensions with these division masses. When the division masses have become very numerous, each protrudes upon the surface, and is converted into a free monadiform germ, pro- vided with an endoplast, a beak, and a long tentacle, which is hardly to be distinguished from a flagelliform cilium. The process of conjugation has been directly observed. Two Noctiluca^ applying themselves by their oral surfaces, adhere closely together, and a bridge of protoplasm connect- ing the endoplasts of the two becomes apparent. The ten- tacula are thrown off, the two bodies gradually coalesce, and the endoplasts fuse into one. The whole process occupies five or six hours. Spheroidal or encysted JYoctihtcce may conjugate in a similar manner. In this case, the regions nearest the endoplasts are those which become applied to- gether. Whether this process is of a sexual nature, or not, is not clearly made out. Cicnkovvsky admits that it may THE FLAGELLATA. 93 hasten the process of multiplication by monadiform germs described above. Nbctiluca is extremely abundant in the superficial waters of the ocean, and is one of the most usual causes of the phos- phorescence of the sea. The light is given out by the pe- ripheral layer of protoplasm which lines the cuticle. The Peridinew (see Fig. 1, f) form another aberrant group of the Flagellata, which lead to the Ciliata. The body is inclosed in a hard case (sometimes produced into rays), which, at one part, presents a groove-like interruption, laying bare the contained protoplasm, in which lies an endo- plast, and in some cases a contractile vacuole. One or more flagelliform cilia, and usually a wreath of short cilia, are pro- truded from the protoplasm, and serve as locomotive organs. The mouth is a depression, whence, in some cases, an oeso- phageal canal is continued and terminates abruptly in the semi-fluid central substance of the body, the food-particles being lodged in vacuoles formed at its extremity, as in the Ciliata. No other mode of multiplication than that by fission has vet been observed in the Peridineoe : but this fission is sometimes preceded by the inclosure of the animal in an elongated, crescent-shaped cyst. (b.) The Tentaculifera. — The Acinetce (Fig. 9, D, F, F, G-) have no oral aperture of the ordinary kind, but filiform processes or tentacula, which are usually slender, simple, and more or less rigid, radiate from the surface cf the body gen- erally, or from one or more regions of that surface. At first sight, these tentacula resemble the radiating pseudopodia of Actinophrys, but, on closer inspection, they are seen to have a different character. Each, in fact, is a delicate tube, pre- senting a structureless external wall, with a semi-fluid granu- lar axis, and usually ends in a slight enlargement or knob. It may be slowly pushed out or retracted, or diversely bent. But, instead of playing the part of mere prehensile organs, these tentacles act, in addition, as suckers; the Acineta ap- plying one or more of these organs to the body of its prey ' — * I Stein ("Der Organismus der Infusionsthiere," L, 70) thus describes the method by which an Acineta seizes its prev : " If an Infusorium swims within reach ot the Acineta, the nearest tentacles are swiftly thrown toward it, and, at the same time, often become much elongated, bent, or irregularly twisted about, lhe knob-like ends of these tentacles, which come into immediate contact with the surface ot the entangled prey, spread out into disks, and adhere fixedly to it. When many of the tentacles haye thus attached themselyes. the im- prisoned animal is no longer able to escape, its moyements become slower, and at length cease. Those tentacles which haye fixed themselyes most firmlv shorten and thicken, and draw the prev nearer to the body. . . . Suddenly, as 94 THE AXATOMY OF IXYERTEBRATED ANIMALS. usually some other species of Infusorium — when the substance of the latter travels along the interior of the sucker into the Fig. 9.— A, Vorticella, active ; J5, C, encysted ; D, U, F, G, Acinetce (after Stein). body of the Acineta. Solid food is not ing-ested throug'h these tentacles, so that the Acinetce cannot be fed with indio*o or carmine. In the interior of the body there is an endoplast 1 with one or more contractile vacuoles, and it may be either fixed by a stalk or free. The Acinetce multiply by several methods. One of these is simple longitudinal fission, which appears to be rare among them. Another method consists in the development of ciliated embryos in the interior of the body. These embryos result from a separation of a portion of the endoplast, and its con- soon as the suckinsr disk has bored through the cuticula of the prey, a very rapid stream, indicated by the fatty particles which it carries, sets along the axis of the tentacle, and, at its base, pours into the neighboring part of the body of the Acineta. . . . The cause of the movement is unknown. It is not accompanied bv any discernible movement of the walls of the tentacle." 1 No endoplastule, such as exists in other Infusoria, has been observed as yet in the Acinetce. Under some circumstances, the Acinetm draw in their radiating processes, and surround themselves with a structureless cyst; but this process does not appear to have any relation to either mode of multiplica- tion. In Acineta mystacina and Poiloplryajim. a peculiar mode of multiplication bv division occurs. At the free end of the body a portion becomes constricted off, together with part of the endoplast, from the remaining stalked part. The tentacula are drawn in, and the segment becoming elongated, develops cilia over its whole surface and swims awav. THE INFUSORIA. 95 version into a globular or oval germ, which, in some species, is wholly covered with vibratile cilia, while, in others, the cilia are confined to a zone around the middle of the embryo. The germ makes its escape by bursting through the body-wall of its parent. After a short existence (sometimes limited to a few minutes) in the condition of a free-swimming animal- cule, provided with an endoplast and a contractile vacuole, but devoid of a mouth, the characteristic knobbed radiating processes make their aj3pearance, the cilia vanish, and the ani- mal passes into the Acineta state. The Acinetce have frequently been observed to conju- gate, the separate individuals becoming completely fused into one and their endoplasts coalescing into the 'single endoplast of the resultant Acineta / but it is not certainly made out whether this process has, or has not, anything to do with the process of the development of ciliated embryos just described. (c.) The Ciliata. — The characteristic feature of the Ciliata is, that the outer surface of the body is provided with numer- ous vibratile cilia, which are the organs of prehension and loco- motion. According to the distribution of the cilia, Stein has divided them into the Holotricha, in which the cilia are scat- tered over the whole body, and are of one kind ; the Hetero- tricha, in which the widely-diffused cilia are of different kinds, some larger and some smaller ; the Hypotricha, in which the cilia are confined to the under or oral side of the bodv: and the Peritricha, in which they form a zone round the body. The great majority of these animals are asymmetrical. In the simplest and smallest Ciliata, the body resembles that of one of the Flagellata in being differentiated merely into an ectosarc and endosarc, with an endoplast and a con- tractile vacuole. In most, if not all cases, however, there is not only an oral region, through which the ingestion of food takes place, but an oesophageal depression leads from this into the endosarc ; and it may be doubted whether, even in the simplest Ciliata, there is not an anal area through which the undigested parts of the food are thrown out. The genus Colpoda, which is very common in infusions of hay, is a good example of this low form of ciliated Infuso- rium. It has somewhat the form of a bean flattened on one side, and moves actively about by means of numerous cilia, the longest of which are situated at the interior end of the body. At the posterior end is the contractile vacuole, while a large endoplast lies in the middle, as Stein originally dis- covered. Colpodaz frequently become quiescent, retract their 96 THE ANATOMY OF INVERTEBRATED ANIMALS. cilia, and surround themselves with a structureless cyst. Each encysted Colpoda then divides into two, four, or more por- tions, which assume the adult form and escape from the cysts to resume an active existence. Allman has described the encystment of a Vorticellidan, followed by division of the nucleus into many germs, with- out any antecedent process of conjugation ; and Everts has observed that the progeny of an encysted Vorticella take on the form of Trichodina grandinella. The Trichodina^ mul- tiply by transverse divisions, and then grow into Vorti- cellce.1 Encystment, whether followed or not by division, is very common among all the Giliata, and a species of Amphilep- tus has been seen to swallow — or rather envelop — a stalked bell-animalcule (Vorticella), and then become encysted upon the stalk of its prey, just as Vampyrella becomes perched upon the stalk of the devoured Gomphonema. In the higher Ciliata, the protoplasm of the body becomes directly differentiated into various structures, in the same way as has already been seen to be the case in Gregarina gigantea, but to a much greater degree. Thus, in the Peritricha, of which the bell-animalcules, or Vorticellm (Fig. 9, A, P, G), are the commonest examples, the oral region presents a depression, the vestibule (Fig. 9, a) from which a permanent oesophageal canal leads into the soft and semi-fluid endosarc, where it terminates abruptly ; and immediately beneath the mouth, in the vestibule, there is an anal region which gives exit to the refuse of digestion, but presents an opening only when fecal matters are passing out. Except where the ciliated circlet, or rather spiral, is situated, the outer wall of the body gives rise to a relatively dense cuticula, and not unfrequently secretes a transparent cup or case, foreshadowing the theca of hydrozoal polyps. Moreover, in the permanently fixed Vorticellm, the stalk of attachment may present a central muscular fibre (Fig. 9,f), by the sudden contraction of which the body is retracted, the stalk being at the same time thrown into a spiral. In the holotrichous Paramoecium (Fig. 10) beneath the thin su- perficial transparent cuticle from which the cilia proceed, there is a very distinct cortical layer, fibrillated in a direc- tion perpendicular to the surface, and, in some species of this or other genera, as St lrombidium and Polyhricos (Butschli),- beset with minute rod-like bodies similarly disposed, which, Allman, " Presidential Address to the Linncean Society," 1875. THE INFUSORIA. 97 under some circumstances, shoot out into long filaments, and have been termed trichocysts. In P. bursaria, minute Fig. 10. — Paramecium bursaria (after Stein).— A, the animal viewed from the dorsal side : a, coi-tical layer of the body ; 6, endoplast ; c, contractile space ; d d', mat- ters taken in as food ; e, chlorophyl granules. B, the animal viewed from' the ventral side: «, depression leading to b, mouth ; c, gullet; d, endoplast ; d\ endoplastule ; a, central protoplasm. In both these figures the arrows indicate the direction of the circulation. C, Paramecium dividing trausversly : a a', contractile spaces ; b b, endoplast divid- ing ; c c\ endoplastules.^ green granules of chlorophyl are dispersed through this layer, and Cohn demonstrated, in 1851, that these yield the same reactions as the chlorophyl grains of the Algae. In Balanti- dium, JSTyct other •us, Spirostomum, and many others, the cor- tical layer is divided by linear markings into bands, which there is reason to believe are rudimentary muscular fibres. In many Cillata, the endosarc appears to be almost fluid. The food, which is driven into the mouth and down the oesoph- agus by the constant action of the cilia, accumulates at the bottom of the oesophagus ; and then, with the water which surrounds it, is passed, at intervals, with a sort of jerk, into the endosarc, where it lies close to the end of the oesophagus, as a food-vacuole, for a short time. But it soon begins to move, and, along with other such vacuoles formed before and after it, circulates in a definite course up one side of the body and down the other, between the cortical layer and the endo- plast. This movement is particularly free and unrestricted in Balantidium ; in JParamcecium, the tract through which the food-vacuoles move is more definitely limited,1 while in Nye- 1 In Paramecium bursaria Cohn observed that the circulation was completed in H to 2 minutes, which gives a rate of rotation of Woo to x*hoo of an inch in a second. 98 THE ANATOMY OF INVERTEBRATED ANIMALS. totherus it appears to be confined to a part of the body be- tween the end of the gullet and the anal region, which in this Infusorium is seated at one end of the body. In fact, the finely granular endosarc of Nyctotherus so limits the passage of the food-vacuoles that the tract along which they pass might properly be described as a rudimentary intestinal canal. The oral cavity is usually ciliated : sometimes, as in Chilo- don, it has a chitinous armature, which becomes somewhat complicated in Emilia (Dysteria *) and the Didinium de- scribed by Balbiani. Torquatella (Lankester) has a plicated membrane around the mouth in the place of cilia. The contractile vacuoles attain their greatest complexity in the JParamoecia, in which there are two — one toward each end of the body. They are lodged in the cortical layer, and, in diastole, a portion of their outer periphery is bounded only by the cuticle, through which it is very probable that they communicate with the exterior. When the svstole takes place, a number of fine canals, which radiate from each vac- uole, are seen to become distended with clear, watery fluid. These canals are constant in their position, and some of them may be traced nearly as far as the mouth ; so that the canals and vacuoles form a permanent water-vascular system. The endoplast is finely granular, like the substance of the endosarc. It is frequently said to be enveloped in a distinct membrane, but I am disposed to think that this is always a product of reagents. Attached to one part of it there is very generally (but not in the Vorticellw) a small oval or rounded body, the so-called "nucleolus" or endoplastule. The endo- plast is commonly said to be imbededd in the cortical layer, but this is certainly not the case in Colpoda, Parametrium, JBalantidiiwi, or JSTyctotherus. The outermost, or cuticular, layer of a large portion of the body becomes hardened and forms a sort of shell, in many of the free Infusoria, In the free marine Dictyocystida and Codonellida of Haeckel, the body has a bell-shaped enve- lope, which in the Dictyocystida (see Fig. 1) is strengthened by a siliceous skeleton like that of a Radiolarian. In both genera the circular lip which surrounds the oral end is pro- vided with numerous long flagelliform cilia.2 Most of the Ciliata, while in full activity, multiply by di- J Huxley, "On Dysteria." ( Quarterly Journal of Microscopical Science, 1857.) 2 Haeckel, uZur Morphologic der Infusorien," 1873. THE INFUSORIA. 99 vision ; this is generally effected by the formation of a more or less transverse constriction, whereby the body becomes divided into two parts, which separate, each developing those structures which are needed for its completion. The endo- plast, however, always elongates and divides, one portion going along with each product of fission. Neither budding nor longitudinal fission occurs among the free Infusora, the appearances which have been regarded as evidence of these processes being due to the opposite operation of conjugation. M. Balbiani,1 its discoverer, thus describes the process of conju- gation in Paramecium bursaria : " The Paramoeoia assemble in great numbers either tow- ard the bottom or on the sides of the vessel in which they are contained. They then conjugate in pairs, their anterior ends being closely united ; and they remain in this state for five or six days or more. During this period the nucleus and nucleolus become transformed into sexual organs. " The nucleolus is changed into an oval capsule, marked superficially by longitudinal striae. Sooner or later, it usually becomes divided into two or four portions, which grow inde- pendently, and form many separate capsules. About the time of separation, each of these is found to be a capsule containing a bundle of curved rods (baguettes), enlarged in the middle, and thinner at the ends. " The nucleus also becomes enlarged, and gives rise — in a manner not clearly explained — to small spherical bodies anal- ogous to ovules. "It is usually about the fifth or sixth day after conjuga- tion that the first germs appear, as little rounded bodies formed of a membrane which is rendered visible by acetic acid, and of grayish pale homogeneous or almost imperceptibly granu- lar contents, in which, as yet, neither nucleus nor contractile vacuole is distinguishable. It is only later that these organs appear. The observations of Stein and of F. Cohn have shown how these embryos leave the body of the mother un- der the form of Aci?ieta3, provided with knobbed tentacles and true suckers, by uieans of which they remain for some time adherent to her, and nourish themselves from her substance. But their investigations have not disclosed the ultimate fate of the young. " I have been able to follow them for a long period after » Balbiani, "Note relative a TExistence d'une Generation Sexuelle chez les Inlusoires." (Journal de la Physiologie, tome i., 1858.) 100 THE ANATOMY OF IXVERTEBRATED ANIMALS. their detachment from the maternal organism ; and I have been able to assure myself that, after having lost their ten- tacles, becoming clothed with vibratile cilia, and acquiring a mouth, which makes its appearance as a longitudinal groove, they return definitely to the parental form, developing in their interior the green granules wmich are characteristic of this Paramoeeium, without undergoing any more extensive metamorphosis." In Figs. 19-22 of Plate IV., which accompanies his paper, Balbiani figures all the stages by which the acinetiform em- bryo becomes a Paramozeium. So far as the fact of conjugation, the changes in the " nu- cleolus," and the development of filaments in it, with the subsequent detachment, by division, of masses from the " nu- cleus," are concerned, these statements have not been modi- fied by M. Balbiani, while they are fully confirmed by the ob- servations made by himself, Claparede and Lachmann, Stein, Kolliker, and others, in Paramoecium bursaria, P. aurelia, and other ciliated Infusoria. In the closely allied Paramoecium aurelia, the occurrence of the various stages of conjugation, conversion of the " nu- cleolus " into bundles of spermatozoa, and subsequent division of the " nucleus," is also established by the coincident testi- mony of Balbiani and Stein. Balbiani affirms that, in this spe- cies, the clear globular bodies which result from the division of the " nucleus " pass out of the body without undergoing any further modification, and he considers them to be ovules. Stein also admits that he has never seen acinetiform embryos in this species. But, as it would seem, on the strength of these negative observations in Paramoecium aurelia, Balbiani, in his later publications, asserts that the " acinetiform embryos " observed not only in Paramoecium, but in Stylonychia, Stent or, and many other ciliated Infusoria, are not embryos at all, but parasitic Acinetm / and he makes this assertion without ex- plicitly withdrawing the statement given above of his own ob- servation of the passage of the acinetiform embryo of Para- moecium bursaria into the parental form. Engelmann and Stein, on the other hand, hold by Balbiani's original doctrine, and give strong reasons for so doing. Among the most for- cible analogical arguments are those afforded by the process of sexual reproduction observed by Stein in the peritrichous In- fusoria. In the Peritricha ( Yorticellida >_, Ophrydidoe, Trichodidai) THE INFUSORIA. 101 conjugation takes place by the complete and permanent fusion of two individuals, which are sometimes of equal dimensions ; though, in other cases, one is much smaller than the other, and, while it is in course of absorption, looks like a bud, and was formerly taken for such (Fig. 9, A, g, h). The small individuals usually take their origin from a group of small stalked Vo)'ticellce, which are produced by the repeat- ed longitudinal division of a Vorticella of the ordinary size. The result of the conjugative act is that the " nuclei" of the two individuals, either before or after their coalescence, break up into a number of segments. The segments may remain separate, or coalesce into a single mass, called by Stein placenta. In the former case, some of the segments become germ-masses, while the others reunite to form a new "nucleus ;" in the latter, the placenta throws out a number of germ-masses, and then assumes the form of an ordinary " nucleus." The germ-masses give off portions of their sub- stance, including part of their " nucleus," and these become converted into ciliated embryos, which escape by a special opening. Knobbed tentacles, like those of the Acinetce, have not been observed in the embryos of the Peritricha, nor has their development been traced out. If the bodies regarded as acinetiform embryos of the Ciliata are really such, they may be taken to represent the myxopod stage of the Catallacta, and the relations of the Acinitm to the Ciliata would appear to be that they are modifications of a common type, differing from the Catal- lacta in having tentacula instead of ordinary pseudopodia. In the Acinetce. the tentaculate stage is the more permanent, the ciliated stage transitory ; while, in the Ciliata, the cili- ated stage is the more permanent, and the tentaculate stage transitory. CHAPTER III. THE PORIFERA AND THE CCELENTERATA. 1. The Porifera or Spongida. — It has been seen that, in the Protozoa, the germ undergoes no process of division analogous to the " yelk division " of the higher animals, and to the corresponding process by which the embryo cell of every plant but the very lowest becomes converted into a cellular embryo. Consequently, there is no blastoderm ; the body of the adult Protozoon is not resolvable into morpho- logical units, or cells, more or less modified ; and the aliment- ary cavity, when it exists, has no special lining. Moreover, the occurrence of sexual reproduction in most of the Proto- zoa is doubtful, and there is, at present, no evidence of the existence of male elements, in the form of filamentous sper- matozoa, in any group but the Infusoria ; and even here the real nature of these bodies is still a matter of doubt. In all the Metazoa, the germ has the form of a nucleated cell. The first step in the process of development is the production of a blastoderm by the subdivision of that cell, and the cells of the blastoderm give rise to the histological elements of the adult body. With the exception of certain parasites, and the extremely modified males of a few species, all these animals possess a permanent alimentary cavity, lined by a special layer of cells. Sexual reproduction always occurs ; and, very generally, though by no means invariably, the male element has the form of filiform spermatozoa. The lowest term in the series of the Metazoa is un- doubtedly represented by the Porifera or Sponges, which, after oscillating between the vegetable and the animal king- doms, have, in recent times, been recognized as animals by all who have sufficiently studied their structure and the manner in which their functions are performed. But the place in the Animal Kingdom which is to be as- signed to the sponges has been, and still is, a matter of de- THE PORIFERA. 103 bate. It is certain that an ordinary sponge is made up of an aggregation of corpuscles, some of which have all the charac- ters of Amoebae, while others are no less similar to Monads ; and therefore, taking adult structure only into account, the comparison of a sponge to a sort of compound Protozoon is perfectly admissible, and, in the absence of other evidence, would justify the location of the sponges among the Protozoa. But, within the last few years, the development of the sponges has been carefully investigated ; and, as in so many other cases, a knowledge of that process necessitates a recon- sideration of the views suggested by adult structure. The impregnated ovum undergoes regular division ; a blas- toderm is formed, consisting of two layers of cells — an epiblast and a hypoblast — and the young animal has the form of a deep cup, the wall of which is composed of two layers, an ec- toderm and an endoderm, which proceed respectively from the epiblast and hypoblast. The embryo sponge is, in fact, simi- lar to the corresponding stage of a hydrozcon, and is totally unlike any known condition of a protozoan. Beyond this early stage, however, the sponge-embryo takes a line of its own, and its subsequent condition differs altogether from anvthino; known amono- the Ccelentercita : all of which, on the other hand, present close and intimate resem- blances in their future development, as in their adult structure. It is not long since the only sponge of the structure and development of which we were accurately informed was the SpongiUafliiviatilis, or fresh-water sponge, the subject of the elaborate researches of Lieberkuhn and Carter. But, recently, a flood of light has been thrown upon the morphology and phys- iology of the marine sponges, particularly of those^ sponges with calcareous skeletons, which are termed Calcispongim, by Lieberkuhn, Oscar Schmidt, and especially Haeckel. It has become clear that Spongilla is a somewhat aberrant form, and that the fundamental type of Poriferal organization is to be sought among the Calcispongim. In the least com- plicated of the calcareous sponges, the body has the form of a cup, and is attached by its closed extremity. The open ex- tremity is the osculum, and leads directly into the spacious ventricidus, or cavity of the cup. The comparatively thin wall of the cup is composed of two layers, readily distinguish- able by their structure — the outer is the ectoderm, the in- ner the endoderm. The ectoderm is a transparent, slightly granular, gelatinous mass in which the nuclei are scattered, but which, in the unaltered state, shows no trace of the primitive 104 THE ANATOMY OF INVERTEBRATED AXTMALS. Fig. li.—Ascetta primordialis (after Haeekel). I. A mature Astcetta^an of one side of the body of which is removed: 0, the exhal- ent aperture ; p. iuhalent pores in the wall of the body ; i, endoderm ; e, ecto- derm ; q, ova. The triradiate spicula are seen imbedded in the ectoderm. II. A portion of the endoderm, with two pores (p>; i, endodermal cells— those round the margins of the pores have their cilia directed inward ; e, ectodermal syncy- tium; , detached hydjo- phyllia ; a, polypites ; b, tentacles ; c, sacculi of the tentacles ; d, hydrophyllia ; /, pneumatophoie. principal function of which is to develop the gonophores from their pedicles. In these two genera the tentacula are separate from the hydranths, and form the outermost circle of appendages. The hydranths of the Siphonophora (Fig. 25, A) never possess a circlet of tentacula round the mouth, which, when expanded, is trumpet-shaped. The endoderm of the hydranth is ciliated, and villus-like prominences project into its cavity. The aboral surface of the umbrella was of a brownish-gray color, variegated with oval white spots ; the oral surface, light brown with eight bluish-green lines radiating toward the lithocysts ; the brachia, gray with brown dots. The brachia divide into two at their origin, and then subdivide into an infinity of small branches. The general color of the smaller branches is light brown, the small interspersed clavatc tentacles being white. The long tentacles which terminate each brachium are blue and cylindrical at their origin, but become trigonal farther on, where they are shaded with brown and green. Is it identi- cal with the Cephea ocellata of Peron and Lesueur? The individual figured- was a young male. 9 128 THE ANATOMY OF INVERTEBRATED ANIMALS. The interior of these frequently contains vacuolar spaces (Fig. 24, B, C). A valvular "pylorus" separates the gastric from the somatic cavity in the Calycophoridce. Long tenta- cles, frequently provided with unilateral series of branches, are developed, either one from the base of each hydranth, or,' independently of the hydranths, from the ccenosarc. In the Calycophoridw and many Physophoridce, complex Fig. 25.—AthoryMa rosacea.— A, a hydranth with villi (a). B, one of the villi in its elongated state, enlarged. 6', a small retracted villus, still more magnified, with its vacuolar spaces and ciliated surface. organs, containing a sort of battery of thread-cells, terminate each lateral branch of a tentacle (Figs. 24 and 26). Each consists of an elongated saccidits, terminated by two fila- mentous appendages, and capable of being spirally coiled up. In this state it is invested by an involucnan^ which surrounds its base. The somatic cavity is continued through the branch, which constitutes the peduncle of this organ, into the saccu- lus and its terminal filaments. In the latter it is narrow, and their thick walls contain numerous small spherical nemato- cysts. In the sacculus the cavity is wider. One wall is very thick, and multitudes of elongated nematocysts, the lateral series of which are sometimes larger than the rest, are dis- posed parallel with one another, and perpendicular to the surface of the sac. Like the other organs, each of these tentacular appendages commences as a simple diverticulum of the ectoderm and endoderm, and passes through the stages represented in Fig. 26. In Physalia the tentacula may be several feet long. They have no lateral branches, but the large nematocysts are situ- THE SIPHONOPHORA. 129 ated in transverse reniform thickenings of the wall of the ten- tacle, which occur at regular intervals. Fig. 26. — Athorybia rosacea.— The ends of the tentacular branches in various stages of development. A, lateral branch, commencing as a bud from the tentacle. In B, terminal papillae, the rudiments of the filaments, are developed at the extremi- ty of the branch ; and, in C, the sacculus is beginning to be marked off, and thread- cells have appeared in its walls ; in Z>, the division into iavolucrum and sacculus is apparent; in B. the involucrum has invested the sacculus, the extremity of which is straight, while the lateral processes have curled round it. Hydrophyllia are generally present, and, like the tentacu- la, are developed either from the pedicle of a hvdranth, in which case they inclose the hvdranth with its tentacle and a group of gonophores (Calycophoridce), or, independently of the hydranths, from the coenosarc (many Physophoridoe). The hydrophyllia are transparent, and often present very beautifully defined forms, so that they resemble pieces of cut glass. They are composed chiefly of the ectoderm (and meso- derm), but contain a prolongation of the endoderm, with a corresponding diverticulum of the somatic cavity. They are, in fact, developed as caecal processes of the endoderm and ectoderm ; but the latter, with the mesodermal layer, rapidly predominates. The gonophores of the Siphonopliora present every varie- ty, from a simple form, in which the medusoid remains in a state of incomplete development, to free medusoids of the Gymnophthalmatous type. As an example of the former 130 THE ANATOMY OF INYERTEBRATED ANIMALS. condition the gonophores of Athorybia may be cited (Fig. 27) ; of the latter, the gonophores of Physalia, JPorpita, and Velella. In Athorybia, groups of gonophores, together with pyri- form sacs, which resemble incompletely developed hydranths (hydrocysts — Fig. 27, A, a), are borne upon a common stem, and constitute a gonoblastidium (Fig. 27, A). The groups of male and female gonophores (Fig. 27, A, b, c) are borne upon separate branches of the gonoblastidium (androphores — e Fig. 27 '.— Athorybia rosacea.— A, gonoblastidium bearing three hydrocysts, a; gyno- phore, b; and two androphores, c. B, female gonophores on their common stem or gynophore, showing toe included ovum, «, and the radical canals, b. C, D, female gonophores enlarged ; a, terminal vehicle ; b. vitellus; c, radial canals of the imperfect nectocalyx ; d, canals ol the manubrial cavity. E, male gonophore. and gynophores). Each female gonophore contains only a single ovum, which projects into the cavity of the imperfectly THE STPHOXOPHORA. 131 differentiated manubrium, and narrowing its cavity at differ- ent points gives rise to the irregular canals (Fig. 27, D, d). In the male gonophore the nectocalyx is more distinct from the manubrium, and its extremity has a rounded aperture (Fig. 27, JE). In the Calycophoridce, as in the elongated Physophoridm, the development of new hydranths and their appendages, which is constantly occurring, takes place at that end of the hydrosoma which corresponds to the fixed extremity of one of the Hydrophora / and, if we consider this to be the proxi- mal end, new buds are developed on the proximal side of those already formed. Moreover, these buds are formed on one side only of the hydrosoma. Hence the appendages are strictly unilateral, though they may change their position so as eventually to appear bilateral or even whorled. In the Calycopjhoridce, the saccular proximal end of the ccenosarc (Fig. 22, A, d) is inclosed within the anterior nectocalyx, at the posterior end of which is a chamber, the hydrceevum (Fig. 22, A, c). The second, or posterior, nectocalyx is at- tached in such a way that its anterior end is inclosed within the hydrcecium of the anterior nectocalyx, while its contrac- tile chamber lies on the opposite side of the axis to that on which the anterior nectocalyx is placed (Fig. 22, A). Sets of appendages (Fig. 22, A, a ; Fig. 23), each consisting of a hydrophyllium, a hydranth with its tentacle, and gonophores, which last bud out from the pedicle of the hydranth — are developed at regular intervals on the ccenosarc, and the long chain trails behind as the animal swims with a darting mo- tion, caused by the simultaneous rhythmical contraction of its nectocalyces, through the water (Fig. 22). From what has been said, it follows that the distal set of appendages is the oldest, and, as they attain their full de- velopment, each set becomes detached, as a free-swimming, complex Diphyzooid (Fig. 23). In this condition they grow and alter their form and size so much, that they were for- merly regarded as distinct genera of what were termed mono- gastric Diphydce. The gonophores, with which these are provided, in their turn become detached, increase in size, become modified in form, and are set free as a third series of independent zooids (Fig. 23, D). But their manubrium does not develop a mouth and become a functional hydranth ; on the contrary, the generative elements are developed in its wall, and are set free by its dehiscence. In the Physophoridce, the proximal end of the hydrosoma 132 THE ANATOMY OF INVERTEBRATED ANIMALS. is provided with a pneumatophore. This is a dilatation, into which the ectoderm is invaginated, so as to form a receptacle, which becomes filled with air and sometimes has a terminal opening, through which the air can be expelled (Fig. 13, 4). It is sometimes small, relatively to the hydrosoma (Agalma, Physophora) ; sometimes so large (Athorybia, Fig. 24 ; Phy- salia, Porpita, Velella), that the whole hydrosoma becomes the investment of the pyriform or discoidal air-sac ; while the latter is sometimes converted into a sort of hard inner shell, its cavity being subdivided by septa into numerous chambers (Porpita, Velella). Nsctocalyces may be present or absent in the Physopho- ridoe. When present, their number varies, but they are con- fined to the region of the hydrosoma which lies nearest to the pneumatophore. In the great majority of the Hydrozoa, the ovum under- goes cleavage and conversion into a morula, and subsequently into a planula, possessing a central cavity inclosed in a double cellular wall, the inner layer of which constitutes the hypo- blast, and the outer the epiblast. In most Hjdrophora the ciliated, locomotive, planula be- comes elongated and fixed by its aboral pole. At the oppo- site end, the mouth appears and the embryo passes into the gastrula stage. Tentacles next bud out round the mouth, and to this larval condition, common to all the JTydrophora, Allman has given the name of Actinula. Generally, the embryo fixes itself by its aboral extremity at the end of the planula stage ; but, in certain Tubularidce, while the embryo is still free, a circlet of tentacles is devel- oped close to the aboral end ; and this form of larva differs but very slightly from that which is observed in the Disco- phora. In the genus Pelagia, for example, the tentacles are de- veloped from the circumference of the embryo, midway be- tween the oral and aboral poles ; but it neither fixes itself nor elongates into the ordinary actinula-form. On the con- trary, it remains a free-swimming organism, and, by degrees, that moiety of the body which lies on the aboral side of the tentacular circlet widens and is converted into the umbrella, the other moiety becoming the hydranth, or " stomach," of the Medusa. In Zfucernaria, it is probable that the larva fixes itself be- fore or during the development of the umbrella, and passes THE DEVELOPMENT OF THE HYDROZOA. 133 directly into the adult condition. But, in most Discophora, the embryo becomes a fixed actinula (the so-called Hydra tuba or Scyphistoma, Fig. 28, 1.), multiplies agamogenetically by budding, and gives rise to permanent colonies of Hydri- form polyps. At certain seasons of the year, some of these enlarge and undergo a farther agamogenetic multiplication by fission (Fig. 28, II.). In fact,, each divides transversely into a number of eight-lobed discoidal medusoids (" JEphyrce " or " Medusce bifidce" ¥\g. 28, II. and III.), and thus passes into what has been termed the btrobila stage. The Ephyrce, becoming detached from one another and from the stalk of the Strobila, are set free, and, undergoing a great increase in size, take on the form of the adult Discophore, and acquire reproductive organs. The base of the Strobila may develop tentacles (Fig. 28, II.) and resume the Scyphistoma condition. Metschnikoff1 has recently traced out the development of Geryonia( Carmarina), Polyxenia,JEginop>$i$, and other Dis- cophora, which differ from the foregoing in possessing a velum ; and in these, as in the Trachynema ciliatum, observed by Gegenbaur,3 the process appears to be of essentially the same nature as in Pelagia, The Scyphistoma of Awelia, Cyanoea, and their allies, is probably to be regarded, like the larva of Pelagia, as a Discophore with a rudimentary disk ; in which case the reproduction of the Eyi/iyra-forms of young Disco- phora will not be comparable to the development of medusoid gonophores among the Hydrophora, but will merely be a pro- cess of multiplication, by transverse fission, of a true, though undeveloped, Discophore. In the Siphonophora? the result of yelk division is the formation of a ciliated body consisting of a small-celled ectoderm investing a solid mass of large blastomeres, which eventually pass into the cells of the endoderm. This body does not take the form of an actinula. On the contrary, it appears to be the rule that buds from which a hvdrophyllium, a nectoealyx, a tentacle, or pneumatophore, or even all of them, will be developed, take their origin antecedently to the formation of the first polypite and of the gastric cavity. As Metschnikoff well remarks, the mode of development of the Siphonophora is wholly inconsistent with the doctrine that the various appendages of the hydrosoma in these ani- |"StTidien fiber die Entwickelung der Medusen und Siphonophoren." (Zeitsehrift fur wiss. Zool., xxiv.) 3 " Zu'r Lehre der Generationswechsel," 1854. 3 See especially the late observations of Metschnikoff, loc. cit. 134 THE ANATOMY OF IXYERTEBRATED AXHIALS. I if:! i \\ i?ij/!:;Msi Mi y ! i !:: s : i i ! !;^ iji.:::; ■ s\ * 1 ! ii ! r. : i • : • : • i ■ ::-.:•::: i :■. ? I • * •a I'Uiini \\\\wi\\\ urn I i m in i Ii i ii vi n ww \ I • ■ » i « til * I • • ' '• ' " ■ ;: ' I i: » • ■ El*ll4tS*:! ! • \\ *' I : '• ! \ • I • I , 1 ! • » J J Si.- ; ■ ; ! J J I \l • -il I I g J ; ! si ! ■• \ ! ' i I' I !!i P I Pig. 28— I. and IL— Cyana>a capittata (after Van Beneden1). I. Two Hyrtrce tubce (Scyphistoma stage), exhibiting their ordinary characters, and between them two {a, b) which are undergoing fission (Strobila stage). II. The two Strobiles, a and 6, three days later. In a. tentacles are "developed be- neath the lowest of the Ephyne, from the stalk of the Strobila, which will persist as a Hydra tuba. III. Half the disk of an EpJiyra of Aurelia aurita, seen from the oral face. The small tentacles which lie between the mouth and the band of circular muscular fibres are inside the somatic cavity, whence sixteen short and wide radial canals extend to the periphery, where they are united by transverse branches. Eight of the radial canals enter the corresponding lobes, and finally divide into three branches: one which enters the peduncle of the lithocyst. and two lateral caeca. Radiating bands of muscular fibres accompany these canals. IV. Side view of one of the litbocyste with its peduncle. The arrow indicates the direction in which the cilia of the exterior work. 1 " Eecherch.es but la Faune littorale de Belgique. Polypes." 1866. THE DEVELOPMENT OF THE HYDROZOA. 135 mals represent individuals. The Hydrozoa are not properly- compound organisms, if this phrase implies a coalescence of separate individualities ; but they are organisms, the organs of which tend more or less completelv to become independent existences or zobids. A medusoid, though it feeds and main- tains itself, is, in a morphological sense, simply the detached independent generative organ of the hvdrosoma on which it was developed ; and what is termed the " alternation of gen- erations," in these and like cases, is the result of the dissocia- tion of those parts of the organism on which the generative function devolves, from the rest.1 In certain Discophora belonging to the group of Trachy- nemata, a method of multiplication by gemmation has been observed, which is unknown among the other Hydrozoa. It mav be termed entogastric gemmation, the bud growing out from the wall of the gastric cavity, into which it eventually passes on its way outward ; while, in all other cases, gemma- tion takes place by the formation of a diverticulum of the whole wall of the gastro-vascular cavity which projects on to the free surface of the body, and is detached thence (if it be- come detached), at once, into the circumjacent water. The de- tails of this process of entogastric gemmation have been traced by Haeckel 2 in Carmcirlna hastata, one of the Geryonidce. As in other members of that family, a conical process of the mesoderm, covered by the endoderm, projects from the roof of the gastric cavity and hangs freely down into its interior. Upon the surface of this, minute elevations of y^To-th °f an inch in diameter make their appearance. The cells of which these outgrowths are composed next become differentiated into two layers — an external clear and transparent layer, which is in contact with the cone, and invests the sides of the elevation ; and an inner darker mass. The external layer is the ectoderm of the young medusoid, the inner its endoderm. A cavity, which is the commencement of the gastric cavity, ap- pears in the endodermal mass, and opens outward on the free side of the bud. The latter, now ^r^th °^ an mcn 1R diameter, has assumed the form of a plano-convex disk, fixed by its flat side to the cone, and having the oral aperture in the centre of its convex free side. The disk next increasing in height, the 1 I have seen no reason to depart from the opinions on the subject of 1 Animal individuality ' enunciated in my lecture published in the Annals and Magazine of Natural History for June, 1852. *" Beitraore zur Natur^eschichte der Hvdromeduserj,"' 1865. 136 THE ANATOMY OF IXVERTEBRATED ANIMALS. body acquires the form of a flask with a wide neck. The belly of the flask is the commencement of the umbrella of the bud- ding medusoid ; the neck is its gastric division. The belly of the flask, in fact, continues to widen out until it has the form of a flat cup, from the centre of which the relatively small gastric neck projects, and the bud is converted into an unmis- takable medusoid, attached to the cone by the centre of the aboral face of its umbrella. In the mean while, the gelatinous transparent mesoderm has appeared, and, in the umbrella, has acquired a great relative thickness. Into this, eight prolonga- tions of the gastric cavity extend, and give rise to the radial canals, which become united into a circular canal at the cir- cumference of the disk. The velum, tentacula, and lithocysts are developed, and the bud becomes detached as a free swim- ming medusoid. But this medusoid is very different from the Carmarina from which it has budded. For example, it has eight radial canals, while the Carmarina has only six ; it has solid tentacles, while the adult Carmarina has tubular tenta- cles ; it has no gastric cone, and has differently disposed lith- ocysts. Haeckel, in fact, identifies it with Cunina rhodo- dactyla, a form which had hitherto been considered to be not only specifically and generically different from Carmarina^ but to be a member of a distinct family — that of the JEginidai. What makes this process of asexual multiplication more remarkable is, that it takes place in Carmarina} which have already attained sexual maturity, and in males as well as in females. There is reason to believe that a similar process of ento- gastric proliferation occurs in several other species of ^Egi- nidas, — JEgineta prolifera (Gegenbaur), Enry stoma rubigi- nositm (Kolliker), and Cunina Kbllikeri (F. Miiller) ; but, in all these cases, the medusoids which result from the gem- mative process closely resemble the stock from which they are produced. As might be expected, the Hydrozoa are extremely rare in the fossil state, and probably the last animal the discovery of fossil remains of which could be anticipated is a jelly-fish. Nevertheless, some impressions of Medusae, in the Solenhofen slates, are sufficiently well preserved to allow of their deter- mination as members of the group of Rhizostomidw? The 1 Haeekel, " Ueber zwei neue fossile Medusen aus der Familie der Rhi- zostomiden." (" Jahrbuch fur Mineralogie," 1866.) THE ACTIXOZOA. 137 apparent absence of the remains of Hydrophora in the meso- zoic and newer palaeozoic rocks is very remarkable. Some singular organisms, termed Graptolites, which abound in the Silurian rocks, may possibly be Hydrozoa, though they present points of resemblance with the Polyzoa. They are simple or branched stems, sometimes slender, sometimes ex- panded or foliaceous ; occasionally the branches are connected at their origin by a membranous expansion. The stems are tubular, and beset on one or both sides with minute cup- shaped prolongations, like the thecae of a Sertularian. A solid thickening of the skeleton may have the appearance of an independent axis. Allman has suggested that the theciform projections of the Graptolite stem may correspond with the mematophores of Sertularians, and that the branches may have been terminated by hydranths. Appendages which ap- pear to be analogous to the gonophores of the Hydrophora have been described in some Graptolites.1 With a very few exceptions (Hydra, Cordylophorci) the Hydrozoa are marine animals; and a considerable number, like the Calycophoridce and JPhysophoridce, are entirely pe- lagic in their habits. The Actixozoa. — The essential distinctions between the Actinozoa and the Hydrozoa are two. In the first place, the oral aperture of an Actinozoon leads into a sac, which, with- out prejudice to the question of its exact function, may be termed " gastric," and which is not, like the hydranth of the Hydrozoon, free and projecting, but is sunk within the body. From the walls of the latter it is separated by a cavity, the sides of which are divided by partitions, the mesenteries, which radiate from the wall of the gastric sac to that of the bod}7, and divide the somatic cavity into a corresponding num- ber of inter mesenteric chambers. As the gastric sac is open at its inner end, however, its cavity is in free communication with that of the central space which communicates with the intermesenteric chambers ; and the central space, together with the chambers, which are often collectivelv termed the " body cavity " or " perivisceral cavity," are, in reality, one with the digestive cavity, and, as in the Hydrozoa, consti- stute an enter ocoele. Thus an ActinozoQn might be com- pared to a Lacernaria, or still better to a Carduella, in which the outer face of the hydranth is united with the inner face 1 Hall, " Graptolites of the Quebec Series of North America," 1865. Nichol- son, " Monograph of the British Graptolitidte," 1872. 138 THE ANATOMY OF INVERTEB RATED ANIMALS. of the umbrella ; under these circumstances the canals of the umbrella in the Hydrozoon would answer to the intermesen- teric chambers in the Actinozoon. Secondly, in the Actinozoa, the reproductive elements are developed in the walls of the chambers or canals of the en- teroccele, just as they so commonly are in the walls of the gastro-vascular canals of the Hydrozoa, but the generative organs thus constituted do not project outwardly, nor dis- charge their contents directly outward. On the contranf, the ova and spermatozoa are shed into the enteroccele, and event- ually make their way out by the mouth. In this respect, again, the Actinozoon is comparable to a Lucernaria modi- fied by the union of the hydranth with the ventral face of the umbrella ; under which circumstances the reproductive ele- ments, which in all Hydrozoa are developed, either in the walls of the hydranth or in those of the oral face of the um- brella, would be precluded from making their exit by any other route than through the gastro-vascular canals and the mouth. In the fundamental composition of the body of an ecto- derm and endoderm, with a more or less largely developed mesoderm, and in the abundance of thread-cells, the Actino- zoa agree with the Hydrozoa. In most of the Actinozoa, the simple polyp, into which the embryo is converted, gives rise by budding to many zooids which form a coherent whole, termed by Lacaze-Du- thiers a zoanthodeme. The Coralligexa. — The Actinozoa comprehend two groups — the Goralligena and the Ctenophora — which are widely different in appearance though fundamental^ similar in structure. In the former, the mouth is always surrounded by one or more circlets of tentacles, which may be slender and conical, or short, broad, and fimbriated. The mouth is usually elongated in one direction, and, at the extremities of the long diameter, presents folds which are continued into the gastric cavity. The arrangement of the parts of the body is therefore not so completely radiate as it appears to be. The enteroccele is divided into six, eig-ht, or more wide inter- mesenteric chambers, which communicate with the cavities of the tentacles, and sometimes directly with the exterior, by apertures in the parietes of the body. The mesenteries which separate these wide chambers are thin and membranous. Two dt them, at opposite ends of a transverse diameter of the Ac- THE CORALLIGENA. 139 tinozoo'n, are often different from the rest. Each mesentery ends, at its aboral extremity, in a free edge, often provided Fig. 29.— Perpendicular section of Actinia holsatica (after Frey and Leuckart).— a, mouth ; &, gastric cavity ; c, common cavity, into which the gastric cavity and the intermesenteric chambers open; d, intermesenteric chambers; e, thickened free margin, containing thread-cells of , /, a mesentery; g, reproductive organ ; h, tentacle. with a thickened and folded margin ; and these free edges look toward the centre of an axial cavity,1 into which the gas- tric sac and all the intermesenteric chambers open. In the Coralligena, the outer wall of the body is not pro- vided with bands of large paddle-like cilia. Most of them are fixed temporarily or permanently, and many give rise by gemmation to turf-like, or arborescent, zoanthodemes. The great majority possess a hard skeleton, composed principally of carbonate of lime, which may be deposited in permanently disconnected spicula in the walls of the body ; or the spicula may run into one another, and form solid networks, or dense plates, of calcareous matter. When the latter is the case, the calcareous deposit may invade the base and lateral walls of the body of the Actinozoon, thus giving rise to a simple cup, or theca. The skeleton thus formed, freed of its soft parts, is a " cup-coral," and receives the name of a corallite. In a zoanthodeme, the various polyps (anthozooids) formed by gemmation may be distinct, or their several enter- occeles may communicate ; in which last case, the common connecting mass of the body, or coenosarc, may be traversed by a regular system of canals. And, when such compound 1 Partially-digested substances are often found in this axial space, and it is not improbable that it may functionally represent the stomach or the com- mencement of the intestine in higher animals. 140 THE AX ATOMY OF INVERTEBRATED ANIMALS. Actinozoa develop skeletons, the corallites may be distinct, and connected only by a substance formed by the calcifica- tion of the ccenosarc, which is termed ccenenchyma ; or the thecae may be imperfectly developed, and the septa of adja- cent corallites run into one another. There are cases, again, in which the calcareous deposit in the several polyps of a compound Actinozoon, and in the superficial parts of the cce- nenchyma, remains loose and spicular, while the axial por- tion of the ccenosarc is converted into a dense chitinous or cal- cified mass — the so-called sclerobase. The mesoderm contains abundantly developed muscular fibres. The question whether the Coralligena possess a ner- vous system and organs of sense, hardly admits of a definite answer at present. It is only in the Actinidw that the ex- istence of such organs has been asserted ; and the nervous circlet of Actinia, described by Spix, has been seen by no later investigator, and may be safely assumed to be non-exist- ent. Prof. P. M. Duncan, F. R. S.,1 however, has recently described a nervous apparatus, consisting of fusiform gan- glionic cells, united by nerve-fibres, which resemble the sym- pathetic nerve-fibrils of the Vertebrata, and form a plexus, which appears to extend throughout the pedal disk, and very probably into other parts of the body. In some of the Actinidce (e. g., Actinia mesembryanthemum), brightly-col- ored bead-like bodies are situated in the oral disk outside the tentacles. The structure of these "chromatcphores," or ''bourses calicinales," has been carefully investigated by Schneider and Rotteken, and by Prof. Duncan. They are diverticula of the body wall, the surface of which is com- posed of close-set " bacilli," beneath which lies a layer of strongly-refracting spherules, followed by another layer of no less strongly-refracting cones. Subjacent to these, Prof. Duncan finds ganglion cells and nervous plexuses. It would seem, therefore, that these bodies are rudimentary eyes. The sexes are united or distinct, and the ovum is ordina- rily, if not always, provided with a vitelline membrane. The impregnated ovum gives rise to a ciliated morula, which may either be discharged or undergo further development within the somatic cavity of the parent. The morula becomes a gas- trula, but whether by true invagination or by delamination, as in most of the Hydrozoa, is not quite clear. The gastrula usually fixes itself by its closed end, while tentacles are de- 1 " On the Nervous System of Actinia." (" Proceedings of the Royal Socie- ty," October 9, 1873.) THE DEVELOPMENT OF THE CORALLIGEXA. 141 veloped from its oral end. It can hardly be doubted that the intermesenteric chambers are diverticula of the primitive en- terocoele ; but the exact mode of their origin needs further elucidation. Lacaze-Duthiers ' has recently thrown a new light upon the development of the Coralligena, and particularly of the Actiniae {Actinia, Sagartia, I? anodes). These animals are generally hermaphrodite, testes and ovaria being usually found in the same animal, and even in the same mesenteries ; but it may happen that the organs of one or the other sex are, at any given time, exclusively developed. The ova undergo the early stages of their development within the body of the parent. The process of yelk division was not observed, and in the earliest condition described the embryo was an oval planula-like body, composed of an inner colored substance and an outer colorless layer. The outer layer (epiblast = ec- toderm) soon becomes ciliated. An oval depression appears at one end, and becomes the mouth 2 and gastric sac, while, at the opposite extremity, the cilia elongate into a tuft. The ectoderm extends into and lines the gastric sac, wrhile the in- terior of the colored hypoblast becomes excavated by a cav- ity, the enteroccele, which communicates with the gastric sac. In this condition the embryo swims about with its oral pole directed backward. The oral aperture changes its form and becomes elongated in one direction, which may be termed the oral axis. The mesenteries are paired processes of the transparent outer layer (probably of that part which constitutes the mesoderm) which mark off corresponding segments of the enteroccele. The first which make their appearance are directed nearly at right angles to the oral axis near, but not exactly in, the centre of its length. Hence they divide the enteroccele into two primitive chambers, a smaller (xA) at one end of the oral axis, and a larger (A') at the other. This condition may be represented by A-^ A' ; the dots indicating the position of the primitive mesenteries, and the hyphen that of the oral axis. It is interesting to remark that, in this state, the em- 1 " Developpemenl des Coralliaires." (Archives de Zoologie experimentale, 2Kowalewsky describes the formation of a gastrula by invagination in a spe- cies of Actinia and in Cereanthus, the aperture of invagination becoming the mouth (Hofmann and Schwalbe, " Jahresbericht," Bd.ll., p. 269). In other species of Actinia and in Alcyonium, the planula seems to delaminate. Ordi- nary yelk division occurs in some Anthozoa, while in others (Alcyonium) the process rather resembles that which occurs in most Arthropods. 142 THE AXATOMY OF INVERTEBRATED ANIMALS. bryo is a bilaterally symmetrical cylindrical body, with a cen< tral canal, the future gastric sac ; and, communicating there- with, a bilobed enteroccele, which separates the central canal from the body- wall. In fact, in principle, it resembles the early condition of the embryo of a Ctenophore, a Brachiopod, or a Sagitta. Another pair of mesenteric processes now makes its ap- pearance in the larger chamber A', and cuts off two lateral chambers, B, B, which lie between these secondary mesenteries and the primary ones. In this state the enteroccele or somat- ic cavity is four-chambered (A-i--r> A'). Next a third pair of mesenteries appear in the smaller chamber (A), and divide it into three portions, one at the end of the oral axis (A), and two lateral (C, C). In this stage there are therefore six A p-^-Ti A' ) ; but almost immediately the number is increased to eight, by the development of a fourth pair of mesenteries in the chambers B, B, which thus give rise to the chambers D, D, between the primitive mesenteries and them- selves. The embryo remains in the eight-chambered condition A pz-^-pv -r> A' ) for some time, until all the chambers and their dividing mesenteries become equal. Then a fifth and a sixth pair of mesenteries are formed in the chambers C, C, and D, D ; two pairs of new chambers, E and F, are produced, and thus the Actinia acquires twelve chambers (A p p~^F D R ^ /' ^ve of which result from the subdivision of the smaller primary chamber, and seven from that of the larger primary chamber. The various chambers now acquire equal dimensions, and the tentacles begin to bud out from each. The appearance of the tentacles, however, is not simultaneous. That which pro- ceeds from the chamber A' is earliest to appear, and for some time is largest, and, at first, eight of the tentacles are larger than the other four. The coiled marginal ends of the mesenteries appear at first upon the edges of the two primary mesenteries ; then upon the edge of the fourth pair, and afterward upon those of the other pairs. For the further changes of the young Actinia, I must refer to the work cited. Sufficient has been said to show that the development of the Actiniw follows a law of bilateral symmetry, and to bring out the important fact that, in the THE OCTOCOKALLA. 143 course of its development, the finally hexamerous Antho- zoon passes through a tetramerous and an octomerous stage. Phenomena analogous to the " alternation of generations," which is so common among the ITydrozoa, are unknown among the great majority of the Actinozoa. But Semper ' has recently described a process of agamogenesis in two spe- cies of Fungio?, which he ranks under this head. The Fiingioe bud out from a branched stem, and then become detached and free, as is the habit of the genus. To make the parallel with the production of a medusoid from a hydroid polyp complete, however, the stem should be nourished by a sexless anthozooid of a different character from the forms of Fungice which are produced by gemmation. And this does not appear to be the case. In one division of the CoralUgena — the Octocoralla — eight enteroccele chambers are developed, and as many ten- tacles. Moreover, these tentacles are relatively broad, flat- tened, and serrated at the edges, or even pinnatifid. The Actinozoon developed from the egg may remain simple (JHaimea, Milne-Edwards), but usually gives rise to a zoan- thodeme. * The ccenosarc of the zoanthodeme in the Octocoralla is a substance of fleshy consistence, which is formed chiefly of a peculiar kind of connective tissue, containing many muscular fibres developed in the thickened mesoderm. The axial cavity of each anthozooid is in communication with a system of large canals. In Alcyoiiiiun, a single large canal descends from each anthozooid into the interior of the zoanthodeme, and the eight mesenteries are continued as so many ridges throughout its entire length,2 so that these tubes have been compared to the thecal canals of the Millepores. In the red coral of commerce ( Corallium ruhrurn, Fig. 30), the large canals run parallel with the axial skeleton. A delicate net- work, which traverses the rest of the substance of the cceno- sarc, appears to be sometimes solid and sometimes to form a system of fine canals opening into the larger ones. The anthozooids possess numerous muscles by which their move- ments are effected. The fibres are delicate, pale, and not striated. Nerves have not been certainly made out. It is in these Octocoralla that the form of skeleton which is termed a sclerobase, which is formed by cornification or 1 " Ueber Generations- Weebsel bei Steinkorallen." Leipsic, 1872. 2 Poucbet and Myevre, " Contribution a 1' Anatomie des Alcyonaires." {Journal

mi rubrum (after Lacaze-Duthiers !). I. The end of a branch with A, B. C, three anthozooids in different desrees of ex- pansion ; k, the mouth ; a, that part of the coenosarc which rises into a cup around t lie base of each anthozoOid. II. Portion of a branch, the ccenosarc of which has been divided longitudinally and partially removed ; 5, B', B", anthozooids in section; B, anthozooid with ex- panded tentacles; k. mouth ; m. gastric sac ; z, its inferior edge; j, mesenteries. B', anthozooid retracted, with the tentacles (d) drawn back into the intermesenteric chamDer-: c, orifices of the cavities of the invaginated tentacles ; e, circum-oral cavity ; b. the part of the body which forms the projecting tube when the antho- zoOid is expanded : a. festooned edges of the cup. B'\ anthozooid, showing the transverse sections of the mesenteries. -4, A. ccenosarc, wiih its deep longitudinal canals (/), and superficial, irregular, reticulated canals (h). P, the hard axis of the coral, with longitudinal grooves (g) answering: to the longitudinal vessels. III., IV. Free ciliated embryos. i " Histoire Naturelle clu Corail," 1864. THE ACTINOZOA. 145 calcification of the axial connective tissue of the zoantho- deme, occurs. It is an unattached simple rod in Pennatula and Veretillum, but fixed, tree-like, branched, and even retic- ulated, in the Gorgonice and the red coral of commerce ( Co- r allium). In the Alcyonia, or " Dead-men's-fingers," of our own shores, there is no sclerobase, nor is there any in Tubi- pora, the organ-coral. But, whereas in all the other Oetoco- ralla the bodies of the polyps and the ccenosarc are beset with loose spicula of carbonate of lime, Tubipora is provided with solid tubiform thecal, in which, however, there are no septa. Dimorphism has been observed by Kolliker to occur exten- sively among the Pennatulidce. Each zoanthodeme presents at least two different sets of zooids, some being fully devel- oped, and provided with sexual organs, while the others have neither tentacles nor generative organs, and exhibit some other peculiarities.1 These abortive zooids are either scat- tered irregularly among the others (e. g., Sarcop>hyton, Vere- t ilium), or may occupy a definite position (e. g., Virgularid). In the other chief division of the Coralligena — the Hexa- coralla — the fundamental number of enteroccele chambers and of tentacles is six,2 and the tentacles are, as a rule, rounded and conical, or filiform. The Actinozoon developed from the egg in some of the JTeozacoralla remains simple, and attains a considerable size. Of these — the Actinidce — many are to some extent locomo- tive, and some (Minyas) float freely by the help of their contractile pedal region. The most remarkable form of this group is the genus Cereanthus, which has two circlets, each composed of numerous tentacles, one immediately around the oral aperture, the other at the margin of the disk. The foot is elongated, subcorneal, and generally presents a pore at its apex. Of the diametral folds of the oral aperture, one pair is much longer than the other, and is produced as far as the pedal pore. The larva is curiously like a young hydrozoon with four tentacles, and, at one time, possesses four mesen- teries. The Zoanthidce differ from the Actinidce in little more than their multiplication by buds, which remain adherent, either by a common connecting expansion or by stolons ; and in the possession of a rudimentary, spicular skeleton. In the Antipathidce there is a sclerobasic skeleton. The proper 1 " Abhandlungen der Senkenbennschen naturforschenden Gesellschaft," xsd. vu., viu. 2 That is to say, in the adult, they are either six or some multiple of six. 146 THE ANATOMY OF INYERTEBRATED ANIMALS. stone-corals are essentially Actinia?, which become converted into zoanthodemes by gemmation or fission, and develop a continuous skeleton. The skeletal parts ' of all the Actinozoa, consist either of a substance of a horny character ; or of an organic basis im- pregnated with earthy salts (chiefly of lime and magnesia), but which can be isolated by the action of dilute acids ; or, finally, of calcareous salts in an almost crystalline state, form- ing rods or corpuscles, which, when treated with acids, leave only an inappreciable and structureless film of organic matter. The hard parts of all the Aporosa, Perforata, and Tabidata of Milne-Edwards are in the last-mentioned condition ; while, in the Octocorcdla, except Tubipora, and in the Antipathidw, and Zoanthidce, among the Hexacoralla, the skeleton is either horny ; or consists, at any rate, to begin with, of definitely formed spicula, which contain an organic basis, and frequently present a laminated structure. In the organ-coral {Tubiponi), the skeleton has the character of that of the ordinary stone- corals, except that it is perforated by numerous minute canals. The skeleton appears, in all cases, to be deposited within the mesoderm, and in the intercellular substance of that layer of the body. Even the definitely shaped spicula of the Octo- coralla seem not to result from the metamorphosis of cells. In the simple aporose corals the calcification of the base and side walls of the body gives rise to the cup or theca ; from the base the calcification extends upward in lamella?, which correspond with the interspaces between the mesenteries, and gives rise to as many vertical septa? the spaces between which are termed loculi ; while, in the centre, either by union of the septa or independently, a column, the columella, grows up. Small separate pillars between the columella and the septa are termed paluli. From the sides of adjacent septa scattered processes of calcified substance, or synapticuke, may grow out toward one another, as in the Fungidm / or the interrup- tion of the cavities of the loculi maybe more complete in consequence of the formation of shelves stretching from sep- tum to septum, but lying at different heights in adjacent loculi. These are interseptal dissepiments. Finally, in the Tabulata, horizontal plates, which stretch completely across the cavity of the theca, are formed one above the other and constitute tabular dissepiments. "« See Kolliker, uIeones Histologic©," 1866. 2 Lacaze-Duthiers's investigations on Astrcea calycularis prove that the septa begin to be formed before the theca. THE " TABULATA." 14? In the Aporosa the theca and septa are almost invariably imperforate; but, in the Perforata, they present apertures, and, in some Madrepores, the whole skeleton is reduced to a mere network of dense calcareous substance. When the Hexacoralla multiply by gemmation or fission, and thus give rise to compound massive or arborescent aggregations, each newly-formed coral polyp develops a skeleton of its own, which is either confluent with that of the others, or is united with them by calcification of the connecting substance of the com- mon body. This intermediate skeletal layer is then termed coenenchyma. The septa in the adult Hexacoralla are often very numer- ous and of different lengths, some approaching the centre more closely than others do. Those of the same lengths are members of one " cycle ; " and the cycles are numbered ac- cording to the lengths of the septa, the longest being counted as the first. In the young, six equal septa constitute the first cycle. As the coral grows, another cycle of six septa arises by the development of a new septum between each pair of the first cycle ; and then a third cycle of twelve septa di- vides the previously existing twelve interseptal chambers into twenty-four. If we mark the septa of the first cycle A, those of the second B, and those of the third C, then the space be- tween any two septa (A A) of the first cycle will be thus rep- resented when the third cycle is formed — A C B C A. When additional septa are developed, the fourth and fol- lowing cycles do not consist of more than twelve septa each ; hence the septa of each new cycle appear in twelve of the previously existing interseptal spaces, and not in all of them; and the order of their appearance follows a definite law, which has been worked out by Milne-Edwards and Haime. Thus, the septa of the fourth cycle of twelve (d) bisect the inter- septal space A C ; and those of the fifth cycle (e) the inter- septal space B C ; the septa of the sixth cycle (f), A d and d A ; those of thes eventh cycle (g), e B and B e ; those of the eighth cycle (h), d C and C d; and those of the ninth cycle (i), C e and e C. Hence, after the formation of nine cycles, the septa added between every pair of primary septa (A, A) will be thus ar- ranged—A fdhCiegBgeiChdfA.1 The stone-corals ordinarily known as Mlllepores are char- 1 That the order of occurrence of the septa of various lengths, at the differ- ent stages of growth of a corallite, is that indicated, seems to be clear, whatever may be the exact mode of development of the septa in each cycle. 148 THE ANATOMY OF IXYERTEBRATED ANIMALS. acterized by being traversed by numerous tubular cavities, which open at the surface, and the deeper parts of which are divided by numerous close-set transverse partitions, or tabular disseptinents, while vertical septa are rudimentary or alto- gether absent. These were regarded as Anthozoa, and classed together in the division of Tabidata, until the elder Agassiz ' published his observations on the living Millepora aleicornis, which led him to the conclusion that the Tabulata are Hydrozoa allied to Hydractinia, and that the extinct Ru- gosa were probably of the same nature. The evidence adduced by Agassiz, however, was insuffi- cient to prove his conclusions ; and the subsequent discovery by Verrill that another tabulate coral, Pocillopora, is a true Hexacorallan, while Moseley 2 has proved that Heliopora coerulea is an Octocorallan, gave further justification to those who hesitated to accept Agassiz's views. The recent very thorough and careful investigation of a species of Millepora occurring at Tahiti,3 by Mr. Moseley, although it still leaves us in ignorance of one important point, namely, the characters of the reproductive organs, yet permits no doubt that Millepora is a true Hydrozoon allied to Hy dr actinia , as Agassiz maintained. The surface of the living Millepora presents short, broad hydranths, the mouth of which is surrounded bv four short tentacles. Around each of these alimentary zooids is disposed a zone of from five to twenty or more, much longer, mouthless zooids, over the bod- ies of which numerous short tentacles are scattered. Each of these zooids expands at its base into a dilatation, whence tubular processes proceed, which ramify and anastomose, giv- ing rise to a thin expanded hydrosoma. The calcareous mat- ter (composed as usual of carbonate, with a small proportion of phosphate of lime) forms a dense continuous crust upon the ectoderm of the ramifications of the hydrosoma, that part of it which underlies the dilatations of the zooids constituting the septa. As the first formed hydrosomal expansion is com- pleted, another is formed on its outer surface, and it dies. The "thecal" canals of the coral arise from the correspond- ence in position of the dilatations of the zooids of successive hydrosomal layers, and the tabulae are their supporting plates. Thus the group of the Tabulata ceases to exist, and its » " Natural History of the United States," vols. iii. and iv., 1860-'62. 2 Moseley, " The Structure and Relations of the Alcyonarian, Heliopora carulea" etc. (" Proceedings of the Roval Societv," November, 1875.) 3 " Proceedings of the Royal Society," 1876. THE REEF-BUILDIXG CORALS. U9 members must be grouped either with the Hexacoralla, the Octocoralla, or the Hydrozoa. The Mugosa constitute a group of extinct and mainly Palaeozoic stone-corals, the thecas of which are provided with tabular dissepiments, and generally have the septa less de- veloped than those of the ordinary stone-corals. The arrange- ment of the parts of the adult Mugosa in fours, and the bilateral symmetry which they sometimes exhibit, are inter- esting peculiarities when taken in connection with the te- tramerous and asymmetrical states of the embryonic Hexaco- ralla. On the other hand, some of the Mugosa possess oper- cula, which are comparable to the skeletal appendages of the Alcyonarian Primnoa observed by Lindstrom, and the te- tramerous arrangement of their parts suggests affinity with the Octocoralla. It seems not improbable that these ancient corals represent an intercalary type between the Hexacoralla and the Octocoralla. All the Actinozoa are marine animals. The Actinia?, among the Hexacoralla, and various forms of Octocoralla, have an exceedingly wide distribution, while the latter are found at very great depths. The stone-corals, again, have a wide range, both as respects depth and temperature, but they are most abundant in hot seas, and many are confined to such regions. Some of these stone-corals are solitary in habit, while others are social, grow- ing together in great fields, and forming what are called " coral reefs." The latter are restricted within that compara- tively narow zone of the earth's surface which lies between the isotherms of 60°, or, in other words, they do not extend for more than about 30° on either side of the equator. It is not conditions of temperature alone, however, which limit their distribution ; for, within this zone, the reef-builders are not found alive at a greater depth than from fifteen to twenty fathoms, while at the equator, an average temperature of 68° is not reached within a depth of 100 fathoms. Not only heat, then, but light, and probably rapid and effectual aeration, are essential conditions for the activitv of the reef-building Actinozoa. But, even within the coral zone, the distribution of the reef-builders appears to be singularly capricious. None are found on the west coast of Africa, very few on the east coast of South America, none on the west coast of North America ; while in the Indian Ocean, the Pa- cific, and the Caribbean Sea, they cover thousands of square 150 THE AXATOMY OF IXVERTEBRATED ANIMALS. miles. It is by no means certain, however, that any one species of West India reef-coral is identical with any East Indian species, and the corals of the central Pacific differ very considerably from those of the Indian Ocean. Different species of corals exhibit great differences as to the rapidity of their growth, and the depth at which they flourish best ; and no one must be taken as evidence for anoth- er in these respects. Certain species of Perforata (Madre- poridce and Poritidce) appear to be at once the fastest grow- ers, and those which delight in the shallowest waters. The Astrceidce among the Aporosa, and Seriatopora among the Tabulata, live at greater depths, and are probably slower of increase. Under the peculiar conditions of existence which have just been described, it would seem easy enough to compre- hend, a priori, the necessary arrangement of coral-reefs. As the reef-building Actinozoa cannot live at greater depths than twenty fathoms, or thereabouts, it is clear that no reef can be originally formed at a greater depth below the surface, and such a depth usually implies no very great distance from land. Furthermore, we should expect that the growth of the coral would fill up all the space between the shore and this farthest limit of its growth ; so that the shores of coral seas would be fringed by a sort of flat terrace of coral, covered, at most, by a very few feet of water ; that this terrace would extend out until the shelving land upon which it had grown descended to a depth of some twenty fathoms ; and that then it would suddenly end in a steep wall, the summit and upper parts of which would be crowned with overhanging ledges of living coral, while its base would be hidden by a talus of dead fragments, torn off and accumulated by the waves. Such a "fringing reef" as this, in fact, surrounds the island of Mauritius. The beach here does not gradually shelve down into the depths of the sea, but passes into a flat, irregular bank, covered by a few feet of water, and terminating at a greater or less distance from the shore in a ridge, over which the sea constantly breaks, and the seaward face of which slopes at once sheer down into fifteen or twenty fathoms of water. The structure of a fringing reef varies at different dis- tances from the land, and at different depths in its seaward face. The edge beaten by the surf is composed of living masses of Forties, and of the coral-like plant, the Nullipore ; deeper than this is a zone of Aporosa (Astrceidce), and of FRINGING REEFS.— ATOLLS. lol Millepores (Seriatopora) ; while, deeper still, all living coral ceases ; the lead bringing up either dead branches, or show- ing the existence of a flat, gently-sloping floor, the true sea- bottom, covered with fine coral sand and mud. Passing from the edge of the reef landward, the Poriticlce cease, and are replaced by a ridge of agglomerated dead branches and sand, coated with N-ullipore / the floor of the shallow basin, or "lagoon,"" inclosed between the reef and the land, is formed by a conglomerate, composed of fragments of coral cemented by mud ; and, on this, Meandrinoe and Funguv rest and flourish, exhibiting the most gaudy coloration, and sometimes attaining a great size. During storms, masses of coral are hurled on to the floor of the lagoon, and there gradually in- crease the accumulation of rocky conglomerate ; but in no other wav can a fringing" reef, which has once attained its limit in depth, increase in size, unless, indeed, the talus ac- cumulating at the foot of its outer wall should ever rise suffi- ciently high to afford a footing for the corals within their pre- scribed limits of depth. Such is the structure cf a fringing reef ; but the great majority of reefs in the Pacific are very different in their character. Along the northeastern coasts of New Holland, for instance, a vast aggregation of reefs lies at a distance from the shore which varies from a hundred to ten miles ; forming a mighty wall or barrier against the waves of the Pacific. At a few hundred yards outside this " barrier reef ' no bottom can be obtained wTith a sounding-line of a thousand fathoms ; between the reef and the mainland, on the con- trary, the sea is hardly ever more than thirty fathoms deep. Manv of the islands of the Pacific, again, are encircled wTith reefs corresponding exactly in their character with the barrier reef ; separated, that is, by a relatively shallow channel from the land, but facing the sea with an almost perpendicular wall which rises from a very great depth. Finally, in many cases, especially among the single reefs, which taken together constitute the great iVustralian barrier, there is no trace of any central island ; but a circular reef, usually having an opening on its leeward side, stands out in the midst of the sea. These reefs, apparently unconnected with other land, are what are called " Atolls." How have these barrier reefs, encircling reefs, and atolls, been formed ? It is certain that the fabricators of these reefs cannot live at a greater depth than in the fringing reefs. How can they have grown up, then, from a thousand fathoms 152 THE ANATOMY OF INVERTEBRATED ANIMALS. or more ? Why do they take so generally tbe circular form? What is the connection, finally, between fringing reefs and atolls? The only thoroughly satisfactory answer to these questions has been given by Mr. Darwin, from whose beauti- ful work on " Coral Reefs " I have borrowed most of the fore- going details. Consider for a moment what would be the effect of a slow and gradual submergence of the island of Mauritius — a submergence, perhaps, of a few feet in a century (at any rate, not greater than the rate of upward growTth of coral), continued for age after age. As the edge of the fring- ing reef sank, new coral would grow up from it to the sur- face; and, as the most active and important of the reef- build- ers nourish best in the very surf of the breakers, so the margin of the reef would grow faster than its inner portion, and the discrepancy would increase as the latter, sinking deeper and deeper, became farther removed from the region of active growth. Nevertheless, the sea-bottom within the reef would constantly tend to be raised by the accumulation of frag- ments, and by the deposit of fine mud, in its sheltered and comparatively calm waters. On the other hand, on the sea- ward face of the reef, no possible extension could take place by direct growth; and that by accumulation must be exceed- ingly slow, the incessant wash of tides, waves, and currents, tending incessantly to spread any talus over a wider and wider area. Thus, then, the edge of the reef unceasingly compensates itself for the depression which it undergoes, while, inside the reef, only a partial compensation takes place, and, outside, hardly any at all. Continue the sinking process until its highest peak was but a few hundred feet above the surface, and all that would be left of Mauritius would be an island surrounded by an encircling reef ; carry on the depression further still, and a circular reef, or atoll, alone would remain. But the region of the coral-reefs is, for the most part, that of constant winds. During the whole process of growth of the reef, therefore, one of its sides — that to windward — has been exposed to more surf than that to leeward. Not only will the greater quantity of debris, therefore, have been heaped up by storms upon the windward side, but the coral-builders themselves will here have been better fed, better aerated, and consequently more active. Hence it is that, other things being alike, there is a probability that the leewrard side of the reef will grow more slowly, and repair any damages less easily, than the windward side ; and hence, again, as a result, ANCIENT REEFS. 153 the known fact that the practicable channels of entrance into encircling reefs or atolls are usually to leeward. The winds and waves are singularly aided in grinding down the corals into mud and fragments by the Scari and Holothurioe which haunt the reefs ; the former browsing upon the living polyps, with their hard and parrot-like jaw^s, and passing a fine calcareous mud in their excrements ; the latter, more probably, swallowing only the smaller fragments and mud, and, having extracted from them such nourishment as they may contain, casting out a similar product. It is curious to reflect upon the similarity of action of these worm- like Holothurioe upon the sea-meadows of coral, to that which the Earthworms, as Darwin has shown, exert upon our land-meadows ! In the Palaeozoic period reefs like »those which have just been described appear to have abounded in our own latitudes ; and there is the most striking superficial resemblance be- tween the ancient beds of calcareous rock which record their existence, and the masses of coral limestone, hard enough to clink with a hammer, which are now being formed in the Pacific, by the processes of accumulation of coral mud and fragments, and their consolidation by percolating water. Closer examination, however, shows an important difference in the nature of the corals which compose the two reefs. The modern limestones are made up of Perforata, Millepores, and Aporosa. The ancient ones contain Millepores, but usu- ally neither Perforata nor Aporosa — both these groups being replaced by the Pugosa, none of whose members (with some doubtful exceptions) have survived the Palaeozoic period. On the other hand, Palwocyclus and Pleurodictyon are the only genera belonging to the Aporosa or Perforata, which have yet been discovered in strata of greater than mesozoic age. The Ctexophoea.1 — These are freely-swimming marine animals, which never give rise by gemmation to compound organisms, and are always of a soft and gelatinous consist- ence, their chief bulk being made up by the greatly -devel- oped mesoderm. Many are oval or rounded (Berde, Pleuro- • Allman (" Monograph of the Tubularian Hydroids," 1871, page 3) consid- ers that the Ctenophora are more properly arranged among the Hydrozoa. I confess, however, that I see no reason to depart from the conclusion to which I was led by the study of the structure of Pleurobrachia, many years ago, that the Ctenophora are peculiarly modified Actlnozoa. 154 THE ANATOMY OF INVERTEBRATED ANIMALS. brachia, Fig. 31), while in others the body is produced into lobes (Callianira), or may even be ribbon-shaped (Cestum) ;• but, whatever their form, they present a distinct bilateral symmetry, similar parts being disposed upon opposite sides of a median plane, which is traversed by the axis of the body. The mouth is situated at one end of this axis, which may be termed the oral pole. At the opposite, or aboral pole, there is no median aperture, but usually, if not inva- riably, a pair of apertures a short distance apart. The faces of the halves of the body present four longitudinal bands of long and strong cilia, disposed in transverse rows, like so many paddles; these constitute the chief organs of locomo- tion. Each half is also often provided with a long retractile tentacle ; and lobed processes of the body, or non-retractile tentacula, may be developed on its oral face. The mouth leads into a wTide, but flattened, gastric sac, the aboral end of which is perforated, and leads into a chamber termed the infundibulum. From the aboral face of this, a canal w7hich bifurcates, or two canals, lead to the aboral apertures. On opposite sides of the infundibulum a canal is given off toward the middle of each half of the body, which sooner or later divides into two, and these two again subdivide, so that four canals, which diverge and radiate toward the inner faces of the rows of paddles, are eventually formed. Having reached the surface, each radiating canal enters a longitudinal canal, which underlies the row of paddles, and may give off branches, or unite wTith the other longitudinal canals in a circular canal at the aboral end of the body. In addition, two other canals, which run parallel with each flat face of the gastric sac, open into the infundibulum. And, when retractile tentacula are present, their cavities also communicate with the same cham- ber. The entire system of canals is in free communication with the gastric cavity, and corresponds with the enteroccele of an Actinia. Indeed, an Actinia with only eight mesenter- ies, and these exceedingly thick, wThereby the intermesenteric chambers would be reduced to canals ; with two aboral pores instead of the one pore, which exists in Cereanthus / and with eight bands of cilia corresponding with the reduced intermesenteric chambers, would have all the essential pecu- liarities of a Ctenophoran. The question whether the Ctenophora possess a nervous system or not is still under debate. Between the aboral aper- tures there is a rounded cellular body, on which there is THE CTEXOPHORA. 155 seated, in many cases, a sac containing solid particles, like one of the lithocysts of the medusiform Hydrozoa. I see no reason to doubt that the rounded body is a ganglion and the sac a rudimentary auditory organ. Bands which radiate from the ganglion to the rows of paddles may be regarded as nerves ; though they may contain other than nervous structures.1 The ova and spermatozoa are developed in the lateral walls of the longitudinal canals, which correspond with the faces of the mesenteries in the Coralligena, and the sexes are usually united in the same individual. Fig. 31.— Diagram of Pleurobrachia.—a. month : b. stomach : c, in fundi buluin ; tf, horizontal canal ; e, one of its branches dividing again at / into two branches which open into the longitudinal canals, g g, parallel with which the ciliated area runs: h, sac of the tentacle, L with one of its branches, k ; /, canal run- ning by the side of the stomach: m, tentaculigerous canal; n ;?, canals opening at theaboral apertures, o, on each side of p, the ganglion and lithocyst. 1 Grant originally described a nervous ganglionated ring, whence longitu- dinal cords proceeded in Cydippe (Pleurobrathia), but his observation has not been verified by subsequent investigators. According to Milne-Edwards, fol- lowed by others (among whom I must include myself), the nervous system consists of a ganglion, situated at the aboral pole of the body, whence nerves radiate, the most conspicuous of which are eight cords which run down the corresponding series of paddles ; and a sensory organ, having the characters of an otolithic sac, is seated upon the ganglion. Agassiz and Kolliker, on the other hand, have denied that the appearances described (though they really exist) are justly interpreted. And again, though the body, described as an otolithic sac, undoubtedly exists in the position indicated in all or most of the Cterwphora, the question has been raised whether it is an auditory or visual organ. These problems have been recently reinvestigated with great care, and by the aid of the refined methods of modern histoloorv, by Dr. Eimer, whose de- scnption of the nervous system has already been quoted {supra, p. 63). 156 THE ANATOMY OF INVERTEBRATED ANIMALS. The development of the Ctenophora has recently been thoroughly investigated by Kowalewsky and by A. Agassiz (" Memoirs of the American Academy of Arts and Sciences," 1874). The laid egg is contained in a spacious capsule, and con- sists of an external thin layer of protoplasm, which, in some cases, is contractile, investing an inner vesicular substance. After fecundation, the vitellus thus constituted divides into two, four, and finally eight masses ; on one face of each of these the protoplasmic layer accumulates, and is divided off as a blastomere of much smaller size than that from which it arises. By repeated division, each of these gives rise to still smaller blastomeres, which become distinctly nucleated when they have reached the number of thirty-two, and form a layer of cells, which gradually spreads round the large blas- tomeres, and invests them in a complete blastodermic sac. At the pole of this sac, on the face opposite to that on which these blastoderm-cells begin to make their appearance, an ingrowth or involution of the blastoderm takes place, which, extending through the middle of the large yelk-masses tow- ard the opposite pole, gives rise to the alimentary canal. This, at first, ends by a rounded blind termination ; but from it, at a later period, prolongations are given off which be- come the canals of the enteroccele. At the opposite pole, in the centre of the region corre- sponding with that in which the cells of the blastoderm first make their appearance, the nervous ganglion is developed by metamorphosis of some of these cells. The invaginated portion of the blastoderm, which gives rise to the alimentary canal, appears to answer to the hypo- blast, while the rest corresponds with the epiblast. The large blastomeres which become inclosed between the epi- blast and hypoblast in the manner described seem to serve the purpose of a food-yelk ; and the space which they origi- nally occupied is eventually filled by a gelatinous connective tissue, which possibly derives its origin from wandering cells of the epiblast. In those Ctenophora the bodies of which depart widely from the globular form in the adult state, the young undergo a sort of metamorphosis after they leave the egg, and have acquired all the essential characters of the group to which they belong. As might be expected from their extreme softness and perishable nature, no fossil Ctenophora are known. CHAPTER IV. THE TURBELLARIA, THE ROTTFERA, THE TREMATODA, AND THE CESTOIDEA. The Turbellaria. — The animals which constitute this group inhabit fresh and salt water and damp localities on land. The smallest are not larger than some of the Infusoria, which they approach very closely in appearance, while the largest may attain a length of many feet. Some are broad, flattened, and discoidal, while others are extremely elongated and relatively narrow. None are divided into distinct seg- ments, except the genus Alaurina, in which there are four ; and the ectoderm, which constitutes the outer surface of the body, is everywhere beset with vibratile cilia. Rod-like bodies, similiar to those met with in some Infusoria and in many Annelida, are often imbedded in its substance, and in some genera (e. g., Microstomum, Thysanozoon) true thread- cells occur. Stiff setae project from the ectoderm in some species. The aperture of the mouth is sometimes situated at the anterior end of the body, sometimes in the middle, or toward the posterior end, of its ventral face. In many, the oral aperture is surrounded by a flexible muscular lip, which some- times takes on the form of a protrusible proboscis. A definite digestive cavity can hardly be said to exist in the lowest Turbellaria (e. g., Convoluta) in which the endo- dermal cells are not arranged in such a manner as to bound a central alimentary cavity, and the food finds its way through the interstices of an endodermal parenchyma. In the higher forms, the alimentary cavity, which may be simple or rami- fied, provided with an anal aperture or without one, is lined by the endoderm, between wThich and the ectoderm is an in- terspace more or less completely occupied by the connective and muscular tissues of the mesoderm. Hence there is no definite perivisceral cavity. 158 THE ANATOMY OF INVERTEBRATED ANIMALS. The Turbellaria possess vessels of two kinds : 1. Water- vessels, which open externally by one or more pores, and are ciliated. When these vessels are present, there are usually two chief lateral trunks, from which many branches are given oif. It is probable that the ultimate ends of these branches open into lacunar interspaces between the elements of the tissues of the mesoderm. 2. JPseud-hcemal vessels, which ap- pear to form a closed system, usually consisting of one median dorsal and two lateral trunks, which anastomose anteriorly and posteriorly. The walls of these vessels are contractile and not ciliated, and their contents are clear, and may be colored. These two systems of vessels have been shown by Schulze to coexist in Tetrastemma. The nervous system con- sists of two ganglia placed in the anterior end of the body, from which, in addition to other branches, a longitudinal cord extends backward on each side of the body. In some cases, these lateral trunks exhibit ganglionic enlargements, from which nerves are given off; and they may become approxi- mated on the ventral side of the body, thereby showing a tendency to the formation of the double ganglionated chain characteristic of higher worms. Most possess eyes, and some have auditory sacs. The Turbellaria are both monoecious and dioecious, and the reproductive organs vary from the utmost simplicity of structure to considerable complexity. In most, the embryo passes by insensible gradations into the form of the adult, but some undergo a remarkable metamor- phosis. The Turbellaria are divisible into two groups. In the one, the Aprocta, the digestive cavity is csecal, having no anal aperture ; in the other, the JProctucha, it is provided with an anal opening. The two groups form parallel series, in each of which organization advances, from forms which are little more than gastrulae provided with reproductive organs, to animals of relatively high organization. In the simplest of the Aprocta, such as Macrostomum,1 the oral opening is devoid of any protrusible muscular proboscis, and the aliment- ary sac is a simple straight bag. The male and female gen- erative organs are united in the same individual, and each consists of an aggregation of cells; which, in the former case, gradually enlarge, fill with yelk-granules, and become ova; while, in the latter, they are converted into spermatozoa. The generative cells are contained within a sac, which opens 1 E. Van Beneden, " Recherches sur la Composition et la Signification de I'CEuf," 1870, p. 64. THE TURBELLARIA. 159 externally by a median pore on the oral face of the body, the male aperture being posterior to the female. The margins of the male aperture are produced into a curved prominence, the penis. Those Turbellaria which resemble Macrostomum in having a straight, simple digestive cavity, are termed Rhabdocoela. They, for the most part, possess a buccal proboscis, which is capable of being protruded from, or retracted into a chamber mm '?4!»w mv;. H m Fig. 32. — Opisthomum (after Schulze).— a, central nervous system : ramifications of the water-vessels are seen close to it; b. mouth; c, proboscis; d, testes; e, vasa deferentia; /, vesicula seminalis ; g, penis ; h, sexual aperture ; i, vagina ; k, sper- raatheca ; ?, germarium; m, viteliarium ; n, uterus with two ova inclosed within their hard shells. formed by the walls of the circum-oral region of the body (Fig. 32, c). In some (e. g., Prostomum) the anterior end of the body is 11 160 THE ANATOMY OF INVERTEBRATED ANIMALS. provided with a second hollow muscular proboscidiform organ, which may be termed the frontal proboscis. In all the higher rhabdoccelous Turbellaria, the female generative apparatus becomes complicated by the presence of a special gland, the vitellarium (Fig. 32, m), in which an accessory vitelline substance is formed. There is a single or double germarium (Fig. 32, I), having nearly the same struct- ure as the ovary of Macrostomum, and the ova are formed in it in the same way. When detached, however, they con- tain no vitelline granules ; but the two vitellaria, which are long and simple or branched tubes, open into the oviduct ; and the vitelline matter which they secrete envelops the proper ovum, and becomes more or less fused with it, as it passes into the uterine continuation of the oviduct connected with the outer, or vaginal, end of the uterus. There is usually a spermatheca, or receptacle for the seminal fluid (Fig. 32, k), and the eggs, after impregnation, are inclosed within a hard shell (Fig. 32, n). The testes and vasa deferentia (Fig. 32, d, e) generally have the form of two long tubes. The penis is often eversible and covered with spines (Fig. 32, g). In some genera a difference is observed between the eggs produced in summer, which have a soft vitelline membrane, and those produced later. These so-called winter ova have hard shells. The water-vascular system consists of lateral trunks, which open by a terminal pore, or by many pores, and give off numerous ramifications. They are not contractile, but their inner surface is ciliated. Many of the Mhabdoccela multiply by transverse fission ; and, in the genus Catemda, the incompletely separated ani- mals produced in this way swim about in long chains. The vitellus of the impregnated ovum undergoes complete yelk-divison, and the embryos pass directly into the form of the parent ; but the precise nature of the steps of the devel- opmental process requires further investigation. However, there seems little reason to doubt that the ectoderm and en- doderm are formed by delamination. In the remaining Aprocta, termed Dendrocoela, the diges- tive cavity gives off many caecal, frequently branched, pro- cesses into the mesoderm, one of which is always median and anterior (Fig. 33) ; and the mouth is always provided with a proboscis. Some (Procotyld) have a frontal proboscis, and others (JBdellura) a posterior sucker. The animals commonly THE DENDROCCELA. 161 known as Planarim belong to this division. Some are ma- rine, some fresh-water, and some terrestrial. In the fresh-water forms, the female reproductive appa- ratus has a distinct vitellarium, as in the higher Hhabdocoela, and there is only one common genital aperture. But, in the marine Planariaz (Fig. 33), there is no vitellarium ; the ova- ries and testes are numerous, and scattered through the meso- derm, being connected with the exterior by ramifications of the oviducts and of the vasa deferentia. A ramified gland, which secretes a viscid albumen or envelope for the eggs, Fig. 33.—Polycelis (Leptoplana) laevigata (after Quatrefage?).— a, mouth; £>, buccal cavity; c, oesophageal orifice ; d, stomach ; hnentale," 1873.) THE STRUCTURE OF THE EARTHWORM. 197 ated one on each side of every segment except the first, and attached to the posterior mesenteric septum of the segment. Each canal communicates internally, by a wide funnel-shaped ciliated aperture, with the perivisceral cavity, while external- ly it opens by a minute pore, which is usually close to the in- ternal pair of seta?.1 A colorless fluid, containing colorless corpuscles, and an- swering to the blood of other invertebrated animals, occu- pies the perivisceral cavity ; but, in addition to this, there is a deep-red fluid, devoid of corpuscles, which fills a very large- ly developed system of pseud-haemal vessels. These consist of longitudinal and transverse principal trunks, and of very numerous branches which proceed from them and ramify in all parts of the body, except the cuticle and hypodermis. The longitudinal trunks are three : one supra-intestinal^ which lies along the dorsal aspect of the alimentary canal ; one sub-intestinal, which corresponds with this on the ven- tral aspect of that canal ; and one sub-neural, which lies be- neath the ganglionic cord. The supra-intestinal and sub-intestinal vessels are con- nected in the greater number of the segments by pairs of com- missural transverse trunks, which embrace the intestine, and give off numerous branches to it. The supra-intestinal and sub-neural vessels give off transverse trunks into the mesenter- ic septa, which branch out into the muscular layers, and some of which anastomose so as to form a second set of transverse communications. Moreover, the sub-neural trunk and the sub-intestinal trunk respectively send branches to each seg- mental organ, upon which they are distributed, and, anastomo- sing, give" rise to another series of communications between the longitudinal trunks. In the seven most anterior segments, the longitudinal vessels break up into a network, and there are no distinct transverse commissural vessels. Behind these, and in the region of the generative apparatus, the commissural vessels are greatly dilated, and form from five to eight pairs of so- called hearts which are attached to the anterior faces of as many mesenteries. These contract from the dorsal toward the ventral side. The nervous system consists of two cerebral ganglia lodged above the pharynx in the third segment, and united 1 Gegenbaur, " Ueberdie sogenanntenRespirationsorgane des Regenwurms." (Zeitschriftfur wiss. Zoologie, 1852.) 198 THE AX ATOMY OF IXVERTEBRATED ANIMALS. by commissural cords with the anterior ganglia of the chain, which extends through the whole length of the body on the ventral wall of the perivisceral cavity. There are no eyes, nor are any other organs of special sense known. The Earthworm is hermaphrodite. The testes are two pairs of large sacs, each of the anterior pair being bilobed. The testes of opposite sides are united in a common median reservoir, situated in the tenth and eleventh segments, from which, on each side, ducts take their origin. The two ducts of the testes of the same side unite into a single vas deferens, and these two vasa deferentia open externally on the ventral aspect of the fifteenth segment. The ovaries are two minute solid bodies, not more than -^ of an inch long, attached to the posterior face of the mesenteric septum which separates the twelfth and thirteenth segments. They therefore lie in the cavity of the latter. The oviducts are quite distinct from the ovaries, and open internally by wide, funnel-shaped aper- tures, situated in the cavity of the thirteenth segment. From these funnel-shaped ends the oviducts are continued, as slender tubes, through the mesenteric septum which separates the thirteenth from the fourteenth segment, and open on the ventral face of the latter. Four globular spermathecag, or receptacles of the sper- matozoa, are situated, two on each side, in the tenth and eleventh segments, and open on the ventral face between the ninth and tenth and the tenth and eleventh segments respectively. These are filled when copulation takes place, during which process the two worms are said to be bound together by a tough secretion of their clitella. The development of the Oligochceta has recently been carefully investigated by Kowalewsky. The eggs of the Earthworm are laid in chitinous cocoons cr cases, which are probably secreted by the clitella. In addition to the eggs, the cocoons inclose an albuminous fluid, and packets of sper- matozoa. The vitellus is invested by a membrane, and con- tains a germinal vesicle and spot. Complete yelk-division takes place, and eventually the blastoccele becomes reduced to a mere cleft. The blastomeres are disposed in two layers — one consisting of small and the other of large blastomeres. The embrvo thus formed becomes concave on the side formed by the large blastomeres, until it assumes the form of a sac, ciliated externally, with an opening, the future mouth, at one end ; the cavity of the sac being the primitive alimentary THE POLYCH.ETA. 199 canal, and the layer of large blastomeres, the hypoblast. Be- tween the two, a mesoblastic layer appears, but the exact manner of its origin is not known. On one face of the sac- cular embryo the mesoblast becomes divided into a series of quadrate masses, like the protovertebrae of a vertebrate em- bryo, disposed symmetrically on each side of a median line, which corresponds with the future ventral median line of the body. Along this line, the epiblast becomes thickened in- ward, and the thickening is converted into the ganglionic chain. At the same time, each quadrate mass of the meso- blast is excavated by the development of a cavity in its in- terior, whereby it becomes converted into a sort of sac. The adjacent anterior and posterior walls of successive sacs unite, and give rise to the mesenteric septa, while their cavities become the chambers of the perivisceral cavity. The seg- mental organs commence as cellular outgrowths from the posterior face of each septum thus formed, and only subse- quently become excavated and communicate with the exte- rior. The development of the Earthworm, therefore, closely re- sembles that of the ITirudmea, and more especially that of the Medicinal Leech, in which the digestive cavity of the embryo would seem to be formed, as in the Earthworm, by a process which is, in a sense, invagination. It would appear that the first-formed aperture is the mouth ; while the anus is a secondary perforation; and the segmentation of the body commences in the mesoblast. In the fresh-water Oligochceta, Euaxes and Tubife-x, the vitellus also becomes divided into large and small blastomeres. The latter extend over the larger blastomeres, and form the epiblast (= ectoderm). A mesoblast (= mesoderm), divided into two broad longitudinal bands, is developed, and the oral cavity is said to be formed by invagination of the epiblast between the anterior ends of the two bands of the mesoblast. In this case, the mouth in these genera is a secondary forma- tion. The innermost layer of large blastomeres becomes the hypoblast (= endodermj.1 The Polych^eta. — Except that the Polychceta are almost invariably dioecious and marine, while the OlU/ochceta are monoecious, and inhabitants either of land or fresh water, it 1 Kcrwalewskv, " Ernbryologische Studien." (" Memoires de l'Academie de St. P6tersbourg," 1S61.) 200 THE ANATOMY OF INVERTEBRATED ANIMALS. is hard to say what absolute characters separate these two groups. The lowest forms of the Polychceta, such as Capi~ tella and Polyophthalmus, might be regarded as marine dioe- cious Naidw. But, in the higher Polyehmta^ each segment of the body develops lateral processes — the parapodia, or rudimentary limbs, which are usually provided with abundant strong setae ; a distinct cephalic segment, the prcestomium, appears in front of and above the mouth, and bears eyes and tentacles ; while those parapodia which lie in the vicinity of the mouth may be specially modified in form and direction, foreshadowing the jaws of the Arthropoda. Ciliated, some- times plumose, processes of the dorsal walls of more or fewer of the segments may perform the office of external branchiae / and, occasionally, the dorsal surface gives rise to flat shield- like processes, the so-called elytra. The following detailed description of a very common species of Polynde will give a fair conception of a polychae- tous Annelid, in which the highest degree of complexity of organization known in the group is attained : Polynde squamata is an elongated vermiform animal, about an inch long, the body of which is divided into a suc- cession of portions, for the most part similar and equivalent to one another, but presenting peculiar modifications at the anterior and posterior extremities. Each such portion is properly termed a somite / while the term "segment" may be retained to indicate generally a portion of the body, with- out implying its precise equivalency to one somite or to many. Thus, then, the body of the Polynde is composed of a series of twenty-six " somites," terminated anteriorly by a "segment," the pra?stomium ("Kopf-lappen," Grube), and posteriorly by another, the pygidium, which may or may not represent single somites. If one of the somites from the middle of the body (Fig. 51, C, D) be examined separately, it will be found to be transversely elongated, so as to be about three times as broad as it is long, and to be slightly convex above and below, presenting a deep, median, longitudinal groove inferiorly. Laterally the somite is produced into two thick processes, the "parapodia" Each parapodium divides at its extremity into two por- tions, a superior and an inferior, which may be denominated respectively the notopodium (Fig. 51, i) and the neitropodium (&), the one occupying the " haemal " or dorsal, the other the "neural" or ventral aspect. The latter is, in this species POLYNOE SQUAMATA. 201 so much the larger, that the notopodium appears like a mere tubercle projecting from its upper surface. In other Anne- lida, however, and in the young state of Polynoe, the notopo- Fig. 51.— Polynoe squamata. A. Viewed from above and enlarged : a. 5, c, etc., as in Fig. 53, B ; £, elytra ; /, space left between the two posterior elytra ; g, setae and fimbria? of the elytra. B. Posterior extremity, inferior view: d, pygidial cirri; h, inferior tubercle; c, c', notopodial and neuropodial cirri. C. Section of half a somite with elytron : i, notopodium ; A", neuropodium. D. Section of half a somite with cirrus. dium is as large as the neuropodium. Both divisions of the parapodia are armed with peculiar stiff, hair-like appendages (g), composed of cbitin, and developed within diverticula of the integment, or trichophores, in which their bases always remain inclosed. These can be protruded and retracted by muscles attached to their sacs, and they vary exceedingly in form. Three distinct kinds are observable in Polynoe alone. The notopodium and the neuropodium carry each a single, sharp, style-like aciculum, the greater part of the length of which is imbedded in the parapodium and its divisions, while the point just projects at about the centre of the latter. The neuropodial is very much longer than the notopodial aciculum. 202 THE ANATOMY OF IN VERTEB RATED ANIMALS. Superiorly, the notopodium carries two transverse rows oi more slender organs of a similar nature, the setae : the proxi- mal set are much shorter than the distal, but even the latter do not attain a length of more than -f^ of an inch (Fig. 52, Gr). The proximal set are somewhat knife-like in shape if viewed in profile, consisting of a comparatively short, straight "han- dle," by which they are imbedded in their sacs, and of a thick, rounded, curved blade, tapering to a fine point at its extrem- ity. Close-set transverse ridges, finely serrated at their edges, and inclined obliquely to the surface of the blade, traverse its convex anterior circumference, leaving the back free. The distal setae (Fig. 52, G) have a very similar structure, but they are much elongated and very slender. The handle is longer ; and the blade, little curved and simply set on an angle with m Fig. Z&.—Polynoe squamata. A, elytron viewed from above. B, a tooth. C, D, neuroporlial setae. E, F, parts of the blade of the same, more highly magnified. G, free extremity of a notopodial seta. the handle, is produced at the end into a long and delicate filament. The base of the blade (E) is beset with incomplete POLYXOE SQUAMATA. 203 ridges, like those of the short setae, but toward the middle (F) these ridges appear to encircle the blade completely, as- suming the aspect of so many closely -imbricated concentric scales, before finally becoming obsolete upon the extremity of the seta. The neuropoclial aciculum needs no special notice, except that the extremity of its trichophore projects as a sort of papilla, less obvious in full-grown specimens, which divides the neuropodium into an upper and a lower portion, the for- mer containing about half as many setae as the latter. The apertures of the trichophores are placed between lobe-like prolongations of the neuropodium, to which the special term of labia (Grube) may be applied. In this species they pre- sent no remarkable peculiarity beyond their inequality. The neuropodial setae (Fig. 52, C, Z*), although at first sight very different from the notopodial setae, are, in truth, constructed on essentially the same plan, the blade being short, while the handle is proportionally elongated. The blade is subcylindrical at its base, pointed and slightly curved. Eight or nine transverse ridges extend around about two-thirds of the circumference of its proximal half ; the basal ridges are narrow, and merely serrated, but toward the apex the ridges become deeper, and the serrations pass into strong teeth ; at the same time, one side of the ridge is elongated into a strong point. Attached to the under surface of the parapodium by a somewhat enlarged base, with which it is articulated, is a smooth, conical, very flexible filament — the neuropodial cir- rus (Fig. 51, c') ; it hardly reaches to the end of the neuro- podium. Again, springing from the neural surface of the somite, close to the parapodium, there is a small pyriform tubercle (A), divided by longitudinal grooves into about eight segments. This is possibly connected with the reproductive function. The appendage of the notopodium, or rather of the noto- podial side of the parapodium and somite, varies according to the particular somite which may be examined. In some somites this appendage is a cirrus (Fig. 51, D, c) similar to the neuropodial cirrus, but much larger, equaling the semi- diameter of the body in length, and presenting an enlarged pigmented bulb of attachment to which the filament of the cirrus, which is cylindrical for about two-thirds of its length, and then becomes enlarged and suddenly tapers to its extrem- ity, is articulated. 204 THE ANATOMY OF INVERTEBRATED ANIMALS. In the other somites the notopodial appendage is a large, thin, oval plate — the elytron (Fig. 51, C\ c). It is attached by a thick peduncle, and has its long axis directed obliquely outward and backward. The surface of the elytron (Fig. 52, A) is covered with an ornamentation of larger or smaller tubercular prominences, granulated and ridged upon their surface. A part of the inner and anterior edge of each ely- tron overlaps or is overlapped by its fellows for a certain ex- tent of its circumference, which is so far smooth, but in the rest of its extent it is fringed with coarse brownish filaments or jimbrice, which arise from the upper surface just within the edge, and are obviously outgrowths of the same order as the tubercles. Such is the structure of one of the middle somites of Polynbe squamata. The anterior and posterior somites, with the exception of the first and second, present only minor dif- ferences, as in the proportion of the setas, or in the figure of the elytra. The first somite, which contains the mouth, is the peristomium (" Mund-Segment " of Grube). The parapodia of this somite are narrow and elongated (Fig. 53, B, (7, m) ; they are obscurely divided at their extremity into a rudimen- tary neuropodium and notopodium, and give attachment to a pair of large peristomial cirri (cr c) (" cirrhes tentaculaires," Audouin and Milne-Edwards ; " Fuhler-cirren," Grube), of the same structure as the notopodial cirri, which stretch for- ward by the sides of the mouth. The apex of a single small aciculum issues rather above the point of division of the peristomial parapodium, and two minute curved setae accompany it. These have been generally overlooked ; ] but they seem to demonstrate, in a very inter- esting manner, the nature of the appendages of the peristo- mial segment. The second somite differs from the rest only in the great elongation of its neuropodial cirrus, which is directed forward and applied against the mouth. The peristomium and the praestomium together are ordi* narily confounded under the common term of " head." The latter (Fig. 53, JB, (7, I) is an oval segment flattened superior- ly, placed altogether in front of and above the mouth, pre- senting on its postero lateral edges four dark spots, the eyes, and possessing five cirriform appendages, two pairs and a 1 At least, in the descriptions of the adult Polynoe. They are particularly- mentioned, however, by Max Midler in his valuable paper, " Ueber die Ent- wickelung und Metamorphose der Polynoen." {Mailer's Archiv, 1841.) POLYNOE SQUAMATA. 205 single median one. The latter (a), or the prcestomial tentacle (" antenne mediane," Milne-Edwards), is similar in structure to an ordinary cirrus. Of the other appendages, the upper one upon each side (supero-lateral praestomial cirrus, " an- tenne mitoyenne ") also resembles an ordinary cirrus (b) ; but the lower (infero-lateral prasstomial cirrus, " antenne ex- terne ") (b1) is much larger, and is capable of extreme elon- Fig. 53-—Polynoe squamata. A. Posterior extremity from above : c, notopodial cirrus of last somite; d, pygidial cirri ; x, anus. B. Anterior extremity from above : a, prgestomial tentacle ; &, superior and b' inferior prsestomial cirrus ; c, c', notopodial and neuropodial cirri ; e, peduncle of first elytron ; I, praestomium ; m, parapodium of peristomium. C. Inferior view of anterior extremity, letters as before. gation and contraction,1 while the ordinary cirri are merely flexible. Although at first sight probable, yet it would ap- pear, from Max Muller's account of the development of Poly- noe, that these two appendages do not, like the two peristo- mial cirri which they essentially resemble, correspond with the notopodial and neuropodial cirri of a single parapodium, inasmuch as they arise from perfectly distinct portions of the praestomium. It is very possible that each represents the appendage of a somite, and in this case the praestomium would be composed of at least two somites. Whether the praestomial tentacle indicates another, or whether it is merely 1 1 have never observed any invagination such as is stated to occur by Audouin and Milne-Edwards, 1834. (" Histoire Naturelle du Littoral de la France,'* p. 10.) 206 THE ANATOMY OF INVERTEBRATED ANIMALS. an appendage of such a nature as the labrum or the rostrum of a Crustacean, there is no evidence at present to show. It is highly interesting to remark that thus, in the Poly- noe, as in the Arthropoda, the "head " results from the modi- fication of a number of somites, some of which lie in front of, and others behind, the mouth. The movements and evident extreme sensitiveness of the inferior praestomial cirri during life indicate that they perform the functions, as well as occupy the position, of antennae. The hindermost segment of the body, or pygidium (Fig. 51, B, Fig. 53, A), is narrow, and divided at the end into two supports for the pygidial (d) cirri which are as long as the three last somites, and resemble the notopodial cirri in form and structure. They extend directly backward, almost paral- lel with one another, and with the notopodial cirri of the last somite, wThich are thrown backward and downward (Fig. 53, A, c). It seems probable that the pygidium represents only a single somite. The anus is not terminal, as in many Annelids, but is seated in the middle of a strongly-raised papilla (Fig. 53, A, a?), which projects from the dorsal surface of the penulti- mate somite ; its sides are produced into about fourteen folds. The two last elytra have their edges excavated, so as to leave a space over the anus (Fig. 51, A,f). The notopodial cirri and the elytra do not coexist upon the same somites ; and the order of arrangement of the ely- trigerous and cirrigerous somites is very curious. The 1st or peristomial somite is cerrigerous, and so are the 3d, 6th, 8th, 10th, 12th, 14th, 16th, 18th, 20th, 22d, 24th, 25th, and 26th ; while the 2d, 4th, 5th, 7th, 9th, 11th, 13th, loth, 17th, 19th, 21st, and 23d, somites bear elytra, making twelve pairs in all. In no polychaatous Annelid is the structure of a somite more complex than in Polynoe ; and there are but very few parts not found in Polynoe to be met with in other Annelida. The careful study of this species, therefore, furnishes us wTith an almost complete nomenclature for the external organs of the whole group ; and it will be found that the other forms of Annelida differ mainly in the greater or less development and modification of the organs which have just been de- scribed. A large proportion of the Polychceta are like Poly- noe, free and actively locomotive animals, which rarely fabri- cate tubular habitations, and are therefore termed Errantia / they possess a praestomium, usually provided with eyes and feelers, and have many parapodia, which are not confined to THE POLYCILETA. 207 the anterior region of the body. They very generally have a proboscis, provided with chitinous teeth. The singular genus, Tomopteris, is a transparent pelagic Annelid, with numerous parapodia, each terminated by two lobes representing the neuropodium and notopodium, but with setae, two of which are very long, only in the cephalic region. The sedentary Annelids (Tubicola) fabricate tubes, either by gluing together particles of sand and shells, or by secret- ing a chitinous or calcined shelly substance, in which they remain (e. g\, Protula, Fig. 54). The praestomium is small or wanting ; none have a proboscis ; there are no cirri ; and the parapodia are short or rudimentary. The branchias are devel- oped only on the anterior somites, and the latter are often markedly different from those which constitute the posterior part of the body. In some (Serpulidoe) a tentacle is enlarged and its end secretes a shelly plate which serves as an operculum, and shuts down over the mouth of the calcareous tube inhabited by the animal, when it is retracted. The dilated end of the opercular tentacle sometimes serves as a chamber in which the young undergo their development (species of Spiro7,bis). The alimentary canal of the polychsetous Annelida rarely presents any marked distinction between stomach and intes- tine, and is almost always of the same length as the body, ex- tending, without folds or convolutions, from its anterior to its posterior extremity ; but in Sip>honostomum (Chlorcema), Pectinaria and others, it is more or less convoluted. It is attached by membranous bands, or more complete mesenteries, to the walls of each somite, and very commonly presents a dila- tation between every pair of mesenteries. In most Polychoeta, the intestine acquires in this way merely a moniliform appear- ance, but in Polynoe, Aphrodite, Sigalion, and their allies, long caeca are given off upon each side of the alimentary canal, and, sometimes becoming more or less convoluted, ter- minate at the upper part of each segment (Fig. 51, D) close beneath, or in the branchiae, wrhere such organs exist. The anterior portion of the alimentary canal is, in a great number of the Polychmta, in fact in all the typical Errantia, so modified as to constitute a distinct muscular pharynx, the anterior portion of the wall of which can be everted like the finger of a glove, from the aperture of the mouth, and the posterior portion protruded, so as to form a proboscis. In Polynbe sqi/amata, the proboscis is one-fourth as long as the 14 208 THE ANATOMY OF INVERTEBRATED ANIMALS. Fig. 54.— Protula Dysteri. A, the sexual, mature animal, extracted from its calca- reous tube : «, branchial plumes ; 6, hood-like expansion of the anterior end of the body; c, the mouth; d, the stomach; e, the anus ;/, the testes ; g, the ova. B, a Protula in course of proliferation ; £>, the branchiae of the zooid. body, and its walls are very thick and muscular. At its an- terior extremity it is surrounded with a circle of small papil- lae, immediately behind which are four strong, pointed and curved horny teeth, implanted in the muscular wall (Fig. 52, THE POLYCHJETA. 209 J3). Each tooth has a little projection upon its convex edge, which is connected by a short strong ligament with the cor- responding projection of another tooth ; and the one pair of teeth, thus connected, works vertically against the opposite pair. In Nereis, there are two powerful teeth which work horizontally, besides minute accessory denticles. In Syllis, the chitinous lining of the pharynx is produced into a circle of sharp teeth anteriorly, and there is, in addition, a much stronger triangular median tooth. In Glycerol, which pos- sesses a pair of teeth, the extremity of the protruded pro- boscis is covered with very remarkable papillae. The most complex arrangement of teeth, however, is that presented by the Eunicidce. In Eunice, there are altogether nine distinct pieces : two large, flat, more or less calcified portions united together below, and three cutting and tearing teeth on the right side w7orking against four on the left. As has has been already stated, the tubicolar Annelids possess neither probos- cis nor teeth. No special hepatic gland appears to exist in the Annelida, unless the intestinal caeca perform that function, and the secretion of the bile is doubtless effected by the glandular tract, which extends for a greater or less distance in the walls of the alimentary canal. A pair of glandular caeca, the func- tion of which is not known, is appended to the base of the proboscis in Nereis. The general cavity of the body, or perivisceral cavity, which is included between the parietes of the alimentary canal and those of the bodv, is filled with a fluid which con- tains corpuscles, which are usually, as in«the Invertebrata in general, colorless. They are red, however, in Glycera, and in a species of Apneumea (De Quatrefages). The parapodia, the cirri, the branchiae, and all the other important appendages of the Polychceta, contain a cavity continuous with the peri- visceral cavity, and are therefore equally filled with the blood. The circulation of this fluid is effected partly by the contrac- tion of the body and its appendages, partly by the vibratile cilia, with which a greater or less extent of the walls of the perivisceral cavity is covered. In a great number of the Polychceta no part of the body is specially adapted to perform the function of respiration, the aeration of the blood probably taking place wherever the integument is sufficiently thin ; and, even when distinct branchiae ordinarily exist, members of the same family may be deprived of them. In Polynbe squamata, ciliated spots 210 THE ANATOMY OF INVERTEBRATED ANIMALS. which appear to represent branchiae, may be discovered on the dorsal side of the bases of the parapodia, at any rate, in young specimens. In some species of Polynoe the parapodia give rise, at corresponding points, to large, richly ciliated, malleiform tubercles, in which the caeca of the alimentary canal terminate. In Sigalion, a filiform, ciliated branch ia depends from the upper part of the somite, beneath the ely- tron ; and, besides this, curious little ciliated palettes are arranged upon the dorsal surface of the parapodia, and upon the sides of the anterior somites. But the best-developed branchiae among these Annelids are possessed by the Amphl- nomidoe, and the Euniddce among the Errantia ; the Tere- bellidw, and the Serpulidoe among the Tubicola. In the three former families the branchiae are ciliated branched plumes, or tufts, attached to the dorsal surface of more or fewer of the somites. In the last (Fig. 54) they are exclu- sively attached to the anterior segment of the body, and present the form of two large plumes, each consisting of a principal stem, with many lateral branches. The stem is supported by a kind of internal skeleton, of cartilaginous consistence, which sends off processes into the lateral branches. I have been unable to find any pseud-haemal vessels in Polynoe squamata, and, as Claparede x could discover none in the transparent P. lunidata, it is safe to assume their non- existence. Cla.parede, in fact, denies them to the whole of the Aphroditidm. When it is present, the pseud-haemal svstem varies very much in the arrangement of its great trunks ; but they com- monly consist of one or two principal longitudinal dorsal and ventral vessels, which are connected in each somite by trans- verse branches. Where branchiae exist, loops or processes of one or other of the great trunks enter them. The dorsal and the ventral trunks are usually rhythmically contractile, and contractile dilatations at the bases of the branchiae (Eunice), in portions of the lateral trunks (Arenicola), or in those which supply the proboscis (Eunice, Nereis), have received the name of " hearts." The direction of the contractions is usually such that the blood is propelled from behind forward in the dorsal vessel, and in the opposite direction in the ven- tral vessel ; but the course which it pursues in the lateral trunks is probably very irregular. In Chlorcema, in which even the smallest ramifications of the vessels are contractile, I » "Annelides Ch&opodes du Golfe de Naples," 1868, p. 65. THE POLYCELETA. 211 have observed crecal branches depending into the perivisceral cavity in which the contained fluid underwent merely an alter- nate flux and reflux. Ramified caeca of a similar kind appear to exist in the oligochaetous genera, Euaxes and X/umbriculus. The principal trunks give off a great number of branches, which ramify very minutely in some Annelids (Eunice) and may give rise to retia mirabilia (Nereis) ; but in many (e. g., Protula) there are hardly any branches and no minute capil- lary ramifications. In many Polychceta no segmental organs have yet been discovered, and in others they appear to be represented by mere openings in the parietes of the body. I have observed short ciliated canals opening externally upon the ventral sur- face at the bases of the parapodia in Phyllodoce viridis, and there are indications of the existence of similar organs in Syllis vittata. True segmental organs have, however, been found by Ehlers and Claparede in many Polychceta. In some cases their walls are thick and glandular, and they probably have a renal function. In addition, the}*- frequently play the part of oviducts and spermiducts. Whether the ciliated canal extending along the ventral surface of the intestine, which I have described in Protula, is a structure of the same order or not, I am not prepared to say. The nervous system of the Polychceta usually consists of a chain of ganglia — one pair for each somite — connected together by longitudinal and transverse commissural bands, which diverge between the cerebral ganglia and the succeed- ing pair, to allow of the passage of the oesophagus. The most important differences presented by the nervous systems of the Polychceta result from the varying length of the transverse commissures. In Vermilia, Serpula, Sabella, these commis- sures are very long, so that two distinct and distant series of ganglia appear to run through the body, while, in Nepthys, the two series of ganglia are fused into a single cord enlarged at intervals. Every transitional condition between these is observable in Terebella, Aonia, Glycera, Phyllodoce, and Aphrodite. In most Polychceta a very extensive series of visceral nerves supplies the alimentary canal. The recognizable organs of sense in the Annelida are eyes and auditory vesicles. The former are usually very simple, consisting of an expansion of the extremitv of the optic nerve, imbedded in pigment, and provided occasionally, but not in- in variably, with transpnrent spheroids or cones. Alcinpe and Torrea have very well-developed and large eyes. The eyes 212 THE ANATOMY OF IXVERTEBRATED ANIMALS. are usually confined to the anterior extremity of the body, and to the praestomium where it exists ; but, in the remarkable genus Polyophthalmus, De Qua tref ages discovered, besides — c Fig. 55.— J., anterior end of the nervous system of Polynoe squama to (after De Qua- trefages) : a, cerebral sanglia ; b, oesophageal commissures; c, longitudinal com- missures of the ventral ganglia. B, anterior end of the nervous system of Sabella fiabellata (after De Quatrefages) : a, cerebral ganglia ; b, oesophageal commissures ; c, longitudinal commissures of the ventral gaDglia. Those of opposite sides are united by long transverse commis- sures. the ordinary cephalic ej'es, a double series of additional visual organs, one pair being allotted to each somite. In JBran- chiomma, eyes are situated at the ends of the branchial plumes. Ehrenberg has described two caudal eyes in Amphi- cora, and De Quatrefages has shown that similarly placed eyes exist in three other species of Polychceta, two of which are closely allied to Amphicora, while the other is an errant form, related to Lumbrinereis. These curious worms are said to swim about with the caudal extremity forward. Auditory sacs, containing many otoliths, have been ob- served upon each side of the oesophageal ring in Arenieola, and similar organs have been noticed in other TuMcola ; but hitherto their existence has not been certainly determined in the Errantia. The genitalia of the polychaetous Annelida are excessively simple in their structure ; indeed, special reproductive organs can hardly be said to exist in most, the generative products THE DEVELOPMENT OF THE POLYCH.ETA. 213 being merely developed from some part of the walls of the perivisceral cavity, in which they eventually freely float, mak- ing their way out in a manner which is not quite understood at present ; probably, however, through temporary or perma- nent apertures at the bases of the parapodia. In many, the segmental organs appear to serve as excretory ducts. As a rule, the polychsetous Annelids are dioecious ; but some (e. g., Protula, Fig. 54) are hermaphrodite. The ova undergo their development within the body of the parent in some species of Eunice y in pouches attached to the body in JExogone ; in masses of gelatinous matter which adhere to the tubes of the vermidom in Protala ; beneath the elytra in Polynbe cir- rata ; in the cavity of the opercular tentacle in some Spir- orbes ; while, in other* cases, they appear to become, almost immediately, free ciliated embryos. The vitellus undergoes division, and is converted, as in the case of the Oligochceta and Hirudinea, into blastomeres of two kinds. This contrast between the two components of the embryo commences with the division of the vitellus into two, inasmuch as the first fissure is usually so directed as to divide the yelk into unequal portions. Both subdivide, but the smaller much faster than the larger ; so that the former becomes converted into very small blastomeres, which grad- ually envelop the larger blastomeres resulting from the sub- division of the latter. The larger included blastomeres are destined to form the alimentary tract ; the smaller peripheral ones, on the other hand, give rise to the ectoderm, and to the nervous ganglia,1 As in the Oligochceta and JPirudinea, again, the mesoblast forms a thick band on each side of the median ventral line, and its transverse division originates the segmentation of the body. But, generally, the development of the protosomites, as these segments might be called, does not occur until some time after the embryo has been hatched. The somites increase in number by the addition of new ones between the last and the penultimate somite. The embryos of the Polychceta differ from those of the Oli- gochceta and Hiradinea in being ciliated. In some cases, the cilia form a broad zone which encircles the body, leaving at each end an area, which is either devoid of cilia, or, as is fre- quently the case, has a tuft of long cilia at the cephalic end. Such larvas are termed Atrocha. In other embryos the cilia are arranged in one or more 1 Claparede and metscknikoflf. " Beitrage zur Kenntniss der Entwickelungs- gesckickte der Chaetopoden," 1863. 214 THE ANATOMY OF IXYERTEBRATED AXIMALS. narrow bands, which surround the body. A very common arrangement is one in which a band of cilia encircles the body immediately in front of the mouth, the region in front of the band bearing eyes, and becoming the praestomium of the adult (e. g., Polynoe). In such embryos, there is very commonly a second band of cilia around the anal end of the embryo, and a tuft of cilia is attached to the centre of the praestomium. These larvae are called Telotrocha. In other cases, one or many bands of cilia surround the middle of the body, between the mouth and the hinder extremity. These are Mesotrocha. In the telotrochous larva of Phyllodoce, a shield-shaped, mantle-like elevation of the integument covers the dorsal region of the body behind the prae-oral ciliated ring-. In the larvae of the Serpulidce a process of 'the integument grows out behind the mouth, and surrounds the anterior part of the body of the larva like a turned-back collar. It persists, as a kind of hood, in the adult. Some larvae are provided with setae of a different charac- ter from those which are possessed by the adult, and which are cast off as development advances. Many Polychceta multiply by a process of zooid develop- ment, which, in some cases, appears to be a combination of fission with gemmation ; in others, to approach very nearly to pure fission or pure gemmation. The result is, not infre- quently, the formation of long chains of connected zooids. The method of multiplication which De Quatrefages ob- served in Syllis prolifera, is nearly simple fission, the animal dividing near its middle, and the posterior division acquiring a new head. In Myrianida, Milne-Edwards has described the occur- rence of a sort of continuous budding between the ultimate and penultimate segments, in which region new segments are formed until the zooid has attained its full length. Frey and Leuckart and Krohn have shown that Autolytus prolifer multiplies in a somewhat similar manner ; but, in- stead of each new zooid being formed at the expense of an entire somite, it is developed from only a portion of one. Finally, I found in Protula Dysteri that, "when the Protula had attained a certain length, all the somites behind the six- teenth became eventually separated as a new zooid ; but the development of the latter is not mere fission, inasmuch as one of the earliest steps in the process is the enlargement of the seventeenth somite, and its conversion into the head and AGAMOGEXESIS AMONG POLYC1LETA. 215 thorax of the bud (Fig. 54, B). Sars has described a similar mode of multiplication in his Filograna implexa, a very close- ly allied form. In Syllls and in Protula, the producing and the produced zooids alike develop generative products, but, in Autolytus, Krohn has shown that the primary producing zcoid remains sexless, the secondary produced zouids having a somewhat different form, and alone giving rise to ova and spermatozoa. In some species of the genus Nereis, the worm, after the development of its genital organs has taken place, takes on the characters of what was formerly considered a distinct genus, Heteronereis ; and the males and the females of the same species of Nereis have even been regarded as different species of Heteronereis.1 The series of forms represented by the Turbellaria, the Hirudinea, the Oligoehceta, and the Polychceta, illustrates the manner in which a type of organization, which, in its simplest condition, exhibits but little advance upon a mere Gastrula, passes into one in which the body is divided into many segments, each provided with a pair of appendages or rudimentary limbs. . The segmentation, or serial repetition of homologous somites, extends to the nervous system, and, more or less, to the vascular and reproductive organs, in the higher forms of these " Annulose ' animals ; from which a further extension of the same process of segmentation, with a fuller develop- ment of the appendages and a more complete appropriation of some of them to manducatory purposes, leads us to the Arthropoda. The Gephyrea. — These are marine vermiform animals without distinct external segmentation or parapodial append- ages. The ectoderm has a chitinous cuticle, and is often provided with tubercles, hooks, or seta?, of chitin (Fchiurus, Sternaspis). No calcareous skeleton is found in any of the Gephyrea. The integument frequently contains numerous simple glands, the apertures of which perforate the cuticle. In one genus {Sternaspis), twTo shield-shaped plates, fringed with seta?, are developed upon the hinder part of the ventral surface of the hpdv. There are external circular and internal longitudinal muscular fibres beneath the ectoderm. An inner i Ehlers, " Die Gattung Heteronereis. ," (" Gottingen Nachrichten," 1867.) 10 216 THE ANATOMY OF IXVERTEBRATED ANIMALS. layer of circularly disposed muscular fibres may be added. The oral end of the body may have the form of a retractile proboscis (Priapulus), or be provided with tentacular append- ages. These may be arranged in a circle round the mouth, and short (Sipunculus, Fig. 56, I., T), or long (Phoronis), or there may be a single long, sometimes bifurcated and ciliated, tentacular appendage (Ponellia). Filamentous appendages, which are probably branchiae, are given otf at the hinder end of the body in Stemaspis and Priapulus. The endoderm is usually ciliated throughout. The intestine is straight in most genera, but is coiled and bent upon itself, so as to terminate in the middle of the body, in Sipunculus (Fig. 56, I.). In Phoronis the anus is close to the mouth. The anal aperture is always situated upon the dorsal aspect of the body. There is a spacious perivisceral cavity, undivided by mesenteries, which in some cases {Priapulus, Sipuncuhts) opens externally by a terminal pore. In Echiurus, Bonellia, Thalassema, a pair of tubular, sometimes branched organs, which are ciliated internally, and communicate by ciliated apertures with the perivisceral cavity, open into the rectum. These appear to represent the water-vessels of the Potifera and the respira- tor v tubes of the Holothurice. A pseud-haemal system exists in most (/Sipunculus, Stemas- pis, Bonellia, Ejhiurus, and Phoronis), and, when fully devel- oped, consists of two longitudinal trunks — one dorsal, or su- pra-intestinal, the other ventral, with their terminal and lateral communications. The pseud-haemal fluid is colorless, or may have a pale reddish tinge, in most. In Phoronis it is said to contain red corpuscles. In Sipunculus, the cavities of the tentacles communicate with a circular vessel provided with caecal appendages ; and this circular vessel is said to open into the pseud-haemal vessels. The nervous system presents a collar, which surrounds the oesophagus, and from which a simple or ganglionated cord proceeds backward in the ventral median line, giving off lat- eral branches. The ventral cord contains a central canal, and the collar usually presents a cerebral ganglionic enlargement. Rudimentary eyes are sometimes connected with the cerebral ganglion. The sexes are distinct, and the reproductive elements are developed either from the parietes of the perivisceral cavity or in simple caecal glands. In Sipunculus, tne ova and sper- matozoa float freely in the perivisceral cavity. The actively locomotive embryo of Sipunculus (Fig. 56, II.) THE GEPHYREA. 217 is surrounded by a circular band of cilia placed immediately behind the mouth ( TV, IV), and resembles a Rotifer or a meso- trochal Annelidan larva. As development advances it loses Fig. 56.—Sipvncuhifi nudus (after Keferstein and Elders').1 I. The animal laid open longitudinally — |n.s. T, tentacles: r, the four retractor muscles of the proboscis; r, the points at which they were attached to the walls of the body ; ee, oesophagus ; *. intestine; a, anus ; J, J\ loops of the intestine ; », y, appendages of the rectum ; s, fusiform muscle ; w, ciliated groove on the inner side of the intestine ; g, anal muscles ; s, caecal glands ; t. caeca which open on each side of the nervous cord, and are generally considered to be testes ; p, pore at the hinder end of the body; w, nervous cord, which ends in a lobed gan- glionic mass, close to the mouth, and presents an enlargement, g\ at its poste- rior end ; m, m\ m", muscles associated with the nervous cords. II. A larval Sipunculus about T\ of an inch long: o, mouth; «, srullet ; *, csecal eland ; i, intestine with masses of fatty cells ; a, anus ; w. ciliated groove of the intestine ; g, brain with two pairs of red eye-spots ; n, nervous cord, p, pore; t, ?, so-called testes ; W, W, circlet of cilia. this apparatus, and passes gradually into the adult form. In Phoronis, the embryo is also mesotrochal, but it has two ciliated bands, one circular, round the anus, and the other im- mediately behind the mouth. The post-oral band of cilia is produced into numerous tentaculiform lobes, and fringes the free edge of a broad concave lobe of the dorsal side of the body, which arches over the mouth. In this state the embryo 1 " Zoologische Beitrage," 1861. 218 THE ANATOMY OF INVERTEBRATED ANIMALS. is the so-called Actinotrocha? An invagination of the ven- tral integument of the larva connects itself with the middle of the intestine, and then, becoming evaginated, pulls the in- testine, in the form of a loop, into the ventral process thus formed, which gives rise to the body of the Phoronis, while the tentacles of the larva grow into those of the adult. Schneider has suggested that the bell-shaped larva, with long seta?, termed Mitraria by Mailer, is the embryo of /SternasjriSo The affinities of the Gephyrea with the Turbellaria, with the Annelida, and with the Potifera, are unmistakable. In fact, it may be doubted whether Sternaspis should not be associated with the Polycficeta, and Ponellia is in man}7 re- spects comparable to a colossal Rotifer. Their usually as- sumed connection with the Echinodermata is more question- able. The circular canal which communicates with the cavi- ties of the tentacles in Sipunculus has been compared to the ambulacral system of the Echinoderms, but the manner of its development is not yet sufficiently understood to justify the expression of an opinion on this subject. Krohn has de- scribed a bilobed organ on the ventral face of the gullet of the larva of Sipunculus, which opens externally in front of the ciliated band by a narrow ciliated duct 2 (Fig. 56, II., S). It has a striking similarity to the " water-vessel " of the larva of Palanoglossus, which, however, lies on the opposite side of the body. 1 " Schneider, "Ueber die Metamorphose der Actinotrocha IrancMata." (" Archiv fur Anatomie," 1862.) 2 " Ueber die Larve des Sipunculus nudits." (" Archiv fur Anatomie," 1851.) CHAPTER VI. THE ARTHROPOD A. The segmentation of the body, that is, its division into a series of somites, each provided with a pair of lateral ap- pendages, which is so characteristic a feature of the higher Annelids, is exhibited in a still more marked degree by the Arthropoda. In these animals, moreover, the appendages, themselves are usually divided into segments, while one or more pairs of the appendages in the neighborhood of the mouth are modified in form and position to subserve man- ducation. Segmental organs, at least in their Annelidan form, are wanting in the Arthropoda, and neither in the em- bryonic nor the adult condition do they ever possess cilia. The process of yelk-division may be complete or incom- plete, but no known Arthropod ovum gives rise to a vesicular morula, nor is the alimentary cavity ordinarily formed by in- vagination.1 The precise mode of origin of the mesoblast has yet to be worked out, but the perivisceral cavity appears always to be developed by its splitting. In other words, it is a schizocoele. As with Annelids, the segmentation of the body results from the subdivision of the mesoblast by transverse constric- tions into protosomites ; and there is every reason to believe that the ganglionated nervous chain arises from an involution of the epiblast. The neural face of the embryo is fashioned first, and its anterior end terminates in two rounded expansions — the pro- cephalic lobes — which are converted into the sides and front of the head. The appendages are developed as paired out- 1 The recent observations of Bobretzky on the development of Oniscus and Astacus (Hofmann and Schwalbe, u Jahresberichte," Bd. ii., 1875), however, tend to show that the hypoblast arises by a sort of modified invagination of the primitive blastoderm. And in other Arthropoda there are indications of a similar process. 220 THE ANATOMY OF INVERTEBRATED ANIMALS. growths from the neural aspect of each somite, and, whatever their ultimate form, they are, at first, simple bud-like pro- cesses. Very generally, a broad median prolongation of the sternum of the somite which lies in front of the mouth gives rise to a labrion y while a corresponding, but often bifid me- dian elevation, behind the mouth, becomes a metastoma. In many Arthropods, the haemal or tergal face of the body grows out into lateral processes, w7hich may either be fixed, or more or less movable. The lateral prolongations of the carapace in the Crustacea and the wings of Insecta are structures of this order. In a number of Insects belonging to different orders of the class, an amnionic investment is developed from the extra-neural part of the blastoderm by a method similar to that which gives rise to the amnion in the higher Vertebrata. In all the higher Arthropods, a certain number of the somites which constitute the anterior end of the body coa- lesce and form a head, distinct from the rest of the body ; and the appendages belonging to these confluent somites un- dergo remarkable modifications, whereby they are converted into organs of the higher senses and into jaws. In many cases, the somites of the middle and posterior parts of the body become similarly differentiated into groups of poly- somitic segments, which then receive the name of thorax and abdomen. The somites entering into each of these groups may remain distinct or may coalesce. The tergal expansions of the somites of the head, or of both head and thorax, may take the shape of a broad shield, or carapace. This may con- stitute a continuous whole (e. g., Apus, Astacus) ; or its two halves may be movably connected by a median hinge, like a bivalve shell ( Cypris, Limnadia) ; or, finally, the tergal pro- cesses of each side may remain distinct from one another and freely movable on their respective somites (wings of In- sects). Limbs, or appendages capable of effecting locomotion, are always attached either to the head or to the thorax,1 or to both. They may be present or absent in the abdominal re- gion. In adult Arachnida and Insecta, there are no abdomi- nal limbs, unless the accessory organs of generation, the stings of some insects, and the peculiar appendages of the abdomen in the Thysanura and Collembola, be such. The alimentary apparatus presents very wide diversities 1 The extinct Trilobites possibly form an exception to this rule- THE ARTHROPODA. 221 in form and structure, and in the number and nature of its glands. The anus, which is very rarely absent, is situated in the hindermost somite. In like manner, the blood-vascular system varies from a mere perivisceral cavity without any heart ( Ostracoda, Cirri- pedia) up to a complete, usually many-chambered heart wTith well-developed arterial vessels. The venous channels, how- ever, always have the nature of more or less definite lacunae. The blood-corpuscles are colorless, nucleated cells. Special respiratory organs may be absent, or they may take one of the following* forms : 1. Branchiae. Externally projecting processes of the body or limbs, supplied with venous blood, which is thus brought into contact with the air dissolved in water. 2. Trachea?. Tubes which traverse the body and gen- erally open upon its exterior by apertures termed stigmata, and thus bring air into contact with the blood and the tissues generally. Saccular reservoirs of air are often formed by dilatations of these tubes. The so-called Tracheo-branchiae of some aquatic Insect larvae are usually laterally projecting processes of more or fewer of the thoracic or abdominal somites, containing abun- dant tracheae, wThich communicate with those which traverse the body {Ephemeridae, Perlaridae). They are in no sense branchiae, but simply take the place cf stigmata. The ex- change of constituents between the air contained in the tracheae of these animals and that of the surrounding* medium is effected indirectly, by diffusion through the walls of the tracheo-branchiae, instead of directly, through the stigmata, as in other cases. In the aquatic larvae of many Dragon-flies (Libelhdida), the function of the tracheo-branchiae is performed by folds of the lining* membrane of the rectum, which contain abundant tracheae. Water is drawn into, and expelled from, the cavity of the rectum by rhythmical contractions of its walls, so as to secure the exchange of gaseous constituents between the air which it contains and that which fills the tracheae. 3. Pulmonary sacs. These are met with only in some Arachnida. They are involutions of the integument, the walls of wThich are folded in such a manner as to expose a large surface to the air, which is alternately taken into, and expelled from, their apertures. The blood is brought to these sacs by venous channels. The exact mode by which the separation of the nitro- 222 THE ANATOMY OF INVERTEBRATED ANIMALS. genous products of the waste of the tissues from the blood is effected in Arthropods requires further elucidation. In many, however, such products, notably uric acid, have been found to abound in the corpus adiposum — a cellular mass which lies in the walls of, and more or less fills, the peri- visceral cavity — and in the Malpighian glands. In the latter case, they are conveyed out of the body by the intestine. The nervous system consists primitively of a pair of gan- glia for each somite, but the number of ganglia discoverable in the adult depends on the extent to which these primitive ganglia coalesce. There is usually, if not always, a well- developed system of ganglionated visceral nerves, connected with the cerebral ganglia and distributed to the gullet and stomach. Eyes are usually present ; and, when they exist, they are almost always situated in the head and are connected with the cerebral ganglia. Among the Crustacea, however, tti- phausia has eyes in some of the thoracic limbs, and in some abdominal somites. The eyes may be simple or compound. In the latter case there are, in correspondence with the num- ber of parts into which the transparent corneal continuation of the chitinous cuticula over the eye is divided, a number of elongated bodies which lie between the outer surface of the ganglionic expansion of the optic nerve and the inner face of the cornea. These bodies consist of two parts: an external transparent crystalline cone and an internal pris- matic rod. The broad end of the cone is external, and is ap- plied to the inner surface of the corneal facet; its narrow end is continuous with the outer extremity of the prismatic rod, which, by its inner end, is connected with the ultimate ramifications of the optic nerve. Each of these crystalline cones and prismatic rods is separated from the rest by a pig- mented sheath.1 Distinct auditory organs have been observed in Crus- taceans and Insects. They are not exclusively confined to the head. In the opossum shrimp (My sis), for example, they are placed in the appendages of the last somite of the ab- domen. And, in Insects, the only organs to which the audi- tory function can be certainly assigned are situated in the thorax or in the legs. 1 Leydig, " Das Auge der Gliederthiere,'1 1864. Schulze, " Untersuch- ungen," 1868. Mr. E. T. Newton has given a very good account of the struct- ure of the eye of the lobster, accompanied by full references to the literature of the subject, in the Quarterly Journal of Microscopical Seience for 1875. THE ARTHROPODA. 223 There is some reason to think that the antennas of Insects are the seat of the olfactory function, but no certain infor- mation of this head has been obtained. The very fine setae to the bases of which nerves can be traced, which abound on the antennary organs of Insecta and Crustacea, but are found in other regions of the body, are probably partly tactile and partly auditory organs. As a general rule, all the muscles of the Arthropoda, even those of the alimentary canal, are striated. Those of the body and limbs are often attached by chitinized tendons to the parts which they have to move. As the hard skeleton is hollow and the muscles are inside it, it follows that the bod37, or a limb, is bent toward that side of its axis which is oppo- site to that on which a contracting muscle is situated. Sounds are produced by many Insects ; but in most cases they cannot be properly referred to a voice, in the sense in which that term is applied to the sounds produced in the higher animals, by the vibrations of the atmosphere arising from the impact of a current of air upon the free edges of membranes bounding the aperture of exit of the current. The chirping and humming of Insects often arise from the friction of their hard parts against one another, or from the rapid vibration of their wings : in some instances, however, recent investigations render it probable that they are pro- duced by the action of expiratory currents on tense mem- branes which bound the stigmata. Agamogenesis is very common among some groups of the Arthropoda, such as the Crustacea and the Insecta, but has not yet been observed in the Myriapoda or the Arachnida. It may be effected in one of two ways : 1. Either individuals which are, by their structure, inca- pable of being impregnated and are therefore physiologically sexless, though it may happen that they more or less approxi- mate females morphologically, give rise to offspring ( Cecido- myia larva?, Aphis) ; 2. Or individuals which are capable of being impregnated, and are thus both morphologically and physiologically true females, give rise to eggs which develop without impreg- nation. (The queen-bee, so far as the production of drones is concerned ; many Lepidopterct). The cases of Apus, Daphnia, and Cypris, would belong to the latter category, if it were certain tliat the very same females which, for a certain period, produce young agamo 15 224 THE ANATOMY OF INVERTEBRATED ANIMALS. genetically, at another time undergo fecundation. Multipli- cation by fission or external gemmation is not known to take place in any Arthropod. Hermaphrodism occurs as a rule in some few Arthropods (e. g., the Cirripedia and Tardigrada), and as an abnormal "sport" in sundry Crustacea and in manv Insecta. In absolute number of species, the Arthropoda far ex- ceed all the rest of the animal kingdom put together. Thus Gerstaecker,1 while allowing 50,000 species for the latter, estimates the number of species of Arthropoda as rather above than below 200,000 ; by far the larger proportion of these, probably more than 150,000, being Insects. The Arthropoda are commonly divided into the Crustacea, the Arachnida, the Myriapoda, and the Insecta ; and though it is impracticable to give a definition which shall absolutely separate the first two groups, it is perhaps not worth while to disturb an arrangement which has much practical con- venience. But, for purely morphological purposes, it may be instructive to regard them from another point of view. The Arthropoda may, in fact, be divided into two series. One of these consists almost wholly of air-breathing forms, which, if they possess special respiratory organs, have either pulmonary sacs or tracheae, or both combined ; while the other includes a corresponding predominance of water-breath- ing animals, w^hich, if they possess respiratory organs, have branchiae. The latter series contains the Crustacea ; the former comprises the Arachnida, Myriapoda, and Insecta. In the course of the development of the higher Arthro- poda, there is a stage in which the body begins to be seg- mented, but the appendages are not developed. This is followed by a stage in which appendages make their appear- ance, but the antennary and manducatory appendages (gna- thites) are like the other limbs : and, finally, there is a stage in which the gnathites are completely converted into jaws. Now, among the water-breathing Arthropoda, no trace of limbs has yet been certainly discovered among the Trilo- bita ; in the Merostomata {Eurypterida and Xiphosura) the gnathites are completely pediform ; while, in the Ento- mostraca and Malacostraca, more or fewer of the gnathites are so modified as to subserve manducation and no other function. 1 Bronn's "Kiassen und Ordnungen des Thierreiclis," vol. v., p. 273. 1868. THE GROUPS OF THE ARTHROPODA. 225 In the air-breathing series no completely apodal forms are known. The Tardigrada and the Pentastornida appear to have no jaws ; but the presence of oral stilets in the former, and the position of the hooks which represent the limbs in the latter, throw some doubt upon this point. In the Arachnida and the Peripatidea the gnathites are completely pediform. But in the Myriapoda, and still more in the Insecta, the gnathites lose the character of legs, and are completely converted into manducatory organs. Thus we arrive at the following arrangement of the Arthropoda : Abtheopoda. I. Without Gnathites. Teilobita. Taedigeada (?) Pextastomida (?) 77. With Pediform Gnathites. Meeosto'viata. Aeachnida. Peeipatidea. III. With Maxilliform Gnathites. ExTOMOSTEACA. MtEIAPODA. Malacosteaca. Insecta. "Water-breathers. Air-breathers. For the most part. Of the four great groups, the Crustacea are those which present the greatest and the most instructive variations upon the fundamental type of structure ; while the modifications of the Insecta, Arachnida, and Myriapoda, are less exten- sive, and may be regarded as of secondary morphological im- portance. The Crustacea will, therefore, be treated of at some length, while the other groups will be passed over more lightly. THE CRUSTACEA. The Trilobita. — These ancient Arthropods, which have been extinct since the latter part of the Palaeozoic epoch, oc- cur in the fossil state in great numbers, and in conditions very favorable for their preservation ; but, up to this time, no certain indications of the existence of appendages, nor even of any hard, sternal body-wall, have been discovered, though 226 THE ANATOMY OF INVERTEBRATED ANIMALS. a shield-shaped labrum, which lies in front of the mouth, has been preserved in some specimens. The body consists of a cephalic shield (Fig. 57, A) ; of a variable number of mov- ably-articulated thoracic somites (Fig. 57, B) ; and of a, py- gidium, composed of a variable number of the somites which succeed the thorax, united together (Fig. 57, C). Each thoracic somite presents a median portion, convex from side to side, termed the axis or tergum, and two flat- tened lateral portions, the pleura. The former overlap one another largely when the body is extended, the latter when it is flexed, and the freedom of motion permitted by this ar- rangement is so great that many Trilobites were able to roll themselves up like wood-lice, and are found fossilized in that condition. At the lateral edge of each pleuron, the cuticular substance of which it is composed folds inward, and can be traced on the ventral or sternal side for some distance. But in the middle of the ventral region no indication of a sternum is discoverable. It may, therefore, be concluded that the sternal region of the somite was of a soft and perishable na- ture ; and that the thoracic somite of a Trilobite resembled one of the abdominal somites of a crab in this and in some other respects. The glabellum (Fig. 57, 4), or central raised ridge of the cephalic shield, is a continuation of the thoracic axis, the lo- cation of its sides perhaps referring to the number of primi- tive somites it represents. The limb, or lateral area on either side, answers to a thoracic pleuron / its thickened margin (Fig. 57, 1) is produced into two longer or shorter posterior angles (g) ; interiorly, the marginal band is reflected inward for a short distance, as the subfrontal fold, the remaining sternal area being incomplete. A median movable plate answers to the labrum of Apus and Limulus. On the occip- ital or lateral margin of the limb a suture (Fig. 57, 5) com- mences, and, passing between the eye and the glabellum, meets that of the opposite side either in front of the latter, or on the margin of the limb, or on the subfrontal fold, and is connected with the labral suture by one or two sutures. The limb is thus divided into two parts — one fixed (the fixed gena, Fig. 57, a), attached to the glabellum ; the other sep- arable (the movable gena, Fig. 57, b), on which the eye is placed. The eyes are absent in some genera. In others they occur as isolated ocelli ; or in groups, their interspaces being occupied by the common integument; or they may resemble the compound eyes of other Arthropods. THE TRILOBITA. 227 M. Barrande ! has succeeded in tracing out the develop- ment of some species of Trilobites. He finds that the small- Fig. 57.— Diagram of DaJmanites (alter Pictet). — A, head ; 1, marginal band ; 2, mar- ginal groove, internal to the band ; 3, occipital segment ; 4, glabellum ; 5, great suture ; 6. eyes ; a, fixed gena ; b, separable gena ; g, genal angle ; 2?, thorax ; 7, axis ortergum ; 8, pleuron ; C, pygidium ; 9, tergal ; 10, pleural portions of the pygidium. est, and therefore the youngest, forms are discoidal bodies, without any clear evidence of segmentation. The division into somites takes place by degrees, the number increasing up to the adult condition. It is possible that still younger conditions may have escaped fossilization, but the analogy of Limuliis suggests that these small discoidal forms really represent the condition in which the Trilobite left the egg. The aIerostomata.2 — The only existing representative of this division of the Crustacea is the genus Limulus (the King Crabs or Horseshoe Crabs), the various species of which are 1 " Systerne Silurien du centre de Boheme." tome i. Trilobites. 1852. 3 H. Woodward, " A Monograph of the British Fossil Crustacea belonging to the Order Merostomata," 1866. 228 THE ANATOMY OF INVERTEBRATED ANIMALS. found in America and in the Moluccas. They are usually classed as a distinct order of the Crustacea, termed Xipho- sura or Pcecilopoda. The body of Limulus (Fig. 58) is naturally divided into three parts, which are movably articulated together. The most anterior is a shield-shaped portion, curiously similar in form to the head of a Trilobite. Its convex dorsal surface is similarly divided into a median and two lateral regions ; its edges are thickened, and its posterior and external angles are produced backward. At the anterior end of the median re- gion two simple eyes are situated, and at its sides are two large compound eyes. The sternal surface presents, ante- riorly, a flattened subfrontal area, behind which it is deeply excavated, so that the labrum and the appendages are hidden in a deep cavity formed by its shelving walls. The middle division of the body of Limulus exhibits markings which in- dicate that it is composed of, at fewest, six coalesced somites; its margins are spinose, and its excavated sternal face lodges the appendages of this region. Fig. 58.— A, Limulus moluccanus (dorsal view). B, L. rotundicauda (ventral view) (after Milne-Edwards): a. anterior; 6, middle division of the body ; c, telson , d; subfrontal area; e, antennules ; f, antennae ; g, operculum ; h, branch iferous ap- pendages. The terminal division is a long, pointed, and laterally ser- rated spine, which is termed the telson. THE MEROSTOMATA. 229 The mouth is placed in the centre of the sternal surface of the anterior division ; the anus opens on the same surface, at the junction between the middle division and the telson. A movable, escutcheon-shaped labrum projects backward in the middle line, immediately behind the subfrontal area (d) ; and on each side of it is a three-jointed appendage, the second joint of which is prolonged in such a manner as to form with the third a pincer or chela. The attachment of this appendage is completely in front of the labrum, which separates it from the mouth. In each of the next live pairs of appendages, the basal joint is enlarged ; and, in the anterior four, its inner edge is beset with numerous movable spines. The attachment of the basal joint of the foremost of these appendages (the second of the whole series) is in front of the mouth ; but its pro- longed, spinose, posterior and internal angle may be made to project a little into the oral cavity. The basal joints of the following three appendages are articulated at the sides of the mouth, and the inner angle of each is provided with a spinose process which projects into the oral cavity. The second, third, fourth, and fifth appendages in the females are chelate ; in the males of most species the second, and sometimes the third, are not chelate. The large basal joint of the sixth ap- pendage is almost devoid of spines, and bears a curved, spat- ulate process, which is directed backward between the ante- rior and middle divisions of the body. The fifth joint of this limb carries four oval lamellae. The appendages which form the seventh pair, very unlike the rest, are short, stout, and single-jointed. The eighth pair of appendages, again, are of a totally dif- ferent character from those which precede them. They are united in the middle line into a single broad plate, which forms a sort of cover, or operculum, over the succeeding ap- pendages, when the animal is viewed from the sternal side. On the dorsal face of this plate are seated the two apertures of the reproductive organs. From the inner face of the anterior, or sternal, wall of each half of the operculum a strong process arises, and passes upward to be attached to a corresponding process of the ter- gal wall of the anterior division of the body. By far the greater part of the large levator muscle of the appendage arises from the tergal wall of the anterior division cf the body, and the nerve which supplies the limb is derived direct- ly from the posterior part of the multiganglionate cord which 230 THE ANATOMY OF LNYERTEBRATED ANIMALS. surrounds the gullet and supplies the appendages which lie in front of the operculum. The five pairs of appendages which remain resemble the operculum in their general form, and have ascending process- es, which are connected with inward prolongations of the ter- gal wall of the middle division of the body. Their nerves are derived from the ganglia which lie in this region of the body. Thus there are altogether thirteen pairs of appendages, eight of which are connected with the anterior, and five with the middle division of the body ; and the appendages in the region of the mouth are essentially ordinary limbs, the basal joints of some of which are so modified as to subserve man- ducation. The determination of the homologies of the parts hither- to spoken of as the anterior and middle divisions of the body, and of their appendages, is a matter of some difficulty ; but, on comparing the disposition of the limbs and their nervous supply with what obtains in the higher Crustacea, it seems hardly doubtful that the first pair of appendages answer, to the antennules ; the second, to the antennae ; the third, to the mandibles ; the fourth and fifth, to the maxilla? ; and the sixth, seventh, and eighth, to the maxillipedes of Astacus or JTomarus ; and, in this case, the anterior division is a ceph- alo-thorax. If the position of the genital openings marks the hinder boundary of the thorax, the middle division of the body represents an abdomen, composed cf five somites. But, on the other hand, it may be that the genital organs open in front of the hinder extremity of the thorax, as in female Pod ophthalmia, and that the five somites which form the middle division correspond with the remaining five somites of the thorax of a Podophtbalmian. In this case, the region which corresponds with the abdomen in the higher crusta- ceans is undeveloped. The alimentary canal of Limidus is very peculiarly ar- ranged. The gullet passes directly forward and upward, and gradually widens into the stomach, the walls of which are provided with many longitudinal folds. The pylorus is prolonged into a narrow tube which projects into the intes- tine. The two biliary ducts on each side are far apart, and branch out into minute tubules, which form a mass occupying the greater part of the cavity of the body. The rectum, a slender canal with plaited walls, and very short, opens into a sort of cloaca situated between the telson and the sternal wall of the abdomen. THE MEROSTOMATA. 231 The heart, in Limulus polyphemus, is an elongated mus- cular tube, divided into eight chambers, and having as many pairs of lateral valvular apertures. It lies in a large peri- cardial sinus, which, in its abdominal portion, presents on each side five apertures, the terminations of the branchial veins. The branchiae consist of numerous delicate semicir- cular lamella?, attached transversely to the posterior faces of the five post-opercular appendages, and superimposed upon one another like the leaves of a book. The nervous system appears, at first sight, to be very con- centrated, its principal substance being disposed in a ring, embracing the oesophagus ; but, on closer inspection, it is found to consist of an anterior mass, representing the prin- cipal part of the cerebral ganglia in most other Crustacea, and of two ganglionic cords which proceed from the outer and posterior angles of that mass, and extend as far as the interval between the last and penultimate pairs of append- ages. These cords are thick, and lie on each side of the oesophagus, around which they converge, so as to come into close union and almost confluence, immediately behind it. In front of this point, howTever, they are connected by three or four transverse commissures, which curve round the poste- rior wall of the oesophagus, and become gradually shorter from before backward. The first of these commissures unites the two cords oppo- site the origin of the nerves to the third pair of appendages, which I regard as the homologues of the mandibles. In front of this point, the cerebral ganglia give off, from their ante- rior edges, the nerves to the ocelli, eyes, and frontal region ; and, from their posterior and under surfaces, those to the an- tennules. The nerves to the antenna? arise from the cord close to the outer and posterior angles of the cerebral gan- glia, and some distance in front of those to the mandibles. Close behind the latter arise the large nerves to the fifth and sixth cephalo-thoracic appendages. The nerves to the rudimentary seventh pair of append- ages are slender, and arise rather from the under part of the post-cesophageal ganglia ; those which supply the eighth pair of appendages, constituting the operculum, are also slender, and seem to come off from the two longitudinal com- missural cords, which connect the post-cesophageal ganglia with those which are situated in the second division of the body, though they are, in truth, only united in one sheath with them for a short distance, and can be readily traced to 232 THE ANATOMY OF INVERTEBRATED ANIMALS. the post-oesophageal ganglia, internal to the nerves of the seventh pair of appendages. The longitudinal commissures are very long, and are inclosed in a continuation of the same sheath ; they pass back into the second division of the body, and there present four ganglionic enlargements, whence the nerves of the post-opercular appendages proceed. The last of these ganglia is much larger than the others, and appears to consist of several confluent masses. The nerves diverge from it in such a manner as to resemble a cauda equina. The reproductive organs of both sexes consist of a mass of glandular caeca, which ramify through the body amid the hepatic tubules, and eventually open on papillae situated on the posterior face of the operculum. The males are much smaller than the females, and present, in many species, an external sexual distinction in the peculiarity of their second and third appendages already referred to. The young of Limulus acquires all its characteristic features while still within the egg. The interesting obser- vations of A. Dohrn 1 have shown that, in an early stage, the embryo is provided with the nine anterior pairs of append- ages, and is marked out into fourteen somites by transverse grooves upon its sternal face. The body has the form of a thick rounded disk, divided into an anterior shield composed of six somites, and a posterior, likewise shield-shaped region, formed by the union of eight somites. The telson has not made its appearance. In this condition, its resemblance, apart from the limbs, to such a Trilobite as Trmucleus is, as Dohrn points out, most remarkable. The JCiphosura were represented in the Carboniferous epoch (Bellinurus). The Eurypterida (Fig. 59) are extinct Crustacea of Pa- laeozoic (Silurian) age, which sometimes attain a very large size and in many respects resemble Limidus, while, in others, they present approximations to other Crustacea., especially the Copepoda. An anterior, eye-bearing, shield-shaped di- vision of the body is succeeded by a number (12 or more) of free somites, and the body is ended by a broad, or narrow and spine-like, telson. Five pairs, at most, of limbs, pro- 1 " Untersuchunffen fiber Bau und Entwickelung der Arthropoden." (Jena- isrhe Zeitsclirift, Bd. vi.) See also the observations of Loekwood and Packard, American NatwaWi. vol. iv., 1871, vol. vii., 1873, and " Memoirs of the Boston Society of Natural History," 1872 ; with the discussion of the systematic place of Limulus by E. Van Beneden, Journal de Zoologie, 1872. THE MERCSTOMATA. 233 vided with toothed basal joints, are attached to the sternal surface of the shield, and the mouth is covered, behind them by a large oval plate which appears to represent a meta- stoma (Fig. 59, B, g). Some of the anterior limbs are fre- quently chelate (Pterygotus) ; the terminal joints of the most Ct/L. Fig. 59.—Enriwterus remipes (after Nieszkowski).1— A. dorsal aspect. B, ventral aspect. Cth, the cephalo-thoracic shield bearing a, the eyes, and b, c, d, e,f, the locomotive limhs ; T7, telson ; g, the metastoma; h, the sternal plates of the an- terior free somites. posterior pair are generally expanded and paddle-like. The integument often presents a peculiar sculpture, simulating minute scales. The sternal surface of one or more of the anterior free somites is occupied by a broad plate, with a median lobe, and two laterally-expanded side-lobes (Fig. 59, 1 " Der Eurypterus remipes, aus den obersilurischen Schichten der Insel Oesel." 1859. 234 THE AXATOMY OF INVERTEBRATED ANIMALS. B, A), having a remote resemblance to the operculum of Limulus. The Extomostraca. — All the remaining Crustacea have completely specialized jaws ; and as many as six pairs of appendages may be converted into gnathites. In the Entomostraca, if the body possesses an abdomen (reckoning as such the somites which lie behind the genital aperture), its somites are devoid of appendages. Moreover, the somites, counting that which bears the eyes as the first, are more or fewer than twenty. There are never more than three pairs of gnathites. The embryo almost always leaves the egg in the condition of a Nauplius / that is, an oval body, provided with two or three pairs of appendages, which become converted into antennary organs and gnathites in the adult. The division of the Entomostraca comprises the Copepoda, the Epizoa, the Branchlopoda, the Ostracoda, and the Pectostraca. The Copepoda. — In these Entomostraca, which come nearest to the Eurypterida, the cephalic shield, which is dis- coidal and not folded longitudinally, is succeeded by a certain number of free thoracic and abdominal somites. The anten- nules and antennas are large, and, as in the Eurypterida, are organs of locomotion and sometimes of prehension. The an- terior thoracic members are converted into foot-jaws ; the posterior serve as paddles, the limbs of each pair being often united together in the middle line, as in Limulus. The em- bryo leaves the egg as a Nauplius. The various species of the genus Cyclops, which abound in fresh water, afford excellent illustrations of the structure of the Copepoda. The minute animal (Fig. 60) is shaped something like a split pear, the larger end corresponding with the head, and the convex side with the dorsal surface. The anterior third of the body is covered by a large carapace, which, at the sides, extends downward as a free fold over the bases of the ap- pendages, but is hardly at all free posteriorly. Anteriorly, in the middle line, it curves forward and downward, and is produced into a short rostrum, on each side of which a con- siderable excavation lodges the base of the long antennule, by the vigorous oar-like strokes of which the animal darts through the water. At the anterior boundary of the head, the double, black, median eye, which, unless very closely ex- THE COPEPODA. 235 amined, appears single, shines through the carapace, and at the sides of the latter, two coiled tubes with clear contents, the so-called shell-glands, are seen. Four distinct and movable somites succeed the carapace, and gradually diminish in diameter. The body then suddenly enlarges, and becomes divided, in the female, into four seg- ments, the last of which gives attachment to two long setose styles, which possibly represent another somite. There is a well-developed and prominent labrum (or conjoined epistoma and labrum) in front of the mouth, and behind it is a bilobed metastoma. The first pair of appendages are the. long and Fig. 60.— Cyclops.— Side-view of an adult female carrying a pair of ovisacs, and ven- tral view of the head, showing the labrum, metastoma and appendages of the left side. 1', eye ; II'. antennule ; III', antenna; IV, mandible; V, first maxilla: VI', second maxilla (erroneously marked YIP) ; a. outer; ft, inner division : 1, 2, 3, 4, 5, thoracic limbs ; R, rostrum ; lb, labrum ; mt, metastoma. many-jointed antennides, which are the chief organs of loco- motion. These are succeeded by the short and few-jointed antenna?. The third pair of appendages, or first pair of gna- thites, differ from the corresponding limb in Limxdus in the re- duction of the greater part of the appendage to a rudiment terminated by seta?, while the strong basal part is the princi- pal gnathite or mandible. The second pair of gnathites are strong and incurved ; following upon these is a third pair of 236 THE ANATOMY OF IXVERTEBRATED ANIMALS. appendages, each divided into two portions, an inner and an outer. The latter is by far the larger, and is so constructed that the three distal articulations can be bent back upon the proximal ones, and opposed to the internal division, consti- tuting a prehensile organ, the " hand " of Jurine.1 Thus the gnathites of Cyclops are a pair of mandibles followed by two pairs of maxillae. At some distance behind the third pair of gnathites the first pair of thoracic appendages is attached to the hinder part of the cephalo-thorax. Each consists of a two- jointed basal part (protojiodite), terminated by two three- jointed divisions {exopodite and endopodite). Three similar pairs are appended to the three anterior free somites, while a fifth rudimentary pair is connected with the next and small- est of these somites. The suddenly-enlarged following seg- ment of the body carries the apertures of the reproductive organs in the female, and supports the ovisacs. It is com- monly regarded as the first abdominal somite ; but, according to Claus, it is composed of two distinct somites, which be- come united only after the last moult. The alimentary canal is straight and simple, and without any distinct liver. There is no heart nor any special respira- tory organ. The single ovary, situated in the thorax, is provided with two oviducts, which open on the sides of the coalesced first and second abdominal somites. On the ventral face, between the apertures of the oviducts, is the median aperture of a colleterial gland which secretes the viscid matter which forms the coat of the ovisac. Short lateral ducts connect the gland with the extremities of the oviducts. The male is much smaller than the female, and the two enlarged somites of the abdomen remain distinct. There ic a single testis provided with two vasa deferentia. A special- ly glandular portion of the latter secretes the material of the spermatophores, or cases in which the spermatozoa are inclosed. The antennas are thickened, and provided with a peculiar hinge-joint, by means of which the male firmly seizes the fourth pair of swimming-legs of the female during copulation, and then, bending up his abdomen, deposits two of the spermatophores on the median opening of the colle- 1 That these are two divisions of the third gnathite, and not two separate appendages, has been demonstrated by tracing out their development. ( Claus, " Organization und Verwandtschaft der Copepoden," Wurzburger naturwiss. Zeitsckrift, 1862.) Under these circumstances I do not know why they should be termed " maxillipedes." THE EPIZOA. 237 terial gland, into which the spermatozoa pass on their way to the oviducts. The gland thus plays the part of a sperma- theca. The eggs are carried about in the ovisacs until they are hatched. The vitellus undergoes complete division, and a morula results, the blastomeres of which soon become differentiated into a superficial epiblast, surrounding a deeper-colored mass, which gives rise to the hypoblast and mesoblast. The whole embryo then becomes divided by two constrictions into three segments, and the hypoblast arises by delamination around a central cavity, which becomes the alimentary canal. There is a large labrum on the ventral side of the first segment in front of the mouth. The eye appears on the tergal aspect of the most anterior segment, as two pigment-spots which soon coalesce into one ; and a pair of jointed setose limbs grows out of each segment. In this uYduplius-stsite the young Cy- clops leaves the egg. The posterior part of the body elongates and becomes divided into the somites of the thorax and abdomen, from which their respective appendages bud out ; and these changes are accompanied by exuviation of the cuticle. The three pairs of appendages of the JVauplius are converted into the antennules, antennas, and mandibles of the adult. There are a few other fresh-water and many marine genera of Copepoda. Among the latter, the Pontellidce are remarkable for the separation of that part of the head which bears the antennules and the antennas, from the rest, a pecu- liarity to which a parallel can be found only among the Sto- matopoda. Corycceus has two large, more or less lateral eyes in addition to the median eye, subchelate antenna?, and a rudimentary abdomen. The beautifully iridescent Sapphi- rina has an extremely depressed body, short filiform an- tennas, two eyes, and rudimentary gnathites. A short tho- racic heart is present in some genera. The Epizoa. — Insensibly connected by such genera as Ergasilus and Caligus with the typical Copepods, are a great number of very singular Crustacea, which, from their habit of living parasitically upon aquatic animals (whence their vulgar name of "fish-lice"), have received the title of Epi- zoa. Chondr acanthus gibbosus, commonly found in great abundance on the walls of the branchial chamber of the Fishing-frog {Lophius piscatorius), may serve very well as 238 THE ANATOMY OF INVERTEBRATED ANIMALS. an illustration of the most remarkable peculiarities of this aberrant group. The female (Fig. 61) is not more than half an inch long, but, posteriorly, two long slender cylindrical filaments (like the rest of the animal, of a whitish or yellowish color) are attached to its body, which is broad and flattened, and as it were crimped at its edges, so as to present two principal transverse folds. The angles of the folds are elongated into lateral processes (A, i,f)> and similar processes (d, e) proceed from the middle line of the body, which by these outgrowths and foldings becomes singularly distorted ; and the grotesque- ness of the animal's appearance is not a little enhanced by the bowing motion, accompanied by a flapping backward and forward of its gouty limbs, which it executes when detached from the integument of the Lophhis. The head is expanded into a sort of hood, the convex anterior margin of which bears the antennules and antennae, the latter being metamorphosed into the strong curved hooks by which the Chondr acanthus is securely anchored to the infested animal. A subquadrate labrum overhangs the mouth, but does not inclose the mandibles and form a suctorial ap- paratus, as it does in some Epizoa. The mandibles and the two pairs of maxillae resemble curved hooks or claws. Two pairs of appendages (Fig. 61, b, c), composed each of a protopodite, terminated by an endo- podite and exopodite and exhibiting hardly any trace of articulation, are attached to the anterior part of the body behind the head. The body ends in a rounded segment, situated in the deep notch between the hindermost marginal processes, and bear- ing the two projecting vulvae. Above each of these is a small triangular papillose lobe (Fig. 62, w)< probably a modified ap- pendage, to which, as wTe shall see, the male attaches himself, while below them are two other rudimentary appendages (Fig. 62, y). The alimentary canal is a straight tube running from the mouth to the opposite extremity of the body. No heart is discoverable, and the nervous system and organs of sense (if any) are equally undistinguishable. The interspace between the alimentary canal and the walls of the body is almost wholly occupied by the ovarium, which consists of four tubes, situated on each side of the intestine, and giving oif ramified caeca, in which the ova are developed. Ante- riorly, each pair of tubes opens into the oviduct of its side, which passes down along the side of the body to terminate at CHONDRACANTHUS GIBBCSUS. 239 the vulva. The lower part of the oviduct contains a clear gelatinous substance, and is very similar in aspect to the ce- ment duct of a cirripede ; this substance is secreted by the Fig. §\.—Chondracanthus gibbosus.— Female: A, lateral view. JB, ventral view, en- larged : a. head ; b, c, appendages ; d, median dorsal process ; e. median ventral processes; /, i, h, lateral processes; k, terminal segment; I, male; g, ovisacs ; m, w, medio-dorsal ovarian tubes : p, lateral ovarian tubes ; o, oviduct. 2, 3, an- tennules ; 4, 5, 6, antennae, gnathites. walls of the oviduct, and forms the walls of the ovigerous sac. The latter, as has been stated, has the form of a long cylin- drical filament, the upper end of which is firmly held between the prominent lips of the vulva (Fig. 62, ac). The male Chondr acanthus does not attain to a twelfth the length of the female, and looks, at first, like a papilla upon 16 240 THE ANATOMY OF IXYERTEBRATED ANIMALS. her body near the vulva. On close examination, however, he is seen to be firmly fixed by his antennary hooks to one of the two triangular lobes described above. The hooks are doubt- less at first attached to the lobe by muscular contraction ; but the connection once effected seems indissoluble — at least ma- ceration in caustic soda does not cause the male to become detached. It does not appear that more than one male is attached to a female. The body of the male (Fig. 62) is pyriform, and exhibits indications of a division into six segments beside the head. Fig. 62.— C, Male Chondr acanthus, in situ, enlarged: x, vulvae of female; w, trian- gular papillose lobes ; q, anrennge of male ; r, eye-spot ; t, testis ; u, vas deferens ; v, genital aperture ; y, rudimentary appendage's of the female ; g, ovisacs. The anterior extremity presents a black eye-spot imbedded in its substance, and gives origin to a pair of rudimentary antennules, and to the strong, hooked, prehensile antenna?. Behind and below them is a large labrum and three pairs of hook-like gnathites. These are succeeded by two pairs, of subcylindrical appendages, which apparently represent ambu- latory limbs. The caudal extremity is terminated by two styles, and there are two prominent tubercles on the ventral surface of the penultimate somite, in which the genital apertures are seated. The alimentary canal is a delicate, irregular tube, having many brownish granules imbedded in its walls. A wide oesophagus is connected with its anterior extremity ; but the opposite end appears to be rounded, and to be united with the ventral surface of the integument onlv bv connec- tive tissue. A complex muscular system, composed of striped fibres, is visible through the integument, and the eye-spot CHOXDRACAXTHUS GIBBOSUS. 241 seems to be connected with a subjacent ganglionic mass. The body is sufficiently transparent to allow the pulsations of a heart to be seen, but none can be discovered. The testis is a large oval bilobed mass (£), lying like a saddle upon the anterior part of the intestine. From this body a thick vas deferens runs back upon each side of the intestine, and di- lates in the penultimate and antepenultimate somites into a thick walled pyriform sac — a sort of vesicula seminalis. The embryo leaves the egg as a Nimplius, like that of Cyclops. There are many genera of these parasites, some of which, such as the almost completely vermiform Jjerncece, deviate even more widely than C ho ndr acanthus from the ordinary form of Crustacea, while others, such as Ergasilus and JYoto- delphys, differ but little from the free Copepoda. In Caligus, the labium and metastoma are elongated and united into a tube in which the sharp styliform mandibles are inclosed ; and from the prevalence of this suctorian form of mouth in some of the best known species of parasitic Cope- poda, they are frequently termed "suctorial" crustaceans. Suctorial disks for attachment are developed from the coa- lesced posterior pair of thoracic members in Achtheres ; and, in this genus, the head, as a distinct part, becomes almost entirely obsolete. Argulus, the parasite so common on the Stickleback, is worthy of notice as one of the most curious modifications of the epizoic type.1 It is extremely flattened, and is composed of an anterior cephalo-thoracic disk, behind which lies a very short and broad, notched abdomen. A median styliform weapon lies in a sheath in front of the mouth, and the small mandibles and maxillae are inclosed in a short tube formed by the labrum and the metastoma. Six pairs of appendages lie behind the mouth, the anterior being metamorphosed into suckers, the next pair into strong limbs with a toothed sec- ond joint, and the four others constituting biramous swim- ming-feet. There are two pairs of antennary organs, and twro compound eyes. According to Leydig, the males are pro- vided with cups on their penultimate swimming-feet ; and, during copulation, these are filled with the seminal fluid, which is thus transferred to the vulva of the female, and thence to the spermatheca. The eggs are laid, and not car- ried about in ovisacs. The larva is provided with two pairs 1 Claus (" Ueber die Entwickelunar, Organization und systematische Stellung der Arsruliden," 1875) has proved the close affinity of Argulu* with the Cope- poda., but proposes to regard it as the type of a special group, the Branchiura. 242 THE ANATOMY OF INVERTEBRATED ANIMALS. of principal swimming appendages, the future antennse and the mandibular palps, the latter eventually entirely dis- appearing. There is a pair of small antennules, a pair of strong legs in the place of the suckers, and, behind them, the rudiments of the prehensile legs and the first pair of bira- mous appendages, the others being rudimentary. Notodelphys, which may be found very commonly in the branchial sac of Ascidians, closely resembles an ordinary Copepod, except that it becomes much distorted, and that it carries its ova in a chamber formed by the dorsum of the carapace. However strangely modified the adult form may be (and it must be remembered that it is always the female which un- dergoes the greatest amount of Change), the larvae of all these epizoic parasites resemble those of the ordinary free Copepoda in possessing only two (Achtheres, Tracheliastes) or three pairs of appendages (which appertain to the anterior region of the head) ; and they are endowed with considerable powers of locomotion. The Branchiopoda. — The genera JVeballa, Apus, Bran- chipuSy Limnetis, Daphnia, and their allies, are usually di- vided into two orders, the Bhyllopoda and the Cladocera / but these pass into one another so gradually, and have so many structural peculiarities in common, that the subdivision of the group of Branchiopoda appears to me to be a step of doubtful propriety. Closely resembling the lower Bodoph- thalmia, such as 3fysis, in some respects, these Crustaceans are invariably distinguished from them by the possession of a greater or less number of somites than twenty; Nebalia, which most nearly approximates the higher Crustacea, hav- ing twenty-two somites. Furthermore the thoracic and ab- dominal appendages of the Branchiopoda are, in the majority of cases, more or less foliaceous, resembling in many respects the anterior maxillipede of an Astaeus, and being constructed on essentially the same plan. Apus glacialis (Fig. 63) presents an elongated vermiform body, terminated by two long, multiarticulate, setcse styles, and covered anteriorly by a great shield-like carapace, deeply excavated behind. The posterior three-fifths of the carapace are free, and merely overlap the segments of the body ; the anterior portion, on the contrary, is united with and forms the tergal surface of the corresponding region of the head ; the free portion of the carapace shelves away laterally from a THE BRAXCHIOPODA. 243 median ridge, on each side of which a curious concentric mark- ing, indicating the position of the shell-gland (Fig. 63, J5, x), is visible. This gland is a coiled tube with clear contents, which, according to Claus, opens on the base of the first pair of thoracic appendages, immediately behind the second max- illae. Where the free joins the fixed portion of the carapace, the ridge is abruptly terminated by a transverse depression. A little distance in front of this is another deeper transverse groove, close to which, in the middle line, are the two reni- form compound eyes, converging toward one another ante- riorly (Fig. 63, B, i'). The ventral surface of the anterior division of the carapace (Fig. 63, G) presents a flattened, semilunar, subfrontal area, as in Limulus, behind which it slopes upward on all sides into the posterior division, thus forming a wide chamber, in which the anterior thoracico-abdominal segments are lodged. In the middle line, the subfrontal plate sends back a long and wide process, movably articulated with it, and rounded at its free end — the labrum ; above and behind which the mouth and gnathites are situated. Behind these follow twenty-six spinulose thoracico-abdominal segments ; the anterior twenty of which bear the swimming-feet, while the twenty-sixth, much larger than the others, is produced into an incurved point posteriorly, and carries the anus and the terminal setae. The compound eyes, as has been said, are seated upon the upper surface of the anterior division of the carapace. On the under surface, just above and behind the posterior boundary of the subfrontal area, and on each side of the labrum (Fig. 63, C, lb), is a delicate jointed filament — the antennule (Fig. 63, (7, n'). Behind this Zaddach found, in some specimens of Apus cancriformis, a second very small filament, the rudiment of the antenna, which in the larva is so large and important an organ ; but I have observed nothing of the kind in A. gla- cialls. On each side of the labrum is a large, convex, strong, toothed mandible, and the aperture of the mouth is bounded posteriorly by a profoundly divided plate, the metastoma. Succeeding this are two pairs of small maxillae, the second pair being foliaceous, and almost rudimentary. Behind these appendages, a cervical fold marks off the boundary between the head and the thorax, and at the same time corresponds with the commencement of the free portion of the carapace. Whether the carapace is also to a certain extent attached to the first thoracic somite, as Grube states,1 or whether it is en- 1 u Bemerkungen iiber die Phyllopoden," p. 81. 244 THE ANATOMY OF INVERTEBKATED ANIMALS. tirely cephalic, as Milne-Edwards considers, is a point upon which I have been able to come to no very clear determina- tion ; indeed, it is a question rather for the embryologist than the anatomist. Of the twenty pedigerous segments, the first eleven have each one pair of appendages ; but, behind the eleventh, each segment gives attachment to a gradually increasing number of limbs, so that the twentieth carries five or six pairs. Alto- gether twenty-eight pairs of appendages are attached to these nine posterior thoracic segments ; these, added to the eleven preceding, make thirty-nine appendages in all. While each of the anterior eleven segments must be regarded as single somites, the nature of the posterior ones is open to doubt ; they may be single terga, the sterna and appendages of which have multiplied; or, more probably, they each represent a number of coalesced terga. Each appendage consists of three divisions — an endopo- dite, exopodite, and epipodite, supported on a protopodite or basal division (Fig. 63, D, E, E). The latter consists of three joints — a coxopodite produced internally into a strongly setose prominence (not represented in the figures), a basi- podite, and an ischiopodite, the latter elongated internally into a lanceolate process, and bearing on its outer side two appendages, of which the proximal — the epipodite or branchia (Fig. 63, D, E, 7) — is pyriform and vesicular in specimens preserved in spirit. The distal appendage, which appears to represent the exopodite (6), is a large flat plate, provided with long setae on its margin. The endopodite consists of four joints, the two proximal ones being much the longer, and, like the penultimate, giving off internally a long process. Finally, the terminal joint is claw-like and serrated on its concave edge. The average form of these appendages is represented by (E), taken from the middle of the series; anteriorly the limbs become more slender and leg-like (D)', posteriorly, on the other hand, theyuire completely foliaceous, as (E) ; but the same elements are recognizable throughout. The eleventh pair of appendages alone depart, in any im- portant respect, from the rest of the series, each of these being modified so as to serve as a receptacle for the ova. To this end the joints of the endopodite are greatly ex- panded, and converted into a hemispherical bowl ; the exo- podite, metamorphosed into another such bowl, shuts down over the endopodite ; and into the box thus formed the THE BRANCHIOPODA. 245 Fig. 63.— Amis g!acia!i?.—A, Lateral view, with the risfht half of the carapace cut away. B, Dorsal view. C, Anterior parr of the body, ventral aspect. D, One of the anterior, E, one of the middle, and F, one of the posterior limbs, without their coxopodites. x, convoluted " shell-inland" in the carapace: y, coudal filament; lb, labrum. 1, 2. 3, 4. Enriopodite. 6. Exopodite. 7. Epipodite or branchia. I', eye; IP, antennule ; IV, labrum; V, YP, maxillae. 246 TBE AXATOMY OF INVERTEBRATED AXIMALS. ova are conducted by means of the oviduct, which opens into it. On the dorsal surface of each side of the terminal seg- ment of the body there is a tubercle produced into five spines anteriorly, and carrying, posteriorly, a long and delicate se- tigerous filament (Fig. 63, J5, g). The alimentary canal of Apus is very simple, consisting of a vertically ascending oesophagus, which bends back into the small stomach, situated immediately behind the com- pound eyes, in the middle of the region bounded by the two transverse furrows on the dorsum of the carapace ; from the hinder end of the stomach the straight intestine passes back to the anus, which is seated beneath the terminal segment. The liver consists of caeca, which branch off from the stomach and lie on each side of it, in the head. Zaddach describes a pair of glands which he regards as salivary, placed above, and opening into, the stomach itself, like the salivary glands of the Scorpion. The heart occupies the tergal region of the eleven ante- rior thoracic somites, presenting as many chambers, with lat- eral venous apertures. The nervous system consists of a quadrate cerebral mass, placed immediately under the compound eyes, and giving off large nerves to them and to the remains of the single eve of the larva, which lies in front of their anterior extremities. Commissures pass downward and backward on either side of the oesophagus, and connect the cerebrum with a chain of numerous ganglia placed on the median line of the ventral surface. It is worthy of remark, that the antennary and antennulary nerves are given off from the commissures, far behind the chief cerebral mass. In the female, the ova are developed in the caecal branches of two long tubes, situated one on each side of the body, and opening, as above described, in the eleventh pair of append- ages. Apus usually propagates agamogenetically, and the examination of thousands of individuals, extending over more than thirty years, failed to reveal to Von Siebold the ex- istence of a male form. In 1856, however, Kozubowski1 dis- covered a small proportion of males (16 in 160), among the specimens taken in the neighborhood of Cracow ; and near Rouen, in 1863, Sir John Lubbock found the largest pro- 1 " Ueber den miinnlicheu Apus carter if or mis." (" Arcliiv fur Naturge- schichte," 1857.) THE BRAXCHIOPODA. 247 portion of males to females yet known, viz., 33 in 72. On the other hand, between 1857 and 1869, Von Siebold ex- amined many thousands of specimens of the Bavarian Apus without finding a single male.1 The testis is similar to the ovary in form, and its duct opens upon the eleventh pair of appendages, as in the case of that of the female organs. The spermatozoa are oval and without motion. The young Apus (cancriformis), when just hatched, is a JSTauplius. The body is oval, indistinctly divided into a few segments, and entirely destitute of appendages, except a shorter anterior, uniramous, and a longer posterior, biramous, pair of oar-like organs, situated at the anterior extremity, on either side of the single median eye. The carapace is rudi- mentary, and there are no caudal filaments. The little ani- mal soon casts its skin, and the mandibles, which are provided with long palps, make their appearance.2 With successive ecdyses, the larva assumes more and more the form of the adult, and acquires the pair of compound eyes ; the anterior pair of appendages being converted into the antennules, the posterior pair disappearing, or remaining as rudimentary an- tenna?, and the mandibular palps also vanishing. Singular and highly instructive modifications are exhib- ited by the other genera of the Branchiopoda, such as Xeba- lia, Branchipus (Cheirocephalus), Limnetis, and Daphnia. In Daphnia and its allies (Fig. 64), the thoracic members are reduced to six, five, or even four pairs, some or all of which may take the form of ordinary limbs ; the abdomen is rudimentary ; the heart is short ; and the carapace presents a posterior division (omostegite), obviously developed from the anterior thoracic somites, the lateral halves of which are deflexed so as to resemble a bivalve shell, into which the hinder part of the body can be withdrawn. The anterior division of the carapace (cephalostegite) in Daphnia has, on the contrary, the same structure as the corresponding part of the carapace of Apus, but the compound eyes, represented by a single mass, are situated at the anterior extremity of the head, rather than on its upper surface, and the single eye is quite distinct, and far posterior to them (Fig. 64, B, i', n"). The antennules (Fig. 64, A, n') are small, rudimentary, 1 " BeitrSge zur Parthenogenesis der Arthropodem1' 1871. It appears that, in Apvs, the impregnated ova alone give rise to males. 2 According to Claus's recent investigations, this third pair of appendages Is present from the time the young Apus leaves the egg. 248 THE ANATOMY OF IXVERTEBRATED ANIMALS. and placed at the sides of the produced frontal rostrum, but the antennae are very large, and coiastitute the principal loco- Fig. &i.—Daphriia.—A, Side-view; the appendages not ficured except II', the an- tennules; IV', the mandibles; and V', the maxillae. Ill', The place of attach- ment of the antennas. B, Front view of head. cs, cephaloetegite, or tnat part of the carapace which covers the head ; ms, omoste- gite, or thoracic portion of the carapace ; c, heart ; st, cervical depression ; lb, labrum ; I', compound eye; II', simple eye; a-, the "shell-gland," which opens behind the maxillae. motive organs. The posterior, or second, maxillae are obso- lete. In Evadne, Polyphemus, Siday and other genera, sucker- like organs of adhesion are situated on the anterior region of the carapace. The eggs are developed in the cavity of the carapace, and the embryos pass directly into the form of the parent, except in Leptodora, where they are, at first, NaupMus- like. Limnetis and Estheria present a Daphnia-like carapace, though more completely bivalve, combined with the numer- ous segments of the body and the foliaceous appendages of the typical Phyllopods (Fig. 65). Nebal'ia has a large carapace, provided with a movable rostrum, like that of Squilla, and arising entirely from the head, which is remarkable for its very slight sternal flexure. In this genus the eyes are large and pedunculated ; there are well-developed antennules, antenna?, mandibles, and two pairs of maxillae, the anterior of which ends in a long palp. Branchipus, finally, develops no carapace either from THE BRANCHIOPODA. 249 the head or the thorax, the segments of the latter being en- tirely free, while the former is similar in shape to that of an Insect, or Edriophthalmous Crustacean, and carries two large stalked eyes, two antennules (singularly modified in the male), two antennae, a pair of mandibles, and two pairs of m axillae. In JEJstheria and Limnetis, the males are met with in full proportion to, and may be even more numerous than, the females. No males are known in Limnadia gigas, although thousands have been examined, while, in L. Stanleyana, more males than females have been found. In Bvanchipus, males are fewer than females; in Artemia, they occur only at rare intervals. In Daphnia, the males are few, and appear Fig. 65.— Ltmnetis brachyurus (after Grube).— The upper left-hand figure is the male, the other the female; one valve of the carapace in each case being removed. A\ Antennules. Aa, Antennas. A, Young larva. B, The same farther advanced, c, head ; o, eye ; d, carapace ; <*, bodv. A '. AntenDae. J/, Mandibles, d', great plate (labrum ?) which covers the mouth. only at certain seasons of the year. But notwithstanding the rarity or absence of the males in many of these genera, reproduction proceeds with great rapidity. The ova are capa- ble of development without fecundation ; and isolated females 250 THE AX ATOMY OF IXVERTEBRATED ANIMALS. of the genus Daphnia will thus go on producing broods for generation after generation, without any known limit.1 Under certain circumstances, however, bodies of a differ- ent nature from these " agamic ova," as they have been well termed by Sir John Lubbock,1 are developed within the ovary, the substance of which acquires an accumulation of strongly refracting granules at one spot, and forms a dark mass, the so-called " ephippial ovum." When fully formed, two of these bodies pass into the dorsal chamber of the cara- pace, the w7alls of which have, in the mean time, become altered. The outer and inner layers of the integument ac- quire a peculiar structure, a brown color, and a more firm consistency, over a large, saddle-like area. When the next moult takes place, these altered portions of the integument, constituting the "ephippium," are cast off, together with the rest of the carapace, which soon disappears, and then the ephippium is left, as a sort of double-walled spring-box (the spring being formed by the original dorsal junction of the two halves of the carapace), in which the ephippial ova are inclosed. The ephippium sinks to the bottom, and, sooner or later, its contents give rise to young Dajyhnioe. Jurine's and Sir J. Lubbock's researches have proved that the development of the ephippial ova may commence with- out the influence of the male, and they seem to indicate that these ova may even be fully formed and laid without the male influence. On the other hand, there appears, under ordi- nary circumstances, to be a certain relation between the com- plete development of ephippial ova and the presence of males ; and, as yet, no ephippial ova produced by virgin females have been directly observed to produce young. The question, therefore, seems to stand thus, at present: the agamic ova may certainly be produced, and give rise to embryos, without impregnation ; the ephippial ova may certainly be produced without impregnation ; but whether impregnation is or is not absolutely necessary for their further development, there is, at present, no evidence to show. The great majority of the JBranchiopoda inhabit fresh waters. Artemia, however, delights in brine-pools. The genus Estheria is of Devonian age, and it seems probable lUUeber die Gattungen Estheria und Limnadia." (" Archiv fur Natur- geschichte," 1854.) 2 " An Account of the Two Methods of Reproduction in Daphnia, and of the Structure of the Ephippium." (" Transactions of the Royal Society," 1875.) THE OSTRACODA. 251 that the Silurian Hymenocaris and its allies were related to Apus. The Ostracoda. — This group contains several genera of both recent and fossil Crustacea, for the most part of very small size, and distinguished by their hard, often calcined, and completely bivalve shell, provided with a distinct hinge. The valves of this shell consist of the lateral moieties of the carapace ; they are commonly unequal and unsym metrical, and present a peculiar ornamentation. The shell-gland is very small. The Ostracoda are also remarkable for the ex- tremely rudimental condition of their abdomen, and for the paucity of their thoracic appendages, which, instead of being foliaceous, are strong and 'subcylindrical, like the ambulatory legs of the higher Crustacea. The cephalic flexure is as well marked as in the highest Crustacea, so that the eye, obscurely divided, and median in Cypris (Fig. 66, A), but double and lateral in Cythere (B), is situated in the upper part of the anterior region of the body. The antennules and antennas, attached to their respective somites, the sterna of which constitute the anterior boundary of the body, are similar in form and function to ambulatory limbs. The ducts of a peculiar gland open, according to Zenker, at the end of the strong spine with which the an- tenna of Cythere is provided. The labium is conspicuous, and the mandibles are strong, and possess a well developed palp. The first maxilla is provided with a large foliaceous se- tose appendage (epipcdite ?). The second maxilla in Cythere is represented by the first of the three pairs of ambulatory limbs (Fig. 66, B, e, e, e) present in this genus. In Cypris, which possesses a second pair of maxilla?, there are only two pairs of ambulatory limbs (Fig. 66, A, p, i., ii.). The aper- tures of the reproductive organs, provided in the male with a wonderfully complex, horny, copulatory apparatus (described with great minuteness by Zenker), are situated between the last pair of thoracic members and the large caudal hooks. Strong adductor muscular bundles pass from one valve of the carapace to the other, and leave impressions discernible from without, the form and arrangement of which furnish valuable systematic characters. The alimentary canal of the Ostracoda is provided ante- riorly with an apparatus of hard parts, resembling in many re- spects the gastric armature of the Isopoda, and gives origin to two hepatic casca. Cypris and Cythere have no heart ; 252 THE ANATOMY OF INVERTEBRATED ANIMALS. but, in Cypridinci) Conchoecia, and Halocryptis there is, ac- cording to Claus, a short saccular heart with one anterior and two lateral apertures. The nervous system is difficult to make out ; but, in Cythere lutea, the same observer found a large cer- ebral ganglion in front of the mouth, whence filaments passed to an ophthalmic ganglionic mass, and to the antennary or- gans. A double ganglion, behind the mouth, supplies the o-nathites ; three ganglia, situated in the thorax, send fila- ments to its appendages, and a terminal ganglion supplies the caudal appendage and genitalia. In the female, the ovaries lie in the valves of the carapace, and terminate in oviducts which open by distinct apertures in front of the caudal ap- pendage. Immediately anterior to them are the openings of Fig. 66.— A. Cypris.—A. i. n. Antennules and Antennae. M. i. ti . in. Mandibles and maxilla;. P.i. n. Thoracic members ; c, caudal extremity ; b, mandibular palp ; o, eye. B. Maxillary appeudage. B. Cythere.— o, eye; a, antennule; 6, antenna; c, mandible; d. first maxilla; e, e, e, second maxilla and two thoracic members ; /, caudal extremity. (Alter Zenker.) x two horny canals, called vaginae by Zenker, each of which is continued into a long convoluted transparent tube, and event- ually terminates in a large vesicle, the spermatheca, into which the spermatozoa- of the male are received. In the males, the antenna?, the second maxillae or some of the thoracic limbs, are modified in such a manner as to enable them to seize and hold the females. The testes are elongated caBca in Cypris, globular vesicles in Cythere, and communi- cate with a long vas deferens, which opens into the copula- tory apparatus. In Cypris, a very singular cylindrical mu- cous gland is connected with the vas deferens"; but perhaps the most remarkable peculiarity about the genital apparatus in the male consists in the size of the spermatozoa, which in Cypris ovum are, according to Zenker, more than three times as long as the body. They possess a spirally-wound coat, and are totally deprived of mobility. 1 u Monographic der Ostracoden." (" Arckiv far Naturgeschichtc," 1854.) THE PECTOSTRACA. 253 The Ostracoda either attach their eggs to aquatic plants, or carry them about between the valves of the carapace. Claus ' hac worked out the development of Cypris, which passes through nine successive stages, distinguished from one another, not merely by the shape of the carapace, but by the number and form of the limbs. An ecdysis of the chitinous cuticle of the body and carapace terminates each stage of de° velopment. When the Cypris leaves the egg, it resembles a JVaiqjlius, in possessing a single median eye and only three pairs of limbs (the future antennules, antenna?, and mandi- bles) ; but none of these are divided into two branches. The body is laterally compressed and has a bivalve carapace. The changes undergone by the marine Ostracoda after they leave the egg are much less marked. Fossil Ostracoda abound in strata of all ages, from the older palaeozoic formations, onward ; and, so far as the char- acters of the carapace furnish evidence, the most ancient forms differed very little from those which now exist. The Pectosteaca (Rhizocephala and Cirripedhi) leave the egg as a JYauplius, provided with three pairs of limb-like appendages, of which the anterior pair are simple, while the two posterior pairs are bifurcated (Fig. 68, A). An addi- tional pair of filiform appendages subsequently makes its ap- pearance in front of the undivided pair of members, in most cases ; and there is a discoidal carapace, the antero-lateral angles of which usually become greatly produced. Subse- quently, the carapace becomes bivalve (as in many Phyllo- poda, and in the Cladocera and Ostracoda), and the anterior undivided pair of limbs are converted into relatively large, jointed appendages, provided with a sucker like organ. The thorax grows and usually develops six pairs of appendages. Finally, the bivalve-shelled larva fixing itself by the suckers of its anterior limbs, the pra?-oral region of the head becomes enlarged, and is converted into the base, or pe- duncle, in ordinary Cirripedes; while it gives off the root- like processes which grow into the tissues of the animals on which the Rhizocephala are parasitic. The Pectostraca are almost all hermaphrodite, a condition which is very excep- tional among Arthropods. They possess no heart. The Cireipedia. — It can hardly be a matter of reproach 1 " Entwickelungsgeschichte von Cypris " (1868) ; and u Grundzuge," p. 487. 254 THE ANATOMY OF IXVERTEBRATED ANIMALS. to the older naturalists if they failed to discover the affinity connecting the sedentary " Acorn-shells " of a rocky coast with the active Shore-crab which runs among them ; or if they classed the Barnacles with Mollusca, instead of admit- ting them to that place amid the Crustacea which is now assigned to them by every naturalist of competent judgment. Nothing, in fact, at first sight, is less suggestive of a Crusta- cean than a Balanus, or a Lepas y the former firmly fixed by the base of its multivalve conical shell, the latter by its fleshy and contractile peduncle ; the only sign of life in either being the alternate protrusion and retraction, from the valvular opening of the animal's case, of a bundle of curved filamentous cirri, which sweep with a brushing motion through the water, and scoop the floating nutritive matters toward the mouth. The valves through which the cirri make their egress are strengthened, in both Balanus and Lepas, by four calcified pieces, two on each side ; those of each half being united to- gether by an oblique suture, or by a regular articulation ; while the two pieces of opposite sides are connected only along one margin, either immediatelj' (Balanus), or by means of an intermediate piece (Lepas). The upper, or distal, pieces are termed the terga, the lower, or proximal, pieces the scuta, the intermediate piece is the carina. In Lepas, there are no other hard external pieces; but, in Balanus, the conical shell, into which the valves can be more or less completely retracted, is composed of six portions or compartments. Of these, one is situated on the same side as the opening between the valves and another at the precisely opposite point, or on the same side as the line of union of the valves. The latter is the homo- logue of the intermediate piece, or carina, in Lepms ; the former, in Balanus, consists of three pieces united together, the median rostrum and the two rostro-lateral compartments. On each side of the carina is a compartment termed carina- lateral, and between them and the complex rostrum lies a lateral compartment. If the shell consisted of its eight typical pieces (as it does in the genus Octomeris), it w^ould be found that each pre- sented a triangular free middle portion and two lateral wrings. The former is always termed the paries, but the latter re- ceive different names, according as they overlap or are over- lapped by others. In the former case, they are termed radii, in the latter, alee. Thus, typically, the carinal and the ros- THE CIRRIPEDIA. 255 tral compartments are overlapped on both sides, and their wings are consequently both alee ; the lateral and carino- lateral compartments are overlapped on one side, and overlap on the other, hence thev have an ala on one side, a radius on the other ; while the rostro-lateral compartment overlaps on both sides, and hence its wings are both radii. In Balanus, however, the rostrum and rostro-lateral compartments being replaced by a single compartment formed by their confluence, this piece has radii on both sides. Different as is the appearance of Lepas from that of Balanus, they closely resemble one anotner in essential structure. Thus, to commence with Lepas. On cutting away the scutum and tergum of one side (Fig. 67, £), the hinder part of the body of the animal is seen within the sac of the capitulum, formed by the valves of the shell, to which it is attached only on the rostral side and inferiorly by a com- paratively narrow isthmus. Immediately behind this point the body widens, to constitute what Mr. Darwin1 has termed the prosoma, but the thoracic segments, which succeed the prosoma, gradually taper posteriorly. Six pairs of appendages (a) are attached to the thorax, each limb consisting of a basal joint (protopodite), terminated by two long multi-articulate cirri, the representatives of the endopodite and exopodite ; and a rudimentary abdominal segment, terminated by two short caudal appendages, succeeds the thorax, and is pro- duced in a long setose annulated penis (/*). Filamentous appendages depend from some of the thoracic somites, and, projecting from the inner wall of the sac on each side, is a triangular process, the ovigerous froenum (m). The mouth is situated at the posterior part of a protuber- ant mass, seated on the rostral face of the prosoma. This is principally composed of a large, bullate labrum, behind which are a pair of mandibles with large and setose palps, and two pairs of maxilla. Anteriorly, the prosoma passes by a nar- row isthmus into the rostral part of the peduncle, into which it, as it were, expands ; while the posterior margins of the peduncle become continuous with the walls of the sac. The extremity of the peduncle is fixed by a peculiar cementing substance to the body to which the Lepas ad- heres ; but, if it be carefully detached, there will be found connected with the rostral portion of the surface a pair of very minute, singular-looking, organs, consisting of two proxi- } " Monograph of the Cirripedia," 1851, 1854. 17 256 THE ANATOMY OF IXVERTEBRATED ANIMALS. mal joints, succeeded by an articulation which is dilated into a sucker, and terminated by an elongated setose joint (Fig. 67, A, B) I). These are the remains of the anterior append- ages of the larva. From what has been said, it follows that the fixed end of the peducle is, in fact, the anterior extremity of the body of the JOepas, and that a Barnacle may be said to be a Crus tacean fixed by its head, and kicking the food into its mouth with its legs. Fig. 67.— A, Diagrammatic section of Balanus ; B, of Lepas. — a is placed in the cavity of the sac. and lios over the labrurn ; 6, prosoma ; c, carina: c, I. carino-lateral compartment: I. lateral compartment: r, rostrum: s. scutum: t. tergum:/, penis; g. erut-formed gland : h. duct connecting this with i. k. cement-duct and elands; I', antennae ; i, peduncular or ovarian tubules; w, ovigerous f'raenum; d, anus. The mouth of TiPpas looks toward the posterior extremity of the body, and leads into a tubular oesophagus, which passes forward, and opens by a wide superior extremity into the globular stomach. From this point, the alimentary canal bends back upon itself, and gradually narrows into the in- testine, which terminates in the anus, situated at the ex- tremity of the abdomen, on the tergal side of the penis. Two considerable branched cseea, probably hepatic, proceed as diverticula from the stomach, corresponding very closely THE CIRRIPEDIA. 257 in position with those of Daphnia. No heart or other cir- culatory organs are known to exist ; and it may be doubted if the ovigerous fraena of Lepas exert, as they have been sup- posed to do, a branchial function. The nervous system consists of a pair of cerebral ganglia situated in front of the oesophagus, and connected by long commissures with the anterior of five pairs of thoracic gam glia, whence nerves are given off to the limbs. In the mid- dle line the cerebral ganglion gives off two slender nerves, which run parallel with one another in front of the stomach and enlarge into two ganglia, wdience they are continued to a double mass of pigment, representing the eyes. From the outer angles of the cerebral ganglion arise the large nerves which proceed into the peduncle and supply the sac. These appear to correspond with the antennary and frontal nerves of other Crustacea ; and Mr. Darwin describes an extensive system of splanchnic nerves. Lepas, like the majority of the Girripedia, is hermaphro- dite. The vesiculse seminales are readily seen in fresh speci- mens, as white cords distended wTith spermatozoa, which run from the canal of the penis, into which they open, forward, on each side of the body, to the prosoma, where they end in dilated extremities, which are connected with a multitude of ramified caeca forming the proper testis. The ovaries are ramified tubes provided with caecal dila- tations, and lodged in the peduncle. The oviducts pass into the body, and, according to Krohn, terminate in apertures situated on the basal joint of the first pair of cirri.1 Two " gut-formed " glands, as they are termed by Darwin, lie, one on each side of the stomach, and are probably accessory glands of the reproductive organs, analogous to those which secrete the walls of the ovisac in the Copepoda. The mode of exit of the ova from the ovary is not cer- tainly known, nor is the place of their impregnation ascer- tained ; but they are eventually found cemented together by chitin into large lamella?, which adhere to the ovigerous fraena, and, ordinarily, at once strike the eye when the ca- pitulum of a Cirripede is opened. Yelk division is complete, and the embryo attains to its earliest larval condition within the egg. If a series of the fresh ovigerous lamellae be taken and pulled to pieces with i The position of these apertures corresponds with that of the openings, supposed to appertain to the shell-glands in Limnadia and Apus. 258 THE ANATOMY OF IXYERTEBRATED ANIMALS. needles in a watch-glass full of sea-water, one is pretty sure to be found whence a number of active little JVaiqrfius -like animalcules are set free (Fig. 68, A). Each presents a some- Fig. 68.— A. Larva of Balanus balanoides on leaving the egs (after Spence-Bate). B. Attached pupa of Lepas Australis (after Darwin): n, antennary apodemes ; t, gut-formed gland, with cement-duct running to the antenna. whit triangular body, produced in the middle line posteriorly and at its anterior lateral anp-les. The mouth is situated on a proboscidiform projection placed nearly in the centre of the body, and in the midst of three pairs of natatory limbs, of which the two posterior pairs have bifid extremities. In front of the mouth, either in this stage, or after one or two moultings, two filaments are often developed. A single eye- spot is situated in front of the bases of the anterior append- ages. After moultino; several times the larva assumes a new form, passing into its second stage. The carapace is now oral and compressed, so as more nearly to resemble that of a Daphnia or Gypris. There are two eyes. The first pair of swimming appendages of the Nauplius are converted into antenniform organs, each provided with a sucker, and the rudiments of the six pairs of cirri make their appearance behind the mouth.1 In the third stage, the larva is, as Mr. Darwin states, " much compressed, nearly of the shape of a Cypris or mus- cle-shell, with the anterior end the thickest, the sternal sur- face nearly or quite straight, and the dorsal arched. Almost the whole of what is externally visible consists of the cara- 1 According to Claus (" Grundziige der Zoologie," 3te Auflage, p. 460), the second pair of appendages disappears, and the third gives rise to the mandi- bles. In this case the antennary organs represent antennules, and the limbs of the Cirripede Nauplius correspond with those of the Copepod and Branchi- opod Nauplius. DEVELOPMENT OF THE CIRRIPEDIA. 259 pace ; for the thorax and limbs are hidden and inclosed by its backward prolongation ; and, even at the anterior end of the animal, the narrow sternal surface can be drawn up, so as to be likewise inclosed." The larva, in this stage, is provided with two large compound lateral eyes, while the median eye is arrested in its development. The oral tubercle exhibits all the gnathites of a Cirripede, but they are covered by an imper- forate integument, so that this " locomotive pupa," as Mr. Darwin terms it, is unable to feed. There are six pairs of legs, and the thorax ends in an abdomen, consisting of three somites terminated by two caudal appendages. There is no penis. The most remarkable structures in the pupa, however, are the "gut-formed glands," which are already well devel- oped, and from which the cement ducts can be traced to the disks of the antenniform organs, on the faces of which they open. The pupa, after swimming about for a while, at length selects its permanent resting-place, to which it adheres, at first, only by the action of the suctorial disks. The tempo- rary attachment, however, is speedily converted into a per- sistent one, the cement pouring out from its excretory aper- tures on the disks, and firmly gluing them and the anterior end of the body down to the surface on which they rest. Coincidently with these changes, several other important alterations take place, during the passage of the locomotive pupa into the fixed young Cirripede. The compound eyes are moulted, and with them the antennary apodemes, furnished by the integument of the deep fold which separates that part of the body of the pupa which corresponds with the beak of a Daphnia, or of a Limnetis, from the prosoma. The fold is thus enabled to straighten itself; and, as a consequence, the carapace of the Cirripede, instead of remaining more or less parallel with the surface of attachment, becomes perpendicu- lar to it. Again, in the pupa, the axis of the carapace and that of the body are identical in direction ; but, during the last moult, the chamber of the carapace extends forward far more on the tergal than on the sternal side, separating the tergal part of the prosoma from the " beak," with which it was at first continuous, and thus allowing the body of the Cirripede to take its final position, which is nearly transverse to the axis of the carapace. The terga and scuta now appear as horny thickenings, and, afterward, as calcifications in the wall of the capitulum. The fraena and the penis make their appearance, and the genitalia become developed in the prosoma and in the pe- 260 THE ANATOMY OF INYERTEBRATED ANIMALS. duncle, which is produced by the gradual elongation of the " beak " of the pupa. With the assumption of its perfect form, the Cirripede ceases to moult its carapace, ecdysis being hereafter confined to the inner lining of the sac, and to the integument of the contained body. Such is the structure and development of a typical pedun- culate Cirripede. In other genera, such as Follicipes, calca- reous plates are developed on the peduncle, foreshadowing the compartments of the sessile forms. The latter, of which JBalanus may be regarded as the type, differ in structure from Lepas in no very essential particular. The peduncle, very short and broad, instead of slender and elongated, is incased by its compartments, and is sometimes fixed by a shelly basis. The arrangement of the layers of cement is often extremely complicated ; the scuta and terga are articulated together ; the fraena are much larger organs, and posssibly subserve the respiratory function ; the thoracic ganglia are concentrated into a single mass ; and the cementing apparatus is much more complicated. The pedunculate and sessile Cirripedia, taken together, constitute by far the largest of the three great groups which Mr. Darwin recognizes ; namely, the Thoracica, characterized by having limbs attached to the thoracic somites, while the abdomen is rudimentary. The second group, the Abdominalia, contains only one genus, Cryptophiahis (Fig. 69, 5, 6), which has no thoracic limbs, but is provided with three pairs of abdominal append- ages. The larva is very imperfect in its first and second changes, which are undergone within the sac of the parent. The third group, Apoda, likewise contains only one genus, the remarkable Proteolepas (Fig. 69, 7), which is devoid of either thoracic or abdominal limbs ; it has a vermi- form body, and a rudimentary peduncle, represented by two threads terminated by the characteristic antenniform organs. In the great majority of the Cirripedia the sexual appa- ratus is disposed as in Lepas, but Cryptophialus and Aleippe are unisexual, the male differing very widely in form and size from the female (Fig. 69, 3, 6). The Balanidce, or sessile Cirripedes, all present the nor- mal sexual relations ; but the other division of the TJioracica, the Lepadidoe, contains two genera, Ibla and JScalpelhwt, which not only possess species having the sexes in distinct individuals, but others presenting the unique combination of THE CIRRIPEDIA. 261 males with hermaphrodites. Thus, Scalpellum vulgar e is hermaphrodite, possessing well-developed male and female organs. Nevertheless, on the inner side of the occludent margin of its scutum there is a fold, over which and imbed- ded in the spinose chitinous border of the scutum, a minute, oval, sac-like creature is commonly found, firmly attached by Fro. 69.— 1. Alcippe lampas ; female. 2. The same in sectional view: E, Horny disk of attachment : in 1, the males are visible as dark specks on either Ride of the upper part of the sac ; c, ovary ; h, first pair of cirri ; k. I, n, three seg- ments of the thorax without cirri : the other three segments, bearing the three pairs of terminal cirri, are very short. 3. Male Alcinpe : a, antennary append- ages ; b, vesicula seminalis ; o, eye; d, testis; k. orifice of the sac; m. pe- nis. 4. Burrow of Alcippe in a portion of a Fu.ms shell. 5. Cryptophialus minulus (female) with the outer internment removed: e. labrum : /, palpi; g, outer maxillce ; h, rudimentary maxillipede; c, wall of sac continued above into the rim of the aperture a, b; L m, abdominal cirri : k, apoendages of un- known nature. 6. Male Cryptophialus. 7. Proteolepas bivincta ; m. month ; g, h, peduncle and antenna; i, k, vesicula seminalis and penis. (After Darwin.) cement which covers the characteristic antennnles of a Cir- ripede. Within the sac is a thorax, with four pairs of rudi- mentary appendages terminated by a short abdomen. There is neither mouth, alimentary canal, nor gnathites, the cavity of the body being principally occupied by a great seminal 262 THE ANATOMY OF INVERTEBRATED ANIMALS. vesicle ; and no trace of female organs exists. This is, there- fore, an accessory, or " complemental " male. In Scalpellum ornatwn the individuals are males and females, two of the former being lodged in cavities of the scuta of one of the latter, as in the preceding species, and in S. rutilum. The males have no mouth. S. rostratum has complemental males, provided with alimentary organs attached to the interior of the sac of the hermaphrodite, while S. Per on H and villosiim have still more perfect complemental males fixed in a like position. In lbla Cumingii, the female has a vermiform male, provided with well-developed alimentary organs at- tached within her sac ; but, in the only other species of this genus, I. quadrivalvis, a similarly constructed, but here only complemental male, is lodged in a relatively large hermaph- rodite form. With regard to the habits of the Cirripedia, the majority are merely cemented to foreign bodies. Anelasma and Tubi- cinella, however, partially bury themselves in the integuments of the shark and whale, and thus prepare us for the com- pletely boring habit of Cryptophiahts Lithotrya^ and Alcippe, the latter of which (Fig. 69, 1, 2, 3) burrows in dead shells on our own coasts. Proteolepas lives within the sac of Alepas cornuta, and Ftg. 70.— A. 2?aupliu8-s\&se of Sacculina purpurea: cp, carapace. B. Cypris-st&ze of Lernomdiscus porcellanxz. C Adult condition of Peltogaster paguri : a. anterior end of the body ; b, aperture ; c, root-like processes. (After P. Muller.) appears to be truly parasitic upon it, sucking the nutritive juices from the soft prosoma of the animal which it infests. THE MALACOSTRACA. 263 The Cirripedia are almost exclusively marine, only a few species tolerating even brackish water. The Thoracica alone have yet been found in the fossil state. The oldest known genus, Pollicipes, occurs in the lower oolite ; there is a single cretaceous species of Verruca, but the sessile Cirripedes be- come numerous only in the tertiary epoch. The Rhizocephala {Peltogaster, Sae&ulina) are sn:alland parasitic ; usually upon the abdomen of other Crustacea {Podophthalmia). The body is like a sac or disk, and devoid of segmentation and of limbs. The aperture of the sac is funnel-shaped, and supported by a ring of chitin. The circum- ference of the funnel gives off a number of root-like processes, which branch out through the body of the infested animal. The alimentary canal is obsolete, and there are no cement- glands. They are hermaphrodite, and the 37oung, like those of the other Pectostraca, pass through a JVauplius and a Cypris stage.1 The Malacostraca. — The groups of Crustacea known as the Podophthalmia , the Cicmacea, the Edriopjhthcdmia, and the Stomatopoda, are here included under this head. The body consists of twenty somites (counting that which bears the eyes as one), and, of these, six (bearing the eyes, antennules, antennae, mandibles, and two pairs of maxillae) constitute the head ; eight enter into the thorax, and bear the foot-jaws and ambulatory limbs ; and six form the abdo- men and swimming limbs. In some few instances the num- ber of somites is reduced, but they never exceed twenty. The JVctuplius-iorm of the free embryo is rare, but occurs in some cases (Peneus). In others (3Iysis) it is represented only by a temporary condition of the embryo, during which, however, a chitinous cuticula is formed, and subsequently shed ; and what appear to be remains of such a transitory record of an original Nauplius state, are seen in many Am- phipoda and Isopoda, which nearly attain their adult form within the egg. In most Podophthalmia the embryo leaves the egg not as a JVauplius, but as a Zocea, which has thora- cic, but no abdominal, appendages, and in many respects re- sembles a Copepod. 1 The term Cypn's-stage, usually applied to that condition of the larvse of the Pcctostraca in which they are provided with a hivalve carapace, must not be taken to imply any special affinity with the Ostracoda. On the contrary, the larva in the Cypris-st&ge is much more similar to a Copepod or Branchi- opod. 264 THE ANATOMY OF INVERTEBRATED ANIMALS. The Cumacea take an intermediate position between the JPodophthalmia and the Edriophthalmia on the one hand, and the Phyllopoda (Nebalia) on the other. They thus serve to connect the Malacostraca with the Entomostraca. The Pod ophthalmia. — It will be convenient to commence the study of the Malacostraca with the Podophthalmia / and as excellent examples of this division of convenient size are readily obtainable in the fresh -water Crayfish [Astacus flum- atilis) and the Lobster (Homarus vulgaris), and as they fur- nish a very intelligible guide to the general plan of structure of the higher Arthropoda, the organization of Astacus will be described at length. With some unimportant modifi- cations, what is said about it will be found to apply to the Lobster. The upper and anterior portion of the dense and more or less calcified exoskeleton which covers the body of Astacus, has the form of a large, expanded, shield-like plate, the cara- pace, produced into a strong frontal spine between the eyes, and bent down at the sides, so as to reach the bases' of the legs. The posterior division of the body, on the other hand, presents a very different aspect, being divided into a series of distinct movable somites. This is called the abdomen ; while the anterior division, covered by the carapace, corresponds with the head and thorax of other Arthropoda, and receives the name of cephalo-thorax. On turning to the ventral surface of the Crayfish, a great number of limbs or appendages, twenty pairs in all, are seen to be attached to the cephalo-thorax and abdomen, six pairs belonging to the latter and fourteen pairs to the former re- gion of the body. The six pairs of abdominal appendages are commonly known as the "false" or "swimming" feet ; and it will be observed that they are attached to the six anterior segments of the abdomen only, the seventh being unprovided with any such organs. Of the fourteen pairs of cephalo-thoracic ap- pendages, the five posterior are called the " ambulatory " legs, being the organs by which the Crayfish is enabled to walk. Strictly speaking, however, the anterior of the five pairs is not more ambulatory than prehensile, being so modified as to constitute the great claws, or " chelae." Of the six next pairs of appendages, passing from behind forward, five are not at first sight apparent, the posterior pair, which are applied over the mouth and cover the others, ASTACUS FLUVIATILIS. 265 being alone visible. These, and the two pairs which lie im- mediately under or in front of them, are called maxillipedes, or "foot-jaws." The next two pairs, delicate and foliaceous, are the maxillae ; while beneath or rather in front of them are two strong, toothed organs, the mandibles. These, the maxillae and the maxillipedes, thus constitute six pairs of gnat hit es. The remaining three pairs of appendages occupy the sides of the forepart of the cephalo-thorax, in front of the mouth. The most posterior pair, or the long feelers, are the antennae; the next, or the short feelers, are the antennulae; while the most anterior pair are the movable stalks, which support the eyes upon their extremities — the " ophthalmic peduncles," or " ophthalmites." To arrive at an understanding of the composition of this complex body with its multiform appendages, we must first detach and study carefully one of the abdominal segments — say the third. Such a segment is nearly semi-circular in ver- tical section, the dorsal wall, or tergum, being very convex, and where it reaches the level of the almost straight ventral wall, or sternum, sending down a flattened lobe, which is re- flected at its free edges into a corresponding prolongation of the ventral wall, so that each infero-lateral angle of the seg- ment is prolonged into a hollow process, the pleuron. Near the outer extremities of the straight ventral portion of the segment two rounded articular cavities, which receive the basal joints of the appendages, are situated. A transverse groove will be seen on the tergum, separating rather more than the anterior third of its surface, as a smooth, convex, lenticular facet, which is completely overlapped by the pos- terior margin of the preceding segment, when the abdomen is extended, and is left uncovered only in complete flexion. This is the tergal facet. A corresponding flattened and rath- er excavated surface upon the anterior half of the pleuron, which is similarly overlapped by the preceding pleuron, and is left uncovered only in complete extension, may be termed the pleural facet. It will be observed that there is a close correspondence between the skeleton of an abdominal somite of a Cray-fish, and that of a thoracic somite of a Trilobite ; except that, in the latter, the sternal region is not calcified. The appendages of the segment (Fig. 71, A ) are very sim- ple, consisting of a cylindrical basal portion, divided into two joints, a shorter proximal, and a longer distal, to the latter of which two terminal many-jointed filaments are articulated. 266 THE ANATOMY OF INVERTEBRATED ANIMALS. ASTACUS FLUVIATILIS. 267 Fig. 71. — Astacus fluviatills. A. Mandible: a, 6. endopodite ; o, its terminal joints constituting the palpus of the mandible. B. First maxilla. €'. Second max- illa. D. First maxillipede. E. Second maxillipede. F. Third ruaxillipede. All the preceding, except B, are left limbs. Q. Ambulatory leg. H. Appendage of first, and /of second, abdominal somite in the male. K. Appendage of third ab- dominal somite. L. Sixth abdominal somite, with its appendages and telson : a, b, endopodite; c, exopodite; d, epipodite; e, setaceous filaments attached to coxopodite ; x, tergum of sixth abdominal somite ; y. z. the two divisions of the telson. In G : 2, basipodite ; 3, ischiopodite ; 4. meropodite ; 5, carpopodite ; 6, propodite ; 7. dactylopodite. In .4, d marks the tendon of the adductor muscle, and in K the joints of a b and c are uot sufficiently numerous. M. Transverse section of half a thoracic somite (a) : b, coxopodite : c. basipodite ; d, ischiopo- dite ; h, branchiferous epipodite ; /, g, branchiae : e, filiform appendage. N. One of the branchiferous epipodites : a. its point of attachment; 6, basal enlarge- ment ; c, branchial filaments ; d, terminal lobes. The inner of these is distinguished from the outer by possess- ing a more elongated and wider basal joint. The whole basal division of the appendages is the protopodite; while the in- ternal and external terminal filaments are the endopodite (a, b) and exopodite (c). An abdominal segment, or somite, then, is composed of a tergum, two pleura, and a sternum ; but it must be remem- bered that these terms rather indicate regions than anatomi- cal elements, the whole segment being continuously calcified, and no sutures or other absolute demarcations separating one portion from another. Furthermore, the somite carries two appendages, each divided into a proximal portion or pro- topodite, terminated by two branches, the endopodite and exopodite. The whole exoskeleton of the Astacus, however various may be the appearance of its different parts, consists of so- mites and appendages essentially similar to those which have just been described, but which are more or less masked by the connation, the coalescence, the abortion, or the extreme modification of their primitive elements. If, in the first place, we follow out these modifications in the posterior somites, we find the fourth, fifth, and sixth abdominal somites to be, in all essential respects, similar to the third ; but the appendages of the sixth (Fig. 71, L) are singularly changed, the protopodite being represented by a single strong, short joint, and the exopodite and endopodite having the form of wide, oval setose plates. The exopodite is again divided into two portions by a transverse joint. The seventh division of the abdomen (Fig. 71, X, y, z) is the telson. This telson bears no appendages ; dorsally it is completely calcified, but is divided by a transverse suture into two portions, the posterior of which is movable upon the other; ventrally, on the contrary, it is only the posterior part which is fully calcified, the middle of the anterior portion, in wThich 268 THE ANATOMY OF IXVERTEBRATED ANIMALS. the anus is situated, being completely membranous, and the sides only being strengthened by calcareous plates extend- ing inward from the dorsal hard skeletal element, or sclero- dermite. The powerful tail-fin of the Astacus is formed by the tel- son, combined with the two distal divisions of the sixth ab- dominal appendages on each side. The other abdominal appendages can have very little influence on locomotion. In the female, howTever, they play an important part as the car- riers of the eggs ; and in this sex there is nothing worthy of special notice about the first and second abdominal somites or their appendages, except that those of the first are rudi- mentary. In the male the appendages of these two somites have undergone a very interesting metamorphosis, whereby they are fitted to subserve copulation. Those of the second somite (Fig. 71, I) are enlarged, and the protopodite and basal joint of the endopodite are much elongated ; the latter being produced internally into a plate rolled upon itself, and thence concave outward and forward. It is as long as the rest of the endopodite (which, like the exopodite, is many-jointed), and serves as a sort of sheath for the reception of the append- age of the first abdominal somite (Fig. 71, -H), which con- sists of a single plate rolled upon itself in a similar manner, so as to resemble a grooved style. These organs, doubtless, help to convey the spermatophores from the male genital apertures to the body of the female. The compact and firm cephalo-thorax seems at first to dif- fer widely from the flexible, many-jointed abdomen ; but the most posterior of its somites offers an interesting transition from the one to the other. This somite is, in fact, only united by membrane to that which precedes it, and is hence, to a certain extent, movable. Its sternal portion is completely calcified, but the epimera ' are only partially calcified. The appendages of this somite differ widely from those of the abdomen, representing (as their development shows) only the protopodite and endopodite of the latter. Each is a long, firm leg, composed of seven joints, the proximal one being thicker than any of the rest, while the terminal joint is nar- row, curved, and pointed. To these seven joints Milne-Ed- wards has applied the following terms (Fig. 71, G) : The proximal one, which articulates with the somite, is the coxo- ^The term epimeron is here employed in a more special sense than that commonly used, to denote that part of the lateral wall of a somite which is situated between the articulation of the appendage and the pleuron. ASTACUS FLUVIATILIS. 269 podite (1) ; the next, small and conical, is the basipodite (2) ; the third, cylindrical, short, and marked by an annular con- striction, is the ischiopodite (3) ; next comes a long joint, the meropodite (4) ; then the carpopodite (5) and pjropodite (6) ; and, finally, the terminal dactylopodite (7).1 The next four somites, proceeding anteriorly, have a sim- ilar general character to that which has just been described, but they cease to be movable upon one another, partly by reason of the calcification of the interepimeral and inter- sternal membranes, partly on account of the development of these membranes by a folding inward, or involution, into processes, the opodemes, which project inward and unite with one another in the cavity of the thorax. In an Astacus which has been macerated — or, better, boiled in caustic alkali — the floor of the thoracic cavity is seen to be divided into a number of incomplete cells, or chambers, by these apodemal partitions, which will be observed, on careful examination, to arise partly from the intersternal, partly from the inter- epimeral, membrane connecting every pair of somites. The former portion of each apodeme is the endosternite, the latter the endopleurite, of Milne-Edwards. As a general rule, each endosternite is distinguishable into three apophyses : the arthrodial, which passes outward and unites with the de scending division of the endopleurite to form one boundary of an articular cavity for a limb ; the mesophragmal, which is directed inward, uniting with its fellow, and forming an arch over the passage left in the middle line between each pair of endosternites — the so-called sternal canal / lastly, the para- phragmal division is a small process, which passes forward, upward, and outward, and unites with the anterior division of its own endopleurite, and with the posterior division of the endopleurite in front of it. The endopleurite likewise divides into three apophyses, one descending or arthrodial, and two which pass nearly horizontally inward : the anterior horizontal apophysis unit- ing with its own paraphragmal apophysis, the posterior with the paraphragmal of the antecedent endosternite. The pos- terior horizontal apophysis, therefore, crosses the space be- tween every pair of apodemes diagonally, whence the ap- pearance of a double row of longitudinal cells opening above, on each side of the sternal canal. It will be understood, 1 Probably the coxo- and basipodite together answer to the protopodite of the abdominal appendages, the remaining joints representing the endopodite. 270 THE ANATOMY OF INVERTEBRATED ANIMALS. however, that these cells are very incomplete, communicating with one another anteriorly and posteriorly by the large apertures left between the endosternites and endopleurites ; and laterally, by the spaces between the endosternites, through which each series opens into the sternal canal ; while above, they are in free communication with the thoracic cavity. The apodemes give attachment to the muscles of the appendages, while the chain of ganglia and the sternal artery lie in the sternal canal. The appendages of the penultimate resemble those of the last thoracic somite, but the three preceding pairs differ from them by being chelate — that is, by having the posterior distal angle of the propodite produced so as to equal the dactylopodite in length, and thus constitute a sort of oppos- able finger for it (Fig. 71, 6r, 6, 7). The first ambulatory or prehensile limb, again, is remarkable for its great size and strength, and for the ankylosis of its basipodite with the ischiopodite. The four anterior pairs of ambulatory limbs differ from the last pair in possessing a long curved appendage (Fig. 71, JV), which ascends from the coxopodite, with which it is artic- ulated, and passes into the branchial chamber, in which it lies. This is the epipodlte ; its relation to the function of respira- tion will be adverted to presently. The sterna, which are wide in the three hindmost thoracic somites, become very narrow and almost linear in the ante- rior ones. They and their apodemes, however, remain per- fectly recognizable. The sternal regions of the three maxillipedary somites have the same characters, their appendages and articular cavi- ties becoming smaller ; while, by the contemporaneous exces- sive narrowing of the interarticular regions of the sterna, these cavities are closely approximated. The sternum of the next anterior somite (bearing the seconl pair of maxillae), on the other hand, though very nar- row from before backward, has a considerable width, and its articular cavities, already much larger than those of the ante- rior maxillipedary somites, are consequently thrown outward. Hence results a sudden widening of the second maxillary, as compared with the first maxillipedary somite ; and, as a con- sequence, we find a deep fold or depression on the sides of the body where these two somites join. This fold is directed upward and backward on the flanks of the body, parallel with an important impression on the carapace, the cervical groove. ASTACUS FLUVIATILIS. 271 Not only on this ground, but because the fold really repre- sents a true neck, or separation between the head and thorax, it may approximately be termed the cervical fold. The scaphognathite (Fig. 71, O, c, d), an important appendage of the second maxilla, lies in this cervical fold. The appendages of the three maxillipedary somites (Fig. 71, D, E, I ) are highly interesting, inasmuch as they afford transitional forms between the ambulatory limbs and the gnathites. Each maxillipede is composed of three divisions, articulated with a stout protopodite. The outermost of these divisions is a curved, elongated lamina (d), precisely resem- bling the epipodite of the posterior thoracic limbs in the two hinder maxillipedes {E, F) ; but, in the anterior (-0), not modified so as to serve as a branchia, and rather approaching the scaphogriathite in form. The middle division of each maxillipede (c), answering to the exopodite, is long, slender, many-jointed, and palpiform ; while the inner division, or endopodite (a, b), not only corre- sponds with one of the ambulatory limbs, but in the posterior maxillipede (Fig. 71, F) very closely resembles one, and con- tains the same number of joints. In the next maxillipede, however (Fig. 71,-27), the endopodite is proportionally shorter, and in texture and form rather approaches the foliaceous en- dopodite of the anterior maxillipede (Fig. 71, D), in which a flat plate is applied to the posterior surface of the slender exopodite. A perfect transition is thus produced between the corresponding divisions of the second maxillipede and of the second maxilla. The intermaxillary apodeme, or that developed from the connecting membrane of the two maxillary somites, is very remarkable for its stoutness and for the great size and ex- panded form of the mesophragmal processes, which unite into a broad plate, whence prolongations are sent forward and outward, in front of the tendon of the great adductor mandi- bulcB muscle on each side. These prolongations appear to be the calcined posterior horizontal apophyses of the mandibulo- maxillary apodeme, which elsewhere remains membranous. The second maxilla (Fig. 71, C) much resembles the an- terior maxillipede, but the epipodite (d) and exopodite (c) appear to be combined into a wide oval plate, the scapho- c/nathite, of which mention has already been made.1 In the first maxilla (Fig. 71, JB) the epipodite and exopodite appear 1 Until the development of these appendages has been worked out, the de- termination of the homologies of their parts must be regarded as provisional. 18 272 THE ANATOMY OF INYERTEBRATED ANIMALS. to be undeveloped, and the joints of the endopodite are com- pletely foliaceous. The somite which supports the mandibles is, to a great extent, membranous in its sternal region; it is united with the corresponding region of the first maxillary somite, itself represented merely by a narrow, distinctly cal- cified, band, in front of the second maxillary sternum, by mem- brane only. In this membranous space the elongated aper- ture of the mouth is situated. On each side of and behind the mouth are two little elongated oval calcified plates, between which an oval pro- cess, setose at its extremity, proceeds downward and for- ward, and lies in close apposition with the posterior face of the mandible of its side. This is one-half of what is termed by most authors the labium, but, to avoid confusion with the labium of Insecta, from which it is wholly different, it may be called the metastoma (Fig. 72, f). It obviously answers to the structure so named in the Copepoda. The mandibles fill up a large space in the sternal mem- brane, with which their edges are continuous on each side of the oral aperture ; externally, the sternal membrane bends suddenly downward into the pleural ridge, continuous with the branchiostegite of the carapace, and becomes calcified ; while, anteriorly, it is very difficult to say where the mandi- bular sternum terminates. In front of the mouth the sternal membrane becomes developed into a large median lobe, con- taining three small calcified plates on each side of the middle line. This is the labrum (Fig. 72, e). The mandible itself (Fig. 71, A) is thick and strong at its inner end, where it is divided by a deep excavation into an upper and a lower portion (a, b), the edge of each being toothed. The outer division of the mandible extends along the whole width of the somite, and tapers to its extremity, which presents an articular head, the outer condyle. At- tached to its anterior margin is the palp (o), which represents the terminal joints of the mandibular endopodite. The ex- opodite and the epipodite have no representatives in this ap- pendage. Superiorly, the outer portion of the mandible is concave, and its posterior edge gives attachment to the cal- cified tendon of the adductor mandibular (c7). In front of the labrum and mandibles is a wide, somewhat pentagonal area, prolonged into a point in the middle line forward, and presenting a small spine on each side; this is the epistoma (Fig. 72, _Z?, I), and it is chiefly, if not entirely, formed by the sternum of the antennary somite. On each ASTACUS FLUVIATILIS. 273 side of its triangular anterior extremity it presents a wide articular cavity for the articulation of the antennae. In these organs (Fig. 72, B^ d) the same parts can be recognized as in Fig. 72.— A. Anterior extremity of the cephalo-thorax of Astacus, with a portion of the carapace removed. B. Vertical section of the anterior part of the cephalo- thorax : a, rostrum ; b, ophthalmic peduncles ; <°, antecnulae ; d, antennae ; e. la- brum ; /, metastoma ; g, oral aperture ; h, procephalic processes ; i, ophthalmic sternum ; k, antennulary sternum : I. antennary sternum or epistoma. the other appendages, viz., an imperfect basal joint, produced into a prominent cone, perforated behind and internal to its apex, and here called coxocerite. Next, a basicerite, to the outer portion of which a flattened plate, the representative of the exopodite, and here called the scaphocerite, is articu- lated; while to its inner portion an ischiocerite is connected, bearing a merocerite and carpocerite, while the last segment, or prooerite, consists of a long multi-articulate filament. The sterna of the next two somites are narrow and elon- gated ; that of the antennary somite is well calcified, but that of the ophthalmic somite is almost entirely membranous.- The antennules (Fig. 72, B, c) present an enlarged trigonal basal joint, succeeded by two others. These represent the protopodite, and carry at their extremities two many-jointed filaments, which probably represent the exo- and endopodites. The peduncles of the eyes (Fig. 72, b), lastly, are com- posed of two joints, a small proximal basiophthalmite, and a larger terminal podophthalmite. Such are the structure and arrangement of the sternal por- tions of the several cephalo-thoracic somites, and the nature of their appendages. On regarding the sternal region as a whole, there are yet some very important points (the morpho- logical value of which has been fully pointed out by Milne- Edwards) to be noticed. A longitudinal median section, in fact, shows that, while a line drawn through the sterna of the somites behind the mouth is nearly straight and parallel 274 THE ANATOMY OF INVERTEBRATED ANIMALS. with the axis of the body, a similar line drawn through the sterna of the somites, in front of the mouth, ascends as it passes through the antennary, antennulary, and ophthalmic sterna, and thus takes a position at right angles to the former line (Fig. 72, JB). The sterna of the somites, in front of the mouth, are, therefore, bent up so as to look forward instead of downward ; and it is of essential importance to bear in mind this cephalic flexure, in considering the structure of the head in these and other Arthropoda. Just as the lateral regions of the abdominal somites are produced into the pleura, so are the lateral regions of the cephalo-thorax similarly prolonged. Thus the membranous lateral walls of the posterior cephalo-thoracic somite are re- flected superiorly, and bent down again to the level of the bases of the legs, where they become continuous with a calci- fied layer corresponding with the tergal half of the pleura, and forming the posterior part of the carapace. In like man- ner, the more or less calcified epimera of all the other somites are reflected superiorly into a membrane which passes down- ward, and the free lower edge of which is continuous with the edges of the carapace. The carapace, therefore, corre- sponds in position with the terga and tergal halves of the pleura of all the somites which are thus reflected into it, and these somites include all, without exception, from the last thoracic to the ophthalmic. Posteriorly, the edges of the carapace are a little prolonged beyond the last thoracic somite, and take the form of a fold, with an under layer distinct from the upper. Anteriorly, in the middle line, the carapace is prolonged in a similar manner, but to a much greater extent; it thus gives rise to the long rostrum, which overhangs the sterna of the ophthalmic and antennulary somites. At the sides of the antennulary and antennary somites the rostral pro- longation of the carapace is the direct continuation outward of the epimera of those somites, and there is nothing to be com- pared to an apodeme ; but the sternum of the ophthalmic so- mite, after giving off the lamella which forms the inferomedian region of the rostrum, is prolonged on each side of the middle line backward and outward into a free, expanded, thin, cal- cified process, which applies itself against the carapace by its upper surface, and by its under surface gives attachment to the anterior gastric muscles. Corresponding processes are developed from the carapace itself, in some Podophthahnia (e. g., Galathea, Carclnus), for the attachment of the poste- rior gastric muscles. From the last thoracic to the maxilli- ASTACUS FLUVIATILIS. 275 pedary somites, the pleural, or free part of the carapace, termed, from its function, the branehiostegite, or cover of the gills, incloses a wide space, bounded internally by the epimera of the somites. This is the branchial chamber. In front of the maxillipedes and cervical fold, however, the chamber sud- denly becomes narrowed by the rapid widening of the sterna of the maxillary and mandibular somites, and by the lowering of the point at which the reflection of their epimera into their pleura takes place. Finally, on the antennary somite, and in front of it, the pleuron becomes a mere fold separated by a shallow groove, the representative of the branchial chamber, from the epimera. On the dorsal surface there is no indication of an}7 divis- ion cf the carapace into terga corresponding with the sterna of the somites, but it is marked by a well-defined, curved groove, the posterior convexity of which extends across the carapace, rather behind its middle, and the lateral portion of which runs downward and forward, toward the anterior part of the antennary sternum. This is the cervical groove / that part of the carapace which lies in front of it is the cephaloste- gite, while that which is behind is the omostegite. The omostegite, again, is divided into three portions by a groove on each side of the middle line — the branchiocardiac grooves. The branchiocardiac groove, and the lateral por- tion of the cervical groove, on the dorsum of the carapace, correspond very closely with the line at which the epimeral is reflected into the pleural membrane, on its ventral surface. The transverse portion of the cervical groove, on the other hand, corresponds with the posterior boundary of the stom- ach and the anterior extremity of the heart, and continues inw^ard the line of the cervical fold ; so that, in a longitudi- nal section of an A.stacus, the direction of the cervical fold, if followed upward and backward, strikes against the inner surface of the carapace, at a point corresponding with the summit of the cervical groove, on its outer surface. By cut- ting through the cervical fold, therefore, through the mem- brane joining the second maxillary with the first maxillipe- dary sternum, and through the carapace in the transverse part of the cervical groove, it is possible to separate an ante- rior portion of the cephalo-thorax, containing the whole of the cephalostegite, and the first six somites, with their ap- pendages, from a posterior portion, consisting of the omos- tegite, and the last eight cephalo-thoracic somites. And, in making this artificial separation, we should be merely carry- 276 THE ANATOMY OF INVERTEBRATED ANIMALS. ing out a distinction between these two sets of somites, already very clearly indicated by the cervical fold and groove. It is for this reason that I differ from Milne-Edwards in regarding the somite which bears the first maxillipedes as the first of the thorax, and not as the last of the head. And the acceptance of this natural delimitation of the head in the higher Crustacea has the advantage of bringing its structure into accordance with that of the same region in the JEnto- mostraca, in which it is the rule that the head possesses eyes, antennules, antennae, mandibles, and two pairs of maxillae. Another mark upon the carapace is a large and rounded convexity, occupying nearly a third of the whole width of the posterior half of the cephalostegite. This impression is bounded internally by a line drawn from the outer angle of the base of the rostrum, directly backward, and externally by a curved depression, deepening into a pit anteriorly ; it corresponds with the attachment of the base of the adduc- tor muscle of the mandible. The mouth of the Crayfish is a wide aperture, situated between the labrum in front, the metastoma behind, and the mandibles on each side. It serves as the entrance to an equally wide oesophagus, a short tube with plaited Avails, which takes a slightly curved direction upward and a little backward, to open into the large stomach, which is not only situated directly over, but extends forward in front of the gullet. The stomach, in fact, occupies almost the whole cavity of the body in front of the cervical suture, and is divided by a constriction into a large anterior moiety, the cardiac division, and a smali posterior, pyloric portion. The anterior half of the cardiac division has the form of a large membranous bag, the inner surface of which is closely set with minute hairs ; but in the posterior half of this, and on the whole of the pyloric division, the walls of the stomach are strengthened by a very peculiar arrangement of un calci- fied and calcified plates and bars articulated together, which are thickenings of the chitinous cuticula of the epithelium of the alimentary canal, and constitute the gastric skeleton. The most important part of this apparatus is that which is developed in the posterior cardiac region. It consists, in the first place, of a transverse, slightly arcuated cardiac plate (Fig. 73, ca), calcified posteriorly, which extends across the whole width of the stomach, and articulates at each extremity by an oblique suture with a small curved triangular antero-lateral or pterocardiac (pt) ASTACUS FLUVIATILIS. 277 ossicle. On each side a large, elongated postero-lateral or zygocardiac ossicle (se), wider posteriorly than anteriorly, is connected with the lower end of the antero-lateral ossicle, and, passing upward and backward, becomes continuous with a transverse arcuated plate, calcified in its anterior moiety, and situated in the roof of the anterior dilatation of the py- loric portion; this is the pyloric ossicle (Fig. 73, py). These pieces, it will be observed, form a sort of six-sided frame, the anterior and lateral angles of which are formed by movable joints, while the posterior angles are united by the elastic pyloric plate. From the middle of the cardiac piece a strong calcified urocardiac process (caf) extends backward and downward, and, immediately under the anterior half of the pyloric ossi- cle, terminates in a broad, thickened extremity, which presents in feriorly two strong rounded tuberosities, or cardiac teeth. With this process is articulated, posteriorly, a- broad pre- pyloric ossicle, which passes obliquely upward and forward, in the front wall of the anterior dilatation of the pyloric portion, and articulates with the anterior edge of the pyloric ossicle, thus forming a kind of elastic diagonal brace between the urocardiac process (caf) and the pyloric ossicle. The inferior end of this pre-pyloric ossicle is produced downward into a strong bifid urocardiac tooth (ac). Finally, the inner edges of the postero-lateral ossicles are flanged inward horizontally, find, becoming greatly thickened and ridged, form the large lateral cardiac teeth (cc). The membrane of the stomach is continued from the edges of the pre-pyloric to those of the postero-lateral ossicle in such a manner as to form a kind of pouch with elastic sides, which act, to a certain extent, as a spring, tending to approximate the inferior face of the pre- pyloric ossicle to the superior face of the median process of the cardiac ossicle. The result is, that there is a certain position of equilibrium of the whole apparatus, when the urocardiac process and the pre-pyloric ossicle make a small angle with one another, and the antero-lateral ossicles form an almost unbroken transverse curve with the cardiac. When undisturbed, the apparatus tends to assume this position. Two pairs of powerful muscles are attached to this gastric skeleton. The anterior pair arise from the procephalic pro- cesses, and are inserted into the roof of the stomach, some- what in front of the cardiac ossicle; the posterior have their origin in the carapace immediately above and behind the 278 THE ANATOMY OF INVERTEBRATED ANIMALS. pyloric end of the stomach, and their insertion into the pylo- ric ossicle and the wide posterior part of the postero-laterai pieces. Fig. 73— Astacus— Upper Figure: Longitudinal Section of Stomach.— A, anterior gastric muscle ; B, posterior gastric muscle ; CE, oesophagus ; _P, pylorus ; ca, cardiac ossicle; ca'. its urocardiac process: ac, urocardiac tooth; py, py- loric ossicle ; the ohlique bar, extending from the end of the cardiac to the pylo- ric, is the pre-pyloric ossicle: pt, pterocardiac ; se, postero-laterai cardiac, with its great tooth, cc ; I, small inferior tooth ; c. cardio-pyloric valve ; b, infero- median pyloric ridge ; a, lateral pyloric ridge ; d, superior pyloric ridge ; up, uro-pyloric ossicle ; x y. line of section ; the anterior lace of the posterior segment being shown in the lower figure. From the attachment of these muscles it is clear that their action must, in a general way, resemble that produced by pulling upon the cardiac and pyloric pieces when the stomach is removed from the body. Now, the result of doing this is that, the cardiac and pyloric pieces being divaricated, the pre-pyloric ossicle assumes a vertical position, and the uro- cardiac tooth turns downward and forward. At the same time the antero-lateral or pterocardiac pieces are pulled back- ward, and, owing to their oblique articulation with the car- ASTACUS FLUVIATILIS. 279 diac piece, their inferior ends move downward, backward, and inward, carrying with them the anterior ends of the postero- lateral pieces, the teeth of which (lateral cardiac) come into contact with the urocardiac and cardiac teeth with a force proportional to that exerted in traction. On ceasing to pull, the apparatus returns to its former position, its backward movement being facilitated by the reaction of the elastic pouch mentioned above, and being doubtless also assisted, in the living state, by a pair of small cardio-pyloric muscles, which pass, one on each side, between the cardiac and pyloric ossicles, beneath the membrane of the stomach, the looseness of which, in this region, where it unites the various ossicles of the gastric mill, greatly assists the free movement or the whole apparatus. Nothing can be more easy than to perform the experi- ment, and to convince one's self that these structures do really constitute a most efficient masticatory apparatus ; and it is surprising that Oesterlen, in his elaborate essay on the stom- ach of Astacus, should have questioned the crushing action of the teeth. A great bilobed valvular process (Fig. 73, c) rises up from the sternal region of the stomach, opposite the cardio-pyloric constriction, and apparently prevents the food from passing into the pyloric division until it is properly comminuted. And, in front of this valve, the infero-lateral parietes of the stomach are strengthened by a number of other plates and bars ; one of which on each side bears a small tooth (infero- lateral cardiac, I), and is continued into a broad uncalcified plate, lying in the hinder and lower part of the side-walls of the stomach, and covered with hairs internally. There are, therefore, altogether seven gastric teeth : three median, the cardiac, and the urocardiac, and two lateral on each side, the lateral cardiac and the infero-lateral cardiac. In the pyloric division of the stomach the food has to undergo a further series of comminutions and strainings. A ridge covered with long hairs projects in the median line above ; other hairy ridges extend inward from the sides to meet it, and nearly close the passage laterally. These ridges are very convex inferiorlv, and their convexities abut against the concavities of an inferior median ridge, which rises up to meet them, and is prolonged posteriorly into a sort of valvu- lar process, covered at its termination with long hairs, which bar the space left between the upper parts of the lateral ridges. The concave faces of this median process are covered 280 THE ANATOMY OF INVERTEBRATED ANIMALS. by close-set parallel ridges, which only become free hair-like processes at the posterior margin of the plate, each ridge giving attachment to a regular series of minute hairs. These are directed inward nearly parallel with the surface, which looks at first as if it were merely ruled with close-set trans- verse lines, connected by still finer and closer longitudinal ones. This apparatus constitutes the "ampoule cartilagineux " of Milne-Edwards. Behind it there is yet another infero- median and two lateral setose valvular prominences, which form the last barrier between the food and the intestine. Mr. T. J. Parker, who has recently carefully examined the structure of the stomach of the Crayfish,1 finds that, besides the anterior and posterior gastric and the cardio-pyloric mus- cles, there are intrinsic fibres in the walls of the stomach, some encircling the posterior pyloric region, others passing between the hindermost accessory ossicle and the postero- lateral and pyloric pieces; these must tend to diminish the cavity of the stomach, and the last-named fibres possibly assist in mastication by bringing the lateral cardiac into con- tact with the infero-lateral cardiac tooth. Moreover, there are nine pairs of minor extrinsic muscles, of which two pairs pass from the anterior wall of the stomach and gullet to the anten- nary sternum, passing between the oesophageal commissures and on either side of the azygos nerve of the visceral system ; three pairs pass between the side-walls of the stomach and oesophagus and the mandibular sterna; a sixth pair arises from the forward processes of the intermaxillary apodeme, and is in- serted into the oesophagus ; two more pairs arise, one from the internal thickened edge of the mandible, the other from the intermaxillary apodeme, and are inserted into the inferior surface of the pyloric region ; and a ninth pair arises from the carapace just behind the posterior gastric muscles, and goes to be inserted into the posterior pyloric dilatation. There are also a few more inconspicuous fibres passing between the oesophagus and the neighboring hard parts. All these, at least when acting* together, must antagonize the intrinsic muscles, and dilate the stomach. The pyloric portion of the stomach passes into the an- terior portion of the intestine, which is smooth internally, and presents superiorly a caecal process, the remains, accord- ing to Rathke, of one lobe of the viteliary sac of the embryo 1 Journal of Anatomy and Physiology, October, 1876. ASTACUS FLUVIATILIS. 281 This anterior portion of the intestine is, however, very short, and almost immediately becomes dilated into the wider posterior division, which extends to the anus. The inner surface of the dilatation is produced into six ridges, which are continued into a corresponding number of series of papillae along the rest of the intestine. The only glandular apparatus of any kind which opens into the alimentary canal is the liver, and the apertures of the wide hepatic ducts are seen on each side of the pylorus. Each duct conveys the secretion from the multitudinous caecal tubes, which constitute the principal mass of the cor- responding bilobed half of the liver. The two halves lie on each side of the stomach, and, though they remain perfectly distinct from one another, come into close contact below. Astacus possesses neither salivary glands nor any csecal appendages to the intestine, such as exist in the JBrachyura and some Macrura, unless the short caecum just now de- scribed is the homologue of the longer caeca of Maia and Homarus. In the spring and summer two very curious discoidal cal- careous plates, the so-called "eyes" of the Crayfish, are found imbedded in the walls of the dilated anterior portion of the cardiac division of the stomach, the middle of the lateral surface of which they occupy. These bodies com- mence as calcareous deposits underneath the chitinous gas- tric lining, and increase in size until the period arrives at which the Crayfish casts its skin. They are then cast, to- gether with this lining membrane and the gastric armature ; and it would appear that, like the latter, they become broken up and destroyed within the new stomach. The purpose of these concretions is not understood; the ordinary theory, that they are stores of calcareous matter, ready to be dis- tributed through the young integument after ecdysis, appear- ing to be negatived by their small size. Oesterlen states that they rarely weigh more than two grains, and judiciously suggests that if it be admitted that the Crayfish can derive all the calcareous matter it requires, except two grains, from other sources, it is hardly necessary to look on those two grains as a special supply. The circulatory apparatus of Astacus is well developed. The heart (Fig. 74, C) has the shape of an irregular polygon, and lies immediately behind the stomach and beneath the cardiac region of the carapace, in a chamber which is com- monly termed the " pericardium," to the walls of which it is 282 THE ANATOMY OF INVERTEBRATED ANIMALS. attached by six ligaments, corresponding with the alge of the heart in insects, but not, like them, muscular. Except by 11 \ ' n \ b ijff I* f Fio. 74.— Astacus, Longitudinal Section.— I, II, III, Sterna of first, second, and third somites; oe, oesophagus; lb, labrum : I. metastoma ; 6-, membranous part of the stomach ; c, cardiac ossicle ; pt. pterocardiac ; uc, urocardiac; cl, lateral cardiac; p, cardio-pyloric valve ; pi, inferior pyloric valvular apparatus; m, anterior gastric muscle ; m*. insertion of posterior gastric muscles; pc, procephalic processes; h', opening of hepatic duct ; v, pyloric caecum ; i, k, intestine ; gn, testis ; gnf, gn" ', vas deferens ; C, heart ; ao, ophthalmic artery ; aa, antennary ; ah, hepatic ; as, sternal ; ap, superior abdominal artery ; b, cerebral ganglia ; sg, azygos visceral nerve, these ligaments, and by the arteries, which pass through it, the walls of the pericardial cavity, or blood sinus (for such it ASTACUS FLUVIATILIS. 283 really is), are wholly unconnected with the heart, which thus is, in a manner, suspended freely in the blood. Six apertures, two of which are superior, two inferior, and two lateral, provided with valves which open inward, allow the blood to enter the cavity of the heart during the diastole, and prevent its egress, except by the. arteries, dur- ing the systole. The arterial trunks are six in number, five being given off anteriorly, and the other from the posterior portion of the heart. Of the five anterior arteries, one, the ophthalmic, is single, and situated in the middle line ; it passes forward on the stomach to the head, where it supplies the eyes and anten- nules. The other arteries are in pairs ; two pass on the stomach forward and outward, giving off branches to the carapace, and eventually supplying the antennae ; the other two pass downward, between the anterior lobes of the geni- talia, and divide into a multitude of branches upon the he- patic casca. The posterior trunk, or sternal artery, is the largest of all, and presents a sort of bulbus arteriosus at its commence- ment. It turns almost directly downward, usuallv on the right side of the intestine, to the sternal canal, which it enters, passing between the antepenultimate and penultimate thoracic ganglia to the lower surface of the ganglionic cord ; it gives off two abdominal branches, one superior, close to its origin from the heart, which traverses the middle of the ter- gal region above the intestine, the other inferior, which takes a corresponding course along its sternal region beneath the nervous system. The arterial trunks are provided with valves at their commencement, so arranged as to prevent the regur- gitation of the blood. They ramify minutely, but how far a capillary system can be said to exist, is a question requiring further investigation. In transparent Zocza?, 1 have plainly observed the abrupt termination of the arterial trunks by open mouths, through which the blood was poured into wall- less lacuna?, and into the general cavity of the body ; nor can there be the least doubt that a similarly lacunar condition of the circulation exists in those lower adult Crustacea, the transparency of which allows of their examination with the requisite powers of the microscope. The probability is that a similar state of things obtains in the vascular system of all other Crustacea, and that, after undergoing a greater or less amount of subdivision, the arterial vessels, or their capillary continuations, cease to exist, the blood then making its way 284 THE ANATOMY OF INVERTEBRATED ANIMALS. into lacunas between the organs, and into the general peri- visceral cavity ; and, as in most 31olluscai ceasing to be con- tained in vessels with distinct walls. The blood thus poured out eventually makes its way into irregular sinuses or reservoirs, the chief of which, lodged in the sternal canal, communicates by lateral channels with oth- ers which lie above the bases of the thoracic appendages, and from which the afferent branchial canals pass into the stems of the branchiae, on the exterior faces of which they ascend, giving off branches to the lateral filaments. Corre- sponding canals return the blood from these filaments to the efferent branchial canals, which run down the inner side of the branchial stems, and unite above the bases of the limbs into six trunks, which ascend beneath the epimera and open into the sides of the pericardial sinus. The floor of this sinus is formed by a continuous membrane, which appears to shut it off completely from the general visceral cavity (at least it retains air or fluid thrown into it), and, if this be really the case, it may be said to be functionally a branchial auricle, containing pure unmixed aerated blood. The branchiae are eighteen in number upon each side, and are attached from the eighth to the fourteenth somites in- clusively. Six of these branchiae are attached to the epipo- dites of the eighth to the thirteenth somites, and differ very considerably in appearance from the other twelve. Each epipodite is, in fact, expanded at its upper extremity into a broad, bilobed membrane, which is folded upon itself, so that the two lobes are directed posteriorly, and receive the epipo- dite of the next limb (Fig. 71, N). The membrane of the lobes is obliquely plaited, so that, doubtless, they subserve respiration to a certain extent ; but, in addition, the anterior edge of the epipodite is beset with a number of branchial filaments, similar to those on the other branchiae. The latter (Fig. 71, 31, f, g) are simple plumes, consisting of a stem, to which are attached many delicate, cylindrical filaments. Two of these plumes are attached to the epimera and coxo-epimeral articular membranes of the ninth, tenth, eleventh, twelfth, and thirteenth somites. They increase in size posteriorly. The eighth and fourteenth somites, on the contrary, only carry one plume. A tuft of long byssus-like filaments is attached to the coxopodite of each of the last six thoracic appendages (Fig. 71, F, 31). The respiratory organs of the Crayfish, not being pro- vided with cilia, require some special arrangement for the ASTACUS FLUVIATILIS. 285 renewal of the water with which they are in contact. This object is attained principally by the action of the scapho- gnathite, which lies immediately behind the anterior opening of the branchial chamber ; and, during life, is incessantly in motion, baling out, as it were, the water which has become impure through the anterior opening, and thus compelling the flow of fresh fluid into the branchial chamber through its posterior and inferior opening, constituted by the space left between the lower edge of the branchiostegite and the bases of the limbs. The nervous system of Astacus x is composed of thirteen principal ganglionic masses, of which one, cerebral, lies in the head, in front of the mouth ; six, thoracic, are situated in the sternal canal ; and six, abdominal, lie in the median sternal region of the six anterior somites of the abdomen. The cerebral ganglia (Fig. 74, b ; Fig. 75, a) give off nerves to the eyes and to the muscles of the ophthalmic appendages ; to the antennules and the auditory organs which they contain ; to the antennas and the sac of the antennary gland ; to the carapace in front of the cervical suture ; and, finally, they send posteriorly two long and Fig. 75. —Visceral nerves of Astacus.— a, cerebral ganglia : b. commissures— that of the right side is cut and turned back ; c. transverse cord uniting them behind the oesophagus, OE ; d, d, d, azysros nerve ; h, ganglion; i, lateral branch of azygos, uniting with postero-lateral nerve g ; e, antero-lataral nerve ; /, medio- lateral nerve ; k, hepatic nerve ; P, pyloric ; C, cardiac po:tiou of stomach. stout commissural cords to the anterior thoracic ganglionic mass. These commissures are connected by a transverse 1 For the histology of the nervous system, see an elaborate essay by Haeckel on the minute structure of the tissues of the Crayfish in the ' Archiv fiir Anatomie," 1857. 286 THE ANATOMY OF INVERTEBRATED ANIMALS. cord immediately behind the oesophagus (Fig. 75, c). The size and form of the anterior thoracic ganglion would lead to a suspicion of the complex nature which development shows it to possess. It supplies the somites and their appendages from the fourth to the ninth inclusively, and sends forward delicate filaments to the oesophagus. Posteriorly it is connected with the ganglionic mass of the tenth somite by two commissures, and the other thoracic ganglia are similarly brought into communication, the com- missures of the ultimate and penultimate only being remark- able for their brevity. The abdominal, wmich are much smaller than the thoracic ganglia, are, w7ith the exception of the last two, united by single cords, which represent coa- lesced double commissures. Each of these ganglia supplies the muscles and the appendages of the somite to which it belongs, and the posterior abdominal ganglion sends branches into the telson. The Crayfish possesses a remarkably well-developed sys- tem of visceral or stomatogastric nerves, which has been the subject of special study by Brandt, Milne-Edwards, Krohn, and Schlemm, each of whom has described a larger or smaller portion of the system with accuracy, but has omitted to men- tion, or has denied, the existence of some other part. Each of the great commissures (Fig. 75, b), as it passes over the sides of the oesophagus, becomes slightly swollen, and from the enlargement four nerves arise ; one, external, passes toward the mandibular muscles ; a second postero-lateral branch (Fig. 75, g) runs upw7ard and backward to the infero- lateral regions of the stomach, and eventually enters into the composition of the hepatic nerve (k) ; a third branch (f) turns directly inward and upward, and unites upon the oesophagus wTith its fellow and with an azygos nerve (d), which passes up in the middle line of the anterior face of the oesophagus and stomach, and enters a ganglion placed be- tween the anterior gastric muscles (A), from whence a lateral branch is given off on each side, wThile a posterior median branch (d) continues the direction of the azygos nerve. Having reached the cardiac ossicle, this nerve divides into two branches (£), each of which passes downward and out- ward, unites with the postero-lateral nerve of its side, and thus forms the hepatic nerve (k). The fourth and last, or antero-lateral branch (e), descends at first to near the mouth, and then, curving forward, ascends to unite on the anterior face of the oesophagus with the anterior continuation of the ASTACUS FLUVIATILIS. 287 azygos nerve, which passes forward and upward and enters the cerebral mass. I am inclined to think that this part of the azygos nerve forms a portion of a fine plexus of nervous filaments which pass from the cerebral ganglia backward to the lining membrane of the carapace ; but the dissection of these fine filaments, and the demonstration of their conti- nuity, is a matter of no ordinary difficulty. The intestine is supplied by two nerves which aiise from the last abdominal ganglion, and unite into a single trunk, from which small branches are given off backward, and two principal ones forward, which supply the greater part of the intestine. According to Brandt, the genitalia receive branches of the fourth, fifth, and sixth thoracic ganglia. The only certainly known organs of sense in Astacus are the eyes and the auditory organs. The eyes are seated at the extremities of the ophthalmic peduncles, the integument of the outer extremity of which becomes translucent over a reniform space, and constitutes the corneal membrane. This membrane is divided into a great number of minute quadri- lateral facets, each of which corresponds with the base of a crystalline cone.1 The upper face of the trihedral, proximal, and largest joint of the antennule presents an oval space, covered by a broad brush of complex hairs having their points all directed inward. On cutting these hairs away close to their bases, however, it is seen that they cover an aperture, wider above than below, and about one-sixteenth of an inch long. The hairs are attached to the outer lip of this aperture, and some are directed so as to lie within the inner lip, but the majority cover it. A good-sized bristle passes with great ease into this aperture, and if the inner and outer walls of the basal joint of the antennule be now removed, and the soft parts carefully dissected away, the end of the bristle will be seen to have passed into a wide delicate sac about one-twelfth of an inch long, which is attached by a narrower neck round the aperture, the lips of which are continuous writh its walls. The sac is filled with minute sandy particles, suspended in a mucous, dirty-looking fluid, and when emptied of these con- tents a band, consisting of several lines of very fine hairs, like those which guard the mouth of the sac, but more deli- 1 Mr. E. T. Newton's careful description of the eye of the Lohster in TJie Quarterly Journal of Microscopical Science for 187o, to which I have referred above, may be takeu as a guide to the study of the minute structure of the eye in the Cravfish. 19 288 THE ANATOMY OF INVERTEBRATED ANIMALS. cate, is seen to skirt its inner contour. The hairs, projecting inward, come into close contact with the solid particles sus- pended in the mucous fluid. A nerve may be traced accompanying the antennulary nerve to the sac, and appears to be distributed principally along the setigerous band, so that the extremities of the nerve fibrils come into close relation with the bases of the hairs. Some, if not all, of the sandy particles are insoluble in strong acetic acid, and would appear to be siliceous.1 Two glandular sacs commonly known as the green glands, which were formerly regarded as the auditory organs, lie in the cavity of the head. An aperture is visible on the inner or oral side of a conical prominence, upon the inferior portion of the coxal joint of the antenna. A bristle passed into this aperture enters a large but very delicate and transparent sac, filled with a clear fluid, wThich is usually conspicuous on each side of the anterior end of the stomach, when the cara- pace is carefully removed. A nerve which comes off from the cerebral mass close to the antennary nerve, passes to the neck of this vesicle, and is distributed over its surface be- tween the outer and inner membranes, of which it is_ com- posed. Inferiorly the vesicle rests upon a large greenish, ap- parently glandular mass, but is directly connected with the latter only at two points, firstly by a vascular cord, which passes to the central and usually more yellow portion of the gland, and secondly by a short neck-like continuation of the sac itself, which is attached over a small circular space, mid- way between the centre and the periphery of the gland, and opens into the circular principal duct of the gland. There is, therefore, a free communication between the cavity of the gland and the exterior by means of the sac, which is, in this respect, simply a dilated duct. A section of the gland shows it to be composed of two subtances, a central and a cortical. The latter is composed of minute caeca, filled with a homo- geneous gelatinous matter, containing many large nuclei ; the former is traversed in all directions by large canals, so as to have a spongy appearance. The casca open into the ulti- mate ramifications of the canals, and the spongy, lung-like texture of the central mass seems to arise merely from the very free anastomosis of their larger branches, which event- 1 See, for a full account of the minute structure of the auditory organs in the higher Crustacea, Henseu's " Studien iiber das Gehororgan der Deca- poden," 1863. ASTACUS FLUVIATILIS. 289 ually enter the circular canal which communicates with the vesicle. There is little in these structural features to suggest an organ of special sensation, but much to show that the green mass is a secreting organ, and that the vesicle acts (whatever other purposes it may subserve) as its duct. In all proba- bility the green gland is an organ of the same nature as the shell gland of the Entomostraca. Leydig has attributed an olfactory function to certain groups of delicate setae which occur on the joints of the outer division of the antennule of the Crayfish. The most remarkable part of the muscular system of the Crayfish is the great extensor muscle of the abdomen, a com- plex mass of fibres which is attached in part to the endo- phragms of the thorax in front, and, behind, to the sterna of the abdominal somites, a large part of the cavity of which it occupies.1 The essential parts of the reproductive organs in the male and female Astacus are very similar to one another in form, both ovarium and testis having the figure of a trilobed gland, situated immediately behind the stomach, and below the heart. Two of the lobes are applied together, and pass for- ward ; the other lobe is directed in the middle line back- ward. The ducts take their origin, one on each side, at the junction of each antero-lateral with the posterior lobe. In minute structure, however, the two organs differ widely. Each lobe of the testis is composed of a number of small caeca, in which the spermatozoa are developed, and which open into a central duct. The ovarium, on the other hand, is essentially a wide sac, produced into three large caeca, each of which corresponds with a lobe ; and the ova are developed in the epithelial lining of the sac. The efferent ducts, again, have little resemblance, the oviducts being short, wide tubes which open on the coxopodites of the antepenultimate thora- cic appendages, while the vasa deferentia are canals as long as the body, at first very narrow, but afterward widening, which lie coiled up on either side of the posterior part of the thoracic cavity, where their white contents make them very conspicuous (Fig. 74, gn'). Eventually, they open on the coxopodites of the posterior thoracic appendages. The spermatozoa, like those of many other Crustacea, are 1 For details, see Suclcow, " Anatomisch-physiolofrische Untersuchuneren." Milne-Edwards has described the muscles of the Lobster at length in the " His* toire naturelle des Crustacea," torn. i. 290 THE ANATOMY OF INVERTEBRATED ANIMALS. motionless, and have the form of cells, provided with, a nu- cleus and produced into several delicate radiating processes. They are united in their course down the vas deferens into cylindrical masses, which, becoming invested by a fine mem- branous coat, probably secreted by the walls of that duct, constitute the spermatophores, which may not unfrequently be found adhering to different parts of the body, not only of female but of male Crayfish. The ova are fecundated while still within the parent ; they become surrounded, in their passage down the oviduct, by a coat corresponding with that of the spermatophore, which is produced into a pedicle, the extremity of which becomes at- tached to one or other of the abdominal appendages. Great numbers of ova, attached in this way, may be observed, dur- ing the breeding-season, within the incubatory chamber formed by the flexure of the abdomen upon itself; and it is in this cavity that the embryos pass through the whole of their fcetal existence. The development of the Crayfish has been the subject of one of the most beautiful of the many admirable memoirs on development, for wdiich we are indebted to the genius and patience of Rathke.1 After fecundation a blastoderm arises upon the surface of the yelk, and, gradually extending over the whole yelk, becomes thickened at one part, so as to form an oval germinal disk, with a central depression. This disk next becomes widened and bilobed at its ante- rior extremity, the lobes being identical with the procephalic lobes, to be hereafter described in the embryo of 3Iysis. The edges of the disk are raised into a fold, and within the fold a papilla, the rudiment of the abdomen, and of the greater part if not of the whole of the thorax, makes its appearance ; while, anteriorly, three pairs of transverse elevations constitute the rudiments of the antennules, the antenna?, and the mandibles. The labrum arises as a median papilla, situated at first be- tween the antennules. The ocular peduncles are next devel- oped in front of the antennules as ridges, which only subse- quently become free processes. The thoracico-abdominal process lengthens, and the anal aperture makes its appearance. It is to be remarked that the anus is at first situated on the dorsal side of the extremity 1 "TJeber die Bildung und Entwickelung des Flusskrebses," Bd. 29. See also Lereboullet, "Recherches d'Embryologie comparee sur le Developpement du B rochet de la Perche et de l'Ecrevisse," 1862 ; and the account of Bobret- sky's researches in Hofmann and Schwalbe, " Jahresbericht " for 1873 (1875). THE DEVELOPMENT OF ASTACUS. 291 of the abdomen, and that there is no telson. This is devel- oped only at a much later period from the dorsum of the end of the abdomen, and, by its outgrowth, forces the anus to the ventral side of the body. In the mean while, the oral aperture is developed behind the labrum, which moves backward ; while the maxilhe, max- illipedes, and ambulatory feet appear in succession as eleva- tions or ridges of the substance of the embryo, which are, at first, all alike, and gradually become specialized into their ultimate forms. When these appendages first appear, the maxillse and first pair of maxillipedes are attached to the embryo in front of the thoracico-abdominal process, the second maxillipedes lie in the angle between them, and the third maxillipedes and following appendages are attached to the sternal surface of the thoracico-abdominal process itself; and, as this process is at first bent forward upon the rest of the germ, it follows that the appendages attached to it look upward, while those at- tached to the anterior part of the embryo look downward. As development proceeds, however, the embryo gradually straightens itself, more and more of the anterior part of the thoracico-abdominal process becoming continuous in direction with the anterior part of the embryo ; until, at length, the whole of the cephalo-thoracic portion forms a convex surface, parallel with the vitellary membrane, only the abdomem re- maining bent upon the cephalo-thorax. The middle portion of the carapace is formed by the continuous calcification of the dorsal walls of the cephalo-thorax of the embryo. Its pleura are developed as two distinct folds, one of which, the rudiment of the branchiostegite, encircles the embryo poste- riorly, and extends forward on each side as far as the mandi- bles ; while the other, the rudiment of the rostrum, and an- terior cephalic pleura, is developed in front of the eyes, and extends on each side to meet the former. Rathke's clear ac- count of this matter is in perfect accordance with what I have observed in 3ft/sis, and shows conclusively that the carapace is not developed from any one or two somites in particular, but that its tergal portion corresponds with, and is formed by, the terga of all the cephalo-thoracic somites, while the bran- chiostegites and rostrum are developments of the lateral por- tions of all these somites; in fact, represent their pleura, which, like the terga, are connate and continuously calcified. The appendages are thus, at first, similar to one another, and each consists of a ridge which eventually takes the form 292 THE AXATOMY OF INVERTEBRATED ANIMALS. of a plate, free at the outer end. This plate, in all the mem- bers, except the ophthalmic peduncles and the mandibles, then becomes bilobed externally, the inner lobe representing the endopodite, while the outer is the representative of the exopodite and epipodite. The two latter, when they are in- dependently developed, become separated by the division of the outer lobe. The gills arise partly as outgrowths from the epipodites, partly as distinct processes from the parts to which they are eventually attached. The division of the limbs into articulations takes place from their distal toward their proximal ends. The heart appears late, at the posterior extremity of the cephalo-thorax, and therefore behind the yelk-sac. The nervous system of the post-oral portion of the ceph- alo-thorax consists at first of eleven pairs of ganglia, cor- responding with the mandibles, maxilla?, maxillipedes, and ambulatory legs. The six anterior post-oral ganglia of each side soon coalesce in pairs, so as to' form as many single gan- glia ; and of these the four anterior, namely, the mandibular, the two maxillary, and the first maxillipedary ganglia, unite into a single mass ; the two hinder ganglia, that is to say, those of the second maxillipedary somite, next coalesce in the same way, and it is only subsequently that the two masses thus formed become fused into the single anterior post-oral ganglion of the adult. The other ganglia not only remain separate, but become wider apart with advancing age. A ridge on each side of the oesophagus at first represents the cerebral ganglion and the commissural cords, the latter being developed out of the posterior part of the ridge, and the for- mer from its anterior portion. The cerebral ganglia are at first two on each side, but the posterior, whence the nerves to the antennary organs proceed, is much larger than the other, and would appear to represent two ganglia. The endoster- nites arise as processes from each of the eight posterior ceph- alo-thoracic sterna, which eventually arch over the gangli- onic cord, and unite with one another. The alimentary canal is produced by the gradual differen- tiation and demarcation of the sternal part of the hypoblast, which invests the whole yelk, from the tergal part, which be- comes the velk-sac.1 1 According to Bobretsky (7. c.) there is no proper yelk-sac, the structure so termed by Rathke being the saccular hypoblast, which is formed by invagina- tion of the primitive blastoderm, and encroaches upon the vitellus, until the latter is all absorhed. The hypoplastic sac is converted into the liver and the intestine. The stomach arises independently by invagination of the epiblast. THE PODCPHTHALMIA. 293 After the liver, genitalia, and antennary glands are de- veloped, the yelk-sac eventually becomes reduced to a small caecal diverticulum, situated at the pyloric end of the stom- ach. The genital ducts in both males aud females are origi- nally diverticula from the corresponding regions of the geni- tal glands ; their external apertures and the copulatory ap- pendages of the first abdominal somites in the male are not developed until some time after birth. The modifications of structure observable within the limits of the Podophthalmia are exceedingly interesting. Excluding, for the present, the SquiUidce, the group is divisible on clear morphological grounds into the following subdivisions : 1. The Brachyura ; 2. The Anomura / 3. The Macrura / 4. The Schizopoda. The morphological relations of the Macrura are nearly such as are indicated by their position in this series ; and Astacus, as a central genus of the central group, thus be- comes a sort of natural centre for the whole of the Podoph- thalmia, whence we may trace a gradual series of modifica- tions, leading on the one hand to the Schizopoda, with their large abdomen and small cephalc-thorax ; and on the other to the Brachyura, with their rudimentary abdomen and com- paratively enormous cephalo-thorax. In all the Macrura the branchiae are numerous, and are covered by the branchiostegites. The abdomen is large, and is used as a locomotive organ, the appendages of its sixth somite being well developed. The thoracic ganglia usually form an elongated chain, and the external maxillipedes never form broad opercular plates over the other jaws. In some of the lower Macrura (Pe?zeus, Pasiphcm) the exopodite per- sists as an appendage at the base of the thoracic limbs ; and in two genera, Sergestes and Acetes, the posterior thoracic members become rudimentary, or even entirely abortive, though the abdominal appendages remain. In the higher Macrura, such as Pali mints, the nervous system exhibits a greater degree of concentration, the tho- racic ganglia constituting an elongated oval mass ; and it is in this genus and its allies that the head and its appendages exhibit modifications, which prepare us for those which are presented by the Brachyura. In this respect the Palinurus vulgaris (Rock Lobster, Sea Crayfish, or Spiny Lobster) is particularly worthy of attention. The rostrum is rudimentary and represented by a mere spine, leaving the anterior cephalic 294 THE ANATOMY OF INVERTEBRATED ANIMALS. somites uncovered. The cephalic flexure is so strong as to throw the ophthalmic sternum, which is very wide, completely to the top of the head. The basal joints of the antennae, or coxocerites, are enormous, fixed to the surrounding parts, and united by their anterior extremities in the middle line below. Superiorly, they seemed to have coalesced with the antennu- lary sternum, so as to form a projecting wedge-shaped mass, which separates the antennules from the ophthalmic sternum, and causes them to appear, at first, as if they were inferior to the antennae. In this genus, the basicerite, ischiocerite, and merocerite are much thicker and stronger than the cor- responding joints of any of the other appendages ; and in the closely allied Scyllarus, the facial region of which is, on the whole, similarly constructed, these joints become extremely expanded and flattened, and are succeeded by no procerite. In these genera the scaphocerite, or squame, usually attached to the base of the antenna, is absent ; and, in Scyllarus, there is another approximation to Brachvuran structure in the ex- istence of distinct orbits, formed by a lobe of the carapace, which descends on the inner side of the ocular peduncle, to meet the base of the antenna. No median septum is formed by the rostrum, however, nor are the antennules capable of being folded back into distinct chambers in any Macruran at present known. The Anomura are so completely intermediate in structure between the Macrura and the Brachyura, that they need not be specially noticed, except to draw attention to the singular deviation from the ordinary habits and form of the higher Crustaceans, presented by the Paguridw, or Hermit Crabs, so common on all coasts. Essentially Macruran in their or- ganization, these Crustacea are distinguished from all true Macrura by the uncalcified and soft condition of the integu- ment of their unsymmetrical abdomen, the appendages of which are for the most part abortive, those of the sixth somite being modified so as to serve as claspers. It is by means of these that the Hermit Crab retains firm hold of the columella of the empty gasteropod shell into which it is his habit to thrust his unprotected abdomen, and, covering over his re- tracted body with the enlarged chela, which takes the place of an operculum, resists all attempts at forcible extraction. The internal structure of the Brachyura is, on the whole, similar to that of the Macrura ; but the thoracic ganglia have coalesced to a much greater extent than in Astacus, forming a single rounded mass. The branchiae are few, never exceed- THE BRACHYURA. 295 inof nine on each side, and sometimes not more than seven. The branchiostegite fits closely down upon the bases of the four posterior pairs of thoracic limbs, and sometimes incloses a space which is very large in proportion to the branchiae This is particularly the case in the Land Crabs (Gecarcinns), where the spacious branchial chamber is lined by a thick and vascular membrane, which, in these almost wholly terrestrial 7 7 J Crustacea, either takes on to some extent the respiratory function, or serves to keep the air within the branchial cham- ber saturated with moisture. The abdomen in the Brachyura is comparatively small ; its sixth somite possesses no appendages ; and the others, if they exist at all, subserve only a sexual purpose, the two an- terior pairs commonly forming accessory copulatory organs in the male ; while, in the female, so many of these append- ages as remain give attachment to the ova, which are carried about until hatched, between the thorax and the abdomen, which is bent up against it. The female Brachyura also pos- sess a spermatheca attached to each oviduct, which is absent in the Macrura / and, in this sex, the abdomen is larger and broader than in the males. In accordance with the rudimen- tary condition of this part of the body, the abdominal gan- glia are represented only by a cord, which proceeds from the posterior part of the great thoracic mass. It is in the con- struction of their skeleton, however, that the Brachyura present the most interesting deviations from the Macrura. Thus, if we select the common Shore-crab, Carcinus moenas (Fig. 76), as a typical example of a Brachyuran, we find that the carapace is a wide shield, broader than long, having a somewhat pentagonal shape, and bent sharply inward at the sides, instead of taking an even sweep down to the base of the legs. It is in such close contact with the four posterior pairs of thoracic limbs as to leave no passage or aperture such as exists in Astacus, the only inlet for the water required for respiration being placed above the basal joints of the che- late anterior ambulatory limbs. The edges of the carapace pass completely in front of the basis of the limbs, and then turn sud- denly forward, parallel with one another and with the axis of the body, as the pterygostomial plates of Milne-Edwards, to join the antennary sternum, which is very wide, but short from before backward. The space included between the edges of the pterygostomial plates and the antennary sternum is the "cadre buccal," or peristome; the antennary sternum itself receives, as in the Astacus, the specific appellation of epi- 296 THE ANATOMY OF INVERTEBRATED ANIMALS. stoma ; and the plate which stretches backward and supports the labrum, within its posterior forked boundary, is the en- do stoma. The middle of the dorsal surface of the carapace is marked somewhat nearer its posterior than its anterior boundary by a short transverse depression, which is continued on each side forward and outward, and then curves directly outward to the edge of the carapace (Fig. ?6, cs). Further than this I cannot trace this homologue of the cervical groove of Astacus. Fig. 76.— Of the two upper figures, the left represents the dorsal surface of the cara- pace of Carcinus mcenas : f, rostrum; o, orbit; cs. cervical groove; g1, epigastric lobe; gr2, protogaetric ; <73, nasogastric ; g*, hypogastric: g5, urogastric; c, e1, an- terior and posterior cardiac; h, hepatic; b1, ft3, b3. epibranchial, mesobranchial and metabranchial lobes. The lower figure represents a ventral view of the an- terior half of the same carapace: a, rostral septum; b. autennary sternum ; c, suture between these; d, supraciliary lobe; e. internal suborbital1 lobe ; /, anten- na; g, articular cavity for the ophthalmic peduncle ; h, the same for the anten- nule; o, orbit ; eh, subhepatic region; ep, anterior pleural region. The riglit- hand upper figure gives a side-view of the carapace of Stenorhyhchiis phalangiiim, the common "spider-crab:" o. orbit ; /',/2, rostrum ; al, autennule ; at, antenna ; ep, epistoma. Elevations and depressions upon the surface of the carapace in front of the cervical groove, which, as in Astacus, is com- posed of the connate terga of the six cephalic somites, mark THE BRACHYURA. 297 it out into certain definite regions of considerable systematic importance. An irregular transverse depression, crossing the carapace near the anterior margin, bounds an anterior or fa- cial region, divided into a middle frontal lobe (/), and lateral orbital lobes (o), from a posterior, much larger, gastro-hepatic area, divided into small lateral hepatic lobes (/*), and a large complex gastric lobe (g1, g*, etc.). The latter is again sub- divided into two epigastric lobes (g1), two protogastric lobes (g2), a median mesogastric lobe (g*), two metagastric lobes (#4), and two urogastric lobes (gb), making altogether nine subordinate divisions. The gastric lobes correspond in a gen- eral way to the stomach ; the hepatic lobes, to a portion of the liver. The region behind the cervical suture consists of the connate terga of the eight thoracic somites ; it is divided by two strong longitudinal grooves, the branchio-cardiac grooves, into a middle region, corresponding with the heart, and two lateral regions, forming the roof of the branchial chamber. A transverse depression divides the middle region into an anterior and a posterior cardiac lobe, while the bran- chial region is subdivided into epibranchial (51), mesobran- chial (b2), and met abranchial (b*) lobes. On turning to the inflected inferior portion of the cara- pace, a sutural line or groove is seen running from the epi- stoma, outward and backward, very nearly reaching the outer edge of the carapace, opposite its external angle, and then sweeping backward parallel with, and but little distant from, its postero-lateral boundary, until it cuts its posterior edge. The portion of the carapace internal to this sutural line is called by Milne-Edwards the inferior branchiostegite, and is considered by him to be composed of an anterior (ep) and posterior epimeral piece, corresponding with the subhepatic (sh) and subbranchial regions of the surface of the carapace between the suture and the line of inflection. I cannot regard these parts, however, as having any relation with the true epimera. The suture, or rather groove, seems rather to correspond with that which marks off the pleuron from the rest of the somite in Astacus. The anterior cephalic somites in Carcinus have under- gone some singular modifications, whereby their true relations are greatly obscured. The broad trilobed plate (Fig. 76,/') corresponds with the elongated rostrum of Astacus ; inferiorly it is produced in the median line into a strong ridge or sep- tum, the lower and posterior edge of which is convex, and fits closely into the concavity formed by the antennulary and 298 THE ANATOMY OF IXVERTEBRATED ANIMALS. ophthalmic sterna, as they bend back from the sternal flex- ure. This rostral septum, therefore, abuts below and behind on the epistoma, and constitutes a sort of partition (Fig. 76, a), by which the cavities in which the antennules and eyes of the two sides are lodged are completely separated from one another. The lateral portions of the rostrum form a flattened roof over the inner portions of these cavities, which contain the bases of the ophthalmic peduncles and the antennules ; but the outer angles of the rostrum are produced downward ((/), to form the supraciliary lobe. The outer half of the lateral cavities or chambers is more excavated, and is bound- ed U37 a strong pointed process, the external orbitar lobe, which is divisible into a supraorbital and suborbital portion. The latter passes gradually into a strong process of the sub- hepatic region, called the internal suborbitar lobe (Fig. 76, e) ; this turns forward and upward toward the supraciliary lobe, which it approaches, but does not meet, the base of the antenna being, as it were, wedged between the two. The supraciliary, external orbitar, and internal suborbi- tar lobes, and the antennas, thus together circumscribe a cavity widely open in front, which is called the orbit, inas- much as it lodges the terminal portion of the ophthalmic peduncles, with the eyes which they support. The proximal portions of the peduncles pass through the comparatively narrow opening by which the inner and outer chambers com- municate, between the antenna and the supraciliary process, and are inserted as usual into the articular cavities on each side of the ophthalmic sternum, which is narrow, and hardly wider than the septum. It thus comes to pass that the eyes, lodged in their orbits, appear to be altogether external to the antennules, the enlarged bases of which hide the oph- thalmic peduncles, and appear to be the sole contents of the inner division of the subfrontal chamber ; but the true posi- tion of the eyes is precisely the same as in Astacus, that is to say, anterior and superior to the antennules. Another interesting peculiarity about the facial region of the cara- pace is that the basal joints of the antennas have coalesced with the sternum of the antennary somite, and, consequent- ly, that the bases of the antennas are immovable. There is no vestige of a scaphocerite, and the aperture of the organ which answers to the green gland of Astacus is provided with a peculiar movable plate, provided with a projecting internal stem, to which delicate muscles are attached in Car- emus. It is this structure which has been compared to an THE BRACHYURA. 299 auditory ossicle ; but, as in Astacus, the auditory sacs are, in fact, lodged in the dilated basal joint of the antennule. A cervical fold, lodging the scaphognathite, occupies the same relative position as in Astacus, and marks off the cephalic form of the thoracic region, on the sides of the body. Tbe thoracic sterna gradually increase in breadth, and the posterior ones are marked externally by a strong median, longitudinal depression, answering to a corresponding fold on the inner surface. The apodemal cells are well formed, but the sternal canal, so largely developed in the Macrura, is absent in this, as in all other Brachyura. The structure of the appendages is essentially the same as in Astacus, but the third thoracic appendage, or external maxillipede, has its ischiopodite and meropodite greatly en- larged, so as to form a broad plate, which, with its fellow, covers over the other organs, and hence receives the name of the gnathostegite. The three terminal joints of the limb re- main small, and constitute a palpiform appendage — the en- dognathal palp. In some of the lower Macrura the thoracic limbs are pro- vided with a short exopodite, and the posterior maxillipedes become undistinguishable from the ordinary thoracic limbs. Such forms lead us naturally to the Schizopoda, a group the name of which is derived from the apparent splitting of the limbs produced by the great development of the exopodite, which, in these Crustacea, is as large as the endopodite. In this group, again, a line can hardly be drawn, in many cases, between any of the maxillipedes and the thoracic limbs, the anterior pair only being somewhat smaller than the rest. Hence Thysanopoda is admitted, by Milne-Edwards, to have eight pairs of thoracic limbs (" Crustaces," ii. 464). The branchiae in the Schizopoda are frequently absent ; when well developed, as in Thyscoiopoda, they are not included under the branchiostegite, but hang down freely from the bases of the thoracic limbs. In 3Iysis, the only represen- tative of a branchia (if it be one in reality) is a process at- tached to the first thoracic appendage. Cynthia has its branchial appendages attached to the abdominal members. In Thysanopoda, My sis, and Cynthia, the general struct- ure of the body is similar to that of the Macrura, except that, in Mysis, the greater number of the abdominal append- ages are rudimentary. In Leucifer, the antennary somite is produced into a very long and narrow peduncle, which supports the eyes, on their 300 THE ANATOMY OF INVERTEBRATED ANIMALS. great stalks, the antennules, and the antenna?, at its extrem- ity, separating them from the rest of the cephalo-thorax, which is covered by a delicate carapace, bent down at the sides. The anterior thoracic members are rudimentary, and the posterior pair is absent. The heart is short and rounded, and situated, as usual, in the thorax. It has been seen that in Astacus fluviatilis, as in Limu- lus and Daphnia, the embryo slowly and gradually pass3s into the form of the adult ; to which it is so similar when it leaves the egg, that the changes of the young present noth- ing comparable to the well-known metamorphoses of Butter- flies and Beetles. But most Podophthalmia rather resemble the Copepoda and the majority of the PJntomostraca, in the fact that the young, when they leave the egg, have a totally dissimilar form to that of the parent, and only acquire the adult con- dition after a series of ecdyses. The observations of Fritz Muller ' have shown that the Fig. n.—Ptneus.—A, Nauplius-st&zQ. B, Zocea or Copepod stage. C, Schizopod- stage. (After Muller.) young of a species of Prawn (Peneus) undergo a metamor- phosis which runs parallel with that of the Copepoda. When it leaves the egg (Fig. 77, A), the young Peneus has an » " Fur Darwin," 1864. THE DEVELOPMENT OF THE PODOPHTHALMIA. 301 oval, unsegmented body with a single frontal eye, a large labrum, and three pairs of natatorial appendages — it is in fact, to all intents and purposes, a Nauplius. The Nauplius-ioun next develops a rounded tergal shield, or carapace ; the first and second pairs of appendages, remaining, long, become the antennules and the antennae ; while those of the third pair, their bases enlarging at the expense of the rest of the append- age, become the mandibles. Four pairs of appendages sub- sequently appear behind the mandibles. The hinder three pairs are bifurcated and become the two pairs of maxillae and the first and second maxillipedes. Behind these again are five pairs of short lamellar processes, which eventually are converted into the rest of the thoracic appendages. The six somites of the abdomen are long and distinct, and the last ends in two setose processes. They are at first without ap- pendages. In this stage (Fig. 77, P), which answers to the so-called Zocea-iovm of other Podophthalmia^ the principal locomotive organs are the antennae and antennules, and the resemblance to an adult Copepod is so striking that it may be termed the Coiiepod-st&ge. Next, the antennae, diminish- ing in relation to the rest of the body, cease to be the prin- cipal organs of locomotion, and the rapidly-elongating abdo- men assumes that function. The stalked double eyes, which made their appearance in the Copepod-stage, become more fully developed. The jointed exopodite of the antenna is re- placed by a single plate. The greatly-enlarged thoracic limbs are provided with an endopodite and an exopodite, as in the Schizopoda, the branchiae are developed from them, and the abdominal appendages make their appearance. This may be termed the Schizo2Jod-sta.ge (Fig. 77, C). Lastly, the me- dian eye vanishes, the exopodite of the locomotive thoracic limbs disappears, and the larva assumes all the characters of the adult Peneus. In the great majority of the Podophthalmia the embryo undergoes as remarkable a metamorphosis after it leaves the egg. This fact was first indicated by Siebold, afterward demonstrated by Vaughan Thompson, whose observations have been confirmed and extended by many more recent ob- servers, notably by Spence Bate1 and Claus.2 But the stages of this metamorphosis differ from those observed in Peneus in 1 " On the Development of Decapod Crustacea." (Philosophical Transac- tions, 1857.) 2 " Zur Kenntniss cler Malakostracenlarven." ("Wurzburg " Natunvissen- schaftliche Zeitschrift," 1861.) 302 THE ANATOMY OF INVEKTEBRATED ANIMALS. the apparent absence of the first or Naupliw condition. Pos- sibly, however, this is represented by a delicate cuticular in- vestment which the larva throws off soon after leaving the egg. It then corresponds with the later form of the Copepod stage of Pe?ieus, and is termed a Zocea. The Zocea has a short carapace, often provided in the median frontal and dor- sal regions with long spine-like prolongations. There is a median simple eye between the lateral sessile faceted eyes, a pair of antennules, a pair of antennae, a pair of mandibles, Fig. 78.— Development of Carcinus mcenas.—A, Zocea-stage. Final state. (After Couch.) B, 3fegalopa-Bt&ge. (7, and two pairs of maxillae ; in short, all the appendages of the head. Of the appendages of the thorax, the first two pairs are well developed, and terminate in an exopodite and an en- dopodite. But behind these, which become the first and the second pair of maxillipedes, only short rudiments of the six remaining pairs of thoracic appendages are to be found, and the somites of the long abdomen have no appendages at all. Subsequently these make their appearance, the posterior tho- racic members increase in size, the eyes become raised upon short peduncles, and the larva resembles one of the lower Macrura. The carapace next becomes broader, and its spines shorter, while the ambulatory thoracic limbs take on the characters of those of the adult, the bifurcated first and sec- ond pairs becoming metamorphosed into the first and second maxillipedes. The abdomen becomes relatively short and THE DEVELOPMENT OF MYSIS. 303 slender, and the larva takes on the characters of one of the Anomura. In this stage it has been named 3Iegalopa. By further changes in the same direction, the Anomuran con- dition passes into that of the young Brachyuran. All these modifications of form are accompanied by exuviations of the chitinous cuticula. The successive stages are well exemplified by the young of the Shore-crab, Carcinus moenas (Fig. 78, A, B, C). The larva, on leaving the egg, has sessile eyes, a long pointed rostrum, and a spine projecting from the middle of the cara- pace ; rudimentary antennae, and two pairs of locomotive ap- pendages— the rudiments of the anterior maxillipedes. The abdominal somites are without appendages, and the telson is broad and bilobed (Fig. 78, A). This, the Zocea-sta,ge, after repeated ecdyses, assumes the Megalopa form represented in Fig. 78, B. Finally, the car- apace becomes broader, the abdomen loses its appendages, and is bent up under the thorax ; the peculiarities of the fa- cial region, characteristic of the Brachyura, are developed ; the antennules and ambulatory members acquire their char- acteristic proportions ; and the little Brachyuran by degrees assumes the special peculiarities of Carcinus (Fig. 78, CT). The development of the Opossum Shrimp (Jfysis)1 is par- ticularly interesting, as it appears to indicate the relations between the two modes of development, that with and that without metamorphosis, which obtain in the Crustacea (Fig. 79). The ova consist of a vitelline mass, inclosed within a deli- cate chorion. The blastoderm appears as an oval patch upon the surface of the yelk (Fig. 79, A, c), thickest in the middle, and here presenting a more or less marked depression (Fig. 79, A, B, c). It is sharply defined from the subjacent yelk (b), and consists of a finely granular mass, in which mul- titudes of nuclei, about 18100 to ^qVo of an inch in diameter, are imbedded. The blastoderm next becomes larger at one end than at the other, and a median sinuation gradually divides this ex- tremity into two lobes, which will eventually form the ante- rior parietes of the head, and may be called the proceplialic lobes? 1 Conf. E. van Beneden, " Developperaent des Mvsis." (" Bulletin de l'Academie de Bruxelles," I860.) 2 It is exceedingly interesting to remark the correspomlei ce between the embryonic structure of the head of Mysis (and I may add that of other Artfiro 20 304 THE ANATOMY OF IXVERTEBRATED ANIMALS. The median depression becomes more decided, and, at the end opposite the procephalic lobes, the blastoderm is produced into a sort of papilla, directed forward. This is the rudiment of the caudal extremity. From the anterior part of the blastoderm there arise, on each side, two papillae, the points of which are directed backward, and which will become the antetmules and antenna?. The whole of these parts are in- vested by a delicate cuticular membrane, which gradually ex- tends over and invests the whole yelk beneath the vitellary membrane. At the end of the caudal papilla it forms a broad process, produced into setae, which sometimes appear fan- like, sometimes so deeply bifid as to resemble two styles. The embryo has now reached what we may term its larval stage, and, in this condition, it leaves the vitellary membrane within which it was inclosed, and lies free in the ovigerous pouch of the parent. At the same time, the caudal extremity enlarges, and straightens itself out, so that no indication of its previous inflexion against the thoracic portion of the blas- toderm remains. The larva thus much resembles a pear (Fig. 79, D, E), with four processes (2, 3), the antennules and antennae, which have now become much elongated, on one surface. The young 3Iysis next grows rapidly and undergoes great changes in form : but it is a very remarkable fact, that the primitive integument remains unaltered ; gradually enlarging, to accommodate itself to the increased size of the fcetus, in- Fig. 79. poda) and that of the head of a vertebrate embryo. The procephalic pro- cesses resemble in a remarkable manner the trabecules cranii of the vertebrate embryo ; and the cephalic flexure of the Crustacean or Insect has its analogue, if not its homologue, in the angle which the trabecular region of the base of the skull at first makes with the parachordal region in almost all Vertebrata. THE DEVELOPMENT OF MYSIS. 305 Fig. 79.— Continued. 14&, the last cephalic and first and second thoracic limbs. guish the general course of the development of Mysis from that of Astacus. On the other hand, another Schizopod, Ewphausia, has been shown by Metschnikoff ' to leave the egg as a true Nauplius. 1 Zeitschrift fur wlss. Zool., 1871. 308 THE ANATOMY OF INVERTEBRATED ANIMALS. The Glass-crabs, or Phyllosomata (Fig. 80), are singular marine pelagic Crustacea, in which the body consists almost wholly of two large, extremely flat and transparent disks, devoid of any segmentation. The anterior of these bears the pedunculated eyes, the antennules and the antennae on its anterior margin ; while the labrum, with the mandibles and anterior pair of maxillae, form a small projection poste- riorly on its ventral surface. The second pair of maxillae is situated a little more backward and outward, and bears a scaphognathite ; and just behind these" appendages is the fold of a cervical groove which separates the anterior disk from the posterior. The anterior disk contains the stomach and the liver, and in this respect, as in its, appendages, cor- responds exactly with the cephalostegite of the carapace of an ordinary Crustacean, and its six cephalic sterna. The pos- terior disk, on the other hand, contains the short and almost round heart, with the intestine, and bears the eight pairs of thoracic appendages, the anterior and posterior of which are not uncommonly rudimentary. The abdomen is usually very small, and situated in a notch at the posterior edge of the thoracic disk. It is provided with six pairs of appendages. No generative organs have been found in the Phyllosomata, and there is reason to believe that they are merely larvae of the Macruran genera Palinurus, Scyllarus, Themis, and their allies. The Cumacea. — These are very remarkable forms, allied to the Schizopoda and Nebalia, on the one hand, and on the other to the JEdriophthalmia and Copepoda y while they ap- pear, in many respects, to represent persistent larvae of the higher Crustacea. Cuma Patlikii might, at first, be readily mistaken for a Copepod. It possesses a comparatively small, thick carapace, apparently produced into a rostrum anteriorly, and succeeded by a series of twelve gradually narrowing free segments, the appendages of which are in great part obsolete. The last of these segments is a pointed telson ; the anterior five, belong- ing to the thorax, bear thoracic limbs, while the eleventh, the last true somite of the body, carries its characteristic styli- form appendages. The appendages of the preceding abdom- inal somites may be either absent or very small and rudimen- tary. Dohrn has proved that this is true only of the females among the Cumacea. The males, which were formerly re- ferred to the genera Bodotria and Alauna, often have well- THE CUMACEA. 309 developed abdominal limbs, though they appear late. It is interesting to find that the females, in this respect, retain more of the larval character than the males. On examining the apparent rostrum with care, it is found to be divided along the middle line by a fissure which runs in front of the eye (which is here single and sessile), divides into two branches, which run backward and outward, and termi- nate before traversing half the length of the carapace ; they thus cut off a median lobe, bearing the eye at its apex, from two lateral processes. The lateral processes are simply pro- longations of the antero-lateral regions of the posterior di- vision of the carapace (as it were the antero-lateral angles of the carapace of My sis , excessively produced and meeting in the middle line) ; while the middle lobe corresponds, I believe, with the cephalostegite of the carapace in ordinary Podoph- thalmia, the insertions of the mandibular muscles occupying their normal position, toward its posterior boundary. The hinder part of the carapace will therefore correspond with the terga of the three anterior thoracic somites, the five posterior ones being, as has been seen, free and movable. The five anterior pairs of thoracic appendages are con- structed much on the same plan as those of the ScMzopoda / the three posterior have no exopodite. In the female, the sixth abdominal somite alone has appendages, but in the male the two anterior abdominal somites are provided with styles. Ovigerous plates are attached to the fourth, fifth, and sixth tho- racic appendages in the female. The structure of the head is peculiar. No ophthalmic sternum nor ophthalmic peduncles are discernible, the single, or closely approximated two, eyes being sessile on the median line on the superior surface of the head. The coxopodites and basipodites of the antennules and antenna? are bent down almost at right angles with the axis of the body, and appear to be connate, or confluent, with their sterna. The succeeding joints are free and pass for- ward, the antennules being much longer and stronger than the antennae in the females, while in the males the antennae are very long; the labrum is large ; the mandibles strong and unprovided with a palp. There is a distinct metastoma, and the maxillae are delicate and foliaceous. A papillose bran- chial plate is attached to the base of the first thoracic append- age. The surface of many parts of the body in some species exhibits a very peculiar sculpturing, singularly like that ex- hibited by the Eurypterida. As in the Podophthalmia^ the heart is short or mod- 310 THE ANATOMY OF INVERTEBRATED ANIMALS. erately elongated, and situated in the posterior part of the thorax. Dohrn1 has shown that the development of the Cumacea takes place without metamorphosis. In most respects the embrvo resembles that of 3/ysis / but, instead of the cuticu- lar investment of the transitory JVauplius-stage with its two pairs of appendages, there is only a sort of cuticular sac with a thickening in the middle line of the tergal aspect, which the embryo bursts as it acquires a larger size. In this respect, the resemblance of the embryonic development of the Cuma- cea to that of the Edriophthalmia is, as Dohrn points out, very striking, and no doubt they form a connecting-link be- tween the Podophthalmia and the Edrioplithalmia. Having regard to their whole organization, on the other hand, thev stand at the bottom of the Malacostracan group, and are com- parable to a JPeneus-l&YVEL in the Copepod stage, the limbs and body of which are modified in the direction of the Schizojioda, while the fore-part of the head has remained Copepodous. Fossil Brachyura are abundant in tertiary deposits, but are rare in formations of earlier date. Macrxira of a pecul- iar type (Eryon) occur in the mesozoic rocks, and perhaps the carboniferous Gampsonyx should be referred to the Po- dophthalmia. The Edeiophthalmia. — These resemble the Podophthal- mia in never possessing a greater than the typical number (20) of somites, though, in some members of the group, the body is composed of fewer somites, in consequence of the abortive or rudimentary condition of the abdomen. Eyes may be absent ; when present, they are usually simple, and are either sessile or seated upon immovable peduncles (Munna). The antennules almost disappear in the terrestrial Isopoda, while the antennae become rudimentary or vanish in some Am- phipoda. The mandibles lose their palps in the Woodlice ; which thus, as in the presence of only one pair of well-devel- oped antennary organs, approach Insects. Ordinarily, the posterior seven, and, at fewest, the posterior four, thoracic somites are perfectly distinct from, and freely movable upon, one another. The ophthalmic and antennary somites have coalesced with the rest of the head ; the branchiae depend from the thoracic limbs, or are modifications of the abdomi- nal appendages ; and the heart is elongated and many-cham- 1 " Ueber denBau und die Entwickelung der Cumaceen." (" Untersuchun- gen iiber Bau und Entwickelung der Arthropoden," 1870.) THE EDRIOPHTHALMIA. 311 bered. But the salient characters of the group will be best understood by the study of such a genus as Amphithoe, the principal details of the organization of which are represented in Fig. 81. The body of this animal is compressed, bent upon itself, and divided into fifteen very distinct segments, reckoning the head as the first and the telson as the last. Fig. 81. — Amphithoe.— The letters and figure? have the same signification as in other figures of Crustacea, except o«, oostegite; br. branchiae; C, lateral view of stomach (D) opened from above ; o, b, c, different, parts of the armature. The head presents a rounded tergal surface ; the anterior face is disposed perpendicularly to the axis of the body, and 312 THE AXATOMY OF IXVERTEBRATED ANIMALS. is produced anteriorly into a strong, curved, and pointed ros- trum ; on each side it bears an aggregation of simple eyes, and in front, immediately beneath the rostrum, this face gives attachment to two long, many-jointed antennules. Below these, two antennae, shorter, and fewer-jointed than the an- tennules, are inserted, and the inferior part of the face is completed by a large movable labrum. Behind this come the strong, toothed palpigerous mandibles (IV), and two pairs of more or less foliaceous maxillae. Inasmuch as the eyes are sessile, these five pairs of appendages are all that belong to the head proper ; but, just as in the Podophthal- mia, -certain of the anterior thoracic appendages are con- verted into accessory gnathites, so, in Amphithoe, the first pair of these members are applied against the mouth, and form a large lower lip (VII'). The "head" of A mphithoe\ therefore, is formed by the coalescence of the seven anterior somites of the body, but I believe that the tergum of the seventh (or first thoracic) so- mite is obsolete, as in a Stomatopod, and hence that the ter- gal surface of the head of the Edriophthabnici corresponds exactly with the cephalostegite (or that part of the carapace which lies in front of the cervical groove) in Podophthahnia. Mr. Spence Bate has shown, in his valuable " Report on the Elriophthalmia" that in the Crustacea at present under discussion, a strong apodeme arises on eacli side from the posterior part of the sternal region of the head, and passing inward and forward meets with its fellow to form an endo- phragmal arch, which supports the oesophagus and stomach, and protects the nervous commissure between the first and second sub-cesophageal ganglia, which runs under it. The discoverer of this structure conceives that it repre- sents the terga of the three somites immediately succeeding the mouth ; but I cannot see that it is other than the repre- sentative of the precisely similar mesophragm formed by the anterior apodemes in Astacus. In fact, the correspondence in structure between the head of an Amphithoe and the ceph- alic portion of the cephalo-thorax of Astacus is not a little striking. There is the same sternal flexure, the same relative position of the stomach, and of the insertions of the mandibu- lar muscles. The great difference lies in the abortive condi- tion of the ophthalmic appendages.1 1 A strong endophragmal arch separates the sub-oesophageal ganglia and com- missures from the gullet in Squilla, but has different connections (Fig. 83). _ A very similar endophragmal arch is found in the Insect head. See the descrip- tion of the head of Blatta {infra). THE EDRIOPHTHALMIA. 313 The seven free somites of the thorax each give attachment to a pair of limbs. It is characteristic of Amphithoe, as of the Amphipoda in general, to have the five anterior pairs of tho- racic members directed forward. Each limb consists of an expanded coxopodite, succeeded by the other six joints of the typical crustacean limb. In the male, a single vesicular lamella, the branchia, is attached to the inner side of the coxopodite of the append- ages of the ninth to the fourteenth somites inclusively ; but, in the female, an additional plate, convex externally and con- cave internally, is attached above, and internal to, the branchia of the 9th to the 12th somite. These oostegites, as they may be called, inclose a cavity in which the incubation of the eggs takes place. The abdomen consists of six somites and a very small ter- minal telson. The appendages of the three anterior somites are terminated by two multiarticulate setose filaments (Fig. 81, XV), while in the three posterior the corresponding- parts are styliform, and serve as a fulcrum for the abdomen when the animal leaps, by the sudden extension of that region of the body. The Edriojjhthalmia are ordinarily divided into three groups. The Amphipoda, which resemble Amphit/w'e, are characterized by their compressed form and their ordinarily saltatory habits ; by having thoracic branchiae ; by the for- ward direction of their four anterior locomotive limbs (2d to 5th pairs of thoracic appendages), and by the contrast between the three anterior and the three posterior pairs of abdominal appendages. The common Sand-hopper is the most familiar example of this division. The second group is that of the Zcemodlpoda, distinguished by the rudimentary state of the abdomen, which is reduced to a mere papilla, and by the coalescence of the second, as wTell as the first, thoracic somite writh the head, so that the anterior limbs appear to be, as it were, suspended under the neck. The strangely-formed genera Cyamus, the parasite of whales, and Capretta, which is very common upon our own coast, adhering to corallines, sea-weeds, and starfish, belong to this group. The Isopoda, which constitute the third group of the Edriophthalmia, are usually depressed instead of compressed, and run or crawl instead of leaping. Many, like the common Woodlouse (Oniscus), possess the power of rolling them- selves into a ball when alarmed ; some, like the last-named genus, are terrestrial ; others, like the Asellus, inhabit fresh 314 THE ANATOMY OF INVERTEBRATED ANIMALS. waters, but the great majority are marine ; and among them are many peculiarly modified parasitic forms (Fig. 82, Cymo- thoa ; Bopyrus). The composition of the head and mouth sn -xn irr ..it -XX 1> X£'% Fig. 82.—Cymothoa. — The letters and figures have the same signification as in Fig. 81, except Ab, abdominal appendages in Fig. A. in the Isopoda is essentially the same as in the Amphip)oda^ though differing considerably in details. The branchiae of the thoracic members are absent, their functions being per- formed by the endopodites of some of the abdominal mem- bers, which are soft and vascular. The three anterior pairs of thoracic members are usually directed forward — the four posterior pairs backward. In some Isopoda the abdominal somites, partly or wholly, coalesce with one another. In all the Edrioplitlialmia the alimentary canal is straight and simple, and its anterior gastric dilatation, frequently strongly armed, is situated in the head. The liver is repre- sented by a variable number of straight caeca. Occasionally there are one or two caeca which open into THE EDRIOI HTHALMIA. 315 the posterior part of the intestine, and appear to be urinary organs analogous to the Malpighian caeca of insects. °The respiratory organs vary greatly in structure. In most Eclriophthalmia they are simple plates or sacs, the delicacy of the integument of which permits of the free exposure of the blood circulating in them to the air. In the amphipod genus Phrosina, however, the branchiae are composed of rudi- mentary lamellae, attached to an expanded stem, and resem- ble not a little the epipoditic branchiae of Astacus. In some Sphceromidce, Duvernoy and Lereboullet found the branchial endopodites transversely folded, so as to approach those of the Xiphosura. The exopodites of the abdominal members of the Isopoda frequently cover the modified endopodites, forming opercula, and the first pair of abdominal limbs is, in many genera, al- tered in such a manner as to form one such large operculum for the four pairs which succeed it. In the Idoteidce it is, on the other hand, the sixth pair of abdominal limbs which are so modified as to form the curious door-like opercula which cover the gills. In certain of the terrestrial Isopoda (PoreelHo, Arma- dilUdium), some of the opercular plates of the branchiae, usually the two anterior pairs, contain curiously ramified cav- ities, which open externally, and contain air. The genus Tylos possesses respiratory organs, which present a still more interesting approximation to those of the purely air-breath- ing Artlculata. They are thus described by Milne-Edwards : " The abdomen presents inferiorly a deep cavity, very similar to that of the Sphceromce, in which the five anterior pairs of appendages are lodged ; but this cavity, instead of being completely open below, is imperfectly closed, in its pos- terior half, by two series of lamellar prolongations, which arise from the sides of the inferior faces of the third, fourth, and fifth abdominal segments, and pass horizontally inward ; the first pair of these plates is small, those of the third pair are, on the other hand, very wide, and almost meet in the me- dian line. The four anterior pairs of abdominal appendages, lodged in this cavity, each carry a wide and short quadrilat- eral appendage, the surface of which is raised into a transverse series of laro:e longitudinal elevations, and each of these eleva- tions presents inferiorly a linear aperture leading to a respir- atory vesicle, the parietes of which are covered with a multi- tude of little arborescent caeca. These vesicles when extracted from the interior of the limb closely resemble a brush-like 316 THE ANATOMY OF INVERTEBRATED ANIMALS. branchia, having its longitudinal canal in communication with the atmosphere by a longitudinal stigma. The fifth pair of abdominal members are rudimentary, while the sixth consti- tute the door-like triangular valves covering the anus, and all the inferior face of the last abdominal segment." 1 The nervous system in the Amphipoda consists of supra- cesophageal or cerebral ganglia, united by commissures with an infra-cesophageal mass, whence commissural cords pass un- der the endophragm to the anterior of the thoracic ganglia, of which there are commonly seven pairs, succeeded by five or six pairs of abdominal ganglia. In some Isopoda ( Cymothoa, Idoiea) the abdominal ganglia are also distinct; but in others, such as u3£ga bicarinata (according to Rathke), they are fused into a single mass placed in the anterior part of the abdomen, presenting only traces of a division into five por- tions. In the Cymothoadce and terrestrial Isopoda, again, the abdominal ganglia appear to have completely coalesced with the last thoracic ganglia and form a mass, whence the ab- dominal nerves radiate. Finally, in the short-bodied Lcemo- dipoda, such as Cyamus, there are not more than eight pairs of post-cesophageal ganglia, the posterior commissures of which are so shortened that the nervous system ends in the antepenultimate somite. Brandt describes splanchnic ganglia like the lateral pair of Insects in the Oniscidce. It is one of the many respects in which the Isopoda simulate Insecta. No other organs of sense than eyes have, as yet, been cer- tainly demonstrated to exist in the Edriophthalmia, though the fine setre which beset the antennary appendages have been supposed to be organs of the olfactory sense. The eyes vary in their structure, from the simple, more or less closely aggregated ocelli of Immodipoda, and of many Isopoda and Amphipoda, to the strictly compound eyes, as complex as those of the highest Articulata, which exist in ^Uga and in Phrosina. The female genitalia of the JEdriophthalmia consist of two simple sacs, the ducts of which usually open on the ventral surface of the antepenultimate thoracic somite, or on the bases of the limbs of this somite. In the male, one or more caaca on each side constitute the testis, which ordinarily opens on the last thoracic or first abdominal somite, in connection with one or two pairs of copulatory organs developed from the an- terior abdominal somites. 1 " Histoire Naturelle des Crustacds," vol. iii., p. 187. THE STOMATOPODA. 317 The eggs of the ordinary Bdriophthahnia usually undergo their development in the chamber beneath the thorax inclosed by the oostegites of the thoracic appendages. In most cases, the young differ so little from the adults that no metamorpho- sis can be said to take place. They frequently, however, want the last thoracic somite. The young of the parasitic Edriophthalmia, such as Bopyrus, Phryxns, Cymothoa, Cy- amus, and the Hyperinw, on the other hand, are widely dif- ferent from the adults ; and not only in their metamorphosis, but in the small proportional size and less aberrant form of the male, Bopyrus and Phryxus recall the parasitic Cope- poda. In certain Amphipods (Gammarus loeusta and Besmo- philus) the vitellus undergoes complete division ; while, in closely allied forms {Gammarus fluviatilis and pidex), and still more completely in those Isopoda which have been studied, the part of the vitellus which divides into blasto- meres becomes more or less completely separated from the rest immediately after fecundation, and the so-called partial yelk division, take place.1 In all Edriophthalmia, the development of which has been examined, before any other organs appear, a cuticular investment or sac is formed, which is eventually burst and thrown off. This appears to represent the JVauplius cuticle of 3Iysis, and, in close relation with it, are peculiar tergal structures, such as the bifid lamella? of Asellus, and the un- fortunately named umicropyle apparatus" of other Bdrioph- thalmia. The Edriophthalmia are not abundant in the fossil state ; but thev may be traced back as far as the later Palaeozoic strata [Prosopionisciis, Amplxipeltis). The Stomatopoda. — Of the Stomatopoda of Milne-Ed- wards, two of the three divisions, the Carido'ldes and the Bicuirasses, have since found a place among the Schizopod- ous Podophthalmia, or among the larvae of certain Macru- ra ; but the third, the Stomatopodes imicuirassfo, compris- ing Sqidlla, Gonodactylus, and Coronis, appear to me to differ so widely and in such important structural peculiarities, not only from the Podophthalmia proper, but from all other Crustacea, as to require arrangement in a separate group, for which the title of /Stomatopoda may well be retained. 1 E. van Beneden, "Recherches sur la Composition et la Signification de l'CEuf," 1870. 318 THE ANATOMY OF INVERTEBRATED ANIMALS. The genera named, in fact, stand alone among the Crus- tacea* in that the ophthalmic and antennulary somites are complete rings, movable upon one another and the anten- na: tit- Fig. 83.—SquiUa scabricauda. — A, the entire body, with the thorax and abdomen in longitudinal and vertical section. B, the head in vertical section. I-XX, somites of which the body is composed. I'-XX', their appendages, the bases of most of which are alone represented. AL, alimentary canal ; G, stomach ; An, anus; <7, heart; bi\ branchia. N, ganglia and their commissures. H, rostrum of the cara- pace ; p, the penis. Pn, endophragmal arch. The fifth thoracic appendage XF is figured separately. nary somite, and that their long axis is parallel with that of the body, so that there is no sternal flexure. Numerous pairs of hepatic caeca open into the elongated alimentary canal. The heart, again, is not short and broad, with at most three pairs of apertures, and confined to the thoracic region, as in the proper Podophthalmia / but it is greatly elongated, mul- tilocular, and extends into the abdomen. The branchiae are plumes attached to the abdominal members (Fig. 83, A, br), and, so far as I have been able to ascertain, the carapace is, in all, connected exclusively with the cephalic somites. This . ' Unless the freedom of the anterior segment of the head in the PonUllidce referred to above, when the Copepoda were under consideration, is a parallel case. THE STOMATOPODA. 319 is particularly well seen to be the case in Squilla scabricauda (Fig. 83), where five completely developed posterior thoracic terga can be counted, uncovered by the short carapace, be- neath which the terga of the three anterior thoracic somites are represented by a membrane which passes forward to be reflected into the carapace. The free somites of the thorax, and those of the abdomen, in this species and in the Stomatopoda generally, are so large relatively to the carapace, that the latter is not larger in pro- portion to the body than the tergal covering of the head in many Edr '(ophthalmia , with which order the Stomatopjoda present many marked affinities. Indeed, if we leave the eyes out of consideration, the organization of the Stomatopoda is more Edriophthalmian (and especially Amphipodan) than Podophthalmian. The five anterior pairs of thoracic members are turned forward, and are subchelate. The first pair are small and slender. The second pair are the largest of all, and have the characters of powerful prehensile limbs, the ter- minal curved and spinose joint of which shuts down into a groove in the penultimate joint, as the blade of a pocket-knife does into its handle. The three posterior thoracic limbs, on the other hand, are turned outward, and terminated by an endopodite and an exopodite. Squilla lays its eggs in burro wts in the bottom of the sea, which the animals inhabit. The earliest condition of the free larva is not fully known, but the young larvae have a single eve, and the hinder thoracic and the abdominal appendages are not developed.1 The larvae pass into forms which, under the names of Alima, Erichthys, and Squilleriehthys, were formerly considered to be independent genera. Claus's inves- tigations, however, have rendered it probable that the two latter genera are simply larval stages of Go?iodactylus, and that Alima is a larval stage of Squilla. 1 Fritz Miiller, " Fur Darwin." See also Claus, " Die Metamorphose der Squilliden," 1872. 21 CHAPTER VII. THE AIR-BREATHING ARTHROPODA. Among these Arthropoda, no forms absolutely devoid of limbs are at present known, though the appendages are re- duced to two pairs of minute hooks in the vermiform parasite Linguatula. The Arachnida have pediform gnathites, and the least modified forms of this group (the Arthrogastra or Scorpions and Pseudo-scorpions) exhibit, in many respects, extraordi- narily close resemblances to the Merostomata among the Crustacea. The Arthrogastra. — The anterior part of the body of a Scorpion (Fig. 84) presents a broad, shield-like tergal plate, resembling that of Eurypterus in form. Two large eyes are situated one on each side of the middle line of the shield, while smaller eyes, which van7 in number according to the species, are ranged along its antero-lateral margins. Six wide plates, representing the terga of as many so- mites, follow the anterior shield, and are connected only by the soft integument of the sides of the body with their sterna. 'The seventh is united with its sternum (xv) poste- riorly, while the five following terga and sterna form contin- uous rings, which constitute the joints of the so-called " tail." The anus is situated behind the last sternum. A movable terminal piece, answering to the telson of a Crustacean, which is swcllen at its base, and then rapidly narrows to a curved and pointed free end, overhangs the anus, and constitutes the characteristic weapon of offense of the Scorpion. This sting, in fact, contains two glands which secrete a poisonous fluid, and their ducts convey it to the minute aperture situated at the sharp point of the organ. On the sternal surface of the body there are four wide and long sternal plates (xi-xiv), THE ARTHROGASTRA. 321 which correspond with the third, fourth, fifth, and sixth, of the free terga. Each of these bears a pair of oblique slits, Fig. 84.— Scorpio afer.—A, tersral, and B, sternal, view of the body : At, chelicerse ; rv', pedipalpi ; v', vi', posterior pair of cephalic appendages : vn', vm', anterior thoracic limbs: Pt, pectines ; St, stigma; Cth, cephalo-tborax. (After Milne- Edwards and Duges.1) which are the openings of the respiratory organs (Fig. 85, e). The sterna of the first and second free somites (ix, x) are very small ; that of the first carries the valves which cover the genital aperture ; that of the second bears a pair of very curious appendages, somewhat like combs, which are termed the pectines. The nervous trunks which enter the pectines are distributed to the numerous papillae which cover them, and are probably tactile in function. Thus there are twelve somites behind the eye-bearing shield, and none of these are provided with appendages, unless the pectines be such. The truncated anterior extremity of the body, beneath the 1 " Regue Animal," Illustrated Edition. 322 THE ANATOMY OF INYERTEBRATED ANIMALS. shield, is formed by a very large setose labrum, behind and below which, in the middle line, is the extraordinarily minute yd — X r; > — > _ > irr e ^ ¥ > ^ Fig. 85.— A diagram of the body of a Scorpion, the majority of the appendages be- in:? removed : a. the mouth : b. the alimentary canal : c, the amis ; d, the heart ; e, a pulmonary *ac ; /, the positi >n of the ventral ganglianated cord ; g, the cere- bral ganglia ; ' T. the telson. YII-XX. the seventh to the twentieth somite. IV, V. VI, the basal joints of the pedipalpi. and two following pairs of limbs. aperture of the mouth (Fig. 86, 31). On each side of it is attached a three-jointed, pincer-ended, appendage, the che- licera. Behind these follow the pedipalpi, large chelate limbs, the stout basal joints (iv') of which lie on each side of the mouth. The following four pairs of appendages are seven-jointed ambulatory limbs, each terminated by three claws. The ba- THE ARTHROG ASTRA. 323 sal joints of the first two (v', vi') lie behind the mouth, the posterior and inferior boundary of which they foFm, and are directed forward. The basal joints of the last two (yii', yiii'), on the other hand, directed inward, are firmly united together, and are altogether excluded from the mouth. Thus the mouth is situated between the labrum in front, the bases of the pedipalpi and those of the first two pairs of ambulatory limbs, at the sides and behind ; just as, in Limulus, the mouth lies between the labrum and the basal joints of the third, fourth, and fifth limbs, which answer to the mandibles and first and second maxillae of the higher Crustacea. If this comparison is just, there is one pair of prse-oral appendages, which exist in Limidus, wanting in the Scorpion ; and the difference between the tw?o may be represented thus : Limulus. Antennule. Antenna. Mandible. Maxilla 1. Maxilla 2. Scorpio. Chelicera. Pedipalpus. Leg 1. Leg 2. Again, if the eye-bearing part of the head may be regarded as a somite, then the body of the Scorpion, like that of a mala- costracous crustacean, will consist of twenty somites and a telson. We may regard the six posterior somites (xv-xx) as the homologues of those which constitute the abdomen in the crustacean ; while the eight middle somites (vii-xiy) will Fig. 86 — Scorpio.— Vertical section of the cephalo-thorax: At. chehcera ; lb, labrum; M, mouth; a. pharyngeal sac; N, X>. supra and infra-oesophageal ganglia; 0, oesophagus; tf, opening of the salivary ducts; e, intestine; H, heart. answer to those which enter into the thorax of the latter ; and the head will resemble that of an Edriophthalmian with 324 THE ANATOMY OF INVERTEBRATED ANIMALS. one pair of antennary organs completely suppressed. Upon this view, the eye-bearing shield is a carapace covering a cephalo-thorax, into which the two anterior thoracic somites only enter. These are followed by six free thoracic somites, the four posterior of which are pulmoniferous. But no trace of the supposed missing antennary appendage has been met with in the embryonic condition, so that the alternative pos- sibility that the mouth is situated one somite farther forward in the Scorpion than in the Crustacean must be borne in mind. It is a very interesting fact that Metschnikoff ! has found ru- diments of limbs on those somites of the embryo Scorpion on which the stigmata are situated — a circumstance which sug- gests the suspicion that the Scorpion is derived from some form possessing more numerous limbs. The minute oral aperture leads into a small pyriform lat- erally-compressed sac (Fig. 86, a) with chitinous elastic walls. Muscles pass from these to apodemes of the sternal wall of the head, and doubtless act as divaricators of the wall of the sac. As the Scorpion sucks out the juices of its prey, it is probable that the elastic sac acts as a kind of buccal pump — the nutritious fluid rushing in when the sides of the pump are separated, and being squeezed into the oesophagus when the elasticity of the walls brings them back to their first position.2 The oesophagus (Fig. 86, b) is an exceedingly narrow tube, which springs from the tergal and posterior aspect of the sac just mentioned, traverses the nervous ring, and then, passing obliquely upward and backward, enlarges into a dilatation which receives the secretion of two large salivary glands, by a wide duct' on each side. The alimentary canal narrows again, and, becoming a delicate cylindrical tube which widens posteriorly, passes straight through the body to the anus. The numerous ducts of the liver open into the anterior part of this region of the alimentary canal, and it receives two delicate Malpighian tubuli. The liver is a vast follicular gland, which occupies all the intervals left between the other organs in the enlarged part of the bodv, and even extends for some distance into the nar- row posterior somites. The eight-chambered heart (Fig. 86, H) is a larger and more conspicuous structure than the alimentary canal, above which it lies, in a pericardial sinus situated in the middle 1 " Embryologie des Scorpions." ( ' Zeitschrift fur wiss. Zoologie, 1871.) 2 Huxley, " On the Mouth of the Scorpion." (Quarterly Journal of Micro- scopical Science, 1860.) THE ARTHROGASTRA. 325 line of the tergal aspect, between the eye-bearing shield and the tail ; each chamber is wider behind and narrower in front, and has two valvular apertures, by which blood is admitted from the pericardial sinus at its postero-lateral angles. It gives off small lateral arteries, and ends in front and behind in a wide aortic trunk. Of these the anterior is larger than the oesophagus, and both aortae give oft' branches which are distributed widely through the body. A large trunk lies on the tergal aspect of the ganglionic chain, and is united with the anterior dorsal aorta, by a lateral aortic arch, on each side of the body. The veins, on the other hand, are irregular pas- sages, the blood of which is carried to two afferent pulmonary sinuses, one for each set of respiratory organs. These respiratory organs are four pairs of flattened sacs, which open externally by as many stigmata, on the sterna of the four posterior free thoracic somites (Fig. 85, xi-xiv) in Fig. 87.— .4, pulmonary sac. 5, respiratory leaflets of Scorpio occitanus. (After Blanchard.) front of the tail. Each lies with one flat side sternal and the other tergal, in front of its stigma, and its walls are so folded as to divide its cavity into a multitude of subdivisions, each of which opens into the common chamber which communi- cates with the exterior by the stigma (Fig. 87). The organ, in fact, somewhat resembles a porte-monnaie with many pock- ets. The blood circulates in the folds, and, after being thus exposed to the influence of the air, is carried by efferent pul- monary sinuses to the. pericardial sinus. Expiration is effect- ed by muscles which pass vertically between the sterna and tero-a of the free somites. The bilobed cerebral ganglion supplies nerves to the eyes and chelicera?, and is connected bv thick commissures with the post-cesophageal ganglion, a larofe oval mass, whence branches are given to the maxillae and following somites. A long cord formed by two closely-applied commissures passes 32G THE ANATOMY OF IN VERTEBRATED ANIMALS. to the three ganglia placed in the twelfth to the fourteenth somites. There are four ganglia in the abdomen, two dis tinct cords passing from the last to its extremity. The vis- ceral nervous system is represented by an oesophageal gan- glion receiving roots from the cerebral ganglion, and giving branches to the alimentary canal.1 Two lateral ovarian tubes, connected by transverse anasto- moses with a median tube, end in two oviducts, which open by a fusiform vagina on the first free sternum (ix). The tu- bular testes end in a pair of deferent ducts, on which, before their union at the common orifice, two long and two short caeca are found, the former playing the part of vesiculae semi- nales. Both male and female organs lie imbedded in the hepatic mass in the posterior thoracic region, their ducts pass- ing forward. Partial yelk-division takes place, and the ova undergo development within the ovarian canals, in a manner which is very similar to that of Astacus. Thus there is no metamorphosis, and the young differ but little from the adult in any respect but size. The Pseudo-scorpions ( Chelifer, Obisium) resemble the Scorpions in form and in the nature of their appendages, but they have no aculeate telson nor poison-gland. They possess silk-glands, which open close to the genital aperture, and their two pairs of stigmata are connected, not with pulmonary sacs, but with tracheal tubes. According to Metschnikoff, the eggs undergo complete yelk-division, and the young leave the egg provided only with that pair of appendages, which become the pedipalpi. In the number of the appendages, and in the segmenta- tion of the abdomen, Galeodes (or Solpuga) agrees with the Scorpions and Pseudo-scorpions. But the three somites which bear the three hinder pairs of ambulatory limbs (vi, vn, viii, in the Scorpion) retain their distinctness, and there is no cephalo-thorax, in the proper sense of the word. In form and function the pedipalps resemble the first pair of ambulatory limbs, while the chelicerae are subchelate. The organs of res- piration are tracheal. The Phalangidce (Phalangium, Gonyleptus) have chelate chelicerae, but the pedipalps are filiform or limb-like, and the articulated abdomen is relatively short and broad. They have no silk-glands, and their respiratory organs are tracheal. i Newport, " On the Structure, etc., of the Nervous and Circulatory Systems in Myriapoda and Macrurous Arachnida." (" Philosophical Transactions," 1843.) THE ARANEINA. 327 While the last-mentioned forms lead from the Arthrogastra to the Acarina, the pulmonate Phrynidce, or Scorpion-spiders (Thelyphonus, Phrynus), are in many respects intermediate between the Arthrogastra and the Araneina. -JP>n mi. &&' Fig. 88. — Mygale aementaria.—A, female of the natural size : At, chelicerae ; rv', pedipalpi ; v', vi', maxillary feet; vn', vni', thoracic feet ; Cth, cephalo-thorax. B, the last joint of the pedipalpus of the male much magnified. C, terminal joint of the chelicera At, with the poison-gland. D, the left pulmonary sac viewed from its dorsal aspect: Stg, stigma; Pm, pulmonary lamellae. E, the two arachnidial mammillae of the left side— the smaller Sp 1 is situated on the base of the large one, Sp 2. (After Dugds, " Regne Animal.") The Araneina. — The Spiders stand in somewhat the same relation to the Scorpions as the brachyurous to the macrurous Crustacea. That part of the body which lies behind the cephalo-thorax and answers to the free somites of the body of Scorpio is swollen, and presents no distinct division into somites. The cheliceras are subchelate, that is to say, the distal joint is folded down upon the next, like the blade of a pocket- knife upon the handle. The duct of a poison-gland, lodged in the cephalo-thorax, opens at the summit of the terminal joint. The pedipalpi are filiform, and, in the males, their extremities are converted into singular spring boxes, in which the sper- matophores are received from the genital apertures and con- veyed to the females (Fig. 88, B). The pulmonary sacs, two or four in number, are similar to those organs in Scorpio, and are placed in the anterior part of the abdomen ; a tracheal system is also present, a pair of sternal stigmata, situated either behind the pulmonary sacs, or at the end of the abdomen, leading* into two more or less branched tubes. There is a complex pharyngeal apparatus, 328 THE ANATOMY OF INVERTEBRATED ANIMALS. probably having1 the same function as in Scorpio.1 The stomach gives off cascal prolongations which may extend far into the limbs. There is usually a dilated short rectum, into which the branched Malpighian ducts open. The nervous system, more concentrated than that of the Arthrogastra, is reduced to a supra-cesophageal ganglion and a single post- cesophageal mass, with four indentations on either side. There are six or eight simple eyes in the anterior part of the cara- pace. Auditory organs have not been discovered in these or any other Arachnida. One of the most characteristic organs of the Aranehia is the arachnidium, or apparatus by which the fine silky threads which constitute the wreb are produced. H. Meckel,3 who has fully described this apparatus as it occurs in Epeira dia- dema, states that, in the adult, more than a thousand glands, A B Fig. 89, k.—Mygale Blondii (after Blanchard).— The stomach with its caeca, and the remainder of the alimentary canal with the liver and Malpighian tubes. Pig. 89, B.— The heart and arterial vessels of the same. with separate excretory ducts, seerete the viscid material, which, when exposed to the air, hardens into silk. These 1 Lyonet's "Anatomic de dim' rentes Especes d'Tnsectes" ("Mem. du Mu- seum d'Histoire Naturelle," 1829) contains an elaborate account of this appa- ratus, as well as of the structure of the pedipalps of the mnlc spiders. 2 " Mikrographie einisrer Druaenapparate der niederen Thiere." (Muller'a " Archiv," 1846.) See also Buchholz and Landois. (Ibid., 1868.) THE ACARINA. 329 glands are divisible into five different kinds (aciniform, am- pullate, aggregate, tubuliform, and tuberous), and their ducts ultimately enter the six prominent arachnidjal mammillce, which, in this species, project from the hinder end of the abdomen. The superior and inferior mammillae are three- jointed, the middle one is two-jointed. Their terminal faces are truncated, forming an area beset with the minute arach- nidial papillm by which the secretion of the glands is poured out. The males are smaller than the females, and their ap- proaches to the latter are made with extreme caution, as they run the risk of being devoured ; extending their pedi- palps, they deposit the spermatophores in the female genital aperture, and betake themselves to flight. The Araneina are oviparous, but the development of the embryo takes place as in the Arthrogastra, and there is no metamorphosis.1 The Acarixa. — In the Mites and Ticks, the hinder so- mites are, as in the Spiders, distinctly separated from one an- other, but they are not separated by any constriction from the anterior somites. The bases of the chelicerae, and of the pedipalpi, coalesce with the labrum, and give rise to a suctorial rostrum (Fig. 90). There are usually several gastric caeca, but no distinct liver. Salivary glands occur in some, and Malpighian caeca are occasionally found. No heart has yet been discovered. Special respiratory organs are sometimes wanting (e. g., Sar- coptes) ; when present, they are tracheal tubes, springing brush-wise from a common trunk which opens by a stigma. The stigmata are usually two, sometimes anterior and some- times posterior in position. The ganglia of the nervous sys- tem are concentrated round the gullet, as in the Spiders; and the reproductive aperture is situated far forward, sometimes close to the rostrum. The greater number of the Amrina are parasites upon other animals or upon plants. Most are oviparous, but the Oribatidce are viviparous. The course of the development of the embryo is the same as in the Spiders. The young, when born, are frequently pro- 1 Claparede, " Reeherehes sur l'Evolution des Araignees,",'1862. Also Balbiani, " Ann. des Sc. Nat.," 1873. 330 THE ANATOMY OF INVERTEBRATED ANIMALS. vided with only three pairs of ambulatory limbs, the fourth pair making its appearance only after ecdysis has occurred. Fig. 90.— Ixodes vicinus, female (after Pa^enstecher 1).— a, mandibular hooklet8 ; 6, d, e, fourth, third, and second joints of the palp ; c, hooklets of sternal surface of proboscis ; /, base of the proboscis ; g, stigma ; h, genital aperture ; i, anal valves. In some Acarina, a singular kind of metamorphosis occurs. Thus, in Atax Bonzi, Claparede 2 observed that, before the limbs appear on the blastoderm, a chitinous cuticula is separated and forms an envelope, which he terms the "sac of the deutovum." The proper vitelline membrane bursts into two halves, much as in Limulus, and the deutovum emerges. In the mean while, the anterior end of the blasto- derm becomes fashioned into two procephalic lobes ; while five pairs of tubercles, answering to the rudiments of the oheliceras, pedipalpi, the two posterior gnathites, and one i " Anatomie der Milben," 1860. 3 " Studien an Acariden." {ZeUschrift/Qr wiss. Zoologie, 1868.) THE PYCNOGOXIDA. 331 pair of thoracic limbs of the Spiders, make their appearance beneath the sac of the deutovum. The rudiments of the cheliceras and pedipalpi apply themselves together, and coa- lesce into a proboscis. Thus the first larval form is com- pleted. It tears the pseudoval sac, emerges, and buries itself in the branchiae of the fresh-water mussel ( Unio), upon which it is parasitic. The cuticular investment of the first larva now becomes distended by absorption of water, and forms a globular case, the limbs being drawn out of their sheaths. The second larval stage completes itself within the sac formed by this singular ecdysis. The two palpi are de- veloped from the pedipalpal portion of the proboscis; two horny hooks from the cheliceral portion ; and, finally, the hinder pair of thoracic limbs is added. This second larva gradually passes into the adult Atax. In the Acarus (Myobia coarctata) of the Mouse, Claparede observed that the deutovum stage is followed by a tritovum ; the chitinous sac, which invests the embryo within the deuto- vum, apparently representing the cuticle of the first larva of Atax. In this case, it presents a parallel to the JVaupliicS cuticle of Mysis. The Arthrogastra, the Araneina, and the Acarina (with some doubtful exceptions among the latter), possess the same number of appendages, and do not differ from one another so much as do trie different forms of the Copepoda, among the Crustacea. But the remaining groups, which are usually in- cluded among the Arachnida, namely, the Pycnogonida, the Arctisca, and the Pentastomida, diverge widely from the Ar- throgastra and the Araneina, though each exhibits certain approximations to the Acarina. The Pycxogoxida. — These are marine animals, with short bodies terminated in front by a rostrum like that of the Mites, but with a mere tubercle in place of the posterior thoracic and abdominal somites. The adult has four pairs of enormously- elongated, many-jointed, ambulatory limbs, in front of which are three pairs of short appendages, the anterior of which may be chelate, while the posterior are more or less rudimentary (Fig. 91). The alimentary canal sends off very long caeca into the legs. There is a short heart, but no distinct respiratory or- gans exist. A cerebral, nervous mass is connected with a ventral chain of four or five pairs of ganglia. Four eyes are seated upon a dorsal tubercle above the brain. The sexes are 332 THE ANATOMY OF INVERTEBRATED ANIMALS. distinct, and the testes and ovaria are lodged in the legs and open upon their basal joints. Fig. 91.—Ammothea pycnogonides, female (after Quatrefage?).— a, oesophagus; d, antennae ; b, stomach with its prolongation into the antennae and limbs e • c, rectum. The embryo emerges from the egg as a larva provided with a rostrum, and with three pairs of appendages, which repre- sent the short, anterior three pairs in the adult.1 The four pairs of great limbs of the adult are produced by outgrowths from a subsequent posterior elongation of the body. The comparison of the embryos of the Pycnoyonida with those of the Acarina, especially such as leave the egg with three pairs of appendages, appears to me to leave little doubt that the rostrum of the larva Pycnogonum is formed, as in the Mites, by the coalesced representatives of the cheliceras and pedipalpi. If so, the seven other pairs of limbs are, by three pairs, in excess of the number found in any Arachnidan. 1 A. Dohrn, " Untersuchungen uber Bau und Entwickelung der Arthro- poden." Erstes Heft. 1870. THE PYCXOGOXIDA. 333 On the other hand, the hexapod larva of the Pycnogonida differs from the hexapod Ncmpllus of the Crustacea, inasmuch as the three pairs of appendages of a Nauplius always repre- Fig. 92.—JfacrobiotusSchulfzei(x 100).— a, mouth with six oral papillae ; b, snllet, calcified stylets; c, salivary glands; d, muscular pharynx ; e, ovary;/, vesicula semiualis ; g, testis ; 1, 2, 3, 4, limbs. (After Greeff.1) sent antennary and mandibular appendages, and these, by the hvpotbesis, are to be sought in the rostrum of the Pycnogo- nida. The fact to which reference has already been made, that the embryo Scorpion has six pairs of rudimentary appendages, attached to as many of the anterior free somites, of which one pair only remain (as the pectines) in the adult, leads me to suspect that the Pycnogonida may represent a much modified 1 " Untersuchunsren iiber den Bau der Barthierchen." (" Arckiv fur mikr. Anat,," 1866.) 334 THE ANATOMY OF IXYERTEBRATED AXIMALS. early Arachnidan form, from which the Arthrogastra, Ara- neidea, and Acaridea, have branched off. T he Arctisca, or Tardigrada, are microscopic animals, found, in association with Potifera, in moss and in sand, rare- ly in water, which present many points of resemblance to the Acarina. The body (Fig. 92) is vermiform, with four pairs of tubercles, representing limbs, terminated by two or more claws. The fourth pair is directed backward at the hinder end of the body, so that if these appendages answer to the hinder pair of limbs in the typical Arachnida, the hinder tho- racic, and all the abdominal, somites are undeveloped. The mouth is situated at the extremity of a rostrum provided with two stylets, wThich is so like that of the Acarina, that it may probably be regarded as formed by the coalescence of cheli- ceral and pedipalpal tubercles. There is a muscular pharynx leading into a wide alimentary canal, which gradually narrows to the anus. No organs of circulation or of respiration exist. The paired ventral ganglia, which correspond in number with the appendages, are large. They are connected by longitu- dinal commissures wTith one another, and with a prae-cesopha- geal cerebral mass which sometimes bears two eyes. The Arctisca are hermaphrodite, the ovarian sac and the two testes opening together into a cloacal dilatation in which the intestine terminates. The ova are relatively very large, and the cuticle of the parent is cast off and incloses them when they are laid, as a sort of ephippium. Complete yelk-division takes place. The young have one-third the size of the adult when they are hatched, and they undergo no metamorphosis beyond the addition, in some cases, of one pair of limbs after birth.1 The Pentastomida. — A still more aberrant form is the parasitic Zdnguatula, or Pentastomum, which is found in a sexless condition in the lungs and liver of herbivorous mam- mals and of reptiles, and in the sexual state in the nasal cav- ities and maxillary antra of Carnivores. Thus, as Leuckart's investigations have proved, Pentastomum tcenioides, which inhabits the latter cavities in the dog and the wolf, is the sexual state of the P. denticulatum, which occurs in the liver of hares and rabbits.8 1 Kaufmann, " Entwickelung und systematische Stellung der Tardigraden." {Zeit. wiss. Zoolorrie, 1851.) s '* Bau und Entwickelungsgeschichte der Pentastornen," 1860. THE PEXTASTOMIDA. 335 The Pentastomida are elongated vermiform animals, the bodies of which are divided by close-set transverse constric- FiG.93.—Pentasto}?iumtcenioi(les.—A.'Ma\e. B. Female. C. Anterior end of the body: a, anterior hooks ; 6, posterior hooks ; d, mouth ; c, rudimentary palpiform or- gans. (After Leuckart.) tions into numerous short segments. At first sight they appear to be entirely devoid of appendages, but, on careful inspection, four curved hooks are found, two on each side of the mouth, which is situated rather behind the anterior ex- tremity of the body. Each hook is solid, and its base pro- Fig. 94.— Embryo of Pentastomum tcenioides. jects into the cavity of the body and gives attachment to the muscular bands by which it is moved. The mouth is surrounded by a chitinous ring ; a narrow 22 336 THE ANATOMY OF INYEUTEBRATED ANIMALS. oesophagus leads from it into a nearly cylindrical, straight, alimentary canal, which terminates in the anus, in the middle line of the posterior extremity of the body. A mesentery is attached to the whole length of the alimentary canal and holds it in place. A nervous ring surrounds the oesophagus, and posteriorly presents a ganglionic enlargement whence nerves are given off to the body. The muscles are striated. The sexes are distinct, and the males are usually much smaller than the females. The testicle is an elongated sac which lies on the ventral aspect of the intestine, and is connected anteriorly with two vasa deferentia. These terminate on the fore-part of the ventral aspect of the body, each having a saccular dilatation which contains a very long, coiled, chitinous penis. In the female, the ovary is also a large sac and the oviducts come off from its anterior end, but the genital aperture is close to the anus. The ova undergo their development in the ovary. The embryos are oval, but taper to the posterior end. In the middle line, in front, are three sharp protractile styles, of which the middle is the longest. Two pairs of articulated limbs are attached to the middle of the ventral aspect ; each is terminated by a double hooked claw. The embryo of Lin- guatula thus resembles those of the Acarina, on the one hand, and those of such parasitic Crustacea as A?ichorella, on the other. In the case of Pentastomum tcenioides, the embryos, in- closed in their vitelline membranes, pass out of the bodies of the dog or wolf, along with the nasal mucus. Taken into the body along with the food of the hare or rabbit, they emerge from the eggy penetrate the walls of the intestine, and lodge themselves in the liver. Here they become encysted, grow, and go through a series of changes of form, accompanied by repeated ecdyses, until they pass into the state known as Pentastomum denticulatum. If the flesh of the rodent con- taining P. denticulatum is devoured by a dog, the parasite passes into the frontal sinuses or maxillary antra of the latter, gradually takes on the form of P. tmnioides, and ac- quires sexual organs. The parasitism of the Pentastomida, therefore, is verv similar to that of the Cestoidea. Spiders and Mites abounded in the tertiary epoch, as their remains, preserved in amber, show. Various Arthro- gastra occur in the mesozoic formations, while Spiders and THE MYRIAPODA. 337 Scorpions of large size have been found in the carbonifer- ous rocks. The Myriapoda. — In these Arthropods, the body is di- vided into many segments, the most anterior of which takes on the characters of a distinct head ; and almost all these segments bear articulated limbs terminated by claws. In the Centipedes (Chilopoda), the segments of the body have broad sterna, and the bases of the limbs are far apart ; but, in the Millipedes (Chilognatha), the sternal region is rudi- mentary, and the bases of the limbs are close together. Fig. 95. — A. Scolojiendra borbonica {Ghilopoda). B. lulus fiawzonatus {Chilo- gnatha).1 Moreover, in the latter s^roup, the majority of the segments of the body bear two pairs of limbs, and probably represent two somites. 1 " Eegne Animal." Illustrated edition. 338 THE ANATOMY OF INVERTEBRATED ANIMALS. The head is either flattened from above downward (Chi- lopoda), or from before backward (Chilognatha). Some species are blind, but the majority have eyes, which are gen- erally small and not very numerous ocelli, though, in some cases, they are large compound eyes. There is always a pair of jointed antenna?. The majority have the mouth constructed for biting, and are provided with a pair of mandibles, the most important peculiarity of which is that they are jointed, and thus depart less from the type of the ordinary limb than do those of in- JV Fig. %.—Scolopendra Hopei (after Newport). A. Dorsal view of the anterior part of the body : a, antennae ; A, cephalic segment ; B, basilar segment. B. Ventral view of the head ' «, B, as before. C. Under view of the cephalic segment, showing the antennas, a ; the eyes, *; the labrum and the mandibles, IV. D. The second pair of gnathites V, and the first pair of appendages of the basilar segment VI'. sects, while, to the same extent, they approach the gnathites of the Peripatidea. The mandibles are more modified in the Chilopoda (Fig. 96) than in the Chilognatha. In the latter, the second pair of gnathites form a broad four-lobed plate which plays the part of an under-lip, while, in the Chilopoda, they are soft and jointed, and united at their bases by a bi- lobed median process (Fig. 96, v'). In the Chilognatha the THE MYRIAPODA. 330 four segments which follow the head are free, and their append- ages resemble ordinary limbs. The anterior pair is turned forward and comes into relation with the mouth, and the ter- o-utn of the first somite is often enlarged ; of the other three somites, the appendages of one appear to be always abortive. Thus there are three segments with single pairs of legs. The rest each bear two pairs. In the Chilopoda, on the contrary, the head is followed by a basilar segment (Fig. 96, B), formed, according to Newport, by the union of four embryonic somites, and carrying three pairs of appendages. Of these the first are limb-like, but are turned forward beneath the mouth (Fig. 96, D, vi') ; the second pair are the strong recurved poison-claws, and the hindermost pair may become functional legs, resembling those which are attached to the succeeding somites, but are always smaller than the others, and may be altogether aborted in the adult. The somites of the body never bear more than one pair of limbs. The alimentary canal is usually straight and simple, like that of an insect larva. There are large salivary glands, and the intestine is provided with Malpighian tubules. The heart extends through the greater part of the length of the body, and is many-chambered, there being one cham- ber for each of the somites in which it lies. Each chamber is somewhat conical in shape, being broader behind than in front, and admits the blood by a pair of lateral clefts, while the blood leaves it, in part by the communication with the adja- cent chamber, in part by lateral arterial branches. A medi- an aortic trunk continues the heart forward, and the lateral trunks encircle the oesophagus and unite into an artery which lies upon the ganglionic chain. The arterial system in the Chilopoda is, in fact, as complete as that of the Scorpions.1 The respiratory organs are trachea?, which open by stig- mata on the lateral or ventral surface of more or fewer of the somites. In Scutigera the stigmata are situated in the me- dian dorsal line of the bodv. The nervous system presents a ventral chain, with a pair of ganglionic enlargements for each segment of the body, the most anterior of which are connected by commissures, which embrace the oesophagus, with the cerebral ganglia. The ovary in both Chilognatha and Chilopoda is long, ' Newport, " On the Structure, Relations, and Development of the Nervous and Circulatory Systems in the Myriapoda and Macrurous Arachnida." (" Phi- losophical Transactions," 1863.) 340 THE ANATOMY OF IXVERTEBRATED ANIMALS. single, and tubular in form. It lies above the alimentary canal in the latter, between the alimentary canal and the ner- vous system in the former. The double vaginae open on, or close behind, the bases of the second pair of legs in the Chi- lognatha ; at the posteri jr end of the body, beneath the anus, in the Chilopoda. Two spermathecae and colleterial glands are very generally present. The testes in the Chilognatha are tubular glands, which occupy the same position as the ovary, and open in the same region. They have lateral caeca, and are connected by trans- verse ducts. Two copulatory organs, or penes, are developed on the sternal face of the sixth segment which follows the head, or are connected with the bases of the seventh pair of legs. In the Chilopoda there is a good deal of variation in the structure of the testis. Thus, in Lithobius,1 the testis is a single filiform tube, connected at the hinder end with two deferent ducts which embrace the rectum. A large caecum, apparently a vesicula seminalis, opens into each deferent duct. But, in most Chilopods, the testes are fusiform acini, united by delicate ducts with a median vas deferens. Two, or four, pairs of accessory glands are connected with the opening of the male apparatus. The spermatozoa are inclosed in spermatophores in Scolo- pendra, Cryptops, and Geophihis. The Chilognatha copulate. In Glomeris and Polyxenns the genital apertures of the two sexes are brought together during copulation ; but, in lulus, the penes of the male are charged with the spermatic fluid before copulation takes place, and it is by their agency that the female is impregnated. The Chilopoda have not been observed to copulate ; in- deed, the female shows a tendencv to destrov the males, as among spiders. The male Geophihis spins webs like those of spiders across the passages which he frequents, and depos- its a spermatophore in the centre of each. -M.jtschnikoff3 has recently shown that, in the Chilognatha, the process of yelk-division is complete, and confirms the obser- vation of Newport (" Phil. Trans.," 1841), that the sternal face of the blastoderm becomes sharp]}' infolded down its centre, in such a manner that the anterior and the posterior halves of 1 Favre, "Anatomic ties oriranes reprodueteurs des Myriapodes." (" Annales des Sciences Naturelles," 1855. 1 2 " Embryolosrie der doppelfussigen Myriapoden {Chilognatha)." (Zeit- tchrift fur wiss. Zoologie, 1874.) THE MYRIAPODA. 341 that face of the embryo become closely applied together. Metschnikoff further points out that only two pairs of ap- pendages are converted into gnathites, and that a chitinous cuticula, apparently identical with what Newport describes as the " amnion " in lulus, is early thrown off from the em- bryo. In some species it develops a median tooth-like pro- cess, which serves to burst the vitelline membrane. New- port describes a short cord, or funiculus, which connects the anal extremity of the embryo with the so-called " amnion." It is not improbable that this is simply the continuation of the first larval skin into the rectum. The embryo lulus at first bursts the vitelline membrane, and is inclosed only in the embryonic integument. At this period its body is divided into eight segments, of which the first represents the head. Traces of the antennae are visible on the sides of the head, and the four following segments exhibit papillae ; those of the second, third, and fifth segments develop into the three pairs of functional limbs, with which the young myriapod is at first provided. Between the terminal segment and the seventh the body grows and becomes divided into six rudimentary new seg- ments. The terminal segment also becomes divided into two. Thus, when the young escapes from the embryonic integument, it consists of nine complete segments, including the head, with six rudimentary segments interposed between the penultimate and the antepenultimate — making fifteen in all ; which is the full number of segments (head + three tho- racic -f eleven abdominal somites) possessed by an insect larva. There is this difference, however, between the insect and the larval myriapod : that since, in the latter, there are only two pairs of gnathites, which must answer to the mandibles and first maxillae of insects, the ambulatory appendages of the second segment must represent the second maxillae of insects ; and hence, though there is apparently the same number of somites in the two cases, there must in reality be one fewer in the myriapod. The myriapod larva, therefore, notwithstanding its hexapod character, is essentially differ- ent from an insect larva. The sixth and the seventh segments develop two pairs of legs, as do all the newly-formed segments ; and it is worthy of notice that the male copulatory apparatus, inasmuch as it is situated in the seventh (sixth postcephalic) segment in the adult, is developed from one of the primary segments of the 342 THE ANATOMY OF INVERTEBRATED ANIMALS. embryo, and not from the • subsequently-added segments. New segments, each giving rise to two pairs of limbs, are developed by sixes in the germinal region between the penul- timate segment and the hindermost of the newly-formed seg- ments, until the full number of the adult is complete. In all other Ghilognatha of which the development has been traced, the young, at first, possess only three pairs of functional legs ; and one of the four segments which follow the head is apodal. According to Fabre, the apodal seg- ment in Polydesmus complanatus is the second, and not, as in lulus, the third. In the Ghilopoda the young leave the egg with seven (Lithobins, Scutigera) or a greater number of pairs of ambu- latory limbs. Scolopendra is said to be viviparous. The early stages of development of Geophilus have been de- scribed by MetschnikonV Complete yelk-division takes place, and when the young leaves the egg it has a cylindrical body, like that of one of the Ghilognatha, and possesses many pairs of limbs. Newport2 has pointed out that, in Geophi- lus longieomis, the basilar segment is formed by the conflu- ence of four somites, of the appendages of which only two are ultimately developed. Thus the basilar segment of the head of the Ghilopoda appears to correspond very closely with the four somites which follow the head in the Ghilogna- tha. Under these circumstances, the difference in the posi- tion of the reproductive apertures in the two groups is ex- ceedingly remarkable. Fossil Myriapoda occur both in the tertiary and secondary formations, and there seems no reason to doubt that the Xylobius sigillariw discovered in the coal of Nova Scotia by Lyell and Dawson is to be referred to this group. The Ixsecta. — Notwithstanding the vast number and the singular diversity of form of Insects, the fundamental unity of their structure is remarkable, and, in this respect, the group exhibits a striking contrast to the Crustacea. The division of the body into three regions — head, thorax, and abdomen — is usually well marked, not only by the peculiar modifications which the cephalic and thoracic somites under- go, but by the attachment of the three pairs of ambulatory 1 Zeitschrift fur iviss. Zoologie, 1875. 2 Monograph of the class Myriapoda, order Chilopola. (" Transactions of the Linnsean Society," xix.) THE INSECTA. 343 limbs exclusively to the latter. The head possesses four pairs of appendages, that is to say, one pair of antennas and three pairs of gnathites ; and, as a general rule, there is a pair of compound eyes, sessile upon the sides of the head ; sometimes simple eyes are added to them. The first pair of gnathites are the mandibles, which are always devoid of a palp. The second pair are the maxillce, which, in those insects in which the mouth is least modified, are distinct from one another and laterally movable ; while the third pair of gnathites are united together in the median line, and constitute the labium of entomologists. In front of the oral aperture is a median plate, the laJbrum y while from the floor of the mouth formed by the labium another median process, the lingua, is usually developed. It is hardly open to doubt that the mandibles, the maxillae, and the labium, answer to the mandibles and the two pairs of maxillae of the crustacean mouth. In this case, one pair of antennary organs found in the latter is wanting in insects, as in other air-breathing Arthropods, and the existence of the corresponding somite cannot be proved. But if it be sup- posed to be present, though without any appendage, and if the eyes be taken to represent the appendages of another somite, the insect-head will contain six somites, the praeoral sterna being bent up toward the tergal aspect, as in the higher Crustacea. The three somites which succeed the head are termed re- spectively prothorax, mesothorax, and metathorox. A pair of legs is normally attached to each ; and, wThen wings exist, the j7- are lateral expansions of the tergal region (correspond- ing with the pleura of Crustacea) of the mesothorax or the metathorax, or of both. In the abdomen there are, at most, eleven somites, none of which, in the adult, bear ambulatory limbs. Thus, assum- ing the existence of six somites in the head, the normal num- ber of somites in the body of insects will be twenty, as in the higher Crustacea Araehnida.1 One of the commonest of insects, the Cockroach (JBlatta (Periplaneta) orientalis) is fortunately one of the oldest, least 1 It is open to question whether the podical plates represent a somite ; and therefore it must be recollected that the total number of somites, the existence of which can be actually demonstrated in insects, is only seventeen, viz., four for the head, three for the thorax, and ten for the abdomen. 344 THE ANATOMY OF IXVERTEBRATED ANIMALS. modified, and in many ways most instructive forms ; at the same time, it is not too small for convenient dissection.1 In this insect the head is vertically elongated, flattened from before backward, and connected by a distinct neck with the prothorax. The antennae are slender, as long as, or rather longer than, the body. Large reniform compound eyes are situated at the sides of the head. The tergal portion of the prothorax (pronotum) is a wide shield, which overlaps the head, in front, and the tergal portion of the mesothorax, or meso?iotum, behind. The legs are strong, and increase in length from the first pair to the last. The abdomen is flat- tened from above downward, and bears a pair of elongated, many-jointed, setose styles (cerci) at its hinder extremity. The males differ very considerably from the females. They have two pairs of wings, of which the anterior are brown, and are of a stiff and horny texture. As they serve to cover the posterior wings, they are termed tegmina. When closed, the left overlaps the right, and they extend back as far as the posterior edge of the tergum of the fifth abdominal so- mite. The posterior wings, on the contrary, are thin and mem- branous ; and, in a state of rest, are folded longitudinally upon themselves, the folded edge being internal. In this condition they are triangular, the base of the triangle lying close to the posterior edge of the fourth abdominal somite, and the right a little overlapping the left. When forcibly unfolded and made to stand out at right angles to the body, each of these wings is seen to have a nearly straight, thick- ened, anterior edge, while its rounded outer and posterior edges are very thin. The wing is strengthened by radiating thickenings, or nervures, united by delicate transverse ridges ; and, when left to itself, it springs back into its folded state with some force. The abdomen of the male is not very broad. The sterna of the abdominal somites are all flattened ; and, to the hind- ermost, twro minute un jointed styles are attached, while some singular hooked processes are seen, on close inspection, to protrude between the hindermost tergum and the hindermost sternum. The abdomen of the female is very much broader, especially toward the middle of its length. The hindermost sternum is convex and boat-shaped, and its posterior half is separated along the middle line into two halves, united only 1 Sfa, for an excellent figure and description, Rolleston, " Forms of Animal Life," p. 199, plate vi. THE COCKROACH. 345 by a thin and flexible membrane. Sometimes the great egg- case, which the female carries about for some time before it is laid, is seen protruding between the posterior terga and sterna. The female has movable tegmina, but they are very small, inasmuch as they do not extend bej-ond the middle of the metathorax, and are widely separated in the middle line; they are, in fact, mere rudiments of the anterior wings. The posterior wings appear, at first, to be altogether wanting. But the outer extremities of the metanotum, or tergal portion of the metathorax, present triangular areas, in which the in- tegument is very thin, and exhibits markings which simulate the nervures of the wings. There can be no doubt, in fact, that these are undeveloped wings, and they show, in a very instructive manner, that the wings are modifications of that part of the insect skeleton which answers to the pleura, and therefore to the lateral parts of the carapace, of a crustacean. The convex dorsal wall of the head of the Cockroach (Fig. 97) is termed the epicranium. A median suture — the epicra- nial suture — may be seen, especially in young Cockroaches, traversing it from before backward, and dividing between the eyes into two branches, one of which passes toward the artic- ulation of each antenna. The basal joint of the antenna is attached to a transparent flexible membrane, which occupies an oval space, the antennary fossa, and allows of the free play of the antenna. A little projection of the hard chitinous skel- eton, when it bounds the inferior margin of the fossa, helps to support the joint. On the inner side of, and above the an- tennary fossa, there is an oval fenestra, covered only bv a thin and transparent portion of the integument, which allows a subjacent tissue of glistening wdiite appearance to be seen (Fig. 97, I., II., b). These have been regarded as rudimentary ocelli by some entomologists ; but their structure needs care- ful examination before this view can be adopted. The transparent cornea of the eye, situated external to and behind the antennary fossa, is elongated, wTider above than below, and has a concave anterior, and sliffhtlv convex posterior, margin. The numerous facets into which the cor- nea is divided are hexagonal in shape, and very small. The broad flattened region of the fore-part of the head, on the oral side of the epicranial suture, is the clypeus. It is prolonged in front of the mouth, and with the truncated edge of this prolongation the flap-like labrum is freely articulated. Behind the labrum are two, very stout, curved mandibles, strongly toothed at their extremities (Fig. 97, II., ran). Each 346 THE ANATOMY OF IXVERTEBRATED ANIMALS. mandible is articulated with the truncated edge of the lateral part of the skeleton of the head, beneath the eyes, which is termed the gena, in such a manner as to be freely movable <^k^J> -m> Fig. ST.—Blatta oriental^.— 1, II. Side and front views of the head : «, the epicranial suture, at the ends of the lateral branches of which are &, the fenestra?;/, the antennae ; g, the eyes ; lb, the labrum ; mn, the mandible; ca, the carclo ; st. the stipes ; ga. the galea ; r>h the palpus of the maxilla ; p. the palpus ; q. the men- tum and suhmentnm of the labium; #, the margins of the occipital foramen : ic. inferior cervical sclerites; Ic, lateral cervical sclerites; pn, pronofum. III. The labium anil the riorht maxilla, viewed from below; letters as before, except la, lacinia of the maxilla ; pg, paraglossia; li, ligula ; m, mentum; sm, subrnentum of the labium. toward and from the median line, but in no other direction. The proximal end of the maxilla (Fig. 97, III.) is formed by an elongated basal articulation, the cardo, which is directed transversely to the axis of the head, and is connected with the inferior margin of the epicranium, or rather with a thin skeletal band which runs round the posterior margin of the THE COCKROACH. 347 epicranium, and is firmly united with it only on its dorsal side. This band forms the boundary of the so-called occipital fora- men, bv which the cavitv of the head communicates with that of the neck, the chitinous wall of the latter region being con- tinuous with it. Articulated at right angles with the cardo is the stipes, or second joint of the maxilla. This is freely movable in the lateral direction, and its outer distal angle bears the continuation of the limb, or palpus, formed by two short and three long joints. Two processes terminate the stipes ; of these, the anterior and outer — the galea — is soft, rounded, and possibly sensory in function, while the posterior and inner — the lacinia — is a curved cutting blade with a toothed and spinose inner edge. The labium (Fig. 97, III.) consists of two incompletely- separated median plates, the submentum behind, and the men- turn in front ; upon the latter follows a bilobed terminal piece, the ligula, each lobe of which is again divided longitudinally into two portions, which have considerable similarity to the galea and lacinia of the maxilla. The outer is usually termed the paraglossa. Between the mentum and the ligula, on each outer edge of the labium, a small piece, the palpiger, is articulated ; it bears the three-jointed labial pat ]jus , which is to be regarded as the proper free termination of the second maxilla. The resemblance between the labium and a pair of maxillae which have coalesced, is obvious. The submentum is not directly articulated with the cranial skeleton, but its posterior edge is close to one of the cervical sclerites,1 or skeletal elements observable in the chitinous in- tegument of the neck, of which there are altogether seven. One is dorsal, median, and marked by a deep longitudinal depression. It articulates with the dorsal margin of the oc- cipital foramen. Four are lateral, two on each side (Fig. 97, I., Ic) ; these take an oblique course from the dorsal part of the boundary of the occipital foramen, with a tubercle of which the anterior piece is articulated, to the anterior edge of the episternum of the prothorax. The inferior cervical sclerites (Fig. 97, I., ic) are two narrow transverse plates, one behind the other in the middle line. They appear to repre- sent the part called gula, which, in many insects, is a large 1 I use this terra in the sense in which it has been employed by Milne- Edwards, to denote any definite hardened part of the chitinous skeleton. It is to the latter what a' distinct ossification is to the skeleton of a vertebrated animal. 348 ^HE ANATOMY OF LWERTEBRATED ANIMALS. plate, confluent with the epicranium above and supporting the submentum anteriorly. I think it is probable that these cer- vical sclerites represent the hindermost of the cephalic somites, while the band with which the maxillae are united, and the genae, are all that is left of the sides and roof of the first max- illary and the mandibular somites ; the epicranial expansions being mainly formed by the upward and backward extension of the ophthalmic and antennary sterna, which arise out of the procephalic lobes of the embryo. In addition to these externally-visible sclerites, there is a sort of internal skeleton (endocranium or tentorium), which extends as a cruciform partition from the inner face of the lateral walls of the cra- nium, close to the articulation of the mandible, to the sides of the occipital foramen. The centre of the cross is perforated by a rounded aperture, through which the oesophageal nerve- commissures pass. The commencement of the oesophagus traverses the interspace between the anterior processes of the cross ; the tendons of the great adductors of the mandible pass through the lateral apertures ; and the backward contin- uation of the gullet enters the thorax through the posterior aperture, included between the tentorium and the margins of the occipital foramen. In each somite of the thorax a distinct median sclerite, the sternum, may be observed ; and a much larger tergal piece, the notum. At the sides of the somite are other defi- nitely-arranged sclerites, the anterior of which appear to an- swer to the episternum and epimera in the Crustacea, while the posterior, perhaps, properly belong to the attached limb. Forked or double apodemes, the antefurca, medifurca, and postfurca, project from the sternal wall of each somite of the thorax into its cavity. They support the nervous cord and give attachment to muscles. The legs present a large basal joint, the coxa, between which and the third, termed femur, a small articulation, the trochanter, is interposed. Upon the femur follows an elon- gated tibia ; and this is succeeded by the tarsus, which con- sists of six joints. Of these, the proximal joint is long and stout, the three next are short, the fifth is elongated and slen- der ; the sixth, very short, is terminated by two curved and pointed claws [ungues).1 1 Mr. Wesstwood (" Modern Classification of Insects," vol. i, p. 416) says that the tarsi are five-jointed, and that there is a pulvillus between the ungues. The sixth joint appears to he what Mr. Westwood terms puhillus, hut it is a true joint, provided with a special flexor, the slender tendon of wrhich, how- ever, traverses several of the joints of the tarsus. THE COCKROACH. 349 The broad differences in the structure of the abdomen of the male and female have been already pointed out. Of the eight terga externally visible in the female (Fig. 98), the first is shorter than those which succeed it ; and the hindermost (Fig. 98, 10) is escutcheon-shaped, deflexed at the sides, thin in the middle, and notched at the end. When this tergum is gently pulled backward, two other very narrow terga (Fig. 98, 8, 9), of which the anterior overlaps the posterior, and which were hidden between it and the antepenultimate or seventh tergum, become visible. The apparent eighth tergum is therefore really the tenth. Beneath the tenth tergum are two triangular podical plates (Fig. 98, 11), one on each side of the anus. Provisionally, I take them to be the sclerites of the eleventh abdominal somite. The first sternum is confluent with the second, and largely hidden by the coxae of the metathoracic limbs. The seventh is greatly enlarged, and its posterior edge is produced into a boat-shaped process, nearly divided into two portions by an inward fold of the integument along the median line. Completely hidden by the seventh sternum is a thin plate, narrower in front than behind, where it is produced on each side. Anteriorly, it is articulated with the sternum of the following somite, so as to form a sort of spring-joint, which ordinarily keeps it applied against the latter, and therefore directed obliquely upward and a little forward. The large aperture of the vulva (Fig. 98, v) lies in the middle of this plate. On the sternal region behind the vulva, between it and the anus, arises a pair of elongated processes, divided into two portions, of which the outer is thick and soft, the inner slender, pointed, and hard. They embrace and partly ensheath two other processes, having somewhat the shape of knife-blades, the anterior fixed ends of which are curved, and, being attached to the sides of the somite to which they be- long, are distant, while the blades meet, and are applied to- gether in the middle line. Of these, which may be termed gonapophyses, the study of their development shows that the posterior bifid pair belong to the ninth somite", while the an- terior pair belong to the eighth. The cerci (x) are attached to the dorso-lateral part of the tenth somite. In the abdomen of the male JBlatta (Fig. 99) the ten terga are readily discernible ; but the eighth and ninth are very short, and the former overlaps the latter. The tenth tergum is flat, and has a freely-projecting, truncated, posterior mar- 350 THE ANATOMY OF INVERTEBRATED AXIMALS. gin. Articulated beneath its lateral edge are two multiartic- ulate eerci (x), similar to those of the female. Beneath the tenth tergum, and hidden by it, are the two podical plates (11) between which the anus opens. The first sternum is small, and may easily escape notice. The second to the sixth sterna are of nearly equal width and length. The seventh and eighth are narrower; the ninth still narrower and longer, about half of its length being covered by the eighth. The covered half is different in texture from the uncovered, being thinner and more transparent, and its anterior margin is deeply notched. The uncovered half is strong, horny, and dark-colored, convex below and concave above ; its free pos- terior margin is obscurely trilobed by two lateral, shallow notches. On each side, a slender, unjointed, setose style, which projects backward and outward, is attached to this sternum. Thus, the tergal surface of the abdomen of the male essen- tially resembles that of the female, while the sternal surface differs in exhibiting two sterna more (namely, the eighth and ninth) without dissection. Hence, while in the female the opening of the recto-genital chamber lies between the tenth tergum and the seventh sternum, in the male it lies between the tenth tergum and the ninth sternum. When the tenth tergum and the podical plates are removed, a very singular apparatus, the male genital armature, comes into view. It consists of a number of chitinous processes having the form of plates and hooks, the exact form and dis- position of which could be made intelligible only by numerous figures. It may be stated generally, however, that these plates and hooks terminate processes of the sternal region of the tenth somite, on each side of the aperture of the vas deferens, and therefore, though they are of the same nature as the gona- pophyses of the female, they are not their exact homologues. The most conspicuous division of the right gonapophysis is a broad plate divided at the extremity into two portions, the inner of which curves inward and ends in two or three sharp spines, while the outer is coiled upon itself so as to resemble a short corkscrew. The left gonapophysis is pro- vided with a long process like a tenaculum, the incurved extremity of which is denticulated. The alimentary canal of the Cockroach commences by the oral cavity, situated between the labrum in front, the mandi- bles and maxillae at the sides, and the labium, with the large THE COCKROACH. 351 Fig. 98. -fit— Mil mt VIA -o — XIV 7 t!rckRffiz 23 xw fdrl-8 XV// £- win 352 THE ANATOMY OF INYERTEBRATED ANIMALS. Fig. 98. — Longitudinal and vertical section of a female Cockroach (Blatta). — 1 to xx, somites of the hody ; 1 to 11. somites of the abdomeu ; A. antenna : lb. labium ; a. mouth; b, oesophagus : c, crop ; d. proventricnlus : e. pyloric caeca : /. chyliflc ventricle : g. insertion of the Malpighian caeca : h. intestine : i. rectum ; r, vulva ; /. salivary trland : k. salivary receptacle. By an error, the duct is made to termi- nate above instead of beneath the lingua. H. position of heart : m. cerebral gan- glia ; .V, thoracic ganglia ; x. cerci. lingua, or hypopharynx^bohmd. The oesophagus, beginning as a narrow tube, passes between the anterior crura of the tentorium, and then, leaving the head by the occipital foramen Fig. 99.— Longitudinal and vertical section of the abdomen of a male Cockroach {Blatta). — 1. 2. 3. 4. etc.. terga and sterna of the abdomen ; t. mushroom-shaped gland ; v. aperture of the vas deferens ; A. anus. and traversing the neck and thorax, gradually widens into the large crop or ingluvies (Fig. 98, c), which lies in the ab- domen. This is followed by the small thick-walled proven- triculus (Fig. 98, d), shaped like a pear, with its broad end applied against the crop. The narrow end of the proventricu- lus opens into a wide canal, the so-called chylific ventricle or THE COCKROACH. 353 ventriculus (Fig. 98, f), an elongated tube, the junction of which with the intestine is marked by the insertion of the numerous Malpighian tubes. The anterior end of the ven- triculus is provided with seven or eight caecal diverticula of un- equal lengths (Fig. 98, e), the pyloric cceca. The first portion of the intestine {ileum) is narrow. The next, termed the colon, is very wide, and somewhat sacculated. A constric- tion marks off the region of the colon from the straight short rectum (Fig. 98, i), which terminates in the anus, situated at the hinder extremity of the body between the podical plates.1 The aperture by which the mouth communicates with the gullet is small, and situated at the superior and anterior part of the buccal cavity. A broad projection of the posterior and inferior wall of the buccal cavity occupies all the space between the oesophageal opening of that cavity and the labium, and ends in a free subcylindrical process. This is termed hypo- pharynx or lingua, but it might be well to reserve the term lingua for the free end, and hypopjharynx for the attached posterior portion. The anterior surface of the hypopharynx slopes downward and forward; its sides are supported by two sclerites, which are narrow and rod-like above and broad be- low, where they unite in an arch on the dorsal face, just where the free part, or lingua, begins. On the under side of the lin- gua are two broader sclerites, which also unite and form an arch, which lies over the opening of the salivary duct. The anterior surface of the lingua and hypopharynx is beset with fine hairs. The two salivary glands, with their receptacles, are greatly developed in the Cockroach.2 The glands (Fig. 98, I) lie on 1 M. F. Plateau ("Eecherches sur les phenomenes de la digestion ehez les Insectes," 1S74 ; "Note sur les phenomenes de la digestion chez la Blatta amerieaine [Periplaneta Americana]," 1870 ; and " Recnerches sur les pheno- menes de la digestion chez les Myriapodes," 1876) divides the alimentary canal of insects and myriapods into a buccal, a median, and a terminal portion. The buccal portion consists oftne oesophagus, crop, and proventriculus — which last he considers to be a mere strainer, and to have no masticatory function. The middle division lies between the proventriculus and the insertion of the Mal- pighian tubes. The terminal division extends from tire latter point to the anus. With the solitary exception of lulus, the secretions of the alimentary canal are always alkaline,*and that which etfects the transformation of the albuminoid elements of the food into peptones appears to be furnished by the middle di- vision, which is lined by epithelium, devoid of any cuticle. In carnivorous in- sects digestion may take place in the crop by the flow of the secretion of the middle intestine into it. The salivary fluid of Blatta rapidly effects the trans- formation of starch into sugar. 2 The salivary glands are well described by Bascli, " Untersuchungen uber die chvlopoietische und uropoietische Svsteme der Blatta orientalis." ("' Sitzb. Wiener Akad.," 1858.) 354 THE ANATOMY OF INVERTEBRATED ANIMALS. each side of the cesophagus and crop, extending through the thorax, as far as the commencement of the abdomen. Each gland is a white mass, as much as a quarter of an inch long, and composed of numerous acini. The ducts which arise from these acini unite first into a single trunk on each side, and then, beneath the sub-cesophageal ganglion, the two trunks join to form the single short salivary duct which opens be- neath the lingua. The ducts of the salivary glands are lined by a transversely-ribbed chitinous membrane, so that they greatly resemble tracheae. The salivary receptacles (Fig. 98, k) are elongated oval sacs, three-eighths of an inch long, each of which is situated at the extremity of a long duct. The ducts unite in front with one another, and with the duct of the gland, to form the short terminal common duct. The receptacle and its ducts have a chitinous lining similar to that of the duct of the glands, but the spiral marking does not extend over the walls of the re- ceptacle. The proventriculus has a thick muscular coat, and the chi- tinous iinino; which is continued into it from the in^luvies is greatly thickened, and produced into six hard, browm, ridge- like principal teeth. Posterior to these is a circle of six prominent cushions covered with setae, and similar setae beset the lining membrane of the funnel-shaped cavity into which they project. Between each pair of principal teeth are five smaller tooth-like ridges, of which the median is the largest, and a variable number of still finer lono-itudinal elevations lie between them. The proventriculus leads posteriorly into a narrow, thick- coated canal, the tubular extremity of which projects freely into the much wider anterior end of the chylific ventricle, and constitutes a very efficient valve. The short and narrow anterior division of the intestine (ileum) is separated from the colon by a circular valve, the surface of which is beset with small spines. The Malpighian glands are very numerous (20-30), deli- cate, caecal tubules, of even diameter throughout, and lined by a small-celled epithelium inclosing a central cavity. The communication between the colon and the rectum is very narrow, but is not valvular. The walls of the rectum itself are raised into six ridges, which project into its inte- rior and are abundantly supplied with trachea? ; these are the so-called rectal glands. Anal glands appear to be absent. The histology of the alimentary canal has been particu- THE COCKROACH. 355 larly studied by Basch.1 From the oral cavity to the funnel- shaped extremity of the proventriculus, it is lined by a chiti- nous coat continuous with the chitinous layer of the integu- ment, and beset for the greater part of its extent with fine setiform processes. Beneath this is the proper endoderm, consisting of a layer of cells. Next follows a structureless membrana propria or basement membrane ; and this is suc- ceeded by two layers of striped muscular fibres, the internal disposed longitudinally and the external circularly. In the proventriculus, the muscular layers become much thicker, and some of those of the outer layer acquire a radial arrangement, while the longitudinal muscles are disposed in bundles which correspond with the six principal ridges. In the chylific ven- tricle, the muscular layers and the basement membrane are disposed much as before. The basement membrane presents pits on its free surface in which rounded cells are lodged, and is beset between these by the elongated cells of a cylinder epithelium. The free ends of these present a thick wall, marked by vertical striations. There is no chitinous layer. The caeca are merely diverticula of the wall of the chylific ventricle. The intestine, finally, repeats the structure found in that part of the alimentary canal which lies in front of the chy- lific ventricle and is provided with a setose chitinous lining. Basch found the secretion of the salivary glands and the contents of the crop acid,2 and that an infusion of the sali- vary glands, acidulated with hydrochloric acid, digested fibrin. The contents of the chylific ventricle were neutral or alka- line : and an infusion of the chvlific ventricle at once turned starch into sugar. The same effect wras produced by an infu- sion of the salivary glands. The heart (Fig. 98, h) is a slender inconspicuous tube, which occupies the middle line of the dorsal wall of the abJomen, and presents, at intervals, pairs of lateral apertures. The wall of the abdomen internal to the chitinous integu- ment is lined by a soft cellular substance (hypodermis), the outer layer of which represents the ectoderm or epidermis, while the deeper part is the parietal layer of the mesoderm. This last contains a stratum of longitudinal muscular fibres, divided into segments or myotomes, in correspondence with the somites, and numerous tracheae. The heart is inclosed in 1 _ Sitzunsrsberichte der Wiener Akademie," xxxiii., 1858. 2 Plateau denies that the salivary secretion of Blatta is ever acid, and as- cribes the occasional acidity of the contents of the crop to the food. 356 THE ANATOMY OF INVERTEBRATED ANIMALS. the abdominal wall which surrounds it on all sides, leaving only a small pericardial space.1 Beyond the slender aortic canal in which the heart terminates anteriorly, and which passes into the thorax and the head, no vessels appear to be given off from the heart. Delicate triangular sheets of muscular fibre, the alary muscles, are attached in pairs by their bases to the wall of the pericardial chamber, while their apices are inserted into the hypodermis. They occupy the interspaces left by the principal dorsal branches of the tracheae, which form arches on each side of the heart. From the inner face of the abdominal wall, processes are given off, some of which appear to hang freely into the ab- dominal cavity, while others accompany the numerous tracheae which pass to the alimentary canal. When the abdominal cavity is laid open, its inner lining has a villous appearance, and often seems to be full of free granular matter, as the pro- cesses very readily break up into fragments. The substance which thus fills up the interspace between the parietes of the abdomen and the contained organs is the corpus adiposum. It is made up of cells often so arranged as to form a network, and it usually has a milk-white color, which arises partly from the air contained in the tracheae, and partly from innumerable, strongly refracting granules contained in its component cells. There are ten stigmata on each side of the body of Blatta, eight in the abdomen, and two in the thorax. The latter are situated between the prothorax and mesothorax, the meso- thorax and the metathorax, respectively ; above the attach- ment of the coxae and beneath the terga. The abdominal stigmata lie in the soft integument which connects the sterna and terga of the somites. All the stigmata are situated in conical thickened elevations of the integument. The thoracic stigmata are the largest, and the anterior pair have a distinctly two-lipped aperture, the anterior lip being notched in the cen- tre. The openings of the abdominal stigmata are more ova], and are inclined backward. Immediately within each stigma the tracheal trunk into which it opens is provided with a val- vular arrangement, by which the passage can be closed or opened. 1 Cornelius (" Beitrage zur niiheren Kenntniss von Periplaneta (Blatta) ori~ entalis," 1853) found that the pulsations of the heart could readily be observed in Blattce which had recently undergone ecdysis. They were as frequent^ as eighty in the minute ; but allowance must be made for the disturbed condition of the insects under observation. THE COCKROACH. 357 The large tracheae which take their origin from these stigmata immediately divide and give off dorsal and ventral branches ; the former unite in a series of arches on each side of the heart, while, on the ventral side, the branches are connected by trunks which run parallel with the abdominal gano-lia. Large tracheae pass from the anterior thoracic stigma through the neck into the head, and, in the abdomen, the vis- cera receive an abundant supply of air-tubes. \-e 9 Ah lh h Fig. 100.— Blatta orientalh.—C, the brain with the antennary (a) and optic (b) nerves ; c, e, /, g\ f>, etomato-gastric nerves. B. the anterior end of the gullet. A, the crop. Z>, the gizzard. The lobes of the corpus adiposum are also plentifully supplied with tracheae, while fine trunks enter the substance of the ganglia and nerves and there ramify. Tracheae accom- pany the nervures of the wings and are abundantly distrib- uted to the muscles. The nervous system consists of the supra-oesophageal ganglia (Fig. 100, A), commonly termed the brain, united by 358 THE ANATOMY OF INVERTEBRATED ANIMALS. thick and short commissures with an infra-cesophageal gan- glionic mass, situated in the head; of three pairs of large coalesced ganglia in the thcrax, one for the prothorax, one for the mesothorax, and one for the metathorax ; of six pairs of closely-united smaller ganglia in the abdomen ; and of a set of visceral or stomato-gastric nerves. The several pairs of thoracic and abdominal ganglia are united by double com- missural cords. In the males the commissures which unite the abdominal ganglia are not straight, but are bent, as if it were needful to make allowance for the possible elongation of the abdomen. The supra-oesophageal ganglia give off the nerves to the antennas from their antero-lateral angles ; while their postero-lateral angles are produced into the great optic nerves. Above the margin of each antennary nerve there is a small rounded tubercle which is in immediate relation wTith the silvery patch which shines through the fenestra on the inner side of the antennary fossa. Beneath this tubercle, and on the inner side of the antennary nerve, arises the root of the stomato-gastric system of nerves. Each root passes forward for a short distance, then turns inward, and in the middle line enters a heart-shaped ganglion situated on the gullet (Fig. 100, c). From this a median cord passes back- ward beneath the brain and enters a ganglion, which is con- nected on each side with two others (e, e). The continuation of the median cord passes back along the tergal wall of the oesophagus, and where this begins to dilate into the crop ends in a small triangular ganglion (g), whence lateral branches are given off, which can be traced as far as the gizzard. The exact form and arrangement of the male organ of generation has only recently been made out. The most con- spicuous of these organs is a mushroom-shaped gland (Fig. 99, t) composed of a great number of short caeca attached to the extremity of the also very short vas deferens. It is lodged in the hinder end of the abdomen, and covers the posterior abdominal ganglion. The contents of the caeca are viscid, granular, and usually brilliantly white. The anterior end of the vas deferens is dilated, and the caeca are arranged in two groups which open into each side of the dilatation. The contents of the vas deferens are also white and viscid, and evidently consist in great measure of the secretion of the caeca. In the adult male, however, innumerable sperma- tozoa with straight rod-like heads, and long flagella, are to be found intermingled with the contents of the vas deferens and THE COCKROACH. 359 its dilatation. On the sternal side of the mushroom-shaped gland, between it and the last abdominal ganglion, there is an accessory gland composed of dichotomous monilated tubes, lined by a columnar epithelium, all bound together by a com- mon investment into a flattened elongated mass. As the duct of the mushroom-shaped gland in the adult male always contains spermatozoa, and no other organ con- tainino- spermatozoa is to be found, this gland has naturally been taken for the testis. Rajewsky,1 however, has recently pointed out that the true testes are situated in the tergal region of the abdomen, and that they may be found in this region in the young and yet wingless males, though they are much obscured by the corpus adiposum which invests them. He traces the efferent duct of the testis to the glands just mentioned. In the adult male the testes atrophy, and are hardly to be discovered among the masses of the corpus adi- posum. I have found the testes in the young males in the position assigned to them by Rajewsky. They consist of numerous oval or pyriform sacs attached by short pedicles to a common duct. The ovaries (Fig. 101) are two groups of eight tubes, sit- uated on each side of the hinder half of the abdomen. The ovarian tubes, or ovarioles, of each group communicate with a short oviduct, which soon unites with its fellow in the mid- dle line and opens externally by the very short and wide vagina. The finely tapering anterior ends of the ovarioles of each side are continued forward by delicate cellular pro- longations. These finally unite together into one long fila- ment, which can be traced for some distance forward among the lobes of the corpus adiposum. It is a cellular cord, which appears to be nothing but a process of the mesoderm. Nu- merous nucleated cells, from some of which the ova take their origin, while others remain as interstitial cells, which are eventually converted into an epithelium, make up the sub- stance of the slender anterior terminations of the ovarioles. The ova situated behind these enlarge, and become disposed in a single series. Further on, the epithelial cells form a thick stratum round each egg, and possibly assist in the formation of the large vitellus with which it is ultimately provided. As the egg advances towrard maturity, the vitellus acquires first a finely and then a coarsely granular structure, and the ger- minal vesicle and spot, previously conspicuous, are no longer 1 Hofmann and Schwalbe, " Jahresberickt," 1875. The original paper ia in Russian, and 1 have not seen it. 360 THE ANATOMY OF FNVERTEBRATED ANIMALS. to be seen. Behind the junction of the oviducts with the vagina and the last abdominal ganglion which lies upon the latter, there is a small sac with a long neck from which a short Fig. 101.— Blatta orientalis.— Female genital organs : a, the posterior abdominal ganglion ; 6, the oviducts ; c, d, e, the ovarian tubes ; /, the filament by which their extremities are united ; g, the spermatheca ; h, the colleterial glands. cascal process is given off. It has a thick chitinous lining and a muscular investment, and is the spermatheca. Behind it are two large, ramified, tubular colleterial glands, which prob- ably give rise to the substance of which the egg-case is formed. Their conjoined ducts open behind the spermatheca. The eggs are inclosed, sixteen together, in strong capsules of a horny consistency, shaped somewhat like a cigar-case, and presenting a longitudinal slit, the raised and serrated edges of which are closely applied to one another. It is through this slit that the fully-developed young make their exit. The eggs attain one-sixth of an inch in length. Each has its own thin but tough brownish shell, the surface of which is beauti- fully ornamented with hexagonal patches of minute tubercles. They are arranged parallel with one another in two opposite series, one series occupying each half of the case. The eggs, adapting themselves to the form of the case, are convex out- ward and concave inward, and thus, though their ends touch, a median space is left between the two sets. The inner con- cave face of the egg is that on which the sternal face of the embryo is situated. The female carries the egg-case about THE COCKROACH. 361 for a week or more, before depositing it. The young leave the eggs as minute active insects, colorless, except for the large dark eyes. Before they are hatched they acquire eyes, antennae, gnathites, legs, and short cerci, which differ only in detail from those of the perfect Blatta, into which the larva passes by successive ecdyses. According to Cornelius {I. c, p. 29), the Cockroach undergoes seven ecdyses : the first im- mediately on leaving the egg, the second a month later. After the second ecdysis the insect sheds its skin only once a year ; so that it attains its adult condition only in its fifth summer. The chitinous cuticula splits along the median line of the tergal aspect of the head, thorax, and abdomen, before it is cast. Thus the Cockroach is said to be an insect without meta- morphosis. For although the male, in the later stages of its growth, acquires wings, and thus does become very sensibly metamorphosed from a merely cursorial animal to one which has, at any rate, the capacity for flight, there is no period in the life of this insect in which the larva passes into a resting condition, during wrhich it takes no food, and in the course of which it develops its wings. In other words, the Cockroach passes through no pupa-state, which the insect enters as a larva, and leaves as an imago, such as is so well known to occur in the course of the development of Moths and Butter- flies. The term metamorphosis, in its technical entomologi- cal sense, is applied only to that succession of changes of which such a definite pupal condition forms the middle term. It is obvious that a metamorphosis, in this sense, is a sec- ondary complication superinduced upon the direct and grad- ual process of development exhibited by such insects as the Cockroach ; 1 and that the Metabola, as insects having a metamorphosis are termed, are, so far, more differentiated than the Ametabbla, or those which have no metamorphosis. Again, in each of these divisions it is clear that the insects which never possess wings are less differentiated, or more embryonic, than those which are winged. And, finally, insects with the parts of the mouth in the condition of ordinary gnathites are less differentiated than those in which such 1 Sir John Lubbock has shown that the young Chloeon {Ephemera) lUmidi- atum undergoes more than twenty ecdyses, each accompanied by a slight change of form in its passage to the adult state. (" Transactions of the Lin- nsean Society," 1863.) 362 THE ANATOMY OF INVERTEBRATED ANIMALS. gnathites are changed in form and function, or become con- fluent. The insects which, in this view of their morphological re- Fig. 102.— Campodea staphyHnus, one of the Thysanura (after Lubbock).1 lations, occupy the lowest position in the group, are the Col- lembola and Thysanura, the Mattophaga, and the Pedicu- lina, inasmuch as they possess no trace of wings and undergo no metamorphosis. The Collembola and Thysanura undergo no metamorpho- sis, and are always wingless. The abdomen contains six seg- ments only in the Collembola (Pochira, Smynthurus, Tomo- ceros), in which group the mouth is usually provided with mandibles and maxilla?, though these, instead of being artic- ulated with the sides of the head, are capable of being re- tracted into its interior.2 In the genus Anoma the mouth is suctorial. 1 "Monograph on the Collembola and Thysanura" pi. liii. 2 Ibid., p. 37. THYSAXURA.— PEDICULINA. 363 The Thysanura {Lepisma, Gampodea, Japyx) resemble the young Blattm. They have ten well-marked abdominal so- mites (Campodea, Fig. 102), ancf the gnathites conform to the mandibulate type. The abdomen in Machetes has a pair of elongated cylindrical appendages attached to every seg- ment except the first ; while Campodea and Japyx have seven pairs of such abdominal appendages.1 The Collembola are provided with a curious tube or sucker, which is attached to the sternum of the first abdominal so- mite, and gives exit to a glandular process, which secretes a viscid matter. Most of the insects belonging to this group possess a curiously-contrived " spring and catch " attached to the sternal region of the penultimate or antepenultimate so- mites of the abdomen, by the help of which they execute their vigorous leaps. Sir John Lubbock could find no trace of trachea? in any of the Collembola except Smynthurus, though they are easily seen in many of the Thysanura. According to the same au- thority, Lepisma has four Malpighian tubes, while Campo- dea, Japyx, and many Collembola, have none. The Mallopjhaga are parasites upon mammals and birds, on the hairs and feathers of which they feed. The head and body are depressed, the eyes simple, the gnathites of the mas- ticatory type. The abdomen has nine or ten visible segments. The Pedlculina, or Lice, subsist upon the blood of the mammals on which they are parasites. The gnathites are converted into a piercing and sucking apparatus. The under- side of the head presents a soft protrusible proboscis, pro- vided externally with minute horny hooks, and traversed by a canal which leads into the oesophagus. The proboscis in- closes two grooved chitinous styles, which are applied to- gether by their concave sides ; and, within the sheath thus formed, lie two finely-pointed chitinous seta1, which can be moved up and down in the sheath.2 The proboscis is, in all probability, formed by the union of the labrum with the second pair of maxillae, while the two halves of the horny sheath are the mandibles, and the setae, the first maxilla?. The prothorax, mesothorax, and meta- 1 The rayriapod ScolopeiulrUla has similar appendages attached to each 6egment along with legs. (Lubbock, I. c.) 2 Gerstfeldt, " Ueber die Mundtheile der saugenden Insecten," 1853.. 364 THE ANATOMY OF INVERTEBRATED ANIMALS. thorax are hardly distinguishable, and the abdomen has nine visible segments. Fig. 103.— Perla nigra— k. The aquatic apterous larva. B. One of the transitional stages between this and the perfect insect, C. (-t Kegne Animal.1') The Orthoptera (Fig. 103) and the Hemiptera (Fig. 104) are ametabolous. The majority have two pairs of similar or more or less dissimilar wings in the adult state, and in the apterous forms it is probable that the wings are aborted, not typically absent. In the Ortliopterax (the Termites, Cock- roaches, Grasshoppers, Crickets, Day-flies, Dragon-flies, and Fig. 104.— Aphis pelargonii. Apterous agamogenetic form. Earwigs) the mouth is constructed upon the same plan as that of Blatta ; but the Physopoda or TJiysanoptera {Thrips 1 The Thysanura and the PJn/snpoda are often united with the Ortlwptera in modem classifications, while the Ephemeridsiptera, in which the thoracic and abdominal ganglia are fused into a common mass. A system of stomato-gastric nerves, similar in its general arrangement to that of Blatta, is very generally present. A special system of nerves, termed respiratory or trans- verse, is found in very many insects, both in the larval and in the perfect condition. The principal nerves of this system are arranged in pairs on the sternal aspect of the body, and their outer extremities anastomose with branches of the or- dinary peripheral nerves, and are distributed to the muscles 378 THE ANATOMY OF INVERTEBRATED ANIMALS. of the stigmata. Their inner ends unite into a plexus, which lies over the interval between two of the ganglia of the cen- tral nervous cord, and they are connected by longitudinal cords with one another, and with these ganglia. In insects, as in other arthropods, the branches of the nerves which are distributed to the integument, and especially those which pass to the bases of the larger or smaller seta? with which the integument is provided, frequently end in minute ganglia. Hensen has shown that in the Crustacea similar setas in all probability have an auditory function ; and Leydig, Hicks, Lespes, Landois, and others, Lave ascribed functions of special sensation to these structures in insects. But whether these setae, on the antennae or elsewhere, sub- serve either hearing or smell, is still very doubtful ; and the only organs which can safely be regarded as auditory in in- sects are those which occur in Grasshoppers (Acridido:), Crickets (Achetidce), and Locusts (Locustidoe), and which were first accurately described by Von Siebold.1 Recently, they have been studied by Leydig, Hensen, Ranke,2 and Os- car Schmidt,3 but it must be confessed that much obscurity still hangs over their minute structure. In the Acrididw, the chitinous cuticula of the metathorax presents on each side, above the articulation of the last pair of legs, a thin tympaniform membranous space surrounded by a raised rim. On its inner face, the cuticular layer of the tympaniform membrane is produced into two processes, one of which is a slender stem ending in a hollow triangular dila- tation. A large tracheal vesicle lies over the tympanic mem- brane, and between its wall and the latter, a nerve derived from the metathoracic ganglion, passes to the region occupied by the processes, and there enlarges into a ganglion, the outer face of which, beset with numerous glassy rods arranged side by side, is in contact with the tympaniform membrane. A nerve arising from this ganglion passes along a groove to the " stem " and ends in a ganglion in its dilatation. From this ganglion certain fine filaments proceed. In the Achetidce and Locustidm the tibiae of the fore-legs present similar tympaniform membranes which are easily seen in the common Cricket, but, in other forms, become hidden 1 " Archiv fur Naturgeschichte," 1864. 2"Beitrage zu der Lehre von den Uebergangs-Sinnesorganen." (Zeit- schriftfiiT wiss. Zoologie, 1875.) 3 'Schmidt, "Die Gehororgane der Heuschrecken." (" Archiv fur mikr. Anatomie," 1875.) THE PHOTOGENIC ORGANS OF INSECTS. 3T9 by the development over them of folds of the cuticle of the adjacent region of the limb. Two spacious tracheal sacs oc- cupy the greater part of the cavity of the tibia, and a large nerve ends in a ganglion in the remaining space. Upon this ganglion a series of peculiar short rod-like bodies are set. The compound eyes of insects differ only in detail from those of the Crustacea. In the ocelli, or so-called simple eyes, a sclerotic, a cornea, a lens, a vitreous humor, and a choroid coat, have been dis- tinguished, and the whole organ has been compared to the vertebrate eye. But the "lens " appears to be always a mere thickening of the cuticle which constitutes the cornea, and the so-called "vitreous humor " is partially or wholly made up of crystalline cones analogous to those which are found in the compound eye. In this respect the ocellus of the insect resembles the simple eye in Arachnida and Crustacea.1 Many insects, as the Glow-worm and Lantern-flies, are re- markable for their power of emitting light. According to Schulze,2 the males of Lampyris splendidula possess two photogenic organs, which lie on the sternal aspects of the penultimate and antepenultimate abdominal somites. Each is a thin, whitish plate, one face of which is in contact with the transparent chitinous cuticula, while the other is in relation with the abdominal nerve-cord and the viscera. The sternal gives out much more light than the tergal face. The photogenic plate is distinguishable into two layers, one occu- pying its sternal and the other its tergal half. The former is yellowish and transparent, the latter white and opaque, in consequence of the multitude of strongly refracting granules which it contains. Trachea? and nerves enter the tergal layer, and for the most part traverse it to terminate in the sternal layer, which alone is luminous. Each layer is composed of polygonal nucleated cells. The granules are doubly refrac- tive, contain uric acid, and probably consist of urate of ammo- nia (Kolliker). Hence the cells of the layer which contain them are termed by Schulze the " urate ceils," while he calls the others the " parenchyma cells." The branches of the tracheal which ramify among the parenchyma cells end, like those of other parts of the body, in stellate nucleated cor- 1 Ley dig, "Das Auge der Gliederthiere," 1864. Landois, " Das Raupen- auge " {ZemchHft fur wis*. Zooloc/ie, 1866), and " Zur Entwickelungsgeschichte der facettirten Augen von Tenehris molitor" (ibid., 1867). 2 " Zur Kenntniss der Leucktorgane von Lampyris %pUndidula.n (" Archiv far mikr. Anatomic, " 1855.) See also Kolliker, " Wiirzburg Pkys. Med. Ge- sellsckaft," 1857. 380 THE ANATOMY OF IXYERTEBRATED AXDIALS. puscles, one process of the corpuscle passing into a ramifica- tion of the trachea. Sclmlze is inclined to think that the other processes end in parenchyma cells. The nerves of the photogenic plates are derived from the last abdominal ganglion ; they branch out between the paren- chyma cells into finer and finer branches, which eventually escape observation. The female reproductive organs of insects consist of the ovarian tubes, or ovarioles, with their so-called peritoneal in- vestments, and of the oviducts, which unite into a vagina ; while a spermatheca, and, generally, accessory glands open into, or close to, the vagina. The ovarioles may be few or very numerous. Each con- sists of an external structureless membrana propria, within which lies a solid columnar mass composed of cells. The an- terior, usually tapering, end of this ovarian mass is composed of protoplasmic substance in which nuclei are imbedded, but in which the contours of the cells which they indicate are not distinguishable. Further back, some of these nuclei enlarge, become surrounded by an accumulation of protoplasm, and constitute the primitive ova. Each primitive ovum is sepa- rated from its fellow by a layer of nucleated protoplasm wThich thus forms a capsule around it. In some insects, such as JBlatta, the capsule is hardly distinguishable in those ova which lie between the smallest and those of middling size, which follow the former in order from before backward. But, in the larger ova which succeed these, the cells of the ovicap- sule rapidly enlarge in a direction perpendicular to the sur- face of the ovum, and constitute a very well-marked epithelial layer. I am inclined to believe that, for some time, an addi- tion is made to the vitellus of the egg by these epithelial cells, and that they, in fact, play the part of vitelligenous cells. But however this may be, before long, a delicate struct- ureless lamella appears on the surface of the vitellus and incloses the egg as a vitelline membrane. The epithelial cells of the ovicapsule next secrete from their surface a thicker, often ornamented, layer of chitinous substance, which consti- tutes the chorion, and the egg is complete. The ovarian mass, therefore, as Waldeyer has justly pointed out, corresponds with one of the epithelial tubes of the ovary of a vertebrated animal, and the ovicapsules answer to Graa- fian follicles. In some insects, as Aphis, the indifferent tissue of the an- terior end of the ovarioles gives rise not only to ova and ovi- THE OVARIA OF INSECTS. 381 capsular epithelium, but to large vitelligenous cells. These stay in the dilated anterior chamber of the ovarian tube. But each ovum is originally connected by continuity of sub- stance with one of these cells, and the pedicle of connection may be traced even to the second and third ovum. It seems probable, therefore, that these " vitelligenous cells," for some time, supply material to the growing ova. * In most insects, similar vitelligenous cells are found ; but they are situated at the anterior end of each ovieapsule, so that, as the column of ovicapsules lengthens by the addition of new ovicapsules to its anterior end, the vitelligenous cells are interposed between every two ova. The vitelline mem- brane and the chorion first invest the posterior extremity and the sides of the ovum ; and, for some time, leave an opening at the end of the ovum adjacent to the vitelligenous cells. This opening- is usually only partially closed, and what re- mains of it constitutes the aperture or apertures, termed the micropyle, through which the spermatozoa enter when the egg is fecundated. The vitelligenous cells usually remain outside the ovum, and eventually undergo degeneration ; but, in many Diptera, they become inclosed within the coats of the ovum and their substance is merged in that of the vitellus. Dr. A. Brandt has proposed the term panoistic for ovaries of the first mode, and meroistic for those of the second and third modes of development of the ova here described. So far as is at present known, only the Orthoptera and the Pidi- eidcs possess panoistic ovaria. The peritoneal coat of the ovarioles is a cellular struct- ure, containing many trachea? and, frequently, muscular fibres. It is usually extended beyond the anterior end of each ovari- ole into a filamentous process, which, after uniting with those of the other ovarioles of the same side, is continued into the pericardial tissue. At its opposite extremity it passes into the walls of the oviduct, which are muscular and are lined by an epithelium. The development of the ovaria has been traced in Diptera and Lepidoptera. Each ovary is, at first, a rounded mass of indifferent tissue, from which a filiform prolongation is given off backward ; this has not been traced into connection with any other organ, and appears to terminate by a free end. The mode of origin of this rudimentary, or primary, ovarium is unknown, but the first step toward the formation of the genital organs is the separation of the peripheral indifferent tissue from the central portion, and the division of the latter 332 THE ANATOMY OF LNVERTEBRATED ANIMALS. into as many elongated solid cellular bodies as ovarioles are to be formed. The peripheral cells become the peritoneal layer. Each cellular rudiment surrounds itself with a struct- ureless membrane, and then elongates into an ovariole, some of the cells filling the posterior end of which then becomes differentiated into the first primary ovum and its capsule, wTith or without vitelligenous cells. The contents of each ovariole must therefore be regarded as a column of generative cells, which instead of burrowing in the stroma of an ovary, and becoming divided into ovisacs, as in a vertebrated animal, grows straight backward, and, as it growTs, becomes divided into ovisacs, of which the oldest and most advanced is the hindermost. Nothing is certainly known respecting the origin of the vagina or the oviducts, though it may be suspected that the posterior prolongations of the primary ovaries give rise to the latter. The development of the testes takes place in the same manner as that of the ovaries, but the contents of the testic- ular tubes become converted into spermatozoa. The origin of the vasa deferentia is unknown.1 In most insects, the vitellus undergoes partial yelk-divis- ion. In some JPoduridce, however, complete division has been observed. The development of the blastoderm takes place in the same way as in other Arthropods, and the cephalic end of the embryo terminates in two procephalic lobes. In many insects, the periphery of the blastoderm, external to the lon- gitudinal thickening which gives rise to the sternal region of the body, and which may be termed the sternal band (" Keim- streif " of the German embryologists), gives off a lamina which grows inward over the sternal face of the embryo, and eventually forms a complete investment thereto. The lamina may be formed by a single layer of cells, or it may, 1 The account given above of the structure of the ovarian tubes in Blatta and Aphis is based on rav own observations, which are in pretty close accord- ance with those of A. Brandt, " Ueber die EirOhren der Blatta (Periplaneta) orientates" (" M6m. de l'Acad. St.-Petersbourg," tome xxi., 1874). The liter- ature of the subject is somewhat extensive. See especially Leydig, " Der Eierstock und die Samentasche der Insecten " ("Nova, Acta," xx'xiii., 1867); Lubbock, *« The Ova and Pseudova of Insects " (" Phil. Trans.," 1858) ; Weis- mann, "Die nachembryonale Entwickelung der Musciden" (ZelUchrift fur wiss. Zoologle, xiv.) ; Bessels, " Entwickelung der Sexualdrnsen bei den Lepidopteren " {ZelUchrift fur wiss. Zoologie, 1857); and Von Siebold, " Beitriige zur Parthenogenesis der Arthropoden," 1871. The various forms of the micropyle and the structure of the chorion are dealt with by Leuckart, in his elaborate memoir, " Ueber die Micropyle und den feineren Bau der Schalenhaut bei den Insekteneiern " (" Mailer's Archiv," 1855). AGAMOGEXESIS IN INSECTS. 383 from the first, be a fold of the blastoderm and thus consist of two layers, the inner of which is continuous with the sternal band, and the outer with the blastoderm which invests the tergal surface of the vitellus. In the latter case, it becomes strictly comparable to the amnion of a vertebrated animal ; and, when the folds have united in the middle line, the invest- ment in question is distinguishable into an outer membrane, which answers to the lamina serosa ^ and an inner, which cor- responds with the amnion proper of the vertebrate embryo. In some cases, the vitelline substance fills up the interval be- tween the lamina serosa and the amnion, so that the sternal band and the latter form a sac plunged into the interior of the yelk. The development of a more or less complete amniotic in- vestment has been observed in Orthoptera (Libellula), Cole- optera, Hemiptera, Hymenoptera, LepidopAera, and Dip- tera, but it does not appear to be universal. Agamogenesis is of frequent occurrence among insects, and occurs under two extreme forms ; in the one, the parent is a perfect female, while the germs have all the morpho- logical characters of eggs, and to this the term parthenogene- sis ought to be restricted.1 In the other the parent has in- complete female genitalia, and the germs have not the ordi- nary characters of insect eggs. In Coccus {Lecanium) hespertdum, in Chermes abietis and pint, no males have been observed; but the perfect females produce ova, out of which only females proceed. It is probable that many species of gall insects (Cynip)s) are in the same predicament. The unimp.regnated, apterous, caterpillar-like females of the Lepidopterous genera Psyche and Solenobia lay eggs out of which only females issue. The males occur but rarely and locally, and, from the impregnated eggs, males and females issue in about equal numbers. Leuckart discovered that the ovaries of so-called neuters among wasps, hornets, humble-bees, and ants, often contain more or less well-developed eggs, and that in the wasps and humble-bees such eggs a.re laid and develop young, the sex of which was not ascertained. Von Siebold has observed that the neuters of Polistes gallica are distinguished from the per- 1 The excellent '; Beitrasre zur Parthenogenesis" (1871) of Von Siebold is my chief authority for the statements in the text respecting Agamogenesis in Insects. 25 384 THE AXATOMY OF LXVERTEBRATED ANIMALS. feet fertilizable female, by little more than their smaller size, and that they possess completely developed female organs. These neuters, or rather, small females, laid eggs which de- veloped, and gave rise only to male JPolistes. The unim- pregnated females of a Saw-fly, Nematus ventricosus (the larvae of which are known as gooseberry caterpillars), regu- larly lay eggs, which develop and produce male offspring. The terms arrenotokous and thelytokous have been pro- posed by Leuckart and Von Siebold to denote those par- thenogenetic females which produce male and female voung respectively. In the case of the Hive-bee, it has been ascertained that the queen either impregnates, or does not impregnate, the eggs when they are laid. The sj)ermatheca, in which the spermatic fluid, introduced by the single act of copulation which takes place, is contained, contracts as the eggs pass along the vagina, in the former case, and remains passive in the latter. The unimpregnated eggs give rise to males or drones ; the impregnated eggs to females, which become neuters with imperfect reproductive organs, or queens, with perfect organs, according to the nutriment which they re- ceive. In the Aphides, ova deposited by the impregnated females in the autumn are hatched in the spring, and give rise to forms which are very generally wingless, and bring forth living young. These may be either winged or wingless, and are also viviparous. The number of successive viviparous broods thus produced has no certain limit, but, so far as our present knowledge goes, is controlled only by temperature, and by the supply of food. Aphides kept in a warm room and well supplied with nourishment have continued to propa- gate viviparously for four years. On the setting in of cold weather, or, apparently, on the failure of nourishment alone, in some cases, males and females are produced by the viviparous forms. The males may pos- sess wings, or may be devoid of them. The females appear invariably to be apterous. Copulation takes place and the eggs are laid. Sometimes viviparous forms coexist with the male or fe- male forms, and some viviparous Aphides are known to hi- bernate.1 1 Huxley, " On the Agamic Reproduction and Morphology of Aphis." (" Linnsean Transactions," 1857.) The papers of M. Balbiani (" Ann. des Sciences Naturelles," 1869, 1870, and AGAM0GEXE3IS IN APHIDES. 385 The viviparous forms differ essentially from the oviparous forms in the structure of their reproductive organs. They pos- sess neither spermathecre nor colleterial glands, both of which, as Von Siebold first demonstrated, are present in the females. The young are developed within organs which resemble the ovarioles of the true females in their disposition and may be termed p>seudovaries. The terminal or anterior chamber of each pseudovarian tube is lined by an epithelium, which in- closes a number of nucleated cells. One of the bindermost of these cells enlarges and becomes detached from the rest as a pseudovum. It then divides and gives rise to a cellular mass, distinguishable into a peripheral layer of clear cells and a central more granular substance, which becomes surrounded by a structureless cuticula. It is this cellular mass which gradually becomes fashioned into the body of a larval Aphis. A portion of the cells of which it is composed becomes con- verted into a pseudovarium, and the development of new pseudova commences before the young leaves the body of its parent. It is obvious that this operation is comparable to a kind of budding. If the pseudovum remained adherent to the parental body, the analogy would be complete.1 The agamogenetic multiplication of Cecidomyia -larvae is an essentially similar process. Professor Nicolas Wagner, of Kasan,2 discovered that the larva3 of a Dipterous insect be- longing to the genus Cecidomyia, or to a closely-allied form (3fiastor)) multiply agamogenetically in the autumn, winter, and spring. In summer, the final terms of the successive broods of grubs thus produced are metamorphosed into males and females, which copulate and lay eggs. From these, larvae which exhibit the same phenomena, emerge. In this case, the young are all developed from germs which are found lying loose in the perivisceral cavity of the parent, the body of which they destroy and burst in order to become free. 1872) should be consulted, not only on account of their richness in details, hut for the peculiar views which the author entertains respecting: the nature of the reproductive process in the Aphides. 1 Leydig (" Der Eierstock und die Samentasche der Insecten," "Nova Acta," 1867) affirms that, in November, he has met with Aphides in which, in the same animal, some of the ovarian tubes contain fully-formed ova, and others pseudova, under^oiiis? their ordinary method of development. Unfortunately no information is atforded as to whether these aphides possessed a sperma- theca, aud showed evidence of impregnation or not. The occurrence of aga- mogenesis alongside of sexual propagation is in itself nothing unprecedented, e. g,, Pyrosoma. 2 K. E. von Baer, " Bericht." (u Bulletin Acad. St.-Petersbourg," 1863.) 386 THE ANATOMY OF INVERTEBRATED ANIMALS. Leuckart, Metschnikoff, and Ganin,1 have shown that these germs are detached from the pseudovarium, which occupies the place of the rudimentary ovarium ordinarily found in larvae ; and that each represents the egg-chamber of an ordinary in- sect ovariole with its epithelial capsule, ovum, and vitelligenous cells. In the ordinary process of growth of an insect, from the time it leaves the egg until it attains the adult condition, every marked change in the outward form of the body, or of its ap- pendages, is accompanied by a shedding of the cuticula. In some cases the modification effected at each ecdysis is very slight, and the moultings of the cuticle are numerous, amount- ing in a species of Day-fly (Chloeon), described by Sir John Lubbock, to as many as twenty. In such a case as this, the structure of the adult is gradually substituted for that of the larva, and the organs of the larva, for the most part, pass into those of the adult. The like holds good of some insects which undergo meta- morphosis, that is to say, in which a quiescent pupal condi- tion is interposed between the active larval and the active imaginal states. Herold and Newport have described at length the series of changes by which the elongated gangli- onic chain of the Lepidopterous caterpillar is converted into the much more highly concentrated nervous system of the Butterfly ; and Weismann has shown by what gradual steps the apodal Corethra-\a.rva, acquires the character of the Dip- terous imago. But, in the Flesh-flies (Mused), and probably in many other members of the division of the Dlptera to which they belong, the apodal maggot, when it leaves the egg, carries in the interior of its bodv certain regularly ar- ranged discoidal masses of indifferent tissue, which are termed imaginal disks.2 Of these, twelve are situated in the thora- cic region, two on each side of each thoracic segment, while two others lie in front of the pro-thoracic disks. These imagi- nal disks undergo little or no change until the larva incloses itself in its hardened last-shed cuticle, and becomes a pupa. But they then rapidly enlarge ; each of the sternal thoracic disks gives rise to a leg and to its half of the sternal region of 1 Leuckart "Die ungeschlechtliche Vermehrung rler Cecidamvienlarven " (G'tt'ui'icr NachrieMen, 1865") ; K. von Baer, " Ueher Prof. Nic. Wasrner's Ent- deckTmg," etc. (*( Melanges l>iolo 2 See the remarkahle memoir of Weismann, " Die nachembryonale Ent- wickelun^' der Musciden." THE PARASITISM OF INSECTS. 387 the thorax, while the tergal disks develop into the tergal halves of the corresponding somites, with their appendages, the wings and the halteres. The anterior pair of disks origi- nate the head and proboscis of the fly. As the imaginal disks develop, the preexisting organs contained in the head and thorax of the larva undergo complete or partial resolu- tion. On the other hand the abdomen of the fly is produced by the continuous modification of the constituents of the lar- val abdomen. As in the Crustacea, so in Insecta, the parasitic habit is Fm 111 The left-hand figure reprepe^t* an adult female of Stylopa aterrvmm con- FlGtainu;7^oe nf r!y hat?he\reS and the right-hand I figure a newly bon. larvj of Sbulvm on a hair of Andrama Tnmmerana. A. ventral sm lace of the thorax B. .the abdomen; a. mandibles ; b. labial plate, and mouth ; c, vulva ; 1, 2, 3, the three thoracic segment? united. (Alter Newport.) accompanied by extreme modification of form. In this re- spect the Strepsipter a, which are parasitic upon Bees, present a remarkable history. The female (Fig. Ill) has the form of a sac with a short neck, and never leaves the body of the Hymenopteran in which she is parasitic. The males, on the contrary, are exceedingly active insects provided with a sin- 388 THE ANATOMY OF IXVERTEBRATED ANIMALS. gle pair of wings, which are attached to the metathorax, while the mesothorax has a pair of twisted appendages in the place of wings. The larvae of both males and females when they leave the egg are minute active hexapod insects (Fig. Ill), with rudi- ment-try manducatory organs, and are found creeping about between and on the hairs with which the abdomen of their host is provided. In this condition they are carried into the nests of the bees, and they attack the larvae of the latter, bor- ing their way through the integument into the abdominal cavity of the grub. Here they cast their cuticle and become changed into sluggish apodal grubs, provided with a mouth, with rudimentary jaws, and with an alimentary sac, but de- void of an anus. About the time that the Hymenopterous larva passes into its imago state, the Strepsipteral larva thrusts the anterior end of its body (the so-called cephalo- thorax) between two of the abdominal segments of the bee, so that it projects externally. The male becomes a pupa, and eventually makes its way out as a winged insect.. The fe- male, on the other hand, undergoes little change of outward form, but presents an opening, which plays the part of a vulva, and enables the male to effect the fecundation of the eggs. These are developed within the body of the female, and make their way out by the cleft in question.1 - The Ichneumon-flies deposit their eggs within the bodies of the larvae of other insects, and the grubs thence hatched devour the corpus adiposum of their host. The larvae of some of these parasites (Platygaster, Teleas), described by Ganin,2 are extraordinarily unlike other insect larvae, and have a certain resemblance to Copepoda. 1 See Von Siebold " Ueber Strepsipteren " (" Archiv fur Naturgeschichte," 1843), and Newport, "Natural History, etc., of the Oil-beetle, Meloe" (uLinn. Trans," 1847). 2 Zeitschrift fur Zoologie^ 1869. CHAPTER VIII. THE P0LYZ0A, THE BRACHIOPODA, AXD THE MOLLUSCA. However diverse in outward appearance and in com- plexity of organization the multitudinous forms of animals which have been described in the preceding four chapters (Chap. IV. to VII.) may be, the student passes from one to the other, by easy and natural gradations, from the simple Turbeilarian at the bottom to the most highly differentiated Arthropod at the summit of the series. But with the higher Crustacea, Araehnida, and Lisecta the scale ends ; from none of these are we led to any higher form of animal life. The Cuttle-fish, the Whelk, the Snail, and the other in- numerable forms of animals wTith univalve, bivalve, and mulr tivalve shells, which are commonly known as 3follusca, are so widely different, not only from the Arthropoda, but from all the higher members of the group of Worms (Chap. V.), that any connection with these seems, at first, to be wanting. The segmentation of the body, which is so conspicuous a fea- ture of the greater number of the members of the series which ends with the Arthropods, is absent ; limbs are want- ing ; instead of the equality .of the neural and haemal faces of the bilaterally symmetrical body, and the consequent remoteness of the oral and anal apertures, which is usual among the Arthropods and Worms, these two faces are usual- ly unequal. The haemal face is often produced into a longer or shorter cone ; the anus is, as a rule, approximated to the mouth ; and, very often, the haemal face of the body is asym- metrical. The higher Mollusks, in fact, form the final term of a series of their own, which commences in the Polyzoa, with animals which have many resemblances to the Rotifera. The Polyzoa or Brtozoa. — In outward form these ani- mals bear a general likeness to the Sertularian Hydrozoa^ 390 THE ANATOMY OF IXVERTEBRATED ANIMALS. with which they were formerly confounded under the name of " Corallines." Like the Sertularians, they almost always form compound aggregations, produced by repeated acts of gemmation from the primitively single embryo, and have a hard cuticular exoskeleton, which remains when the soft parts decay. The compound organism thus formed is termed Fig. 112.— A portion of the polyzoarium of PlumateUa repens (after Allman).1 a Polyzoarium (Fig. 112), and each zooid which buds from the common stock is a jPolypide. The outer, chitinous or calcified, cuticular exoskeleton is termed the ectoeyst, and, as the rest of the body of the polypide is contained in, or can be retracted into, the hard case thus formed, it is commonly termed a "cell." The proper ectoderm, with the parietal layer of the meso- derm which lines and secretes this cell, is termed the endo- cyst. The mouth is situated on a disk, termed the lopho- phore, at the free end of the polypide ; and the margins of the lophophore are produced into a number of richly ciliated tentacula. At the oral aperture, the ectoderm passes into the endodermal lining of the alimentary canal, which is almost always divided into three portions, a long and wide pharynx, a spacious stomach, and a narrow intestine. The latter is al- ways bent up nearly parallel with the phaiynx, and termi- nates in an anus situated beside the mouth. As the nervous ganglion is placed between the mouth and the anus, the flex- ure of the intestine is neural? and the ha?mal face of the 1 " Monograph of the Fresh-water Polyzoa," 1856. 2 In dealing with the morphological relations of the parts of Mollusks, it is very necessary to employ a terminology which shall be independent of the or- THE POLYZOA. 391 bodv is developed greatly in excess of the neural face. A wide perivisceral cavity occupies the interval between the alimentary canal and the parietes of the body, and sometimes ■ZS Fig. 113.— Plumatella repens.—A single cell more magnified : a, ectocyst ; 6, endo- cyst ; m, calyx at the base of the ciliated tentacula borne by the disk or lopho- phore ; k, mouth; /, gullet ; g g, stomach; h, intestine ; i, anus; ?;, muscles; w, nervous ganglion ;' z, statoblasts ; 0, funiculus. (After Allman.) the walls of this cavity are ciliated. Very generally, the gastric division of the alimentary canal is connected with the parietes of the body by a sort of ligament, the funi- culus, or g 'astro-parietal band. Circular and longitudinal mus- cular fibres, which frequently exhibit distinct transverse stria- tums, may be developed in the body-wall ; and there are usual- ly special muscles for the retraction of the lophophore within the cell, and others for the closing and opening of the oper- cular apparatus, with which many species are provided. dinary position of the animals. I therefore term that face of the hotly on which the chief nervous centres, or the pedal ganglia ( when such are separately distinguishable), are placed, neural, and the opposite hcemal. 392 THE ANATOMY OF IXVERTERBATED AXIMALS. The single nervous ganglion is situated, as has been stated, between the oral and the anal apertures. In jSeria- laria, Scrupocellaria, and some other genera, nervous cords and plexuses connecting the ganglia of the several polypides, and constituting what F. Muller1 terms a "colonial nervous system," have been described. But it is not yet certain that these cords and plexuses are really nerves. It is doubtful if there are any special organs of sense, unless a lobed process — the epistoma — which overhangs the mouth in many fresh- water Polyzoa, be of this nature. The ectoderm of that region of the body which lies immediately beneath the tentacula is always soft and flexible ; and when the lophophore is re- tracted, becomes invaginated, so as to form a sheath, by which the tentacles are protected. Sometimes, as in the Ctenosto- mata,2 this sheath is surrounded by a circle of chitinous fila- ments, which, when the tentacles are retracted, furnish a pro- tective outer covering to them. And, sometimes, as in the Cheilostomata,* part of the ectocyst of the polype cell is dis- posed in such a manner as to constitute a movable lid, which Fig. 114. — Scrvpocellaria ferox.—k small portion of a polyzoarium, showing the vibracula (a). (After Busk.) * 1 " Archiv rur Anatomie," I860. 2 Farre, " Observations on the Minute Structure of some of the Higher Forms of Polypi" | •• Phil. Trans.," 1837\ Reichert, "Ueber Zoobotryon pellucidus" (" Abh. d. konigl. Akad. der Wissenschaften," Berlin. 1869). -3 Busk, " Catalogue oi the Marine Polyzoa in the British Museum: Cheilo- stomata," 1 852-' 54. 8esfor this group Niteche's recent important "Beitrage zur Kenntniss der Bryozoen" {Znbsehrifi fur wiss. Zoologie, 1869-'71). 4 " Catalogue of Marine Polyzoa," 1852. AYICULARIA AND VIBRACULA. 393 shuts down on the retracted polypide. This operculum is placed on the opposite side of the polypide to that on which the nervous ganglion is situated. In many genera, the cells are provided with flagelliform appendages— the vibracula (Fig. 114). These are usually articulated with short dilated processes of the ectocvst and execute constant lashing movements. In others " bodies shaped like birds' heads, with a movable mandible, and either seated upon slender and flexible peduncles or sessile, snap incessantly. Sometimes these last, which are termed avicu- laria (Fig. 115), are present along with vibracula. Fig. \\z.—Bvq\iIo avicularia.— A. Part of tha polyzoarium viewed from the neural side, showing the tentacles of a polypide protruded from its cell (A) ; the intestine (7) and the stomach and gullet (/) ; q. retractor muscles ; d. d. avicularia. One of these is holding a minute worm which it lias seized. In front of this is seen an ovicell. B. A retracted polypide withan avicularium (d), viewed from the haemal or dorsal side. The dilated bases of the vibracula contain muscles by the contraction of which the flajrelliform appendage is moved. In the avicularia, a large adductor muscle, which takes its origin from the greater part of the inner surface of the 394 THE AtfATOMY OF INTERTEBRATED ANIMALS. " head," is attached by a slender tendon to the " mandible M on the one side of the hinge line, while a smaller divaricator muscle is fixed to the other side. The mechanism of adduc- tion and divarication of the mandible is quite similar to that by which the dorsal valve of the shell of an articulated Bra- chiopod is moved upon the ventral valve. Male and female reproductive organs are usually com- bined in the same polypide. They are cellular masses, devel- oped in the funiculus, or in the parietes of the body, whence the ova or spermatozoa are detached into the perivisceral cavity. They sometimes pass thence, and undergo the first stages of their development in dilatations of the wall of the body, termed ovicells. Multiplication by gemmation occurs throughout the group, but the buds usually remain adherent to the stock. In Loxo- soma and Pedicellina, however, the buds become detached. Some Polyzoa multiply agamogenetically by a kind of gemmule developed in the funiculus, provided with a pecul- iar shell, and termed a statoblast. With these general characters, the Polyzoa present an interesting series of modifications. They have been divided by Nitsche into two groups — the Entoprocta, in which the anus lies within the circle of tentacles ; and the Ectoprocta, in which it lies outside this circle. In the former division, the genus Loxosoma,1 which attaches itself to Sertularians and to other Polyzoa, is particularly noteworthy. It is a small stalked animal, and the superior wider end of the body is an obliquely truncated disk, the margins of which are elon- gated into ten ciliated processes. The mouth is a trans- versely elongated, slit-like aperture on the lower side of the tentacular circlet. A long oesophagus connects this with a globular caecal gastric sac. From the midst of the disk, a conical prominence, the summit of which bears the anus, is situated. The sexes are united, the ovaries and testes being situated on each side of the stomach, and the spermatozoa pass directly into the ovaries. No nervous system has yet been made out in Loxosoma. The animal is fixed by the truncated extremity of its narrow stalk-like end ; and this stalk contains a gland, the duct of which opens in the centre of the face of attachment. Loxosoma appears to multiply by budding, but the ap- 1 Kowalewsky, " Beitrage zur Anatomie unci Entwickelungsgeschichte des Loxosoma neapolitanum " (kt Mem. de l'Acacl. de St.-Petersbourg," 1866). Os- car Schmidt " Die Gattung Loxosoma" ("Archiv liar mikr. Anatomie," 1875). THE POLYZOA. 395 parent buds are really one of two kinds of embryos devel- oped from the impregnated ova. The other kind of embryo becomes a gastrula, with a large post-oral ciliated disk, like a mesotrochal annelid larva, and its ultimate fate has not yet been traced. The Ectoprocta are divided into the Gymnolcemata, which have a circular lophophore, and no epistoma ; and the Phylac- tolmmata* which possess an epistoma, and usually have the lophophore prolonged into two lobes, so as to be horseshoe- shaped; whence the term hippocrepian applied to such Po- ly zoa. Among the Gymnoleemata are distinguished : the Cyclo- stomata, in which the opening of the cell is round and has no opercular structures ; the Ctenostomata (supra), and the Che ilostomata {supra) . All the PJiylactolcemata are inhabitants of fresh water ; while all the Gymnolaimata, except Paludlcella, are marine. The polyzoarium of Cristatella is free and creeps about as a whole ; and that of Lunulites is free, at any rate in the adult condition. In the fresh-water Polyzoa, the impregnated ovum gives rise to a saccular planuliform embryo, which is covered external- ly with cilia. From one end of this cystid, one or more poly- pides are developed from thickenings of the wall of the sac. In the Gymnohematous genera Biic/iila, Scrupocellaria, and Bicellaria, the embryo is ciliated, and provided with a mouth and with eve-spots. After swimming about for some time, it loses its cilia, fixes itself, acquires a chitinous outer coat, and becomes a mere sac or cystid, in which a polypide is developed by gemmation, and gives rise to the first cell of the polyzoa- rium. Schneider2 has showm that the anomalous Cyphonautes, which he considers to resemble Actinotrocho, and which is inclosed in a bivalve shell, is the larva of Membranipora pir losa. It is provided with an intestine, and with largely de- veloped ciliated motor bands. But when it attaches itself, all these organs disappear, and the larva passes into the condi- tion of a cystid, from which a polypide is developed, as in the foregoing cases. 1 See Dumortier and Van Beneden, " Ilistoire Xaturelle d. Polypes compo- ses d'eau douce" ("Mdm. de l'Aad. Royale de Bruxelles," 1850) ; the mono- graph of Allman cited above ; and Nitsche's " Beitrc"i£re." 2 " Zur Entwickeluncrsoreschichte Und svstematischen Stellung der Bryczoen und Gephvreen." (u Archiv fur mikr. Anat.," 1869.) 39G THE ANATOMY OF IXVERTEBRATED ANIMALS. Hence, it has been pointed out that the characteristic poly- pide of the ectoproctous Polyzoa is a structure developed from the cystid, in much the same way as the Tcenia-head is developed from its saccular embryo; or as the Cercaria is de- veloped from the sporocyst, or Redid ; the cystid of the Phy- lactoloemata being comparable to a sporocyst, and that of Mem- branipora to a Media. But, without altogether denying the justice of this compaiison, it may be suggested that the cys- tid is comparable to a vesicular morula, and that the mode of development of the alimentary canal of the polypide corre- sponds with that of the formation of an alimentary sac by in- vagination. Tf this view of the case be correct, the perivisce- ral cavity in the Polyzoa is a blastoccele, more or less modified by the development of the mesoderm. The only known representative of the genus Phabdopleu- ra 1 is an aberrant Polyzoon which presents many interesting peculiarities. The polyzoarium consists of a creeping stem from which erect branches, each of which ends in a circular aperture and constitutes the cell of a polypide, arise. The cavity of the stem is divided by transverse septa, and its centre is traversed by a hollow chitinous cord, which passes through and is attached to the septa. The lophophore resembles that of the hippocrepian Phy- lactolmmata in being produced into two arms, fringed with a double series of tentacula. These arms are longer, narrower, and more cylindrical than in any other Polyzoa, and, thus far, approach the arms of the Prachiopoda. Furthermore, the tentacula are confined to the arms, which are verv flexible. Betweeen the bases of the arms there is a rounded or pen- tagonal disk with raised and ciliated edges, which occupies the place of the epistoma in the phylactolsematous Polyzoa. The mouth is situated beneath the free margin of this disk, on the opposite side to the anus, and to that toward which the arms are turned. The animal is attached to the bottom of its cell, or rather to the endosarc of the stem, by means of a long contractile pedicle, by which its retraction is effected. According to Sars it protrudes itself by climbing up the wall of its tubular cell by means of the disk. Prof. Lankester's comparison of the polypide of Phabdopleura to the embryo PUidium* appears to me to be fully justified. The branchiae of Nucula, in form and position, present no little resemblance 1 See the papers of Alltnan and G. 0. Sars, Quarterly Journal of Micro- scopical Science, 1869 and 1874. 2 " On the Developmental History of the Mollusca." (" Phil. Trans.," 1874.) THE BRACHIOPODA. 397 to the arms of Bhabdopleura, though these, like the arms of the Brachiopoda, are probably more strictly comparable to the labial palpi of the Lamellibranchs. Polyzoa occur in the fossil state from the Silurian epoch to the present day, and the oldest forms are referable to the groups which now exist. The Brachiopoda. — The Brachiopoda are all marine animals provided with a bivalve shell, and are usually fixed by a peduncle which passes between the two valves in the centre of the hinge line, or the region which answers to it, in those Brachiopods which have no proper hinge. They never multiply by gemmation, nor give rise to compound organisms. The shell is always inequivalve and equilateral ; that is to say, each valve is svm metrical within itself and more or less un- like the other valve. The shell is a cuticular structure se- creted by the ectoderm, and consists of a membranous basis, hardened by the deposit of calcareous salts, sometimes con- taining a large proportion of phosphate of lime (Lingula). In many Brachiopods, variously-formed calcareous spic- ula, or minute plates, are found in the walls of the peri- visceral cavity, and of the greater sinuses ; and also in the arms and cirri, and sometimes these unite together so as to form an almost continuous skeleton.1 The body, or rather that part of it which contains the chief viscera, is often small relatively to the valves of the shell, and the integument is produced into two broad lobes, which line so much of the interior of the valves as the visceral mass does not occupy. The free edges of these lobes are thickened, and are beset with numerous fine chitinous sette like those found in Annelids, and like them lodged in sacs. Between the two lobes of the mantle, or pallium, is the pallial cham- ber, bounded behind the anterior wall of the visceral mass. In the middle line, this wall presents the oral aperture, wmich is seated in the midst of a wider or narrower area, the mar- gins of which are provided with numerous ciliated tentacula. In Argiope, the oral area occupies a large part of that lobe of the mantle which is ordinarily termed dorsal, and its mar- gins are simply indented by three deep sinuations. In Theci- dium, the sinuations are deeper, and the folds of the oral area thus produced narrower. But in most Brachiopods the oral 1 These have been described by Woodward, Laeaze-Duthiers. and especially by Eudes Deslongchamps, " Recherches sur 1' organisation du Manteau chez les Brachiopodes articules," 186-4. 398 THE ANATOMY OF IXYERTEBRATED ANIMALS. area is narrowed to a mere groove, and is produced on each side of the mouth into a long spirally-coiled arm, fringed with tentacles ; whence the name of Brachiopoda, applied to the group. In this case the tentacula disappear from the anterior margin of the oral disk in the region of the mouth, and are re- placed by a lip-like ridge. Each arm contains a canal, which ends in a sac at the side of the mouth. In Waldheimia (Fig. 116), the two arms are united to- gether and their distal portions coiled into a horizontal spiral. In many genera, the margins of the oral area or arms are fixed to processes of the dorsal valve of the shell.1 In this case the arms are not protrusible ; but, according to the ob- servations of Morse,2 they can be straightened and extended beyond the shell in Rhynchonetta, which has no brachial skeleton. The alimentary canal consists of an oesophagus, a stomach, provided with hepatic follicles, and an intestine. In the ma- jority of existing genera the latter is short, and ends in a caecum in the middle line of the body (e. g., Waldheimia) ; in others it is long, and opens into the pallial chamber on the right side of the mouth (e. g., IAnguld, Discina, and Crania). The alimentary canal is invested by an outer coat — the so- called peritoneum — by which it is suspended, as by a mesen- tery, in a spacious " perivisceral " cavity. The walls of this cavity are provided with cilia, the working of which keeps up a circulation of the contained fluid. Lateral processes of this coat — the gastro-parietal and ileo-parietal bands — connect the gastric and intestinal divisions of the alimentary canal respectively, with the parietes.3 From the perivisceral cavity, sinus-like, branched prolonga- tions extend into each lobe of the mnntle, and end crecally at its margins. The lobes of the mantle are probably, together with the ciliated tentacula, the seat of the respiratory func- tion. The sinuses of the pallial lobes of Lingida give rise to numerous highly contractile, teat-like processes, or ampul- lar. During life the circulating fluid can be seen rapidly cours- ing into and out of each ampulla in turn (Morse, I. C, p. 33). 1 See, for excellent figures of these arrangements, and for the shells and ex- ternal form of the body in general, Woodward's " Manual of the Mollusca." 2 " On the S\ Btematic Position of the Brachiopoda." (" Proceedings of the Boston Society of Natural History," 1873.) 3 Huxley. " Contributions to the Anatomy of the Brachiopoda" ("Proceed- ings of the Roval Societv," 1854); and Hancock, " On the Organization of the Brachiopoda" ("Phil. Trans.," 1858). THE BRACHIOPODA. 399 Fig. 116.— Lateral view of the viscera of Waldheimia australis (after Hancock. ;i On the Organization of the Brachiopoda," " Phil. Trans.," 1858). a, " dorsal " layer of mantle; 6, "ventral " layer; c, anterior walls of the body between the mantle lobes; d, arms; p, gullet; q, stomach with cut biliary ducts of the left side; r, rierht hepatic mass; s, intestine ending ca?cally below; v, so-called " auricles; " o, the right " pseudo-heart,11 the left being almost wholly removed ; w. pyriform vesicle fixed at the back of the stomach; z, oesophageal ganglia; «',,/', adductor; *, divari- cator ; I, adjustor muscles ; n, peduncles. 26 400 THE ANATOMY OF IN VERTEB RATED ANIMALS. The perivisceral cavity communicates with the pallial chamber by at fewest two, and sometimes four (Rhynchonel- la), tubular organs, which have been described as hearts,1 but are now known to have no such nature. Each of these organs is shaped like a funnel, the wide por- tion which opens into the perivisceral cavity being much plait- ed and folded, and separated by a constriction from the nar- rower part, which answers to the pipe of the funnel. The lat- ter, passing obliquely through the anterior wall of the visceral chamber, ends by a small aperture in the pallial cavity. Prof. Morse has observed the passage of the eggs through these organs in Terebratulina septentrionalis. They are drawn into the open end of the funnel by the action of the cilia with which its surface is covered, and enter the pallial cavity by the aperture just mentioned. It is probable that these " pseu- do-hearts " subserve the function both of renal organs and of genital ducts ; and that they are the homologues of the organs of Bojanus of other mollusks, and of the segmental organs of worms. Between the ectoderm and the lining membrane of the prolongations of the perivisceral cavity in the mantle, and between the endoderm, the ectoderm, and the lining membrane of the perivisceral cavity itself, there is an interspace broken up into many anastomosing canals, which I conceive to rep' resent a large part of the proper blood system. Vesicular dilatations of the walls of these canals found at the back of the stomach, and in so ne other localities, in the Brachiopods with articulate shells, have been regarded as hearts; but observations on the living animals, made by various investigators, show that they are not contractile, and their function is unknown. Although the existence of a direct communication between the perivisceral chamber and the blood canals has not been demonstrated, it is very probable that the perivisceral chamber really forms part of the blood- vascular system. Muscles for the adduction and divarication of the valves of the shell, and for effecting the other movements of the ani- mals, are well developed in the Brachiopoda? They are to a great extent striated. 1 Owen, " Lettrc but l'appareil de la circulation chez les Mollusques de la classe des Brachiopodes." (u Annates des Sciences Naturelles," 1845.) 2 See Hancock (1. c). Owen, Introduction to Davidson's "Fossil Brachi- onoda." ("Memoirs of the PaLeontographical Society," and "Transactions of the Zoological Society of London," 1835.) THE DEVELOPMENT OF THE BRACHIOPODA. 401 The nervous system of the articulated Brachiopods, in which it has been best made out, consists of a relatively thick ganglionic band on the ventral side of the mouth, the ends of which are united by a commissural cord, which surrounds the gullet, and bears two small ganglionic enlargements. The lat- ter probably answer to the cerebral, the former to the pedal, ganglia of the Lamellibranchiata. Immediately behind the pedal mass, from which two large nerves to the dorsal or ante- rior lobe of the mantle are given off, are two elongated ganglia, connected by a commissure of their own, which possibly cor- respond with the parieto-splanchnic ganglia of the higher Mol- lusks. The nerves to the ventral lobe of the mantle and those to the peduncle arise from these ganglia. In the inarticulated Brachiopods, our knowledge of the ner- vous system is very imperfect. In Lingula, Professor Owen has described two lateral nerve-cords, and the observation has been confirmed by Gratiolet and Morse. The latter anato- mist finds similar cords in Discina, and Gratiolet has de- scribed an oesophageal ring in Lingula.1 The reproductive organs are lodged in the perivisceral cavity or its prolongations, and are apparently always con- tained in processes of the lining membrane of that cavity. It is not clear whether hermaphrodism is the rule or the ex- ception. Theeidium, however, has been shown by Lacaze- Duthiers to be dioecious ; and, according to Morse, the sexes are distinct in Terebratulina and Discina. The development of the Urachiopoda, notwithstanding the important observations of F. Muller,2 Lacaze-Duthiers,3 and especially of Morse,4 stood much in need of further eluci- dation (especially in regard to the earlier conditions of the embryo), until quite recently, when the investigations of Kowalewsky5 filled up the hiatus in our knowledge for the genera Argiope, Theeidium, Terebratula, and Terebratulina. The ew becomes converted into a vesicular morula, in which an alimentary sac is developed by invagination, and this sac gives off, as i-n Sagitta, two diverticula, which become shut 1 " Recherches pour servira l'histoire cles Brachiopodes." (" Journal cle Conchvliolo.de," 1860.) 2 u Besctireibuns* einer Brachiopoden-Larva." (" Arcliiv fnr Anat.," 1860.) 3 " Histoire de la Thecidee." (" Ann. d'llist. Nat.," 1861.) 4 " On the early stages of Terebratulina septentrional is." (" Memoirs of the Boston Society of Natural History," 1809, and the memoir already cited). 6 Contained in a memoir, published at Moscow in 1874. for winch I am in- debted to the courtesy of the author. It is in Russian ; but I have been able to acquaint myself with its contents, to some extent, by the aid of a friend. 402 THE ANATOMY OF IXYERTEBRATED ANIMALS. off from the alimentary canal, and are converted into the peri- visceral cavity. The latter, therefore, is an enteroccele. The embryo elongates, and constrictions divide it into three seg- ments, of which the anterior becomes fringed with long cilia, and develops eye-spots. Thus the young Brachiopod acquires a great resemblance to an ordinary Annelid larva. The re- semblance is increased by the appearance of four bundles of setae on the middle segment, which becomes produced into a sort of hood, the free edges of which are at first turned back- ward and bear these setae. As the larva grows, the third segment becomes truncated at the end, and furnishes a sur- face (provided with a shell gland ? vifrd), by which the larva attaches itself. At the same time, the first, or pra?stomial segment, atrophies, and the setigerous hood developed from the middle segment is retroverted, rapidly grows, and gives rise to the lobes of the mantle, on which the valves of the shell are developed. The resemblance of the larval Brachiopod to a Polyzoon, and especially to Loxosoma, is striking-, and fully bears out the conclusion as to the affinity of the Polyzoa with the Brachiopoda which results from the study of their adult structure. On the other hand, the development of the JBra- chiopoda no less strongly testifies to their close relations with the Worms.1 In the course of the previous pages the terms dorsal and ventral have been employed in the sense in which they are conventionally used by conchologists. But an interesting question, and one not easy to settle, is, What relation do these dorsal and ventral regions of a Brachiopod bear to the neural and haemal regions of a Polyzoon, or to those of a Lamelli- branch, or of a Gasteropod ? If we compare one of the articulated Brachiopods, such as Waldheimia. in its shell, with a polypide of a Cheilostoma- tous Polyzoon in its cell, the dorsal valve will appear to an- swer to the operculum, and the ventral valve to the cell. If this comparison be just, the two lobes of the mantle of the Brachiopod must both belong to the dorsal or haemal aspect of the body ; that which corresponds with the so-called dor- sal valve of the shell being the anterior, and that which lines 1 The acceptance of the view originally propounded by Stecnstrup, and 80 ably urged by Prof. Morse, respecting the affinities of the Brachiopods with the' Worms (" Proceedings of Boston Society of Natural History," 1873), el not to my mind weaken the opinion I have always held as to their affinities with the Polyzoa, on the one hand, and with the higher Mollusca , on the other, THE BRACHIOPODA. 403 the ventral valve of the shell being the posterior lobe. And the region of the anterior wall of the pallial cavity which lies behind or below the mouth will answer to the neural aspect of the Polyzoon. On the other hand, if the segments of the body of the larval Brachiopod are true somites, and the discoidal surface of the hindermost corresponds with the similarly formed end of the larva of Lacinularki, as Prof. Morse suggests, the dorsal lobe of the mantle will, as before, represent part of the hremal surface of the body, but the ventral lobe will be- long to its neural surface — and can no longer properly be termed mantle, but will rather answer to the foot of one of the higher Mollusca. The Brachiopoda are distinguishable into two groups, the Articulata and the Inarticulata. In the Articulata, the two valves are united by a hinge, and the ventral valve is usually provided with teeth, which are received in sockets of the dorsal valve. The gullet ascends in the middle line toward the dorsal valve, and the intestine descends toward the opposite, or ventral, valve, and there ends in a cascum. The dorsal valve often gives rise to spiral or looped shelly pro- cesses to which the arms are attached. The valves are brought together by a pair of adductor muscles, which pass directly from valve to valve; and they are separated by di- varicator muscles, which run obliquely from the ventral valve to a median process (the cardinal jirocess) of the hinge-line of the dorsal valve. The impressions of the attachments of these muscles on the inner surfaces of the valves have con- siderable systematic importance. Very often the ventral valve is produced into a sort of spout, through which passes the peduncle by wmich the animal is attached to rocks. At the sides of the visceral chamber the thickened edge of the dorsal lobe of the mantle passes into that of the ventral lobe. The substance of the shell is very often traversed by numerous canals perpendicular to its surface, which contain prolongations of the mantle.1 This division contains the families of (1) The Terebra- tulidce, (2) the Spiriferidm, (3) the Rhynchonellklv, (4) the Orthidce, and (5) the ProducMdce, of which the second, fourth, and fifth are extinct and almost wholly pakeozoic, no species 1 The structure of the shell has been particularly studied by Carpenter. ("Reports of the British Association," 1 844-' 47. and Introduction to David- son's " Fossil Brachiopoda.") See also King, " Trans. Royal Irish Academy," 404 THE ANATOMY OF IXVERTEBRATED ANIMALS. extending beyond the lias, while the majority of the species of the other two families are also extinct. The family of the Terebratulldce, which is not certainly known to occur in formations older than the Devonian, is the only one in which, since the end of the palaeozoic epoch, numerous new generic types appear.1 The Inarticulata have no hinge ; the intestine opens into the cavity of the mantle, the margins of the lobes of which are completely separate. Some have a long peduncle [Lin- gula), others are fixed by a plug which passes through an aperture or notch of one valve (Discina), or by the surface of one valve (Crania). There is no brachial skeleton, and the arrangement of the muscles is in many respects different from that which obtains in the articulated division. Species of all these families, except the Spirifericke, Orthidw, and Productidce, exist at the present day, but they are also represented in the older palaeozoic epochs, and Lin- gular are among the oldest known fossils.2 The Mollusca. — The term Mottusca may be used as a convenient denomination for the Lamellibranchiata and Odontophora (= Gasteropoda, Pteropoda, and Cephalopoda, of Cuvier), which can be readily shown to be modifications of one fundamental plan of structure. This may be represented by a body, symmetrical in relation to a median vertical plane, at one end of which is the oral and at the other the anal aperture of the alimentary canal. In the body a ventral, or neural, face, an opposite dorsal, or hcemal, face, and a right and left side may be distinguished. The neural face usually gives rise to a muscular foot. The integument of the haemal face is generally produced at its ed«;es into a free fold, and the term mantle, or pallium, is applied to the region of the integument thus circumscribed. Between the free portion of the mantle and the rest of the body is a cavity, the pallial chamber, from the walls of which, processes which subserve respiration, the branchiae, may be developed. In the median line of the surface of the mantle of the em- bryo a shell-gland is very generally formed, and from the surface of the mantle a cuticular secretion, the shell, is pro- duced. . l Suess, " Ueber die "Wohnsitze der Brachiopoden." (" Sitzb. d. "Wiener Akad.," 1857.) 2 See Davidson's " Monographs of British Fossil Brachiopoda," in the Pa- lseontographical Society's publications. THE MOLLUSCA. 405 A systemic heart usually exists, and when present is situ- ated in the middle of the posterior haemal region, and consists of, at fewest, two chambers, an auricle and a ventricle. Arte- rial vessels often ramify extensively through the body, but more or fewer of the venous channels remain in the condition of lacunae. The blood-corpuscles are colorless and nucleated. Distinct respiratory organs may be absent, or they may take the form of branchiae or pulmonary sacs. When present, they lie in the course of the blood which is returning to the heart. Beside the heart and the intestine are situated the renal or- gans, which, on the one side, open externally, and on the other communicate with the blood system. The nervous system consists of, at least, one pair of ganglia [cerebral) at the sides, or on the haemal aspect of, the mouth, and of two other pairs of oesophageal ganglia (pedal and parieto-splanchnic). The latter are situated at the sides, or on the neural aspect, of the alimentary canal, and are con- nected by commissures with the former. In the majority of the Mollusca, the embryo passes through a stage in which it is provided with bands of cilia or with a simple, bifid, or multifid fold of the integument (veh(m), the edges of which are ciliated, developed on the haemal aspect of the cephalic region of the body, in front of the pallial region. The special peculiarities of the different groups of the Mollusca result chiefly — 1. From the form of the pallial region, and the extent of the mantel-lobes relatively to the body. 2. From the number and arrangement of the pieces of the shell to which the mantle gives rise. 3. From the proportional size and the form of the fcot and the production, or non-production, of chitinous, or shelly, matter by it. 4. From the development of sense-organs on the anterior end of the body, and the absence or presence of a distinguishable head. 5. From the disproportionate growth of the haemal region of the bodv into a visceral sac, followed by a change in the primitive direction of the intestine, and often accompanied by asymmetrical lateral distortion. The Lamellibraxchiata.1 — In these Mollusks there are 1 For a description of the anatomy of a Lamellibranch in detail, the student is referred to Huxley and Martinj " Elementary Biology," and Rolleston, " Forms of Animal Life." 400 THE ANATOMY OF INVERTEBRATED ANIMALS. always two large pallial lobes, the margins of which are de- void of setae ; and which are lateral, or right and left, in rela- tion to the median plane. Each .lobe gives rise to apiece, or valve, of the shell ; and to these, accessory pieces, developed upon the median haemal face (Pholas) or the posterior end of the mantle (Teredo), are in some cases added; or, in addition to its valves, the mantle may secrete a shelly tube (Teredo, Asperg ilium). The shell itself consists of superimposed lamellae of organic matter, hardened by the deposit of calca- reous salts. It is a cuticular excretion from the surface of the mantle, and never presents any cellular structure. But, from the disposition of its lamellae, and from the manner in which the calcareous deposit takes place in them, it may present varieties of structure which have been distinguished as nacre- ous, prismatic, and epidermic.1 The two valves are generally united over the median line of the haemal surface of the body by an uncalcified chitinous cuticular matter, termed the ligament, which is usually very elastic, and is so disposed that, when the valves are closed, it is either stretched or compressed. In either case, it antago- nizes the action of the adductor muscles, and divaricates the valves when these muscles are relaxed. Conch ologists com- monly draw a distinction between an internal and an external ligament ; but, in relation to the body of the animal, all liga- ments are external, and their internality or externality is in respect of the hinge-line, or the line along which the edges of the valves meet. In symmetrical, or equivalve, Lamelli- branchs, each valve is concave internally and convex exter- nally; it has, in fact, the form of a very depressed cone, the apex of which, termed the umbo, is incurved and is situated on, or projects beyond, the haemal, or, as it is termed, dorsal edge of the valve. Moreover, it is usually inclined forward, and situated nearer the anterior than the posterior end of the valve. Sometimes the umbonic cone is prolonged and bent inward, or may even form a short spiral turn (Tsocardia, Diceras), so that the valve acquires a certain resemblance to the shell of some gasteropods. As the shell of a Lamellibranch increases in thickness by the deposition of new layers on the interior fice of the old ones, and, in area, by the extension of the new layers beyond the old ones, the summit of the umbo represents the original shell of the embryo, and the outer sur- 1 See Carpenter, article " Shell," Todd's " Cyclopaedia." Huxley, " Tegu- mentary Organs," ibid. THE LAMELLIBRANCHIATA. 407 face is usually marked by concentric lines of growth, which indicate the boundaries of the successively added new layers of shell-substance. / Pio. 117.— Sectional diagram of a Fresh- water Mass -1 (Anodonia). — A, A. mantle, the right, lobe of which is cut away; B. foot ; C, branchial chamber of the mantle cavity; D. anal chamber; 7, anterior adductor muscle; IF, posterior adductor muscle; III. retractor muscle of the foot; a, mouth; b, stomach; c. intestine, the turns of which are supposed to be seen through the side wall* of the meso- soma; d, rectum: e, anus: /, ventricle; g, auricle; h, ffills, except i, riirht exter- nal sfill, largely cut away and turned back; k, labial palpi ; /, cerebral; m, pedal; n, parieto-splanchnic ganglia; o, aperture of the kidney or or<>an of Bojanus; p, pericardium. The applied edges of the two valves are very often pro- duced into elevations and depressions which interlock with one another. The form and arrangement of these teeth and sockets are of much use in systematic conchology. 408 THE ANATOMY OF IXVERTEBRATED ANIMALS. The muscles which are attached to the valves, viz., the ad- ductors, retractors of the foot, and pallial muscles, give rise to impressions on the inner faces of the valves, which are very obvious when the latter are removed and cleaned. With the growth of the animal, the distance of these impressions from the hinge-line and from one another is necessarily in- creased, and it is not difficult in some cases (e.g., Anodontci) to trace a faint triangular mark, which has its base in each adductor impression and its apex in the umbo, and which in- dicates the successive shiftings of position of the muscle. In some cases (e. g., Lima) a Lamellibranch may perform a sort of aquatic flight by the flapping of the valves of its shell. The hard and sharp-edged valves of the shell in Teredo are probably the agents by which the mollusk carves its pas- sages through the wood which it inhabits. Whether the valves of the shell of the Pholades and Saxicavce are the in- struments hy which they excavate their burrows in hard rock, or whether, as has been suggested, the foot, armed with sand, is the boring instrument, is a question which has been much discussed, but hardly brought to a satisfactory decision. The haemal face of the body is either flat or slightly arched, whence, in side view, the haemal contour is either straight or convex. In most Lamellibranchs the body is symmetrical in relation to the median plane, but, in those which have inequivalve shells, as the Scallop (Pecteii) and the Oyster ( Ostrcea), the one half is more convex than the other. No Lamellibranch has a distinct head ; but, in those which possess two adductor muscles (e. g., Anodonta), the reoion in which the anterior adductor lies and which is situ- ated in front of the mouth may be distinguished as the pro- soma, from the middle region (mesosoma) which gives rise to the foot ; while the part which lies behind the foot and con- tains the posterior adductor may be termed the metasoma. The foot may be rudimentary, but it is usually large, flex- ible, and employed as an organ of locomotion. The posterior face of tlie foot not uncommonly presents a gland which se- cretes a chitinous, or shelly, substance — the byssns. From the sides of the mesosoma, close to the attachment of each mantle-lobe, the branchiae project into the pallial cavity. In its simplest form, the branchia of a Lamellibranch con- sists of a stem fringed by a double series of filaments (e. g., Nucula). The next degree of complication arises from these The lamellibraxchiata. 409 filaments becoming as it were doubled upon themselves at their free ends, the reflected portions lying on the outer side of the outer, and on the inner side of the inner, series of primary filaments. But the free, or haemal, ends of the re- flected filaments contract no adhesion either with the mantle on the outer side, or with those of the opposite gill on the inner side. Delicate bands stretch from the primary to the reflected filaments across the interspace which they inclose (Mytilus, Pecten). In most Lamellibranchs the gills are four elongated plates, each of which is in fact a long and narrow pouch, with its open end turned toward the haemal face of the body. Two pouches are situated on each side of the mesosoma ; one of these pouches is internal, the other external.1 Their walls are united by transverse septa ; they are richly ciliated, and are perforated by numerous apertures. As the outer wall of each pouch is united with the mantle, and the inner with its fellow of the opposite side, behind the foot, the whole branchial apparatus forms a sieve-like parti- tion extended between the mantle and the foot (Fig. 117), and thus divides the pallia! cavity into a supra-branchial and an infra-branchial chamber. Inasmuch as the haemal edge of the inner wall of each inner branchial pouch is, for the greater part of its extent, not united with the mesosoma, but only closely applied against the latter, the supra-bran- chial and infra-branchial chambers may communicate by the cleft thus formed, as well as by the apertures in the lamellar walls of the branchial pouches. The anterior part of the supra-branchial chamber is divided into a right and left cavity by the interposition of the mesosoma, on the sides of which the apertures of the renal and generative organs are situated. The products of these organs therefore readily pass into these right and left cavities. The posterior part of the supra-branchial chamber, into which these two lateral divisions open, contains the termination of the rectum, and receives the faeces, as well as the urinary and generative prod- ucts : it is therefore a sort of cloaca. Its external opening is usually termed the anal opening of the mantle cavity. The margins of this opening may be produced into a tube which is termed the anal siphon. In front of the anal, or rather cloacal, opening, the margins of the mantle may be com- pletely disunited. Very frequently, however, they are con- ^The external gill-pouch is often smaller than the internal. In species of Lucina, Cytherea .and Telli/ui, only one gill-pouch, the internal, is present. 410 THE ANATOMY OF INVERTEBRATED ANIMALS. joined, so as to leave only an opening for the exit of the foot, and another behind this, which is termed the branchial opening. The edges of this aperture may be prolonged into a tube, which is termed the branchial siphon. When a La- mellibranch is in its natural element and undisturbed, the valves of the shell gape sufficiently to allow of the free en- trance or exit of water to or from the pallial cavity ; or, when siphons exist, they are fully protruded. The cilia with which the branchiae are beset work in such a manner as to drive the water from the infra -branchial chamber, through the open- ings of the branchiae, into the supra-branchial chamber. From hence its only way of exit is by the cloaca and the anal siphon, when the latter exists. In order to make up for the water thus driven out, a new supply of water enters by the interspace between the lobes of the mantle, which bound the infra-branchial chamber, or by the branchial siphon. These currents may readily be made obvious by allowing a stream of finely -divided coloring matter to pass slowly toward the branchial siphon of a Lamellibranch. It will be seen to be swiftly sucked in, and after a very short time a colored stream will flow out of the anal siphon. The same agency brings the nutritive matters suspended in the water within reach of the labial palpi, by which they are guided to the mouth. Whatever form the branchiae may possess, they are sup- ported by a chitinous skeleton, in the form of a partial or complete investment to the transverse branchial vessels. The mouth is bounded by lips, the angles of which are usually produced on each side into two labial palpi. Some- times the lips are represented by a circular fold produced into numerous tentacula (Pecten). There are no organs for the prehension or mastication of food. A wide and short gullet leads into a stomach surrounded by the liver, which consists of numerous caeca united into ducts which open into the stom- ach. Very generally a diverticulum of the pyloric end of the stomach contains a transparent rod-like body — the crystalline style. The intestine usually makes many convolutions, but, finally reaching the middle line of the dorsal region of the body, it terminates by the anus in the posterior part of the pallial chamber. The heart lies in the region traversed by the termi- nation of the intestine. It consists of an auricle and a ventri- cle, or of a ventricle and two auricles, or may be divided into two separate auricles and ventricles (Area). Aortic trunks distribute the colorless blood to the bod v, whence it is carried THE LAMELLIBRAXCHIATA. 411 to a large median venous sinus; from this it passes through the walls of the renal organs to the gills, and is returned from these to the auricular division of the heart.1 Very generally the ventricle invests the rectum, but in Ostrcea, Teredo, and Anomia, the ventricle is quite detached from the intestine. The renal organs, or organs of Bojamis, are usually two in number, often more or less united together, of a dark color, situated beneath and behind the pericardium and in front of the posterior adductor muscle, extending forward on each side of the mesosoma, and traversed by such numerous blood- channels, that they have a spongy texture. The walls of the cavernous blood-sinuses are lined with cells which secrete the urinary matters from the blood. These take the form of cal- careous concretions, containing uric acid. The gland commu- nicates at one extremity with the pericardium ; at the other, it either opens directly on to the surface of the body, or into a vestibular cavity which has an external aperture. In Ostroea and Teredo the renal organ seems to be present in only a very rudimentary form.2 The mesodermal region, between the endoderm and the ectoderm, is for the most part occupied by vascular, connec- tive, and muscular tissues, and by the reproductive organs, so that there is no large perivisceral space. But there is — 1. The large median sinus already mentioned, which receives the blood returned from all parts of the body, and is com- monly termed the vena cava. 2. A spacious pericardial chamber which incloses the heart. It is in communication with the venous system, and, consequently, directly or in- directly, with the vena cava. 3. The cavities of the renal organs, which usually freely communicate with one another, while they open into the pericardium on the one hand, and on the exterior of the bodv on the other. 4. In some Lamelli- branchiata, canals open on the exterior of the body, especiallv on the surface of the foot. In this way the blood-system is placed in direct, though circuitous, communication with the surroundino* water. These so-called icater-vessels com muni- cate internallv with the venous system, of which, indeed, they seem to form a part. It is probable that all these cavi- ties, taken together, represent the perivisceral cavity, pallial sinuses, and pseudo-hearts of a Brachiopod. 1 The circulatory organs of the fresh-water Mussel have been very fully de- scribed by Langer. (" Denksckriften der Wiss. Akademie, " 1855 and 1856.) 2 See, for the strueture of the renal organs and many other points connected with the anatomy of the LamellibrancMata. the series of valuable papers of La- caze-Duthiers. '(" Annates des Sciences Naturelles," 1854 to 1861.) 412 THE ANATOMY OF INVERTEBRATED ANIMALS. Strong bundles of muscular fibres, usually unstriated, pass transversely from one valve of the shell to the other, and bring them together ; while they are divaricated by the Fig. 118.— Anor?onta.~ Vertical and transverse section of the body through the heart; /, ventricle ; g, auricles ; c, rectum ; p. pericardium ; h, inner, ?*, outer gill; (/, ves- tibule of q, the organ of Bojanus ; £, foot, AA, mantle lobes. elastic reaction of the ligament. Of such adductor muscles there may be either one or two. When there are two (Di- myaria), the anterior adductor lies in front, and on the hae- mal side, of the oesophagus; while the posterior adductor lies in front, but on the neural side, of the rectum. Hence the alimentary canal, as a whole, lies between those two muscles. When only one adductor muscle exists (Monomyaria), it is the posterior. The foot is retracted between the valves of the shell by two or three pairs of retractor muscles, of which the anterior and posterior pairs are usually attached to the shell, close to the anterior and posterior adductor impressions. The pro- traction of the foot appears to be effected by the compression of the blood by the intrinsic muscles of the walls of the meso- soma and of the foot itself. Each lobe of the mantle is attached to the corresponding valve of the shell by a series of muscular fibres, the attach- ments of which give rise to a linear impression, which runs from one adductor to the other, and constitutes the pallial THE LAMELLIBRANCHIATA. 413 line. When the siphons are largely developed they have re- tractor muscles, the insertions of which are so disposed as to cause the posterior part of the pallial line to be more or less deeply curved or angulated. Hence the distinction of integro- palliate and siniipalliate as applied to Lamellibranchs which have the pallial line evenly rounded or notched. The cerebral ganglia lie at the sides of the mouth, and are connected by a commissure, which passes in front of it. They give branches to the anterior region of the mantle, to the gills, to the anterior adductor muscle, to the labial palpi, and to the parts about the mouth. The pedal ganglia are situ- ated in the foot; or in the corresponding region on the neu- ral side of the alimentary canal, when no foot is developed. Each is united by a commissure with the cerebral ganglion of the same side, and gives off branches to the muscles of the foot. The parieto-splanchnic ganglia lie on the neural face of the posterior adductor muscle. The long commissures which unite them with the cerebral ganglia usually traverse the renal organ, and lie beneath the floor of the pericardium. Each of these ganglia gives off a nerve to the branchia of its side, and supplies the posterior and middle part of the man- tle. This posterior pallial nerve may anastomose with the anterior pallial nerve from the cerebral ganglion. The gan- glia also furnish nerves to the posterior adductor muscle, to the heart, to the rectum, and to the muscles of the siphons, when the latter are present. Eyes are never developed in the cephalic region of the Lamellibranchs, but, in many (e. g., Pecten), numerous simple eyes terminate papillae of the mar- gins of the mantle. Auditory sacs are almost invariably at- tached by longer or shorter peduncles to the pedal ganglia. The Lamellibranchiata are usually dioecious, but some- times hermaphrodite ' (e. g., Ci/clas, some species of Cardium and Pecten, Ostraea, Clavarella, and Pandor a). The genera- tive organs are ramified glands of simple structure and simi- lar in both sexes, the ducts of which open into, or close to, the renal organs. The process of yelk-division s usually gives rise to smaller 1 The testes and ovaria are distinct in the hermaphrodite Perth/ ex. In Cat" dium serratum, adjacent c;eca of the sexual gland contain spermatozoa or ova, or both products may be developed in the same caecum. In the common Oysj ter the genital cax'a in any given individual are found to be either almost all ovigerous or almost all spermigerous ; and it appears probable that the pre- dominantly male precedes the predominantly female condition. See Lacaze- Duthiers, " Organes genitaux des Acephales Laniellibranches." (" Annates des Sciences Naturelles" 1854.) 2 See Loven, ArcMvfur NaturgeschicMe, 1849. De Quatrefages, " M^moires sur l'Embryogenie des'Tarets." (" Annales des Sciences Naturelles," 1849.) 414 THE ANATOMY OF IXVERTEBRATED ANIMALS. and larger blastomeres, of which the former, as an epiblast, invest the latter as a hypoblast. At the cephalic end of the embryo of most Lamellibranchs, a velum, or disk with richly ciliated edges, and, usually, a central tuft of longer cilia, is formed. On the dorsal face of the embryo the integument rises into a patch with raised edges, which is the rudiment of the mantle. The separation of the shell into two valves, united by an uncalcified hinge, must probably be ascribed to the manner in which the calcareous matter subsequently added to the shell is deposited. The foot appears as a median outgrowth of the neural face of the embryo behind the mouth. The branchiae have, at first, the form of separate fila- mentous processes, which are developed from the roof of the anterior part of the pallial cavity, at the point of junction of the mantle with the mesosoma, and gradually increase in number from before backward. In those Lamellibranchs wThich have pouchlike gills, it appears that the processes which are first formed become the outer lamella of the inner gill-plate, their free ends uniting together; the inner lamella of this plate is produced by the upgrowth of a thin lamina, which subsequently becomes perforated, from the united ends of these processes. The inner lamella of the outer gill is formed of branchial processes, which grow out from the at- tached ends of the first set ; and the outer lamella of this gill is produced in the same fashion as the inner lamella of the inner gill.1 Recent observations tend to show that in these, as in other Invertebrata, the nervous ganglia are modified in- growths of the epiblast. The simplest form of development of the Lamellibranchi- ata has been observed in Pisidium? By the process of cleavage, the vitellus is divided into a number of equal blas- tomeres. The morula thus formed undergoes invagination, and is converted into a gastrula. The blastopore, or aperture of invagination, closes, and the epiblast, or ectodermal layer of the embryo, growing much faster than the hypoblast, or en- dodermal layer, the latter forms a small shut sac, the primi- tive alimentary sac (or archenteron) attached to one point of the inner surface of the much larger ectodermal sac. The 1 Lacazc-Duthiers, " Sur le developpement des brnnchies des Mollusques ac^phales Lamellibranches." (" Annales des Sciences Naturelles," 4, iv. ) 2 Lankester, " On the Developmental History of the Mollusca." (" Phil. Trans.," 1874.) THE DEVELOPMENT OF LAMELLIBRANCHS. 415 mesoblastic cells appear to be derived both from the epiblast and the hypoblast. The mouth is formed by a depression of the ectoderm at the anterior end of the body, which grows toward and opens into the archenteron. The anus is developed at the opposite end, in the region of the primitive invagination. On the neural face of the embryo the foot grows out, while the mantle appears on the opposite face ; and, in the centre of the man- tle, a transversely oblong depression lined by elongated cells is the " shell gland." In the median line this answers to the ligament, and, at the sides, to the middle region of the future valves of the shell ; but the precise share, if any, which it takes in the formation of these parts does not appear. jPisi- clium has no velum. The development of one of the fresh-water Mussels ( JJnlo pictorum) has recently been worked out very fully by Rabl.1 The vitellus divides into two unequal masses, of which the larger is termed by Rabl the " vegetative " and the smaller the " animal" cell — somewhat inconvenient names, which may be replaced by " macromere " and " micromere." Each of these becomes subdivided, partly by ordinary fission, partly, as in the case of the macromere, by a process of budding, into blastomeres, of which those which proceed from the macromere long remain larger and more granular than those which pro- ceed from the micromere. The blastomeres arrange them- selves into a hollow sphere — the blastosphere. This is a vesic- ular morula, composed of a single layer of blastomeres, of which those of one hemisphere have proceeded from the micro- mere, and those of the other from the macromere. Two blas- tomeres of the macromeral hemisphere remain much larger than the rest. The macromeral hemisphere next undergoes invagination, and its invaginated part becomes the hypoblast. The two large blastomeres just mentioned, which are disposed symmetrically, one on each side of the median plane at the anterior margin of the area of invagination, become inclosed between the hypoblast and the epiblast, and by their division give rise to the mesoblast. This last, therefore, may be re- garded as an indirect product of the hypoblast. The endodermal sac formed by the hypoblast now loses its connection with the region of the embryo of which it is an invagination, and applies itself to the anterior wall of the body, where an involution of the ectoderm, which gives rise 1 C. Rabl, " Ueber die Entwickelungsgeschicbte der Malermuschel," Jena, 1876. 27 416 THE ANATOMY OF IXERVTEBRATED ANIMALS. to the oral cavity, takes place. The greater part of the meso* blastic cells become the adductor muscle, which is at first sin- gle and answers to the posterior adductor of the adult. There seems to be no shell gland. The shell appears at first as a membranous cuticula, continuous from side to side, and there- fore undivided into two valves. Subsequently it becomes calcified and bivalve. The byssus gland is developed as an involution of the octoderm at the posterior end of the body; and the ventral hemisphere, or that" opposite the shell, be- comes divided by a deep median fold into the two lobes of the mantle on which the characteristic pencil-like papillae appear. In front of the rudimentary mouth are two ciliated depres- sions of the ectoderm, which are possibly the rudiments of the nervous ganglia. In Unio and Anodonta the young are hatched in the outer gill pouches of the parent, from which they are so dissimilar that they were at one time considered to be parasites ( Glochi- diuni). The valves of the shell are triangular, and have in- curved and serrated apices, by the help of which the larvae, after they leave the parent, attach themselves to fishes and other floating bodies. In this position they undergo a sort of metamorphosis, and eventually fall off and sink to the bottom as minute fresh-water Mussels. On comparing the Lamellibranchiata with the JBrachio- poda, it is obvious that the two have, in common with one another and with the Annelida, the ciliated or veligerous larval form. If the shell gland is, as Mr. Lankester suggests, the homologue of the peduncular gland of Loxosoma and of the Brachiopod larvae, it follows that the peduncle of the Brachiopod corresponds with the centre of the pallial surface of the Lamellibranch, and that the so-called dorsal and ven- tral lobes of the .mantle in the Brachiopod correspond with the anterior and posterior halves of the mantle in the Lamel- libranch. The Brachiopod hinge will therefore be transverse to the axis of the body, while the Lamellibranch hinge is parallel with it. If this comparison be just, however, the three segments of the Brachiopod larva cannot answer to the segments of an Annelid larva, but the two posterior seg- ments of the Brachiopod larva must represent an outgrowth of the haemal side of the body ; and this would correspond very well with the arrangement of the intestine in the artic- ulated Brachiopoda. In the simplest forms of the Lamellibranchiata, as Tri- gonia, Nucxda, and Pecten, the mantle-lobes are almost, or THE LAMELLIBRANCHIATA. 417 completely, disunited from one another and from the branchiae, and the latter are either simple plumes or have undergone but little modification. The haemal face of the body is short relatively to its vertical height. In most Lamellibranchs the haemal face of the bodv is longer; the gills are lamellar, and the mantle-lobes are united with one another and with the gills, so as to separate a supra- branchial from an infra-branchial chamber (Anodonta). In yet others, the posterior margins of the mantle are produced backward into short siphons, but the mantle-lobes remain separate for the rest of their extent ( Cardium) ; in others, the siphons are greatly elongated and the ventral margins of the mantle-lobes unite, so as to leave only a small median aperture for the foot (Pholas). In the most modified forms, the body becomes more and more elongated, until, in Teredo, it is completely vermiform, and the valves of the shell cover but a very small portion of the body. The foot is wanting as a distinct structure in Ostrcea ; while in Cardium and Trigonia it is a large muscular organ, by the* aid of wThich the animal is able to leap for some dis- tance. The byssus may be present in the young and absent in the adult (e. g., Anodonta). It may have the form of strong chitinous filaments (Mytilus), or of a plate of horny or shelly texture (Area, Anomia). The inequality of the valves at- tains its maximum in the Hippuritidce, in which one valve may have the form of a long cylinder, or cone, while the other is a flattened plate.1 The shells of Lamellibranchs are among the most abun- dant of fossil remains in all epochs of the world's history. In the Palaeozoic formations, however, the proportion of these mollusks relatively to the JBrachiopoda is the reverse of what obtains at the present day, the latter being very numerous, while the Lamellibranchs are comparativelv scantv. The in- tegropalliate are far more numerous than the sinupalliate forms in the older rocks. The JERppuritidoB of the Cretaceous epoch is the only family of ancient Lamellibranchs which is extinct at the present day, and the only one which diverges to any considerable degree from existing forms. The Odoxtophoea. — In the Mollusks which belong to this division, the mantle, always present in the newly-hatched young, may abort in the adult condition. It is never divided 1 For an excellent account of the Lamellibrancliiata from the conchological side, see Woodward's " Manual of the Mollusca," 418 THE ANATOMY OF INVERTEBRATED ANIMALS. into two lobes, though it may be slit or perforated where it forms the wall of the branchial chamber (Haliotis, Fis- surella). Very generally, the prosoma bears tentacula and eyes ; and a distinct head being thus recognizable, these Mollusks have been named Cephcdophora, in contradistinction to the acephalous Lamellibranchs and Brachiopods. The mantle commonly gives rise to a shell, which may either be a more or less calcified cuticular product of the epi- dermis, covering the outer surface of the mantle, when it constitutes an external shell, as in the Lamellibranchiata and Bracluopoda ; or it may be developed within a sac in the interior of the mantle, as an internal shell. In neither of these cases is it ever a bivalve shell divided into two lateral portions.1 Usually it is in one piece (univalve), but in one group, the Ghltonidoe, it consists of a number of pieces (not exceeding eight), arranged in longitudinal series along the middle line. Calcareous matter is very commonly diffused, in the form of granules, through the connective tissue, and often takes the form of spicula (e. g., Doris). The mesosoma is generally prolonged into a muscular foot, which may be provided with lateral appendages, the epipodla. And, on the haemal aspect of the posterior portion of the foot, a chitinous or shelly plate, termed the operculum, may be developed. This operculum appears to be the analogue, if not the homologue, of the byssus of the Lamellibranchs, and is certainly not homologous with either of the valves of the shell of the latter, which are pallial structures. The edge of the mantle forms a free fold which nearly or entirely sur- rounds the mesosoma ; and in one genus, Dentcdium, the margins of the mantle unite for the greater part of their length : in all the rest they remain free. A space is inclosed between the lobes of the mantle and the mesosoma. Usually this space is much larger on one face of the body, and con- stitutes the pallial chamber. As a rule, the branchiae are lodged in this chamber, and the anus opens into it. In a very few Odontophora, the symmetry of the body is undisturbed; that is to say, the mouth and the anus are situ- ated at opposite ends of the axis of the body, and the haemal J The singular bivalve plates, termed Aptychw, which occur in the Ammo- nitirtce, whatever their nature may be, are obviously not homologous with the shell of ordinary Mollusks, which is represented by the chambered shell of the cephalopod. THE ODOXTOPHORA. 419 face is not produced into a visceral sac (e. g., Chiton, De?ita- llum). But, in the great majority, such a visceral sac is formed. In the Cephalopoda it coexists with bilateral sym- metry, inasmuch as the mantle and the anus lie in the plane which divides the body into two similar halves. But, in most Odontophora, the anus is twisted to one side (usually the right), and in many it is situated, together with the pallial chamber in which it is contained, on the anterior face of the bodv. The mouth lies at the anterior end of the body, on the haemal side of the anterior part of the foot (except in the Cephalopoda). It may be provided with variously-disposed jaws, or cutting-plates, of a chitinous or calcined substance. But the structure which is most characteristic of the Odon- tophora, and which is absent in only very few genera (e. g., Tethys, Doridium, Hhodope), is a peculiar rasping and some- times prehensile apparatus, the odontophore, or, as it is often termed, the tongue, which is attached to the floor of the mouth (Figs. 119,120). This apparatus consists of a skeleton / of a subradular membrane, which is continuous with the lining of the oral cavity ; of the radula ; and of intrinsic and extrinsic mus- cles. The skeleton is composed of two principal masses of par- tially fibrous, or completely cartilaginous, tissue {odonto- phoral cartilages), which may be more or less confluent, and are further united together in the middle line by fibrous and muscular tissue. Their anterior ends and oral faces are free and smooth, and are usually excavated so as to present a trough-like surface to the subradular membrane, which rests upon them. Accessory cartilages may be added to these. Behind, the subradular membrane is continued into a longer or shorter sac, lined by a continuation of the buccal epithe- lium. The radula is a cuticular chitinous product of the epithelium of the subradular membrane. It is armed with tooth-like processes arranged in one or many series ; and ad- ditions are constantly being made to its posterior end, which is lodged in the sac of the subradular membrane. Thus the teeth are replaced from behind, as fast as they are worn away by friction against the food which they rasp, at the anterior end of the ribbon. The intrinsic muscles of the odontophore are attached, on the one hand, to the posterior and under faces of the odonto- phoral cartilages, and, on the other, to the subradular mem- 420 THE ANATOMY OF INVERTEBRATED ANIMALS. brane, some being inserted into its posterior and lateial por« tions, and others into its anterior extremity, after it has turned over the anterior extremities of the principal cartilages. Fig. 119.— Buccinam undatum.—A, radula. B, one of the transverse rows of teeth ; a, anterior, b, posterior end ; c, central, I, lateral teeth. (After Woodward, '" Man- ual oi the Mollusca.") Fig. 120.— A, Troclnis cinerarius ; the median tooth and the teeth of the right half of one row of the radula. B, Cyprcea, Europcea, one row of teeth of the radula. (Woodward, ibid.) Certain of the muscular bundles are also attached to the fore- part of the odontophoral cartilages themselves. The con- traction of these muscles must tend to cause the subradular membrane, and with it the radula, to travel backward and forward over the ends of the cartilages in the fashion of a chain-saw,' and thus to rasp any body against which the teeth may be applied. When undisturbed, the radula is concave from side to side, and the teeth of the lateral series, being perpendicular to the surface to which they are attached, are inclined inward toward one another. But when the intrinsic muscles come into action, the radula, as it passes over the ends of the cartilages, becomes flattened, and the lateral teeth are consequently erected or divaricated. The extrinsic mus- cles pass from the odontophore to the lateral walls of the head, and protract or retract the whole apparatus. They THE ODOXTOPHORA. 421 may give the protruded extremity of the radula a licking mo- tion, which is quite independent of the chain-saw action due to the intrinsic muscles.1 The odontophore is developed very early, and it would be interesting to know whether it exists in the jToung of those few Odontophora in which it is wanting in the adult state. Salivary glands are very generally present in the Odonto- phora* and the liver is usually large. As in the Mollusca in general, the blood-corpuscles are colorless and nucleated. The blood plasma is red in Pla- norbis. The heart may be wanting (Detitalium), or it may resemble that of the Lamellibranchs in having two auricles ( Chiton, Haliotis), and even in being perforated by the rectum (Ha- liotis, Turbo, JVerita) ; most commonly it consists of a single auricle and a single ventricle. In the Cephalopods, it is hard to say whether the two or four branchio-cardiac trunks which open into the ventricle should be regarded as veins or as auricles. An accessory "portal" heart has been described in Doris.3 Special respiratory organs may be wanting, their place being taken by processes of the body, or by the walls of the mantle cavity, or by the general surface. The branchiae, when present, are numerous lamellar pro- cesses, or from one to four plume-like gills. Aerial respira- tion is effected by the walls of a pulmonary sac, which is a modification of the pallial cavity. The presence of renal organs, in the form of one or more sacs situated close to the heart, open to the exterior on one side, and, on the other, in relation, usually by means of a glandular structure, with the returning current of blood, is very general ; and, in many cases, these renal sacs communi- 1 In my memoir " On the Morphology of the Cephalous Mollusca " (" Phil. Trans.," 1852) I described the chain-saw action of the odontophore, as I ob- served it in the transparent Firoloides and Atlanta, while living. But, as Tro- schel has remarked in his excellent monograph (" Das Gebiss der Schnecken," erste Lieferuns?, pp. 19, 20? 1856), I did not sufficiently dwell on the frequency and importance of the licking action produced by the extrinsic muscles. I am still of opinion, however, that this action cannot be rightly described as a movement of the radula following secondarily upon that of the cartilages, inas- much as it is a motion of the whole odontophore. On the other hand, it may be, as has been susrsfested to me by Mr. Geddes — who at my suggestion has undertaken a reexamination of the structure of the odontophore — that the flex- ure of the anterior ends of the odontophoral cartilages, bv the intrinsic mus- cles inserted into them, plays an important part in the motion of the radula. 2 In Dolinm the salivary secretion contains free sulphuric acid. 3 Hancock and Embleton, " On the Anatomy of Doris." (" Phil. Trans.," 1852.) 4:22 THE ANATOMY OF IXVERTEBRATED ANIMALS. cate directly with the blood sinuses through the pericardium. In many Pteropods and Heteropods they are rhythmically contractile. As in the Lamettibrarichiata, so in many Odontophora, simple or branched canals traverse the substance of the foot and open externally by a more or less conspicuous pore, which is usually situated upon its inferior face. These aquiferous canals, as they have been termed, appear, in many cases, to open by their inner ends into the blood sinuses, and thus to establish a direct communication between the blood and the surrounding water. In species of Pyrula, Agassiz found that colored fluids injected into the pore passed into and filled the blood-vessels generally. But it may be doubted whether these canals should be regarded as a special system of ves- sels, rather than as blood sinuses which open externally. The arrangement of the centres of the nervous system in Dentalium J most nearly approaches that which exists in the Lamellibranchiata. Two cerebral ganglia lie close together on the haemal side of the oesophagus. A long commissural cord connects each of them with one of the pedal ganglia, which are also closely united. A second long commissure passes backward from the cerebral ganglia, and often presents a ganglionic enlargement at its origin. It unites with one of two ganglia, situated close to the anus, and connected, in front of it, by a rather long transverse commissure. The nerves distributed to the posterior half of the mantle are given off from these ganglia, and those to its middle region from the anterior end of the commissure or its ganglionic en- largement. There seems no reason to doubt that the ganglia close to the anus, together with the ganglionic enlargements at the anterior ends of the commissures which connect them with the cerebral ganglia, correspond with the parieto- splanchnic ganglia of the Lameilibranchs, and that the cere- bral and pedal ganglia are the homologues of those so named in the latter Mollusks. In addition to this approximation of part of the gangli- onic mass of the parieto-splanchnic system to the cerebral ganglia, Dentalium differs from the Lameilibranchs and re- sembles other Odontophora, in the possession of a system of buccal nerves, which arise from the cerebral ganglia, and in which minute ganglia are developed. The nerves which pro- 1 See Lacaze-Duthiers, " Organisation du Dentale." THE ODOXTOPHORA. 423 ceed from the buccal ganglia are distributed to the oclonto- phore and its muscles. In other Odontqphora, the two cerebral and two pedal ganglia, with their commissures, are always to be recognized ; but the number of the ganglia which represent the parieto- splanchnic system may be increased, and the anterior ganglia of this system may attain a large size, and may come into close relation not only with the cerebral but with the pedal ganglia. In Lymncmis palustris,1 for example, there are five such ganglia situated close to the cerebro-pedal ring. The most anterior of these, on each side, is united with both the cere- bral and the pedal ganglion of its side, and appears, indeed, like an enlargement upon a second commissure between those two ganglia. The ganglia which constitute the second pair are united, in front, by a short commissure, with the preced- ing; and, behind, with the fifth or azygos ganglion. The second pair of ganglia give off the nerves to the right and left sides of the mantle respectively. In Zrimax, and apparently in the terrestrial Ptdmonata generally, the arrangement is essentially the same, except that all the ganglia of the parieto splanchnic system coalesce into one mass, between which and the pedal ganglia the aorta passes. In Haliotis? on the other hand, while the anterior parieto- splanchnic ganglia are situated close to the pedal ganglia, and are connected with them and w7ith the cerebral ganglia in such a manner as to give rise to an apparent second cere- bro-pedal commissure, the ganglia which represent the second pair in Lymncmis are situated at the base of the branchiae, and are united by a long commissure with one another, and also with the anterior parieto-splanchnic ganglia. Of the latter commissures, that from the left branchio-pallial gan- glion goes to the right anterior parieto-splanchnic ganglion, and vice versa. With respect to the position of the cerebral and pedal ganglia in the Odontophora, the commonest arrangement is that in which the cerebral ganglia are supra-cesophageal, and are„connected by two longer or shorter commissures, on each Compare Laeaze-Duthiers, "Du systeme nerveux des Molluaques gas- teropodes pulmones aquatiques " ("Arch, de Zoologie," 1872), and tha numer- ous figures of the arrangement of the cerebral ganglia of the nervous system given in his memoir on the otocysts. (Ibid.) 2 See Lacaze-Duthiers, " Sur le systeme nerveux de Ilaliotide." 424 THE ANATOMY OF INVERTEBRATED ANIMALS. side, with the pedal and anterior parieto-splanchnic ganglia, both of which are infra- or post-oesophageal. But in many cases (most JVudibranehiata) the pedal and parieto-splanch- nic ganglia are approximated to the cerebral ganglia (the latter being supra-cesophageal), and are united by long sub- oesophageal commissures. In others, as in most Pteropoday the pedal and parieto-splanchnic ganglia are sub-cesophageal; while the cerebral ganglia, brought close to them, are united by a supra-cesophageal commissure. Accessory ganglia are frequently developed in the region of the heart and branchiae, on the nerves of the parieto- splanchnic system. A complicated system of visceral nerves is distributed over the whole length of the alimentary canal, the genital organs, and various parts of the vascular system, in many Odontophora.1 Two auditory vesicles usually exist, and very generally ap- pear to be sessile upon the pedal ganglia. In the Heteropoda, in many JVudibranehiata, as shown by Hancock, and in nu- merous genera of Branchio- and Tulmo-g aster opoda, which have been carefully examined by Lacaze-Duthiers,2 however, there seems to be no doubt that the auditory nerves arise from the cerebral ganglia, even though the vesicles may be situated close to the pedal ganglia. Olfactory organs certainly exist in the Cepthalopoda in the form of saccular involutions of the integument near the eyes ; and it is very probable that the integument of the ten- tacula, or of the lips, may subserve the same function in the Gasteropods. Eyes are generally present, and are limited to two, situ- ated in the head. They resemble the vertebrate eye in struct- ure, so far as they possess a concave retinal expansion, and usually, in front of this, a vitreous humor, lens, and cornea. 1 See especially Hancock and Embleton, " The Anatomy of Doris." (" Phil. Trans.," 1852.) 2 " Otocystes des Mollusques." ("Archives de Zoologie ExpeVimentale," 1872.) In the memoir the origin of the acoustic nerves from the cerebral ganglia is determined in so many Pulmo-gasteropoda (Limax, Arion, Testacella, Clausilia, Zonites, Helix, Succinea, Phym, Lymncevs, Ancylns) and Bravchio- gasteropoda {Neritma, Ptludina, Cyclostoma, Pileopsis, Calyptroea, JVatlca, Nassa, Trochus^ Murex, Cassidaria, Purpura, Patella, Haliotis, Pkiline, Aply- sia, Lam ell aria), that there is a large basis for the generalization that this mode of origin is universal. Moreover, according to Lacaze-Duthiers, the same law holds good for the Cephalopoda. Such being the case, the question suggests itself whether the connection of the nerves of the otocysts with the pedal ganglia, which obtains universally among the Lamellibranchs, indicates their real or only their apparent origin. THE ODOXTOPHORA. 405 But they differ from the eyes of Vertebrata^ and resemble those of other invertebrated animals, in that the structures which answer to the rods and cones are situated on that face of the retina which is turned toward the light, while the fibres of the optic nerve traverse the pigment layer to reach them. The reproductive organs of the Odontophora present very great diversities of structure. They may be either dioecious or monoecious, and each type of reproductive organs may pre- sent various degrees of complexity. Of the dioecious repro- ductive organs there are two chief forms : the one in which the duct of the ovarium or testis is continuous with the gland; and the other in which the duct opens into a sac, into which the ova or spermatozoa are set free by the dehiscence of the follicles in which they are developed. The latter arrange- ment is met with in the Cephalopoda / the former appears to prevail among all the other dioecious Odontophora. In these, the racemose generative gland is usually situ- ated close to the liver. In the female, the oviduct ordinarily presents a uterine dilatation toward its termination, which is generally situated in the pallial cavity on the right side of the body. In some rare cases (Paludina, JVeritina), a dila- tation or a sj)ecial vesicular appendage of the uterus may serve as a vesicula seminalis / and in Paludina, according to Leydig, an albumen-gland opens into it. A penis is not always present. When it exists, it is a muscular process of the mesosoma, to which the semen may be led from the opening of the vas deferens by a groove ; or it may be traversed by the vas deferens which opens near, or at, its apex. In all the monoecious Odontophora which have as yet been thoroughly examined, there is a generative gland termed the ovotestis, in which both spermatozoa and ova are produced. Only in the anomalous genus Phodope (Kolliker) are the spermatozoa and ova formed in distinct caeca ; in all the rest, each caecum is hermaphrodite, the spermatozoa and the ova being usually developed in different parts of the caecum. The duct of the ovotestis may remain single to its termination at the genital aperture, or become only incompletely divided into two semicanals (Pteropoda, Pleurophyllidia, Umbrella, Aplysia) ; or it may become, at first partially, and then com- pletely, divided into an oviduct and a vas deferens (JViidi- branchiata, Pleurobranohia, Pulmonata). In the former case there is but one genital aperture. The common duct usually receives the secretion of a uterine gland 426 THE ANATOMY OF INVERTEBRATED ANIMALS. which may take the form of a special albumen gland, and a spermatheca opens into it near its outer extremity; while, on the male side, a vesicula seminalis and an eversible peuis may be added. The penis, however, may be distant from the genital opening, and then a groove on the side of the body leads to it (Aplysia). In the latter case there are two geni- tal apertures, one for the male and one for the female organs, though they may open into a common vestibule. The penis is an eversible involution of the integument, on which the vas deferens opens. A prostate gland is usually connected with the latter, and, near its opening, there may be a saccular ap- pendage, in which a hard pointed body, the spiculum amoris, is contained (Don's, Helicidce). An albumen-gland opens into the uterus, and a spermatheca is connected with the vagina. Spermatophores, by the aid of which the spermatozoa are transferred into the female organs, occur in the Cephalopoda, and in the Palmonata. In the latter they are grooved bands, or incomplete tubes of hardened mucus secreted by the penis, which become filled with spermatozoa during copulation ; while, in the former, they are closed cases which may have a very complex structure. In the great majority of the Oclontophora the young leaves the egg as a vellger, very similar to that of the Lamel- libranchiata. The velum usually becomes bilobed, and some- times (Hetero})oda) its margins are produced into many ten- taculiform processes ; and, in all Pteropoda and Pranchio- (j'lsteropoda, whether the adult possess a mantle and a shell or not, the larva is provided with both, the shell being at first a simple conical symmetrical cap, developed in the middle line of the mantle. The eyes make their appearance behind the velum, and the tentacles in front of or upon it. While the course of the development of the embryo in the O lontophora presents a general uniformity, there are wide differences in detail. In Paludina* the blastomeres produced by yelk-division are of equal size. They arrange themselves into a vesicular morula, which undergoes invagination and becomes a gas- trula of the simplest tyjie. The aperture of invagination (blastopore) becomes the anus, while the mouth is formed by an involution of the ectoderm of the anterior end of the 1 Lankester, " On the Coincidence of the Blastopore and Anus in Paludina vivipara." {Quarterly Journal of Microscopical Science, 1876.) THE DEVELOPMENT OF THE ODOXTOPHORA. 427 body, which extends toward and eventually opens into the blind end of the archenteron or primitive alimentary sac. A ciliated velum is developed on the hasmal side of the mouth, and a " shell gland " appears in the centre of the area which gives rise to the mantle. In LymnceusJ also, cleavage ends in the production of blastomeres of equal size, whether with or without a transi- tory stage of inequality, and the vesicular morula undergoes invagination to give rise to the archenteron. The blastopore is elongated, and it appears to be likely that its anterior and posterior ends may coincide with, if they do not give rise to, the mouth and anus respectively. In most 0 lontophora, the process of yelk-division goes on unequally, and results in the production of large and small blastomeres (macromeres and micromeres). The latter form a layer which gradually extends over the macromeres and in- closes them. Obviously, this comes to the same result as invagination ; and the included macromeres and their progeny either become converted into the archenteron with its ap- pendages, and more or less of the mesoblast, or a portion of them may serve as food-yelk. In the Pteropoda and Heteropoda* and in JVassa, JVatica, and Pusus* the blastopore, or aperture circumscribed by the edges of the micromeral layer as it grows round the macro- meres, closes, but corresponds in position to the invagination of the ectoderm which gives rise to the future mouth ; and the anus is a new formation. In such land Ptdmonata as Llmax, the process of velk- division gives rise to macromeres and micromeres, and the latter inclose the former. What becomes of the blastopore is not clear, though I am inclined to think that it corresponds in position with the mouth. The latter is seen very early as a funnel-shaped invagination of the epiblast bounded by lat- eral lips. Behind it, the foot grows out and rapidly attains a considerable size. Its posterior extremity becomes flattened from above downward, and converted into an orbicular ap- pendage, the opposite walls of which are connected by retic- ulated muscle-cells. This appendage undergoes rhythmical 1 Lankester, " Observations on the Development of the Pond-Snail" (Quar- terly Journal of Microscopical Science, 1874), and C. Rabl, " Die Ontogenie der Siisswasser Pulraonaten " (Jen. Zeitschrift, 1875). 2 Fol, " Etudes sur le developpement des Mollusques." (" Arch, de Zoologie experimental e," 1875, 1876.) 3 Bobretskv, " Studien fiber die embrvonale Entwickelung der Gasteropo- Jen." f" Archiv f. Mikr. Anat.," 1S7«>.) * 428 THE ANATOMY OF INVERTEBRATED ANIMALS. movements of dilatation and contraction. The macromeres form a large mass inclosed within a spheroidal dilatation of the greater part of the haemal wall of the body, which deserves the name of velk-sac even better than the structure so named in the Cephalopoda, inasmuch as it more nearly corresponds, morphologically, with the vitelline sac of vertebrated animals. Between this sac and the foot the small remainder of the haemal wall becomes converted into the mantle. The walls of the vitelline sac undergo contractions which sometimes, but not always, alternate with those of the pedal appendage. On each side of it appears the " primitive kid- ney," consisting of a curved elongated series of cells within which concretions are developed, and terminating in a duct which opens on the posterior face of the vitelline sac, close to the mantle. The exact mode of origin of the alimentary canal has not been made out ; but, in any case, only a very small portion of the endodermal cells can take part in its formation, and the archenteron is, at first, a sac which nearly fills the small projection formed by the rudimentary mantle. The oral involution of the ectoderm gives rise to the odon- tophore, and extends across the base of the foot, to open, eventually, into the archenteron. The fold of the mantle which overhangs the respiratory aperture makes its appearance very early; and, immediately behind it, the intestine is visible as a short tube, which ex- tends from the archenteron to the surface, but does not, at first, open there. As development proceeds, a movement of the macromeric part of the vitellus takes place in exactly the opposite direc- tion to that of the food-yelk of the Cephalopoda ; that is to say, from the vitelline sac into the constantly enlarging foot. The alimentary canal accompanies it, the anus alone remain- ing in its primitive position. The constantly lengthening alimentary canal becomes disposed in folds ; between these the macromeric part of the vitellus, which gradually forsakes the diminishing vitelline sac, disposes itself around the coils of the intestine. Eventually, for the mcst part, it becomes converted into the liver. The rudimentary shell first makes its appearance in the form of a few subcrystalline calcareous plates, on the inner side of the ectoderm.1 The development of Helix is similar to that of Limax ; 1 Compare Gegenbaur, " Zur Entwickelimgsgescliichte der Land-Gastero- poden." {Zeitschriftfiir Wiss. Zoologie, 1852.) THE DEVELOPMENT OF THE ODOXTOPHORA. 429 but the intestine passes into the large visceral sac instead of into the cavity of the mesosoma. The shell is stated by Gegenbaur to be at first internal, as in Limax. In neither case has the relation of the shell to the shell-gland been determined. The process of development appears to present a consider- able range of variation in the Pulmonata. Semper1 states of a species of Vaginulus, that, after the process of cleavage, the embryo assumes the form of a cylinder, at one pole of which the rudiments of the tentacula and of the lips appear; while, at the sides, a longitudinal ridge indicates the edge of the mantle, and marks off the more convex pallia! region from the flat foot. No shell is formed. In LymncBiis? as has been already stated, the vitellus undergoes complete division, and the resulting vesicular morula undergoes invagination to produce the hypoblast. Only the middle part of the archenteron becomes the alimen- tary canal, however. The lateral portions, which take on the form of rounded sacs, may not improbably, as in the Brachio- pods, give rise to the perivisceral cavity, though this has not been proved. The mouth is produced by the formation of an opening in the coalesced endoderm and ectoderm, at a point near the anterior end of the body. Upon each side of the mouth a transverse ciliated ridge of the ectoderm is developed, and represents the edge of the velum in other molluscan em- bryos. Behind this, and on the opposite side of the embryo to that on which the mouth is placed, a raised patch of the ectoderm represents the mantle. The foot commences as a papilla immediately behind the mouth. An involution of the centre of the pallial ectoderm gives rise to a shell-gland, but the proper shell is developed, independently of this, as a cu- ticular secretion from the whole surface of the mantle. Thus the embryo of Lymnceus possesses an incompletely developed velum, and is, in all essential respects, similar to the veligerous embryo of Larcellibranchs, Pteropods, and Gasteropods ; while the Slugs and Land-snails have neither the velum (unless it be represented by the anterior contrac- tile sac) nor the external embryonic shell. The development of the Cephalopoda is very unlike that of other Mollusks, and will be dealt with under the head of that group. 1 " Entwickelungsgeschichte der Ampullaria polita." 2 Lankester, "Observations on the Development of the Pond-Snail, Lym- nceus stagnalis." {Quarterly Journal of Microscopical Science, vol. xiv., Isew Series.) 430 THE ANATOMY OF IN VERTEBRATED ANIMALS. The lowest forms of the Odontophora are the Polyplaco- phora, or ChitonidcB, and the Scaphopoda, or Dentalidm. The bilateral symmetry of the body is completely, or almost IV Pig. 121.— I. Chiton Wossnessenskii. (After Middendorf.) II. Chiton dissected to show o, the mouth; g. the nervous ring; ao, the aorta; c, the ventricle : c\ an auricle; br, the left branchiae ; od, the oviducts. (After Cuvier.) IIL, IV., V. Stages of development of Chiton cinereus. (After Loven.) completely, undisturbed, while the haemal wall is flat, or near- ly so, and there is no visceral sac. The Polyplacophora. — The Chitons (Fig. 121, I.) are elongated, slug-like animals, having the mouth at one end of the body, and the anus at the opposite extremity. A rounded lobe surmounts the mouth, but it bears no eyes nor tenta- eula, and there is no definite head. The edges of the mantle are thickened, but little prominent, so that the pallial cavity is not much more than an elongated groove, beneath and internal to the thickened edge, which is sometimes beset .with setae. In the region in which these setae occur, the surface of the mantle is covered by a thick cuticula. The setae, which THE SCAPHOPODA. 431 may be merely chitinous or completely calcified, or partly in the one and partly in the other condition, are developed in sacs lined by the cells of the ectoderm.1 In the pallial groove lie the short lamellar processes which represent the branchiae. The shell is unlike that of any other Alollusk. It consists of eight, transversely elongated, symmetrical pieces, arranged one behind the other, overlapping in such a manner that the posterior edge of the one covers the anterior edge of the next, and articulated together. Sometimes the valves are partially or completely inclosed in the mantle. The heart, composed of a single median ventricle and two lateral auri- cles, is placed in the middle line, above the rectum, at the posterior end of the body. The aorta is continued forward from its anterior end, while the auricles receive the blood from the branchiae. In Chiton piceus, according to Schiff,2 each auricle communicates by two openings with the ven- tricle, and the two auricles are united behind. The repro- ductive organ is median and symmetrical, and its two ducts open on each side of, and not far from, the anus. The embryo leaves the egg as an oval body, surrounded near its anterior end bv a circular ciliated band, behind which an eye-spot appears on each side (Fig. 121, III.). The seg- ments of the shell appear while the young Chiton is still locomotive, and the disk in front of the ciliated band becomes converted into the lobe above the mouth (Fig. 121, IV., V.). The Chitons have existed from the Silurian epoch to the present day, apparently with very little modification. The Scaphopoda.3— In DentoUum, the shell is elongated, conical, and curved, like an elephant's tusk, with the apex broken off, and it is open at both ends. The animal has a large mantle corresponding in form with the shell, and also open at both ends, the margins of the anterior, larger, aper- ture being much thickened. The mouth, placed at the extrerm ity of a sort of cup, the margin of which is fringed with pa* pillae, is situated far behind the anterior opening of the man- tle. Behind the oral cup, where the body joins the mantle, is a transverse muscular ridge, from which proceed a great 'Reincke, " Beitriiere zur Bildungsgeschichte der Stacheln, u. s. vr." (Zeit- schrift fur wimenschaftliche Zooloaie.) 2 Zeitschrift fur wissenschaftliche Zool.octu^ 1858. 3 A very complete and accurate account of the organization of Dental i urn is given in the monograph of Lacaze-Duthiers, u Ilistoire de l'onranisation, du developpement, des moeurs et des rapports zoologiques des Dentales," 1858. 28 432 THE ANATOMY OF INVERTEBRATED ANIMALS. number of long tentacles. These protrude through the an- terior opening of the mantle, and play the part of prehensile organs. Behind and below the oral cup the very long sub- cylindrical foot proceeds. Near its extremity are two lateral fleshy lobes which perhaps correspond with the epipodia of other Mollusks. The oral cup leads into a buccal chamber containing the odontophore, whence the oesophagus passes to the stomach. The liver consists of two symmetrically- branched divisions ; and the intestine, after becoming coiled upon itself, ends in a prominent anal papilla, in the median line, behind the root of the foot. There is no heart, but the blood fills spacious sinuses. There are no special respiratory organs distinct from the wall of the pallial cavity. The two renal organs open one on each side of the anus. The renal blood sinus communicates directly with the pallial cavity by two apertures, situated close to those of the renal organs. In the nervous system, the commissures of the parieto- splanchnic ganglia pass directly to the cerebral ganglia, as in the Lamellibranchs. The sexes are distinct, and the geni- tal gland is single and symmetrical, though its duct opens into the right renal organ. The embryo is at first surrounded by a number of ciliated rings, its anterior end presenting a tuft of long cilia. By degrees the cilia become restricted to the edges of a disk, into which the anterior end of the embryo expands, and which represents the prre-oral ciliated velum of the Lamellibranchs. The mantle now appears on the dorsal aspect of the body, behind this disk. Its ventral edges are free, and it secretes a shelly plate of corresponding form. But, as development advances, the edges of both manile and shell unite in the median ventral line, leaving the anterior and the posterior ends open. The Scajyhopoda are an ancient group, remains of them occurring as far back as the Devonian epoch. The higher Odontophora (or the Gasteropoda, JPteropoda, and Cephalopoda of Cuvier) fall into two divisions, according to the structure and arrangement of the parts of the foot. In the one division (the Gasteropoda and Pteropoda) it may be a simple disk, or it may be divided into three portions — an anterior (the propodium), a middle (the mesopodiwn), and a posterior (the metapodhon) ; and it may be still further complicated by the development from its sides of muscular expansions — the epipodia. But, whatever the shape of the foot in these Mollusks, its margins are not produced into THE GASTEROPODA AXD PTEROPODA. 433 prehensile processes, and its antero-lateral portions do not extend beyond the sides of the head, and unite in front of the mouth. In the other division (the Cephalopoda}, the margins of the foot are produced into prehensile processes or arms, and the antero-lateral regions of the foot extend over, and unite in front of, the mouth, in such a manner that the latter is placed in the centre of the discoidal foot.1 In the former division — that is, in all Pteropoda — in all those Gasteropoda which breathe the air dissolved in water (Branchiogasteropoda), and in some of those which breathe air directly (Pulmogasteropoda), the embryo is, as in the Scaphopoda and Polyplacophora, a veliger ; or, at any rate, it has ciliated bands which subserve locomotion. But in the Cephalopoda no such velum is formed, and the animal ac- quires the general characters of the adult before leaving the A shell-gland is often, if not always, present in the em- bryo of the higher Odontophora / and, in all Pteropods and Branchiogasteropods, the mantle secretes a cuticular shell, which, however, may exist only during the larval condition. If the arrangement of the alimentary canal in a Cephalo- pod, or a Pteropod, be compared with that which obtains in such a Branchiogasteropod as Atlanta, it will be observed that, in the former, the oesophagus enters the outgrowth of the hsemal region of the body which constitutes the visceral sac, to reach the stomach ; and that the intestine passes, at an acute angle with the anterior portion of the alimentary canal, along the posterior face of the visceral sac, to end in the pallial chamber, which is situated on the posterior face of the body. The pedal ganglia consequently lie between lines traversing the anterior and the posterior divisions of the ali- mentary canal respectively ; and hence the alimentary canal has a neural flexure, or is bent toward the neural face of the body. In Atlanta, on the other hand, the intestine, when it leaves the stomach, passes along the anterior face of the visceral sac, to reach the pallial cavity, which is situated on the an- terior face of the body. Hence lines traversing the two di- visions of the alimentary canal would inclose not the pedal 1 See, for a valuable discussion of the homologies of the arms and the funnel of the Cephalopoda, in which the view here taken is ably, though I do not think satisfactorily, controverted. Grenadier, uZur Entwickelun^sgeschichte der Cephalopoden." {ZeitscJiriftfur wiss. Zoologie, 1374.) 434 THE ANATOMY OF INVERTEBRATED ANIMALS. but the cerebral ganglia. In other words, the intestine is bent in the opposite direction to that which it takes in the Cephalopod, or has a hcemal flexure.1 The haemal flexure of the intestine is very characteristic of the Branchiogasteropoda, and is completed at an early stage of their development. In such a slightly ^modified Odontophoran as Chiton, the heart presents its normal position in the posterior region of the haemal face of the body, and has its aortic end turned for- ward. Although the branchiae are situated at the sides of the body, the blood which passes through them must take a backward course to reach the heart; and thus the branchia3 may be said to be virtually behind the heart, and the animal is truly opisthobranchiate. It appears to be otherwise with such a Gasteropod as Buccinum, in which the gills lie actual- ly in front of the heart, and the animal is therefore said to be prosobranchiate. It must be recollected, however, that, strictly speaking, no Odontophoran is other than opistho- branchiate. The anus represents the morphological hinder end of the body ; and the auricle of the heart, into which the current of blood from the branchiae passes, is never, morpho- logically, posterior to the branchiae. This is perfectly obvious in the Cephalopoda. In the position which the animal frequently assumes and in which it is ordinarily represented, the gills are in front of the heart. But if the Mollusk is placed in its morphologically correct position with the oral face of the arms downward, it will at once be seen that what is commonly called the ventral face of the animal is the posterior half of its haemal face, and that the heart lies, morphologically, anterior to the branchiae. In such Branchiogasteropods as are prosobranchiate, the gills come to lie in front of the heart in consequence of their having followed the twisted intestine forward and to the haemal side of the body. The Pteropoda.2 — In this group of small pelagic animals there is no distinct head, the eyes and the ordinary tentacles remaining rudimentary. Auditory sacs are attached to the pedal ganglia. Sometimes (Pneumodermori) two eversible 1 Huxlev, "On the Morphology of the Cephalous Mollusca." ("Phil. Trans.," 1852.) . 2 See Kang and Souleyet, " Histoire naturelle des Mollusquea Pteropodes ; " and Gegenbaur, " Untersuchungen fiber die Pteropoden und Heteropoden," 1855. THE PTEROPODA. 435 spinose tentacular organs are developed at the sides of the mouth, and, in addition, two acetabuliferous tentacles take their origin on the inner side of a cup-like hood, which sur- rounds the anterior end of the body.1 Cymbulia is stated to possess no radula. The epipodia are large muscular ex- pansions, by the flapping of which the Pteropods swim ; but the rest of the foot is always small, and often rudimentary, in correspondence with the small size of the neural face of the bodv. The haemal face, on the contrary, is always produced, as in the Cephalopoda, into a relatively large visceral sac ; and in some (the Thecosomata) this visceral sac is coextensive with the mantle, which is protected by a shell. In others [Gymnosomata) the mantle early disappears, and there is no shell. In Cymbulia, the delicate transparent chitinous shell is internal, and is invested by an epithelial layer derived from the mantle. In Spirialis, the foot bears an operculum. Chromatophores similar to those of the Cephalopoda occur in Tiedemannia. In the Thecosomata, the free lobe of the mantle, which incloses a spacious pallial cavity, usually lies on the posterior aspect of the visceral sac, as in the Cephalopoda, and the rectum terminates in it, on one side of the middle line. In these there is a simple neural flexure of the alimentary canal, as in the Cephalopods, although the turning of the rectum to one side destroys the symmetry of the body. In Limacina and Spirialis, the intestine appears to be bent round to the anterior face of the visceral sac, the mantle-cavity accom- panying it, so that the opening of the mantle is placed on the anterior, instead of on the posterior, face of the visceral sac. There are no distinct gills in the Thecosomata, but the lining of the mantle-cavity subserves the function of respira- tion, and is sometimes produced into folds, which doubtless aid in the performance of that function. Processes of the body, to which the office of gills is ascribed, are found in soine Gymnosomata (Pneumodermon fcpongobranchia). The heart consists of a single auricle and a single ventricle. The auricle lies close to the pallial cavity, and receives the aerated blood from its walls. The ventricle is sometimes directed forward (as in all Gymnosomata), and sometimes 1 See, for the somewhat similar arrangements in Clione, Esehricht, " Ana' tomische Untersuchungen iiber Clione borealis," 1858 ; and Maedonald, " Od the Zoological Characters of the Living Clio caudata." (" Trans. Royal Society of Edinburgh," 1863.) 436 THE ANATOMY OF INVERTEBRATED ANIMALS. • backward, so that nearly-related forms are sometimes opistho- branchiate, sometimes prosobranchiate. The branches of the aortic trunk soon terminate in lacunae, by which the blood is conveyed back to the walls of the mantle-cavity. The renal organ is a contractile sac with delicate walls, which opens on one side into the pallial chamber, and on the other into the pericardial sinus. The Thecosomata have the principal ganglia concentrated around the gullet — the cerebral ganglia being lateral, and united by a long commissure. In the Gymnosomata the ganglia are more scattered, but the arrangement of their nervous system needs reexamina- tion. All the Pteropoda are provided with an ovotestis. This is a racemose gland, in the ultimate caeca of which. both ova and spermatozoa are developed. The spermatozoa make their appearance at the closed end of the caecum and accumu- late in its cavity ; the ova are developed from the epithelial tissue of the caecum, somewhat lower down; nevertheless fecundation does not take place in the ovotestis, probably in consequence of the ova and spermatozoa attaining maturity at different times. The ovotestis has a single excretory duct, the termination of which may be provided with a reeeptaculum seminls and connected with a penis. The young of the Pteropoda leave the egg provided with a velum, with a rudimentary shell, and probably with an operculum. In most of the Thecosomata the shell is re- tained and forms the commencement of that of the adult, while the vela disappear and the epipodia are developed. In Cymbidla, the primary external shell is shed and the chitinous internal shell is a secondary development. In the Gymnosomata, the primary shell is also cast off, but is not replaced, and three girdles of cilia are developed on the sur- face of the bodv.1 The Silurian genera Tentacidltes, Tlieca, Pterotheca, Comdaria, Ecculiomphalus, are referred to the Pteropoda, but they differ much from all existing forms. Unquestionable Pteropoda are not know earlier than the tertiary formations. The Branchiogasteropoda. — In all the members of this 1 Gesrenbaur, I. c. ; Krohn, " Beitrage zur Entwickelungsgescliichte der Pteropoden und Heteropoden," 1860; and Fol, "Etudes" ("Archives de Zool. Experimeutale," 1875 and 1876). THE BRAN'CHIOGASTEROPODA. 437 group, the development of which has hitherto been studied, the intestine becomes twisted round on to the anterior face of the body, in such a manner that the alimentary canal has a completely haemal flexure, even in the veligerous embryo. Hence, in the adult, the intestine springs from the haemal or dorsal, and not from the ventral or neural, aspect of the stomach ; and the pallial cavity, when it exists, is placed upon the anterior haemal face of the body. In the embryo, the shell always makes its appearance as a conical, symmetrical, median cap. This embryonic shell usually persists at the apex of that of the adult, the form of which is modeled upon that of the visceral sac, and hence, like the latter, is usually spiral. The embryo is also very generally, if not universally, provided with an operculum. The shell and operculum of the embryo disappear in the naked Branchiogasteropods ; but the primitive external shell is sometimes replaced by an internal shell lodged in a cavity of the mantle (e. g., Aplysia). Usually, the Branchiogastero- pods possess a distinct head provided with a pair of tentacles and with two eyes, which may either be sessile or mounted upon peduncles of their own. The mouth may be armed with chitinous jaw-plates, in ad- dition to the radula. The heart is generally composed of a ventricle and a single auricle, but sometimes there are two auricles. The Br anchio gasteropoda fall into two distinct series, of which the one is hermaphrodite (the genital gland being an ovotestis) and invariably opisthobranchiate ; while the other is unisexual and usually prosobranchiate. In each series there are some forms which are provided with a large mantle, and others in which the mantle is altogether abortive (Nudi- branchiata, Firolc). These chlamydate and achlamydate Branchiogasteropods correspond with the Thecosomata and Gymnosomata among the Pteropods. The chlamydate Branchiogasteropods are usually provided with branchiae, which either take the form of numerous la- mellae, or of two plume-like organs, sometimes reduced to one functional gill and a rudiment of the second. In the achlamy- date forms true gills are usually absent, though they may be replaced functionally by processes of the haemal body-wall. Among the Ophthobra nchiata, PhyUidia is nearly sym- metrical, the anus being situated at the posterior end of the body, and there is a large mantle, devoid of a shell. There 438 THE ANATOMY OF IN VERTEBRATED ANIMALS. is no pallial cavity, and the branchiae are numerous lamellae, placed on each side of the body, between the free edge of the mantle and the foot. In Aplasia, the mantle is relatively small, and possesses an internal shell ; the branchiae, the anus, and the reproductive apertures, are placed on the right side of the body. In this genus, and in G aster opter on, there are very large epipodial lobes, by the aid of which some species propel themselves like Pteropods. The Nudibranchiata have no mantle, and the anus is usually situated on the right side of the body ; sometimes, however, as in Doris, it is terminal. In the pelagic Phylli- rhde, the foot aborts, as well as the mantle, and the body has the form of an elongated sac. The gastric portion of the alimentary canal becomes com- plicated by division into several portions, some of which are provided with chitinous or calcareous plates, or teeth, in Aplysia, Bulla, and other genera. In many Nudibranchs, as Eolis, the liver is represented by a much-branched tubular organ, the caecal ultimate ramifications of which end in the elongated dorsal papillae. The apices of these papillae contain thread-cells. In the series of the Prosobranchiata, the great majority are not only chlamydate, but there is a spacious branchial chamber, and the pallial wall of the body is produced into a conical visceral sac, which contains the stomach, liver, and genital organs. It is usually asymmetrically coiled, and is protected by the shell. No Opisthobranch possesses a large visceral sac of this kind. On the other hand, no Prosobranch is, like Phyllidia, symmetrical, with the anus at the posterior end of the body. Patella and Fissurella are nearly sym- metrical, but the anus is anterior. The Prosobranchiata have, at most, rudiments of epipodia, bat the rest of the foot often acquires a much greater develop- ment than in the Opisthobranchiata, and a chitinous or shelly plate — the operculum — is frequently developed from the dor- sal or haemal aspect of the metapodium. The differentiation of the foot attains its highest degree in the so-called Hetero- poda, in which the propodium, mesopodium, and metapodium differ widely in form ; the propodium being broad and fin-like, and constituting the chief organ of locomotion in these free- swimming; oceanic animals. In the Limpets (Patellidce), the visceral sac forms merely THE HETEROPODA. 439 a conical projection of the haemal surface, and the numerous lamellar, or filamentous, respiratory organs, are lodged be- tween the free edges of the mantle and the sides of the body. In the other chlamydate Prosobranchiata, except the Cyclos- tomata, there are two plumose gills lodged in a pallial chamber situated on the anterior face of the visceral mass, which is usually large and spirally coiled. Sometimes, as in the di vision of the Aspidobranchia, the two branchiae are equal, or nearly equal, in size. Sometimes one is so much smaller than the other as to be nearly abortive ( Ctenobranchia). Arnpul- laria has a pulmonary cavity as well as gills. On the other hand, the Cyclostomata have no branchiae, but breathe air by means of the parietes of the pallial chamber, whence they are ordinarily reckoned among the Pulmonata, which they resemble in their terrestrial habits. In many Prosobranchiata, the wall of the branchial chamber is produced into a muscular spout-like prolongation, termed the siphon, which serves to direct the branchial current. The presence of this siphon is usually accompanied by a notch or grooved process of the shell, and by carnivorous habits. In the Hetcropoda, there is a gradual reduction of the mantle, from Atlanta, in which the mantle and shell have the ordinary proportions, and the departure from the ordinary Gasteropod type is but little greater than that observed in Strombus and Pteroceras, through Carinaria, in which the mantle is much reduced, and the shell is a mere conical cap, to Flrola, in which the mantle and shell are wanting in the adult, and which, therefore, corresponds with the achlamydate Pteropoda and Opisthobranchiata. In many genera of the Ctenobranchia, and especially among the carnivorous forms, the mouth is situated at the end of a long proboscis, which contains the odontophore, and a great part of the long oesophagus. This proboscis is pro- truded and retracted by special muscles.1 The eggs are often laid in capsules secreted by the walls of the oviduct. In Neritina, Purpura, and Bucc'inum, each capsule contains a considerable number of ova, but of these only a few (one in Neritina) become embryos, and devour the rest.2 1 See the description of the proboscis of the Whelk in Cuvier's " Memoires sur les Mollusques." 2Koren and Daniellssen, "Recherches sur le developpement des Pectini- branches" ("Fauna littoralis Norvegiae," ii.. 1856), and Carpenter, "On the Development of the Embryo of Purpura lapUlus" ("Trans. Micr. Society," 1854, and " Annals of Nat. Hist.,7' 1857). Claparede, " Anatomic und Entwicke- lungsgeschichtc der Neritina fluviatilis." (" Archiv fur Anatomie," 1857. ) 440 THE ANATOMY OF LWERTEBRATED ANIMALS. The parasitic habit which is so rare among the Mollusca occurs in the genus Stylifer, which infests Star-fishes and Sea-urchins, sometimes imbedding itself in the perisoma ; and, under a very remarkable and not yet thoroughly-under- stood form, in the singular parasite of another Echinoderm, Synapta digitata, termed by its discoverer, Muller, Ento- concha m irabills. l In some few of the Synaptce (not more than one, or per- haps .two, in a hundred), elongated tubular molluskigerous sacs are found attached by one extremity to one of the intes- tinal vessels; while the opposite end either hatigs freely into the perivisceral cavity, or may be entangled among the bases of the tentacles, at the cephalic extremity of the body of the Synapta. The sac is closed, but, at its attached end, a long invagination extends into its interior. The cavity of the sac beyond the closed extremity of the invagination contains an ovary ; and, beyond this, a certain number of free seminal capsules. The ova are detached from the ovary, and under- go their development in envelopes, each containing many ova, which gradually fill the cavity of the molluskigerous sac. From these ova, embryos, provided with a velum, shell, and operculum, proceed. A large pallial cavity is soon apparent ; but, in the most advanced stages of development observed, it contained no branchiae. What becomes of these larvae is unknown, nor is it even certain to what group of the Odontophora Entoconcha be- longs. The Pulmonata. — These are odontophorous Mollusks which breathe air directly, by means of a respiratory surface furnished by the wall of the pallial cavity. In some, such as the Peroniadce (Fig. 123) and Veroni- cellidce, the body of the slug-like animal is very nearly sym- metrical; the anus and the lung-sac being situated close to- gether at the posterior extremity of the body. The mantle is large, and extends over the whole haemal or dorsal surface. In all the other JPulmonata, the pulmonary and the anal apertures lie on the right side of the body, and the mantle is provided with at least the rudiments of a shell. The pallial region is sometimes very small in proportion to the rest of the body, and then forms a flattened disk, as in the common Slug ; while, in some Eimacidce and Testacellidce, and in the 1 "Die Erzeu2nn£ von Schnecken in Ilolotlmrien," 1S52. Baur, " Ueber Synapta digitata." (" Nova Acta," xxxi., 18G4.) THE PULMOXATA. 441 Janellidce, the mantle is so much reduced that they are al- most achlamydate. In the Snails, the mantle is large and is produced into an asymmetrically coiled visceral sac, in which the stomach, liver, and genital gland lie. The mantle-cavity lies on the fore-part of the sac, and the anus opens on its margin. Thus, in all the ordinary jPulmonata, the termina- tion of the intestine is twisted from its normal position at the hinder end, forward to the right dorsal, or haemal, aspect of the bod}7. AVhen the pulmonary sac is posterior, and the pallial re- gion small, the ventricle of the heart is anterior, and the auricle posterior, and the animal may be said to be opistho- pulmonate. On the other hand, when the pallial region is large, and gives rise to a visceral sac, with the concomitant forward position of the pulmonary chamber, the auricle is inclined more or less forward and to the right side, and the apex of the ventricle backward and to the left side. The animal is thus more or less prosopidmonate. The mouth is commonly provided with a horny upper jaw, as well as with a well-developed odontophore. Large salivary glands are usually present. The heart consists of a single auricle and a single ventri- cle. The aortic trunk, which proceeds from the apex of the latter, divides into many branches, but the venous channels are altogether lacunar. A renal organ lies close to the pul- monary sac in the course of the current of the returning blood. There are usually two simple eyes, often lodged in the summits of retractile tentacula. The Pulmonata are hermaphrodite. The generative gland is an ovotestis, and is composed of branched tubuli, from the cellular contents of which both ova and spermatozoa are de- veloped (Fig. 123, III.). A narrow common duct leads from the ovotestis, and, soon dilating, receives the viscid secretion of a large albumen- gland. The much wider portion of. the common duct beyond the attachment of this gland is incompletely divided by longi- tudinal infoldings into a sacculated, wider, and a straight, narrower, division. The former conveys the ova, and the latter the spermatozoa. At the end of this part of the ap- paratus, the wider portion, which represents the oviduct, passes into the vagina, which opens at the female genital aperture, while the narrower portion of the common duct is continued into a separate, narrow, vas deferens, the end of wThich opens into a long invagination of the integument — the u-> THE ANATOMY OF INVERTEBRATED ANIMALS. penis. In Peroyiia, the vas deferens and the oviduct open together by the genital aperture, and, as in some Branchio- gasteropods, a groove, along which the seminal fluid is con- Fig. 132.—- Diagram exhibiting the disposition of the intestine, nervous system, etc., in a common Snail (Hdix). — a, mouth ; b. tooth ; c, odontophore ; tf, gullet ; e, its dilatation into a sort of crop;/, stomach; g, coiled termination of the visceral mass ; the latter is also close to ttie commencement of the intestine, which will he seen to lie on the neural side of the oesophagus ; h, rectum ; *, anus ; k, renal sac ; J, heart: m, lung, or modified pallial chamber; w, its external aperture; 0, thick edge of the mantle united with the sides of the body; p, foot; r, s, cerebral, pedal, and parieto-splanchnic ganglia aggregated round the gullet. ducted, leads to the outer opening of the eversible penis (Fig. 123, I., II.). In connection with the female genital aperture, there is always a spermatheca, or sac (which is sessile in the Slugs, but in the Snails is placed at the extremity of a long duct), for the reception of the semen of the other individual when copulation takes place. The Helicidm alone possess, in addition, the so-called sac of the dart, a short muscular b;tg, in which pointed chitinous or calcined bodies — the spicula amor Is — are formed ; and certain glandular caeca, generally arranged in two digitate bundles, termed mucous glands, which give rise to a milky secretion. Sometimes prostatic glands are developed on the THE PULMOXATA. 443 vas deferens, which may be dilated in part of its course into a vesicula seminal is. Fig. 123.— I. Peronia verrucnlata.—a. anus : pi, palmonary aperture ; g, genital aper- ture ; fs, semiual eroove : p. openine for the penis. II. Generative orsans of the same animal, the ovotcstis beinar omitted. — gal, gland which furnishes a glairy secretion : od. oviduct : rd, vas deferens ; ?'. intestine : a. anus; rs, receptaeulnm seminis : p. aperture of the penis; p> , penis: cs. seminal duct : ap. glandular appendage ; m, retractor muscle of the penis. (After Kefer- stein.) III. Blind end of a follicle of the ovotestis of Helix poniatia . At the apex the sperma- tozoa are seen in different stages of development, Ihe fully-formed spermatozoa floating in bundles in the cavity of the follicle. Lower down, ova are developing in the walls of the follicle. (After Keferstein and Ehlers.) The ova are impregnated high up in the oviduct, and are invested by a relatively very large mass of albumen and in- closed within a thick, sometimes calcified, chorion. The mass inclosed by the latter may be a tenth of an inch or more in diameter, while the proper ovum may have not more than a twelfth of that size. There is no trustworthy evidence of the existence of the opisthobranchiate Gasteropods before the epoch of the Trias, but it is to be remembered that the great majority of these animals have no shells. Of the rest of the preceding- groups of Otlontophora, representatives are known as far back as the middle of the Palaeozoic epoch, while Pteropoda, Hetero- 444 THE ANATOMY OF IXYERTEBRATED ANIMALS. poda, and Prosobranc/uata, occur in the Silurian formations. Among the Prosobranchiata, the PateUidce and the Aspido- hranchia are the characteristic forms of the older formations, the Ctenobranchia appearing later, and acquiring their pres- ent relative abundance only in the later secondary and the tertiary epochs. The Cephalopoda. — The bilateral symmetry which is so obvious in the Polyplacophora and the Scaphopoda is but Fig. 124.— A, Sepia officinalis. B, lateral view of the horny ring of an acetabulum. little disturbed in this group of the Odontophora. The mouth and the anus are situated in the median plane, which divides the bodv into corresponding halves ; while the bran- chiae, two or four in number, are disposed svmmetrically on each side of this plane, as are the brachial prolongations of the margins of the foot. The haemal fa,ce of the body, how- ever, is not flat, as in the mollusks which have just been men- tioned, but is elongated perpendicularly to the neural face, so as to form a sort of sac, invested by the mantle. On the pos- THE CEPHALOPODA. 445 terior, or anal, face of the sac, the mantle incloses a large pallial cavity, in which the branchiae are protected. On the anterior aspect of the sac, on the contrary, the mantle may have no free edge, or, at most, forms a comparatively small flap.1 The integument is provided with chromatophores, which are sacs with elastic walls, full of pigment, and provided with radiating muscles, by which they may be drawn out to a size many times greater than that which they possess in their contracted state. In their dilated condition, the color proper to the contained pigment becomes plainly visible, while in their contracted state they appear as mere dark specks. It is to the successive expansion and contraction of these chro- matophores that the Cephalopoda owe the peculiar play of "shot" colors, which pass like blushes over their sur- face in the living state. These blushes of color are especial- ly well displayed by young Cephalopods just freed from the But that which particularly distinguishes the Cephalo- pod is the form and disposition of the foot. The margins of this organ are, in fact, produced into eight or more pro- cesses, termed arms, or brachia y and its antero-lateral por- tions have grown over and united in front of the mouth, which thus comes, apparently, to be placed in the centre of the pedal disk. Moreover, two muscular lobes which cor- respond with the epipodia of the Pteropods and Branchio- gasteropods, developed from the sides of the foot, unite pos- teriorly, and, folding over, give rise to a more or less com- pletely tubular organ, the funnel, or infundibuhim. The open end of the funnel projects between the posterior face of the body and the pallial wall of the branchial cavity, and serves to conduct the water, when it is driven out of the latter by the contraction of the mantle in ordinary expira- tion ; and when the animal swims, the stream forcibly driven out in this way causes it to dart swiftly backward. The aperture of the mouth (Fig. 125, a) is provided with a hard, chitinous beak, like that of a parrot, the two divis- ions ' of which are anterior and posterior. Of these, the anterior is always the shorter, and is overlapped by the other. 1 Cephalopods are usually described as if the oral end of the bodv were the upper end, and the face on winch the pallial chamber is placed ventral— a method which seriously interferes with the comprehension of their relations with other Mollusks. 44G THE AX ATOMY OF IXVERTEBRATED ANIMALS. Within the cavity of the mouth is an odontophore, with its raclula (Fig. 126, II.) ; and the long gullet passes back on the middle line to open into the stomach, which is situated Fig. 125.— Diagrammatic Section of a female Sejna.— a, Buccal mass surrounded by the lips, and showing the horny jaws and tongue; 0, oesophagus ; c, salivary srlanil : d. stomach: e. "pyloric caecum : g. the intestine ; h. the aims; i, the ink- ba<_r: k. the place of the systemic heart: L the liver; n, the hepatic duct of the left side; 0, the ovary: p, the oviduct; q\ one of the apertures by which the water- chambers are placed in communication with the exterior ; r, one of the branchiae; s, the principal ganglia asrsregated round the oesophagus; /, the funnel; m. the mantle; »ft, the internal shell, or cuttle-bone ; 1, 2. 3, 4. 5, the produced and modi- fied margins of the foot, constituting the so-called arms of the Sepia. toward the middle, or the end, of the mantle-sac. From the stomach, the intestine, more or less bent upon itself, passes tow»»d the neural aspect of the body, and ends in the median THE CEPHALOPODA. 447 anus. Hence the alimentary canal has a well-marked neural flexure (Fig. 125). Except in Nautilus, one or two pairs of salivary glands are present (Fig. 126, I. s'). The liver (Fig. 126, I. h) is al- ways large ; and there are two hepatic ducts (Fig. 126, 1, d/t), beset for a greater or less extent with glandular follicles, gen- erally considered to be pancreatic in function. Very often a large, sometimes spirally wound, caecum is developed from the commencement of the intestine ; into this the hepatic ducts open. The heart (Fig. 127, c) is placed upon the posterior face of the body on the haemal side of the intestine, and receives the blood by branchio-cardiac vessels, which correspond in number with the gills, and, as they are contractile, might be regarded as auricles. The gills themselves have no cilia, and are, in some cases, if not always, contractile. The arteries end in an extensively-developed capillary system, but the venous channels retain to a greater or less extent the char- acter of sinuses.1 The venous blood, pn its way back to the heart, is gathered into a large, longitudinal sinus — the vena cava — which lies on the posterior face of the bod}7, close to the anterior wall of the branchial chamber, and divides into as many afferent branchial vessels as there are gills. Each of these vessels traverses a chamber which communicates di- rectly with the mantle-cavity, and the wall of the vessel which comes into contact with the water in this chamber is saccu- lated and glandular2 (Fig. 127, re). Each chamber, in fact, represents a renal organ. The pericardium, and the sacs in which the testes and ovaria are lodged, may communicate 1 Milne-Edwards, " Recherches Anatoniiques et Zoologiques. Premiere Par- tie." " Observations et Experiences sur la Circulation chez les Mollusques, 1845 2 On account of the transparency of the tissues in the living Loligo media, this species affords an easv opportunity of observing the rhythmical contrac- tions of the branchiae, and their afferent and efferent vessels. 1 or this pur- pose the mantle should be laid open, and the nidimental glands carefully removed. The sacculated afferent veins and the branchial hearts contract about sixty times a minute. The pulsations of these veins, and of the bran- chial hearts, are not synchronous. The branchial veins, and the lamellae ot the branchiae, also contract rhythmically, but I could observe no contraction in the branchial arteries. The portion of the branchial vein which lies between the base of the gill and the systemic ventricle is very short, and it is hard to say whether it contracts independently or not. Mechanical irritation causes contraction both of the afferent branchial veins and of the branchial hearts. In the living Eledone cirrhosus I have observed regular rhythmical con- tractions of the vena cava itself as well as of its divisions, the sacculated affe- rent branchial veins, of the branchial hearts, and of the branchio-cardiac ves- sels. 09 448 THE ANATOMY OF INVERTEBRATED ANIMALS. with the pallial cavity either directly or through these cham- bers. Thus, in Sepia officinalis, Krohn * observed that the Fig. 126.— Sepia officinalis.— 1. The alimentary canal, with the ink-bag: mb, buccal mass; gb, inferior buccal ganglion ; s', posterior salivary glanrls ; oe, oesophagus; A, liver ; dh, hepatic duct ; v, stomach ; v', pyloric caecum ; i, intestine ; a, anus; bi, ink-bag; gsp, splanchnic ganglion on the stomach. (After Kef er stein.) II. Longitudinal ana vertical section through the buccal mass: mxi, posterior beak ; mx$, anterior beak ; ?nbc, buccal membrane ; ml. lip ; x, gustatory (?) organ ; rd, radula ; z, sac of the radula ; s', salivary gland ; gl, superior buccal ganglia. (After Keferstein.) III. A single transverse row of teeth from the radula. (After Troschel.) renal chambers communicate not only with the cavities in which the branchial hearts are lodsred, but with a chamber which contains the stomach and the spiral pyloric appendages ; and that all these cavities are distended when air is blown into one renal chamber. In Eledone, on the contrary, he found, and I have repented the observation, that one renal 1 " Ueber das wasserfuhrende System einiger Ceplialopodeu." (" Archiv fur Anatomic," 1839.) THE CEPHALOPODA. 449 chamber can be fully distended without the air passing into the other. Fig. 12?. — Sepia officinalis. — c. systemic heart : ao. anterior aorta ; ao', posterior aorta ; 1, vena cava; 2, afferent branchial vessels: re. renal organs; 2. appendages of these vessels ; 3, 4, lar^e posterior veins bringing blood to the afferent branchial vessels ; 5, 6. 7, efferent branchial vessels, branchial veins,-and branchio-cardiac or auricular trunks. (After Hunter.) In Nautilus pompilius there are, as Valenciennes discov- ered, three pairs of openings which lead from the branchial sac into chambers contained in the interior of the body. Of these chambers there are five : the anterior and posterior pairs are situated on each side of the rectum, and each has its own opening ; the fifth, a very much larger chamber, has two openings, one on each side. It is coextensive with that part of the mantle which lies behind the insertion of the shell- muscles and the horny band which connects them. It is separated from the paired chambers by their inner walls, and these walls are traversed by the afferent branchial veins. Appendages of these veins project on the one hand into the paired chambers, and on the other into the single chamber. The latter appendages are elongated papilla?, while the for- mer are lamellar. Earthy concretions, composed mainly of phosphate of lime, but which yield no trace of uric acid, are usually found in the paired sacs.1 1 Owen, " Memoir on the Pearly Nautilus." Van der Hoeven, " Beitrasr znr Anatomie vom Nautilus pompilius" (" Arehiv far Xaturgeschiehte,1' 1857). Huxley. "On some Points in the Anatomy of Nautilus pompiUusn (" Proceedings of the Limvean Society," 1858). See also Keferstem, Bronn's 44 Klassen u. Ordnungen," Bd. iii. (18G2-'66), pp. 1390, 1319. 450 THE ANATOMY OF IXVERTEBRATED ANIMALS. The nervous system in the Cephalopoda, as in other Mol- lusca, consists of cerebral, pedal, and parieto-splanchnic gan- glia, aggregated around the gullet, and connected by com- missural cords. In addition to these, buccal, visceral, bran- chial, and pallial ganglia may be developed on the nerves which supply the buccal mass, the alimentary canal, heart, branchia, and mantle. In the Dibranchiata (Fig. 128), the three principal pairs of ganglia are usually large, and so closely aggregated to- gether that the commissures are not readily distinguishable. The optic nerves are very large ; one or two nerves are given off to the superior or anterior buccal ganglia, which have co- alesced into one mass, and are united by commissures, which encircle the oesophagus, with the coalesced inferior or pos- terior buccal ganglia. The pedal ganglia lie on the pos- terior side of the gullet, and supply the large nerves to the arms, and those to the funnel, while the auditory nerves are immediately connected with them. Each parieto-splanchnic ganglion gives off a nerve which runs along the shell-muscles to the anterior wall of the mantle, and there enters a large ganglion, the ganglion stellatnm. A large median branch, or branches, from the parieto-splanchnic ganglia, accompanies the vena cava, and is distributed to the branchiae and sexual organs. The inferior buccal ganglion sends a recurrent nerve along the oesophagus, which ends in a ganglion on the stom- ach.1 The nervous svstem of Nautilus differs in some important particulars from that of the Dibranchiata. The cerebral ganglia are represented by a thick transverse cord, which lies in front of the oesophagus, and from the outer angles of which the optic and olfactory nerves are given off, while nerves to the buccal mass proceed from its anterior edge. The pedal ganglia lie close to the cerebral ganglia, and are united by a slender commissure, which passes behind the gullet. They supply all the brachial processes and the funnel with nerves, and the short auditory nerves are connected with them. The parieto-splanchnic ganglia are, like the cerebral ganglia, elon- gated, and together constitute a thick cord, which, united at each end with the cerebral ganglia, forms a hoop round the gul- let, distinct from the pedal nerve-arch, and separated from it by a process of the cartilaginous skeleton. The largest nerves 1 See Hancock, " Anatomy of the Nervous System of Ommastrephes." (" Ann. Nat. History," 1852./ THE CEPHALOPODA. 451 given off from these ganglia are those which go to the bran- chiae. Eyes, olfactory organs, and auditory sacs, are always present. The eyes of the Cephalopoda may be lodged in orbital cavities at the sides of the head, as in all the Dlbran- chiata ; or may be pedunculated, as in Nautilus. In the former case, the eye is inclosed partly by the cephalic car- Fig. 128.— Sepia officinalis.— The nervous mass which surrounds the gullet ; N, the cerebral; IP, the pedal ; A", the parieto-splanchnic gandion ; ao, the aorta; oe, the oesophagus ; (/. buccal nerves ; P\ nerves to the anus ; M, pallial nerves ; g, superior; g', inferior buccal ganglion. (After Garner.) * tilage, to which sometimes special orbital cartilages are add- ed, and partly by a fibrous capsule continuous with these. The fibrous capsule becomes transparent over the eye, and gives rise to what is variously interpreted as the representa- tive of the cornea, or as that of the eyelids of vertebrated ani- mals. This transparent coat is sometimes entire, or presents only a small perforation ( Octopus, Sepia, Loligo, and the other Myopsidce of D'Orbigny) ; sometimes it has a wide opening, through which the crystalline lens may project (Lo- ligophes, Ommastrepsis, and the other Oigopsidw of D'Or- bigny) ; and sometimes it is altogether absent, and the capsule of the eye becomes an open cup (Nautilus). In the Dibranchiata* a great part of the chamber of the capsule of the eye is occupied by the ganglion, into which the optic nerve enlarges after entering it ; by muscles ; and by a peculiar white glandular substance. Lining the capsule, but i "Trans. Linnaean Society," 1836. _ 2 See Hensen, " Ueber das Auge einiger Cephalopoden." (Zeitschrift fur wissenschaftliche Zoologie, 1865.) 452 THE AXATOMY OF INYERTEBRATED ANIMALS. not adhering to its inner, surface, in front, is the silvery tape- turn^ formed of two layers. These pass into one another at the edges of the free prolongation of the tapetum, which forms the iris. Longitudinal muscular fibres are interposed between the two layers of the tapetum. Under the tapetum is a layer of cartilage, which forms the inner capsule of the eye, extends as far as the iris externa 11 \", and is perforated by the fibres of the optic nerve on its inner side. The free edge of the inner capsule gives attachment to a thick rim of connective tissue, containing muscular fibres. This so-called ciliary body enters the deep groove which surrounds the lens ; the latter is, in fact, made up of layers of structureless membrane, which are cuticular productions of the ciliary bod}r. In shape, the lens is elongated in the direction of the axis of the eye, so as to be almost a cylinder with convex ends, and thus, with its deep equatorial groove, into which the ciliary body fits, it has a wonderful resemblance to a Coddington lens. The vitreous humor is a transparent fluid. The retina lines the inner cap- sule, and may be divided into an outer and an inner stratum, separated by a pigment layer. The inner stratum is formed of prismatic or cylindrical rods, the outer ends of which abut upon the pigment, while their inner ends, turned toward the cavity of the eye, are covered by a thick hyaloid membrane. The outer stratum contains the plexus of the fibres of the optic nerves, and numerous cells (ganglionic), supported by connective tissue. The terminations of the nerves, therefore, must traverse the pigment layer to reach the rods. It will be observed that the apparent resemblances between the cephalopodous and the vertebrate eye are merely super- ficial, and disappear on detailed comparison. In Nautilus, the eye has neither cornea, lens, nor vitreous humor, but is a mere cup, lined by the retina. The aperture for the admission of light is exceedingly small. The olfactory organs, the true nature of which was dis- covered by Kolliker,1 are sometimes pits, sometimes papilla? of the integument, situated behind or above the eyes. In the TeuthidcB and Sepiadce, they are depressions above the eyes; in the Octopoo la ', they are either depressions or papillae (Ar- f/onaiita and Tremoctopus) in the same position, but nearer the anterior face of the body. In Nautilus, they are elon- gated, tentaculiform, and situated immediately behind the eyes. 1 " Entwickelungsgeschiclite der Cephalopoden," 18-41, p. 107. THE CEPHALOPODA. 453 In the Dihranchiata, the auditory sacs are lodged in cavi- ties of the cephalic cartilage, and contain a single large otolith, composed of carbonate of lime, and of rounded or irregular but definite and characteristic form. In JS^autihts, Dr. Macdonald discovered that the auditory sacs are attached to the pedal ganglia, and are not lodged in the cranial cartilage. They contain numerous otoliths. An endoskeleton formed of true cartilage is developed in the region of the principal ganglia, and sometimes furnishes them with a complete investment. It gives attachment to the most important muscles. In some Cephalopods additional cartilages appear in the mantle and in the funnel. The mus- cular fibres of the Cephalopoda are un striated. The sexes are distinct, and the reproductive organs are un- like those of other Mollusks. They consist, in both sexes (Fig. 129), of lamellar or branched organs, the cellular con- tents of which are metamorphosed into ova or spermatozoa, Fig. 129.— Sepia officinalis.— 1. male organs: f. testis : vd. vas deferens ; tw, vesicula seminalis : pr, proBtate ; bsp, receptacle of the spermatophores ; p, penis with the genital aperture. (After Duvernoy.) II. Female orenital organs : a, anus ; i, intestine: oo, ovary ; odf. ovi ducal aperture; od. oviducal gland ; gn, nidamental gland ; gn', accessory glands. (After Milne- Edwards.) and which are attached to one point or line of the wall of a chamber, which communicates with the pallial cavity by two 454 THE ANATOMY OF INVERTEBRATED ANIMALS. symmetrically - disposed oviducts, in the females of some species ; but" in most female, and almost all male, Cephalo- pods 1 it has only one duct, the termination of which is usually situated on the left side, but may be near the middle line (male Nautilus), or even on the right side (female Nautilus). Tn the female, the oviduct, or oviducts, present glandular en- largements. In addition, two lamellar nidamental glands are developed upon the walls of the branchial cavity, and to these accessory glands may be added. These glands secrete a vis- cid fluid, which invests the ova, and connects them, when laid, into variously-shaped aggregations. In the male, a prostatic gland furnishes the material of the cases, or sp>ermatophores, in which packets of spermatozoa are contained, and which sometimes possess a very complicated structure. In the Dibranchiata, the spermatophores are slender cylindrical bodies which may reach half an inch in length. They have an external structureless case, thinner at one end than the other, and often ending in a fine filament at the thin end. Within this case, filling its thicker end, and as much as half or two-thirds of the rest of its cavity, is a delicate sac full of spermatozoa. The rest of the case is occupied bj' a very singular elastic body, in form somewhat resembling the sponge of a gun with a spiral screw turned on the handle. The enlarged " sponge ,: end of this body is fastened by a delicate prolongation to the spermatic sac, while the " handle," being too long to lie straight, is coiled up at the end opposite to the sponge, and then fastened to the outer case. When these bodies come into contact with water they undergo strange contortions, and finally, the thin end of the case giving way, the spring frees itself, starts out of the case, and drags with it the sper- matic sac.9 In Nautilus, according to Van der Hoeven, the spermato- phores have a much simpler structure. The male Cephalopods are distinguished from the females by the asymmetry of their arms, one or more of which, on one side, are peculiarly modified, or hectocotylized. Some Cephalopods are devoid of any shell, but most pos- sess a pallial shell, which is either external or internal. In the former case, the visceral sac is lodged within that part of 1 Keferstein found two ducts in a male Eledone moschata. 2 For the minute structure of these curious spermatic cartridges, see Milne- Edwards' s elaborate essay, " Observations sur les Spermatophores des Mol- lusques Cephalopodes." (" Annales des Sciences Naturelles," 1840.) THE CEPHALOPODA. 455 the cavity of the shell which lies nearest its open end, and the rest of the cavity is divided into chambers, which contain air, by transverse septa. The septa are perforated, and a pro- longation of the mantle — the sip/iuncle — is continued through the series of perforations, as far as the apical chamber of the shell. The internal shells of the Cephalopods may have very various forms, and may even be chambered and siphun- culated ; but, in this case, the chamber nearest the mouth of the shell is small, and incapable of lodging the viscera. Our knowledge of the development of the Cephalopods is confined to that of the Dibranchiata.1 In these, the yelk undergoes partial division, and the blastoderm, formed upon one face of it by the smaller blastomeres, spreads gradually over the whole ovum, inclosing the larger and more slowly- dividing blastomeres. The mantle makes its appearance as an elevated patch in the centre of the blastoderm, while the future arms appear as symmetrically-disposed elevations of the periphery, on each side of the mantle. Between these and the edge of the mantle, two longitudinal ridges mark the rudiments of the epipodia, while the mouth appears in the middle line, in front of the mantle, and the anus, with the rudiments of the gills, behind it. The rest of the blastoderm forms the walls of a vitelline sac, inclosing the larger blasto- meres. The pallial surface now gradually becomes more and more convex, the posterior margin of the mantle growing into a free fold, which incloses the pallial chamber and covers over the gills. The internal shell is developed in a sac formed by an in- volution of the ectoderm of the mantle. The epipodia unite behind, and give rise to the funnel, while the anterolateral portions of the foot grow over the mouth, and thus gradually force the latter to take up a position in the centre of the neu- ral face, instead of in front of it. The yelk-sac gradually diminishes, and the contained blastomeres are finally taken into the interior of the visceral sac, into which the alimentary canal is gradually drawn. The Cephalopoda are divided into two very distinct groups, the Tetrabranchiata and the Dibranchiata. The Tetrabranchiata possess an external chambered si- 1 Kolliker, " Entwickelunsrsoreschichte der Cephalopoden," 1841. Gre- nadier, u Zur Entwickelunarscreschichte der Cephalopoden " (Zeitschrift fur wiss. Zoolonie, 1876). Lankester, "Observations on the Cephalopoda" (Quarterly journal of Mlcr. Science, 1875). 456 THE ANATOMY OF IXVERTEBRATED ANIMALS. phunculated shell. The terminal chamber is much larger than any of the rest, and the body of the animal can be almost completely retracted into it. When, as in the only existing genus, Nautilus l (Fig. 130), the shell is coiled into a flat, symmetrical spiral, its apex lies on the anterior face of the body, and the outermost chamber, into which the whole body can be retracted, is consequently posterior to the axis of the helix. In Nautilus, the brachial processes are short, and pos- sess no acetabula such as exist in the Dibr and data, but the margins of the foot are produced externally into a sort of sheath, which, in front, has the form of a broad hood with a tuberculated surface ; while, at the sides, it is divided into many processes of unequal lengths. Behind, the halves of the sheath are separated throughout the greater part of their length by a wide interval, but are united above by a thick 7ttT an, Fig. 180.— JVautili/8 pompilius, female. -C, hood; mx, jaws: J, funnel : p, p', man- tle ; br, branchiae ; gn. nidamer.tal "land ; r>, r, position of the renal appendaps ; ann, horny ring ; u, shell-muscle ; ov, ovary ; gal. oviducal gland ; sph', siphun- cle; ch. black part of the shell under the mantle/)' ; kn, process of the cartilagi- nous skeleton into the funnel. (After Keferstein.) muscular isthmus. The central portion of the sheath is a broad, triangular, hood-like plate, the apex of which is free. It contains two long, narrow cavities, each of which lodges a tentacle. The tentacle consists of a slender stem, on which . » Owen, " Memoir on the Pearly Nautilus," 1832. Van der Hoeven, " An- nates des Sciences Naturelles," 1856. Keferstein in Bronn's " Klassen u. Ordnunwen." THE TETRABRA.XCHIATA. 457 are set a great number of transverse plates, in such a manner that the axis of the stem passes through the centre of the plates. The anterior and lateral regions of the hood are completed by two narrower processes, each of which contains a similar tentacle, and the lateral portions of the sheath are formed by sixteen or seventeen smaller tentaculiferous pro- cesses, the surfaces of which are more or less distinctly an- nulated. When the sheath is opened out, there is seen to be attached to its inner surface, on each side, close to the reen- tering angle between it and the lip which surrounds the beak, and along the line of junction of the lateral part of the sheath writh the isthmus, a thin, free, quadrate lobe, which carries twelve tentacles. The isthmus joins the posterior edges of these outer tentaculiferous lobes, as well as those of the two halves of the sheath, and it exhibits on its anterior, or inner, surface a broad area beset with delicate, close-set, curved lamina?. Two other similar, but much thicker, inner tenta- culiferous lobes, which also carry twelve tentacles, lie be- tween these and the lip. They are quite free from the outer tentaculiferous lobes, and unite with the sheath only above and behind. Like the halves of the sheath, these two lobes are united behind by a thick isthmus, the surface of which presents a number of parallel longitudinal laminae. The beak, which is hidden by the sheath and the lobes, is sur- rounded by the thin circular lip already mentioned, the free margin of which is papillose. Besides these, there is a short, conical tentaculiferous process above the pedunculate eye, and another below it. In the male, the internal tentaculiferous lobes are wanting, and the outer tentaculiferous lobes are divided into two portions, an anterior which bears eight, and a posterior with four, tentacula. On the left side, the four tentacles of the posterior division have undergone much mod- ification, and are converted into a peculiar organ termed the spadix, which bears a discoidal follicular gland upon its outer surface. There is thus a kind of hectocotvlization in the Tetrdbranchiata, The margins of the united epipodia are not united into a tubular funnel. They constitute a muscular membrane, nar- row on the anterior face of the body, but becoming wide, and folded in such a manner that its posterior edges overlap, be- hind. The mantle has a broad anterior fold, which covers the anterior convexity of the shell, and the region which it thus invests is black. The pallia! chamber does not extend for 458 THE ANATOMY OF INVERTEBRATED ANIMALS. more than three-fifths of the length of the body, and is there- fore much less deep than in the Dibranchiata. The anus opens in the middle line on the posterior wall of the pallial cavity, close to its junction with the anterior wall. The four branchiae are attached, two on each side of the anus, to the posterior wall of the branchial chamber, and the inner branchia is shorter than the outer. The nidamental glands, composed of numerous vertical lamella?, partly covered by a fold of the lining membrane of the pallial cavity, are situated on the posterior wall of that cavity, almost midway between its union with the anterior wall and its free edge. The paired renal chambers lie immediately above them also, in the pos- terior wall of the pallial cavity. The buccal mass is very large, its length amounting to one-third that of the body. The apices of the great horny beaks are obtuse, and are coated with a calcareous deposit. The oesophagus dilates into a wide crop and is separated by a constriction from the stomach, the chitinous lining of which is thick and ridged. The pyloric caecum is small and rounded, and the intestine makes two bends upon itself before reaching the anus. Salivary glands appear to be wanting, unless cer- tain glandular bodies placed within the buccal mass should be of this nature. The liver is a loosely racemose gland, divided into four lobes, and is lodged in the anterior part of the perivisceral cavity. There is no ink-bag, and there are no branchial hearts. The quadrate systemic heart is situated on the left side of the posterior face of the body, close to the junction of the posterior with the anterior wall of the pallial cavity. It receives four branchio-cardiac veins ; and, attached to it, is a pyriform sac, which, according to Keferstein, opens into the pallial cavity. The cartilaginous skeleton supports the pedal and parieto- splanchnic ganglia, but does not encircle the gullet, or roof over the cerebral ganglia. Two long processes of the skele- ton pass into the funnel and give attachment to its muscles. Two large shell-muscles are attached to it; and, passing up- ward and outward, are inserted into oval chitinous patches visible on the outer surface of the mantle, and connected to- gether by a thin ring of the same substance (the annulus) which encircles the mantle. The oviduct does not arise directly from the sac in which the ovary is lodged, but from a distinct chamber, into which the ovarian sac opens. A large albumen-gland pours its THE TETRABRANCHIATA. 450 secretion into the ovarian sac. The vas deferens similarly takes its origin, not from the sac of the testis but from a smaller chamber communicating therewith. The commence- ment of the vas deferens is enlarged and glandular. Nothing is known of the development of the Tetrabranchiata. The only existing representatives of the Tetrabranchiata are the different varieties of " pearly nautilus " (Nautilus pompilius), which are found in the southern seas, living at the bottom at a considerable depth. The genus is one of the oldest in existence, since it is traceable through the whole series of fossiliferous rocks as far back as the Silurian epoch. Along with it, in the Palaeozoic formations, occur numer- ous closely-allied forms, which differ from Nautilus mainly in the different curvature (Lituites, Gyroceras, Trochoceras) or straightness (Orthoceras, Gomphoceras) of the shell, and in the varying position, proportions, and degree of calcification of the siphuncle. In the middle of the Palaeozoic strata (Devonian), Tetra- branchs (Ammonitidce) appear, in which the margins of the septa are strongly bent, whence their edges appear as zigzag transverse lines, folded into lobes and saddles, when the outer layer of the shell is worn away (Goniatites, Ceratites); and, in the Mesozoic epoch, the lobes and saddles become extreme- ly complicated, while the shells may be straight, simply curved, or bent, or turbinated (Ammonites, Bacidites, Turri- lites). The Ammonitidce are extraordinarily numerous in the Mesozoic epoch, but no trace of them has been found in tertiary or quaternary formations. Associated with Ammonites, and not unfrequently lodged in the terminal chamber of the shell, are the so-called Aptychi. These are plates of a shelly substance, three-sided, with rounded-off angles, and applied together by their straightest edges so as to resemble bivalve shells. They consist of two layers, an inner and an outer, of which the inner presents lines of growth, concentric with the angle of each plate which is situated on that side of its broad end which is applied to its felknv. The outer layer is composed of many laminae, and is traversed by pores. Its free surface frequently presents longitudinal ridges. The heart-shaped plates, undivided by a suture, wilich are found in some Goniatites and Ammonites, are termed Anaptychi. The Aptychi, when undisturbed, occupy the middle of the posterior wall of the terminal chamber of the Ammonite, and 460 THE ANATOMY OF INVERTEBRATED ANIMALS. have their bases toward its mouth. Nothing is certainly known as to the nature of the Aptychi or Anaptychi.1 In the Dlbranchiata, the margins of the foot are pro- duced into not fewer than eight, nor more than ten, arms, which are provided with acetabula, or suckers. Each ace- tabulum is a sessile or stalked cup, from the bottom of which rises a plug, which nearly fills the cup, but can be retracted by the action of muscular fibres attached to it. When the margins of the acetabulum are applied to any surface, and the plug is retracted, a partial vacuum is created, and the acetabulum is caused to adhere to the surface by atmospheric pressure. The edges of the acetabula are frequently strength- ened by chitinous rings, and these may be serrated (Fig. 124, B), and are sometimes produced into long, curved hooks. The margins of the united epipodia are not only folded inward, but coalesce so as to give rise to a tubular funnel, throusrh which the water taken into the branchial sac for respiratory purposes is ejected. Very often, a valve which prevents the flow of water back into the mantle cavity is de- veloped within the funnel. There are two branchiae, and the anus terminates between them in the anterior wall of the branchial sac, on which also the nidamental glands are situ- ated. The apices of the horny beaks are acutely pointed, and not ensheathed in calcareous matter. The liver is usual- ly a compact mass. A peculiar gland, which secretes an ex- tremely dark fluid — the so-called ink — and has the form of an oval or pyriform sac (the ink-bag), with a long duct which opens into, or close to, the rectum, is lodged sometimes in the liver, sometimes further back (Fig. 126, I.). The ink is ejected when the animal is alarmed, and gives rise to a dark cloud in the water, by which its retreat is covered. There are two branchial hearts. The eye is lodged in an orbit and is provided with a lens. The cartilaginous endoskeleton forms a ring surrounding the gullet and enveloping the principal ganglia. There is usually an internal pallial shell. It may "be chambered and siphun- culated, but in this case the last chamber is small, and hardly larger than the others. The Dibmnchiata are divided into the Octopoda and the Decapoda. The Octopoda have eight arms, and possess no pallial shell. But, in the female of one genus {Argonauta, the "paper Nautilus," Fig. 131), the extremities of the an- 1 See the discussion of this question by Keferstein, in Bronn's " Thierreich." THE DIBRAXCHIATA. 461 terior pair of arms are greatly expanded, and, being turned back" over the mantle, secrete an elegant shelly structure which covers the bodv, and serves for the attachment of the Fig. \Z\.— Argonaut a argo.—A, female with the expanded amis in their natural position, embracing the shells,* d, the other six arms; a, the tunnel. B, ace- tabula. Fig. \%Z.—Argonauta argo, male, with the Hectocotylus-^vm attached. eggs. In this genus, and in some other Octopods ( Octopus carina, Tremoctopus violaceus, and T. Q 7 coy anus), the male is very much smaller than the female, and gives rise to a Hectocoti/lus. In Argonautn arr/o (Figs. 132, 133), it is the third arm on the left side which becomes thus modified. At first it has the form of a sac, within which the slender terminal part of the arm is coiled up (Fig*. 133, B). The sac splits to give exit to the latter (Fig. 132), and its two halves reunite on the outer face of the base of the arm to form a chamber, which becomes filled with spermatophores in a manner not yet un- derstood; During sexual union the arm thus charged with 462 THE ANATOMY OF INVERTEBRATED ANIMALS. semen is detached and left in the mantle cavity of the female (Fig. 133, A). When first discovered it was regarded as a parasite, and termed Trichocephalus acetabularis by Delle Fig }. 133.— Argonaut a argo.—B, male, with thehectocotylized arm inclosed in its sac; 1, 2, 3, 4, the other arms of the right side ; and 1', 2', 4', those of the left side. A, the hectocotylus detached. Chiaje, while the corresponding body found in an Octopus was called Hectocotylus octopodis by Cuvier. In Tremoctopus, it is the third arm on the right side which becomes the Hectocotylus. In other Octopods? one or other arm is peculiarly modified, but does not become detached or serve as a receptacle for the spermatophores. The JDecapoda have ten arms, two of which are usually much longer than the rest, and can be protruded from, or re- tracted into, sockets. The acetabula have horny rims, which may take on the form of hooks. Hectocotvlization does not go further than a modification of the form of one of the arms. There is always an internal shell, which is either a pe?i, a sepiostaire, a phragmocone, or a combination of the latter with a pen. 1 Steenstrup, " Die Hectocotylenbildung bei Argonauta und Tremodopus erkliirt durch Beobachtun', intestine; c. rudimentary Echino- derm; c', the ambulacral sac; c", the external opening of its duct; A A, FF, B, the processes of the body. In the Echimdea, as in the Ophiuridea, the Echinopae- THE CRINOIDEA. 497 dium is a Pluteus, and has a skeleton formed of calcareous rods, which support the processes into which the body, in the region of the ciliated bands and elsewhere, is prolonged. The origin of the ambulacral system, before it has the form of a caecum with a dorsal pore, has not been made out. The blind end of this cascum lies on the left side of the ali- mentary canal, and is connected with a discoidal body, which is situated on the left side of the stomach ; a similar body ap- pears on the right side. Doubtless these discoidal bodies an- swer to the peritoneal diverticula of the alimentary canal of the Echinopsedium in other Echinoderms. The blind end of the tube enlarges, and gives rise to a rosette, whence the ambulacral vessels proceed ; and a de- pression of the integument of the larva, forming the so-called limbo, extends inward to this. At the bottom of the umbo, a new mouth opens through the centre of the rosette into the gastric cavity of the larva, the primitive oesophagus being abolished. The larval skeleton undergoes resorption, but the rest of the Echinopaedium passes into the Echinoderm.1 Loven has recently drawn attention to the fact that, in youno- Echinids,2 the plates of the apical region are not only more conspicuous in relation to the corona, but differ some- what in their arrangement from those of the adult, Thus the anus is at first wanting, and the anal plate, which occu- pies the centre of the apical area, is relatively large ; it is united by its edges with the five plates, which, imperforate in the young, will become the genital plates in the adult. The five ocular plates are also imperforate, and are disposed in a circle outside that formed by the genital plates, their inter- spaces being occupied by interambulacral plates. The apical region of an Echinid has thus, as Loven points out, a most striking resemblance to the calyx of a Crinoid ; the anal plate representing the basal la, the genital plates the para- basalia, and the ocular plates the first radialia. The Ckinoidea. — This remarkable group, which abounded in former periods of the world's history, is represented at the J See, in addition to the memoirs of Midler and Metschnikoff already cited, A. Agassiz, " On the Embryology of Echinoderms." (" Mem. American Acad- emy of Sciences," 1864. ) '2 The admirable monograph of A. Agassiz, " Revision of the Echini," pub- lished in the " Illustrated Catalogue of the Museum of Comparative Zoology at Harvard College," is also full of information respecting the young states of the Echinids. 498 THE ANATOMY OF IXYERTEBKATED ANIMALS. present day only by the genera Antedon (Comatula), Acti- nometra, Comaster, JPentacrinus, Rhizocrinus, and Holopus. The first three genera are capable of locomotion, while the next two are attached by long articulated stems to sub- marine bodies. Holopus, which is but imperfectly known, appears to be fixed by a short, thick, unjointed prolongation of its base. JZhizocri?ius lofotensis (Fig. 146), which has been very carefully and elaborately described by Sars,1 is a small animal which does not attain more than three inches in length, and lives at great depths (100-300 fathoms or more) in the sea. It consists of a relatively long, many-jointed stem, from many of the articulations of which, branched, root-like filaments, or cirri, are given off ; at the summit of this is seated a cup- shaped body, the calyx, from the margins of which five to seven arms (brachia) radiate. To each arm is attached a double series of alternating pinnules. The mouth is situated in the centre of that part of the perisoma which forms the surface of the calyx opposite to the stem. The oral aperture is cir- cular, but five (or sometimes only four) triangular lobes of the perisoma, wTith rounded free ends, project over it, and, when shut, close it like so many valves. From the inter- vals between these oral valves five (rarely four) grooves trav- erse the oral surface of the calyx, and extend thence throughout the whole length of each arm, giving offsets as they go to the pinnules. Thus the oral surface of each arm and of each pinnule is deeply excavated. Between the circular lip and the oral valves, soft flexible tentaculiform pedicels are attached in a single series. Two pairs of pedicels correspond to every valve, each pair aris- ing opposite the basal angle of a valve. These pedicels are hollow, their surface is papillose, and the outer or radial pedi- cel of each pair is very contractile. Pedicels of the same general character are continued throughout the brachial and pinnular grooves. The anus is situated at the end of a conical prominence between two of the grooves on the oral face of the calyx, and is therefore interradial in position (Fig. 146, III. an). The skeleton consists of very numerous pieces resulting from the calcification of the perisoma. In the stem they have the form of elongated, subcylindrical, or hour-glass-shaped, joints (articuli), the opposed faces of which are united by * " Memoires pour servir a la connaissance des Crino'ides vivants," 1868. THE CRIXOIDEA. 499 is strong elastic ligamentous fibres. The centre of each traversed by a longitudinal axial canal, which extends through the whole length of the stem and is occupied by a Fig. 146. — Rhizocrinw lofotensis. (After Sars. ) I. Rhizoainus entire: a, enlarged upper joint of the stern ; b, larval joints of the stem ; c, cirri ; d, brachia. II. Calyx and arms, with the summit of the stem of a Rhizocrinvs having five well- developed braehia : a, as before ; s, first radials ; r-, r3, second and third radials ; b\ first brachial : p, p, pinnules. III. Upper part of the stem and oral face of the calyx, viewed obliquely : v, lower part of visceral mass : st. tentacular grooves ; b, oral valves ; t, oral tentacles ; an, anus. soft but solid substance. The distal joint of the stem is not directly fixed to the surface to which the Crinoid is attached, but is connected therewith by the branched cirri which pro- ceed from it. Each cirrus has a skeleton composed of joints or articuli, somewhat like those of the stem, and traversed by a prolongation of the axial canal. Similar cirri are developed from a larger or smaller number of the articuli of the distal portion of the stem. The proximal joints become gradually shorter in propor- tion to their length, until they assume a discoidal form. It appears that new articuli are continually added at that end of the stem which lies nearest the calyx. The summit of the stem, or the base of the calyx, is formed by an enlarged, solid, pear-shaped ossicle, which is probably formed by-the coalescence of several articuli. Upon 500 THE ANATOMY OF INVERTEBRATED ANIMALS. this follow five pieces (first radialia) closely united together and with a central piece, which probably represents the basa- lia of other Crinoids. The first radial corresponds in direc- tion with the origin of one of the arms, and is followed by a second and third radial. With the third radial is articulated the first of the brachial ossicles, which constitute the skele- tal support of the unbranched brachia. The pinnules are also supported by a series of elongated calcified joints, the basal joint being articulated with a brachial ossicle and the distal joint pointed. The axial canal dilates in the enlarged pyriform ossicle above mentioned ; and, from the dilatation, branches, which traverse the radial and the pinnular ossicles, are given off. There is a calcareous plate in the substance of each oral valve, and minute reticulated calcifications are scattered through the perisoma of the oral face of the disk. The sides of the radial grooves are provided throughout with a double series of oval calcareous plates — the marginal lamellce — which are disposed transversely to the groovTe, those of opposite sides alternating with one another. They can be erected or depressed ; and, in the latter case, overlap one an- other like tiles. In Pentacrinus, the long stem is fixed by its distal end, and the pentagonal artieidi of its skeleton give off, at inter- vals, whorls of unbranched cirri. No distinct basal piece is known, but the calyx appears to begin with the five first radi- alia. At the third radiale, the series bifurcates into two series of brachialia, and these again bifurcate to give rise to the palmaria, which support the free arms. There are mar- ginal lamellae along the sides of the tentacular grooves, and a longitudinal series of calcareous ossicles occupies the floor of each groove. The anus is situated upon an elevated inter- radial cone. The body of an adult Comatula (Antedon) answers to the calyx, with its brachia, in other Crinoids. The centre of the skeleton is constituted by a large centro- dorsal ossicle, articulated with the aboral face of which are the numerous cirri, by which the Antedon ordinarily grasps the bodies to which it adheres, though it is able, on occasion, to swim freely about. This centro-dorsal ossicle appears to be the homologue of the uppermost part of the stem in the Pentacrinus. There are five divergent series of radialia, each containing three ossicles. The first radials, or those nearest the centro-dorsal plate, are close'ly adherent to one THE CRINOIDEA. 501 another and to the centro-dorsal plate, and are not visible on the outer surface of the calyx. The space left between the apices of the five first radials is occupied by a single plate, the rosette,1 which is formed by the coalescence of the five basal la present in the larva. The anatomy of the soft parts of the Crinoidea has been most thoroughly investigated in the genus Comatula (An- tedori).* The mouth leads, by a short, wide gullet, into a spacious sacculated alimentary canal, which is coiled upon itself in such a manner as to make about one turn and a half around the axis of the body, and then terminates in the projecting rectal cone, which, as has already been seen, is situated inter- radially on the oral face of the calyx. The central cavity, included by the coil of the alimentary canal, is occupied by a sort of core of connective tissue, and has received the name of columella, but it must be understood that it is not a dis- tinct structure. Bands of connective tissue connect the outer periphery of the alimentary canal with the perisoma. The five triangular lobes of the perisoma, which surround the mouth like so many valves, contain no calcareous skele- ton in the adult Antedon. Within these lobes, attached to the oral membrane, there is a circle of tentacula. From the interval between each pair of oral valves, a groove radiates outward over the surface of the calvcine perisoma and speed- ily bifurcates ; one branch goes to the oral surface of each of the arms, and runs along it to its extremity, giving off alter- nate lateral branches to the pinnules in its course. These grooves are the ambulacral grooves. Their sides are, as it were, fenced by small, lobed processes of the peri- soma ; and, on the inner sides of these processes, groups of minute pedicels take their origin from the sides of the floor of the groove. A thickened band of the ectoderm occupies the middle of the floor, and so strikingly resembles the ambu- lacral nerve of the Star-fish that the homology of the two, 1 Carpenter, " On the Structure, Physiology, and Development of Comatula. ^ ("Phil. Trans.," 1866.) 2 E. Perrier, " Recherches sur l'Anatomie de la Comatula rosacei" ("Arch, de Zooloarie Experimentale," 1873). Semper, " Kurze anatomische Bemerkun- gen nber Comatula" (*' Wiirzburg Arheiten," 1874). Ludwio-, " Zur Anatomie der Orinoideen" (ZwUehrift f:>r wiss. Zw/., 1876X Carpenter, "On tne Struct- ure, Physiology, and Development of Antedon" ("Proc. Royal Society," 1876\ Greef, '• Ueber den Bui der Crinoideen " (" Marburg Sitzungsherichte," 1876). P. H. Carpenter, " Remarks on- the Anatomy of the Armsof the Cri- noids" (Journal of Jnat-. and Physiology, 1876). 502 THE ANATOMY OF IXVEETEBRATED ANIMALS. first asserted by Ludwig,1 cannot be doubted. Immediately beneath it runs a small canal, discovered by Dr. Carpenter, and termed by him the tentacular canal, which gives off lat- eral branches to communicate with the cavities of the pedi- cels. A second much wider canal — the subtentacular canal — lies beneath this, and is divided by a longitudinal septum. But the septum is incomplete at intervals, and thus the two canals communicate. A third, still larger — cceliac canal — is interposed between the floor of the subtentacular canal and the axial skeleton of the arm. Where the arm joins the calyx the tentacular canals run beneath the ambulacral groove to the gullet, around which they are united by a circular canal, from which numerous short diverticula, resembling the vasa ambulacralia cavi in the Ophiurids, described by Simrock {I. c), depend. The subtentacular and cceliac canals communicate with channels in the perivisceral tissue, on the oral or the aboral face of the visceral mass ; and these channels appear, eventually, to open freely into the cavities by wThich the columella is trav- ersed. In the partition between the subtentacular and the cceliac canals there lies a cellular cord, or rachis, which can be traced back into a reticulation of similar tissue in the visceral mass. The genital glands contained in the pinnules are enlargements of lateral branches of this rachis. But the rachis is appar- entlv only an extension of the mesodermal tissue of the vis- ceral mass, comparable to that in which the genitalia are lodged in the Star-fishes ; and the multiplication of the geni- tal glands may be regarded as a further extension of the structure which obtains in Brislnga. Thus it would seem that the position of the genital glands in the Crinoids is not so anomalous as it at first appears to be. The centro-dorsal tubercle contains a cavity with which the canals which traverse the ossicula of the cirri, the calyx, the brachia, and the pinnules communicate. This cavity was considered by Mttller to be a heart. It proves, however, to be largely filled by solid tissue, which is continued not only into all the canals which traverse the ossicula, but also into the columella, or tissue which occupies the centre of the coils of the alimentary canal. Dr. Carpenter2 is of opinion that so much of this axial 1 Zeiischrift fur wiss. Zoologie* 1876. 2 " Proceedings of the Royal Society," 1876. THE DEVELOPMENT OF THE CRINOIDEA. 503 tissue as occupies the cavity of the central tubercle, and is continued throughout the ossicula of the calyx and arms, is the proper central organ of the nervous system ; founding this opinion partly upon the fact that, when this mass is irri- tated in a living Antedon, a sudden contraction of all the muscles of the arms takes place, and partly upon the distri- bution of the ultimate ramifications of the axial tissue in the arms. Greef, on the contrary,1 affirms that all these tracts can be injected, and retains the name of "heart" for the cavity of the centro-dorsal tubercle. The perisoma of the oral surface of Comatula exhibits a great number of minute circular pores, with thickened cellu- lar margins. Greef has discovered that these are the external apertures of canals, with ciliated walls, which open into the body-cavity, and readily allow fluids to pass into, or out of, that cavity. Each mature ovary of Antedon has a distinct aperture, through which the ova are discharged, and to which they ad- here for soms days like bunches of grapes. The testis devel- ops no special aperture, but the spermatozoa appear to be discharged by dehiscence of the integument. Since the discovery by Vaughan Thompson that Comatula passes through a Pentacrinoid larval condition, the develop- ment of the free Oinoids has been the subject of various in- vestigations,2 and the following results may be regarded as established : Complete yelk-division takes place. The morula acquires an oval form, and develops four hoop-like bands of cilia, with a tuft of cilia at the hinder end. Between the third and fourth bands of cilia, counting from the anterior end of the Echinopaeclium, the blastoderm becomes invaginatecl, and gives rise to an archenteron. In the interspace between this blind sac, the wall of which is the hypoblast, and the epiblast, constituted by the rest of the blastoderm, a mesoblast com- posed of reticulated cells makes its appearance. The blasto- pore closes, while the archenteron detaches itself from its attachment to the posterior ventral face of the larva, and be- comes connected with an oesophageal involution formed at its anterior end. The archenteron next throws out three diver- ticula, of which two are lateral and one is ventral. The lat- 1 " Ueber das Herz der Crinoideen" (" Marburg Sitzungsberiehte," 1876). a See Wyville-Thompson ("Phil. Trans.," 1865), Metschnikoff (" Bulletin de PAcad. Imp. des Sciences de St.-PStersbourg," 1871), and especially G-otte (u Archiv far Mikroskopisclie Anatoinie," 1S76J. 22 504 THE ANATOMY OF LNYERTEBRATED ANIMALS. eral diverticula enlarge, and apply themselves to the rest of the archenteron, now become the intestine, from which they are soon completely shut off, and converted into peritoneal sacs. The left sac thus formed lies on the ventral side of the intestine, the right sac on its dorsal side. The walls of the two sacs become applied together, and form a circular mesen- tery. The peritoneal sac of the aboral side sends a pro- cess into the hinder end of the body, which has begun to elongate, in order to give rise to the stem of the Pentacrinoid form. The third, or ventral, diverticulum is shut off from the alimentary canal much later than the other two. It grows round the mouth, and gives rise to the circular ambulacral vessel, whence the tentacular canals are given off. Ten plates, each consisting of a calcareous network, and arranged in two rows of five each, next appear in the sub- stance of the Echinopcedium around the alimentary canal. From the centre of the posterior row, eight calcareous rings extend through the length of the body of the larva, inclosing the backward prolongation of the aboral peritoneal sac ; and the series terminates by a broad, discoidal network, which lies on one side of the posterior end of the larva. This discoidal plate is that which occupies the attached end of the stem of the future Crinoid ; the rings become the stem, and the two circles of plates the basal and oral ossicula of the calyx, re- spectively. As the stem elongates, new rings (articuli) are added at the junction of the stem with the calyx. The larva now fixes itself bv the discoidal end of its stalk, which becomes relatively longer and narrower ; while the part of the body which contains the basal and oral plates, and is to be converted into the calvx, remains thick and short. Its broad end becomes five-lobed, each lobe answering to an oral plate. These plates separate like the petals of a flower- bud, and discover, in the centre, the wide, permanent oral aperture. Between the margins of this and the oral plates, tentaculiform pedicels, at first only five, but eventually ar- ranged in groups of three, between every pair of oral plates, make their appearance. The alimentary cavity is still a mere sac, without intestine or anus. Five radial plates next appear in the wall of the calyx, be- tween the basal and the oral plates, and alternating with both; and, in correspondence with them, the arms grow out as rap- idly-elongating processes, in which the other radials are sue- THE AFFINITIES OF THE ECHINODERMATA. 505 cessively developed. The entire zone of the calyx, which is occupied by the origins of the arms, at the same time widens, so that the oral plates, which remain round the mouth, and the basal plates, which encircle the stem, become widely sep- arated. The intestine grows out as a diverticulum of the alimentary cavity, and opens on an interradial elevation of the calyx, inwdiichan anal plate is developed. The young Echi- noderm has now passed into the stalked Pentacrinoid stage. In Comatida, the oral and anal plates disappear altogether, and the basals, coalescing into the rosette, are hidden by the first radials, on the one hand, and the centro-dorsal tuber- cle, which represents coalesced joints of the stem, on the other. The arms bifurcate and acquire their pinnules ; and the calyx, with its appendages, eventually becomes detached from its stem as a free Comatula. In the existing stalked Crinoids, such as Pentacrinus, on the other hand, the seg- ments of the stem acquire whorls of cirri, at intervals, and no such modification of the uppermost segments into a centro- dorsal tubercle takes place. On comparing the facts of structure and development which have now been ascertained in the five existing groups of the Ecliinodermata, it is obvious that they are modifications of one fundamental plan. The segmented vitellus gives rise to a ciliated morula, and this, by a process of invagination, is converted into a gastrula, the blastopore of which usually be- comes the anus. A mouth and gullet are added, as new for- mations, by invagination of the epiblast. The embryo normally becomes a free Echinopasdium, which has a complete alimen- tary canal, and is bilaterally symmetrical. The cilia of its ectoderm dispose themselves, in one or more bands, which surround the body ; and, w'hile retaining a bilateral sym- metry, become variously modified. In the Holothuridea, As- teridea, and Crinoidea, the larva is vermiform, and has no skeleton ; in the Echinidea and the Ophiuridea it becomes pluteiform, and develops a special spicular skeleton. If an Echinopasdium were to attain reproductive organs, and reproduce its kind, I think that it cannot be doubted that its nearest allies would be found among the Turhelhirla, the Rotifera, the Gepliyrea, and the Enter opneusta* But that 1 In a report upon the "Researches of Prof. Mailer into the Anatomy and Development of the Echinodermo," published in the Annals- of Natvral His- tory for July, 1851, I drew attention to the am rities of the Eohinoderms with the Worms'; and in a paper on Lacinularia socialis, read before the Micro- 506 THE ANATOMY OF INVERTEBRATED ANIMALS. which characterizes the Echinodermata is the fact that the alimentary canal of the Echinopaedium gives rise to an en- terocoele, which again is subdivided into two systems of cav- ities, on? ambulacral and the other peritoneal, and that the mesoblast becomes modified in accordance with the arrange- ment of these systems. The enterocoele may be formed by one diverticulum or by three. In the former case, the first formed becomes subdivided into three, of which one is ante- rior, and two lateral, as in the latter case. The lateral di- verticula give rise to the peritoneal cavity and its lining ; the medium diverticulum is converted into the circular ambu- lacral vessel and its dependencies ; and it is in consequence of the radiating disposition of the latter, and of the nerves and muscles which are related to it, that the Echinoderm pos- sesses so much radial symmetry as it displays. It is clear, therefore, that the radial symmetry of the Echinoderm results from the secondary modification of an animal, which is primi- tively bilaterally symmetrical; and that the apparently radi- ate Echinus, or Star-fish, is a specially modified " Worm " (using that term in its widest sense), in the same sense as the apparently radiate Coromda\s a modified Arthropod. Haeckel goes farther than this, and supposes that each ray of a Star-fish or Ophiurid, for example, represents a Worm, and that the Echinoderm consists of coalesced vermiform buds, developed in the interior of the Echinopaedium. I must confess my inability to see that this hypothesis is sup- ported hy valid reasons. On the contrary, the more closely one compares the structure of the ray of an Echinoderm with the body of any known Annelid, the more difficult does it ap- pear to me to be to find any real likeness between the two. In order to find any analogy for the production of the Echinoderm within the Echinopaedium, on the contrary, it ap- pears to me that we must look to the lower, and not to the higher, morphological types. Among the Hydrozoa, nothing is commoner than the distribution of the functions of life be- tween two distinct zooids, one of which alone develops repro- ductive organs. In the former — the hydranth — radial sym- scopical Society in the same year, I expressed the view that the Ttotifera " are the permanent forms of Echinoderm larvae, and hold the same relation to the Echinoderms that the Hydriforra Polypi hold to the Medusae," and that they "connect the Echinoderms with theNematide and the Nematoid Worms." When they were published, those who did not ignore these views, ridiculed them. Nevertheless, though somewhat crudely expressed, I think it will be admitted that they have been substantially justified by the progress of knowl- edge during the last Quarter of a century. THE AFFINITIES OF THE ECHINODERMATA. 507 metry is often hardly discernible (e.g., Calycoplioridce)\ in the latter — the medusoid — it is very marked, and especially char- acterizes the arrangement of the gastro-vascular canals, which are offshoots of the alimentary cavity, and, if they became shut off therefrom, would answer to the enteroccele of the Echinoderm. Suppose that, from a hydranth such as that of a Diphyes^ a medusoid were developed, and that, instead of projecting from the exterior of the body, it remained hypodermic, spread- ing out between the ectoderm and the endoderm of the hy- droid, and consequently superinducing a very marked radial symmetry upon it. The resulting form would give us a Ccelenterate which would be a close analogue of an Echino- derm. In a certain sense, an Actinozoon may be fairly regarded as such a combination of a hydroid with its medusoid ; and, hence, it must be conceded that the parallel between the gas- tro-vascular system of the Ctenophora and the ambulacra! system of the Echinoderm s, instituted by the elder Agassiz, was well worthy of consideration. Shut off the gastro-vascu- lar canals of a Cydippe from the alimentary canal, and they become an enteroccele, of which the prolongations along the stomach may be compared with the peritoneal sacs, and those beneath the paddles with the ambulacral vessels of the Echino- derm. But there is a long step between the admission of the force of these analogies, and the conclusion that the Echinoderms and the Coelenterata are so closely allied as to be properly associated in one natural assemblage of " Radiate " animals. On the contrary, the Echinoderm, by its Echinopgedium stage, shows an advance in organization far bevond anvthino* known in the Coelenterata , and in the nighty-characteristic mode of development of its enteroccele (the elucidation of which in the " Star-fishes," by Prof. A. Agassiz, is the most important advance in our knowledge of the Echinoderms made since the time of Muller), the Echinoderm agrees with the higher, and not with the lower, Metazoa. Echinodermata abound in the fossil state. Calcareous plates, referred to the Uolotfutridea, occur in the Mesozoic rocks, but are not known earlier. The Star-fishes are met with in the older Palreozoic strata, under forms very similar to some of those which now exist. The Eehinidea abound from the Upper Silurian (Palcechinus) onward. The Palreo- 508 THE ANATOMY OF INVERTEBRATED ANIMALS. zoic forms are spherical, and have multiple interambulacral plates and simple ambulacra. Echinidea of the modern type appear in the Mesozbic strata — the Echinoida first, while the Spatangoida and Clypeastroida are of later date. This order of occurrence agrees with the embryonic development of the two latter groups, which are more nearly spherical when young than subsequently. The Crinoidea abound in the Palaeozoic and older Meso- zoic rocks, gradually diminishing in number in later forma- tions. The oldest appear to have all been stalked, and of peculiar and extinct types. Three groups are wholly extinct, and are unknown in strata newer than the Carboniferous formation. These are the Cystidea, the Edrioasterida, and the Blastoidea. The Ctsttdea. — In their general characters the Cystidea come ve«y near the Crinoids. Cryptocrimts, the simplest form of the group, possesses a calyx supported on a stem, and composed of five basalia, five parabasalia, and five radialia. An interradial aperture is surrounded by a cone of small plates, termed the pyramid. The antambulacral surface has no pores, but these were present in other genera, and sometimes are scattered irregularly (Caryocrinus); sometimes disposed in pairs (Sphceronites) ; while sometimes they take the form of parallel slits arranged in " pectinated rhombs." The arms were free (Comarocystites), or recurved and closely applied to the calyx. They bore pinnules, which, in consequence of the non-development of the arms, were sometimes sessile on the radialia. In the species with recurved arms, the latter simulate calycine ambulacra. There is an aperture placed in the centre of the calyx at the point of convergence of the ambulacra ; another small one on one side of this ; and, third- ly, the aperture of the pyramid. The first of these is com- monly regarded as the mouth, the second as the anus, the third as the reproductive aperture. The Cystidea would, on this interpretation, differ from all other Echinodermata, except the Edrioasterida and Holo- thuridea, in the genital outlet being single ; but around the central aperture five pores are seen, in some species at least, to which a genital function has been ascribed. In any case, the Cystidea would appear to come very close to the Cri- noidea. The Edrioasterida. — This group contains several genera THE BLASTOIDEA. 509 of extinct Echinoderms (Edrioaster, Agelacrinites, Hemicys- tites), which, in general form, somewhat resemble what the Asterid Goniaster would be if its angles were rounded off. Like the Cystidea, they possess an interambulacral^/ram/^, but they differ from them in that they have ambulacra per- forated by canals which open directly into the cavity of the calyx, and that they possess no arms. The Edrioasterida have no stem, but seem to have been attached by the abo- ral face of the body. The Blastoidea. — In Pentremites, the representative of this order, the ambulacral and antambulacral regions are nearly on an equality : the body is prismatic or subcylin- drical. The pedunculated calyx is composed of three basal plates, two of which are double. The aboral plates receive in their intervals five plates deeply cleft above. In the clefts lie the apices of the ambulacra, the oral portions of which are included between the five deltoid interradial pieces which surround the mouth. The cleft plates are not radials, but portions of the perisomatic skeleton of the aboral region. Surrounding the central, probably oral, aperture, are four double pores, and a fifth divided into three. The median of these three seems to be anal, the others and the paired pores being genital. Each ambulacrum is lanceolate in form, and presents superficially a double row of ossicles, which meet in the middle line and support pinnules at their outer extremi- ties ; beneath them lies a single plate, perhaps the homologue of the vertebral ossicles in the Ophiuridea ; beneath it again are parallel canals, the nature of which is unknown. CHAPTER X. THE TUNICATA OR ASCIDIOIDA. This remarkable and, in many respects, isolated group of marine animals contains both simple and composite, tixed and free, organisms. None attain a length of more than a few inches, and some are minute and almost microscopic. The simplest members of the group, and those the struct- ure of which is most readily comprehensible, are the Appen- dicularice y minute pelagic organisms, which are found in all latitudes, and are propelled, like tadpoles, by the napping of a long caudal appendage at the surface of the sea. Appendicularia flabellurn (Fig. 147) has an ovoid or flask- shaped body (A), one-sixth to one-fourth of an inch in length. The appendage (J3) is from three to four times as long as the body, to one face of which it is attached near, but not at, the posterior extremity. It is flattened, and is supported by a firm central axis, which may be termed the urochord (Fig. 147, £). The greater part of the body is usually invested by a structureless gelatinous substance, but, on its rounded hinder extremity, this ceases to be distinguishable from the ectoderm. On the caudal appendage the polygonal contours of the cells of which the ectoderm is composed are plainly discern- ible. The mouth has an overhanging lip. It leads into a large pharyngeal sac, the walls of which are formed by the endo- derm. Posteriorly this sac narrows into the oesophagus, which bends toward the haemal side of the body, and then opens into a spacious stomach, which takes a transverse direc- tion, and is divided into two lobes, a right and a left. From the left lobe the intestine arises, and, bending in- ward, turns abruptly forward in the middle line, where it terminates midway between the oral aperture and the attach- ment of the caudal appendage. The intestine, therefore, has APPENDICULAR!.! FLABELLUM. 511 a haemal flexure. In the middle of its haemal aspect the en- doderm of the pharyngeal cavity is raised into a fold, which projects into the blood-cavity contained between the endo- Fig. 147.— Appendicularia flabellvm. I. The entire animal, with the caudal appendage in its ordinary position, or turned forward. II. Side view of the body, with the caudal appendage forcibly bent backward. A, the body; J5, the caudal appendage; a, oral aperture; b, the pharynx; c, an atrial opening; d, the corresponding stigma, with its cilia ; e, anus; /, rectum ; g, oesophagus; h, i, stomach ; k, testis; I, urochord ; m, cellular patch at the side of the oral end of the body; n. endostyle ; p. ganglion ; q, ciliated sac; r, otocyst ; s, posterior nerve with its ganglia, t ; en, endoderm ; ec, ectoderm. derm and ectoderm. The walls of the bottom of the fold are thicker than the rest, so that, viewed sideways, it has the aspect of a hollow cylinder. This is the endostyle.1 (Fig-. 147, n.) 1 So described and named in my " Observations upon the Anatomy and Physiology of Salpa and Pyrosoma, together with Remarks upon Doliolum and Appendicularia." ("Phil. Trans.," 1851.) In 1S5G, however, I stated: " With regard to the endostyle, I have nothing important to add to ray pre- vious account, except that I believe it to be here, as in other Ascidians, the optical expression of the thickened bottom of a fold or groove of the branchial sac." ( Quarterly Journal of Microscopical Science, April, 1856.) In my memoir on Pyrosoma (" Linn. Trans.," 1860, p. 205), the endostyle is stated to be " in 3-3 512 THE ANATOMY OF INVERTEBRATED ANIMALS. The endoderni of the pharynx is ciliated, and the cilia are especially large over a narrow tract, or peripharyngeal band, which encircles the oral aperture at the level of the anterior end of the endostyle, and is continued back, as a hypopha- ryngeal band, along the middle of the neural face of the pharynx to the oesophageal opening. On each side of the endostyle, the posterior part of the haemal wall of the pharynx presents two oval apertures or stigmata (Fig. 147, d), encircled by cells, which are provided with very long and active cilia. Each stigma leads into a funnel-shaped atrial canal, the open end of which terminates beside the rectum.1 (Fig. 147, c.) The heart is a large sac, which exhibits rapid peristaltic contractions, and is placed transversely between the two lobes of the stomach. In the species which I observed no blood-corpuscles could be seen, and the direction of the pul- sations of the heart was not reversed at intervals, as it is in the Ascidians in general. M. Fol,2 however, states that, in other Appendicular io?, the reversal of the contractions of the heart takes place. Like myself, he has been unable to dis- cover any blood-corpuscles. There are no distinct vessels, but the colorless fluid which takes the place of blood makes its way through the interspaces between the ectoderm and endoderm and the various viscera. The nervous system consists of a ganglion (Fig. 147, p) situated nearly opposite the anterior end of the endostyle ; in front, this gives off" the nerves to the sides of the mouth, while, behind, it is continued into a long cord (s), which runs back beside the oesophagus, and between the lobes of the stomach, to the base of the appendage. It then passes along one side of the urochord to its extremity, giving off nerves at intervals. At the origins of these nerves as;p;re°;ations of ganglionic cells are situated. (Fig. 147, t.) The most an- terior of these ganglia is the largest.3 reality a longitudinal fold or diverticulum of the middle of the tuemal wall of the pharynx, which projects as a vertical ridge into the hn?mal sinus, hut re- mains in free communication with the pharynx by a cleft upon its neural side." 1 These stigmata were first described by Gegenbaur (" Bemerkungen fiber die Organisation der Appendicularien," Zeitschrift fur wiss. Zoologie, 1855), who supposed that they communicated with canals of the interior of the body. However, by feeding Appendicular ice with indigo, I demonstrated the commu- nication of these stiirmatic funnels with the exterior of the body. (Quarterly Journal of Microscopical Science, I. c.) 2 " Etudes sur les Appendiculaires," 1872. 3 Quarterly Journal of Microscopical Science, 1856, pp. 8, 9. M. Fol, who finds the same arrangement in other Appendicular Ue, counts this as the second ganglion of the nervous system, and states that a fine canal traverses both the ganglia and the longitudinal nerve. APPENDICULARIA FLABELLUM. 513 A rounded octocyst containing a spherical otolith is at- tached to the ganglion, and a small ciliated sac, which opens into the pharynx, is in close relation with it (Fig. 147, r, q). M. Fol describes a number of fine tactile setse situated around the oral aperture. The urochord, which constitutes the axial skeleton of the appendage, is transparent, rounded at each end, and bounded by a delicate membrane. The remains of the cells of which it is composed are to be seen in it, here and there, as ramified corpuscles lodged in its periphery. The only muscles hitherto observed in Appendicularia are two sheets of striped fibres interposed between the uro- chord and the cellular ectoderm of the appendage. The reproductive organs occupy the rounded projection formed by the posterior part of the body behind the digestive canal. The testis (Fig. 147, k) is a large cellular mass which fills the greater part of the cavity of this projection in the adult. When fully formed, it is resolved into spermatozoa with rod-like heads about yoVo °f an ^ncn long and very fine filiform tails. They escape by the dehiscence of the testis. I have never met with Appendicularim containing ova, nor do any other observers, except M. Fol, appear to have been more fortunate. The latter, however, states that these animals are hermaphrodite ( Oikopleura dioica apparently is dioecious), and that the ovary is developed later than the testis.1 Two singular rounded patches of a cellular structure (Fig. 147, II. m) are interposed between the ectoderm and the en- doderm on each side of the anterior end of the endostyle. Similar bodies occur in other Ascidians, but their function is unknown. One of the strangest peculiarities of the Appendicularim is the power which they possess of excreting from the surface of the ectoderm, with extreme rapidity, a mucilaginous cu- ticular investment, in the interior of which, as in a spacious case, the whole body is lodged. This is what was originally described by Mertens as the " house " of the Appendicularia. 1 I must confess that M. Fol's figures and descriptions of the ovary and ova are not satisfactory to me, and his dismissal of the subject of their development in the following paragraph is tantalizing : " Le developpement, que j'ai pu suivre jusqu'a la formation de la larve, ne me parut differer en rien do celui des Ascidies ; et comme d'autre part la peti- tesse de ces oeufe et la difficult^ qu'on a de les obtenir les rendent peu favo- rables a l'etude, je n'ai pas juge a propos d'approfondir davantage ce sujet." (I. c,p.l.) 514 THE ANATOMY OF INVERTEBRATED ANIMALS. It is obviously the homologue of the test of other Ascidians, which is often adherent to the ectoderm by only two or three points; but no cellulose has been discovered in it. Accord- ing to M. Fol, who has studied the formation of the " house " with great care, the Appendicular ice have no proper test, and what I have described as the structureless gelatinous invest- ment of the anterior part of the body is the commencement of the " house." It increases, assumes a peculiar fibrous structure, and in the course of an hour, in a vigorous animal, it is separated as an envelope in which the whole body is capable of free movement. In front, it presents two funnel- shaped apertures supported by a fibrous trellis-work, which lead down to the cavit}T in which the body is contained. A spacious median chamber allows of the free motion of the tail. After a few hours the animal deserts its test and forms an- other. In the great majority of those Tunicata which are fixed in the adult state, the young leave the egg in an active lar- val condition, and resemble Appendicular la in being pro- pelled by a muscular appendage in the axis of which lies an urochord. The body and appendage, however, are invested by a coat, or test, impregnated with cellulose, and the former presents some important structural differences from that of Appendicularia. After a free existence of a certain dura- tion, the body of the larva fixes itself, the appendage withers awajr, and the y6ung animal assumes the ordinary form of a fixed Ascidian. It may remain simple, or it may develop buds and give rise to a compound organism or Ascidiarium, consisting of many Ascidiozooids united together. All the fixed Tunicates present two, more or less closely approximated, apertures : one, oral, leads into the alimentary cavity ; the other, atricd, opens into a chamber, the atrium, into which the faeces and genital products are poured. During life, when these apertures are open, a current sets into the oral and out of the atrial opening. But if the animal is irri- tated, the sudden contraction of the muscular walls of its body causes the water contained in the brachial and atrial cavities to squirt out in two jets, while both apertures are speedily closed. The apertures are much farther apart in some forms than in others, and in certain of the JBotryllidcB they are almost terminal. In the pelagic genera Pyrosoma (Fig. 150), Dolio- lum (Fig. 151), and ScUpa (Fig. 152), the atrial and oral aper- THE TUXICATA. olo tures are at opposite ends of the longest diameter of the body ; and, in the two latter, locomotion is effected by the contraction of transverse muscular bands, which drives the Fig. H8.—Pha!Zusiament>/la.—The test is removed, and harrlly more of the animal represented than would be seen in a longitudinal section: a. oral aperture; b, ganglion; c, circlet of tentacles ; d, branchial s:ic— the three rows of aoertures in its upper part indicate, but do not represent, the stioid. IX. Foetus, the most advanced stage observed. The remains of the conjoined cyathozoQid and ovisac are hiddeu by the circle of ascidiozooids. The letters have the same signification in all the figures, a, test; a3, labial process ; ai. lip of the cloacal aperture; a5, cells of the embryonic test; e. oral aperture ; p1. atrial aperture; i, ecdostyle; l2, I3, branchial sac and siumata; r, heart ; /'2, stolons of the adu!t ascidiarium ; r4, stolons of the embryonic ascicliarium ; », ovi- sac; t, testis ; u, u' ovum ; w\ peduncle of a bud ; x, the alimentary portion of the endoderm entering into a bud; it'1, its generative portion; ce2, Me ectoderm entering into a bud ; ce, the celeoblast ; z, ganglion. I., II.. III., IV., V. Segments of the blastoderm. I. Cyathozociid. IV.-V. Ascidio- zooids. B, mouth of the cyathozoOid. « which develop new ascidiozooids, are given off at intervals ; but, more commonly, the ascidiarium is massive, and the as- cidiozooids retain no permanent connection with one another. In the Botryllidcv, the zooids are arranged in whorls around a common central cavity, or cloaca, into which the atria of all the members of the whorl open. In Pyrosoma, which is a sort of floating Botryllus, the process of budding is highly instructive, as it exemplifies the manner in which gemmation occurs in the Tanicata in general.1 The ascidiarium of Pyrosoma (Fig. 150, I.) has the form of a hollow cylinder, rounded and closed at one end, truncated and open at the other, formed of a firm transparent test, in which the zooids are arranged in whorls. Their oral apertures open on the exterior surface, and their atrial apertures into the interior of the cylinder. The haemal aspect of each zooid is turned toward the closed end of the cylinder. The bran- chial sac has the ordinary structure, and each zooid is provid- ed with a testis and with an ovisac, containing a single ovum. Every zooid multiplies by gemmation from a region of the body which lies immediately behind the extremity of the en- dostyle. Close to the heart, attached to a short ccecal process of the endoderm which constitutes the extremity of the endo- style, and which I have termed the endostylic cone, is a cellu- lar mass — the remains of that mass of indifferent tissue which I have called the generative blastema, and from which the gen- erative organs of the gemmiparous zooid have been developed (Fig. 150, II.). The endostylic cone elongates, and, curving toward the hsemal side of the body, applies itself closely to the ecoderm (Fig. 150, III.). The latter grows out into a conical elevation, which projects into the surrounding substance of the test, and contains a mass of mesoblastic cells, one of which 1 Huxlev, " Anatorav and Development of Pyrosoma.'1'' (" Trans. Liansean Society," i860.) Kowalewsky CI. c, infra, p. 616). THE BUDDIXG AND FISSION OF ASCIDIAXS. 525 (it) has already taken on the character of an ovum, and is surrounded by a rudimental ovisac. The conical bud elongates and dilates at its extremity, and the dilatation gradually takes on the form of a new zooid united by a narrow neck, or pe- duncle, with the parent (Fig. 150, IV.). The endostylic cone gives rise to the whole alimentary canal of the bud, while the ectoderm of the latter proceeds from the ectoderm, and its ovisac and testis from the mesoblastic cells, of the parent. Thus the organs of the bud are all the direct product of the corresponding parts, or of the primitive layers of the germ from which they are derived, in the parent.1 After the terminal bud is formed, a second is usually de- veloped immediately below it (Fig. 150, VI.) by the growth of the ectoderm, endodermal axis, and mesoblastic cells of the peduncle ; and it would appear that this process is frequently repeated. The fully-formed bud becomes detached, and takes its place among the other zooids in the test, there to repeat the process of gemmation. The observations of Krohn, Metschnikoff, and Kowalewsky, have shown that two components enter into the buds of ascid- ians in general ; first, an outer layer consisting of the ecto- derm of the region in which the budding takes place, and, secondly, an inner layer derived from the endoderm of the branchial sac (Perojihora) ; or, as in Botryllus, according to Metschnikoff, from the atrial tunic.2 To these must be added 1 In my second memoir on Pyrosoma ('• Trans. Linn. Society," xxiii., p. 211) I have said: " Gemmation does not take place in Pyrosoma as in so many of the lower animals (e. g., the Hydrozoa and Pohjzoa, or Salpa and Clavelina, among the ascidians ), by the outgrowth of a process of the body-wall, whose primarily whol- ly indifferent parietes become differentiated into the organs of the bud ; but, from the first, several components, derived from as many distinct parts of the parental organism, are distinguishable in it, and each component is the source of certain parts of the new being, and of them only. Thus the body-wall or external tunic of the parent gives rise to the external tunic of the bud : while a process of the endostvlic cone of the parent is converted into the alimentary tract of the bud, and the reproductive organs of the latter are furnished by a part of that tissue whence the reproductive organs of the parent took their origin." As will appear further on. however, recent investigations show that the whole process of budding in the great majority of the Tunicata, and at any rate the first steps of that process in Salpa, are essentially similar to those in Pyro- soma ; and it remains to be seen whether there is anv difference in other As- cidians. And as regards even the ffpdroaoa, the expression that the parietes of a bud are at first " wholly indifferent " in structure is not quite accurate, in- asmuch as they are composed of an ectodermal and an endodermal layer, which are continuous with those of the parent, and give rise to homologous organs. 2 If, as some observations tend to show, the atrial tunic itself is a diver- ticulum of the primitive endoderm, this case would form no exception to the general law of budding in the Tunicata. 526 THE ANATOMY OF INVERTEBRATED ANIMALS. a third component, derived from the indifferent tissue, out of which the reproductive organs of the parent have been de- veloped. In Amouroucium proUfemm, agamic multiplication takes place when the larva has fixed itself and grown into a soli- tary ascidian. The long post-abdomen (as the prolongation of the abdomen beyond the alimentary canal is termed) sepa- rates itself from the body, carrying with it the heart, and divides into a number of segments which rise to the summit of the test of the parent, range themselves around it, and be- come converted into independent zooids. The parent devel- ops a new heart and post-abdomen. The process appears to be repeated in the post-abdomina of the new zooids. The post-abdomen is a process of the ectoderm, the inner cavity of which is divided by a septum into two chambers, contain- ing many fatty cells. The septum itself incloses a cavity, and there appears to be no doubt that it is a prolongation of the pharyngeal sac. When the segments of the post-abdo- men develop, the cavity of the anterior end of the septum dilates and divides, as in Didemnum, into three chambers, of which the median becomes the branchial sac, and the lateral the atrial chambers. The rest remains as the septum of the post-abdomen of the foetus, and its cavity at first communi- cates with the branchial sac, between the endostyle and the oesophageal aperture. Kowalewsky 1 has observed the formation of buds from free cellular masses in the common test of JDidemnum styli- ferum; the origin of these masses is undetermined. They multiply by division, after the rudiments of the alimentary cavnry and of the reproductive organs have made their ap- pearance. The alimentary cavity gives off a process whence the oesophagus, stomach, and intestine are developed, and then becomes divided by longitudinal partitions into three chambers, a median and two lateral. The latter give rise to the lateral chambers of the atrium, which subsequently open into one another on the neural side of the body, and finally communicate with the exterior by a median atrial opening. Gegenbaur2 has described the detachment of the ova of a species of Didemnnm into the substance of the common test, where they are developed into caudate larvae provided with an eye. Before the development of the larva is nearly com- 2 " Ueber die Knospung der Ascidien." (" Archiv fur Mikr. Anatomie," 1874.) i " Ueber Didemnum gelatinosum." (" Archiv fur Anat.," 1862.) THE DEVELOPMENT OF BOTRYLLUS. 527 plete, a zooid is formed from it, so that, at one time, the em- bryo appears to have two branchial sacs. Metschnikoff 1 and Krohn2 have shown that the caudate larvae of Botryllus are not composite, as Savigny and Sars supposed, but that the bodies imagined by these observers to be buds are simply diverticula of the ectoderm, and become converted into the vascular processes, which ramify through the common test, and commonly end in dilatations. In the adult, the buds are developed, one, or sometimes two, at a time, at the sides of the body, and consist of an outer layer, derived from the ectoderm, and an inner layer, which, accord- ing to Metschnikoff, proceeds from the atrial tunic. From the inner layer the alimentary canal of the bud proceeds, and between the inner and the outer layers the rudiments of the genitalia appear. The ovaria advance toward their develop- ment much more rapidly than the testes. The zooids thus developed, as they enlarge, rise to the surface, taking the place of those from which they proceed and which die away. The ova are impregnated from without, and undergo their de- velopment in the atrium of the parent. Subsequently the testes attain their full development ; and, at the same time, the buds are formed which will give rise to a third generation, supplanting the second. After the larva (which may be called A) has attached it- self, the first sets of zooids which are developed are sexless. The first bud arises on the right side of the body of the larva (A) in the neighborhood of the heart ; as it increases in size, the parent withers away, and the zooid (B) thus developed takes its place. Two buds, a right and a left, are developed from (B) and become zooids (C, C), B disappearing. The two zooids (C, C) are so disposed that their atrial ends are close together, and their oral ends turned away from one another. These each develop two lateral buds, which become four zo5ids (D, D, D, D). The zooids (C, C) disappear as before, and their successors arrange themselves in a circle. Each of these develops two, or sometimes three, lateral buds ; these grow into zooids, which supplant their predecessors, and are them- selves, in turn, supplanted. Every new system of the later successions is, at first, de- 1 " Entwickelungsgeschichtliche Beitrage." ("Bulletin do 1' Academic dea Sciences de St.-Pelersbourg," xiii, 18(>8). 2"Ueber die Fortpflanzungsverhaltnisse bei den Botrvllidcn " ("Arohiv fi'ir Natursreschichte," 1869). " Ueber die friihesto Bildung der Botrvllen- itocke'1 {ibid.). 34 528 THE ANATOMY OF IXVERTEBRATED AXIMALS. void of a common cloaca ; and the zooids which compose it may arrange themselves into one or several circles, each of which then acquires its cloaca. It thus appears that, in Botryllus, the ascidiozooid which results from the metamorphosis of the caudate larva serves merely as a kind of stock, from whence the other zooids which build up the ascidiarium proceed ; and this leads to the still more singular process of development in Pyrosoma, in wrhich the first-formed embryo attains only an imperfect development, and disappears after having given rise to four ascidiozooids. In Pyrosoma, the ovisac is attached by a short oviduct to the wTalls of the atrium, into which it eventually opens, and thus allows of the entrance of the spermatozoa. Of the process of yelk-division I could see nothing in my specimen, which wTas preserved in spirit, but it has since been traced, in fresh specimens, by Kowalewsky,1 who compares it to that which takes place in osseous fishes. The result is the formation of an elonaated, flattened blastoderm, which occu- O 7 7 pies one pole of the egg, and is converted into what I termed the cyathozooid, which is showTn by Kowalewsky to be a sort of rudimentary ascidian (Fig. 150, VIII.). From this a pro- longation or stolon is given off, which becomes divided by lat- eral constrictions into four portions, each of which gives rise to a complete ascidiozooid. As these increase in size, they coil themselves round the cyathozooid, with their oral openings outward and their cloacal openings inward, and thus lay the foundation of a new ascidiarium (Fig. 150, VIII.). The cyatho- zooid eventually disappears, and its place is occupied by the central cloacal cavity (Fig. 150, IX.). Thus, in Pyrosoma, the usual first stage of an Ascidian — the caudate larva — is abor- tive, and serves only to found the colony by the buds which are developed from it. In the pelagic genus Doliolum* the cycle of life of the species is represented by distinct sexual and sexless forms. The egg produced by the sexual form (A)3 gives rise to a caudate larva which passes into the first sexless form (B) ; this gives off from the neural side of the body an outgrowth 1 " Ueber die Entwiekeluncrsgesckichte der Pyrosoma." (" Archiv fur Mikr. Anatomie," 1ST". I 2 Tluxlev, " Remarks upon Appenn licvloria and Dolhlvm." (" Phil. Trans.," 1851.) Krohn, " Ueber die Gattung DoMolvm." (" Archiv fur Natur- eeschiclite," 1852.) Geerenbaur, "Ueber die Entwickelung von Doliolum." \ZeiUchrift fur wis*. Zoolrcji<>^ 1853.) 3 Keferstein and Ehlers', "Zoologiscke Beitrage," 1861. THE DOLIOLUM. 529 or stolon, from which buds are developed. These buds are arranged in three rows, two lateral and one median, and grow into zooids of two different forms, of which the median may be indicated by C m, the lateral by C I. All these zooids are detached, and swim about as independent organisms. What becomes of the lateral zooids (C I) is unknown. But the median zooids give off a stolon from the haemal side of the body on which buds are developed, which pass into the sex- ual form (A). The sexual zooid (A) (Fig. 151) is shaped like a cask with an opening at each end ; these are the oral and cloacal aper- tures. According to Keferstein and Ehlers there is no test, the outer wTall of the body being formed, as in most Appe?i- dicidarice, by the ectoderm. Eight muscular bands encircle the body, and by their contractions expel the water from either the oral or the cloacal ends. The body is thus pro- pelled either backward or forward. The branchial sac is much simplified. In Doliohnn Mulleri, the atrial cavity does not extend further forward than the hinder end of the wide pharynx, and this is perforated only by two rows of stigmata, four or five in each. In JDoliolum denticulatum (Fig. 151), on the other hand, the atrial cavity extends forward at the sides of the pharynx, both on the hasmal and the neural side, and the stigmata are numerous and vertically elongated. An opening in the middle line of the haemal face of the Fig. m.—Doliolum denUcvlatum.—a, ganglion: c. endostyle ; d, oral opening; g, oesophagus; L stomach; I, iutestiue; p,p', testis; r, heart; t, t, muscles. pharynx leads, by a short gullet, into a dilated stomach, whence the slender intestine proceeds to terminate in the atrial cavity. The nervous ganglion is situated in the third intermuscular space in D. denticulatum. There is a ciliated sac, but no auditory organ, in the sexual form. The testis 530 THE ANATOMY OF INVERTEBRATED ANIMALS. is a long tube (Fig. 151, />,£>), which lies on one side of the haemal face of the body and opens on a papilla in the atrium. The ovary, small, rounded, and situated close to the hinder end of the testis, contains many ova. According to Kefer- stein and Ehlers, the ova and spermatozoa appear often to become ripe at the same time. The first sexless zooid (B) resembles A in general form, but has nine muscle-rings. The long stolon, which trails in the water, is attached in the seventh intermuscular space to the middle of the neural face 'of the body. The stigmata are arranged as in the form A, of Doliolum Jfttlleri, and one of the anterolateral nerves terminates in an otolithic sac. It is spherical, and contains a single otolith. The zooids produced by the lateral buds of the stolon (C I) have wide oral apertures, and the body is shaped somewhat like the bowl of a spoon. They possess neither auditory or- gans nor genital organs, nor have they been observed to de- velop buds. The median zooids (C m) closely resemble the sexual zooids. The stalk by which each is attached, and the insertion of which is in the middle line of the haemal face in the sixth intermuscular space, remains as a prominence after the animal is set free ; and, from the base of this prominence, buds are developed which take on a sexual form (A). In the SalpoB, the divergence from the ordinary Tunicata reaches its maximum. The oral and atrial openings are situ- ated at opposite extremities of the body, as in Pyrosoma and Doliolum ; and the branchial stigmata are represented by wide vacuities at the sides of the branchial sac, the walls of which are thus represented only by the epipharyngeal folds on the one side, and a narrow trabecula, which occupies the region of the hypopharyngeal band, on the other side. The relatively small alimentary and reproductive viscera are some- times asroregated into a mass — the so-called nucleus — at the posterior end of the haemal side of the body. The chief mus- cular bands, by the contraction of which the water is driven out of the branchial and atrial apertures, and the propulsion of the animal is effected, are transverse, but do not form com- plete hoops, as in Doliolum. In all the Salpce, each species is represented by two sets of zooids, the one sexual and the other sexless. The sexual zooids are produced by budding from a stolon, which is given off from the body of the sexless form in the immediate neigh- borhood of the heart. When the sexual zooids thus formed are detached, they are at first connected into chains of vari- SALPA DEMOCRATICA-MUCRONATA. 531 ous forms, but these eventually break up, and the constituent zooids are set free. Fig. 152 shows the two zooids of the species /Salpa democratica-?nucronata, viz., the sexless zooid, Salpa democratica (Fig. 152, I.), and the free sexual zooid, Salpa mucronata (Fig. 152, IT.). The recent investigations of Dr. Todaro,1 in accordance with those of Kowalewskv. show that the stolon is formed, as in Pyrosoma, by the conjunction of a process of the endo- derm which forms the extremity of the endostyle, with an outgrowth of the ectoderm, and with certain cells of the meso- blast. But, according to Todaro, there is this essential differ- ence : the young Salpa?, which make their appearance in double series along the stolon, are developed altogether from the mesoblastic cells. These cells, in fact, besome aggre- gated into masses, of which four are arranged in the circum- ference of each segment into which the stolon is divided ; and two of these masses, one on each side of each segment, are converted into young Salpa3 by a process analogous to that by which a morula becomes an embryo. If this account of the matter be correct, the agamic development of the Salpce would rather resemble that of the germ masses of the sporo- cysts of Trematoda, or the pseud-ova of insects, than ordinary budding. Each sexual zooid possesses a testis and a single ovum. The latter is contained in an ovarian follicle, the slender duct of which is attached to the wall of the atrium and opens into the atrial cavity. The testis attains its full growth and func- tional perfection only after the ovum has undergone develop- ment. It follows, therefore, that impregnation must be ef- fected by the spermatozoa of some other zooid. The sexless form which is developed from the egg goes through the early stages of its development in the atrial cavity of the parent, to the walls of which it is attached by a peduncle (Fig. 152, III.), the centre of which is occupied by a diverticulum of the vascular canals of the parent, inclosed within a cup-shaped cavity in free communication with the blood-sinuses of the foetus. It is, in fact, a true placenta ; and, during life, the independence of the fcetal and maternal circulations is readily observed, as the blood-corpuscles of the two organisms course through their respective channels. The early stages of the development of the embryo Salpa have been investigated by numerous observers, most recently 1 " Sopra lo Sviluppo e l'Anatomia delle Salpe," 1S75. 532 THE ANATOMY OF INVERTEBRATED AXIMALS. by Kowalewsky,1 Todaro, Brooks,2 and Salensky.3 The ob- servations of the last-named author relate chiefly to Salpa Fig. 152.— Salpa democratica-mucronata. — I. Salpa democratica. II. Salpa mucro- nata. III. A foetal Salpa democratica attached by its placenta to the wall of the atrial cavity of a Salpa mucronata. IV. Part of the stolon of Sulpa democratica with attached Salpa mucronata buds. The letters have the same signification throughout : «, oral ; b, atrial orifices : c, endostyle; d, ganglion ; e, liypopharyngeal band in a so-cailed " branchia ; "/» languet ; g, heart ; h, gemmiparous stolon ; i, visceral mass or nucleus ; A\ mus- cular bands ; m, placenta ; n, blood -sinus ; g, ovisac and ovum ; t, stomach ; w, ciliated sac ; ce, oaleoblast ; a, ectoderm and test ; j8, endoclerm. democratica-mucronata, and his account of the process ap- pears to me to be the most satisfactory. 1 " Naehrichten der Koniglichen Gesellschaft zu Gottingen," 1868. 2 "Bulletin of the Museum of Comparative Zoology," No. 14. 8 Zeitschriftfiir wiss. Zooloffie, 1876. THE DEVELOPMENT OF THE SALPJE. 533 The egg is impregnated in the ovarian follicle, as in Pyro- soma ; and the oviduct, shortening, gradually draws the ova- rian follicle, with its contents, into a sort of incubatory pouch, which is a diverticulum of the wall of the atrium, and pro- jects into the atrial cavity. For distinction's sake the incubatory pouch may be termed the ovicyst. As the oviduct shortens, it widens, and consti- tutes, together with the ovarian follicle, a single uterine sac, the outer or oviducal half of which applies itself to the wall of the ovicyst, while the inner half contains the ovum. The vitellus undergoes complete division, and the superficial layer of blastomeres constitutes itself into an epiblast, investing the solid mass formed by the other blastomeres, which repre- sent the hypoblast. A mesoblastic layer subsequently ap- pears between the two. The nervous ganglion results from an involution of the epiblast, while the branchial sac, the alimentary canal, and the asrium, are the product of the sub- division of a cavity which appears in the midst of the hypo- blast. The maternal and the fcetal parts of the placenta arise, respectively, from the wall of the ovarian sac, and from certain large blastomeres on the adjacent haemal face of the embryo. Todaro agrees with other observers in stating that the vitellus undergoes division, and that a small celled blastoderm invests the large remaining cells, which he terms the germinal mass. But his account of the further stages Of development is very different. A circular thickening of the blastoderm separates the hemisphere which is directed outward from that which is turned inward, and gives rise to a lamellar outgrowth. It is, at first, directed toward the inner end of the ovisac, having reached the bottom of which, it becomes reflected ; and the reflected portions lining the inner wall of the ovisac, and meeting over the outer hemisphere, form a sort of am- niotic investment of the embryo. It is the cavity left be- tween this " amnion and the inner hemisphere of the blasto- derm, which becomes the parental blood-sinus. An involu- tion of the outer hemisphere of the blastoderm gives rise to the alimentary canal, which becomes shut off, as the endoderm, from the remaining blastoderm, which constitutes the ecto- derm. A mass of cells which appears in the middle of the outer half of the embryo, between the alimentary sac and the ectoderm, and which has only a transitory existence, is re- garde 1 by Todaro as the representative of the urochorcL CHAPTER XI. THE PERIPATIDEA, THE MYZOSTOMATA, THE EXTEROPXETJSTA, THE CHJ3TOGXATHA, THE XEMATOIDEA, THE PHYSEMARIA, THE ACAXTHOCEPHALA, AXD THE DICYEMIDA. J have reserved for discussion in this chapter the Peripa- tidea, which have heretofore been referred by most authors to the Annelida / and certain groups of the lower Metazoa, the precise morphological relations of which are as yet uncertain, although it is pretty clear that several of them are allied with the lower Annelida, the Hot if era, and the Turbellaria. They are, for the most part, totally devoid of segmentation ; while the Chcetognatha and the Myzostomata alone present any structures resembling limbs, though the nature of these is doubtful. So far as the nervous system is clearly made out, it exhibits no such chain of post-oral ganglia as characterizes the higher worms. The Peripatidea. — At p. 225, I have referred this group to the Arthropoda, Mr. Moseley's memoir on Peripatns l hav- ing left no doubt upon my mind, that he had satisfactorily proved the justice of the surmise respecting its affinities originally made by Gervais. It is only recently, however, that I have been able, thanks to Mr. Moseley, to examine one or two specimens of Peripatus JVovw Zelaniw, and to satisfy myself of the main point, namely, the existence of the tra- cheal system which he has described. Of the genus Peripatus several species are now known, from the West Indies, South America, the Cape of Good Hope, and New Zealand, where they are found among the de- caying wood of damp and warm localities. They have the 1 " Philosophical Transactions," 1874. See, also, the valuable memoir of Grube, " Ueber den Bau von Peripatus EdwardsiV (" Archiv fur Anatomie," 1853;. THE PERIPATIDEA. 535 curious habit of throwing out a web of viscid filaments when handled or otherwise irritated. The head is distinct, and is provided with a pair of many- jointed antenna-like tentacula and two simple eyes. The mouth, situated upon the under surface of the head, is sur- rounded by a prominent lip, which incloses a pair of jaws, each of which is terminated by two curved chitinous claws, similar to those of the feet. On each side of the mouth, the head supports a short obscurely-jointed "oral papilla," which is somewhat like one of the feet, but is devoid of claws and perforated at its extremity. The head is followed by an un- segmented body produced laterally into paired appendages, which vary in number from fourteen to more than thirty, ac- cording to the species ; and each of these appendages is in- distinctly articulated, the terminal joint being provided with two small curved claws. The anus is terminal, and the genital aperture is situated on a papilla?, a little distance in front of the anus, on the neural or ventral face of the body. The alimentary canal commences by an ovoid muscular pharynx. The oesophagus, continued from this, gradually dilates into a wide and long stomach, from which a very short intestine is continued to the anus, situated at the posterior end of the body. There are no Malpighian ca?ca. Two very large ramified tubular glands, which secrete the viscid matter of which the web is composed, lie at the sides of the alimen- tary canal, and open outward by the perforations of the oral papillae. A vessel occupies the middle line of the dorsal body-wall, and is probably a heart. The respiratory organs are the trachea? discovered by Mr. Moseley. The numerous pores, or stigmata, from which the trachea? take their origin, are scattered all over the surface of the body, one row being median and ventral. Each stigma is the outward termination of a short, wide tube, which, at its opposite end, branches out into a pencil of fine trachea?, which rarely divide, and are distributed in great abundance to the viscera. They are very delicate tubes, which often take an undulating course, and are rarely more than ^-^j of an inch in diameter. In optical section, their walls have a finely-beaded appearance, as if from the presence of trans- verse thickenings, though distinct transverse markings are rarely to be seen. The nervous system, as Milne-Edwards discovered, con- sist^ of two ganglia in the head, closely united above the 536 THE ANATOMY OF INVERTEBKATED ANIMALS. oesophagus. From each of these a relatively stout longitudi- nal cord proceeds, overlying the bases of the feet (and hence widely separated from its fellow), to the posterior extremity of the body. As Grube has stated, there are no distinct ganglia on this cord. On the contrary, ganglionic cells ap- pear to be pretty evenly distributed along its ventral face, throughout its length ; and nerves, which pass transversely outward and inward, are given off from opposite sides of it at short intervals. Grube has shown that many of the branches that take the latter direction are commissures be- tween the two cords. The muscles of Peripatus are not striated, which is a curious exception to its generally well-marked arthropod char- acteristics. Mr. Moseley has proved that the sexes are distinct. The ovary is small, divided by a median septum into two lobes, and lies beneath the alimentary canal. The oviduct, at first single, divides into two branches, which are long, and, pos- teriorly, present uterine dilatations. They then unite, and ter- minate by a short vagina on the ventral aspect of the rectum. The testes are ovate bodies, each with a cascal appendage. The long and coiled vasa deferentia unite into a common duct, which opens in the same position as in the female. The ova are developed within the uterine dilatations of the ovi- ducts.1 Mr. Moseley has made out the chief points in the develop- mental history of Peripatus. In an early condition, the emb^o is very like that of a Scorpion, but is folded upon itself, so that the ventral aspects of the anterior and posterior halves of the body are turned toward one another. As in the Scorpion, there is a pair of large procephalic lobes, succeeded by a series of segments, from the sides of which, processes — the rudiments of the limbs — bud out. The procephalic lobes give rise to a kind of hood, the lateral angles of which extend over the bases of the first pair of limbs, and join with those of the second pair, which are the oral papillae of the adult. The first pair of limbs thus become inclosed within the hood (the margins of which form the suctorial lip of the adult), and developing two chitinous claws upon their extremities, like those of the 1 One of the specimens which I examined was a pregnant female, but the viscera were glued together, apparently by the action of the spirit in which it had been preserved, in such a manner, that little could be made of their struc- ture or of that of the embryos. THE MYZOSTOMATA. 537 other limbs, they are converted into the jaws of the adult animal. It is remarkable that the antennae are developed from the anterior part of the procephalic lobes ; while the chelicerae of the Scorpion appear at the posterior margin of these lobes, in a position corresponding with that of the first pair of limbs, or jaws, of Peripatus. It is obvious that whether we consider the appendages, the respiratory and reproductive systems, or the development of the embryo, Peripatus is a true Arthropod, apparently nearly allied to the suctorial Myriapoda. The Myzostomata. — The genus Jfyzostomiim l compre- hends certain small animals, the largest species not exceeding one-fifth of an inch in length, which are parasitic upon the Feather-stars. The body has the form of a flattened oval disk, the surface of which is ciliated, while its margins may be produced into as many as twenty short filamentous processes or cirri. Within the margin of the ventral face are eight suckers, four on each side, and, internal to these again, are ten short conical " feet," five on each side ; each of these lodges two strong seta?, which can be protracted and re- tracted in the same way as those of Annelids. Just within the middle of the anterior margin lies a rounded aperture, through which a muscular proboscis, the free end of which is beset with papilla?, can be protruded. A straight alimentary canal runs through the body, and terminates in a sort of cloaca, which opens in the middle line on the posterior mar- gin. From each side of the alimentary canal long ramified caeca are given off. No vessels or organs of circulation have been discovered. All that is known of the nervous system is an elongated gan- glionic mass, from which branches are given off on each side, situated in the middle line of the ventral face of the body. The sexes are combined in the same individual. The acini of the generative glands are scattered through the body. Those of the testes pour their contents into ducts, which unite together and open by a separate vas deferens on each side of the body, about the middle of its ventral face. The two oviducts convey the ova to the cloacal chamber. The development of Myzostomum has been worked out by Semper and by Metschnikoff.2 The vitellus undergoes 1 See Loven, " Arcliiv fur Naturgescliichte," 1842. 2 Semper, " Zur Anatomie unci Entwiekekmgsgeschichte der Gattung My- zostomum." {Zeitschrift fur wiss. Zoologie, 1875.) "Zur Entwiekelungsge- schichte von Myzostomum." (Ibid., 180(3.) 538 THE ANATOMY OF INVERTEBRATED ANIMALS. complete division, and the embryo leaves the egg* as an oval morula, covered with vibratile cilia. In the next stage ob- served, the embryo is cylindroidal, and is provided with a mouth at one end and an anus at the other. The commence- ment of the straight and simple alimentary canal has the form of a muscular bulb or proboscis. There are two pairs of rudimentary appendages, each containing two seta?. The number of the setigerous appendages increases up to five pairs, and the intestine begins to show indications of diver- ticula ; but, in the latest stage observed, the cirri had not made their appearance, and the body was still comparatively narrow. MetschnikoiT regards Myzostomum as a parasitic form of a polychaetous Annelid ; and there is much to be said in favor of this suggestion ; though, in some respects, it rather approaches the Hirudinea. The presence of cilia on the surface of the body and of protractile setae in the parapodia excludes Myzostomum from the Arthropoda y while Metschnikoff has justly com- pared its larval state with that of Syllis. Sufficient doubt, however, still adheres to the determination of the true place of Myzostomum, to lead me to discuss it apart from the An- nelids. The Enteeopneusta. — The very singular animal Balano- glossus, which is the only known example of this group, is an elongated, apodal, soft-bodied worm, with the mouth at one end of the body and the anus at the other (Fig. 153, III.). The mouth is surrounded by a sort of collar, or promi- nent lip, within the margin of which springs a long probos- cidiform median appendage, which is hollow within and has a terminal pore. On the same side as that from which the pro- boscis springs, the anterior region of the body presents an elongated, somewdiat flattened area, bounded by raised longi- tudinal folds. On. each side of this area is a longitudinal se- ries of apertures — the branchial apertures. The latter com- municate with saccular dilatations of the anterior part of the alimentary canal, and those branchial sacs are supported by a peculiar skeleton. No nervous svstem, nor any organs of sense, have yet been certainly made out. According to Kowalewsky,1 who was the first to elucidate 1 " Anatomie des Balanoglossus." (" Mem. de l'Acad. Imp. de St. -Peters- burg," 1866.) THE DEVELOPMENT OF BALAXOGLOSSUS. 539 the true nature of Balanoglossus, the vascular system con- sists of a dorsal and a ventral vessel. At the posterior end of the branchial region the former divides into a superior and an inferior dorsal, and two lateral, trunks. The superior trunk passes forward, and, at the anterior end of the body, divides into two descending branches, which unite with the ventral trunk. The inferior dorsal trunk supplies the bran- chiae, of which the lateral trunks are the efferent vessels. For the pharyngeal branchiae of Balanoglossus, the only parallels to be found are among the Tunicata and the Verie- brata. On the other hand, the larval form of this anomalous creature is generally Annelidan or Turbellarian, with very close and special resemblances to the Echinopaedia of some Ech inodermata. The young of Balanoglo&sus was first observed by Miiller, who called it Tornaria, and regarded it (as did all succeeding observers until its true nature was discovered) as an Echino- derm-larva, on account of its extraordinary resemblance to the larvae of Star-fishes (Fig. 153, I.). 1 9"VS0- ~&ir\i Fig. 153.— Balanoglossus. (After A. Agassiz.) I. The Torn-aria larva, side-view (about TV of an inch Ion?): a. anus; 6, vessels leading to the dorsal pore (d) from w. the sac of the water-vascular system ; w\ prolongation of the sac; A, heart; I, intestine; s, stomach; 0, oesophagus; m, mouth; v, u\ lobes of the alimentary canal; mb, muscular band running from the eye-speck (e) to the water-vascular sac. II. A young Balanoglossus— letters as before, except g, the first formed branchial stigmata. III. A more advanced Balanog ossus ; e, the collar; p, the proboscis. It is an elongated ovoid body, provided with three bands of cilia, one of which is prae-oral, while the other two are 540 THE ANATOMY OF IXVERTEBRATED ANIMALS. post-oral. Of the latter, one, at the posterior end, is circular, while the other is inclined obliquely to the axis of the body, so that anteriorly and superiorly it reaches the anterior ex- tremity, while posteriorly it occupies nearly the middle of the body. On the ventral face a deep groove separates it from the prse-oral ciliated band, and in this groove the mouth is situated. The margins of the prse-oral and post-oral ciliated bands are deeply sinuated, and they come into contact in the median dorsal line. A wide gullet leads from the mouth, and opens into the gastro-intestinal portion of the alimentary canal, which passes backward in the middle line to terminate in the anus, at the hinder end of the body. About the middle of the dorsal face of the body there is a circular pore (Fig. 153, I. d), whence a canal leads to a rounded sac which lies on the junction between the gullet and the stomach. The sac gives off two lateral short diverticula, which embrace the oesophagus. A delicate band, apparently of a muscular na- ture, connects the summit of the wTater-sac with that part of the dorsal aspect of the body at which the prae-oral and post- oral ciliated bands unite. Here two eye-spots are developed. A constriction separates a rounded gastric from a tubular in- testinal division of the alimentary canal. Diverticula of the gastro-intestinal part of the alimentary canal give rise to two pairs of discoidal bodies, from which, apparently, the meso- blast and the perivisceral cavity of the JBalanoglossus are de- veloped. From the sides of the oesophagus a series of diverticula are given off, which unite with the ectoderm, open external^, and become the gill-pouches. When only two of these bran- chial apertures are formed, they are said by Metschnikoff to have a striking resemblance to those of Appendicular 'ia. A pulsating vesicle — the so-called " heart " — makes its appear- ance close to the water-sac. The anterior end of the body, in front of the mouth, now elongates, and is converted into the proboscis ; while the post-oral region loses its ciliated bands, and, lengthening, becomes the long body of the adult worm.1 (Fig. 153, II., III.) The Cii^etognatha. — The genus Sagitta? which is the 1 See Agassiz, " The History of Balanoglosfiiis and Tornarla " (" Memoirs of the American Academy of Arts and Sciences," 1873) ; and Metschnikoff, " Untersuehungen fiber die Metamorphose einiger Seethiere." (Zcitschrift fur taiss. Zoologie, xx..lS7A). 2 See Busk, Quarterly Journal of Microscopical Science, 1856. Leuckart and Pagenstecher, " Archiv iiir Anatomie," 1858. THE CH^ETOGXATHA. 541 only member of this group, comprises several species of small animals which are found swimming at the surface of the ocean in all parts of the world. Although the whole structure and course of development of Sagitta are now very well known, its true affinities are not definitely settled. Anatomically, it approaches the Nematoid worms and the oligochagtous Anne- lids in some respects ; but its development presents pecul- iarities which are as yet unknown among these animals, while they occur among the JSrachiopoda and the Echinodermata. The body of Sagitta (Fig. 154), rarely more than an inch long, is elongated, subcylindrical, and unsegmented ; it is en- larged at one end into a rounded head, while at the other it tapers to a point. There are no parapodial appendages, but the chitinous cuticle is produced into a finely-striated lateral fin on each side of the body and tail, and into delicate seta?. On each side of the head there are a number of strong, curved, claw-like, chitinous processes, which can be laterally divari- cated and approximated, and serve as jaws. Between them is the mouth ; and at the sides of the mouth are four sets of short but strong spines. The mouth leads into a simple and straight intestine, which opens by an anus situated on the ventral face of the body, where the tapering caudal region commences. A dorsal and a ventral mesenteric band connect the intestine with the wall of the body, and divide the peri- visceral cavity into two chambers. Beneath the ectoderm lies a layer of longitudinal, striated, muscular fibres. The nervous system consists of a large oval ganglion, which lies in the middle of the ventral wall of the body, and sends off anteriorly twTo commissural cords, which unite with a supra- cesophageal ganglion. Among other branches, this gives off two to the dorsal side of the head ; these dilate at their ex- tremities into spheroidal ganglia on wThich the eyes rest. The ovaries are elongated tubular organs, which lie one on each side of the intestine, attached to the parietes of the body. Their ciliated ducts open close to the vent and are provided with dilatations which serve as receptacula seminis. Behind the anus the mesenteric laminae unite and form a vertical partition, which divides the cavity of the caudal part of the body into two chambers. On the lateral walls of these, cellu- lar masses are developed, which become detached, and, float- ing freely in the perivisceral fluid, are developed into sper- matozoa. The latter escape by spout-like lateral ducts, the dilated bases of which may be regarded as vesiculag semi- nales. 542 THE ANATOMY OF INVERTEBRATED ANIMALS. Thus far, although the organization of Sagitta is very peculiar, it presents analogies both with the JSfematoidea and with the Annelida. But its development, as described by y —--»«■• fZ Fig. 154.— Sagitta Mpunctata.—a, the head, with its eyes and appendages ; J, the anus ; c, the ovary ; d, the testicular chambers. Kowalewsky,1 is, in some respects, unlike anything at pres- ent known in either of these groups. Yelk-division takes place as usual, and converts the eggs into a vesicular morula, 18T1 1 " Memoires de l'Academie Imperiale des Sciences de St.-Petersbourg," THE DEVELOPMENT OF SAGITTA. 543 with a large cleavage cavity, or blastocoele. One face of the vesicle thus constituted now becomes invaginated, with the effect of gradually obliterating the blastocoele, and converting the spherical single-walled sac into a hemispherical, double- walled, cup-shaped gastrula. The cavity of the cup is the future digestive cavity ; the layer of invaginated blasto- dermic cells which lines this cavity is the hypoblast, which will become the endoderm ; and the outer layer of cells is the epiblast, and will become the ectoderm. In this condi- tion the embryo resembles that of the Leech in its early state. The embryo elongates, and the aperture of invagi- nation, or blastopore, eventually ceases to be discernible. Whether it becomes the anus, or whether the anal aperture is formed anew, is not certain. The nervous ganglia result from the modification of cells of the ectoderm. The anterior end of the primitive alimentary cavity, or archenteron, is at first closed. It soon sends out an enlargement on each side, so that the archenteron is divided into a central and two lat- eral divisions. The central division opens externally and an- teriorly by the development of the oral aperture ; and, as the body elongates, it becomes the tubular intestine. The lat- eral diverticula at first communicate with it, but they are eventually shut off, and constitute the right and left perivis- ceral cavities, their walls becoming converted into the cellu- lar and muscular lining of those cavities. It results, from the mode of development of the perivisceral cavity of Sagitta, that this cavity, like the perivisceral cavity of the Brachio- pods, and the " peritoneal " cavity of the Echinoderms, is an enteroccele, comparable to that of the Hydrozoa and Actino- zoa : but which, instead of remaining: in communication with the alimentary cavity, is shut off from it, its wall becoming the mesoderm, and its cavity the perivisceral cavity.1 Nothing of this kind is known to occur in the Tnrbellaria, Annelida, Nematoidea, or Hot if era ; but when a perivisceral cavity exists in these animals, it appears always to result from 1 Kowalewsky's account of the development ofSagUta has been confirmed by Butsehli,* who has further determined the origin of the reproductive or- gans, which arise as outgrowths from the hypoblast ; and the division of each primitive enteroc, unicellular cutaneous glands at the anal extremity ; D'. glandular mass, with its excretory duct above the gizzard ; ov, ovaiium ; T', testis ; S, seminal corpuscles. 1 " Ueber einige in Humus lebende Anguilluliuen." (Zeitschrift fi'ir wiss. Zoologiey xii.) 546 THE ANATOMY OF INVERTEBRATED ANIMALS. The outermost layer of the body is a dense chitinous cuticula, usually divisible into several layers. These lavers may be fibrillated, the direction of the fibrillation being dif- ferent in the successive layers. Cilia are found neither on the surface, nor elsewhere, at any period of life. The mouth is situated at one extremity of the body, the anus at, or near, the other end. The first portion of the alimentary canal is a thick- walled pharynx, lined by a continuation of the chiti- nous layer of the integument, which may be raised up into ridges or tooth-like prominences. Transverse fibres, appar- ently of a muscular nature, radiate from the lining of the pharynx through its thick wall, and probably serve to dilate its cavity. A straight and simple tubular alimentary canal, without any distinction into stomach and intestine, extends through the axis of the body, a narrow oesophageal portion usually connecting it with the pharynx. The endoderm, or wall of the alimentary canal, consists of a single layer of cells, disposed in few or many longitu- dinal series ; and lined, both internally and externally, by a cuticular layer. On each side, the intestine is fixed through its whole length to the " lateral area," to be described below. The cuticle, which lines the inner faces of the endodermal cells, and circumscribes the digestive cavity, appears, on verti- cal section, to be divided into rods, which are possibly merely the intervals of minute vertical pores. In some cases, muscu- lar fibres invest the posterior portion of the intestine. Beneath the layers of the chitinous cuticle there is a proper integument, or ectoderm, internal to which again is a single layer of longitudinally-disposed muscles, which may or may not be divided into distinct series of "muscle-cells." The space between these and the outer face of the intestine is occupied by a spongy or fibrous substance, which must probably be regarded as a kind of connective tissue. The muscles and this tissue, taken together, constitute the meso- derm. In the typical JVematoidea, the muscular layer does not form a complete investment of the bod}^, but is interrupted along four equidistant longitudinal lines. One of them is termed dorsal, the opposite ventral, and both these are very narrow. The other two are much broader, and are termed the lateral areas. They often (Fig. 156) present two or more series of conspicuous nuclei, and each is traversed by a canal with well-defined contractile walls and clear contents. Op- posite the junction of the oesophageal with the gastric por- THE NEMATOIDEA. 547 tion of the alimentary canal, each of these lateral canals passes inward and toward the mid-Ventral line, and, joining with its fellow, opens by a pore on the exterior. In some cases, con- tinuations of the lateral canals extend forward into the head, A ring of fibres and nerve-cells surrounds the gullet, about Fig. 156. — Oxyuris.—a, mouth; b, pharynx; c, commencement of intestine, and d, its termination. The intermediate portion is not figured, e, genital aperture ; /, opening of vessels ; g, their receptacle; h, one of the vessels ; i, cellular matter enveloping them. A portion of one of the contractile vessels is represented more highly magnified in the upper figure. the level of the opening of the water-vascular system, and gives off filaments forward to the head, and backward to the muscles and to the lateral area ; while two coids pass back, along the dorsal and ventral median lines, to the hinder end of the body. In the males of some species, nervous ganglia have been observed in the neighborhood of the sac of the spicula.1 Organs of sense are not certainly known to exist, unless the pigmented spots on the nervous ring of some free Nematoids have this character. The JVematoidea are for the most part dioecious. In the females, the reproductive aperture is usually placed toward the centre of the body ; in the males, it is always situated at or near the posterior extremity. The female apparatus (Fig. 155, ITI.) consists of a vagina, with which is connected a single, or double, elongated, tubu* la'r, organ, which tapers to a point at its blind extremity, and is at once ovarium, oviduct, and uterus. The cgecal end is 1 The question of the structure and disposition of the nervous system in the Ntmatoidea is, perhaps, not even yet completely decided ; but there is much evidence in favor oi what is here stated. See Leuckart, "Die menschliehen Parasiten;" the monograph of Schneider, cited below ; and especially Butschli, "Beitragc zur Kenntmsa des Nervensystems der Nematoden" ("Archivfur Mikr. Anatomic," 1873). 548 TBE ANATOMY OF IXVERTEBRATED ANIMALS. occupied by a nucleated protoplasmic mass. Further on, this mass becomes differentiated into an axile cord of protoplasmic substance — the rhachis — and peripheral masses, each contain- ing a nucleus and connected by a stalk with the rhachis, which are the developing ova. Still further on, in the ovi- ducal portion of the tube, the ova become free ; while, in the uterine portion, they are impregnated, and acquire a hard, often ornamented, shell. The testis is, generally, a single ca?cal tube, in the blind end of which cells are developed, much in the same way as in the ovary : they become free in that part of the tube which plays the part of a vas deferens. Contrary to what happens in most animals, these spermatozoa retain the character of cells, and may even exhibit amoeboid movements. The defer- ential end of the testicular tube opens into a sac close to the anus, from the dorsal wall of which one or two curved chiti- nous spicula are developed. These are introduced into the vulva of the female when copulation takes place, and appear to distend it, in order to allow of the free passage of the sem- inal corpuscles into the vagina, and thence into the uterus. In the female organs, the seminal cells undergo further changes, and eventually enter into, and coalesce with, the substance of the ova. Yelk-division follows impregnation. The oval morula be- comes indented on one side, and the embryo, as it grows, folds itself in accordance with this indentation. In most, it would appear that the central cells of the solid morula are differentiated from the rest to form the endoderm, which thus arises by delamination. But Blitschli ' has recently shown that the morula, which results from the division of the vitellus of Cucullanus elegaas, has the form of a flattened plate, com- posed of two layers of blastomeres, the blastoccele being re- duced to a mere fissure. The lamellar blastoderm next be- comes concave on one side, convex on the other, and passes into the gastrula form. The blastopore, at first very wide, gradually narrows, and appears to be converted into the oral opening of the worm. The mesoblast takes its origin from certain cells of the hypoblast, which lie close to the mouth, and grow thence toward the caudal extremity. The resem- blance of this developmental process to that of Lumbricus is obvious. J " Zur Entwickolunofsoresdiichte des Cucullanus ehganx." (Zeitschrift fur wiss. Zooloqie^ 1876.) Hallez (" Kevue des Sciences Naturelles," 1877) has ob- served a similar process in Anguillula aceti, but he denies that the blastopore becomes the mouth. THE DEVELOPMENT OF THE NEMATOIDEA. 549 The female reproductive apparatus is, at first, represented by a solid cellular body which lies in the mesoderm ; though whether it originally belongs to this, or to the ectoderm, or to the endoderm, is not clear. The cellular body acquires a tubular form, and eventually opens externally by uniting with an inward process of the ectoderm, which gives rise to the vagina. The young cast their cuticle twice — first, when they leave the eggf and, again," when they acquire their sexual organs. The JVematoidea have been divided into three principal groups 1 — Poly my aria, Meromyaria, and Holomyaria — char- acterized by the nature of their muscular system. In the jPolymyaria, the muscles of the parietes of the body are divided into many series, each made up of many " muscle-cells." In the Meromyaria there are only eight longitudinal series of such muscle-cells, two between each lateral area and the dorsal and ventral lines respectively. In the Holomyaria the muscles are not divided into series of muscle-cells. The first two divisions contain only such genera as an- swer to the general description just given ; but, in the JTolo- myariay there are included several aberrant forms. Thus, Trichocephalus has no lateral areas ; Ichthyojiema has no anus ; 3Iermis has no anus, and the alimentary canal is rudi- mentary, though it possesses the lateral areas, and the males have spicula. Gordius has no lateral areas, and only the ventral line ; the alimentary canal is reduced to a rudiment, without either oral or anal aperture, and the male has no spicula. In both these genera the anterior ends of the em- bryos are provided with spines', which aid them to bore their way into the bodies of the insects on which they are para- sitic. In SphcerulaHa the alimentary canal is similarly rudi- mentary, and Sir John Lubbock discovered that the small male becomes permanently adherent to the female. Some Nematoidea (e. g., Leptodera, Pelodera) live in water or damp earth, and are never actually parasitic; but the}* require abundant nitrogenous food in order to develop their sexual organs, and hence they are found in the sexual • * Schneider, " Monographic cler Neraatoden," 1866. See also Bastian, "Monograph of the Anguillulidae " (" Trans. Linnsean Societv," 1865^ ; and, " On the Anatomy and Physiology of the Nematoids " (" Phil. Trans.," 1866) ; and several memoirs by Bntschli. The latter affirms that the muscles are as much made up of muscle-cells in the Holorayarin^ as in the rest. (" Giebt es Holomyarier ? " Zeitschrift \fvr wiss. Zoologie, 1873.) 550 THE ANATOMY OF INVERTEBRATED AXIMALS. state only among putrefying vegetable or animal matters. The sexless worms, which live in moist earth, are at once at- tracted hy nutriment, such as a few drops of milk.1 Here they multiply with great rapidity as long as the store of food lasts ; but, when it is exhausted, the last-hatched young wander away. In the course of their wanderings, the em- bryos enter into their larval condition ; but, before doing so, they become twice as large as those which attain the larval state in putrefj'ing substances. The embryonic cuticle be- comes thickened, and its oral and anal apertures closed, so that it forms a cyst for the larva. The larva, however, is not restrained by this cyst from moving about and continuing its wanderings, though, at length, it passes into a quiescent con- dition. Its inner substance, at the same time, becomes dark by transmitted light, in consequence of the accumulation of small fatty granules ; and, if this state of things lasts long, the larva dies. If the larvae should dry up, the circumstance tends to their preservation. The embryonic cuticle is sepa- rated, and forms a protective cyst ; and, when moistened, the larvae resume their vital activity. Nematoid worms belonging to naturally free and nonpara- sitic genera may enter, and become encysted in, worms and slugs ; but they only attain their sexual state when their host dies, and they are nourished by the products of its putre- faction. Anguilhda scandens, the Nematoid which infests and gives rise to a diseased condition of the ears of wheat, is a true parasite. The young are hatched from the eggs laid by the parent in the infected ear, and there become encysted. When the wheat dies down, the larvae are set free, and wander on the moist earth, until they meet with young wheat plants, up which they creep, and lodge themselves in the developing ears. Here they acquire the sexual condition, nourishing themselves at the expense of the inflorescence, which becomes modified into a kind of gall. Most Nematoids found in the alimentary canal of animals are parasitic in the sexual state, but have a longer or shorter period of freedom as larvae or as eggs. But some, as Cucal- lanus elegans, are parasitic both in the sexless and the sexual condition ; inhabiting Cyclops, while in the former state, and sundry fresh-water fishes, particularly the Perch, in the latter. Trichina sj)iralis a acquires its sexual state in the alimen- » Schneider, I c, pp. 302-' 3. 2 Leuckart, ' ' Untersucnungen liber Trichina spiralis" 1866. THE XEMATOIDEA. 551 tary canal of Man, of the Pig, and other mammals ; but the young, set free in the alimentary canal, bore their way through its walls, and enter the fibres of the voluntary muscles, in which they become encysted in the sexless state. If the flesh thus trichinized be eaten, the Trichince are set free, acquire their sexual state in the alimentary canal, and the thousands of embryos which are developed immediately bore their way into the extra-alimentary tissues of their host. The insect parasites, Gordius and Jfermts, are sexless so long as they are parasitic ; but, when they have attained their full growth, they leave the body of their host, acquire sexual organs, copulate, and lay eggs. From these, embryos proceed, which bore their way into the bodies of insects. It has been stated that the Nematoidea are, for the most part, dioecious. Schneider has, however, discovered certain species of the nonparasitic genera, Leptodera and Pelodera, which always have the external appearance of females, but in the ovarian tubes of which spermatozoa are developed, and impregnation takes place. This was placed beyond doubt by isolating embryos of these Nematoids, and tracing out the development of the spermatozoa, which result from the sub- division of the first cells developed from the rhachis. After a time, the development of spermatozoa ceases, and the cells separated from the rhachis become ova, which are impregnated by the already formed spermatozoa. These Nematoidea are probably the most complete and necessary hermaphrodites known in the animal kingdom. Ascarls nigrovenosa is parasitic in the lungs of Frogs and Toads, and attains a length of three-quarters of an inch. It has the characters of a female, and no male has ever been met with, but spermatozoa are developed in the ovaries in the same manner as in the preceding form?. The esfars of this Ascarls are discharged, and the embryos find their way into the intestines of the Amphibian in which they are parasitic. Here they become males and females which are very much smaller than the hermaphrodite form (not exceeding one-twentieth of an inch in length), and other- wise different from it. They are evacuated with the fa?ces of the frog, and passing into damp earth or mud, the females give rise to a few eggs. Embryos are developed from these eggs within the body of the mother, the organs of which they destroy, until her cuticle forms a mere case for them. The free embryo?, introduced into the Frog's mouth, pass into the lungs, and take on the characters of the large hermaphrodite 552 THE ANATOMY OF IXVERTEBRATED ANIMALS. forms. It is not unlikely that the Guinea worm (Filaria medinensis), which infests the integument of Man in hot cli- mates, may answer to the hermaphrodite stage of a similarly dimorphous Nematoid, though its multiplication has hitherto been supposed to take place agamogenetically. The many points of resemblance between the Nematoi- dea, the Oligochceta, and the Polychceta, have been indicated by Schneider. They differ, however, from these no less than from the Turbellaria and Potifera, in possessing only longi- tudinal parietal muscles. In this respect they agree with Phamphogordius and Polygordius (united by Schneider into the group of Gymnotoma),1 which are segmented worms, devoid of seta?, but possessing mesenteries, segmental organs, and pseud-haemal vessels. Polygordius has a telotrochous larva, and in its development, as in other respects, it is ex- traordinarily like a polychaetous Annelid. Butschli,2 on the other hand, dwells upon the connection between the JSfernatoidea and the Gasterotricha {see Chap. IV., p. 170) and Atricha {Echinoderes\ which he includes in the group of N^ematorhyncha, on the one side, and the lower Arthropods, such as the Tardigrada, on the other. The Physemaeia. — Since the completion of the third chapter of this work, Haeckel 3 has published an account of certain low Metazoa, constituting the two genera, Haliphy- sema and Gastrophysema, which had previously been con- founded, partly with the Sponges and partly with the Pro- tozoa. These are minute marine bodies, having the form of cups with longer or shorter stalks, by which they are attached. The cavity of the cup into which the wide or narrow oral opening leads is either simple (HaVphysema) or divided by circular constrictions into two or more communicating cham- bers ( Gastrophysema). The wall is composed of two layers, an ectoderm and an endoderm — the latter being- formed by a single layer of flagellate cells, like those of sponges ; and a series of larger flagellate cells are disposed in a spiral, on the inner face of the endoderm near the mouth. The ectoderm is a syncytium, which attaches foreign bodies, such as sponge 1 See supra, p. 165, note. 2 u Untersuchuncren iiber freilebende Nematoden und die Gattung Chceto- votus." (Zeitschrift fur wis*. Zoologie," 1876.) See also Ludwig, " Ueber die Ordnunsr Gastrotrieha " (ibid.). - 3 " BioloirUche Studien," Heft 2, 1877. THE ACANTHOCEPUALA. 553 spicula or skeletons of Ifbraminifera, to itself, and thus be- comes provided with an adventitious skeleton, the nature of which varies in different species, but is constant for each. Reproduction is effected by ova, which are said to be modi- fied cells of the endoderm. In Gastrophysema, the endo- deriu of the innermost chamber alone gives rise to ova. The place of development of the spermatozoa has not been made out. Yelk-division is complete and regular, and gives rise to a vesicula morula (archiblastula of Haeckel), each cell of which is provided with a flagellate cilium. A gastrula arises by in- vagination, but the final stages of development have not been made out. As Haeckel points out, the Thysemaria are obviously re- lated, on the one hand to the PoHfera, and on the other to the Ccelenterata ; in fact, they very nearly represent the morphological common plan of which these two groups are modifications. The Acaxthocephala. — In their sexual state the para- sites which constitute the genus Echinorhynchus inhabit the various classes of the Vertebrata, while they are found in the Invertebrata only in a sexless condition. The Echinorhynchus of the Flounder (Fig. 157), the structure of which may serve as an illustration of that of the group, inhabits the rectum of that fish, which it pierces in such a manner that the anterior extremity or head projects, inclosed within a cyst, upon the peritoneal surface, while the body hangs freely into the cavity of the intestine. Where the worm traverses the wall of the rectum it presents a much constricted neck (Fig. lo1,f). It would appear that, eventually, the JEchinorhynchi completely pass out of the intestine, as they are found inclosed in detached cysts lying in the peritoneal cavity. The anterior extremity of the Echi- norhynchus is produced into a short cylindrical proboscis, covered with many rows of recurved hooks, and, behind this, it forms a dilatation, in which the integument and the mus- cular coat are separated by a considerable interval. The body, behind the constricted neck, which separates it from this anterior dilatation, has a thick, yellowish outer wall, between which and the inner muscular tunic lies a system of vessels, consisting of two longitudinal trunks, connected by a network of anastomosing canals. These canals do not appear to possess distinct walls, nor 554 THE ANATOMY OF INVERTEBRATED ANIMALS. are any cilia visible in them ; but the minute molecules which float in the clear fluid which they contain are driven to and fro, apparently by the contraction of the body. Infer iorly, X Fig. 157. — EcMnorhynchus.—k. Diagram exhibiting the relative position of the or- gans : a, proboscis ; 6, its stem ; c, anterior enlargement of the body ; /, neck or constriction between the anterior enlargement and the rest of tbe body, d; e, posterior ''funnel ;" ^meniscus; h. superior oblique tubular bands; k, inferior muscles of the proboscis : I, m. genitalia ; o, penis, or vulva. B. Lower extrem- ity of the stem of the proboscis : a, ganglion ; 6, vascular space ; d, outer coat ; c, inner wall ; e, tubular band, with the nerve ; A,/, muscular bands ; g, suspen- sorium of the genitalia. C. Part of the female genitalia : a, ovary ; b b, ducts leading from ovary to uterus, spermiducts (?); c, open mouth of oviduct; d, e, uterus and vagina. the vessels all terminate in blind canals, disposed around the margin of the posterior funnel. Internal to the vessel lies a double layer of anastomosing muscular fibrils, the external of which are circular, while the internal are longitudinal.1 The cavity of the body is filled with a fluid, in which the ova, or spermatozoa, float, and, at its anterior extremity, two elon- gated oval bodies depend from the parietes, and hang freely in it. These are the lemnisci ; they are traversed by vessels continuous with those of the parietes. The axis of the pro- boscis is continued downward into an elongated subcylindrical stem, rounded below, which hangs down like a handle into the cavity of the body. The extremity of the stem is con- nected by broad retractor muscles with the parietes, and 1 See, for an account of the remarkable structure of these muscles, Schneider, " Ueber den Bau der Acanthocephalen." (" Archiv fur Anatomie," 1868.) THE ACAXTHOCEPHALA. 555 gives attachment to the suspensory ligament of the repro- ductive apparatus (Fig. 157, B). Two other bands are at- tached a little above these, and run obliquely forward to the parietes ; they are uot mere muscles, as they are ordinarily described, but contain a wide vessel, continuous with a large sinus, which separates the axile portion of the stem of the proboscis from its investing coat. In the axis of the stem of the proboscis is the oval ganglion, which sends off some small branches upward, and two larger lateral trunks, which can be followed into the vessels of the oblique bands ; and, in other species, have been traced to the walls of the body and to the genital openings. Two ganglia have been found by Schneider in this region in the males. There is no mouth or alimentary canal in Echinorhynchus, the animal being probably nourished by imbibition through the walls of the body. The reproductive organs are, both in the male and in the female, attached by a suspensory liga- ment to the extremity of the proboscis, and extend thence, through the axis of the body, to the posterior extremity. Here they open in a papilla at the bottom of a funnel-shaped terminal dilatation of the body, which exists both in the male and in the female, though it is much more marked, and sepa- rated by a constricted neck from the body, in the former. On each side of the papilla is an organ which has much the appearance of a sucker, but which is apparently noncontrac- tile, while the funnel itself undergoes constant and rhythmi- cal contractions. In the male the testes are two oval sacs, one behind the other, connected by vasa deferentia, often provided with pe- culiar accessory glands, with the genital outlet, which is pro- vided with a long penis. In the female the ovary is a single, long, thin-walled, cylindrical tube, the anterior end of which is usually empty for a short distance. Further back, clear, pale, rounded masses appear, containing cavities in which cor- puscles, like the germinal spots of ova, lie. More posteriorly still, these masses become elliptical, and are surrounded by a membranous coat, which gradually thickens, and gives rise at each end to a spiral filament which surrounds the inclosed egg. The ova thus constituted then pass into the cavity of ths body, where they accumulate in great numbers ; but, in this species, I have not found the free floating ovarian masses described in other Eehinorhynchl. From the lower end of the ovarium two short oviducts, or rather spermiducts, arise, and almost immediately unite into a sort of uterus, which is 556 THE ANATOMY OF INVERTEBRATED ANIMALS. continued into the vagina (Fig. 157, C). The uterus passes above into a short, open, funnel-shaped canal, which lies be- tween the two oviducts (Fig. 157, C c), and, according to Von Siebold, takes in the ova from the perivisceral cavity by a pe- culiar swallowing action. The embryos of the different species of Echinorhynchi vary somewhat in structure. Von Siebold has described those of E gigas, which are provided with hooks disposed like those of the Cestoidea, but only four in number. Sexless Echino- rhynchi have been found in Cyclops and in the muscles of fishes. Leuckart states that they acquire sexual organs in the alimentary canal of Gadus lota. The same excellent ob- server has succeeded in tracing the development of Echino- rhynchus proteus, a common parasite of many river fishes, es- pecially the Perch.1 What appeared to be the sexless con- dition of the same Echinorhynchus had previously been seen by Leuckart in Gammarus pulex. Into water containing specimens of this Crustacean, ova from E. proteus were trans- ferred. After a few days these ova could easily be detected in the digestive tube of the Gammarus, while numerous em- bryos, escaped from the egg-shell, were found within the ap- pendages of the Crustacean. Each ovum has two coats — an outer, albuminous, and an inner, chitinous. The first is digested in its progress through the alimentary canal ; the second is afterward ruptured by the embryo, which bores through the intestinal walls into the cavity of the body, and is thence conveyed to the site proper for its development. The body of the embryo is somewhat fusiform in shape, and consists of a colorless, transparent parenchyma, protected by a cuticle. The parenchyma may be resolved into an outer, homogeneous, contractile layer, and a semi-fluid medullary substance. Within this is lodged an ovoid, central mass, made up of large, highly-refracting granules. Isolated gran- ules of the same kind may also be found scattered throughout the soft medullary substance. At its posterior end the em- bryo tapers to a point, while its opposite extremity is obliquely truncated toward the ventral aspect. On this oblique surface may be observed two series of straight spines, five (rarely six) 1 " Ueber Echinorhynchus " (" Gottinger Nachrichten," 1862). Results of further investitrations and a history of the subject are contained in Leuckart's " Programm," " De statu et embrvonali etlarvali Echiuorhynchorum eorumque metamorphosi," 1873; and, further, in the concluding part of " Die mensch- lichen Parasiten," 1876, which has reached me too late for use in this place. THE DEVELOPMENT OF ECHINORHYNCHUS. 557 in each. The two series meet near the middle line to form an arch, the central and largest spine constituting its summit. Two short, ridge-like elevations of the cuticle, close to the middle Hue, separate the spines on either side from one an- other. Behind, the peripheral layer gives rise to a knob-like process. At the end of fourteen days, the embryo is found to have increased much in size, but presents few changes of form. The anterior extremity displays two rounded elevations, the spines retaining their original position. The peripheral layer has become thicker and more distinct ; its knob-like process has by this time disappeared. The central mass, now much larger, has assumed a spherical figure. No longer granular, it is seen to be composed of numerous pale cells, which con- tinue rapidly to increase. During the third week, numbers of large yellow granules begin to appear within the outer layer of the embryo. No other changes, save those of growth, take place in its walls : but the central mass, still continuing to enlarge, gradually puts on the aspect of a young Echinorhynchus. This mode of development has been compared by Leuckart to that of certain Echinoderms, or to the production of the Nemertid larva within its pilidium. The first part to become differentiated is the cavity of the future proboscis, which appears as a transparent lenticular vesicle at the anterior end of the spherical mass. Behind this are soon seen rudiments of the central axis and its contained ganglion; and the suspensorial ligament, with the reproduc- tive organs, are, at the same time, marked out. The muscles of the outer wall have also commenced their development. Next, the central region of the young Echinorhynchus rapid- ly elongates ; its walls become thinner, and, separating from the included structures, show the first trace of the visceral cavity. About this time distinctions of sex first make them- selves evident. The posterior end of the body undergoes a disproportionate increase of size, the muscles become more distinct, and the rudimentary generative organs are clearly manifest. At length the young Echinorhynchus occupies almost the whole interior of the embryo, the walls of which have, meanwhile, undergone but slight histological change. The spines, however, have disappeared, together, it would seem, with the cuticle to which they were attached. No rup- ture of the other embryonic structures takes place, but they gradually attach themselves to the body of the contained 558 THE ANATOMY OF INVERTEBRATED ANIMALS. Echinorhynchus, becoming closely fitted to its surface, and apparently persisting throughout its entire life. The devel- opment of the Echinorhynchus now approaches completion. The lemnisci appear. Hooks arise on the surface of the pro- boscis, not, as might be supposed, from its outer cuticle, but from specially modified cells of an inner membrane. The in- ternal organs begin to assume their final aspect. The ex- ternal form of the adult organism is rather slowly reached, and a few changes which take place after transference of the Echinorhynchus to its final host have yet to be observed. The Acanthocephala undoubtedly present certain resem- blances to the JYematoidea, and more particularly to the Gor- diacea, but the fundamental differences in the structure of the muscular and nervous system, and in that of the repro- ductive organs, are so great, that it is impossible to regard them as Nematoids which have undergone a retrogressive metamorphosis. In their case, as in that of the Cestoidea and that of the Dicyemida, it is, I think, desirable to keep one's mind open to the possibility that anenterous parasites are not necessarily modifications of free, enterate ancestors. The Dicyemida. — In 1830, Krohn discovered certain cili- ated filiform parasites in the renal organs of Cephalopods, to which Kulliker subsequently gave the name of Dicyema. Recently, these strange organisms have been made the subject of renewed investigation by E. van Beneden, from whose elaborate memoir 1 I take the following account of their structure : The body of a Dicyema (Fig. 158, 1.) consists of one large, cylindrical, or more or less fusiform, axial cell, which extends from the slightly-enlarged head-end, by which the animal is attached, to its posterior extremity, and is invested by a single layer of relatively small flattened cortical cells. These are arranged, like a pavement epithelium, around the axial cell, their edges being juxtaposed ; they are nucleated, and their free surfaces are ciliated. There is no interspace be- tween the cortical cells and the axial cell, and the organism is a simple cell-aggregate, devoid of connective, muscular, or nervous tissues. The cortical cells which invest the anterior or head-end of the Dicyema have peculiar characters, and are distin- guished as the polar cells. They are arranged in such a 1 " Recherelies sur les Dicvemides." (" Bulletin de l'Acad. Rovale de Bel- gique," 1876.) THE DICYEMIDA. 559 manner that the head is bilaterally symmetrical. Sometimes the polar cells constitute the whole of the cephalic enlarge- ment ; but, in others, cells of the adjacent part of the body Fig. 158.— Dicyema. — I. D. typus. The large papillae of the cortical layer and the germs in the interior of the axial cell are noticeable. II. D. typus. Different stages of the development of a vermiform germ. III. Infusoriform embryo found free in the renal organs oiEledone moschata, treated with osmic acid: p, the urn; ca, its capsule; s, its lid; i, multinucleate cells in its interior. (After Van Beneden, /. c.) (parapolar cells) contribute to the investment of the head. Strongly-refracting globules and rods accumulate in some of the ectodermal cells, and cause them to project in the form of papillae. The axial cell is a mass of protoplasm. Its relatively dense outer layer passes into a central reticulation, in the midst of which there is a large oval nucleus. Reproduction takes place by the formation of germs, and the development of embryo from them, in the axial cell. The embryos are of two kinds, the one vermiform, the other in- fusoriform, and are not met with in the same Dicyema^ but in individuals of somewhat different characters. Those which give rise to the vermiform embryos are termed JVematoc/ena, while the others are named Rhombogena. In the N~ematogena, the germs arise in the protoplasmic reticulum of the axial cell, and, at first, are minute spherical bodies, each of which is provided with a nucleus. This germ- cell divides into two, and each of these again becoming bi- sected, four cells are produced, of which one remains undi- vided, while the rest go on dividing. The former enlarges, and gives rise to an axial cell, around which the other cells arrange themselves, until eventually they inclose it. Before 3G 560 THE ANATOMY OF INVERTEBRATED ANIMALS. they meet, they surround an opening through which one end of the axial cell protrudes. This corresponds with the oral pole. Before the young Dicyema thus developed leaves the body, which it generally does by traversing the oral pole (though it may make its way out through the parietes), two embrvos of the same kind appear within its axial cell. Thus the nematogenous Dicyema gives rise by agamo- genetic process to new Dicyemas, In the H/wmbogena the germs are developed in from two to five special nucleated parent cells, the origin of which is not known. They are found imbedded in the protoplasm of the axial cell, and the germs are developed endogenously from the protoplasm of the parent cell, the nucleus of which remains unchanged. The germs undergo division, and be- come spheroidal bodies composed of two kinds of cells, small and large. Each of these bodies is converted into an infu- soriform, bilaterally symmetrical embryo, which consists of an urn, a ciliated body, and two refractive bodies. The urn, situated on the ventral side of the embr}To, is composed of a capsule, a lid, and contents. The latter are four granular masses, each of which con- tains many nuclei, and eventually becomes covered with cilia. The refractive bodies take their origin in two adjacent cells. They partially cover the urn in front, and form the largest portion of the dorsal face of the embryo. The ciliated body consists of ciliated cells, and forms the caudal portion of the embryo. While the vermiform embryo becomes a Dicyema in the body of the Cephalopod on which its parent is parasitic, the infusoriform embryo is set free, and probably serves as the means by which the parasite is transmitted from one Cepha- lopod to another. Professor E. van Beneden compares the cortical layer of a Dicyema to the ectoderm, and the axial cell to the endoderm of a Metazoon ; and the mode of production of the embryo to the process of epiboly in the Metazoa. But, from the complete absence of any mesoblastic layer, he proposes to establish a new division of Mesozoa, intermediate between the Protozoa and the Metazoa, for the Dicyemida. CHAPTER XII. THE TAXONOMY OF INVERTEBRATED ANIMALS. The grouping of the various kinds of invertebrated ani- mals which has been adopted in the preceding pages is to be regarded merely as a temporary arrangement. Each chapter, from the second to the tenth, is devoted to a series of forms, the morphological relations of which are more or less obvi- ous, while Chapter XI. is reserved partly for such groups as do not readily find a place in any of the series which precede them ; and, partly, for such as have been established since this work was commenced. Our knowledge of the anatomy, and especially of the development, of the Invertebrata is increasing with such pro- digious rapidity, that the views of Taxonomists in regard to the proper manner of expressing that knowledge by classifi- cation are undergoing, and, for some time to come, are likely to undergo, incessant modifications. To the beginner, who is apt to make the mistake of look- ing upon classification as the foundation and essence of mor- phology, instead of what it really is, the superstructure and outcome thereof, this state of things is distressing. Every hand-book presents him with a different system of classifica- tion, and he may, not unnaturally, despair of finding any stability in a science, the most general results of which are capable of being stated in such very different ways. If, how- ever, the student will attend to the facts which constitute the subject-matter of classifications, rather than to the modes of generalizing them which are expressed in taxonomic systems, he will find that, however apparently divergent these systems may be, they have a great deal in common. It is possible to divide invertebrated animals into a certain number of groups, each of which will be admitted by every morphologist to be in itself a perfectly natural assemblage. That is to say, all the forms thus associated together will re- 562 THE ANATOMY OF INVERTEBRATED ANIMALS. semble one another, and will differ from all other animals in certain respects. Each such assemblage is, in fact, a " nat- ural order " in the sense in which that word is used by bota- nists ; and, although the number of these natural orders may be increased by the discovery of new forms, or diminished by the ascertainment of closer bonds of union than are at present known to exist between the orders already discriminated, yet, the morphological types which they represent will al- ways remain ; and, therefore, the knowledge of their charac- ters, once acquired, will be a permanent possession. It is not needful that these natural orders should be mor- phologically, still less numerically, equivalent ; and, in form- ing them, it is more important that similarities should not be neglected, than that differences should be overlooked. Those which have been recognized in the preceding pages are enu- merated in the following list, arranged in sections correspond- ing with the chapters in which they are discussed. Under the head of each section I shall proceed to make such obser- vations as have been suggested to me by new information or by further reflection, during the progress of this work. Section I. — Monera \Foraruiniferc{\ \Heliozoa\ JRadio- laria, Protoplasta, Gregarinida?, CataUacta, Infusoria [ Opa- linina, GUiata, FlageUata, Tentaculiferd]. Section II. — Par if era, Hydrozoa, Goralligena [Cteno- phora\ Section III. — Ticrbellaria, Rotifera [Nematorhyneha], Trematoda, Cestoidea. Section IV. — Hirudinea, Oligochceta, Polychceta, Gephy- rea. Section V. — Crustacea, Arachnida \Pycnogonida, Tardi- grada, Pentastomidd], Myriapoda, Insecta. Section VI. — Polyzoa, Braehiopoda, Lamellibranchiata, Odontophora, Section VII. — Eehin odermata. Section VIII. — Tunicata. Section IX. — Peripatidea, 3fyzostomata, Enter opneusta, Chmtognatha, Nematoidea, Physemaria, Acanthocephala, Dicyemida. Section I. — In the commencement of Chapter II., I have expressed a doubt as to the validity of the distinction of the groups contained in this section by the presence or absence THE HELTOZOA. 563 of a nucleus, and the recent investigations of Schulze * and Hertwig 2 have justified my hesitation. These observers have, in fact, demonstrated the existence of one or more nuclei in many Foraminifera (Fntosalenia, Polystomella, Rotalia, Textularia, some Miliolidce). These nuclei may be simple or multiple ; in the latter case, they have no special relation to the cameration of the skeleton, and thev are single in the young. The discovery of the nuclei was effected by treating the Foraminifera in which they were found in a special manner; and, considering the negative results at which the best ob- servers of the Foraminifera have hitherto arrived, and the fact that the other Monera have not been investigated by the same methods, it will probably be wise to consider the question of the nonexistence of a nucleus in them as an open one. Hertwig proposes to include all the Rhizopods which are invested by a coat of chitin, or by siliceous or arenaceous par- ticles, or which possess a skeleton, under the head of Thala- mophora y but the name of Foraminifera is now so widely accepted and so long established that I cannot but think that the better course is to retain it. I have included the Actinophryida and the similar forms found in fresh water, and provided with Radiolarian skele- tons, with the marine Radiolaria. Hertwig and Lesser,3 however, in their important mono- graph upon the Rhizopods, have stated reasons for separating the former as a distinct group (the Heliozoa of Haeckel), though their conclusion that there are, at present, no grounds for assuming even a remote relation between the FLeliozoei and the Reidiolaria {1. c , p. 159) appears to me to have no sufficient warranty. The Heliozoa are defined by these authors to*be unicellu- lar organisms, which occasionally become multicellular, or at any rate multinucleate, by the multiplication of the nucleus. They are usually spheroidal and free, but some are fixed by means of a stalk. In most, the protoplasm of which they con- i " phizopoden-Studien, VI." (" Archiv fur Mikr. Anatomie," 187pears to me to be wholly inadequate to bear out the conclusions deduced from it. 2 "On the Morphology of the Cephalous Mollusca." ("Phil. Trans.," 1852, p. 45 and note.) 3 The mode of development of the central nervous system in Euaxes and Clepsine offers many points of interest. Not the least important of them is the obvious similarity (to which attention has already been directed by Semper) between the germ-bands of Clepsine when they have united throughout the greater part of their length, but surround the blastopore behind, and the Am- phibian embryo with its dorsal ridges, which have exactlv similar relations. (See, for example, Fig. 40, in Plate III. of Gotte's work, " Die Entwickelungs- geschichte der Unke.~") CHjETODERMA, xeomexia, axd chiton. 5;i discovery of L. Graff,1 that the minute spines of Choetoderma are calcined. It is a further peculiarity of this genus that two distinct nerve-cords proceed from the cerebral ganglia parallel with one another on each side of the body, in the place of the single median nerve-cord of other members of the group. Dr. Jbering 2 has directed attention to certain points of resemblance between Choetoderma, with the allied genus Neomenia, and the Chitons, especially in the arrangement of the trunks of the nervous system ; and he proposes to unite the three into a group of Amphineura — thus separating the Chitons from the Mollusca altogether. Section V. — I regret that I have been unable to make use of Claus's recently-published important contributions to the history of the development of the Crustacea* Section VI. — The thorough examination of the structure of Pedicellina and Loxosoma by Nitsche 4 has shown that the differences between the ectoproctous and the endoproctous Polyzoa are of a more fundamental character than had been suspected. In the Ectoproeta, in fact, the endocyst consists of two layers, an outer and an inner, of which the former is the representative of the ectoderm in other animals. The lat- ter lines the wall of the " perivisceral cavity," and is reflected thence, like a peritoneal tunic, over the tentacular sheath and into the interior of the tentacula, whence it is continued on to the alimentary canal, of which it forms the external invest- ment. The endoderm, which lines the alimentary canal, is, of course, continuous, through the oral opening, with the ec- toderm. In the Endoprocta, on the contrary, the endocyst is com- posed of only one layer, and the endoderm of the alimentary canal has no second or external coat. The " perivisceral cavity," or interspace between the endoderm and ectoderm, is occupied by ramified mesodermal cells. Thus the Endoprocta present a structure as simple as that 1 " Auatomie des Choetoderma nitidulum." (Zeitschrift fur iciss. Zoologie, 1876.) 2 " Yercfleichende Anatomie des Nervensystems dcr Mollusken," 1877. 3 " Untersuchungen zur Erforschung der genealogischen Grundlage des CrustaceensystemV 1876. 4 " Beitrfice zur Kenntniss der Bryozoen." {Zeitschrift fur tviss. Zoologie, 1870 and 1875.) Compare Barrois (" Coniptes Eendus," 1875). 572 THE ANATOMY OF INVERTEBRATED ANIMALS. of Nematoid worms ; while the JEJctoprocta, in possessing a perivisceral cavity with a special lining, the inner surface of which may be ciliated, are, so far, comparable to Brachiopods or Echinoderms. Unfortunately, our knowledge of the embryonic develop- ment of the ectoproctous Polyzoa does not enable us to de- termine with certainty the nature of this perivisceral cavity, and of the layer which bounds it. Nitsche shows that the saccular cystid, which results from the first developmental changes of the embryo in the Phylactolcemata, is composed of two layers, which correspond with those of the endocyst in the adult ; and, further, that the polypide (alimentary canal, tentacula, and ganglion) results from an ingrowth of the outer layer of the endocyst, which pushes before it an in- volution of the inner layer. The latter gives rise to the re- flected " peritoneum." But I am not aware that there is any evidence which proves conclusively the manner in which these two layers of the embryonic endocyst take their origin, or with what layers of the ordinary embryo they are homologous. If we make the ordinary assumption that the inner or peritoneal layer of the endocyst is the partial or complete homologue of the hy- poblast in other animals, it follows that the perivisceral cav- ity of the Ectoprocta is really an enteroccele, as it is in the Brachiopoda. The only other alternative appears to be the supposition that the inner layer of the endocyst is a meso- blast, differentiated from the germ earlier than the hypoblast; in which case the perivisceral cavity will be a schizoccele. Dr. Jhering's work on the nervous system of the Molhisca, to which I have already referred, contains a number of valu- able anatomical details, and especially gives a better account of the structure of the nervous system of Chiton than has hitherto existed.1 1 In addition to a great variety of surprising phylogenic speculations, Dr. Jhering puts forward the novel morphological views that the respiratory sac of the Pulmonata {NepliropnevMa, Jhering) is morphologically a sort of urinary bladder, and that the ganglia whence the~arm-nerves of the Cephalopoda arise are cerehral, and not pedal. The arms are thus parts of the head, and only the funnel represents the foot of Gasteropoda. I do not presume to rebel against the authoritative censure of my memoir on the " Morphology of the Mollusca," published now five-and-twenty years ago, which is pronounced by Dr. Jhering. Nevertheless, I may remark that, had he condescended to pay attention to what is said respecting the flexure of the intestine in Mollusks in that antiquated production, he would not have com- mitted himself to the publication of the two diagrams— one of a Cephalopod and the other of a Pteropod — each with its alimentary canal twisted after a fashion of which Nature knows nothing, which illustrate, though they hardly adorn, page 272 of his work. THE HIGHER GROUPS. 573 There is no invertebrated animal at present known which cannot at once be referred to one or other of the natural or- ders which have been discussed in the preceding pages. The next question which arises is, How far are these groups sus- ceptible of arrangement into assemblages of a higher order, distinguished from all others by certain common characters ? It is universally admitted that the Insecta, Myriapoda, Arachnida, Crustacea, Pycnogonida, and Tardigrada, form such an assemblage, termed the Arthropoda, and character- ized by the segmentation of the body ; the chitinous cuticula ; the absence of cilia upon, or in, the body at any period of life ; the segmentation of the central nervous system, and its per- foration by the gullet ; and the presence (with the possible exception of the Jrllobita) of limbs, which, almost always, are themselves subdivided into joints. The reasons for in- cluding the Peripatidea in this division have been given in Chapter XL; and, though the Pentastomida must be regarded as hardly within the limits of the definition, I think that, tak- ing into account the strange modifications which are under- gone by the parasitic Crustacea and Arachnida, it is not needful to depart from the ordinary practice of associating them with the Arthropoda. The Lamellibranchiata and the Odontophora constitute another very well marked division, the Mollusca, the char- acters of which have been discussed in Chapter VIIT. The proposal to separate the Polyplacophora from the Mollusca, to which I have already referred, appears to me to be devoid of any justification. The resemblances between certain Gephyrea, such as Choetoderma and Neomenia, and the Polyplacophora, are accompanied by wide differences ; and even if these resemblances are to be regarded as evi- dences of affinity, some considerations, such as the restriction of the branchiae to the hinder part of the body, and the reduc- tion of the foot in Chitonellus, rather lead to the suggestion that Ghoztoderma and JVeomenia may be extremely modified Mollusks, allied to the Polyplacophora. As to the supposition that the resemblances between the JSfudlbranchiata and the Turbellaria indicate a direct affin- ity between these groups, it seems to be forgotten that the Nud'ibranchiata are all, when young, unmistakable Gastero- pods provided with mantle and shell. Their adult structure is as little evidence of any Turbellarian affinities as that of Lemma is proof of its being allied to the wrorms rather than to the Crustacea. 574 THE ANATOMY OF INVERTEBRATED ANIMALS. The Physemaria, the Porifera, the Hydrozoa, the Coral- Hyena, and the Ctenophora, are obviously modifications of the same fundamental plan. I think it is convenient to re- tain the well-established name of Coelenterata for the last three orders, which are much more closely related to one another than to the other two. Haeckel's proposal to apply the old name of Zoophyta to the whole division appears to me to be well worthy of adoption. The inconvenience of using a term the connotations of which have varied some- what widely since it was first invented, is probably less than that which would attend the invention of a new name. The Monera, Foraminifera, Heliozoa, Badiolaria, Pro- toplasta, Gregarinida, CataMacta, and Pifusoria (Opali- nina, Ciliata, Tentaculifera, Flagellata), again, are so close- ly united together that the difficulty is to distinguish the less differentiated forms of each from one another. They constitute the division of the Protozoa, the common charac- ters of which have been given in Chapter II. If there were no invertebrated animals besides those in- cluded under these four divisions of Arthropoda, Mollusca, Zoophyta, and Protozoa, the task of classification would be very easv, and each of the higher divisions would be sharply defined from the others. But a vast residuum remains to be considered ; and it is with the attempt to arrange these resid- ual orders into higher groups that the difficulties of the Tax- onomist commence. The Polychmta and the Oligochceta, the Hirudinea and the Gephyrea, resemble one another generally in the seg- mentation of the body, indicated at least by the serially mul- tigangliate nervous centres ; ' in the presence of cilia and of segmental organs ; and in the nature of the larvae, which are set free when their embryos are hatched in an early stage of development. And, although no one of these characters is of universal occurrence (cilia, for example, being absent in most adult Hirudinea), yet they are found in such association that the accepted arrangement of these four groups (to which, though not without some hesitation, I add the Myzostomata) into the division of the Annelida is undoubtedly very con- venient. The Trematoda, the Turbellaria, and the Botifera, form 1 This character is wanting in most Gepltyrea, which, as I have remarked at p. 218, incline in many respects toward the next division, and especially toward the Rotifera and Nematorhyncha. THE HIGHER GROUPS. 5|5 another very natural assemblage. But it must be admitted that the highest forms of this division are separated by no very sharp line of demarcation from the Annelida y while the simplest Turbellaria are almost on a level with the Phy- semaria and the lower Hydrozoa. Even a Planaria is com- parable to a free zoophyte ; its proboscis may be likened to the hydranth of a Medusa, the prolongation of the alimen- tary sac to the gastro-vascular canals, the central nervous sys- tem, with its lateral prolongations, to the marginal ganglia and nerves. The water-vascular system and the complication of the reproductive organs, indeed, afford clear marks of dis- tinction ; but both of these s}*stems vary indefinitely in the degree of their development within the limits of the Turbel- laria. On the other hand, the connection of the Hirudinea by such forms as Malacobdella with the Turbellaria and Trema- toda is very close ; Polygordius appears to be a transitional form between the Turbellaria and the Polychceta ; while the Potifera, in many respects, represent larval forms of the Polychceta and of the Gephyrea. The Cestoidea are usually regarded as anenterous Trema- toda, in which case, of course, they must be associated with the latter. I propose to establish a division of Trichoscolices for the natural orders now enumerated, in order to discriminate the morphological type which they exemplify from that of the Nematoscolices, containing the Nematoidea, which are as remarkable for the universal absence of cilia as the former are for their presence ; and which are further so clearly dis- tinguished by the arrangement of their nervous and muscu- lar systems and of their water-vessels ; and by their ecdysis. The connection between the two divisions by way of the Nematorhyncha and the Potifera is undoubtedly very inti- mate, and there is almost as much reason to arrange the JVe- matorhyncha with the Trichoscolices, as with the Nematosco- lices. On the whole, however, I think that, notwithstanding the cilia of the Gastrotricha, the closest affinities of the Nematorhyncha are with the Nematcidea, and I therefore place them among the Nematoscolices. But I may remark, once for all, that the attempt to estab- lish sharply-defined, large divisions of the animal kingdom is futile. The progress of knowledge every day renders it more and more clear that morphological groups are compara- ble to distributional provinces ; each, however well marked 37 576 THE ANATOMY OF INVERTEBRATED ANIMALS. may be its characteristic features, shades off at its margins into some other group ; and the object of classification is simply to bring into prominence the morphological types which embody these characteristic features. It appears to me impossible to compare the structure and the larval conditions of a Polyzoon with those of a Brachio- pod, without arriving at the conclusion that they are more closely allied with one another than they are with any third group. Nevertheless, the Polyzoa approach the Potifera, and the Prachiopoda the Annelida, on the one side ; while on the other they present unmistakable affinities with the low- er Mollusca. At the same time the weight of the resemblances between the Polyzoa and the Tunicata, which led Milne- Edwards to the establishment of the group of " Mollusco'ides " (adopted by myself under the title of Molluscoida), has been much lessened by the progress of investigation. I conceive that we may best keep these resemblances and differences in view by associating the Polyzoa and the Pra- chiopoda into a division apart, for which I propose the name of Malacoscolices ; in order to indicate its relations with the Worms on the one side and with the Mollusca on the other. The Tunicata are absolutely distinguished from all other iuvertebrated animals except Pcdanoglossus, by the perfora- tion of the pharynx and its conversion into a respiratory organ.1 At first sight there appears to be little ground for the approximation of groups apparently so widely different as the Tunicata and the Enteropneusta. But the extraordinary similarity in the structure of the perforated pharyngeal sac in the larvas of Tunicates and of Palanoglossus is a fact of great morphological weight. An ecaudate Appendicularia of those species which have the alimentary canal nearly straight, would be marvelously like a larval Palanoglossus, which is again little more than a specially modified Turbella- rian. I think, therefore, that the Tunicata and the Entero- pneusta may properly constitute a division of Pharyngo- PNETJSTA. 1 I have alluded above to the structures described by Semper in some Oli- gochazta and in Sabella. I do not doubt the accuracy of the description ; but it does not lead me to conclude that the structures in question are homologous with either Vertebrate, Enteropneustal, or Tunicate branchiae. THE ANENTEROUS IXVERTEBRATA. 577 The Tunicate Pharyngopneusta, with their caudate larvae, may be supposed to stand in the same relation to the Turbel- lariform Pharyngopneusta, as the Trematoda, with their cer- cariform larvae, to the Turbellaria. Another very well marked division is that of the Echino- dermata, the characteristics and relations of which have been fully discussed in Chapter IX. Although the structure and development of Sagitta have now been as thoroughly elucidated as those of any animal, the proper Taxonomic place of the Chcetognatha is still an unsolved problem. The issues, however, appear to be nar- rowed to these : either they belong to the Annelida , or to the JYe?natoscolices, or to the Trichoscolices / or the Chceto- gnatha are to be regarded as an independent division, allied to all these, and perhaps to the lower Arthropoda. I am dis- posed to adopt the last view, chiefly on the ground of the mode of development of Sagitta, which is unlike anything at present known to occur in Annelida, Trichoscolices, Nema- toscolices, or Arthropoda. The Acanthocephcda are hardly less anomalous than the Chcetognatha. Taking into account the Gordiacea and the characters of the proboscis in the Nematorhyncha, there is undoubtedly room for the suggestion that they are specially- modified anenterous J\Tematoscolices, and should be classed among the latter. But here, as in the case of the Cestoidea, there are many difficulties in the way of accounting for these anenterous forms by the supposition that they are the results of a retrogressive metamorphosis of enterate animals. This question of the true relations of the anenterous in- vertebrates— bv which I mean not onlv those which, like the male Rotifers, have no functional alimentary canal in the adult condition ; but those which, like the Cestoidea and the Acanthocephala, never exhibit a trace of an alimentary canal, even in the embryo ; which is usually dealt with so summarily by the assumption of retrogressive metamorphosis — acquires still more importance, when we attempt to determine the Taxonomic place of the Dieyemida. Prof. E. van Beneden has proved that these parasites can- not be dismissed, sans fa<;on, as retrogressively metamor- phosed " worms ; '" and though I am not disposed to attach much weight to the absence of a mesoderm, on which Van 578 THE ANATOMY OF INVERTEBRATED ANIMALS. Beneden insists as a distinction between the Dicyemida and the Metazoa, the manner in which the contents of the axial cell give rise to germs is so completely unlike anything which is known to obtain in the Metazoa, as, to my mind, to justify the separation of the Dicyemida from the whole of this divi- sion. On the other hand, the similarity of their development to the formation of metazoic embryos by epiboly, as com- pletely divides the Dicyemida from all the Protozoa. It must be recollected that the changes which are undergone by the ciliated embryos are still to be discovered ; but, provision- ally, I am disposed to agree with Van Beneden, that the Di- cyemida should be regarded as the representatives of a dis- tinct division, the Mesozoa, intermediate between the Pro- tozoa and the Metazoa. And without distinctly j^ledging myself to any such view, I yet think it is worth while to throw out the suggestion that the Cestoidea, if not the Acanthocephala, may be modifications of the same type, differing from the Dicyemida in the development of a meso- derm, but resembling them in the total absence of an alimen- tary apparatus. The Serial Relations of the Inveetebeata. — When the various groups of invertebrate animals are compared, it is obvious that they present very different degrees of morpho- logical complexity ; whence they may be considered as terms in a graduated progression, in which the place of each group corresponds broadly with the degree of its differentiation. The lowest Protozoa will occupy one extreme of such a pro- gression, the Arthropoda and the Mollusca the other, while the remaining groups fall into intermediate places. On at- tempting to carry out this serial arrangement into detail, however, it will be found that no single series will suffice to express the facts, but that, starting from the lowest Protozoa, we are led along various lines, none of which, as far as our present knowledge enables us to judge, can be traced, with- out interruption, throughout the whole length of the scale. If we assume, in the absence of proof to the contrary, that the Monera have the simplicity of structure ascribed to them by Haeckel, then, on comparing the Endoplastica with the Monera, the different groups of the former appear to be re- lated to those of the latter division, as if they were similar forms complicated by the addition of one or many nuclei. Protofjenes may thus be considered as the root of the Foram- iniferal series, Protamoeba of the Protoplasta, Myxastrum THE SERIAL RELATIONS OF LNVERTEBRATA. 579 of the Gfregarinidm, Vtimpyrella of the Heliozoa, Protomo- nas of the Flagellata. A Moneran, ciliated over its whole surface, which might stand in the same relation to the Opa- linina, Catallaeta, Tentaeidifera, Ciliata, is at present un- known. The Protozoa thus fall into the following series : Protozoa. II. III. Protamoeba. Myxastrum. Foraminifera. Protoplasta. Gregarinidce. Heliozoa. I. Protogenes. IV. Vampyrella. Padiolaria. V. I Tentaculifera. VI. I Catallaeta. i Opalinina. Ciliata. VII. Protomonas. Flagellata. I am unable to trace any one of these series of modifica- tions further; that is to say, to find forms which actually bridge over the interval between any one of them and the JSIetazoa. though it is easv enough to imagine what such forms might be. The spheroidal free-swimming monad aggregates, such as Uvella and Polytoma, and Magospho&ra itself, are, in many respects, comparable to Physemarian or Poriferan embryos ; while an animal Vblvox would be a sort of perma- nent vesicular morula. So, one of the higher Infusoria, if it became multinucleate, like an Opalina, would approach the lowest Turhellaria. The axial cell of a Dieyema, from the protoplasm of which its ciliated and nonciliated germs are produced, is, to a cer- tain extent, comparable to the capsule of a Radiolarian ; while, on the other hand, a Radiolarian with a multinucle- ate cortical layer would approach the structure of Dieyema. And if what is at present known of Dieyema gives a just conception of the essential points of its entire history, it un- doubtedly, as E. van Beneden has suggested, represents a type intermediate between the Protozoa and the Metazoa, 580 THE ANATOMY OF IXYERTEBRATED ANIMALS. though it can hardly be said to fill up the hiatus between them. In our further search after the serial relations of animals, we must therefore start afresh from the lowest Metazoa. Here a Zoophytic Series is very well marked ; commencing with the Physemaria, and thence diverging, on the one hand, to the Porifera, and, on the other, to the Coelenterata, with the highest forms of which this series comes to an end. A second gradation, which may be termed the Antxuloid Series, is represented by the Trichoscolices and the Anne- lida. The lowest Turbellaria are upon nearly the same level of organization as the Hydrozoa. It would be hard to dis* tinguish an aproctous Turbellarian, devoid of a ganglion and water-vessels, from a free-swimming nontentaculate Hydro- zoon. On the other hand, as I have already pointed out, the line of demarcation between the higher Trichoscolices and the Annelida is very indistinct, and we may expect it to be speedily obliterated by the progress of discovery. A third gradation is constituted by the Nematoscolices and the Arthropoda. The lowest Nematoidea possess no higher organization than the lowest Turbellaria and the Potifera. The Nematorhyncha, whether they are really transitional forms between the Nematoidea and the Arthropoda or not, at any rate indicate the road by which the transition may be effected ; and I am much inclined to think that the Chceto- gnatha may occupj7 a place in this series. The oral armature of Sagitta may be regarded as a modification of the oral spines of Echinoderes^ and its nervous system is as much Arthropodal as is that of the Pentastomida. This may be called the Arthrozoic Series. A fourth series is that which I shall term the Malacozoic Series. It includes the 31alacoscolices and the Mollusca. The entoproctous Polyzoa form the lowest term of this series. The resemblances of the Polyzoa with the Potifera (e. g., with Step>hanoceros) have often been remarked, and, indeed, insisted upon, with too little regard to the differences which are established bv the water-vessels and the peculiar pharyn- geal armature of the Rotifers. Nevertheless, these resem- blances are important as far as they go, and in grade of or- ganization the two groups are much upon the same level. On THE SERIAL RELATIONS OF IXYERTEBRATA. 581 the other hand, the comparison of a Polyzoon with a larval Lamellibranch or Gasteropod, or with a Pteropod, leaves no doubt in my mind that the Malacoscolices have the same rela- tion to the Mollusca, as the Trichoscolices to the Annelida. A fifth gradation is presented by the Tunicata and the Enteropneusta, which constitute the Phaeyxgopxeustal Seeies. I do not regard the Enteropneusta as of distinctly lower organization than the Tunicata, but rather as a col- lateral group; and I conceive it to be probable that some lower forms, connecting the Enteropneusta and the Tunicata. with one another and with the Trichoscolices, will yet be found. However this may be, Appendicular i a presents a grade of organization but little higher than that of the Polyzoa. A sixth gradation is represented by the Echixodeemal Seeies. Like the foregoing, this series at present stands isolated,1 no annectent forms between the Echinoderms and higher or lower groups being known. On the ground of the uniformity of character of the larvae of the Echinoderms, however, there can be little doubt that, if ever such forms are discovered, they will prove to be allied to the Gephyrea, the Trichoscolices, and the Enteropneusta. Thus the study of the gradations of structure among the Metazoa leads to the conclusion that they fall into six series, which may be arranged in the following tabular shape : Seeies. II. III. ECHIXODEEMAL. PhAEYXGOPXEUSTAL. Ccelenterata. Echinodermata. Enteropneusta. Tunicata. Porifera. Physemaria. I. ZOOPHYTIC. IV. Malacozoic. Mollusca. Malacoscolices. V. AXXULOID. Annelida. Trichoscolices. VI. Aetheozoic. Arthropjoda. Chcetognatha (?). Xematoscolices. 1 I say, at present, inasmuch as the characters of the nervous system sharp- ly separate the most vermiform of the Echinoderms from the most Echinoderm- like Gephyrea, 532 THE ANATOMY OF INVERTEBRATED ANIMAL^. The lowest known term of the Arthrozoic series is a Nema- toid worm ; that of the Annuloid series is a low Turbellarian or Rotifer ; that of the Malacozoic series is an entoproctous Polyzoon ; that of the Pharyngopneustal series is probably most nearly exemplified by the young larva of Balanoglos- sus : that of the Echinodermal series by the vermiform JEchi- nopmdium. But the differences between one of the simpler Nematoid worms, an aproctous Turbellarian, a Rotifer, an Echinopsedium, and a Pedicelllna, are relatively so small, that all six series may be said to converge toward a common form ; and that common form, when the special characters of each group are eliminated, and the alimentary canal is reduced to its primi- tive aproctous condition, would be exceedingly similar to a Physemarian. Hence the consideration of the gradations of structure which are presented by the various series of Invertebrated animals, irresistibly leads to the conclusion that the whole of the Metazoa may be conceived as diverse modifications of a common fundamental plan. The Serial Relations of the Inyertebrata com- pared with the Results of Embryology. — The conception of the unity of organization of the Iuvertebrata thus reached, so far as it is based upon the comparison of adult structures, is purely ideal ; and the study of the development of individ- ual animals is alone competent to decide the question whether this ideal unity has a foundation in objective fact. But the history of the development of animals appertaining to every group of the Invertebrata which has been given, bears out the statement which is made in the Introduction, that the ideal unity has such a foundation in fact ; inasmuch as all these animals commence their existence under the same form — that, namely, of a simple protoj)lasmic body, the ovum or germ. In the Introduction I have said that, "among the lowest forms of animal life, the protoplasmic mass which represents the morphological unit may be, as in the lowest plants, devoid of a nucleus " (p. 18). However, as I have remarked at the commencement of this chapter, until the search for the nucleus has been instituted afresh, with the help of such methods as have recently proved its existence in the Foraminifera, I think it will be wise to entertain a doubt whether any of the Monera are really devoid of this amount of structural differ- THE RESULTS OF EMBRYOLOGY. 583 entiation ; and the tendency of recent investigations appears to render it very questionable whether the nucleus of the ovum ever really disappears, whatever may be the modifica- tions undergone by the germinal vesicle and its contents. I shall, therefore, assume provisionally, that the primary form of every animal is a nucleated protoplasmic body, cytode, or cell, in the most general acceptation of the latter term. Whether the primary cytode possesses a nucleus or net, the important fact remains that, in its earliest condition, every invertebrated animal, if it were competent to lead an independent existence, would be classed among the .Protozoa. The first change which takes place in the development of the embryo from the primitive cytode, or impregnated ovum, in all the Metazoa, is its division ; and the simplest form of division results in the formation of a spheroidal or discoidal mass of equal, or subequal, derivative cytodes, the blasto- meres. Next, the morula, thus formed, generally acquires a central cavity, the blastocoele, and becomes a hollow vesicle, the blastosphere, the wall of which, composed of a single layer of blastomeres, is the blastoderm. The blastomeres of the blastoderm next undergo differen- tiation into two kinds, distinguished by their internal activi- ties, if not bv their outward form. Of these the one set con- stitute the epiblast, the others the hypoblast. The further changes of the embryo are the consequences of the tendencies toward further modification resident in the epiblastic and hy- poblastic blastomeres respectively. Each of these is, as it were, a germ, whence certain parts of the adult organism will be evolved. Every series of the Invertebrata has now yielded a num- ber of examples of the further modification of the blastosphere by the process of invagination, or emboly, the result of which is that the hypoblast becomes more or less completely inclosed within the epiblast. The invagination is accompanied by the diminution, or even abolition, of the blastoccele, and the for- mation of a cavity inclosed within the hypoblast, which is the archenteron, or primitive alimentary cavity. The opening left by the approximated edges of the epiblast, when the pro- cess of invagination is completed, and by which the archente- ron communicates with the exterior, is the blastopore. In this state the embryo is a gastrula. It very commonly happens that the process of develop- ment is modified by an inequality in the size of the blasto- meres ; which inequality may be manifest from the bisection 584 THE ANATOMY of invertebrated animals. of the ovum, or may appear later. In this case, it usually happens that the smaller and more rapidly-dividing blasto- meres belong to the epiblast, and the larger and more slowly dividing to the hypoblast. Moreover, no blastoccele may arise, and the process of inclusion of the hypoblast within the epiblast may have the appearance of the growth of the latter over the former, or what is termed epiboly y while the archen- teron may not be formed within the hypoblast till very late. When, in cases of epiboly, the blastoderm is small in rela- tion to the vitellus, the epiblast and hypoblast, at their first appearance, necessarily adapt themselves to the surface of the yelk ; and thus the gastrula, instead of having the form of a deep cup, becomes more or less flattened and discoidal. I am inclined to believe that all the various processes b}7 which the gastrula or its equivalent are produced, are reduci- ble to epiboly and emboly. Even when the epiblast and the hypoblast appear to be formed by delamination , or the split- ting into two layers of cells of a primitively single-layered blastoderm, there seems little doubt that what happens is either the very early inclusion of the hypoblastic blastomeres within those which give rise to the epiblast, or a very late and inconspicuous ingrowth, or invagination, of the hypoblas- tic region of the blastoderm. If we employ the term gastrula in the broad sense defined above, it may be truly said that every metazoQn passes through the gastrula stage in the course of its development. The question whether the mode of development of the gastrula by emboly is primitive, and that by epiboly secondary ; or whether epiboly is primary and emboly secondary; or whether the two processes have originated independently, is of sec- ondary importance, and belongs to the debatable ground of phylogeny.1 The meaning of the differentiation of the aggregate of cytodes, of which the body of a simple metazoon is composed, into a hypoblastic, or endodermal, and an epiblastic, or ecto- dermal, group, is to be sought in the physiological division of labor, w7hich is the primary source of morphological changes. It is a separation of the aggregate of morphological units into one set with a specially nutritive, and another set with a spe- cially motor and protective, function. It is quite possible to conceive of an adult metazoon having the structure of a sponge- 1 Compare Ilaeckel, " Studien zur Gastrasa-Tkeorie," in his "Biologiscke Studien,''* 1877. THE RESULTS OF EMBRYOLOGY. 585 embryo ; moving by its ectodermal hemisphere, and feeding by its endodermal hemisphere. The next advance in organization of such a metazoSn would doubtless consist in the more complete extension of the protective layer over the nutritive layer, with due pro- vision for the access of the surrounding medium to the latter. It is obvious that this advance might be effected in either of two ways : the one by emboly, the other by epiboly. In the former, the blastopore would be left as the aperture of com- munication of the endoderm with the exterior; and the result would be the formation of an archceostomatous gastrula, such as that which is supposed by Haeckel to be the primitive form of the metazoun. . In the latter, the blastopore would com- pletely close up, and a new aperture or apertures must be formed in the ectoderm to subserve the ingestion of nutri- ment. The resulting organism would be a deuterostomatous gastrula. Undoubtedly it seems natural to suppose that the first process preceded the second, in order of evolution ; but the proof that it did so is at present wanting. And, however this may be, the progress of inquiry seems to throw more and more doubt upon many cases of the supposed persistence of the blastopore as the mouth. It is certain that, in the great majority of invertebrated animals, the blastopore either be- comes the anus, or closes up ; and renewed observations are needed to determine the limits within which the archaeostoma- tous condition prevails. The blastoccele of the gastrula may be obliterated by the approximation of the epiblast and the hypoblast, or it may persist and constitute the perienteron, or primitive perivis- ceral cavity. Those animals which, in their adult condition, most nearly represent simple gastrula with obliterated blastoccele, are the Physemaria and Hydra, cup-shaped bodies with an oval opening at one end, the walls of which are made up simply of an ectoderm and an endoderm.1 In the great majority of the Metazoa, a further advance in complication is effected by the appearance, between the epi- blast and the hypoblast, of cytodes, either isolatedly or in a continuous laj-er, which constitute the mesoblast, and eventu- ally are converted into mesodermal structures. The origin 1 I do not think that Kleinenberg's fibres in Hydra strictly represent a mesoderm, though they occupy the position of one. 586 THE ANATOMY OF INVERTEBRATED ANIMALS. of these is still a matter of doubt, but in many cases it ap- pears to be unquestionable that they are derived from the hypoblast. The perienteron, more or less interrupted and broken up by the constituents of the mesoblast, may give rise directly to the perivisceral space, or channels, of the adult, which thus constitute a schizocoele. It is hardly doubtful, I think, that the perivisceral cavity takes its origin in this manner in the Rotifera, the entoproctous Polyzoa, the Echinopaedia of the Echinoderms, the Tunicata, and the Nematoidea. On the other hand, in many Tnvertebralct, one or more di- verticula of the archenteron extend into the perienteron and its contained mesoblast. Sometimes, as in the Ccelenterata, these remain connected with the alimentary cavity through- out life, and are termed gastrovascular canals. In other cases (Echinodermata, Brachiopoda, Chcetognatha) they become shut off; their cavities constitute a variously -modified enter o- eoele y and their walls give rise, along with the primitive mesoblastic elements, to the mesoderm. To which of these two possible sources of the mesoderm, the mesodermal structures of the Annelida and the Arthro- poda, which so very generally take on the form of two longi- tudinal germ-bands in the embryo, and subsequently undergo segmentation, are to be referred, is a very interesting, but, as yet, unsolved problem. It is possible that they are solid rep- resentatives of the hollow diverticula which, in other animals, give rise to the enteroccele ; in which case the perivisceral cavity in these animals will be a virtual enteroccele. On the other hand, they may merely represent the cells of the meso- blast of the entoproctous Polyzoa and of the Echinopasdia, and their perivisceral cavity would then be a schizocoele. But it is needless to pursue this topic further ; enough has been said to show conclusivelv that, however different one inver- tebrated animal may be from another, the study of develop- ment proves that each, when traced back through its embry- onic states, approaches the earlier stages of all the rest ; or, in other words, that all start from a common morphological type, and even in their extremest divergence retain traces of their primitive unity. It is very important to remark that these morphological generalizations, so far as they are correctly made, are simple statements of fact, and have nothing to do with any specula- tions respecting the manner in which the invertebrated ani- PALAEONTOLOGY AND PHYLOGENY. 587 mals with which we are acquainted have come into exist- ence. They will remain true, so far as they are true at all, even if it should be proved that every animal species has come into existence by itself and without reference to any other. On the other hand, if there are independent grounds for a belief in evolution, the facts of morphology not only present no difficulty in the way of the hypothesis of the evo- lution of the Invertebrata from a common origin, but readily adapt themselves to it. Hence the numerous phylogenic hypotheses which have of late come into existence, and of which it may be said that all are valuable, so far as they suggest new lines of investi- gation, and that few have any other significance. I do not desire to add to the number of these hypotheses ; and I wrill only venture to remark that, in the absence of any adequate palaeontological history of the Jnvertebrata , any attempt to construct their Phylogeny must be mere speculation. But the oldest portion of the geological record does not furnish a single example of a fossil which we have any rea- sonable grounds for supposing to be the representative of the earliest form of any one of the series of invertebrated ani- mals ; nor any means of checking our imaginations of what may have been, by evidence of what has been, the early his- tory of invertebrate life on the globe. Already indications are not wanting that the vast multi- tude of fossil Arthropods, Mollusks, Echinoderms, and Zoo- phytes, now known, will yield satisfactory evidence of the filiation of successive forms, when the investigations of pa- laeontologists are not merely actuated by the desire to dis- cover geological time-marks and to multiply species, but are guided by that perception of the importance of morphological facts which can only be conferred by a large and thorough acquaintance with anatomy and embryology. But, under this aspect, the palaeontology of the Invertebrata has yet to be created. INDEX. Abdominalia, 260. Abiognesis, 88. Abiological sciences, 9. Acanthobdella, 1S9. Acanthocephala, 553, 57T Acanthoteuthis, 165. Acarina, 329. Achetidae, 378. Achtheres, 241. Acineta mystacina, 94. Acineta?, 89. 93, 94, 99-101. Aerididae, 378. Actinia. 53, 140, 141, 154. holsatica, 139. Actinidae, 140, 141. 145. Actinophrys, 83, 86, 93. Aetinosphaerium Eichhornii, 83, 85. Actinozoa, 55, 110, 137, 146, 119, 150. Actinula, 132. ^Eginidae, 136. ^Ethalium septicum, 13. ^Etiology, 16, 37 Agamogenesis, 31, 34, 383. Aglaophenia (Plumularidae). 120. Air-breathing Arthropoda, 320. Alaurina, 157. Albertia. 170. Alcippe lampas, 261. Alcyonium, 143. Alectoromorpha?, 69. Alga?, 14, 20, 32, 97. Alimentary apparatus, 56. AllantoR 67. Alternation of generations, 36, 67. Ambulacral vessels, 54. Ametabola, 361. Amuionitidae. 459. Ammothea pycnogonoides, 332. Amnion, 67. Amceba radiosa. 86. sphaerococcus. 86. Amceba?, 13, S6, 103. Amouroucium proliferum, 526. Amphibia. 58, 50. 64, 70, 86. Amphidiscus, 108. Amphidotus cordatus, 491. Amphioxus, 57, 59. Amphipoda, 313. Amphithoe, 311. Ampullaria, 60. Anatomy, 16, 17. Anenterous invertebrates, 577. Anguillula brevispinus, 545. scandens, 550. Animals, characters, 44; morphology, 47; physiology, 54 ; natural orders, 562. Anisonema, 90. Annelida, 50, 51, 66, 164, 171, 193, 206, 207, 212, 219, 416, 575, 5S0, 586. Annuloid series, 5S0. Annulose differentiation. 52. Anodonta. 407. 408, 412, 416, 417. Anomia, 411, 417. Anomura. 293. 294. Anthophysa, 90. Antinophrys Eichhornii, 13. Antipathida?, 145, 146. Aphis. 36, 37, 3S0-384. pelargonii. 364. Aphroditidae, 210. Apoda, 260. Aporosa. 146, 147, 150, 153. Appendages, 20. Appendicular^, 53, 510-5 IS, 576. flabellum, 511. Aprocta, 158. Aptychus, 459, 460. Apus, 220, 223, 226. 242-245. cancriformis, 243. g-laeialR 245. Arachnida. 59, 221, 224, 320, 573. Araneina, 326. Area. 417. Areella, S6. Arctisca, 334. Argonauta argo, 461, 462. Argulus. 241. "Aristotle's lantern," 492. Arthrosastra. 320. Arthropoda. 21, 32, 36. 52. 53. 57, 64. 66, 193, 206. 219-225. 320. 573, 574, 580, 5S6. Arthrozoic series, 5S0, 590 INDEX. Articulata, 403. Ascaris nigrovenosa, 551. Ascetta primordialis, 104. Ascidians, 45. 53. Ascidioida. 510. Ascones, 106, 110. Ascula, 106. Asellas, 813, 317. Aspergillum, 406. Aspidobranchia, 444. Aspidogaster ccnehiola, 172— ITS. Astaeus. 66. 219, 264-293. fluviatilis, 266. Asterida?, 466. 4T4, 475. Astraea calycularis, 146. A tax Bonzi, 330. Athorybia, 130. rosacea, 127-129. Atolls, 151. Atrocha, 213. Aurelia aurita, 122. Avicularia, 393. B Bacteria, 12-14, 38. Balanidae, 260. Balanoglossus, 53, 539, 576, 582. Balantidium, 97, 98. Balanus, 254-260. balanoides, 258 Bees, 33, 34. Beetles, 366. Belemnitidie, 463-465. Beryx, 40. Blcosoeca, 90. Bilharzia, 178. Biogenesis, 40. Biology, principles, 9 ; divisions, 16. Bipinnaria. 481. Blastoderm, 382. Blastoidea, 509. Blastomere, 20, 22, 28, 32, 34, 48. 317, 415. Blastosphere. 415. Blastostyle, 119. Blatta, 343. 349. 356, 371, 373, 377, 380, 382. orientalis, 346, 357, 360. Blood and circulatory apparatus, 56. Bojanus, organs of, 52, 57, 61, ^, 411. Bombus, 369. 372. Bothriocephalic, 187. latus, 184. Botryllida?, 514, 518, 522, 524-528. Botrvtis Bassiana, 45. Brachionus, 168, 169. Brachiopoda. 389, 396, 397. 417, 586. Brachyura, 281, 293, 294, 295. Branchellion, 189. Branchiae, 58. Branchiogasteropoda, 424, 433, 434, 436, 437. Branchiopoda, 242. Branchipus, 247, 248, 249. Brisinga, 480, 481. Bryozoa, 389. Buccinum, 434. . undatum, 420. Bucephalus polymorphus, 181. Bugula avicularia, 393. Butterflies, 366. C Calcispongi^;, 104-110. Caligus, 241. Calycophoridae, 36, 117, 12S-131. Cambium layer, 21. Campanularia, 119. Campanularida?, 117, 118. Campodea staphylinus, 362. Capitella, 200. Caprella, 313. Carcinus mcenas, 295, 302, 303. Cardium, 417. Carmarina, 115, 135. Caryophyllaeus, 182. Catallacta, 89, 101, 574. Caulerpa, 48. Cecidomyia, 385. Cells, 17, 21, 28, 31, 32. Cell- wall, 18. Centipedes, 344. Cephalopoda, 64, 66, 404, 418, 419, 425-435, 444,44f>. Cephea ocellata, 124-126. Cereariae, 53, 180-182. Cereanthus, 145. Cestoidea, 56, 157, 1S2, 575. Cestracion, 69. Cetacea, 69. Chaetoderina, £71. Chsetogaster, 193, 194. Cha-tognatha, 540, 577, 586. " Challenger " expedition, 68, 70, 79, 81. Changes, cvclical, in living matter, 10. Chara, 21.' Chemical composition of living matter, 9. Chick, 19. Chilodon, 98. Chilognatha, 337, 338. Chilopoda, 337. Chitonidse, 430, 431. 434, 571, 572. Chlamydomonas, 46. ChloraJma, 210. Chlorophyll, 45, 97. Chondrac'anthus gibbosus, 237-241. Chromatophores, 445. Cicadae, 365, 377. Cidaris, 487. Cilia, 29. 73. Ciliata, 93-101. Circulatory apparatus, 56. Cirripedia, 221, 253. Cladocera, 242. Classification of living forms, 23. Clepsine, 190-192, 568-571. Climate in relation to animal life, 69. Cliona, 107. Clionida?, 110. Clypeastroida. 489. Cockroach, 343. Codonoeca, 90. Codonellida, 98. Codosiga, 90. Ccelenterata, 45, 50, 51, 56, 102, 109, 110, 115, 574, 586. Coenurus, 185. Cold, action of, on living matter, 12, Coleochaete, 46. Coleoptera, 366, 375, 377. Collembola, 220, 262, 363, INDEX. 591 Collosphaera, 85. Colpoda, 62, 95, 96, 98. Comatula, 36, 37. (Antedon), 500. Conjugation, 31, T4. Contractile tissue, 29. vacuole, 73. Copepoda, 68, 23-4, 236, 300-302. Coralligena, 138, 574. Corallines, 390. Corallite, 139. Corallium rubrum, 144. Corals, 110. Cordylophora, 36, 37. Coryne, 118. Crayfish, 276, 2S4-286. Crickets, 376, 378. Crinoidea, 466, 497. Crocodilia, 26. Crustacea, 21, 26, 58, 65, 222-225, 573. Cryptogamia, 32. Crytopbialus, 261. Ctenobranehia,439, 444. Ctenophora, 53, 63, 68, 138, 153, 162, 192, 574. Cucullanus elegans, 548, 550. Cuma Eathkii, 308. Cumacea, 264, 308. Cunina, 568. rhododactyla, 136. Cyamus, 313. Cyansea, 36. capillata, 134. Cyclops, 234, 235, 550. Cyclostomata, 439. Cydippe (Pleurobrachia), 155. Cymothoa, 314, 317. Cymothoadae, 316. Cynthia, 299. Cypraea Europaea, 420. Cypris, 251, 252. Cystic worm, 186. Cysticercus, 184, 1S6. Cystidea, 508. Cythere, 251, 252. Dalmanites, 227. Daphnia, 247. 248. Decapoda. 462. Deep-sea fauna. 26, 68, 80, 81. Dendroccela, 160. Dentalidae, 430, 431. Dentalium, 422. Dermis (enderon), 55. Desmidiae, 89. Development, 17, 19, 66. Diatomaceae, 12, 75, 80, 89. Dibranchiata, 450-456. Diceras, 406. Dicoryne conferta, 119. Dictybeysta?, 76. Dictyocystida, 98. Dicyema, 579. Dicyemida, 558, 577, 578. Didemnum styliferum, 526. Didinium, 98. serpula, 13. Differentiation, 20. 38 Dimyaria, 412. Diphydae, 131. Diphyes appendiculata, 126. Diphyllidea, lb7. Diphyozooid, 126, 131. Diplozoon paradoxum, 33, 1S2. Dipnoi, 60. Diporpa, 33. 182. Diptera, 366, 375, 381. Discophora, 118, 121, 132-135. Disintegration of living matter, 10. Distoma, 179. Distribution. 16, 24-26, 67-69. Dog-louse, 1S7. Dogs, retrieving of, 35. Doliolum, 514, 518, 528. denticulatum, 529. " Double circulation," 60. Dragon -flies, 221. Dysteria, 98. E Earthworm, 193. Echeneibotbrium, 187. Echinidea, 56, 466, 4S5, 488, 489. Echinococcus, 184. veterinorum, 185. Echinoderes, 171. Ecbinodermal series. 581, 582. Echinodermata, 26, 36, 53-55, 466, 577, 586. Echinoida. 4S9. Echinopaediuni, 54, 466, 481, 505, 5S2. Ecbinorhynchus, 553. Echinus, 4S6. sphaera, 487, 488. Ectoderm, 55, 56. Ectoprocta, 394, 571, 572. Ectosarc, 74. Edrioasterida, 508. Edriophthalmia, 310. Elytron, 204. Embryology, 42, 50, 582. Empusa, 45. Endoparasites, 182. Endoplast, 48, 74. Endoplastica, 73, 82. Endoprocta, 571. Endosarc, 74. Endostyle, 511. Enteropneusta. 59, 538, 576, 581. Entoconcha mirabilis. 440. Entogastric gemmation, 135, 568. Entomostraca, 224, 234. Entoprocta, 394. Eozoon, 72. canadense, 82. Epiblast, 21, 51. Epidermis (ectoderm), 55. Epigenesis, 19. Epimera, 268. Epizoa. 237. Equidae, 26. Ergasilus, 241. Eristalis floreus. 368. Errantia. 206, 207. Ervilia, 98. Estheria. 243-250. Euaxes, 199, 569, 570. 592 INDEX. Euglena, 12. viridis, 90. Euphausia, 307. Euplectella, 110. Eurypterida, 232, 234. Eurypterus remipes, 233. Evolution, 40. Families, 23. FauDa, oldest known, 72. See Fossils. Fauna?, dissimilar, 24. Fecundation, 33, 34. Ferns, 21. Fibrospongia, 109, 110. Fishes, 59-65. Fish-lice, 237. Fission, 31. Flagellata, 89-95. Flagellum, 73. Fleas, 366. Flies, 366, Florae, dissimilar, 24. Florideae, 33. Flower-buds, 21. Food-vacuole, 90. Food-yelk, 32. Foraminifera, 48, 68, 77-82, 85, 86, 574. Fossils : succession of species, distribution, etc.. 24, 25, 40, 43, 68, 71, 81, 136, 153, 225, 253, 263, 310, 317, 342, 416, 443, 459, 463. 488, 507. * cambrian, 82. carboniferous, 508. cretacious, 82, 110, 417. devonian, 250, 432, 459. laurentian, 82. lias, 465. limestone, 81, 82, 153. mummulitic, 82, — Silurian, 82, 137, 232, 251, 397, 431, 436, 507. — Solenhofen slate, 136. trias, 465. Fowl, 33. Fringing-reefs, 150. Fnnctions, 27, 54. Fungi, 12, 20, 27, 32, 38, 45. Fungidae, 146, 151. Galeodes, 326. Gamogenesis, 32. Ganoids, 60. ( Gasteropoda, 404, 432. Gasterostomum. 182. Gasterotricha, 170. Gastraea, 51. Gastrophysema, 107, 110. Gastrula, 107. Gecarcinus, 295. Gemmation, 30. 525. Generation, 30-32. Genus, 23. Gephyrea, 59, 189, 215, 570, 573, 574. Geryonia, 568. Geryonidae, 117, 135. Glass-crabs, 308. Globigerina, 40, 79-82. Glossocodon, 115. Gnathites, 224, 236. Gomphonema, 75, 96. Gonodactylus, 319. Graafian follicles, 66, 380. Graptolites, 137. Gregarina, 73, 87, 88. gigantea, 88, 89, 96. Gregarinidae, 86-88, 574. Gromia, 78. Gromidae, 79. Growth of animals and plants, 10, Gymnolaemata, 395. Gymnophthalmata, 118. Gymnosomata, 435-437. Gyrodactylus, 182. II Haliotis, 423. Haliphysema, 107, 110. Halisarca, 107, 110. Heat, effect of. on living matter, 11. Hectocotylus, 461. Helicidae, 442. Heliopora caerulea, 148. Heliozoa, 563, 574. Helix, 428, 442. pomatia, 443. Hemiptera, 365, 366. Hereditary transmission, 35, 41, Hermaphrodites, 182, 192, 198, 224, 413, 442, 4S4, 551. Heteromorphae, 69. Heteronereis. 215. Heteropoda, 424, 426, 439. Heterotricha, 95. Hexacoralla, 145-147. Hippuritidae, 417. Hirudinea, 189, 190, 192', 194, 213, 569, 574, 575. Hirudo medicinalis, 190, 191. Histology, 16. Histriobdella, 189-192. Holomyaria. 549. Holothuria, 153. Holothuridea, 59, 466-468. Holotricha, 95. Homarus, 66. Humming of insects, 223. Hyalonema, 110. Hydatina senta, 168. Hydra, 56, 62. 63, 115, 118, 585. Hydractina. 65, 115. Hydranth. 110. Hydrophilus piceus, 365. Hydrophora. 118, 132. Hydrophyllia, 117. Hydrosoma, 116, 117. Hydrotheca. 117. Hydrozoa, 36, 65, 107, 110, 132-138, 153-155, 574, 575. ^ Hymenoptera, 369. Hopoblnst, 21. 51. Hypotricha, 95. ICHTHYOPSTDA, 57. Idoteidae, 315. INDEX. 593 Imperforate, 79. Impregnation, 81. Inarticulata, 403. Infusoria, 12, 20, 33, 45, 48, 74, 77, 89-91, 94, 99-105, 157, 565, 574. ciliata. 89. flagellata, 89, 90. tentaculifera, 89. Insecta, 21, 59, 67, 224, 316, 342, 372-3S6, 573. Insectivorous plants, 44. Integumentary organs, 55. Invertebrata, 'morphological types among, 49. Isocardia, 406. Isopoda, 313. lulus, 342. Ixodes ricinus, 330. JANELLID.E, 441. Jaws, 56. Jelly-fishes, 110. Labium, in insects, 203. Lacinularia, 168-171. Laemodipoda, 313, 316. Laniellibranchiata, 404, 405, 573. Lampyris splendidula, 379. Laomedea, 120. Larvae, 66, 164, 166. 169, 170, 179, 213. 214, 247, 258, 319, 332. 339, 365, 371, 375, 385- 388, 402, 403, 467, 550, 568, 582. Leeches, 189, 570. Lepadidae, 260. Lepas. 254-260. — — australis. 258. Lepidoptera, 366, 375, 377, 381. Leptoplana, 162. Lernae, 241. Lernaeodiscus porcellanae, 262. Leucifer, 299. Leucones, 106, 110. Lice, 363. Lieberkiihnia, 78. Ligula, 182. Lima, 408. Liraacidae, 440. Limax. 423, 427-429. Limnetis, 247-249. brachyurus, 249. Limpets, 438. Limulus. 226-235. 32S. moluccanus. 228. polyphemus, 231. Lineus, 165. Linguatula, 320, 334. Lingula, 397.401. Lithoeysts. 115. Lituitidae. 79. Living matter, properties of, 9-42. Lobster, 264. Locustidae, 378. Loxosomma, 394, 416. 571. Lucernaria, 122, 123, 132, 137, 138. Lumbrieus, 193-195. Lungs. 59. Lymnaous, 427. 429. palustris, 423. Macrobiotus Schultzei, 333. Macrostomum, 158-160. Macrura, 281, 293-299. Madrepores, 147. Madreporite, 489. Magosphaera, 89. Malacobdella, 189-192, 570, 575. Malacoscolices, 576. 580. Malacostraca, 224, 264. Malacozoic series, 580. Mallophaga, 362, 363. Manubrium, 116. Mastigopods, 73. Meandrina, 151. Medusae, 36.37, 110, 115-118, 575. Megalopa, 302, 303. Meromyaria, 549. Merostomata, 224, 227. Mesoblast, 21. Mesoderm, 55, 56. Mesotrocha, 214. Mesozoa, 578. Metabola, 361. Metamorphosis, 66, 3S6. Metazoa, 48, 51-58, 102, 110, 166, 171, 578, 582, 583, 585. Microstomum, 163. Miliolidae, 79. Millepores, 147, 148, 151, 153. Millipedes, 337. Mites, 329. Moisture, effect of, on living matter, 11. Molar motion, 27. Mollusca, 55-61, 76, 389, 404, 572, 573, 580, 581. Monads, 38. 39. 46. 77, 85. 89, 90, 103. Monera, 73, 77, 85, 574, 578, 582. Monomyaria, 412. Monostbmum mutabile. 179. Morphological species, 22. Morphology, 16. Morula, 48. Moths. 33. Mucor, 88. Munna, 310. Muscular tissue, 27, 29. Mussel, 407, 415. Mygale Blondii, 328. casmentaria, 327. Mvriapoda, 59, 224, 342, 573. Mvsis, 291, 299, 303, 317. Mytilus, 409. 417. Myxastrum 77, 86. Mvxodictvum, 75, 77. Mvxomvcetes, 13. 46, 86. Myxopods. 73, 75, 82. 83. Myxosponeiae, 109, 110. Myzostomata, 537, 574. N Nair. 193, 194. Naked-eved medusae. 118. Nauplius. 234. 237, 247, 253, 25S-263, 300-302, 307,317,331,333. Nautilus. 64, 65. 69, 447-465. Nebalia. 242. 247. 24s. Nematoidea. 32, 545, 575. Nematophores, 119. 594 INDEX. Nematorhyncha, 569, 575. Neinatoscolices, 575, 580. Nemertidae, 105, 166. Neomenia, 571. Nephelis, 192. Nerve, 27, 29. Nervous system, 61. Neuroptera, 366, 375, 377. Noctiluca, 46, 90-92. Notodelphys, 241, 242. Notommata tardigrada, 170. Nova Scotian coal fossils, 342. Nucleus, 17, 48, 74. Nucula, 408, 416. Nudibranchiata, 424, 437, 438. Nullipore^l5(), 151. Nummulites, 79. Nyctotherus, S7, 98. OCTOCORALLA, 143, 146. Octopoda, 452, 460. Odontophora, 404, 417, 419, 573. Oligoehaeta, 66, 189, 193, 198, 199, 213, 574. Onchidum, 60. Oniscidaj, 316. Oniscus, 219, 313. 0{>hiodes, 119. Ophiolepis ciliata. 484. Ophiuridea, 466, 482. Opkrydidae, 100. Opisthobranchiata, 437, 438. Opisthomuui, 159. Orbulina, 79. Orders, 23. " Organized," meaning of, 15. Organs, 28, 64, 65. Origin of living matter, 41. " Origin of Species," 37. Orthidae, 403, 404. Orthoptera, 364, 366, 375, 377, 381, 383. Ossicula auditus. 65. Ostracoda, 221, 251. Ostraea, 408-417. Oviparous animals, 67. Ovoviviparous animals, 67. Oxidation, waste of living matter by, 10. Oxyuris, 547. ' Oyster, 408. PAGTTRrD^;, 299. Palseocyclus, 153. Palaeontology. See Fossils. Palinurus vulgaris, 293. Paludina, 180, 181, 425, 426. Pangenesis, 41. Paramoecium. 48, 96-100. Parasites, 45, 67, 171, 181-183, 187, 237, 241, 242, 253, 262, 263, 314, 331, 334, 363, 387, 416,440,550,553. Pasteur's experiments, 12, 13. Patellidae. 438, 444. Pecten. 408-416. Pectostraca, 253. Pedalion, 170, 171. Pedicellina. 571, 582. Pedicels, 54. Pediculina, 362, 368. Pelagia, 132, 133. Peltogaster paguri, 262. Peneus, 300-302. Penicillium, 45. Pennatnlidae, 145. Pentacrinus. 500. Pentastomida, 225, 234, 573. Pentastonmm taenioides, 385. Pentremites, 509. Perennibranchiata. 58. Perforata, 79, 146-150, 153. Peridineae, 76, 93. Peripatidea, 225, 534, 573. Peritricha, 95, 96, 100. Perla nigra, 364. Peroniadae, 440. Peronia verruculata, 443. Peronospora, 45, 46. Phalangidae, 326. Phallusia, 515, 520. Pharyngopneusta, 577. Pbaryngopneustal series, 581. Pholas, 406, 408, 417. Phoronis, 217, 218. Phrosina, 315. Phrynidae, 326. Pbylactolaemata, 572. Phyllodoce, 214. viridis, 211. Phyllopoda, 242. Phyllosomata, 307, 308. Pbylogeny, 42. Pbysalia, 115, 12?, 130. Physemaria, 552, 574, 575, 5S5. Physiologv, 9, 16, 26. Pbysophorida?, 117. 128-132. Pihdium gyrans, 165, 166. Pisidium, 414, 415. Placenta. 67, 101. Piagiostome fishes. 67. Planaria, 161, 192, 575. dioica. 161. Plants. 31-33, 44, 68. Pleurobrancbia, 155. Pleurodictyon, 153. Plumatella repens, 390, 391. Plumularidae, 118. Pocillopora, 148. Podophrya fixa, 94. Podophthalmia, 230, 264. Poduridae, 382. Pcecilopoda, 228. Polian vesicles. 468, 483. Polyarthra, 171. Polycelis, 192. laevigata, 161. Polycba?ta, 66, 189, 199, 200, 207, 574, 575. Polycistina, 83. Polygordius. 575. Polykricos, 96. Polymyaria, 549. Polynoe, 200. hinulata, 210. squamata, 200-207, 212. Polyophthalmus, 200, 212. Polvpes, 31, 62, 110. Polypide, 390. Polypite, 110. Polyplacophora, 430, 433, 573. INDEX. 595 Polyzoa, 53, 56, 3S9, 5T1, 572, 576. Polyzoariurn, 390. Pontellidae, 237, 318. Porifera, 51, 55, 62, 102, 109, 574. Porpita, 51, 127. Porites, 151. Prawn, 300. Priapulus, 216. Primordial utricle, 13. Proctucha, 15S, 162. Prodictidae, 403, 404. Proglottis, 1S6. Prosobranchiata, 433, 439, 444. Protamoeba, 75, 86-89. Protein, 9. Proteolepas bivincta, 261. Proteus, 60. animalcules, 86. Protococcus, 12, 17. Protogenes, 75-78, 86, 89. Protomonas, 77, 85, 89. Protomyxa aurantiaca, 76. Protoplasm, 9, 14. Protoplasta, 86, 574. Protozoa, 38, 47, 48, 54, 61, 62, 73, 102, 103, 103, 574. Protozoic series, 579. Protula, 207, 215. Dysteri, 208. Provinces of distribution, 24. Pseud-haemal system, 57. Pseudo-filaria, 83, 89. Pseudo-navicella, 88. Pseudophyllidea, 187. Pseudopodia. 29, 73, 75. Pseudo-scorpions, 326. Psychology, 9. Pteropoda, 53, 68, 401, 424, 432-434. Pterygotus, 233. Puhcidse, 3S1. Pulmogasteropoda, 59, 424, 433. Pulmonary sacs, 59. Pulmonata, 423-429, 440. Pulvinularia, 79, SO. Pupipara, 367. Pycnogonida, 331. 573. Pyrosoma, 514, 528. giganteum, 523. R Radiolaria, 12, 46-48, 6S( S0-S3, 93, 564, 574. Redia, 179, 131. Reef- builders, 149. Renierinae. 109. Reproductive system, 65. Respiratory system, 53. Rhabaoccela, 159. Rhabdopleura, 396, 397. Rhachis. 548. Rhizocephala. 253, 263. Rhizocrinus lofotensis, 493. Rhizostouiidae, 124. Rhodope. 425. Rhopalodina, 470. Rhynchocoele turbellaria, 163. Rliynchonella, 398, 400. Rhynchonellidas, 403. Rock-builders, 81, 82. Rotalia, 78. Rotifera, 12, 56, 89, 157, 162, 166, 574. Rugosa, 148, 149, 153. Sacculixa purpurea, 262. Sagitta. 6s, 542, 577. Salpa. 36, 514, 518, 531, 532. Salpingoeca, 90. Sarsia prolifera, 120. Saxieava, 40^. Scallop. 408. Scalpellum vulgare, 260-262. Scaphopoda. 430-433. Schizoccele, 51, 52, 219. Schizopoda. 293. 299. Scolopendra borbonica, 337. Ilopei, 338. Scorpio afer, 821. Scorpions. 59, 320, 323, 324. Scrupoeellaria ferox, 392. Scyllarus, 294. Sea anemones, 110. Sensitive plant, 44. Sensory organs, 27. Sepiadae. 446,452, 463, 465. Sepia officinalis, 446, 453. Serial relations of invertebrata. 578. Serpulidae, 207, 214. Sertularidae, 117, 113. Shrimp, 3(13. Siphonophora, 118, 127, 133. Sipunculus nudus, 216-218. Snail, 442. Solenhofen slates, 136. Somatoplem-e, 57. Somites, 200. Sounds from insects, 376. Spatangoida, 439. Spermatophores, 454. Sphaeromidae, 315. Sphaerozoum ovodimare. 84, 35. punctatum, 84. Sphinx ligustri, 367. Spiders. 326. Spiriferidae, 403. Spirillum volntans. 12. Spirorbis. 207. Spirostomum, 97. Spirulidae. 463. Splanchnopleure. 56, 57. Sponeida, 102, 567. Sponsilla,' 104, 111. fluviatilis. 103, 107. Sporocysts. 183. Springs, hot, living things in, 14. Squids. 463. Squilla, 312, 319. scabricauda, 318, 319. Star-fish. 474. Stentor, 100. Stephanoceros. 167, 169, 170. Sternaspis, 215. Stigmata. 59, 325. 339, 356, 374- Stings of insects. 372. Stomatopoda. 237, 317. Stone corals, 147, 149. Strepsiptera, 371, 3S7. Strombidium. 94. " Struggle for existence," 30. 596 INDEX. Stylifer, 440. Stylonychia, 100. Stylops aterrimus, 38T. Sun-animalcule, 82. Sundew, 44. " Survival of the fittest/- 41. Sycandra raphanus, 568. Sycon, 110. ciliatum, 106. Syllis, 214, 215. vittata, 211. Synapta, 440, 467. digitata and inhaerens, 471. Syncytium, 105. Syrphus ribesii, 368. Tabttlata, 146, 148, 150. Taenia, 1S3-188. Tape-worms, 182. Tardigrada, 225, 334, 573. Taxonomy, 16,22,561. Teeth, 56. Tegumentary system, 55. Telotrocha, 164. Temperature in relation to living matter, 11, 39. Tentacula, 51. Tentaculifera, 93. Terebratula, 40. psittacea, 40. Terebratulidae, 404. Terebratulina septentrionalis, 400. Teredo, 406. 417. Testacellidse, 440. Tetrabranchiata, 455. Tetraphyllidea, 187. Tetrarhynchus, 183, 187. Tetrastemma. 163,164. Teuthidae. 452, 463, 465. Thecosomata, 435-437. Thysanopoda, 299. Thysanura, 220, 362, 363. Ticks, 329. Tissues, 17. Tomopteris, 207. Torquatella, 98. Torula, 38. Trachea?, 59. Tracheo-branchia?, 221. Trachynemata, 135. Tradescantia hair, 75. Trematoda, 53, 56, 157, 171-173, 182-188, 190, 574, 575. Tremoctopus, 462. Triarthra, 170,171. Trichina, 550, 551. Trichocvsts, 97. Trichodidse, 100. Trichodina gratidinella, 96. Trichoscolices, 575, 580, 581. Trigonia, 69. 416. Trilobita, 220, 224, 225, 573. Trochus cinerarius, 420. Tubicola, 20 7. Tubifex, 193, 199. Tubipora, 146. Tubularidae, 118, 132. Tunicata, 53, 55, 59, 67, 510, 576, 581. Turbellaria, 45, 51, 56-61, 65, 157, 573-575, Tylos, 315. Types, morphological, 49. Typhlosole, 196, 518. U Unio pictorum, 415. Uropoietic system, 61. Vaginulus, 429. Vampyrella. 75, 96. Vanessa atalanta, 367. Velella, 127. Ventriculites, 110. Veronicellidae, 440. Vertebrata and Invertebrata, 49. Vibracula, 393. Vibrionidae, 89. Vital force, 15. 16. Viviparous animals, 67. Volvocinese, 89. Volvox, 89, 579. Vorticellidae, 5, 33, 48, 62, 64, 94-101. W Waldheimia, 398. australis, 399. Waste of living matter, 10. Water in living matter, 10. Willsia, 120,121. Wolffian duct, 61. Xtphosuea, 228, 232. Yeast, 12. Yeast-plant, 45. Zo.£A, 302, 303. Zoanthida?, 145, 146. Zoanthodeme, 138. Zoological chronology and geography, 70u Zoophyta, 574. 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