ELEMENTARY TEXT-BOOK OF ZOOLOGY. GENERAL PART AND SPECIAL PART: PROTOZOA TO IN8ECTA. BY DB. C. GLAUS, Professor of Zoology and Comparative Anatomy in the University of Vienim; Director- of the Zoological Station at Trieste. TRANSLATED AND EDITED BY ADAM SEDGWICK, M.A., Fellow and Lecturer of Trinity College, Cambridge, WITH THE ASSISTANCE OF LONDON: W. SWAN SpNNENSCHEIN k CO., PATERNOSTER SQUARE. 1884. Priuted by H.zell. Watson. & Viney, Limited, LoBd^T^d Aylesbury. PREFACE TO THE ENGLISH TRANSLATION. I UNDERTOOK the translation of Professor Glaus' excellent "Lehrbuch der Zoologie " with a view of supplying the want, which has long been felt by teachers as well as students in this country, of a good elementary text-book of Zoology. Professor Glaus' works on zoology are already well known in this country ; and I think it will be generally admitted that they take the .first place amongst the zoological text-books of 'the present day. It has been decided to publish the English translation in two volumes. The second volume, which begins with MoUuSca, is in the press, and will, I trust, appear early in the autumn. The German has been, with one or two unimportant exceptions, closely followed throughout. These exceptions, and the few additions which I have thought it necessary to make, have in all cases been indicated by enclosure within brackets. I must ask the indulgence of the reader towards the errors and deficiencies of this translation. I trust that they will be found to be neither numerous nor important. I have to thank Mr. Heathcote for the assistance he has given me in the laborious work of translation. I am also indebted to Professors Newton and Foster, Dr. Gadow, and Mr. W. Heape for advice; and assistance. ADAM SEDGWIGK. Tbinity College, Cambridge, 1884. TABLE OF CONTENTS GENERAL PART. CHAPTEE I. ORGANIZED AND UNORGANIZED SUBSTANCES CHAPTER II. ANIMALS AND PLANTS . ORGANIZATION AND GENERAL CHAPTER III. DEVELOPMENT OF ANIMALS IN Page y— 14 15—24 •24—131 Individual, Obgan, Stock Repetition of organs and parts of the body . Cells and Cell Tissues Nucleus and Nucleolus Cell-membrane Reproduction of Cells and division of Nucleus 1. Cells and Cell Aggregates Isolated cells, e.g., blood corpuscles, ova, etc Epithelium EpideiTQal exoskeleton Glandular tissue 2. Tissues of tlie connective substance Cellular or vesicular Mucous or gelatinous Reticular, adenoid . . _ Fibrillar .... * Elastic Cartilage Osseous tissue 3. Muscular tissue 4. Nervous tissue . . • • • • • ■ Inckease in Size and Peogeessive Diffeeentiation, etc Unicellular stage Multicellular stage COEEKLATION AND CONNECTION OP OBGANS Doctrine of Final Causes '^Type" Scope of Morphology .... Steuctube and Function of the Compound Oegans Digestive organs Salivary glands, liver, pancreas 24 25 29 29 29 30 32 32 34 34 36 37 37 87 38 38 39 39 40 43 45 47 48 49 50 51 52 52 62 53 58 TABLE OF CONTENTS. 5 Organs of oirculatmi Heart Arteries and veins Heart and vessels of vertebrates Organs' of 7'esjpiration . Brauchi', sternal artery. pericardial sinus, from which it parses through the venous ostia into the heart (Scorpions, Decapods) (fig. 54). In other cases (Molluscs) the blood flows directly from the afferent vessels into the heart, the walls of the vessel being directly continuous with the walls of the heart. The heart in such cases consists of two chambers, the one known as auricle serves for the reception of the returning blood, the other known as ventricle for its propulsion (fig. 55). The vessels passing from the ventricle and carrying the blood from the heart are called arteries ; those returning the blood to it are called veins, and, in the higher animals, are distinguished from the arteries by their thinner walls. Between the ends of the arteries and the beginning of the veins the body cavity intervenes either as HEART OF VBETEBBATES. 03 a blood sinus or as a system of blood-lacunse ; or the arteries and veins are connected by a network of delicate vessels, tbe capillaries. If the connection between arteries and veins is effected by capillaries in all parts of the vascular system, and the body cavity, as in the Vertebrata, no longer functions as a blood sinus, the vascular system is spoken of as being completely closed. In the Veii;ebrates and segmented worms the vascular system ob- tains a considerable development before a true heart is differentiated in it. At first rhythmically pulsating sections, very frequently the Fig. 55.— Nei-vous system and circulatory organs of Paludina vivipara (after Leydig). F. tentacle ; Oe, oesophagus ; Cff, cerebral ganglion with eye ; Pff, pedal ganglion with adjacent otocyst ; Vg, visceral ganglion ; Phg, pharyngeal ganglion ; A, aiiricle of heart; Fe, ventricle; ^u, ahdominal aorta; .4c, cephalic aorta ; F, vein; Fe, afferent vessel. Br, gill. dorsal vessel, or the lateral vessels connecting this with the venti-al vessel (fig. 56), serve for the propulsion of the blood. Similarly amongst the Vertebrata, the lancelet (Amphioxus) possesses no distinctly differentiated muscular heart, the function of that organ being discharged by various parts of the vascular system which are contractile. The arrangement of the vessels supplying the pharyngeal section of the alimentary tract, which has a respiratory function and is known as the branchial sac, admits of a comparison with the vascular arrangement of the segmented woi'ms, and repre- sents the simplest form of the vertebrate vascular system. The longitudinal vessel which runs in the ventral wall of the branchial sac gives off numerous lateral branches, which ascend in the branchial walls. These lateral vessels are contractile at their point of oxigin (>4 OHQANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. f I'om the ventral vessel. The anterior pair, placed behind the mouth, mute beneath the notochord to form the root of the median body artery (descending or dorsal aorta) which receives the hinder succes- sive pairs of lateral vessels. This dorsal artery gives off branches to the muscles of the body wall and the viscera, from which the venous blood in part is returned to the ventral pharyn- geal vessel; part of it, however, before reaching the latter, traverses a capillary network in the liver. From the hinder part of the ventral pha- ryngeal vessel there is developed, in the higher Vertebrata, the heart, which at first has the shape of an S-shaped tube, but later acquires a conical form and becomes divided into auricle and ventricle. The former receives the blood returning from the body and passes it on into the more powerful ventricle, from which arises an anterior vessel, the ascending or cardiac aorta, presenting a swelling at its root, known as the aortic bulb. This vessel leads, by means ^ of lateral vascular arches, the arterial arches, into the dorsal aorta, which passes backwards beneath the vertebral column, and supplies the body. Yalves placed at the two ostia of the ventricles regulate the direction of the blood stream ; and they are so arranged as to prevent any lackward flow of blood from the cardiac aorta into the ventricle in diastole, and from the ventricle into the auricle in systole. In consequence of the insertion of the respi- ratory organs on to the system of the arterial arches, the latter, and at the same time the structure of the heart, assumes various degi*ees of complication. In fishes (fig. 57), four or five pairs of gills are inserted in the course of the arterial arches, which break up into a respiratory capillary net- work in the branchial leaflets. From this network the arterialised blood is collected into efferent branchial arches, the branchial veins, corresponding each to a branchial artery ; and these unite to form the dorsal aorta. In such cases the heart remains simple, and receives venous blood. Fig. 5G. — Anterior part of the vascvilar system of an Oligochsete worm (Sffinuris) (after Ge- genbaur) . In the dor- sal vessel the blood moves from behind forward ; in the ven- tral vessel from before backwards (see ar- rows). H, heart-like dilated transverse lateral vessels. PULMONARY OIECULATIOIf. 65 With the appearance of lungs as respiratory organs (Dipnoi, Perennibranchiate Amphibia, larvse of Salamanders and Batra- chians) (fig. 58), the heart obtains a more complicated structure, in that the auricle becomes divided into a right and left division, the latter of which receives the arte- rialised blood, returning from the lungs by the pulmonary veins, Tlie septum between the two divisions of the auricle may, how- ever, remain incomplete (Dipnoi, Proteus). The advehent pulmon- ary vessels, the pulmonary arte- ries, always proceed from the FxG. 57. — Diagram of the circulator7 organs of an osseous fish. V, ventricle ; Ba, aortic bulb with the arterial arches which carry the venous blood to the gilla ; Ao, dorsal aorta into which open the vessels from the gills or branchial veins Ab. N, kidney; B, alimen- tary canal ; Lk, portal circulation. Fig. 58.— Gills {Br) and pulmonary sacs (P) of a perennibranchiate amphibian. Ap, pulmonary artery proceeding from the posterior of the four aortic arches. The other three lead to the three pairs of gills ; D, alimentary tract ; A, aorta. posterior vascular arch, which, as a rule, loses its relation to the branchial respiration. On the disappearance of the gills, which is completed during the metamorphosis in the Salamandrina and Batrachia, the pulmonary 6 GO OBOANIZATION AND DEVELOPMENT OF ANIMALS IN QENEEAL, arteries obtain a mucli more considerable size and become the direct continuation of the hindermost pair of vascular arches, while the remaining and primitively most important portions of the latter, i.e. the portions leading to the dorsal aorta, are reduced to r-udimentary ducts (Ductus Botalli) or completely obliterated. Contemporaneously with these changes there appears a fold in the lumen of the ventral or cai'diac aorta, leading to a separation of the posterior vascular arch (pulmonary artery), which now receives through the ventricle venous blood from the right auricle, from the system of anterior arches which give origin to the cef)halic vessels and dor- sal aorta and receive arterial blood from the left auricle (mixed, how- ever, with venous blood in the ventricle) (fig. 59). In Reptiles the sepa- ration of the arterial from the venous blood is more complete, in that there is an incomplete ventricular septum which foreshadows the later division of the ventricle into a right and a left half. From the left division arises the right aortic arch, which gives origin in its further course, to the arteries to the head (carotid arteries). A vessel to the lungs and a left aortic arch may also be distinguished. The left aortic arch and pulmonary artery receive only venous blood, while the right aortic arch, and therefore the carotids which proceed from it, receive principally arterial blood from the left side of the ventricle (fig. 60). The ventricular septum, and consequently the separation of the right from the left ventricle, is found complete for the first time Fig. 59.— Circulatory organs of the frog. P, left lung, right lung is removed ; Ap, pulmonary artery ; Vp, pulmonary vein ; Vc, vena cava inferior ; Ao, dorsal aorta ; N, kidney ; D, alimentary canal ; U, portal circulation. LYMPHATIC srSTEM. 67 in the Crocodilia, and in these animals the right aortic arch arises from the left ventricle. But the separation of the arterial and venous blood is even now not quite complete, for at the point where the two aortic arches cross one another there is a passage (foramen Panizzc^) leading from one into the other, and through which a commimication may take place. It is only in Birds and Mammals, in which, as in the Crocodilia, the right and left ventricle are completely separated, that a separation between the two kinds of blood is completely effected (fig. 61). In Birds the right aortic arch persists, and the left entirely disappears ; while in Mammalia the opposite obtains, the left arch per- sisting and giving rise to the dorsal aorta. In these animals the blood is essentially diffe- rent from the chyle both in colour and composition, and there is present a special system of chyle and lymph vessels. This system origi- nates in simple tissue spaces, which are without walls, and its main trunks open into the vascular system. The con- tents are derived from the nutrient material absorbed from the intestine (chyle), and from the fluids which have transuded into the tissues from the capillaries (lymph), and they serve to renovate the blood. In the actual course of the lymph and chyle, i.e., in the lymphatic vessels themselves, are placed peculiar glandular organs, known as lymphatic glands (blood glands), in which the lymph receives its form elements (lymph corpuscles = white blood corpuscles). Organs of Respiration. The blood needs for the retention of its properties not only this continued renovation by the addition of nutrient fluids, but also the constaiit introduction of oxygen, with the reception of which is closely connected the excretion of carbonic FiQ. 60.— Heart and great vessels of a Chelouian, Ad, right auricle ; ^.f.left auricle ; Ao.d, right aortic arch ; Ao.s, left aortic arch ; Ao, aorta; C, carotids ; Ap, pulnjonary ai'teries. 68 OllOANIZATION AND DEVELOPMENT OF ANIMALS IN GENEUAL. acid (and water). The exchange of these two gases between the blood and the external medium is the essential part of the respix-atory process, and is effected through organs which are suited for carrying on this process either in air or in water. In the simplest cases the exchange of these two gases takes place through the general surface of the body; and in all cases, even when special respira- tory organs are present, the outer skin also takes part in respiration. Fig. Gl— Diagram of the circulation in an animal with a completely separated right and left ventricle, and a double circulation (after Huxley). Ad, right auricle receiv- ing the superior and inferior venae cavae, Vcs, and Vei; Dtk, thoracic duct, the main trunk of the lymphatic system ; Ad, right auricle ; Vd, right ventricle ; Ap, pulmonary artery ; P, lung ; Vp, pulmon- ary vera ; As, left auricle ; Vs, left ven- tricle; Ao, aorta; D, intestine; L, liver; Vp', portal vein ; Lv, hepatic vein. Fig. 63.-— Diagram of the great arteries of a maromal with reference to the five embi-j'- onic arterial arches (after Eathke) . c, common carotids ; c', external carotid ; c", inter- nal carotid; A, aorta. Ap, pulmonary artery ; Aa, aortic arch. Inner surfaces also may be con- cerned in this exchange, especially those of the digestive cavity and intestine, or, as in the Echi- noderms, in which a separate vascular system is developed, the surface of the whole body cavity. Respiration in water obviously takes place under far more un- favourable conditions for the introduction of oxygen than does the direct respiration in air, because it is only the small quantity of BBSPIEATORT ORGANS. 69 oxygen dissolved in water which is available respiration is found in animals low in the scale of life in which the metabolic processes are less energetic (worms, molluscs, and fishes). Organs of aquatic respiration, or gills, have the form of external appendages possessing as large a surface extension as possible. They consist of simple or antler- shaped or dendritically branched processes (fig. 63 a, b), or of Hence this form of Fig. 63(1.— Head and anterior body segments of a Eunice, viewed from the dorsal sur- face. T, tentacles. Ct, tentacular cirrus. C, parapodial cirrus. £r, parapodial gill. lancet-shaped closely-packed leaves with a large stii-face extension (fig. 64). Fig. Gi. — Transverse section through the gill of a Teleostean fish, h, branchial leaf- let with capillaries ; e, branchial artery con- taining venous blood ; (I, branchial vein con- taining arterial blood. ({, branchial bar. Fig. 635. — Transverse section throngh the body of Eu- nice. Br, gill ; C, cirrus ; P, parapodium with a bundle of setae ; Z>, alimentaiy canal ; iV, nervous system The organs of aerial respiration, on the contrary, are internal. They present likewis'^ the condi- tion favourable for an exchange of gases between the air and the blood, viz., a large extent of surface. They have the form either of lungs or air-beaiing tubes. In the first case (Spiders, Vertebrates) they consist of spacious sacs with aiveolar or spongy 70 onaANizATiON and development of animals in general. walls, travei'sed by numerous septa and folds which bear an extremely rich network of capillaiies. The air tubes or tracheae (fig. 65) consti- tute a branched system of canals which extend througliout the whole body, and carry the aii- to all the organs. Tlius instead of the respi- ratory pi'o- cess being localised, as it is in ani- mals with kings, it is carried on in all tissues and organs of the body, which are surrounded by a fine tracheal network. Nevertheless, the air tubes in the case of the modification known as fan- trachece present an approximation in their structures to lungs, in that the main stems, without further branching, give rise to flat hollow leaves. Fig. 65.— TrachesB with fine branches (after Leydig). Z, cellular, outer wall ; Sp, sph'al thread. Tr- FiG. 66J.— Lateral view of head and body of an Acridium. St, stigmata ; T, Tympanum. Openings in the body wall are present, placing the organs of aerial respiration in communica- tion with the exterior. These openings may be numerous, and paired, placed symmetrically on the sides Fig. 6Go . — Tra ct eal sy s tern of a Dipterous l.'Tva. Tr, Longitudi- nal stem of the right side with tufts of tra - chere; St', and Sf", anterior and posterior stigmata; Mb, oral hooks. TEACHEiE. 71 of the body (fig. QQ a, b) {stigmata of Insects, Spiders), or they may be more restricted in nvimber, and communicate also with cavities of complicated structure which are used for other functions (nasal cavities of "Vertebrates). In the aquatic larvae of certain I 'Ik f 67(t.— Larva of an Ephemeral fly with seven pairs o£_ tracheal gills , slightly magnified ; Tk, isolated tracheal gill strongly magnified. Fig. 67i.— Tracheal sys- tem at the sides of the alimentary canal of an Agrion larva (after L. Dufour). Tit, main tracheal trunk ; Sf, tracheal gills ; Ifa, the three simple eyes. Insects (Ephemeridje, Libellulidae) the tracheae may be without any external openings. In such cases processes of the body filled with a close network of tracheae, which take up oxygen from the watei', and are known as tracheal gills, are developed (fig. 67 a, b). In rare instances tracheal gills are developed on the wall of the rectum, and 72 ORGANIZATION AND DEVELOPMENT OP ANIMALS IN GENEBAL. thus acquire a protected position (rectal respiration of Aeschna Libenula). ' In other respects the branchial and pulmonary respiratory pro- cesses are essentially the same. In the pulmonate snails (Lymnaeus) the pulmonary cavity may be filled with water, and yet continue to function as a respiratory organ (in the young state and also under special conditions in the adult, the animal remaining permanently in deep water). With this fact before us of an air-breathing surface functioning as a gill, it will not surprise us to find that gills and branching folds of skin, which under normal circumstances serve for breathing in water, can, provided they be protected from shrivelling up and desiccation either by their position in a damp space or by their copious blood supply, function as lungs, and allow their pos- sessors to live and breathe on land (Crabs, Birgus latro, labyrintho- branchiate Fishes). A rapid renewal of the medium which carries the oxygen and surrounds the respiratory surfaces is of the greatest importance for the gaseous exchanges. We find, therefore, very often special arrangements, by which the removal of that part of the respiratory medium which has been deprived of oxygen and saturated with carbonic acid and the introduction of another portion con- taining oxygen and free of carbonic acid, is efiected. In the simplest cases this renewal can, although not very efficiently, be brought about by the movements of the body, or by a continuous oscillation of the respiratory surfaces themselves ; a method which is especially common when the gills are placed in the region of the mouth and function also as organs of food prehension, e.g., the tentacles of many attached animals (Polyzoa, Brachiopoda, tubi- colous Worms, etc.) Yery frequently the gills appear as appendages of the organs of locomotion, e.g., of the swimming or ambulatory feet (Crustacea, Annelids), the movement of which brings about a renewal of the respiratory medium around the gills. The move- ments become more complicated when the gills are enclosed in special chambers (Decapoda, Pisces), or when the respiratory organs are placed within the body, as happens in the case of trachese and lungs, in which case also a renewal of the air is effected either by a more or less regular movement of neighbouring parts, or by rhj^th- mical contractions and dilatations of the air-chamber, constituting the so-called respiratory movements. The term resjnration is now not only applied to these movements so obvious to the eye in air- breathing animals, but also to the osmotic processes, secondarily ANIMAL HEAT. 73 dependent upon the entrance and exit of air, which effect the gaseous exchanges. Talcen strictly in this sense it is an incorrect term, inasmuch as in the respiratory movements of animals pro- vided with branchial cavities we have to do with the entrance and exit of water. In the higher animals provided with red blood, the difference in the condition of the blood before and after its passage through the respii-atory organs is so striking that it is possible to distinguish blood rich in oxygen from blood rich in carbonic acid, by the colour. The latter is dark red, and is known as venous blood ; the former, i.e., blood which has just left the gills or lungs, on the contrary, has a blight red colour, and is known as artei-ial blood. While the terms venous and arterial are used in an anatomical sense to express the nature of the blood-vessel, — those carrying the blood to the heart being called venous, and those carrying it from the heart arterial, — they are also used in a physiological sense as an expression for the two conditions of the blood before and after its passage through the respiratory organs, i.e., to express the quality of the blood. Since, however, the respiratory organs may be inserted in the course of either the venous or arterial vessels, it is obvious that, in the first case, there must be venous vessels carrying arterial blood, (Molluscs and some Vertebrates), and, in the latter, arterial vessels carrying venous blood (Vertebrates). Animal heat. The intensity of respiration stands in direct relation to the energy of the metabolism. Animals which breathe by gills and absorb but little oxygen are not in a position to oxidise a large quantity of organic constituents, and can only transform a small quantity of potential into actual energy. They perform, therefore, not only a proportionately smaller amount of muscular and nervous work, but also produce in only a small degree the peculiar molecular movements known as heat. The "source of this heat is to be sought not, as was formerly erroneously supposed, in the respiratory organs, but in the active tissues. Animals in which thermogenic activities are small have no power of keeping independently their own internal heat when exposed to the temperature influences of the surrounding medium. This is also true of those air-breathing animals in which the metabolic and thermogenic activities are great, but which, in consequence of their small size, offer a relatively very large surface for the loss of heat by radiation (Insects). On account of the ex- changes of heat which are continually taking place between the animal body and the surrounding medium, the temperature of the 74 ORGANIZATION AND DEVELOPMENT OP ANIMALS IN GENEHAL. former mvist in such animals be largely dependent on that of the latter, falling and rising with it. Hence, most of the lower animals are poikilothermic* or, as they have less appropriately been called, cold-blooded. The higher animals, on the contrary, in which, on account of their highly developed respiratory organs and energetic metabolism, the thermogenic activity is great, and which are protected from a rapid loss of heat by radiation by the size of their bodies and by the possession of a covering of hairs or feathers, possess the power of maintaining a constant temperature, which is independent of the rising and falling of the temperature of the surrounding medium. Such animals are designated homothermic, or warm-blooded. Since they require a high internal temperature, varying only within small limits, as a necessary condition for the normal course of the vital processes, or one may say for the maintenance of life itself, they must possess within themselves a series of regulators whose function is to keep the body temperature within its proper limits, when the temperature of the surrounding medium is high. This may be effected either by diminishing the prodviction of internal heat (diminishing the metabolism) or by increasing the loss of heat from the surfaces of the body (by radiation, evaporation of secretions, cooling in water) ] and, on the contrary, when the temperature of the outer medium is too low, by increasing the production of internal heat (increasing the metabolic activity by more plentiful food supply, more vigorous movements), or by diminishing the loss of heat by the development of better protective coverings. When the conditions necessary for the action of these regulators are absent (want of food, small and unprotected bodies), we find either the phenomenon of winter sleep, in which life is preserved with a temporary lowering of the metabolic processes ) or, when the metabolic processes of the organism do not enter into abeyance, the remarkable phenomena of migration (migration of birds). Organs of Secretion. The respiratory organs stand to a certain extent intermediate between the organs of- nutrition and those of excretion, in that they take in oxygen and excrete carbonic acid. In addition to this gas a number of excrementitious substances, mostly in a fluid form, which have entered the blood from the tissues, pass out by the lungs. The fvmction, however, of excretion * Col BerKmann, " Ueber die Verhaltnisse der Warmeokonomie der Thiere zu ihrer Gr5sse," Gottinger Studien, 1847; also Bergnmnn uud LcAickart, » Anatomisch-physiologische Uebcrsicht des Thicrreichs, btuttgart, lb52. UEINAUT OEGANS. 75 ( is mainly discharged by the special secretory organs. These have I the form of glands of a simple or complex structure which originate 1 from invaginations of the outer skin or of the intestinal wall, and i consist essentially of simple or branched tubes, or of racemose and ' lobulated glands. Among the various substances which by the aid of the epithelial i lining of the walls of glands are removed from the blood and some- times utilised further for the performance of various functions, the nitrogenous excretory substances are especially important. The organs by which the excretion of these iiltimate products of meta- >i holism are effected are the kidneys. In 1 the Protozoa they are represented by the contractile vacuoles ; in the Worms they appear as the so-called water- vascular vessels, and are constituted of a system of branched canals which •take their origin in delicate internal cUiated funnels, which open into the spaces in the parenchymatous tissues or the body cavity. In the latter case the ciliated f imnels have a wide opening. In the Platyelminthes (flat worms) the eflferent ducts of the system consist of two main lateral trunks (fig. 68, Ex.), which frequently open together at the hind end of the body by means of a medium terminal contractile vesicle (fig. 68, ep). \ In the segmented worms the paired I kidneys are repeated in every segment, and are known as segmental organs (figs. 69 and 70). The shell-glands of Crustacea are in all probability to be traced back to these segmental ■ organs : as are also the paired kidney (organ of Bo j anus) of mussels, and the unpaired renal sac of Snails, both of which communicate by means of an internal opening with the pericardial division of the body cavity. In the air-breathing Arthropods and some Crustacea (Orchestia.) . the urinary organs are tubular appendages (Malpigh^ian vessels) of ■ the hind gut. In the Yertebrata the urinary organs or kidneys obtain a greater independence, and open to the exterior by special Fig. 68.— Young Distomum (after La Valette). Ex, main stems of the excretory system; Ep, ex- cretory pore ; O, nloiith with sticker; S, sucker in the middle of the ventral surface ; P, pha- rynx ; B, alimentary canal. 7P) ORGAlsriZA.TI(m AND DEVELOPMENT OP ANIMALS IN GBNEEAL. openings which are usually common to the generative organs ; they consist essentially of a number of coiled tubes, which in the more pr-imitive types of Vertebrates have a ciliated funnel-shaped opening into tho J body cavity (Dogfish embryo, fig. 71). The individual tubules of which the vertc- Wtr Fig. 70. — Diagrammatic representation of the segmental organs of a segmented, worm (after 0. Semper). Dv, dissepi- ment ; Wtr, ciliated funnels wMcb. lead into the coiled tubes. Fis. 69. — Longitudinal section through the medicinal Leech (after R. Leuckart). D, ali- mentary canal ; , layer of fibres : Gz, layer of ganglion cells ; Sf, retinal fibres ; Fi:, crossing of fibres. rangement on corre- sponding points of the optic nerve and so render possible the perception of an image vary, and are closely dependent upon the whole structure of the eye. Leaving out of consideration the simplest eyes, such as we find in Worms and the lower Crustacea, two types of eye are to be distin- guished. 1. The first form occurs in the so- called facetted eyes* (figs. 85 & 86) of Arthropods (Crustacea and Insects). The retina of such eyes has a hemispherical form, the convex surface being directed out- wards, and consists of large compound nerve rods, the retinulaj * See Job. Mliller, "Zur vergleichondon Pliysiolo3;ie des Gesichtssinne?," Leipzig, 1820). H. Grenadier, Uutersuciiungen iiber das Schorgan der Arthro- podeu," Gcittingen, 1879. UNIOOBNEAL EYE. 89 which, following -if. (figs. 85 & 86 7?/ tfc H), which are separated from one another by pigment sheaths. In front of these rods are placed the strongly refractile crystalline cones (k), and in front of these again the lens- shaped corneal facets (6^ c6 F). The eye is enclosed by a firm chitinous layer, the sheath of the entering optic nerve, surrounds its soft parts and reaches as far as the cornea. That part of the eye which is known as optic nerve corresponds in a great measure to the retina itself, and contains a layer of ganglion cells and of nerve fibres. A reversed and reduced picture of the object is thrown behind each convex corneal facet (lying far from the sensitive layer of nervous rods), and only the perpendicular rays can be perceived since all the others are absorbed by the pigment. Ac- cordingly the light impressions caused by these axial rays, whose number corresponds with the separate nerve rods, form a mosaic on the retina which repeats the arrangement of the parts of the external object emitting light. The picture which is here formed lacks, however, brilliancy and dis- tinctness. 2. The second form of eye, which is widely distri- buted in the animal kingdom (the simple eye. Annelids, Insects, Arachnida, Molluscs, Verte- brates) corresponds to a globular camera obscura with collecting lenses (cornea, lens) on its exposed anterior wall on which the light falls and usually Avith additional dioptric m:edia filling the optic chamber (vitreous humour.) The simple eye of Insects seems to have originated from the simple metamorphosis of part of the integument, beneath which are placed the end organs of the optic nerve (fig. 87). The cuticular covering (CL) projects as a lens-shaped thickening into the subjacent layer of transparent, elongated, hypodermis cells {Gk), within which are placed elongated rod-like nerve- cells with refractile cuticular portions, closely aggregated to form a retina (fig. 87 Hz). The hypodermis cells surrounding the edge of the lens are filled with pigment, and form an iris-like dark ring Fig. 86.— Three fa- cets -with retinulfe from the com- pound eye of a cockchafer (after Grenacher) . The pigment has been dissolved avray from two of them. F, corneal facet. Jl, crystalline cone. P, pigment sheath. P', chief pigment cell. P", pigment cells of the second order. Jt, retinuliE. 90 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. through the opening in which the rays of light enter the eye to fall on the terminal segments of the retinal cells (fig. 87), In the more highly developed forms of this type of eye, especially in the Vertebrate eye, the peripheral portion of the optic nei-ve spreads out so as to form a cup-shaped nervous membrane, the retina, placed immediately behind the refractile media and surrounded by a vascular pigmented membrane, the choroid. The choroid, again, is surrounded by a tough supporting membrane composed of fibrous connective tissue, and known as the sclerotic, which is continued over the anterior part of the eye, i.e., that part through which the light passes, as a thinner transparent membrane. Of the I'efi-actile media which are placed behind the cornea and fill the cavity of the optic bulb, viz., the aque- ous humour, the lens (fig. 88 L), the vitre- ous humour (Gl), the lens is the most powerful. Grasped by the thickened muscular anterior part of the choroid (the ciliary body (Cc) and ciliary processes), the peripheral part of its anterior face is covered by a forward continuation of the choroid, the iris (Jr), which, as a ring-like contractile border, forms a kind of diaphragm perforated by a central contractile opening, the pupil, through which the light enters the eye (fig. 88). The reversed image which is formed in the hinder part of the Vertebrate eye on the cup-shaped retina has a very considerable brilliancy and definition. The eyes of many Cephalopods may , be looked upon as a modifica- tion of this type of eye. In the eye of Nautilus the lens is absent, and the light enters through a small opening. In this case a reversed, but not brilliant, image is formed on the retina placed on the hinder wall of the eye. To enable the eye to see clearly objects in different directions and Fig. 87.— Transverse section through the simple eye of a beetle larva (.partly after Grenacher). CL, corneal lens ; G!c, the subjacent hypodermis cells, the vitreous humour of Authors ; P, pigment in the peripheral cells of the lat- ter ; Sz, retinal cells. St, cuticular rods of the latter. OLrAOTOET ORGAN. 91 at different distances, special apparatuses for its movement and accommodation are necessary. They are represented by muscles which can in the former case move the optic bulb and modify the direction of sight in obedience to the will of the animal, and in the latter act upon the refractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the head on which the eye is placed is prolonged so as to give rise to a movable stalk-like process, which bears the eye at its extremity. The eyes of Vertebrata possess in addition special protective arrangements, e.g., eyelids, lacrymal glands. The position and number of the eyes present very great variations amongst the lower animals. The paired arrangement on the head appears to be the general rule among the higher animals ; nevertheless visual organs sometimes occur on parts of the body far removed from the brain, as for instance, in Euphausia, Pecten, Spondy- lus, and certain Annelids (Sabellidse). In the Radiata the eyes are repeated at the periphery of the body in each radius. In the star fishes they lie at the extreme end of the ambulacral furrow at the tip of the arms, in the Acalephse as the marginal bodies on the edge of the umbrella. The sense of smell appears to be less widely distributed. Its func- tion is to test the quality of gaseous matters and to produce in consciousness the special form of sensation known as " Smell." This sense in aquatic animals which breathe through gills cannot be sharply marked off from that of taste. The small pits, standing in connec- tion with nerves and provided with an epithelial lining of hair-bearing sense cells are to be looked upon as the simplest form of olfactory organ (Medusae, Heteropoda, Cephalopoda). Nevertheless scattered hair cells (Lamellibranchiata) may also have to do with the same sensation. In the Arthropoda the cuticular appendages of the Pig. 88.— Transverse section tbrough the human eye (after Arlt). C, cornea ; L, lens ; Jr, iris with pupil ; Cc, ciliaiy body ; Qi, vitreous humour ; R, retina ; 8a sclerotic ; Ch, choroid. Ml, macula lutea ; Po, papilla optica; i\'o, optic nerve. 92 OEQA.NIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. antennse in which tlie gangliated swollen extremities of nerves occur are to be explained as olfactory fibres. In the Ver-tebrata the olfactory organ usually has the form of a paired pit or cavity placed on the under surface of the head (nasal cavity), on the walls of which the ends of the olfactory nerve are distributed. The higher air- • breathing Vertebrata are distinguished by the fact that in them this cavity communicates with the pharynx, and by the great surface extension (in a confined area) of the much-folded olfactory mucous membrane. The fibres of the olfactory nerve terminate in delicate elongated cells, befiring rods or hairs and placed- between the epithelial cells of this mucous membrane. The special sense of taste is confined to the mouth and pharynx. Its function, from what we know of the higher organisms, is to test the quality of fluid sub- stances, and to bring about the special sensation of taste. The presence of this sense can be demonstrated with certainty in the Ver- tebrata, and it is connected with the distribution of a special nerve of taste, the glossophar'yngeal, which in man supplies the tip, edges. If, nerve; Gk, taste buds in the side-waU of the ^nd root of the tongue and papOla, Pc. b, isolated taste bud from the lateral ^Iso parts of the Soft palate, taste orsjans of a rabbit, c, isolated supporting . . , . cells (2)z) and sense Cells (&) from the same. making these parts Capable of the taste sensation. The so-called taste-buds found in special papillae (papillse circum- vallataj), with their central fibre-Hke cells, are explained as the percipient organs of this sense (fig. 89 a, h, c). Taste is, as a rule, connected with the tactile and temperature sensations of the buccal cavity, and also with the olfactory sensations. Finally, special organs of taste appear to be present also in the Molluscs and Arthropods as a specific sensory epithelium at the entrance to the buccal cavity. In the lower animals the taste and olfactory organs are still less a Fig. 89.— rt Transverse section through a cn-cum' \-allate papilla of a calf (after Th. W.Engehnann). PSYCHICAL LIFE AND INSTINCT. 93 clearly distinguishable than in the higher, and there are numerous senses of an intermediate character for the purpose of testing the surrounding medium. The sense-organs of the lateral line of Fishes and Salamanders, and the organs resembling taste-buds of the Hirudinea and Chpetopoda have been described as organs of a sixth sense. They probably bring about certain sensations referring to the quality of the water. PSYCHICAL LIFE* AND INSTINCT. The higher animals are not only rendered conscious of the unity of their organization by their feelings of comfort and discomfort, pleasure and pain, but also possess the power of retaining residua of the impressions of the outer world conveyed through the senses, and of combining them with simultaneously perceived conditions of their bodily state. In what manner the irritabiHty of the lower pro- toplasmic organisms leads by gradual transitions and intermediate steps to the first affection of sensation and consciousness is as completely hidden from us as are the nature and essence of the psychical processes which we know are dependent on the movement of matter. We are, however, justified in supposing that a nervous system is indispensable for the development of these internal conditions which may be compared with that condition of our own organization called consciousness. Again, as animals have sense-organs capable of receiving impressions of definite quality from external causes, together with a capacity for retaining in their memory residua of their perceptions, and the power of connecting them with present and with the recollection of past states of bodily sensation so as to form judgments and conclusions, they possess all the conditions essential for the operation of the • intelligence ; and, as a matter of fact, they do manifest in an elementary form nearly all the phenomena which distinguish human intelligence. The actions of animals are not only voluntary, the result of experi- ence and intellectual activity, but are also largely determined by internal impulses which work independently of consciousness, and cause numerous, often very complicated, actions useful to the organism. Such impulses tending to the preservation of the individual and the * W. Wundt, " Vorlesungen iiber die Menschen und Thierseele." 2 Bde Leipzig, 1863. W. Wundt, " Grundzlige der physiologischen Psvchologie Leipzig, 1874. , ^ a , 94 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. species are called instincts;* and they are usually regarded as a special property of the lower animals, and contrasted with the conscious reason of Man. But just as the latter must be looked upon as a higher form of the understanding and intellect, and not as something essentially distinct from them, so a closer examination shows that instinct and the conscious understanding do not stand in absolute contrast, but rather in a complex relation, and cannot be sharply marked off from one another. For if, according to the general view, we recognise the essence of instinct in the unconsciov^ and the innate, still we find that actions which were at first performed under the. direction of conscious intelligence become, by constant practice, completely instinctive and are performed unconsciously; and that, in accordance with the theory of descent, which the whole connection of natural phenomena renders so probable, instincts have been developed from small beginnings, and have only been able to reach the high and complicated forms which we admire in many of the more highly organised animals (Hymenoptera), when assisted by a certain amount, however small, of intellectual activity. Instinct accordingly may be rightly defined as a mechanism which works unconsciously, and is inherited with the organization, and which, when set in motion by external or internal stimuli, leads to the performance of appropriate actions, which apparently are directed by a conscious purpose. We must not, however, forget that while the intellectual activities are the direct means whereby higher and more complicated instincts arise from simple ones, they themselves depend upon mechanical processes. We may well suppose that the simplest form of instinct is identical with the definite reaction of living matter following a stimulus, or, in other words, with that special form of molecular change which is caused by an external action (as, for instance, the contraction of an Amoeba when brought into contact with a foreign body). By the theory of partly instinctive, partly intellectual processes, we may explain the phenomena of association in societies so often found among the higher animals,t i.e., the association of numerous * Compare H. S. Reimarius, "Allgemeine Betrachtungen iiber die Triebc der Thiere," Hamburg, 1773. P.Flourens, "De I'instinct et de rintelligence desanimaux," Paris, 1851. t The origin of the so-called animal stocks with incomplete or confined individuality among the lower animals is quite diflEerent, and merely determined by processes of growth ; at the same time the advantage for the pr&servation of the species gained by the fusion is the same. Cf. the animal stocks of the Vorticellidse, Polyps, and Siphonophora, Bryozoa and Tunicata. EEPBODUCTIVE OEGANS. 95 individuals into communities— the so-called animal-polities — which may be complicated by the division of labour (Bees, Wasps, Ants, Termites). In fact here the combined action appears to be mutually assisting or mutually limiting as we find in those forms the so-called animal stocks, the individuals of which are bound together by continuity of body. The -advantages to be gained by this mutual rendering of service are not merely Hmited to the greater facilities for nourish- ment and defence, and therefore for the preservation of the in- di\adual ; but, above all, tend to the maintenance of the offspring, and hence to the preservation of the species. It is for this reason that the simplest and commonest associations, from which the more complicated communities, subdivided by partition of labour, are derived, are generally communities of both sexes of the same species. REPRODUCTIVE ORGANS. On account of the limit set to the duration of the life of every organ- ■ ism, it appears absolutely necessary for the preservation of the animal and vegetable kingdoms that new life should originate. The forma- tion of new organisms might be due to spontaneous generation [generatio equivoca) ; and formerly this was supposed to take place, not only in the simpler and lower organisms, but also in the more complicated and higher. Aristotle thought that Frogs and Eels arose spontaneously from slime ; and the appearance of maggots in putre- fying meat was, till Redi's time, explained in the same manner. With the progress of science the limits within which this supposition could be applied became ever narrower, so that they soon came to include only the Entozoa and small animals found in infusions. Finally it has been shown by the researches of late years that these organisms also must, for the most part, be withdrawn from the region of the generatio equivoca; so that at present, when the question of spontaneous generation is discussed, it is only the lowest organisms, those found in putrefying infusions, that are considered. The greater number of investigators,* supported by the results of * Cf. especially Pasteur, " Memoire sur les corpuscules organises qui existent clans ratmosphere" (Ann. des. Sc. Nat.), 1861 ; also " Exp^nences relatives aux generations dites spontan^es " (Compt. rend, de I'Acad. des Sciences, tome 50). t)6 OllGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL, numerous experiments, have rejected, even for the latter animals, the idea of spontaneous generation, which, however, still finds in Pouchet* a prominent and zealous supporter. Biogenesis, as opposed to abiogenesis, or spontaneous generation, must be regarded as the usual and normal form of reproduction. Fundamentally it is nothing else than a growth of the organism beyond the sphere of its own individuality, and can be always reduced to a separation of a part of the body, which develops into an indi- vidual resembling the parent organism. Nevertheless the nature and method of this process differ extraordinarily ; and various kinds of reproduction can be distinguished, viz,, fission, budding [spore- /o7'mation), sexual reproduction.-^ Reproduction by fission, which, with that by budding and spore- formation, is included under the term monogenous asexual reproduc- tion, is found widely scattered in the lowest animals, and is also of special importance for the reproduction of the cell. It consists simply of a division of the organism into two parts by means of a constriction which gradually becomes deeper, and eventually leads to the separation of the whole body of the organism into two individuals of the same kind. If the division remains permanently incomplete, and its products do not completely separate from each other, con- pound colonies of animals arise. The number of individuals in such colonies increases by a continuation of the process of incomplete and often dichotomous division of the newly-formed individuals (Vorti- cella, Polyp stocks). The division may take place in various direc- tions— longitudinal, transverse, or diagonal. Budding differs from fission by a precedent disproportionate and asymmetrical growth of the body, giving rise to a structure not absolutely necessary to the parent organism which is developed to a new individual, and by a process of constriction and division becomes independent. If the buds remain permanently attached to the parent, we have here also the conditions necessary for the formation of a colony (Polyp colonies). Sometimes the budding takes place at various parts of the outer surface of the body, irregularly or obeying definite laws (Ascidians, Polyps) ; sometimes it is localised to a definite part of the bddy, separated off as a Germ- stock (Salpa, stolo prolifer). The cell-layers distinguished as germinal * Pouchefc, " Nouvelles experiences sur la g6n6ration spontan^e et la resist- ance vitale," Paris, 1864. , „^ ...... 1 t Cf, E. Leuckart's article, "Zeugung" in K. Wagner's « Handworterbuch der Physiologic." EEPRODUOTION BY SPORES. SEXUAL EEPEODUCTIOTf . 97 layers are repeated in the commencing buds, and from them the organs are differentiated. The reproduction by spores is characterised by the production within the organism of cells, which develop into new individuals in situ or after leaving the organism. But this conception of spores, which is taken from the vegetable kingdom, can only be applied to the Protozoa and coincides with endogenous cell-division. The cases of so-called spore-formation amongst the Metazoa (germinal sacs of Trematodes) are probably identical with egg formation, and are to be reduced to a precocious maturation and spontaneous development of ova (Parthenogenesis, Psedogenesis). The digenous or sexual reproduction depends upon the production of two kinds of germinal cells, the combined action of which is necessary for the de- velopment of a new or- ganism. The one form of germ cells contains the material from which the new individual arises, and is known as the egg -cell, or merely egg (ovum). The second form, the spermrcell {spermato- zoon), contains the ferti- lising material, semen or sperm, which fuses with the contents of the egg- cell, and in a way which is not understood gives the impetus to the de- velopment of the egg. The cell structures from which the eggs and sperm arise are called sexual organs, for reasons which will be evi- dent in the sequel ; the eggs being produced in the female organ or ovary, and the semen in the male organ or testis. The egg is the femcde, and the semen the male product. The structure of the sexual organs presents extraordinary diffe- rences and numerous grades of progressive complication. In the simplest cases, both products arise in the body wall, the cells of which give rise at determined places to ova or spermatozoa (CcElenterata). Sometimes they arise in the ectoderm (Hydroid-Medusre), sometimes in the entoderm (Acalepha, Anthozoa). A similar arrangement 7 6. Fis. 90.— Generative organs of a Heteropod (Pterotra- chea) after R. Leuckart. a, Male-organs ; T, testis- Vd, vas deferens, b, female organs ; Ov, ovary ; Ed, albumen gland ; iBs, receptaculum seminis ; Va, va- gina. 98 ORGANIZATION AND DEVELOPMENT OP ANIMALS IN QENEBAL. obtains in tlie marine Polychaeta, in which the ova and spermatozoa are developed from the epithelium of the body-cjxvity (mesoderm), and dehisced into the body cavity. Usually, however, special glands, the ovaries and testes, are developed, which perform no other function than that of secreting ova and spermatozoa (Echinodex-ms). As a rule, however, there are found associated with the male and female generative glands accessory structux-es and a more or less com- plicated arrangement of ducts, which discharge definite functions in connection with the development of the genei-ative products subse- quent to their separation from the glands, and ensure a suitable meeting between the male and female elements (fig 90), The ovaries are provided with ducts, the oviducts, which are not rarely derived a Fig. 91, a. — The female organs of Pulex (after Stein). On, ovarian tubes ; Be, receptaculum eeminis ; F, vagina ; Gl, accessory gland, b. The male generative organs of a water-bug (Nepa) (after Stein). T, testis ; Yd, vasa deferentia ; Ql, accessory glands ; D, ductus ejacu- latorius. from structures serving quite another purpose (segmental organs). The oviducts, in their course, may receive glandular appendages of various kinds which furnish yolk for the nourishment of the ovum, or albumen to surround it, or material for the formation of a hard egg-shell (chorion). These functions may be sometimes discharged by the ovarian wall (Insects), so that the egg when it enters the oviduct has taken up its accessoiy yolk and acquired its firm egg- shell. Very often the ducts also dischai-ge these various functions, and are divided into coi-responding regions ; they are often dilated at part of theii- course to form a reservoir for the retention of the HEKMAPHBODITISM. 99 eegs or of the developing embryos (uterus). Their terminal section presents differentiations subserving fertilization (receptaculum seminis, vagina, copulatory pouch, external generative organs). The efferent ducts of the testis, the vasa deferentia, likewise frequently give rise to reservoirs (vesiculse seminales) and receive glands (pros- tate), the secretion of which mixes with the sperm fluid or surrounds aggregations of the spermatozoa with a firm sheath (spermatophors). The terminal section of the vas deferens becomes exceedingly muscular, and gives rise to a ductus ejaculatorius, which, as a rule, is accompanied by an external organ of copulation to facilitate the conveyance of the semen into the female generative organs. The generative organs present a. Zd either a radial (Coelenterata, Echinodermata) or a bilate- rally symmetiical arrangement (fig. 91), a contrast which is visible in the typical arrange- ment of all the systems of organs. The simplest and most primitive condition of the generative organs is the her- mcqjhroclite. Ova and sper- matozoa are produced in the body of one and the same individual, which thus unites in itself all the conditions necessary for the preservation of the species, and alone represents the species. Instances of hermaphroditism are found in every group of the animal kingdom. But they are especially nume- rous in the lower groups, and also in animals in which the movements are slow (Land-snails, Flat-worms, Hirudinea, OHgochceta), or which live singly (Cestoda, Trematoda), or in attached animals which are without power of changing their position (Cirripedia, Tunicata, Bryozoa, Oysters). The hermaphrodite arrangement of the gene- rative organs presents great variation, which, to a certain extent, forms a gradual series tending towards the separation of the sexes. In the simplest cases, the points of origin of the two kinds of generative products lie close to one another, so that the spermatozoa and ova meet directly in the parent body (Ctenophora, Chrysaora). Fig. 92.— Sexual organs of a Pteropod (OymlDulia) (after Gegenbaur.) a, Zd, hennapliroclite gland with conunon duct ; Bs, receptaculum seminis ; ^Z7, uterus, b, Acinus of the hei-maphrodite gland of the same. 0, ova ; 8, spermatozoa. 100 OROANTZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. The elements of both sexes arise in layers of cells whicli have a definite position beneath tlie entodermal lining of the gastro-vascukr canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gknd, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common to the ova and spermatozoa (fig. 92), but which, as in Helix (fig. 93), may partially separate into vas deferens and oviduct. In other cases the ovaries and testes appear as completely separated glands with separate ducts, which may still open into a common cloaca (Cestoda, Trematoda, rhabdocoile Z,r Turbellarians, fig. 94), or may possess separate open- ings (Hirudinea, fig. 95). Two hermaphrodite in- dividuals may, and this appears to be the rule, mutually fertilise each other at the same time, or cases may occur in such hermaphrodites in which self-fertilization is sufficient for the production of off- spring. But this original condition of self-fertiliza- tion appears to be the ex- ception in almost all hermaphrodites. In those animals in which the ovary and testis are not com- pletely separated from one another cross-fei'tilization is rendered necessary, and self-fertilization prevented by the fact that the male and female elements are matured at different times (Snail?, Salps). From this form of complete hermaphroditism the generative organs pass through a stage of incomplete hermaphroditism, in which, though the organs of both sexes are present, one of them is rudi- mentary, to reach the dioecious condition in which the sexes are completely separated {Distomum fillicolle and hcematohium). Animals in which the sexes are distinct not unfrequently present traces of an Fig. 93.— Sexual organs of the Roman Snail (Helix pomatia). Zd, hermaphrodite gland ; Zg, its duct ; Ed, albumen gland; Od, oviduct and seminal groove ; yd, vas deferens ; P, protrusible penis ; Fl, flagellum ; Bs, reoeptaculum seminis ; X», finger-shaped gland ; L, Spiculum amoris ; Go, common genital opening. SEPARATION OF THE SEXES. 101 of the male hermaphrodite arrangement ; such, for instance, as may be seen in the arrangement of the generative ducts of the Vertebrata. In the Amphibia both male and female generative ducts, which are secondarily derived from the urinary ducts, are developed in each individual. The oviduct (Mullerian duct) in the male atrophies, and is only repre- sented by a small rudiment (fig. 966, Mg); while, on the contrary, in the female, the vas deferens (Wolffian duct) is rudimentary, or, as in Amphibia, functions as the efferent duct for the kidney secre- tion (fig. 96«, hg). With the separation and female gene- rative organs in different indivi- duals the most complete form of sexual reproduc- tion, so far as con- cerns division of labour, is reached ; but at the same time a progressing dimorphism of the male and female individuals be- comes apparent. This is due to the fact that the or- ganization in bi- sexual animals is more and more influenced by the deviating func- tions of the sexual organs, and with the complication of sexual life becomes modified for the performance of special accessory fvmctions connected with the production of ova and spermatozoa. In the first place, the modification of the generative ducts of the two sexes in accordance with the function they have to perform determines the development of secondary sexual characters and of sexual dimorphism. Other organs as well as the generative appa- Fig. 94. — Generative appara- tus of a rhabclocoee Tur- bellarian (vortex viridis) (after M. Schultze). T, tes- tis ; Vd, vas deferens ; Vs, seminal v esicle ; P, pro- trusible penis ; Ou, ovary ; Va, vagina ; M, uterus ; B, yolli gland ; Rs, recep- taculum seminis. Fi&. 95.— Generative appa- ratus of the medicinal leech. T, testis ; Vd, vaa deferens ; Nh, resicula seminalis ; Pr, prostate ; C, penis ; Ou, ovaries with vagina and female generative opening. increasing 102 ORQANIZATION- AND DEVELOPMENT OP ANIMALS IN GENEBAL. ratus present differences in the two sexen, being modified for the a Fig. 960.— Left iirinary and generative or- Pig. 966, Left ui-inary and generative organs gans of a female Salamander without the of a male Salamander, more diagi-ammatic. cloaca. Ov, ovary ; iV^, kidney ; hg, urin- T, testis ; Ve, vasa efferentia ; iV, kidney ary duct corresponding to the Wolffian with its collecting tubules ; Mg, Miille- duct; i%, Miillerian duct as oviduct. rian duct as a rudiment; Wg, Wolffian duct or vas deferens ; Kl, cloaca with ac- cessory glands Dr, of the left side. performance of special functions in the sexual life. The female is FirNCirONS OF MALE AND TEMALB. 103 the passive agent in copulation, merely receiving the semen of the male; the female possesses material from which the offspring Fig. 97a.- Male of Aphis platanoides. oc, ocelli ; Ilr, honey tubes ; P, copulatory organ. develop, and accordingly takes care of the development of the fertilised egg and of the later fate of the offspring. Hence the female usually possesses a less active body and numerous arrangements for the protection and nourishment of her offspring, which develop either from eggs laid by the mother and sometimes carried about with her, or in the maternal body and are born alive. The function of the male is to seek, to excite, and to hold the female during copulation ; hence, as a rule, he possesses greater vigour and power of movement, higher development of the senses, various means of exciting sexual feeling, such as brighter colour- ing, louder and richer voice, pre- hensile organs, and external organs for copulation (fig. 97, a, b). In exceptional cases, the functions relating to the maintenance of Fig. 97i, Apterous oviparous fbmale of the same. 104 OEGANIZA.TION AND DEVELOPMENT OP ANIMALS IN OBNEEAL. tlie ofTspring may be discharged by the male, e.g., Alytes and the Lophobranchia. Male birds also often share with the female the labour of building the nest, of bringing up and protecting the young. But it is a rare exception to find, as in Cottus and the Sticldebick (Gasterosteus), that the care and protection of the young fall exclusively upon the male, that he only bears the brood pouch and alone builds the nest, — an exception which bears strong witness to the fact that the sexual difJerences both in form and function were first acquired by adaptation. In extreme cases, the sexual dimorphism may lead to so great a difierence in the sexes that without a knowledge of their development Pig. 98. — Ohondracanthus gibbosus, magnified about 6 times, a, female from the side, b, female from tlie ventral surface witli the male (F) attached, e, male isolated, under strong magnification. An', anterior antenna ; An", clasping antennag ; F' and F", the two pairs of feet ; A, eye ; Ov, egg sacs ; Oe, oesophagus ; U, intestine j M, mouth parts ; T, testis ; Vd, vas deferens ; Sp, spermatophore. and sexual relations, the one sex would be placed in a different family and genus to the other. Such extremes are found in the Rotifera and parasitic Copepoda (Ohondracanthus, Lernjeopoda, fig. 98, a, h, c), and are to be explained as the result of a parasitic mode of life. The difference in the two kinds of individuals representing and maintaining the species, whose copulation and mutual action was known long before it was possible to give a correct account of the roiil nature of reproduction, has led to the designation "sexes," from which tho term sexual has been taken to apply to the organs and manner of reproduction. PAETHENOGETfESIS . 105 In reality sexual reprocUiction is nothing else than a special form of growth. The ova and spermatoblasts represent the two forms of germinal cells which have become free, and which, after a mutual interaction in the process of fertilization, develop into a new organism. Nevertheless under certain conditions the egg can, like the simple germ cell, undergo spontaneous development; numerous instances of this mode of development, which is known as parther.o- genesis, are found in Insects. The necessity of fertiHzation therefore Fig. 99.— Viviparous form of Aphis platanoides. Oc, ocelli ; Sr, honey tubes. / no longer enters into our conception of the egg-cell, and no absolute physiological test is left to enable us to distinguish it from the germ- cell. It is usual to regard the place of origin in the sextial organ and in the female body as a feature distinguishing the ovum from a germ cell, but even with this morphological test we do not in each individual case arrive at the desired result {Bees, Bark-lice, Psychiclce). We have already given prominence to the fact that ovaries and testes, in the simplest cases, consist of nothing more than groups of cells of the epithelium of the body cavity or of the outer skin. These, however, do not acquire the character of sexual organs until, at a higher stage of differentiation, the contrast between the 'two 106 OEGANIZATION AND DEVELOPMENT OP ANIMALS IN GENEEAL. sexual elements has made its appearance. When the male elements, and with them the nece.«siLy of fertilization, are absent, and when, at the same time, the organ which produces the germ cells possesses, in its full development, a structure similar to that of an ovary, it becomes very difficult to distinguish whether we have to do with a pseudovary (germ-gland), and with an animal which reproduces asexually ; or with an ovary and a true female, whose eggs possess the capacity of developing spontaneously. It is only a comparison with the sexual form of the animal which makes the distinction possible. To take the case of the Plant-lice or Aphides; in these animals we find a generation of viviparous individuals, easily distinguishable from the true oviparous females, which copulate and lay eggs. They resemble the latter in the fact that they are provided with a similar reproductive gland, constructed upon the ovarian type ; but they differ from them in this important peculiarity, that they are without organs for copulation and ferti- lization (in correspondence with the absence of the male animal) (fig. 99). The reproductive cells of the organs known as have an origin precisely similar to that of eggs in the ova- ries, and only differ from ova in the very early commencement of the embryonic development. The viviparous individuals will therefore be more correctly regarded as agamic females peculiarly modified in the absence of organs for copulation and fertilization ; and the reproductive cells are by no means to be relegated to the category of germ-cells (as formerly was done by Steenstrup). We must therefore speak of the reproductive pro- cesses in the Aphides as being sexual and partheno- genetic and not sexual and asexual. A comparison of the mode of reproduction of the Bark-lice with that of the Aphides, especially of the species Pem- phigus terebinthi, puts the correctness of this supposition beyond the sphere of doubt. A similar condition is found in the viviparous larva of Cecidomyia. Here the rudiment of the generative glands very early assumes a structure resembling that of the ovary, and produces a number of ■Tl Pig. 100.— Vivipa- roua Cecidomyia (Miastor) lai-va (after Al. Pagen- stecher). Tl, Daughter 1 a r v se developed from the rudimentary ovary. DEVELOPMENT. 107 reproductive cells which resemble ova in their method of origin, and at once develop into larvae. The pseudovary is clearly derived from the rudiment of the sexual gland, but without ever reaching complete development (fig. 100). The ovary acquires to a certain extent the signification of an organ for producing germ-cells, and it is not improbable that many products (Bedia, Sporoc7/st) regarded as spores or germ-cells correspond to embryonic ovaries which produce ova capable of spontaneous development. 5 Fio. 101.— Ovum of Nephelia (after O. Herfcwig) . a, the ovum half-an-hour after deposition, a projection of the protoplasm indicates the commencing formation of the first polar body ; the nuclear spindle ia visible. 6, The same an hour later, with polar body extruded, and after entrance of the spermatozoon. Sk, male pronucleus, c, The same another hour later without egg membrane, and with two polar bodies and male pronucleus (Sk) ; d, the same an hour later with approximated female and male pronuclei ; Sk, polar bodies. DEVELOPMENT. It follows from the facts of sexual reproduction that the simple cell must be regarded as the starting-point for the development of the organism. The contents of the ovum spontaneously or under the influence of fertilization enter upon a series of changes, the final result of which is the rudiment of the body of the embryo. These changes consist essentially in a process of cell division which implicates the whole protoplasm of the ovum, and is known as segmentation. 108 ORGANIZATION AND DEA^ELOPMENT OF ANIMALS IN GEXEIIAL. For a long time the behaviour of the germinal vesicle at the commencement of segmentation and its relation to the nuclei of the tirst formed segments were obscure, and the knowledge of the changes and fate of the spermatozoa wliich enter the ovum in the process of fertilization was, in like manner, in a very unsatisfactory state. Of late years, numerous investigations, especially those of Biitschli, 0. Hertwig, Fol, etc., have thrown some light on these hitherto completely obscure processes. It was supposed that in a ripe ovum preparing itself for segmentation the germinal vesicle disappeared, a Fig. 102, a, 5.— Parts of the ovum of Asterias glacialis with spermatozoa, embedded in the macilaginous coat (after H. Fol.) e, upper part of the ovxim of Petromyzon (after Calberla). Am, micropyle ; Sp, spermatozoa; Jm, path of the spermatozoon; Ek, female pronucleus ; Eh, membrane of ovum ; Ehz, prominences of the same. and a new nucleus was formed quite independently of it ; and that the persistence and the participation of the germinal vesicle in the for- mation of the nuclei of the first segmentation spheres were exceptional (Siphonophora, Entoconcha, etc.) Thorough investigations carried out on the eggs of numerous animals have, however, shown that as a matter of fact the germinal vesicle of the ripe ovum only experi- ences changes in which the greater part of it, together with some of FERTILIZATIOK". 109 the protoplasm of the ovum, is thrown out of the egg as the so-called directive bodies or polar cells (fig. 101), The part of it, however, which remains in the ovum retains its significance as a nucleus, and is known as the female pronucleus. This fuses with the single spermatozoon (male pronucleus) which has forced its way into the ovum (fig. 102); and the compound structure so formed constitutes the nucleus of the fertilized ovum, or as it is generally called, the first segmentation nucleus. 7 5 3 Fig. 103.— Development of a Star-fish, Asteracanthion berylinus (after Alex. Agassiz). 1, Commencing segmentation of the flattened egg— at one pole are seen the polar bodies ; 2, stage with two segments ; 3, with foui- ; 4, with eight ; 5, with thirty-two segments ; 6, later stage ; 7, blastosphere with commencing invagination ; 8 and 9, more advanced stages of invagination. The opening of the gastrula cavity becomes the anus. This new nucleus, which divides to give rise to the nuclei of the first segmentation spheres, would appear therefore to be the product of the fusion or conjugation of the part of the germinal vesicle, which remains behind in the ovum, with the male pronucleus, which IS a derivative of the spermatozoon which has entered the ovum. Fertilization would ajypear, therefore, to depend upon the addition 110 OEGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. of a neio element bringing about the regeneration of the prirmry nucleus of the ovum or germinal vesicle, and would have impressed its influence on the constitution of the conjugated nucleus. The regenerated ovum is therefore the starting-point of the subsequent generations of cells which build up the embryonic body. Both the origin of the polar bodies which takes place in the ripe ovum independently of fertilization, and the division of the segmen- tation nucleus are accompanied by the appearance of the nuclear spindle and star-shaped figures at the poles of the spindle Avhich are so characteristic of the division of nuclei. The male pronucleus, before it fuses with the female pronucleus, also becomes surrounded by a layer of clear protoplasm, around which a star-shaped figure appears (fig. 101). In those cases in which segmentation takes place without a precedent fertilization (parthenogenesis), the female pronucleus appears to possess within itself the properties of the first segmentation nucleus. The fertilization is followed by the process known as segmentation, in which the ovum gradually divides into a greater and greater number of smaller cells. Segmentation may be total, i.e., the whole ovum segments (fig. 103), or it may be j^cc^tial, in which case only a portion segments (fig. 105). Total segmentation may be regular and equal, the resulting seg- ments being of equal size (fig. 103) ; or it may sooner or later become irregular, the resulting segments being of two kinds — the one smaller and containing a preponderating amount of protoplasm, the other larger and containing more fatty matter. In these cases the seg- mentation is said to be unequal. The process of division proceeds much more quickly in the smaller segments, while in the larger and more fatty segments it is much slower, and may eventually come to a complete standstill. The development of the frog's egg will serve as an example of unequal segmentation, of which there are various degrees (fig. 104), In this egg a dark pigmented and protoplasmic portion can be distinguished from a lighter portion containing much fatty matter or food yolk. The former is always turned uppermost in the water, and is therefore called the upper pole of the egg. The axis which connects the upper pole with the lower is known as the chief axis. The planes of the two first segmentation furrows pass through the chief axis and are at right angles to each other. They divide the egg into four equal parts. The thii-d furrow (fig. 104, 4) is equatorial, taking place in a hoi^izontal plane, and cuttino- the chief axis at right angles. It lies, however, nearer HOIOBLASTIC AND MEEOBLASTIC SEGMENTATION. Ill the upper pole than the lower, and marks the line of division between the upper and smaller portion of the egg from the lower Fig. 104.— Unequal segmentation of the Frog's egg (after Bcker) in ten successive stages. and larger portion, in which the segmentation proceeds much more slowly than in the former. In partial segmentation we find a sharply marked contrast between the formative and nutritive parts of the egg, inasmuch as the latter does not seg- ment. The terms holohlastic and me- rohlastic therefore have been applied to total and partial seg- mentation respec- tively. Nevertheless, in total segmentation also, either groups of segments of a definite quaHty, or, at any rate, a fluid yolk material may be used for the nourishment of the developing embryo. In fact, the contents of every egg consists of two parts-(l) of a viscous albu- minous protoplasm; and (2) of a fatty granular matter, the deutoplasm, or food yolk. The first is derived from the protoplasm Fig. 105. Segmentation of the germinal disc of a Fowl's egg surface view (after Kolliker). A, germinal disc with the first vertical furrow ; B, the same with two vertical furrows crossing one another at right angles; C and B, more ad- vanced stages with small central segments. 112 OKGA.NIZA.TION AND DEVJSLOPMENT OP ANIMALS IN GEKEHAL. of the original germinal cell, while the yolk is only secondarily developed with the gradual growth of the first ; and not unfrequently it is derived from the secretion of special glands (yolk glands, Trcnta- todes) ; it may even be added in the form of cells. In the Ctenophora and other Coelenterata we see already in the first-formed segments the separation of the formative matter or peripheral ectoplasm from the nutritive matter or central endoplasm. In eggs undergoing a partial segmentation the formative matter usually lies on one side of the large unsegmenting food yolk. In accordance with this, the segments of such eggs, known as telolecitJiul, arrange themselves in the form of a flat disc (germinal disc) ; hence this kind of segmentation, has been called discoidal (eggs of Aves, Reptilia, Pisces) (fig. 105). The food yolk may, however, have a central position. In such centrolecithal eggs the segmentation is A B c n Fig. 106.— TJnequal segmentation of the centrolecithal egg of Gammarus locufita (in part after Ed. van Beneden). The central yolk mass does not appear tiU a late stage and undergoes later an " after-segmentation." confined to the periphery, and is sometimes equal (Palaemon) and sometimes unequal (fig. 106). The central yolk mass may at fiLrst remain unsegmented, but later it may undergo a kind of after- segmentation and break up into a number of cells (fig. 106). Again, in other cases the food yolk, at the commencement of segmentation, has a peripheral position, so that the cleavage process is at first confined to the inner parts of the egg, and only in later stages, when the food yolk has gradually shifted into the centre of the egg, appears as a peripheral layer on the surface. This is found especially in the eggs of Spiders (fig. 107). The first processes of segmentation in these at first ectolecithal ova are withdrawn from observation, since they take place in the centre of an egg covered by a superficial layer of food yolk, until the nuclei with theii' protoplasmic invest- BLASTOSPHEEE. 113 ment reach the periphery, and the fatty and often darkly-granular food yolk comes to constitute the central mass of the egg (Insects). As vtirious as the forms of segmentation are the methods by which the segments ai-e applied to the building up of the embryo. Fre- quently in cases of equal segmentation the segments arrange them- selves in the form of a one-layered vesicle, the hlastosphere, the central cavity of which not rarely contains fluid elements of the food yolk ; or they are at once divided into two layers around a central cavity containing fluid; or they form a solid mass of cells without ^ B 0 Fig. 107.— Six stages in the segmentation of a spider's egg (Philodromns limbatus) after Hub. Ludwig. A, egg with two deutoplasmic rosette-like masses (segmentation spheres) ; B, the rosette-like masses with their centrally placed nucleated protoplasm without 'egg membrane ; C, egg with a great number of rosette-like masses ; B, the rosette-Uke masses have the form of polyhedral deutoplasmic columns, each of which has a cell of the blas- toderm lying immediately superficial to it ; E, stage with blastoderm completely formed ; F, optical section through the same. The yolk coliunns form within the blastoderm a closed investment to the central space. any central cavity. In numerous cases, especially when the food yolk is relatively abundant (unequal and partial segmentation) or the food supply continuous, the embryonic development is longer and more complicated. The embryonic rudiment in such cases has at first the form of a disc of cells lying on the yolk; it soon divides into two layers, a,nd then grows round the yolk. 8 114 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The two-layered gastrula is, as a rule, developed from the blasto- sphere by invagination (emliolic invagination). In this process the one half (sometimes distinguished by the larger size and more gi-anular nature of its cells) of the cell wall of the blastosphere is pushed in upon the other half (fig. 108), and on the narrowing of the A Fig. 108. — A, Blastosphere of AmphioxTis ; B, invagination of the same; C, gastrula, invagfi- nation completed ; O, blastopore (after B. Hatschek). aperture of invagination (blastopore, mouth of gastrula) becomes the endodermal layer {hypoblast) lining the gastrula cavity. The outer layer of cells constitutes the ectoderm or ejjiblast. This mode of formation of the gastrula, which is very common, is found, e.g., in Ascidians, and amongst the Vertebrata in Amphioxus (fig. 108). More rarely the gastrula arises by delamination. This process consists of a concentric splitting of the cells of the blastosphere into an outer layer (epiblast), and an inner (hypoblast) (fig. 109). Fig 109.— Transverse sections through three stages in the segmentation of Geryonia (after H Fol ) A, stage with thirty-two segments, each segment is divided into an external finely granular protoplasm (ectoplasm) and an inner clearer layer (endoplasm) ; B, later stage ; C, embryo after delamination; with ectoderm slightly separated from the endoderm, which is composed of lavge cells sm-roimding the segmentation cavity. The central cavity of the gastrula in this case is derived from the" original segmentation cavity, and the gastrula mouth is only secondarily formed by perforation. This method of development PBIMITIVE STEEAK. 115 of the gasti-ula has hitherto only been observed in some hydroid Medus£e (Geryonia). Finally, when the inequality of the segmentation is very pro- nounced, the gastrula is formed by a process known as epibole. In this process of development the epiblast cells, which are early distin- guishable from the much larger hypoblast cells, spread themselves over the latter as a thin layer (fig. 110); and in this, as in the second method of development of the gastrula, the cavity of the gastrula is, as a rule, a secondary formation in the centre of the closely-packed mass of hypoblast cells. The blastopore is usually found at the point where the complete enclosure of the hypoblast is effected. It sometimes happens that a part of the primary blastosphere is developed more rapidly than the remainder, and gives rise to a Pig. 110.—^, Unequal segmentation of the egg of Bonellia; B, epibolic gastrula of the same (after Spengel). bilaterally-symmetrical stripe-like thickening placed on the dorsal or ventral surface of the embryo. Frequently, however, such a germinal or primitive streak is not developed, and the rudiment of the embryo continues to develop uniformly. Formerly great importance was attached to these differences, the one being distinguished as an evolviio ex una parte, and the other an evolutio ex omnihiis partibus. It is not, however, possible to draw a sharp line between these two methods of development, nor have they the significance which was formerly ascribed to them, for closely allied forms may present great differences in this respect according to the amount of food yolk and the duration of the embryonic development. The Coelenterata, the Echinoderms, the lower Worms and Mol- luscs, Annelids, and even Arthropods and Vertebrates (Amphioxus) present us with examples of regular development of all parts of the 116 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN QENEEAL. body of the embryo which, if the yolk membrane fails, has no need of a special protective envelope. In this latter group, however, the formation of the germinal streak, which is in close relation with the formation of the nervous system, is accomplished later, during the post-embryonic development, when the larva is free-swimming and can procure its own food. In like manner many Polychjetes and Arthropods (Branchipus) only acquire a germinal streak in the course of their later growth as larvte. In all cases in which the embryonic development begins by the formation of a germinal streak, the embryo only becomes definitely limited after the yolk has been gradually surrounded, as a result of processes which are connected with the complete entry of the yolk into the body cavity (Frogs, Insects), or with the origin of a yolk sac from which the yolk passes gradually into the body of the embryo (Birds, Mammals). The progressive organization of this latter, up to its exit from the egg membranes, presents in each group such extraordinary variations that it is not possible to give a general account of them. Of primary importance is the fact that in the rudiment of the germ two cell layers first make their appearance — one the ectoderm, which gives rise to the outer integument; and the other the endoderm, from which arises the lining membrane of the digestive cavity and of the glands opening into it. Between these two layers there is formed, either from the outer or the inner layer, or from both layers, an intermediate layer, known as the mesoderm. From the mesoderm arise the muscular system and the connective tissues, the corpuscles of the lymph and blood, and the vascular system. The body cavity may either be derived from the persisting segmentation cavity, i.e., the primitive space between the ectoderm and endoderm (primary body cavity), or it may be developed secondarily as a split in the mesoderm (coelom), or as outgrowths from the rudiment of the alimentary canal (archenteron), in which case it is known as an enterocoele body cavity. The nervous system and organs of sense are probably in all cases derived from the ectoderm, very frequently as pit- or groove-Hke invaginations which are subsequently constricted off. On the other hand, the urinary and generative organs ai-ise both from the outer and inner layers as well as from the middle layer, which is itself derived from one of the pi-imary layers or from the walls of the primary single -layered blastosphere. Accordingly, as a I'ule the rudiments of the skin and glandular HOMOLO&T OF THE aSBMINAL LATBBS. 117 lining of the alimentary canal are the first differentiations in the embryo ; and many embryos, the so-called Planulse and Gastrulas, on leaving the egg, have only these two layers and an internal cavity, the archenteron. Then follows the development of the nervous and muscular systems, — the latter taking place sometimes contem- poraneously with or after the first appearance of the skeleton, — especially in cases in which a germinal streak is developed. The urinary organs and various accessory glands, the blood-vessels and I'espiratory organs do not appear till later. The degree of difference between the offspring on attaining the free condition {i.e., at birth or hatching) and the sexually mature adults, both as regards form and size as well as organization, varies considerably throughout the animal kingdom. It is a very striking fact that an embryo provided with a central cavity and a body wall composed of only two layers of cells appears in different groups of animals as a freely moveable larva capable of leading an independent life. Having recognized this fact, it was not a great step, especially as Huxley* some time ago had compared the two membranes of the body wall of the Medusae (called later by Allman ectoderm and endoderm) with the outer and inner layers of the vertebrate blastoderm (epiblast and hypoblast), to arrive at the conclusion that there was a similar phylogenetic origin for the similar larvte of very different animal types, and to trace back the origin of organs functionally resembling each other to the same primitive structure. It was A. Kowalewskit who, by the results of his numerous researches on the development of the lower animals, first gave this conception the groundwork of fact. He not only proved the occui - rence of a two-layered larva in the development of the Coelenteratr , Echinoderms, Worms, Ascidians, and in Amphioxus amongst Vertt- brates, but also on the ground of the great agreement in the later developmental stages of the larvae of Ascidians and Amphioxus and in the mode of origin of equivalent organs in the embryos of Worms, Insects, and Vertebrata, protested against the hitherto universally received view implied in Cuvier's conception of types, that the organs of different types could not be homologous with one another. * Thomas Huxley, " On the Anatomy and Affinities of the family of Medusae." Philosophical Transactions. London, 1849. t Of. A. Kowalewski's various papers in the " Memoires de I'Acad. de Peters- bourg," on Ctenophora, Phoronis, Holothurians, Ascidians, and Amphioxus, 1866 and 1867. lis ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. Inasmuch as Kowalewski,* from the results of his embryological work, drew the conclusion that the nervous layer and embryonic skin of Insects and Vertebrates are homologous, and that the germinal layers of Amphioxus and Vertebrates correspond with those of Molluscs (Tunicata) or worms, he was in agi-eement with the long recognised fact that anatomical transitional forms and intermediate links between the different groups or types of animals exist, and that these latter do not represent absolutely isolated planes of organization, but the highest divisions in the system, and he only gave in reality an embryological expression to the claims of the descent theory. In fact, the conclusion which Kowalewski reached was completely correct — viz., that the homologies of the germinal layers in the different types affoid a scientific basis for comparative anatomy and embryology, and must be recognised as the starting-point for the proper understanding of the relationships of the types. For this position we find amongst the vertebrata proofs at every step. But while his own comprehensive embryological experiences inspired Kowalewski, the founder of the theory of the germinal layers, with a prudent reserve, other investigators, inclined to bold generalization, appeared at once with ready theories, in which the results of embryo- logical investigations were interpreted in accordance with the theory of descent. Among these E. Haeckel's gastrsea theory is especially prominent, which raises no less a claim " than to substitute, in the place of the classification hitherto received, a new system based on phylogeny, of which the main principle is homology of the germiaal layers and of the archenteron, and secondarily on the differentiation of the axes (bilateral and radial symmetry) and of the coelom." E. Haeckel t designated the larval form used as the point of depar- ture the Gastrida, and believed to have found in it the repetition in embryonic development of a common primitive form, to which the origin of all Metazoa may be traced back. To this hypothetical prototype, which is supposed to have lived in very early times during the Laurentian period, he gave the name of Gastrma, and called the ancient group, supposed to be widely scattered and to consist of many families and genei-a, by the name Gastrceadce. Fi-om this sup- position was deduced the complete homology of the outer and inner * A. Kowalewski, " Embryologische Studien an Wiirmernund Arthropoden.'" Petersburg, 1871, p. 58-60. t E. Haeckel, " GastrEeatheorie," Jen. nat. Zeitschrift, 187i.'' For criticism see C. Glaus, "Die Typenlehre and Haeckel's sogenannte Gastr^atheorie," Vienna, 187 J. DIBECT DEVELOPMENT AND METAMORPHOSIS. 119 terminal layers throughout the whole Metazoa ; the one being traced back to the ectoderm and the other to the endoderm of the hypothe- tical Gastrjea; while for the middle layer, which is only secondarily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this theory, which Ts essentially an extension of the Baer-Remak theory of the germinal layers from the Yertebrata to the whole group of Metazoa, with its pretentious and hasty speculation has created a basis for comparative embryology; svich a basis can. only be obtained as the result of comprehensive investigations. DIRECT DEVELOPMENT AND METAMORPHOSIS. The more complete the agreement between the just born young and the adult sexual animal, so much the greater, especially in the higher animals, will be the du- ration of the embryonic development and the more compUcated the developmental processes of the embryo. The post-embryonic develop- ment will, in this case, be confined to simple processes of growth and perfection of the sexual organs. When, how- ever, embryonic life has, relatively to the height of the organization, a quick and simple course ; when, in other words, the embryo is born in an immature condition and at a relatively low stage of organization, the post-embryonic development wall be more complicated, and the young animal, in addition to its increase in size, will present various processes of transformation and change of form. In such cases, the just hatched young, as opposed to the adult animal, is called a Larva, and develops gradually to the form of the adult Fig. 111.— Larval stages of tlie Frog (after Ecker). a, embryo some time before hatching, with wart-like gill papillse on the visceral arches, h, Larva some time after hatching, with external branchia). c, Older larva, with horny beak and small branchial clefts beneath the integumentary opercuhim, with internal branchiae ; N, nasal pit ; S, sucker ; K, branchiae ; A, eye j Sz, homy teeth. 120 OnOANIZATION AND DEVELOPMENT OP ANIMALS IN GENEEAL. sexual animal. The development of larvae, however, is by no means direct and uniform, but is complicated by the necessity for special contrivances to enable them to procure food and to protect them- selves ; sometimes taking place in an entirely different medium under different conditions of life. Thin kind of post-embryonic development is known as metamorphosis. Well-known examples of metamorphosis are afforded by the deve- lopmental histories of the Insecta and Amphibia. From the eggs of Frogs and Toads proceed larvse provided with tails, but without limbs, the so-called Tadpoles (fig 111). These, with their laterally compressed tails and their gills, remind one of fishes, and they possess organs of attachment in the form of two small cervical suckers by which they can anchor themselves to plants. The mouth is provided with horny plates ; the spirally coiled intestine is surprisingly long ; the heart is simple; and the vascular arches have the piscine relations! Later, as development proceeds, the external branchite abort, and are replaced by new branchi^ covered by folds of the integument, the caudal fin is enlarged, and the fore and hind limbs sprout out ; the fore limbs remain for some time covered by the integument, and only subsequently break through it to appear on the surface. Meanwhile the lungs have developed as appendages of the anterior part of the alimentary canal, and supplant the giUs as respiratory organs, a double circulation is developed, and the horny beak is cast off. Finally the tail gradually shrinks and atrophies ; on the completion of which the metamorphosis of the aquatic tadpole into the frog or toad suited for life on land is accomplished (fig. 112)., We have then to consider two kinds of development, viz., develop- ment with a metamorphosis and direct development, which in extreme cases are distinctly opposed to each other, but are connected by inter- mediate methods. The size of the egg, or, in other words, the amount of food yolk available for the use of the embryo in proportion to the size of the adult animal appears to be a factor of primary importance in any explanation of these two distinct processes (R. Leuckart). Animals with a direct development require — generally in pro- portion to the height of their organization and the size of their bodies — that their eggs should be provided with a rich endowment of food yolk, or that the developing embryo should possess a special accessory source of nutriment ; they arise therefore either from relatively large eggs (Birds), or they are developed inside, and in close connection with the maternal body, by which arrangement they have a continual supply of food material (Mammals). Animals, EELA.TION OF METAMOBPHOSIS TO FEETILITT. 121 on the contrary, which pass through a metamorphosis always arise from eggs of relatively small size, are hatched in an immature con- dition as larvc^, and obtain independently, by their own activity, the materials which have been withheld from them while in the egg, but which are necessary for their full development. The number of embryos produced in the case of a direct development is, in proportion to the total weight of the material applied by the mother for reproductive purposes, far smaller than in the case of a develop- ment with metamorphosis. The fertility of animals whose young Fig. 112.— Later stages in the development of Pelobates fuscus. a, larva without limba with well developed tail ; b, older larva with hind limbs ; o, larva with two pairs of limbs ; d, young frog with caudal stump ; e, yoang frog after complete atrophy of tail. undergo a metamorphosis, or, in other words, the number of offspring produced from a given mass of formative material, is increased to an extraordinary degree, and has, in the complicated relations of organic life, a great physiological significance, though systematically it is of little importance. Some time ago it was attempted to explain these indirect meta- morphoses, in which both processes of reduction and new development take place, as the result of the necessity which the simply organized 122 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. larva, hatched at an early stage of development, laboured under of acquiring special arrangements for its protection and nourishment (R. Leuckart), The proof that such relations do exist between the special larval organs and the peculiar methods of nutrition and protection is an important factor for the full understanding of these remarkable processes, but still is by no means an explanation of them. It is only by aid of the Darwinian principles and the theory of descent that we can get nearer to an explanation. According to this theory, the form and structure of larvje are to be considered in relation to the development of the race, i.e. phylogeny, and are to be derived from the various phases of structure through which the latter has passed in its evolution, and in such a way that the yoimger larval stages would correspond to the primitive, and the older, on the other hand, to the more advanced and more highly organized animals, which have appeared later in the history of the race. In this sense the developmental processes of the individual constitute a more or less complete recapitulation of the developmental histoiy of the species, complicated, however, by secondary variations due to adaptation, which have been acquired in the struggle 'for existence * (Fritz MiiUer's fundamental principle, called by Haeckel the funda- mental law of biogenesis). The greater the number of stages, therefore, through which the larva passes, the more completely is the ancestral history of the species preserved in the developmental history of the individual ; and it is the more truly preserved the fewer the peculiarities of the larva, whether independently acquired, or shifted back from the later to the earlier periods of life (Copepoda.) On the other hand, there are certain larval forms without any phylogenetic meaning which are to be explained as having been secondarily acquired by adaptation (many Insect larvae). The historical record preserved in the developmental history becomes, however, gradually defaced by simplification and shortening of the free development; for the successive phases of development are gradually more and more shifted back in the life of the embryo, and run their course more rapidly and in an abbreviated form, under the protection of the egg membranes, and at the cost of a rich supply of nutrient material (yolk, albumen, placenta). In a,nimals with a direct development, therefore, the complicated devei;- lopment within the egg membranes is a compressed and simplified * Fritz Muller, " Fiir Darwin," Leipzig, 1863, p. 75—81. ALTliENATION OF GENEEATIONS. 123 metamorphosis, and hence the direct development, as opposed to the metamorphosis, is a secondary form of development. ALTERNATION OF GENERATIONS, POLYMORPHISM AND HETEROGAMY. Both in direct development and indirect development by means of a metamorphosis, the successive stages take place in the life- history of the same individual. There are, however, instances of free development, in which the individual only passes through a part of the developmental changes, while the offspring produced by it accompUshes the remaining part. In this case the life-history of the species is represented by two or more generations of indivi- duals, which possess different forms and organization, exist under different conditions of hfe, and reproduce in different ways. Such a manner of development is known as alternation of genera- tions (metagenesis), and consists of the regular alternation of a sexually differentiated generation with one or more generations reproducing asexually. This phenomenon was first discovered by the poet Chamisso* in the Salpida3 ; but the observation remained for more than twenty years unnoticed. It was rediscovered by J. Steenstrup, t and discussed in the reproduction of a series of animals (Medusse, Trematoda) as a law of development. The essence of the process consists in this, that the sexual animals produce offspring, which through their whole life remain different from their parents, but can give rise by an asexual process of reproduction to a gener- ation of animals which resemble in their organization and habits of life the sexual form, or again produce themselves asexually, their offspring assuming the characters of the original sexual animal. So that in the last case the life of the species is composed of three different generations proceeding from one another, viz., sexual form, first asexual form, and second asexual form. The development of the two, three, or more generations may be direct, or may take place by a more or less complicated metamorphosis ; similarly the asexual and the sexual generations sometimes differ but little from each other {e.g. Salpa), and sometimes present relations analogous to those which exist between a larva and the adult animal {p,.g. * Adalbert de Chamisso, *' De animalibus quibusdam e classe vermium Linnseana in circumnavigatione terree auspicante comite N. EomanzofI: duce Ottone dc Kotzebue annis 1815, 1816, 1817, 1818 peracta." Fasc. I. De salpa Berolini 1819. t Job. Jap. Sm. Steenstrup, " Ueber den Generationswechsel, etc," iibersetzt von C. H. Lorenzen. Kopenhagen, 1842. 124 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. Medusfe). Accordingly we have to distinguish different forms of alternations of generations, which have genetically a different origin and explanation. The latter form of alternations of generations resembles metamor- phosis ; and we have in most cases to explain it as having arisen in the following way :— The asexual form corresponds to a lower stage in the phylogenetic history, from which it has inherited the capacity of asexual reproduction, while the sexual reproduction belongs entirely to the higher form. To take as an example the alternation of generations of the Scyphomedus^. The animal is hatched as a free-swimming ciliated planula (gastrula with closed blastopore) (fig. 113 a). After a certain time it fixes itself by the pole of its body, d Fig. 113. — Development of the planula of Chrysaora to the Scyphistoma stage, with eight arms, a, Two layered planula with a narrow gastric cavity; 6, the same after its attachment with just-formed mouth (O), and commencing tentacles ; c, four-armed Scy- phistoma polyp ; Cxk, excreted cuticular skeleton ; d, eight-armed Scyphistoma polyp with wide mouth ; M, longitudinal muscles of the gastric ridges ; Csk, excreted cuticular skeleton. which is directed forward in swimming, and acquires at its free ex- tremity a new mouth, round which 1, 2, 4, 8, and finally 16 long tentacles soon make their appearance; while the broad oral region projects as a contractile cone (fig. 113 b, c, d). Inside the gastric cavity there project four gastric ridges with longitudinal muscular bands extending from the foot or point of attachment to the base of the oral cone. When the polyp, which has now become a Scyphis- toma, has under favourable conditions of nutrition reached a certain size (about 2 to 4 mm.), ring-like constrictions are formed at the SCYPHISTOMA, STEOBTLA, EPHTBA. 125 anterior part of the body, giving rise to a series of segment-like divisions. The anterior part of the body bearing the tentacles is first marked off; and following this a greater or less number of sections, the new segments appearing continuously in the direction from before backward. The hindermost or basal swollen club-shaped end of the polyp's body remains undivided. The Scyphistoma has Fig. 113. — e, Stage of ScypMstoma with sixteen arms (slightly magiiifled); Gto. gastric ridges. /, Commencing strobilization. now become the Strohila, which itself passes through various developmental phases. The tentacles abort ; the successive segments, separated from each other by constrictions and provided with lobe- like continuations and marginal bodies, become transformed into small flat discs, which become separate, and, as Ephyrm, represent the larv£e of the Scyphomedusae (fig. 113 h-h). 120 ORGANIZATION AND BICVELOPMENT OF ANIMALS IN QENEHAL. In the other cases in which the sexual and asexual forms mor- phologically resemble each other, as in Salpa, the origin of the alternation of generations may, as in the case of the origin of the dioecious from the hermaphrodite state, be traced back to the ten- dency towards the establishment of a division of labour acting upon an animal which possessed the capacity of sexual and asexual repro- duction. It was advantageous for the formation of the regular chain of buds (stolo prolifer) that the power of sexual reproduction should be suppressed, and that the generative organs should be- come rudimentaiy and finally vanish in the budding indivi- duals; while, on the other hand, in the individuals united in the chain, the gene- rative organs were early de- veloped, and the stolo prolifer was aborted and completely vanished. Special forms of alternation of generations may be dis- tinguished in which colonies are formed as the result of the asexual reproduction by budding from a single animal, the buds remaining attached and developing into individuals which differ considerably in structure and appearance, and each of which performs special functions in maintaining the colony (nutritive, protective, sexual, etc.) Such a form of altei-nation of generations is known as polymorpJiism* and reaches a great complication in the polymorphous colonies of the Siphonophora. A form of reproduction which closely resembles metagenesis, but which genetically has quite a different explanation, is the lately Fig. 113.-9, Fully developed Strobila with separ rating Ephyi-se. h. Free Ephyra (of about 1 '5 to 2 mm. diameter. * E. Leuckart, " Ueber den Polymorphismus der Individuen oder die Erscheinung der Arbeitstlieilung in der Natur." Giessen, 1851. HETEEO&AMT. 127 discovered process known as heterogamy. It is characterised by the succession of differently organized sexual generations living under different nutritive conditions. Heterogamy, which was first discovered in certain small Nematodes {Rhahdonema nicjrovenoswm and Leptoclera a'p'pendiculata), can scarcely be explained otherwise than as an adaptation to changed conditions. For when the embryo is developed as a parasite in conditions favour- I able for the acquisition of nutriment, it gives rise to a sexual form so different in size and structure from that which arises if the A. .11 B. Fig. 114. — A, Rhabdonema nigfrovenosum of about 3'5 mm. in length at the stage when the male organs are ripe. genital gland ; O, mouth ; D, alimentary canal ; ^1, anus ; iV, nerve ring; X>rz, gland cells ; Z, isolated spermatozoa. B, Male and female Ehabditis, length from about 1 "5 to 2 mm. ; Oo, ovary; T, testis; F, female genital opening; ecies in p7'ocess of formation. Variety and species are connected by continuous series of transitions, and are not absolutely distinct from one another; but ai'e only relatively separated PEOGEESSING DIVEEGENOE OP CHAEACTEES. 149 by the amount of difference in their morphological and physiological characteristics. This conclusion of Darwin's, which extends the result of natural selection from the production of variety to that of species, is ob- stinately and often bitterly opposed by those who subordinate the phenomena of nature to traditional ideas. Even if they do not deny the facts of variability, and even admit the influence of natural selection on the formation of natural varieties, they yet continue true to the belief that there is an absolute separa- tion between species and race-variety. As a matter of fact, however, we are not in a position to draw such a line of separation. Neither the quality of the distinctive characteristics nor the results of cross- ing afford us a distinctive criterion between species and variety. The fact, however, that we are not able to give any satisfactory defini- tion of the conception of species, precisely because we are unable clearly to distingioish between species and variety, adds so much the more weight to Darwin's argument, since neither the variability of the organism and the struggle for existence nor the great antiquity of Hfe upon the globe can be contested. The variability of forms is a firmly established fact ; so, too, is the struggle for existence. Now if we add the operations of natural selection to these two factors, we are able to understand the origin of varieties. If we imagine the same process which has led to the formation of varieties continued through a greater number of genera- tions and effective through a longer period of time — ^and we are the more justified in making use of these long periods of time, since with their help astronomy and geology have been enabled to explain many phenomena — the diverging characteristics will become more and more marked, and will at last gain the value of distinctive species- characters. In still greater periods of time the species will become so far separated from one another by the simultaneous disappearance of the intermediate forms that they will represent different genera. Accordingly the greater differences of organization which are ex- pressed in the higher divisions of the system, such as orders and sub-orders, etc., require a longer interval of time for their accom- plishment, and an extinction of a greater number of intermediate forms. Finally, the different ancestral forms of the classes of a gi'oup may be refen-ed to a common starting-point ; and since the different groups of animals are connected by many intermediate forms, the nvimber of the ancestral forms becomes much reduced. 150 MEANING OF THE SYSTEM, The undifferentiated contractile substance, sarcode or protoplasm, was probably the starting-point of all organic life. If these suppositions are correct, species no longer retain the signifi- cation of independent and immutable units, and appear, according to the great law of evolution, only as transient groups of forms, capable of change, and confined to longer or shorter periods of time, to definite conditions of life, and preserving, as long as these conditions do not vary, a constancy in their essential characters. The different categoi-ies of the system show the closer or more remote degree of relationship ; and the system is the expression of genealogical relationship founded upon descent. All systems, however, must be imperfect and full of gaps, since the extinct ancestors of organisms living at the present time can only be very imperfectly supplied by the geological record ; numerous intermediate forms are wanting and finally no traces of organic remains from the most ancient periods are preserved. Only the ultimate twigs of the enormously ramified ancestral tree are accessible to us in sufficient number. Only the extreme points of the twigs are completely preserved ; while of the numerous rami- fications of the branches only the existence of a stump here and there has been demonstrated. Hence it appeai-s quite impossible, in the present state of our knowledge, to attain to a sufficiently sure representation of this natural genealogical tree of organisms ; and while we admire the bold speculations of E. Haeckel's genealogical attempts, it must be admitted that at present there is room for innumerable possibilities in detail, and that subjective judgment holds a more conspicuous place than objective certainty of fact. Hence we must be contented for the present with an incomplete and more or less artificial arrangement ; although the conce2)tio7i of the natural system theoretically is established. When the fundamental arguments of the Darwinian theory of selection and the transmutation theory founded upon it are submitted to criticism, it is soon apparent that direct proof by investigation is now, and perhaps always will be, impossible ; for the theory is founded upon postulates which cannot be submitted to direct inquiry. Periods of time which cannot be brought within the limits of hrmian observation are required for the alteration of forms under natural conditions of life ; and the extremely complicated interactions, which in the natural state under the form of natvxral selection are tending to change plants and animals, can only be grasped in a general sense, while in their details they are practically unknown to us. Fvirther, plants and animals which are under the influence of EVIDEKOE FROM MORPHOLOaT. 151 natural selection are entirely inaccessible to the experiments of man, and the relatively few forms which man has, in a greater or less space of time, brought completely within his power, have been and are being altered and modified by the so-called artificial selection. The action of the natii,ral selection, in Darwin's sense, is therefore in general incapable of direct proof, and even for the origin of varieties can only be illustrated and rendered probable by hypothe- tical examples. Against this we must, however, set the fact that there is a great probability in favour of the correctness of the theories of descent and transmutation of species, which have never received better support than from the natural selection theory of Darwin; and that this probability is supported, not only by the whole weight of morphological evidence, but also by the testimony of PalfEontology and of geographical distribution. EVIDENCE IN FAVOUR OF THE THEORY OF DESCENT. If the transmutation of species is to be regarded as an hypothesis, because it is incapable of being demonstrated by direct observation, then its value depends upon its correspondence with the facts and phenomena of nature. Evidence from Morphology. — The lohole of Morphology tends to show the correctness of the theory of transmutation of species. The degrees of resemblance between species which was for a long time expressed by the metaphorical term " relatimishijo," and which rested upon an agreement in more or less important characteristics, led to the establishment of systematic groups, of which the highest, the kingdom or type, was founded upon a similarity in the most general features of organization and development. The agreement of numerous animals in the general plan of their organization, e.g., the common possession by fishes, reptiles, birds, and mammals of a rigid column forming the axis of the body, and the dorsal position in regard to this of the central nervous system and the ventral position of the organs of novirishment and reproduction, are very well explained, according to the theoiies of selection and descent, by the derivation of all Vertebrates from a common ancestor possessing the characteristics of the type, while the supposition of a plan of the Creator renounces all explanation. In like manner is explained that similarity of characteristics by which the remaining groups and sub-groups, from class to genus, are distinguished, as well as the possibility of dividing all organized beings into groiips subordinated the one to the other. 162 MEANING OP THE SYSTEM. The impossibility of a sharply defined classification hi also rendered comprehensible by the theory of descent. The theory i-equires the existence of forms transitional between intimately and remotely allied groups ; and explains, as a result of the disappearance, in course of time, of numerous types which have been worsted in the struggle for existence, the fact that gi-oups of equal value are of such various extent, and are often only represented by single forms. It is not only systematic characters, but also the innumerable facts brought to light by the science of Comparative Anatomy which point to a nearer or more remote relationship between the different groups. For example, if we examine the structure of the extremities or the brain of Vertebrates, we find, in spite of considerable differ- ences (sometimes bridged over by intermediate forms) in the various groups, that in all they are built upon a common type of struc- ture. This type is found very variously modified and more or less differentiated in each secondary group, according to the different functions which the organ has to fulfil and according to the exigencies of 'the mode of life to which each species is subjected. In the fin of the whale, in the wing of the bird, in the anterior limb of the quadruped, and in the human arm it can be shown that there are present the same bones, here short and broad and immoveably con- nected, there elongated and jointed in different ways to allow of corresponding movements, sometimes with every part fully developed, sometimes simplified m one way or another, and partly or entirely rudimentary. Evidence from the facts of Dimorphism and Polymorphism. — The phenomena of dimorphism and polymorphism in the same species, and the sexual differences which have been developed in animals originally hermaphrodite, may be quoted as important evi- dence of the extensive infiuence of adaptation. Male and female forms differ not only in the fact that the foi-mer produce spermatozoa and the latter ova, but they exhibit numerous secondary sexual characteristics connected with the different func- tions which the male and female respectively have to perform. The existence of these secondary characteristics can in all cases be satisfactorily explained by means of natural selection. We may therefore, in a certain sense, speak of a sexual selection by means of which the two sexes have been, in course of time, gradually sepa- rated from one another, not only in peculiarities of form and organiza- * Ch. Darwin, " The Descent of Man, and. Selection in Helation to Sex," Vol. I. and II. London 1871. EVIDENCE EB.OM DIMORPHISM. 153 tion, but also iu habits of life, in such a way as to favour the preservation of the race. Since the male sex generally has to take a more active part in the acts of copulation and fertilization it is comprehensible that the male form should differ more from the young than the female which supplies material for the formation and nourishment of the embryo and is charged with the care of the progeny. Very frequently the male sex is capable of quicker and more facile movements; in many Insects the male alone has the power of flight, while the female remains without wings (fig. 97). In the strife which the males of similar species have to wage for the possession of the females, those individuals which are most favoured by their organization (in respect of strength, capability for motion, prehensile organs, beauty, organs for production of sound, etc.) will prove the conquerors; while those females which possess properties especially favourable to the prosperity of the offspring will best fulfil their task. At the same time variations in the duration of development, in the mode of growth and structure, may in a more passive way be favovirable under the special conditions of life of the species. The secondary sexual characters may sometimes acquire such importance as to lead to essential and deeply engrained modification of the organism, and to a true sexual dimorphism (males of Rotifera with no digestive tube, dwarfed males of Bonellia, Trichosomum crassicauda). It is a significant fact that dimorphism of sex reaches its highest extreme in parasites. In many parasitic Crustacea (Siphonostoma) such extreme cases, in which the large shapeless females have lost the organs of sense and locomotion, and even segmentation, while the males are small and dwarfed, are connected by numerous inter- mediate forms; and the circumstances which have operated as the cause of this sexual dimorphism are not far to seek. The influence of favourable conditions of nourishment which parasites enjoy does away with the necessity of rapid and frequent locomotion, increases in the female the capacity of producing reproductive material, and brings about svich an alteration of form that the power of locomotion is diminished and the organs of movement atrophy and may com- pletely vanish. The body acquires an unwieldy, shapeless character in consequence of the enormous size of the ovary which is filled with eggs, and throws out outgrowths and processes into which the ovaries project, or else acquires an unsymmetrical saclike form. The seg- mentation is lost and the limbs degenerate; the slender moveable abdomen which, when the animal was free-swimming, was an essen- 154 MEANING OF THE SYSTEM. tial tlid to locomotion, is reduced more and more till it becomes a short, unsegniented stump. The appearance of such a parasite is so strange that one can easily comprehend how it was that formerly one of these abnormal groups, the Lema'.ai, was placed among the endoparasitic Worms, or even among the MoUusca. The more the female remains behind the type of its fully-developed, free-living allies, so much the more do the two sexes become morpho- logically I'emote from one another, for the form and organization of the male also are affected by the changed conditions of life, but in a different manner.* In the male sex the more favoui-able and abundant nouidshment may not affect the necessity of locomotion and the development of the locomotive organs in so direct a manner, since the sexual activity of the male and the necessity for locomotion in order to select a female remain unaltered. E^'^en when locomo- tion is reduced and rendered difficult, parasitism does not, in the case of the male, lead either to a complete loss of segmentation or to such unsymmetrical growths as we observe in many female parasitic Crus- tacea. The large quantity of generative material produced, which in the female is of the greatest importance for the preservation of the species, and which therefore favours the development of a large, shapeless, unwieldy body, is the less conspicuous in the male because a very small quantity of sperm serves for the fertilization of an enormous nvimber of ova. Thus, then, the extreme degree of parasitism in the male, even when accompanied by a confined and more creeping mode of loco- motion, does not lead to an excessive increase in size nor produce an unsegmented and strange form of body, but, on the contrary, gives rise to the S3TiimetricaUy formed, dwarfed pigmtean males. This extreme state is, however, connected with the normal state by numerous intermediate steps. Thus we find in the Lernaeopods that the size of the male Adheres is only slightly reduced, while the true dwarfed males of the Lernmojwda and Chondracanthidce, are attached, like small parasites (fig. 98), to the posterior end of the female body, which is relatively enormously large. The preparation of a large amount of sperm which implies the possession of a large body, would only be a useless expenditure of material and time in the life of the species, and this must have been avoided by the influence of natural selection. In adchtion to this sexual dimorphism we find in various groups of animals— especiaUy in the insects which live together in great * Compare C. Claus, " Die fi-eilebeiideu Copepoden." 1863. ETIDENCE EEOM MIMICBT. 155 societies, the so-called canimal communities— a third group of indi- \dduals (sometimes even divided into several series of forms) which are without generative organs and are incapable of reproduction, but which assume the functions of protecting, of providing nourish- ment for the community, and of caring for the young. Adaptive peculiarities suitable for the discharge of these functions are apparent in their structure and organization. These sterile indivi- duals are in the HymmoiMra aborted females. Among the ants they are divided into workers and soldiers. Amongst the Termites they are derived from both males and females, in which the genera- tive organs are reduced. Sterile individuals are also found amongst animals (Fishes) which do not form communities, and were formerly taken for particular species and described as such. Polymorphism is most highly developed in the Hydroids which are united in stocks — the Siplionopliora. The numerous cases of dimorphism and polymorphism in eithei' sex of the same species, should be regarded from the same point of view. Dimorphic females among insects have been observed, e.g., in the Malayan Painlioniclce (P. Memnon, Pamnon, Ormenus), in cer- tain species of Hydroporus and Dytiscus, as also in the Eeurotemis, a genus of the Neuro^otera. In these cases, as a rule, one of the female forms is more nearly related in form and colour to the male orm whose peculiarities it has assumed. In other cases the differences are more connected with climate and season (seasonal dimorphism of butterflies), and also affect the male animal. They may be connected with the different forms of reproduction (parthen- ogenesis), and lead to the phenomenon of heterogamy {Chermes Phylloxera, Aphis). Much more rarely we find two kinds of males with dissimilar secondary sexual characters connected with copula- tion, as in the case of the " smellers " and " claspers " described by Fritz Miiller in the Isopoda [Tanais clubius). Evidence from Mimicry. — Another series of phenomena which may probably be referred to useful adaptation is the so-called mimicry. Certain animal forms come to resemble othei' -widely- distributed species, which are protected by any peculiarity of form and colour, so closely that they seem to have copied them. The cases of mimicry which have been principally made known by Bates and Wallace are directly connected with the protective resemblances mentioned above : that is, the resemblance of many animals in colour and body shape to the objects amongst which they * Fritz Muller, " Facts for Darwin," p. 22. 156 MEANING or THE SYSTEM, live. For example, amongst the butterflies certain Leplalidm resemble in outward appearance and in mode of flight a species of the family Heliconius (fig. IIG), which appears to be protected from the pursuit of birds and lizards by a yellow disagreeable-smelling fluid, and share the same locality with the above-mentioned species. The most pei-fect instances of mimicry are found in the Tropics of the Old World, where the Danaidce and Acrceidce are imitated by the Papilionidie [JMnais niavius, Papilio hippocoon — Danais echeria, Papilio cenea — Acrcea gea, Panopcea hirce). Cases of mimicry fre- quently occur between insects of different orders ; butterflies imitate the form of Hymenoptera, which are protected by the possession of spines [Sesia homhyliformis — Bomhus hortorum, etc.) In the same way certain beetles resemble bees and wasps {Gharis melipona, Odontocera odyneroides), and the Oi'thopteran genus dylodera tricondyloides from the Philippines is like a genus of Cicmdelce (Tricondyla). Numerous Diptera have the form and colour of stinging Sphegidce and Wasps. Also among Vertebrates (Serpents and Birds) some examples of mimicxy are known. Evidence from Rudimen- tary Organs, — Rudimentary organs, too, which are so common, are satisfactorily ex- plained by the theory of selec- tion as the result of non- employment of such organs. Organs which were formerly functional have gradually or even suddenly become functionless as a result of adaptation to special conditions of life, and, through want of exercise, have, after the lapse of generations, become weaker and finally aborted or degraded (Parasites). We cannot, however, assert that rudimentary organs are in all cases useless. They have, on the contrary, often gained secondary functions, though this may be diflicult to demon- strate. We find, for instance, in certain snakes {Pythonidce) that there are small processes armed with claws at the sides of the anus (anal Fig. 116. — a, Lepfalis Theoiwe, var. JLeuconoi: (Pieris). b, Ifhomia Ilerdina (the mimicked Heliconius). (After Bates.) EVIDENCE FEOM EMBRYOLOGY. 157 claws). These are the hind limbs which have become rudimentary, and which do not subserve locomotion but, in the male at least, assist in copulation. The bhnd worms possess a rudimentary shoulder girdle and breast bone, although the anterior extremities are want- ing : these bones may be connected with the need of protecting the heart, or may aid in respiration. When we see that the upper incisor teeth are developed in the foetus of many ruminants, and that these teeth are never cut, and that the embiyos of the whalebone whales have the rudiments of teeth in their jaws, which they soon lose and never make use of in mastication, it is much more rational to asciibe to these structures a part in the growth of the jaw than to hold them for wholly useless. The i-udimentary wings of the penguin are employed as oars, those of the ostrich as aids to running and as weapons for protection. The rudimentary stumps of the kiwi, on the contrary, appear valueless. In man}^ cases we are not in a position to assign any function or value to rudimentary organs. Evidence from Embryology. — The results of embryology too, i.e., the individual development from the ovum to the fully developed form, are in complete agreement with the Darwinian theories of selection and descent. The fact that the animals belonging to one type have, as a rule, embryos which are much alike and undei'go a similar developmental process, and that the closer the relationship between the adiilt forms the greater the similarity in theii- develop- ment (with some remarkable exceptions), supports the conception of a common ancestry and the hypothesis of diflFering gradations of blood- relationship. If the groups of different value which correspond to the divisions and subdivisions of ora- classification are genetically derived from more or less remote ancestral forms, then the individual develop- ment will present .so many the more common features the closer the forms stand to their common ancestor. The fact that animals which differ much from one another and exist under very different conditions of life show an unusual agree- ment in their post-embryonic development up to a more or less late period (the free Copepoda, parasitic Crustacea, Cirripedia), is in no wise opposed to the theory, but may be explained by the influence which adaptation has exerted not only during the period of sexual life, but also during each developmental period, causing changes which have been inherited in corresponding periods of life. The phenomena of metamorphosis afford numerous proofs of the fact that the adaptation of the embryonic form is as complete as 158 MEANING or THE SYSTEM. that of the adult ; and we can thus understand how larvse of many insects belonging to diflerent orders can present great resemblances to on© another and be unlike the larv88 of insects of the same ordei-. While as a general rule the development of the individual is an advance from a simpler and lower organization to one more complex which has become more perfect by a continued division of labour among its parts — and we shall later find a parallel to this law of perfection of the individual in the great law of progressive perfection in the development of groups — yet the course of development may, in particular cases, lead to numerous retrogressions, so that we may find the adult animal to be of lower organization than the larva. This phenomenon, which is known as retrogressive meta/raorpJiosis [Ciri'ipeclia and parasitic Crustacea), corresponds to the demands of the selection theory, since under more simple conditions of life, where nourishment is more easily obtained (parasitism), degradation and even the loss of parts may be of advantage to the organism. Again, the facts of embryonic development, when considered in relation to the gradations expressed in the system are in complete accord with the theory of evolution. Numerous examples may be cited to prove that features, not only of the simple and more primitive, but also of the more perfectly organised groups of the same type, are reflected in the successive phases of foetal Hfe. In the case of a complicated free development by metamorphosis, which is usually correlated with an unusual simpHfication of the foetal development within the egg- membranes, the relation of the successive larval stages to the allied smaller groups of the system, to the genera, families and orders, is more direct and striking. For example, in the early stages of the embryonic development of mammals certain structures occur, which in the lower fishes endure throughotit life. Later stages show peculiarities which correspond to the characters of amphibia. The metamorphosis of the frog begins with a stage which in form and organization and mode of locomotion agrees with the fish type ; and this stage is succeeded by numerous other larval stages in which the characters of the other orders of Amphibia (Perenni- branchiata, Salamandrinidse) and of individual families and genera of the same are repeated. This undeniable likeness between the successive stages of individual development and between allied groups of the system allows us to institute a parallel between the former and the evolution of the species. The evolution of the species finds, it is true, a most imper- fect expression in the relationship of the systematic groups, and can GEOGRAPHICAL DISTRIBUTION. 159 only be inferred from the history of the past for which palteon- tology aftbrds us but slight material. . This parallel, which naturally presents numerous greater or smaller variations in detail, is explained by the theory of evolution, according to which the developmental history of the individual appears to be a short and simplified rep)etition, or in a certain sense a recapitulation, of the coici'se of development of the species/'' The historical record preserved in the developmental history of the individual must often be more or less blurred and obscure on account of the many adaptations which have occurred during the embryonic development, or duiing larval life. Especially in those cases where the peculiar conditions of the struggle for existence demand a simplification, the development will take a more direct course from the ovum to the perfect animal, will be thrown back into an earlier period of life, and finally will be completed before the animal is hatched, until, in absence of a metamorphosis, the historical record is completely suppressed. On the contrary, in the cases of progres- sive transformation where the larval states are gradually modified and live under similar conditions of life, the history of the species will be less impei'fectly reproduced in that of the individual. Evidence from the Facts of Geographical Distribution. — Unlike the facts of moi'phology, those of geographical distribution luise great difiiculties for the theory principally because the phenomena are very complicated and our experiences are still too limited to permit of our establishing general laws. The present distribution of plants and animals over the surface of the earth is clearly the combined result of the earlier distribution of their ancestors and of the geologi- cal changes which have since taken place, the modifications in the extent and position of land and water, which must have had an influ.ence on the fauna and fiora. Accordingly the geographical distribution of plants and animals \ appears intimately connected with that part of geology which has for its aim the investigation of the most recent occurrences in the formation of the earth's crust and its contents. It cannot, therefore, be confined to an examination of the areas of distribution of the animals and plants of the present day, but must take cogni- zance of the distribution of the remains, enclosed in the most recent formations, of the nearest relations and ancestors of living forms, in * Fr. Mliller, " Fiir Darwin," Leipzig, 1864. t A. R. Wallace, " The Geographical Distribution of Animals," London 1876 P. L. Sclater, " Address to the Biological Section of the Brit. Association,"' 1875'. 160 MEANING OF TUB SYSTEM. order to find an historical ex[)lanation of the known facts of distribu- tion. Although in this sense the science of animal geography is still in its infancy, yet numerous and important phenomena of geographical distribution receive a satisfactoiy explanation according to the theory of transmvitation of species on the supposition of migrations and gradual changes brought about by natural selection. It is a most important fact that neither the resemblance nor the want of resemblance of the animals inhabiting different localities can be completely explained as the result of climatic and physical conditions. Closely allied species of plants and animals often appear under very different natural conditions, while a completely different fauna and flora can exist in a similar climate and on a similar soil. On the other hand, the extent of the difference between two fauna is closely connected with the limitations of space and the barriers and hindrances to free migration. The Old and New Worlds, which, leaving out of consideration the polar connection, are completely separated, have in part a very different fauna and flora, although with regard to the climatic and physical conditions of existence there are innumerable parallels which would equally favour the prosperity of the same species. In particular if we compare the districts of South America with regions situated in the same latitude and possessing the same climate in South Africa and Australia, we find three fauna and flora which differ considerably, while the natural productions from different latitudes of South America with entirely different climates are closely allied. Here the northern animals are indeed specifically different from the southern, but belong to similar or nearly aUied genera with the peculiar stamp characteristic of South America. Zoological Provinces.— The surface of the earth can be divided into from six to eight regions according to the general features of the terrestrial and f resii-water fauna. These regions can indeed only be considered as a relative expression for large natural districts of distribution, since they cannot be appHed to all groups of animals in the same manner, and it is impossible that they should differ in like degree and in the same direction. There must also be inter- mediate regions combining the characteristics of the neighbouring re-ions with peculiarities of their own ; and the question must arise whether these should not be taken as independent regions.^ The merit of ha^dng first established a natural dmsion of the earth into zoological regions and sub-regions belongs to Sclater. This naturalist founded his system on the distribution of birds, and dis- ZOOLOGICAL PROVINCES. 161 tinguished six i-egiona, the limits of which agreed fairly well with the distribution of Mammalia and Reptilia. These regions are — (1) The Falcearctic Region — Europe, the temperate part of Asia, and North Africa as far as Mount Atlas. (2) Nearctic Region — Greenland and North America as far as North Mexico. (3) The Ethiojnan Region — Africa, south of Atlas, Madagascar, and the Mascarenes with South Aiubia. (l) The Indian Region — India south of the Himalayas, to South China, Borneo and Java. (5) The Australian Region — Celebes and Lombok eastward to Australia, and the South Sea Islands. (6) The Neotropical Region — South America, the Antilles, and South Mexico. Other naturalists (Huxley) have since shown that the four first of these regions have a much greater resemblance to one another than any one of them has to the Australian or South American regions; that New Zealand is entitled by the peculiarities of its fauna to be considered as forming a region by itself ; finally, that a circumpolar^'= province should be formed equal in value to the Palse- arctic and Nearctic. Wallace objects to the establishment either of a Neio Zealand or of a circumpolar region, and advocates the adoption of the six regions of Sclater on practical grounds, but suggests the modification that since the South American and Australian are much more isolated, the regions should not be of equal value. These regions are bounded by extended seas, lofty mountain ranges, or vast sandy deserts, and obviously such boundaries do not constitute effective barriers to the migration of all animals, but allow certain groups to pass from one region to another. The obstacles to immigration and emigration appear in certain places, at all events in the present time, to be insurmountable; * Andrew Muri'ay, on the contrary, in his work on the geographical dis- tribution of Mammalia in 1866, distinguishes only four divisions— the PalsBarctic, Indo-Afncan, the Australian, and the American. Riitimever recognises in addi- tion to the six provinces of delator a Mediterranean and Circumpolar province J. A. Allen ("Bulletin of the Museum of Comparative Zoology, Cambrido'e, vol. ii.) proposes to distinguish eight regions, in connection with " the law of circumpolar distribution of life in zones : "—(1) Arctic realm ; (2) North Temper- ate realm ; (8) Tropical American realm ; (4) Indo-Afi'icaii Tropical realm • (5) Tropical South American realm ; (6) Temperate African realm ; (7) Ant- arctic realm ; (8) Australian realm. 11 162 MIOANING OP THE SYSTEM. but in past ages, wlien the divisions of land and water were different, they must have been, for many forms of Hfe, easily surmountable. The expression "centre of creation," which has long been used in the sense of a tolerably defined district of dis- tribution— or better still, Riitimeyer's word, " centre of distribution " — has as a fundamental idea the endemic appearance of definite groups of typical species and their gradual extension " towards the boundaries of the said region, a conception which harmonizes well with the theoi-y of the origin of species through gradual alterations. The same laws apply also to the distribution of the inhabitants of the sea. G-reat seas studded with islands which serve to confine the land animals may favour the migration of marine species, while extended continents, which allow their inhabitants to wander freely over them, confine the sea animals within limits which cannot be passed. A great number of sea animals live only in the shallow water round the coast, and their distribution thus often coincides with that of the land animals ; whereas the animals found on the opposite coasts of great continents are very different. For example, the sea animals of the east and west coasts of South and Central America differ to such a degree that, with the exception of a series of fishes, which, according to Giinther, are found on both sides of the Isthmus of Pancmuo, only a few forms are common to the two coasts. In the same way we find that the marine inhabitants of the east insular district of the Pacific differ completely from those of the west coast of South America. If, however, we advance to the west of this part of the Pacific till we come to the coast of Africa in the other hemisphere, we find that the fauna of this extensive district cannot be so sharply distinguished. Many species of fish are found from the Pacific to the Indian Ocean. Numerous Mollusca of the South Sea Islands live also on the east coast of Africa, almost beneath the opposite meridian. In this case the limits of distribu- tion are not impassable, as numerous islands and coasts afford a rest- ing place to wandering inhabitants of the sea. In respect of the different haunts of the inhabitants of the sea, we must make a dis- tinction between the littoral animals, which are distributed along the coasts, and live under different conditions and at different depths on the bottom of the sea, and the ijelagic animals, which swim on the surface. * Compare Riitimeyer's Essay, " Ucber die Herlcunft unserer Thierwelt." Basel and Genf, 1867. ETIDENOJS PEOM PALyEONTOLOQY. But there also exists, at considerable depths and on the bottom of the sea, a rich and varied animal life. This has only lately been brought to our knowledge principally by the deep-sea explorations from North America, Scandinavia, and England. In place of that want of animal life which we should on (I priori grounds expect to find, we see that numerous lowly organised animals of the most different groups are able to exist even at the greatest depths. Besides the lowest sai'code animals of the Foraminifera (Globigerina ooze), we find especially silicious sponges, certain corals, Echinoderms, and Crustacea.'^ The representatives of the latter are in part of low type, but gigantic, and ma,ny of them blind. It is also a fact of more than ordinary interest, as showing the continuity of living creatures from successive geological forma- tions up to the present time, that the deep sea animals are allied to ancient types which occvir in Mesozoic formations, especially in chalk. Evidence from Palaeontology. — The results of geological and jxdceontological inquiry give vis a third great series of facts in support of the theory of slow alterations of species and the gradual development of genera, families, orders, etc. The firm crust of our earth is formed of numerous and enormou.s rock strata, which have been deposited in a definite series by water in course of time, and also of the so-called volcanic or plutonic rocks, masses which have been forcibly ejected from the molten interior of the earth. The former or sedimentary deposits, which have under- gone numerous alterations in the originally horizontal arrangement of their strata as well as in the petrographical condition of their rocks, contain a quantity of the fossilized remains of former plants and animals which have become buried in them, and thus afford an historical i-ecoixl of a rich fauna and flora which existed during the earlier periods of the earth's development. Although these so-caUed fossils have made us acquainted with a very considerable number of ancient organisms presenting great diversity of form, yet they only constitiite a very small portion of the enormous quantity of living beings which have at all times existed upon the earth. They suffice, however, to teach us that a different fauna and flora existed at the time when each individual deposit was being formed, and that * Compare Wyville Thomson, " The depths of the sea. An account of the general results of the dredging cruizes of the Poron^rhtc and Liqhtning, during the summer months of 1868, 18(59. 1870." London, 1873. Also the results of the Challenfjer expedition 1874.-1876. 164 MEANING or THJS SYSTJJM. tlie deeper a stratum comes in the series, that is, the earlier it appears in the history of the earth, so much tlie moi-e its fiiuna and lloi-a (litter from those of the present time. The more neai-ly one stratum follows another in the series, the closer the relationship between their respective fossils. Every sedimentary formation possesses characteristic fossils which appear very frequently ; and from these, taking into account the succession of strata and the peti-ogi'aphic characters of the I'ocks, the place occupied by the stratum in the geological system can be defined with tolerable accui'acy. Without doubt the characters of the fossils and the relative posi- tions of the strata are the most important aids to the determination of the geological age of the deposit ; at any rate they furnish a more reliable criterion than does the structure of the rocks. The idea entertained in earlier times that rocks of the same period always possessed a similar, and rocks of a different period a dissimilar structure, has lately been given up as erroneous. Stratified or sedimentary deposits have arisen in every period vmder similar condi- tions. In past times, as at the present time, they were caused by the deposition of clay, of fine or coarse sand, of fine and coarse debris, by chemical precipitation of carbonates and sulphates of lime and magnesia, of silica and oxide of iron, and by accumulation of solid animal and vegetable remains. These have become transformed only in course of time into such hard rocks as argillaceous and calcareous schists, limestone, sandstone, dolomite, and conglomerates of many kinds ; as the result of many causes, such as mechanical pressure of superincumbent masses, increase of temperature, internal chemical processes, and so forth. Even though the peculiar structure of rocks may in many cases afford good ground for conjecture as to the relative age, yet it is certain that deposits of similar age may show an entu-ely different petrographical character; and, on the other hand, that deposits of very diiferent ages may have given rise to rock forma- tions that can be scarcely or not at all distinguished from one another. The old idea that deposits of the same age must everywhere contain the same fossils, could only be maintained as long as geological inves- tigations were confined to small districts. Similarly the idea, closely connected with the former, that the various geological formations, characterised by a series of definite strata, are entirely independent uf one another, no longer obtains credit. Tbe various forma- GEOLOGIOAIi PERIODS. 165 QUARTIAKY PERIOD {Diluvial and Alluvial Formation^:). TERTIARY PERIOD {Caiiiozoie Formations) tions,* as the group of strata of one district of distribution and belong- ing to one period are named, cannot be divided petrographically or * The follo^ving table may serve for a bird's-eye view of the geological periods and their most important formations : — Recent Periods (alluvium, marine and fresh-water formations). Postpliocenc or Diluvial Period (erratic boulders, glacial period). Pliocene Period, (subappenine formations, bone sand of Bppelsheim, etc.) Miocene Period (Molasse, Tegel near Vienna, brown coal in North Germany, etc). j Flysch, Nummulite formation ( of the Paris basin. I" Maestricht strata, white chalk, ■j upper green sand, Gault, lower green sand, Weald. Purbeck strata, Portland stone, Kimmeridge clay. Coral Kag, Oxford clay. Great oolite, Lower oolite, Lias (white, brown, and black jura). Keuper or upper new red sand- stone, Muschelkalk (upper Muschelkalk, gypsum and anhydrite, Wellenkalk, Bun- ter Sandstein). SECONDARY PERIOD (Mesocoic Formation ) . Eocene Period Oretaceo^is Period Jurassic Period SECONDARY PERIOD ( Mesozoio Form atio n s ) . Triassic Period Peo'mian Carloniferom Period \ Zechstein, lower new red sandstone Eothliegendes. — PALiEOZOIC PERIOD (PalcBOZoic Formations). Coal Measures of England, Germany, and North America, Kulmformation, Carboniferous limestone. Devo7iiau Period (Spiriferenscbiefer, Cypridinen- schiefer, Stryngocephalenkalk, etc. — old red sand- stone.) Sihcrian Period: (Ludlow, Wenlock, strata, etc.) \Camhrian Period (slate, etc.) j Thonschiefer, Laurentian formations. Mica schist, ( Older Gneiss formations. According to Professor Eamsay the groups of formations in England have a thickness of 72,.584: feet, i.e., about 13| English miles ; that is, formations of the — Palfeozoic period have a thickness of 57,1541 SecoMary ,. „ 13,190 1 72,584 feet Tertiary „ „ 2,240 ! ARCH.a;AN PERIOD 166 MEAJJING OP THE SYSTEM. palteoritologiciilly from each other in such a manner as to lend support to the hypothesis of sudden and mighty revolutions and ctitastrophes destroying the whole living world. We may rather assert with cer- tainty, that the extinction of old species and the appearance of new ones has hot taken place at the same time at all points of the surface of the earth, for many species extend from one formation into another, and a number of oi'ganisms persist from the tertisiry period to the present time, but little altered or even identical. Just as the commencement of the recent epoch is hard to define, and cjinnot be sharply separated from the diluvial period by the character either of its deposits or of its fossils, so it is with the remoter periods of the earth's history, which are founded, like periods of human history, upon great and unportant occurrences, and yet are in direct con- tinuity. Lyell has proved in a convincing way on geological grounds that there were not sudden revolutions extending over the whole surface of the earth, but that changes took place slowly, and were confined* to separate localities; in other words, that the past history of the earth consists essentially of a gradual process of development, in which the numerous forces which may be observed in action at the present day have, by their long continued operation, had an enormous total effect in transforming the earth's surface. The reason for the irregular development of strata and for the limitations of formations is principally to be sought in the interrup- tion of depositions, which, though widely distributed, were only of local importance. Were it possible that a single basin of the sea should have persisted during the whole period of sedimentary forma- tion and under singularly favourable circumstances have formed new deposits in persistent continuity, then we should find a progres- sive series of strata interrupted by no gaps, which we should be unable to classify according to formations. Such an ideal basin would include only a single formation, in which we shoidd find representatives of all the other formations of the sm^face of the earth. * " Every sedimentary formation was extended at the time of deposition over a confined territory— confined on the one hand by the extent of the sea or fresh- vrater basin, and on the other by the different conditions favourable to the depo- sition inside the basin. At the same time, in other places entirely or at any rate somewhat differentlv stratified formations {/.r., formations of the same age, but of diflEerent composition) resulted. Thus marine, fresh-water, and swamp formations have been deposited at the same time from different rocks and with different fossils, while the laud surface has remained free." Comp. B. Cotta, Die Geologic der Gegenwart." GAPS IN THE GEOLOGICA.L BECOBD. 167 In reality this ideal continuous series of strata is interrupted by numerous and often large gaps, which determine the petrographical and paljeontological differences, often strongly marked, between successive strata, and correspond to periods of inactivity, or, as may happen, to periods when the results of sedimentary action have been again destroyed. These interruptions of local deposits are explained by the constant alterations of level which the sm-face of the earth has undergone in every period in consequence of the reaction of the molten contents of the earth against its firm crust. As we see in the present time that wide tracts of country are gradually sinking (west coast of Greenland, coral islands), while others are beii^g slowly elevated (west coast of South America, Sweden) ; that strips of coast line are suddenly submerged beneath the sea by subterranean forces, and that islands as suddenly appear; so it was in earlier periods. Elevation and depression were at work, perhaps uninterruptedly, causing a gradual, more rarely a sudden (and then locally confined) interchange between land and sea. Basins of the sea rising with gradual movement became dry land and I'ose up first as islands, and afterwards as connected continents, the different deposits of which, with their included fossils, bear witness of the sea which once covered them. On the other hand, great continents sank beneath the sea, leaving perhaps their highest moiin- tain peaks appearing as islands, and again became the seat of fresh deposition of strata. In the first case there would be an interruption of deposit, while in the latter there would result, after a longer or shorter period of inactivity, the beginning of a new formation. Since, however, elevations and depressions, even though affecting districts of great extent, must always be locally confined, the commencement and interruption of formations of equal age have not taken place every- where at the same time. Deposits continued a long time on one tract after they had ceased on another; hence the upper and lower boun- dary of equivalent formations may show great want of uniformity, according to the different locality. This explains how it is that for- mations lying one above the other are composed of strata of ver'y variable thickness, and why we can only in rare cases supply the gaps in the series of these strata from strata found in other countries. The whole succession of formations known to us up to the present time is not sufficiently complete to form an entire and uninteri-upted series of the sedimentary formations. There are still numerous and important gaps in the geological record which we may expect to MEANING OK THE SYSTEM. see filled in future days, when knowledge has increased, and per- haps only when formations now beneath the sea have become known to us. Imperfection of the Geological Record. -After the foregoing dis- cussion we may consider that the continuity of living or-ganisms in the successive periods of the earth's development and their close rehitionship has been proved partly l.y geological and partly l)y pal£eontological facts. The theory of descent, however, ^^ccording to which the natural system must be regarded as a genealogical tree requires still further proof. It requires proof of the presence of numerous forms, transitional not only between the species now existing and those in the more recent formations, but also between the species in all those formations which have immediately succeeded one another in point of time. The theory also demands proof that forms connecting the different groups of plants and animals of the present day have existed. The estalilishment and limitation of these groups can, according to Darwin, only be explained by the extinction, in the course of the earth's history, of ninnerous and intimately connected species. Palajontology is only able imperfectly to comply with these demands ; for the numerous closely graduated series of varieties which, according to the theory of selection, must have existed, are, for the greater number of forms, entirely wanting in the geological record. This want, however, which Darwin himself recognised as an objection to his theory, loses its importance when we consider the circumstances under which organic remains were generally deposited in mud, and preserved for succeeding ages in a fossil form ; when we recognise the facts which indicate the extraordinary incomplete- ness of the geological record, and which show that the intermediate forms must have been in part described as species. First of all we can only expect to find in deposits the remains of those organisms which possessed a firm skeleton supporting the softer parts of the body, since it is only the harder structures of the body, such as the bones and teeth of Vertebrates, the calcarepus and silicious shells of Molluscs and Rhizopods, the shells and spines of Echinoderms, the chitinous skeleton of Arthropods, etc., which are able to resist rapid decay, and to undergo gradual petrifaction. Thus the geological record will fail to provide us with any account of the numerous and principally low organisms which are not pro- vided with firm skeletal structures. But also among those organisms which are capable of becoming IMPEHrDCTION 01? THE GEOLOGICAL BECOBD. 169 fossilized, there are large groups which have only exceptionally left traces of their existence : these are the animals which lived on land. Fossil remains of land animals can only have survived when, during great floods or inundations, or for some reason or othei' their carcasses have been carried away by the water, floated hither and thither, and been surrounded finally by hardening mud. This explains not only the relative scarcity of fossil Mammalia, but also the fact that of the most ancient Marsupials (Stonesfield slate), scarcely anything is preserved but the underjaw, which, as the body decayed, was easily detached, and, on account of its weight, offered most resist- ance to the current of the water, and was the first part to sink to the bottom. Although it has been shown by such remains that Mammalia existed in the Jurassic period, yet the Eocene forms are the first which give us an insight into the details of their structure. Circumstances must have been more favourable to the preservation of fresh-water animals, and most of all to that of marine animals, since the marine deposits have a much greater extent than the locally confined fresh-water deposits. Thick formations seem in general to have arisen under one of two conditions : either in a very deep sea, protected from the operation of winds and waves, no matter whether the bottom was gradually rising or sinking — in this case, however, the strata would be relatively poor in fossils, since only the inhabitants of the deep sea, which is comparatively wanting in animal and vegetable life, would be preserved — -or in a shallow sea, in which the bottom underwent a gradual and continued depression during long periods of time favourable to the development of a rich and varied fauna and flora. In this case the sea would have retained uninterruptedly its rich fauna so long as the gradual sinking of its bottom was counteracted by the continual supply of sediment deposited upon it. Thick formations, all or most of the sti-ata of which are rich in fossils, must have been deposited on extended and very shallow regions of the sea, during a long peiiod of gradual depression. Thus the great gaps which occur in the series of palteontological remains are explained by a consideration of the mode of origin of deposits. These remains must necessarily be confined to the more recent formations. The lower, more ancient, and very thick succes- sions of strata in which the remains of the oldest fauna and flora must have been buried, seem to have been so completely altered by the heat of the molten interior of the earth, that the organic ^'^^ MEANING or THE SYSTEM. residua which they contain have been completely destroyed, or so altered that they cannot be recognised. In any case it may be regarded as certain, that only a small part of the extinct animal and vegetable woi-ld has been preserved in a fossil state, and that of this we only know a small part. Therefore we cannot conclude that, because the fossil remains of intermedi^ite stages cannot be found, they have never existed. If fossilized transitional forms are wanting in the strata where they should have occurred, or if a species suddenly appears in the middle of a series of strata and suddenly disappears, or if whole groups of species make their appearance and quickly vanish, the value of these facts as arguments against the theory of selection is diminished by the circumstance that in certain cases series of transitional forms between more or less remotely related organisms have been found, and that many species have been developed in course of time as links between other species and genera ; and again, that species and groups of species not unfrequently increase veiy gradually till they attain an unusually wide distribution, extend into later formations, and then gradually disappear again. Such positive facts have a higher value when we consider the incomplete- ness of fossil remains. It mil suffice here to refer to the Ammonites and Gasteropods, such as Valvata multiformis, as examples supplied to us by Palison- tology of transitional forms which can be arranged in a gradual series. Relation of Fossil Forms with Living Species. — The close rela- tionship of the plants and animals of the present time to the fossil remains of recent formations is a fact of great importance. In particular, we find in the diluvial period and in the diffei'ent tertiary formations the ancestral forms from which numerous living species are directly descended ; and further the characteristic features of the fauna of any particular geographical province in the present epoch are foreshadowed by the fauna of the epoch immediately preceding in the same region ; a fact which is proved by the fossil remains we find buried in the most recent strata. Many fossil Mammalia from the diluvial period and the most recent (pliocene) tertiaiy formations of South America belong to types of the order of Edentata which are now distributed in that part of the world. Sloths and Armadillos of immense size [Megatherium, Megalonyx, Glyjitodon, Toxodon, etc.) formerly inhabited the same continent, the mammalian fauna of which in the present day is so specially charac- STJCGES3I0N OF SIMILAR TYPES. 171 terisecl by its Sloths, Ai-madillos, and Anteaters. In addition to these gigantic forms, smaU an^l extinct species have been found in the bone caves of Brazil, and some of these are so nearly related to the livmg forms that we may assume them to have been their ancestoi'S. This law of the " succession of similar types " in the same localities is also exemplified by the Mammalia of New Holland ; for in the bone caves of that country are found many species nearly allied to its present Carnivora. The same law holds good for the gigantic birds of New Zealand, and, as Owen and others have shown, for the Mam- malia of the Old World, which, indeed, is continuous by the circum- polar region with North America ; and ancient types were able, in the tertiary period, to pass into North America, and vice versd by that way. The presence of Central American types {Didelphys) in the early and middle tertiary formations of Europe is to be explained in the same way. It is even more difficult to distinguish the regions of distribution of the animals of that time than of those of the later tertiary period. The evolution of the ancient forms into those of the present time was effected in the case of lower, simply organised animals at a much earlier period than in the case of higher organisms. Rhizopods, indistinguishable from species living at the present time {globigerina ooze) were already living in the Cretaceous period. The deep sea explorations * have accordingly yielded the interesting result, that certain Sponges, Corals, Molluscs, and Echinoderms now living in the deep sea existed in the Cretaceous period. We meet ■\vith a number of living species of Molluscs in the oldest tertiary period, though the mammalian fauna of this period differs completely from that of the present day. The greater number of species of MoUuscs found in the recent tertiary period resemble those of the present day, but the Insects of that formation differ considerably from living species. On the other hand, the Mammalia, even in the post-pliocene (diluvial) deposits, differ in part both in genera and species from those of the present day, although a number of forms have been preserved through the glacial period. On this account, and on account of the relative completeness of the tertiary remains, it is * {RMzocrinus Lofotensis — A2niicri)uti;s, Pleurotomaria, SijtJiiDiuc, Micrastcr, Pomocaru, etc.) Types of earlier and even of the older geological formations have been found preserved in the depths of the ocean, which, in spite of the great pressure, the want of light and deficiency in gaseous contents of the water, are more suited to the development of animal life than was formerly believed. 172 MEANING OF THE SYSTEM. especially interesting to trace the recent mammalian fauna back through tlie pleistocene forms to the forms of the oldest tertiary period. It is possible to trace the ancestry of a number of mam- malian species. Riitimeyer was the first to undertake to trace out the ancestral line of the Ungulata, and especially of the Rumincmtia, so as to obtain a palaeontological developmental history, and succeeded in obtaining results, by means of detailed geological and anatomical (deciduous teeth) comparison, which leave no room to doubt tliat whole series of species of existing mammalia are collaterally or directly related with each other and with fossil species. Riitimeyer's investigations have received corroboration in their essential points from the recent comprehensive works of W. Kowalevski, and have resulted in the establishment of a natural classification of the ungulate animals founded on phylogeny. Fig. 117. — Bones of tlie feet of the different genera of the Eqiiidce (after Marsh), a. Foot of Orohippus (Eocene), h, Foot of Anchithermm (Lower ^Miocene), e, Foot of Sipparion (Pleiocene). d, Foot of the recent genus JEguits. In addition to these works we have the recent researches of Marsh, who has completed to an extraordinary degree our knowledge of the genealogy of the genus Bqims, by numerous discoveries (fig. 117) in America (^Wyoming, Green River, White River). The eocene Orohijypus, in which the small posterior toes were present as well as the three principal toes which rested on the ground, was succeeded in the Lower Miocene formation by Anchitheriimi with three hoofs ; and the latter was followed by the Hi'pparion of the Pleiocene formations ; and this is the ancestral form of the existing genus Eqims. The oi'igin of most orders of Mammalia, such as Rodeniia, Cheirop- tera, Prohoscidea, Cetacea, etc., cannot be clearly ti-aced out, but for certain orders, as the Prosimice, Carnivora, Ungulata, and Ro- EVOLUTION or MAMMALIA. 173 dmtia, remarkable transitional forms have been discovered among the remains of extinct types. These also appear most prominently among the tertiary remains of North America. In the Eocene period here (Wyoming) lived the Tillodontia with the genus Tillo- theHum* characterized by having a broad skull like a bear, two broad incisor teeth like a rodent, and molar teeth like Pcdceotherium, and feet having five toes armed with strong claws. It thus united in its skeletal structure peculiarities of Carnivora and Ungulata. The Dinocerata [Dinoceras latice2}s mirabile) were powerful Ungulates with five toed feet with six horns on theii- heads, without incisors in the prtemaxillary bone, with strong sabre-like canine teeth in the upper jaw and with six molars. A third type, that of the Brontotheridce attained elephantine proportions, and was provided with transversely placed horns in front of the eyes. In addition to the foregoing there are a number of other groups of Mammals now completely extinct, the remains of which extend back into far earlier strata. Amongst them are the South Am.eTicax\ MegatheridcB {Mylodon, Megatherium), which belong to the order Edentata, and the Toxodontia, whose skull and dentition show relations to the Ungulates, Rodents, and Edentates. Many other types, however, especially of the Ungulates, which during the tertiary period inhabited both hemispheres, are now extinct in America, but still exist in the East. Elephants, Mastodonta, Hhinoceridse, and Eqiiidse existed in America in the diluvial but not in recent periods. Of the Perissodactyles the group of Tapirs alone is preserved in America, This group has also been preserved in the Eastern hemisphere in the East Indian species. In the palfearctic region also are found the remains of extinct intermediate groups of Mammals which existed during the tertiary period. In the Phosphorites of Quercyt in the south of France are found the remains of the skulls of Prosimiaj [Adapts), the dentition of which is intermediate between the ancient Ungulates and the Lemuridse (^Pachylemuridce), so that the question may be raised whether the Prosimise had not a common ancestry with several * Compare 0. C. Marsh, "Principal Characters of the Tillodontia." Amer. Journal of Seleiwc and Art, Vol. xi., 1876. 0. C. Marsh, " Principal Characters of the Dinocerata." ^1 mer. Journal oj Science and Art, Vol. xi., 1876. 0. C. Marsh. " Principal Characters of the Brontotheridfe. " Amrr. Journal of Science and Art, Vol. xi., 1876. t Compare H. Filhol, " Recherches sur les Phosphorites du Qucrcy, ]&tude des fossils qu'on y rencontre et spdcialement des Mammiferes." Ann. Sciences {/eolof/iqiiex, Vol. vii., 1876. 174 MEANING OF THE SYSTEM. eocene Ungulates {Pachydermata). In the same locality are found the well preserved remains of the bones of peculiar Carnivora whicli are well worthy of remai-k. These are the Hyjenodonta. It was foi- a long time doubtful whether they were Marsupials or not, until Filhol showed from the reserve teeth of their permanent dentition that they were probably of the nature of placental Carnivora. The great agreement of the molars of these Hysenodonta with those of the carnivorous Mai-- supials, as well as the small size of the skull cavity and the rela- tively slight develop- ment of the brain, support the view, which is also rendered probable by many other circumstances, that placental Mam- malia have developed from the Marsupials of the mesozoic pei'iod. In the oldest sti-ata of the Eocene forma- tions in both hemi- spheres, the higher placental Mammalia already appear in a rich variety of forms, which contrast mark- edly with one another (^Artiodactyla, Peris- FiG. 118.— Pterodactyhts crassirodris (after Goldfiiss) about There is one -third natural size. J I' ' however, no ground for regarding the immeasurable period from the oldest Eocene to the Keuper, in which the oldest Mammalian remains (the teeth and bones of insectivorous Marsupials) have been found, as the period in which this higher development of the Mammalian organism has been effected. In other cases also the science of paljeontology has led to the discovery of intermediate forms between groups and even between EVOLUTION 0¥ BIllDS. 175 classes and orders. The Lahyrinthodonta, the most ancient of the Amphibia, found as early as the carboniferous period, present many piscine characters {ventral exosMeton), and have a cartihxginous skeleton. Many fossil orders and sub-orders of Saurians {Halo- scmridce, Dinosauridce, Pterodactylidice (fig. 118), Thecodontidce) have not left a single representative in the present day ; others again are transitional between recent orders. Such a relation has, for example, been recently shown between the " Pythonomorphous " lizards (related to the genus Mosasaurus) from the chalk in America, and serpents so far as the structure of the skull and jaw is concerned. Owen's researches on the fossil Reptiles of the Cape have shown that certain Reptiles {Theriodonta) once lived there which showed a close resemblance to carnivorous Mammalia with regard to their dentition and the structure of their feet. The teeth of these animals, though only furnished with one root, can be divided into incisors, canine teeth, and molars, a fact which induces us to believe it possible that the dentition of the most ancient Marsupials hitherto known (Keuper) may be derived from tha.t of a Theriodon-Wke Reptile. Even as regards birds, a class so uniform in structure and so sharply defixied, a form [Archceopteryx lithograjyhica) (fig. 119) transitional between them and Reptile has been discovered in the Sohlenhofen slate, although the impression was not perfect. In this foi"m the short tail of the bird is replaced by a long reptilian tail composed of numerous (20) vertebrae and provided with two rows of feathers [Saururce). The articulation of the vertebral column and the structure of the pelvis indicated an affinity to the long-tailed Pterodactyls. The discovery of a second and more perfect specimen of Archceop- teryx has made known to us its dentition. It had sharp-pointed teeth wedged into the jaws. Other types of birds have also been found in the American chalk, which diverge more widely among themseves and from the Saurians than do the birds of any living order. These were defined as Odontornithes by Marsh,* and dis- tinguished as a sub-class ; they had teeth in the jaws, which latter were elongated to form a kind of beak. Some of them (Order Ichthyornithes) had bicoelous vertebrse, a crista stei-ni, and well * 0. C. Marsh, " On a new sub-class of fossil Birds {Odontornithes)J" American Jtmrnal of Science and Art, Vol. v., 1873. 0. C. Marsh, " On the Odontornithes, or birds with teeth." American Journal of Scienee and Art, Vol. x.. 1875. -•■'O MEANINa or THE SYSTEM. developed wings {Ichthi/ornis). Others {Odontolcoi) had teeth em- Pig. 119. — ArchcEopteryx lithograpKica. bedded in pits, normal vertebrae, no keel to the breast-bone, and rudimentary wings. They were not capable of flight {HesjJerornis , PEOGBESSIVE PEEPECTION. 177 Lestornis). Possibly in future clays we shall be able by the dis- covery of new types to establish the connection with the Dino- saunans (Compsognatlms), the formation of whose pelvis and feet ofter a closer relationship to those parts in birds. Advance towards perfection. — If we compare the animal and vegetable life of the most ancient formations with that of the sue ceeding periods of the earth's development, it becomes evident that there has been, on the whole, a continual progress from a lower to a higher condition. The oldest formations of the so-called archsean time, the rocks of wlaich are for the most part in a metamorphic state, must from their enormous thickness have occupied immea- surable time in their origin. They contain no fossil remains which can be recognised with certainty as such ; although the presence of bituminous gneiss in the old formations is a proof of the existence of organic bodies at that time. All the organisms of these most ancient periods, which were certainly numerous, have been de- stroyed without leaving any further traces than the Graphite deposits of the crystalline schist. In the most ancient and very extensive groups of strata we find exclusively cryptogamous plants, especially Fuci, which formed extensive forests beneath the sea. The warm seas of the primary period were inhabited by numerous sea animals of very different groups, such as Zoophytes, Molluscs (especially Brachiopoda), Crustaceans (larva-like Hymenocaris, Trilo- hites), and Fishes whose peculiar armoured forms {Ce2Jhalas2ndce) indicate a low stage of organization. In the coal formations we meet for the first time wifch the remains of land animals. Amphibia {A2Xttheon, Archegosaurus), with a notochord and a cartilaginous skeleton ; we also find Insects and Spiders ; and in the Permian formations we meet with large lizard-like reptilian forms [Protero- saurus); while fishes, exclusively Elasmobranchs and Ganoids with a notochord, and vascvilar cryptogamous plants (Tree-ferns, Lepido- dendra, Calamites, Sigillaria, Stigmaria) still predominate. In the carboniferous period isolated instances of the Lizards- amongst Vertebrates and of Coniferas and Cycadite amongst plants- had already made their appearance ; but in the secondary period they obtained such a preponderance that the whole period has been named from them the period of Saurians and Gymnospernis. Amongst the first the colossal Dinosaurians living upon the land, the flying Lizards or Pterodactyls, the Halosaurians, with their best known genera Idiihyosaurus and PlesiosoburiijS, are entirely peculiar to the secondary period. 12 178 MEANING OF THE SYSTEM. Examples of Mammalia, althouf,'h Hcavce, are found in the upper Triassic beds, and also in the Jurassic. Such Mammalia belong without exception to the lowest grade of Marsupials. Flowering plants appear for the first time in the chalk, as do the oldest remains of distinctly bony fishes. Flowering plants and Mammalia — and amongst the latter the highest order of Apes is represented — so preponderated in the tertiary period that it has been called the period of leafy forests and Mammalia. The plants and animals of the upper tertiary beds show a gradually increasing resemblance to those of the present time, the higher we ascend in the series. Numerous lower animals and plants are identical, not only generically but also specifically with those now living, and the genera and species of the higher animals have a greater resemblance to those of the present time. With the transition to the diluvial and recent epoch, the number and area of distribution of the higher types of flowering plants increase, and in every order of Mammalia we find forms whose structure is specialized more and more in definite directions, and which therefore appear more perfect. In the dikivial age we find the first unmistakable traces of the existence of Man. His history and the development of his civilization has occupied only the last portion of the recent period which has been relatively so short. Despite its great incompleteness the geological record affords sufficient material to prove the existence of a progressive develop- ment from simple and lower grades of organization to higher, and to confirm the law of a progress towards perfection in the succession of the groups. We are indeed unable to make use of more than a small period of the time that has been occupied in this progress towards perfection of organisms, since the organic world of the most ancient and extensive periods has completely disappeared from the record. If, after the above discussion, we consider the hypothesis of Trans- mutation of Species and of Descent to have a firm foundation on fact, we must concede a high value to Darwin's theory of Selection as an explanation of the manner in which the transmutation of species has been effected. There are yet natural historians who admit the great changes which the animal and vegetable world have undergone, and yet combat the Darwinian principle of Selection, without being able to give any other explanation. The phenomena of gradual progress towards perfection agree very well with the theory of Selection. INCOMPLETENESS OP THE EXPLANATION. 179 Natural Selection leads, on the whole, to a progressive differentiation of organs (division of labour), since it preserves any peculiarities which are of use in the struggle for existence, and thus tends to the ' perfection of the organism. We can therefore connect the progress of simple types to higher ones with the principle of utility implied by Natural Selection, without being obliged, with Nageli, to have recourse to the obscure notion of an inexplicable tendency towards perfection. It is the latter mystical supposition, and not Natural Selection, which is contradicted by the fact that we find a number of Rhizopods, Molluscs, and Crustacea {e.g., the genera Lincjula, Nautilus, Limulus) have existed almost without alteration from the earliest formations through all the geological periods to the present time, and by the observation of a retrogression of organization in the course of development {e.g., retrogressive metamorphosis of Parasites). Nor again can it be objected that on the hypothesis of Natural Selection the lower types should have been long ago suppressed and have become extinct, while, as a matter of fact, there are higher and lower genera in every class, and the lowest organisms are numerous and widely distributed. It is precisely the great variety in the degrees of organization which brings about and is favourable to the greatest development of life, all the forms of which, both the higher and the lower, being best suited to their peculiar circumstances are able, more or less perfectly, to occupy a special place in nature^ and in a certain sense to maintain it. Even the most simple organisms occupy a place in the economy of nature which can be filled by no other organisms, and are necessary to the existence of numerou^s higher grades. However well grounded we admit the theory of Selection to be, we cannot accept it as in itself sufiicient to explain the complicated and involved metamorphoses which have taken place in organisms in the course of immeasurable time. If the theory of repeated acts of creation be rejected and the process of natural development be established in its place, there is still the first appearances of organisms to be accounted for, and especially the definite course which the evolution of the complicated and more highly developed organisms has taken has to be explained. In the many wonderful phenomena of the organic world, amongst others in the origin of Man in the diluvial or tertiary period, we have a riddle the solution of which must remain for future investigators. SPECIAL PAET. CHAPTER VI. PROTOZOA. Animals of simjjle constitution and small size ; without tissues com- posed of definite cells. Sexual rei^roduction by means of ova and sjnrmatozoa tmknown. From a morphological point of view the Protozoa have remained at the stage of cells, in the protoplasm of which one or more nuclei may be present. The phenomena of segmentation of the egg and formation of the germinal layers are therefore absent from their development. The body is always composed of a contractile granular substance, filled with vacuoles : it may also contain a pulsating vacuole, and present the phenomenon of granule currents. The 2^ulsating vacuole consists of a space without walls filled with a clear fluid. This space apparently diminishes and disappears through the contrac- tion of the surrounding plasma, and then re-appeai-s. There exists, however, in the varying differentiations in the interior of the saroode body, and in the difierences in the external boundary, and in the manner of nourishment, a number of modi- fications which we shall use foi- the foundation of groups. In the simplest cases, the entire body consists of a small lump of sarcode, the contractility of which is confined by no firm external membrane. This lump of sarcode is sometimes semi-fluid, and protrudes and retracts processes. It is sometimes of tougher consistence in parts, and protrudes hair-like rays and threads (Fhizojjoda). ISToui^ishment takes place through the intussusception of extraneous bodies, which can be surrounded and enclosed by the protoplasmic svibstance at any portion whatsoever of the periphery of the body. In other cases the body which sends out slender processes (pseudopodia) secretes silicious or calcareous needles, lattice-work shells, or shells perforated EHIZOPODA. 181 by holes, to shelter and protect the body {Foraminifera, Radiolaria). In the Infusoria the sarcode body is bounded by an external mem- brane, and is capable of quick and varied locomotion by means of the movements of the cilia, hairs, bristles, etc., which it possesses. The solid nourishing matter is taken in thi-ough a mouth, and the remainder, after digestion, passes out through an anal aperture. CLASS I— RHIZOPODA.* Protozoa without external investing membrane, the 'parenchyma of which protrudes and retracts 2}rocesses ; as a rule, a calcareous shell or silicioios skeleton is secreted. The body-substance of these animals, the shells of which were described as Foraminifera or Polythalamia, long before their living contents were known, consists of sarcode, and is without any boundary mem- brane. The body- substance, which is richly granulated and contains pig- ment, contracts slowly and sends out at the same time fine thread-like rays (fig. 120), for the most part of a semi-fluid consistency {^pseudopodia) ; and these serve not only as a means of movement but also for the reception of nourishment. The pseudopodia may, how- * Dujardin, "Observations sur les Rhizopodes" (^Comj^tes rmdua, 18.35). Ehrenberg, " Uber noch jetzt zahlreich lebende Thierarten der Kreidebildung und deu Organismus der Polythalamien " {Ahhandlmuj dor Akad.zu JBcrlm, 1839). Max Sigm. Schultze, "Uber den Organismus der Polythalamien" (Leipzig, 1854). .Joh. Mliller, " Uber die Thalassicolen, Polycystinen und Acan- thometren " (1858). E. Haeckel, " Die Radiolarien " (Eine Monoerauhie Berlin, 1862). s f • Fig. 120.— Optical section through portion of the sarcode body of Actinosphaerium, JEiehhoruii (after Hertwig and Lesser). JV, nuclei in the endosark, from which the vacuolated ectosark is clearly dis- tinguishable. In the centre of the pseudopodia the axial thread is visible. 182 PBOTOZOA. 1/ // //// ever, be broad, lobed, or linger-like processes by means of which a quick and flowing motion can be imparted to the body mass. A tougher, clear homogeneous external layer {Exa'plasm) is usually to be distinguished as the peripheral boundary from a more fluid and more granular internal mass {Endojilasvi). During motion the former is projected in processes into which the granules of the latter stream more or less quickly. In the stifter pseudopodia streams of granules are observable, slow but regular, passing from the base to the extremity and vice versA. The explanation of these movements is to be sought in the contractility of the surrounding portions of sarcode (fig, 120). A pulsating space, the contractile vacuole, is not unfreqently to be found in the sarcode, e.g., Difflugia, Actinophrys, Arcella (tig. 121), Nuclei are also usually present in the sarcode, by which the morpho- logical value of the Rhizopod body as cell or as cell aggregate is placed beyond all doubt. There are also forms in the protoplasm of which no trace of a cell nucleus has been found. In such either the protoplasm of the nuclevis is not yet differentiated as a separate s-tructure (the Monera of E. Haeckel), or we have to do with a transient, non- nucleated stage in the life-history. The sarcode usually seci'etes sili- cious or calcareous structures, either as fine spicula and hollow spines which are directed from the centre to the periphery in regular order and number, or as lattice-work chambers (liadiolaria), which often bear points and spines, or finally as single and many chambered shells with finely perforated walls {Foraminifera) and one larger opening. Through this last (fig, 123), as well as through the countless pores of the small shells (fig. 122), the slender threads of sarcode pass out to the exterior as pseudopodia, changing without intermission in form, size, and number, and often joining themselves together in delicate networks (figs. 122, 123). The .pseudopodia, by their slow, creeping movements, afi"ord a means of locomotion, while they also serve for the taking up of nourishment I \{0 / . // Fig. 121. — Amceba (DactylosphmraJ poly- podia (after Fr. E. Schultze). N, Nu- cleus. Pu. pulsating vacuole. RHIZOPODA. 183 by surrounding and transporting into the interior of the body small vegetable organisms as Bacillaria. Among the shell-bearing forms, the reception and digestion of food takes place outside the shell in the peripheral threads and networks of sarcode ; for each spot on the surface can for the time being assume the functions of mouth, and also of anus, by rejecting the undigested remnants. The Rhizopoda live for the most part in the sea, and contribute by the accumulation of their shells to the formation of the sea sand, and even to the deposition of thick strata. An innumerable quantity of fossil forms from various and very ancient formations are known. 184 PROTOZOA. Order 1. — Foraminifera,* Rhizopoda, either naked or with a shell, the shell almost invariably calcareous and usually j^i^rced with fine pores for the exit of tJie jiseiulopodia. Only in rare cases, for instance Nonionina and PolymorjMrut,, is the shell substance of a silicious nature; in all other forms it is Fig. 123. — Miliola tenera, with network of pseudopodia (aftei- M. Schultze). membranous or consists of a calcareous deposit in a basis of organic matter. The shell is either a simple chamber, usually provided with a large opening, or is many chambered, that is, is composed of numerous chambers arranged upon one another according to definite laws. The spaces of these chambers communicate by means of narrow * Besides D'Orbigny, Max Schxiltze, 1. c.,.compare W. C. Williamson, On the recent Foraminifera of Great Britain," Loudon, 1858. Carpenter, "Introduc- tion to the Study of the Foraminifera," London, 1862. Rcuss, '-Entwurf einer system. Zusammenstellung der Foraminiferon," Wien, 1861. rOKAMINIE'EKA. 185 passages and large openings in the partition walls. In like manner those portions of the living sarcode body which are enclosed in the individual chambers are in direct communication with one another by means of processes which pass through the passages and openings in the septa, and connect one portion with another. The quaUty of the body-substance, the mode of movement and nourishment, agree closely with those which have been depicted as. characteristic of the order. Our knowledge of the mode of reproduction is imperfect. Amongst the forms without a shell, fission has been observed as well as fusion, which may perhaps be referred to a species of sexual reproduction {conjugation). The reproduction of shell-bearing Foraminifera such as Miliola and Rotcdia has also been observed. The former produces from the protoplasm of its body single chambered, the latter three chambered, young. Probably this mode of reproduction is preceded by an increase in the number of nuclei, and the animal divides into as many portions as there are nuclei, each of which becomes a young Foraminifer, and contains but one nucleus. In spite of their small size, the shells of our simple organisms may lay claim to no small consequence, since they not only accumulate in enormous quantity in the sea sand (M. Schultze calculated their nixmber for an ounce of sea sand from Molo di Gaeta at about one and a half millions), but are also found as fossils in different formations (the cretaceous and tertiary), and have yielded an essential material to the construction of rocks. Silicious nodules of Polythalamia are even found in Silurian deposits. The most remarkable, on account of their considerable size, are the Numimdites (fig. 124) in the thick formation of the so-called Nummulite limestone (Pyrenees). A coarse chalk of the Paris basin, which makes an excellent building stone, contains the Trilocidina trigomda {Miliolite chalk). The greater number of Foraminifera are marine, and move by creeping on the bottom of the sea, but Globigerina and Orbulina have been met with on the surface. The bottom of the sea at very consider- able depths is also covered with a rich abundance of forms, especially with Globigerina, the remains of the shells of which give rise to an enduring deposit. 1. Sub-order : Lobosa [Amcebiformes). — Amceba-like fresh -water Rhizopoda, usually with pulsating vacuole, sometimes naked, some- times with a single- chambered firm shell. The sarcode body consists as a rule of a tougher exoplasm and a fluid granular endoplasm. The pseudopodia are lobed or finger-shaped processes of considerable 186 PEOTOZOA. size, occasionally tougher slender processes without granule streams (figs. 125 and 126). Avioeha. prinrrj,, Ehrbg., A. frrriroU Greef., Petalopvx difflv^ivmH Clap. Lachm. Here should also bo placed the famous Bathyhim Haecltcli Huxl., which is found in the' deep se.i mud of the Atlantic Ocean, if it is indeed a living organism (and not simply a deposit of Gypsum). Aroella vuli/aris Ehrbg., JDitHnr/ia proMforwu Ehrbg., Ewjlypha yluhom Cart, have shells and tough, pointed, dichotomously branching pseudopodia (fig. 125). Tig. 124. — Nummulitic Limestone, with hoi'izontal section of N. dittans (after Zittell). Fig. 128. — Difflvgia oiZoH^a (after Stein). Fig. 125. — Svgli/pha globona (after Hertwig and Lesser). Fig. 127. — Aeervulina glohosa (after M. Schnltze). 2. Sub-ordei- : Reticularia {Thalamopliora). Principally mai-ine Rhizopods with extremely slender anastomosing pseudopodia, with granule streams in the latter, rarely naked [Protogenes, Lieber- kiihnia), nsually with membranous or calcareous shell, which is single-chambered (Monol/ialamia) or many-chambered (Polytlialamia) (fig. 127). HBLIOZOA. 187 1 . Impcrfovata. With membrauous or calcareous shell, which is withoiit fine purcs, but i)Ossesscs, in one place, an opening, either simple or sieve-like, through which the pseudopodia project. To these belong the Gromidce, with a mem- branous chitinous shell: Gromia oviforviis Duj., and Miliolidre, with a poroellanous shell : Corimxjnra jflaiiorhu M. Sch., 3niiola cyeloi magnified 550 diam. (after R. Leuckart). c, d, Stages of spore formation which have only been observed outside the cells. CHAPTER YII. CCELENTERATA (Zoophytes).* Radially symmetrical animals toith a body composed of cells. They have a body-cavity which serves alike for circulation and digestion [gastrovascular sjxcce). Amongst the Ccelenterata we meet for the first time with organs and tissues composed of cells. In addition to the external and internal epithelium, cuticular, calcareous, and silicious structures, as well as muscles, nerves, and sense-organs are very generally present. On the other hand, the internal sm-face of the body is not difier- entiated into organs of circulation and digestion distinct from each other. The vegetative processes are performed by the internal sur- face of the gastric cavity, the gastrovascular space, of which the central part functions as stomach and intestine, the peripheral as vascular system. R. Leuckart was the first to recognise the importance of these^ characters, and made use of them to separate the Polyps and the Medusa', from the Echinoderms, thus resolving CuAder's type of Radiata into the types of Coilenterata and Echinodermata. It is only in more recent years that Naturalists have been con- vinced of the close relationship between the Porifera and the Polyp>s. * R. Leuckart. " Ucber die Morphologic und Verwandschaftsverhaltnisse- niederer Tliiere," Braunschweig, 1848. 14 210 COJLENTBEATA. and Meclusce, and have included the former in the group of the Ccelenterata. The Pori/era were for a long time taken for plants and more recently for Protozoon-stocks. While, however, the Polyps and Medusa, are distinguished as Cnidaria and are characterised by the possession of nematocysts and by the higher differentiation of their tissues, the Porifera or Sponcjiaria present more simple forms of tissue m the spongy structure of their body, and are without nemato- cysts. The entire structure of the body may, generally speaking, be described as radial, although the radial symmetry does not appear in most sponges, and among the Cnidaria transitions towards lateral symmetry are ap- parent. Similar organs are usually repeated round the body axis four or six times or in multiples of these numbers. Pour distinct types of body form are met with in the group Ccelente- rata, viz., that of the S2)onge ; of the Pol^j) ; of the Me- dusa ; and of the Ctenopliora. The Sponge type.— The sim- plest form of Sponge is represented by a fixed cylindrical tube, with an exhalent opening, the Oscidum, at the free end (fig. 152). The contractde wall is supported by skeletal spicules, and is pierced by numerous inhalent pores, through which water and small food particles pass into the ciliated internal space. By the fusion of separate indi- viduals, and by reproduction by gemmation, the latter being the more frequent mode, widely different Sponge stocks with compli- cated canal systems are formed. The polyzooid nature of these is made apparent by the presence of many oscula. The Polyp type.— The Polyp has the form of a cylindrical or club-shaped tube, of which the posterior end is fixed and the opposed Fig. 152.— Young Sycon (after Ft. E. Sohulze). O, Osculum or exhalent pore ; P, pore in the wall. MEDUSA — OTENOPHO R. 211 free pole pierced by an oral opening situated on a flat or conical prominence, the oral cone. The mouth is surrounded by one or more circles of tentacles, and leads into a simple cylindrical body ca\'ity {Hydroid'polyjjs), or through an oesophageal tube into a compli- cated gastrovascular cavity [Anthozoa, fig. 153). The disappearance of the tentacles gives rise to the so-called polypoid form, which consists of a simple hollow tube fur- nished with a mouth. The Medusa type. — The free-swim- ming Medusa consists of a flattened disc or arched bell of gelatinous or Fig. 153. ■Sagartia nivea (after Gosse). of the mouth into cartilaginous consistence, from the under surface of which hangs a central stalk, the manibbriimi, bearing at its free end the mouth. This manubrium is frequently prolonged in the neighbourhood numerous lobes and tentacles, while from the edge of the disc arise a varying number of thread-like tentacles. The central cavity of the body, into which the hollow manubrium leads, is called the gastric cavity, and from it peripheral pouches or radial canals, the so-called vessels, run to the edge of the disc, where, as a rule, they are con- nected by a circular vessel. The movements of the Me- dusa are eft'ected by the alter- nate contraction and dilatation of the muscular under surface of the bell, i.e. of the subum- brella. Rudimentary Medusse, in which the manubrium and mai-ginal tentacles are absent, are found. They are called Medusoids, and do not acquire individual independence, but remain attached to the body of the Medusa or Polyp from which they are budded. The Medusae and Polyps, in spite of the important differences between them, are but modifications of the same plan of structiire. A Medusa may be compared to a free, flattened Polyp, possessing a large gastric cavity and a muscular and enlarged oral disc. The Ctenophor type has fundamentally the form of a sphere, Fig. 151— Medusa of the Podocoryne carnea with four tentacles at the edge of the disc, ovaries and manubrium, immediately after separa- tion from the stock. 212 COiLENTEBATA. beset with eight meridional rows of vibratile plates, which, working like oars, serve for locomotion (fig. 155). The body parenchyma in the Sponges consists principally of amoiba-like cells, which frequently bear flagella, but which never produce stinging threads. In the Cnidaria (Polyps and Medusie), in cei-tain cells the • peculiar struc- tui'es known as thread cells (fig, 156)are developed. They consist of small capsules filled with fluid, and containing a sharp-pointed, spi- rally coiled thread; they are developed in cells which may be called cnido- blasts. Under cer- tain mechanical conditions, e.g. under influence of the pressure pro- duced by contact with a foreign body, these cap- sules burst, and the thread is sud- denly protruded, and either fastens on to the cause of disturbance or pierces it, carrying into it a part of the fluid contents of the capsule. In many parts of the body, and especially on the tentacles, which serve for the capture of prey, these small microscopic weapons are collected in masses, and are often united in a peculiar arrangement to form batteries of thread cells. Fig. 155. — Cydippe (Hormiphora) plumosa (after Chun). O, moutli. Fig. 156. — Nematocysts and cnidoblasts of Siphonophora. a and b, with the cnidocil of the cell, e to e, Nemato- cysts with evaginated thread. DEVELOPMENT. 213 The tissues (which are composed of cells) are generally arranged in two or three layers, of which the external layer is known as ectoderm and forms the outer skin, while the internal layer, the endoderm, line-^ the gastric cavity. Between the two there is developed a delicate homogeneous siip- porting membrane or a stronger layer of connective tissue, in which the skeletal elements are developed. This intermediate layer is known as the mesoderm. The skeletal formations present great variations in structure and arrangement. Muscles are formed in the deeper part of the ectoderm as cell- processes {the so-called neuromuscular fibres), but often penetrate within the mesoblast as independent cell structures. Sense epi- thelium, nerve fibrillse, and ganglion cells also appear as differentia- tions of the ectoderm. The endoderm cells, on the other hand, often bear cilia, and are principally concerned in the processes of digestion .and secretion. Where the tissues are upon the whole of homogeneous structure, we find a preponderance of asexual reproduction by fission and gemma- tion. If the individual forms so produced remain united, they give rise to the colonies which are so widely distributed amongst the Polyps and Sponges, and which, by the continual multiplication of their individuals, may in course of time attain a very considerable size. But we also meet eveiywhere with the sexual reproduction, in that ova or spermatozoa are produced in the tissues, usually in the region of the gastrovascular cavity, in a definite portion of the body. As a rule, the ova come in contact with the spermatozoa away fi^om the place where they are produced ; either within the body cavity or outside the parent body, in the sea-water. In a few cases only do both the sexual elements originate in the body of the same indivi- dual, as, for example, in many of the Spongiaria, some Anthozoa, and in the hermaphrodite CtenopJiora. As a rule, in the colonies of Anthozoa the monoecious arrangement of sexes obtains, the indivi- duals of the same stock being partly male, partly female. Some are dicecious, e.g. Veretillitm, Diphyes, Ap)olemia. The development of the Ccelenterata for the most part consists of a metamorphosis. The just hatched young difier from the sexual animal in the form and structure of the body, and pass through larval stages. The greater number of them leave the egg as ciliated larvfe, which resemble somewhat an Infusorian in external appearance. They acquire a mouth, body cavity, and organs for obtaining food, either during their existence as free larv^, or after 214 COiLENTEBATA. attachment to »lid surrounding objects in the sea. If the young forms, wlnd> d,«er from the sexual animal, gain the power of re p oducmg by buddmg, the development leads to various forms of alternation of generation. Sub-groups.— I. SPONGIARIA*=PORIFERA. The body has a spongy consistence and is composed of rmtsses of eel s capable of amoeboid movements and supported by a solid, calcareous s^l^c^ous, or horny skeleton. There are external pores, an internal canal system, a7id one or many exhalent openings (oscula). The sponges are at present universally regarded as Ccelenterata, and m this group they are distinguished from the Cnidaria (Polyps and Medusae). They are composed of a contractile tissue, which IS usually supported by a framework composed of spicules and fibres; the whole being arranged in such a manner that there exists on the external wall of the body larger and smaller openings; and in the interior a system of canals and spaces in which a continuous stream of water is maintained by the vibratHe motion of cUia. Amoeba-hke cells, net-like membranes of sarcode, flageUated cells, spindle cells, ova, spermatozoa, and tissues derived as excretions from cells are present as the histological elements of the Sponge body. The chief mass of the contractile parenchyma is composed of the amoeba- like cells. These are granular cells, which, Fig. 157. — Amoeba-like cell of a i, i, , i , SpongiUa. Amoebse, have no external membrane, can protrude and retract processes, and take into their interior foreign substances (fig. 157). The framework or skeleton, which we find wanting only in the soft * Literature : Nardo G. D., " System der Scliwamme," Isis, 1833 and 1834. Grant, " Observations and Experiments on the Struct, and Funct. of Sponges," Mlin. Phil. Journal, 1825 — 1827. Bowerbank, " On the Auatomj^ and Phvsio- logy of the Spongiadte," PhUox. Traw., 1858 and 1862. Lieberkiihn. '■ Beitriige zur Entwickelungsgeschichte der Spongillen," Miillcrs Archiv., 1856. Lieber- kiihn, " Zur Anatomic der Spongien," Mailer's Archiv., 1857, 1859, 1863. 1865, 1867. 0. Schmidt, " Die Spongien des adriatischen Mecres," Leipzig, W. Eng- elmann, 1862, as well as Supplement. Leipzig, W. Eugelmann, 18C4, 1866, 1868. E. Haeckel, "Die Kalkschwiimme," 3 Bde, Berlin. 1872. Fr. E. Schulze, " Untersuchungen iiber den Bau und die Entwickelung der Spongien," Zeitschrift,fiir vriss. Zool., 1876—1880. POllIFEBA. 215 gelatinous Sponges or Myxospongia, is composed of horny fibres or siliciovis or calcareous spicules. The horny fibres form, mthout exception, anastomosing networks of varying degrees of thickness, and present a lamellated structure (fig. 158), which indicates that they are formed of a number of layers. They are formed by excretion as hardening portions of sarcode. The calcareous needles (fig. 159) are simple or three- and four- rayed spicules, and take their origin, as do the silicious structures, in the interior of cells. The silicious spicules present, however, an extraordinary variety of form : some of them constitute a connected frame- work of silicious fibres, and others are free silicious bodies with simple or branched central canals (fig. 160). The latter are fovmd in the form of needles, spindles, cylinders, hooks, anchors, wheels, and crosses, and arise in nucleated cells, pro- bably as deposits round a hardening of Fig. 158.— Piece of network of organic matter (central fibre). ^^"'^ Euspongia o \ / equina. In order to understand the morphology of the Spongiaria we must begin by examining the structure of a young Sponge, which proceeds from the fixed larva. The young- Sponge, after the formation of a cihated gastric cavity and an ex- halent opening or osculum, has the form of a simple hollow tube, the walls of which are pierced by pores for the passage of small food particles suspended in the water (fig. 152). In this stage we can distinguish three layers — ( 1 ) an entoderm, formed of elongated flagellated cells; (2) a Fi«. ISO.-Calcareous Spicules of %co™. mesoderm, the skeleto- genous cell layer, the structure of which recalls connective tissue ; and (3) an ectoderm, which forms the outer layer of the Sponge, and consists of aflat epithelium. The cylindrical cells of the endo- derm possess at their free ends surrounding the flagellum a delicate 216 CaOLENTERATA. hyaline marginal membrane, which, derived from a prolongation of the hyaline plasma, projects as a hollow cylinder resembling the protoplasmic collar of certain Flagellata ''- {(Jylicoviaatiyes). [This Fig. 160. — Silex bodies from different silicious Sponges, a, Siles needle from Spongilla, inside the cell, h, Amphidisc of a gemmule of Spongilla. c, Anchor from Ancorina. d, Tlook from. JEsperia. e, Star from Ckondrilla. f, ATichortromSuplectella axpergiUiim. g, h, needle rays from the same, i, Six-rayed needle from the same, with central canal. structure is commonly known as the collar, and the cells as the collared cells.] The thick layer in which the skeletal spicules are pi-oduced consists of a hyaline matrix, in which irregularly branched or spindle-shaped amoeboid cells are embedded, and may be regarded, like the gelatinous substance of the Acalepha, as mesoderm, while the external, clearly defined, flat epi- thelium (also in the Asconia, Leucosolenia) is to be considered as ectoderm. The pores or inhalent openings so cha- racteristic of the Sponge body are in reality only intercellular spaces, and are able to close themselves, vanish and be replaced by new pores, which arise by the separation of one cell from another (fig. 161). * Upon this ground Clark declared the Sponges to be nearly allied to the Flagdlata, and regarded them as great colonies of the latter. Fig. 161. — Portion of the exter- nal layer of Spongilla with the pores, P (after Lutaerkiihn) . POBIFEBA. 217 ximongst the calcareous Sponges, the simple Sponge with inhalent pores and terminal osculum {Olynthus-iorm) is represented by the stock-forming Lmcosolenia (Grantia), which is composed of numerous hollow cyliaders. The structure of this sponge has been described by Lieberkiihn. In the Syconidce the body cavity has a more compHcated form. The central space opens into secondary peripheral spaces or radial tubes, which are lined by ciliated cells, and open externally through the inhalent pores (fig. 162). In other calcareous Sponges {Leuconidce) the radial canals have the form of irregular parietal canals, giving off branches to the peripheiy and possessing dilated, ciliated chambers. This form of internal canal system is also found in most of the stock-forming, silicious Sponges (fig. 163). Sponge forms may become more complicated by the formation of stocks ; the originally simple Sponge, which has developed from a single cili- ated larva, gives rise by budding and incomplete fission to a polyzoid sponge body; or several originally separate individu - als, each of which has origi- FiG. 162.— Longitudinal section through Sijcon raphanus, slightly mag- nified. 0, Osculum with collar of spicules ; at, radial tubes which open into the central cavity. nated from a single larva, fuse tosfether to form a com- pound sponge stock. Both these methods of growth are repeated in a similar manner in the formation of the stocks of Polyps (fig. 164). In the same way that the fan-like Fig. 163. — Section of Corticimii candelahrum (after nets of the Pan Coral {Bhipi- parietafSnir^' ^'^'""^^^ °* clogorcjia flahellum) are formed by the i-epeated fusion of its branches, the gastrovascular cavities of which anastomose, so also in the case of the branching sponges, as a result of the same pro- cess, reticulate, or coiled or even massive stocks are formed (fig. 165). 218 CaiLlDNTiiUA.TA. In this case the canal system, in which the modifications before described for each individual 8pouge are repeated, becomes more complex, partly through the formation of anastomoses, and partly because irregular gaps and winding pa.^sages make their appearance between the fused branches of the stock and form spaces which lead into the ciliated cavities. Eeproduction takes place mainly asexually by fission and the production of germs or fjeramules, but also by the formation of ova and sperm capsules. The gemmules are in the fresh-water Sponyilla masses of cells which are surrounded by a firm shell composed of silicious structures {amphidiscs), and, like encysted Protozoa, pass through a long period of rest and inac- tivity. After the expiration of the cold and steiile season of the year, the contents pass out of the opening of the capsule and gene- rally surround the latter, and with increasing growth become differentiated into amoeboid cells and all the essential parts of a new small sponge body. Multiplication by means of gemmules is also common among the marine Sponges. The gemmules take their origin under certain conditions as small globules surrounded by a membrane. The contents are essentially formed of sponge cells and spicules, and, after a longer or shorter period of inactivity, reach the exterior by the ruptm-e of the membrane. Sexual repi-oduction was first demonstrated with certainty by Lieberkiihn for Sj>ongilla, but more recently has been shown to exist in almost every group of Sponges. In most cases the ova and spermatozoa seem to reach maturity at different times in the same Sponge. The spermatozoa are needle-shaped, and lie in small spaces lined with cells. The ova, like the mother cells of the spermatozoa, are modified cells of the parenchyma, and are derived from cells of the same tissue layer (mesoderm) in which the needles and skeletal Btructures take their origin. The ova are naked amoeboid cells, and pass into the canal system, while in the viviparous Sijcons they Fig. 164. — Axinella polypoides (after O. Schmidt). PORITEEA. 219 remain in the mesoderm, and there undergo their development, is only later that the 0 ciliated embryos or larvse fall into the canal system, pass out, and attach themselves, to de- velop into a young- sponge. The embryonic de- velopment among the calcareous sponges is most accurately known for the It Fi&. 165. -Ens2}ongia officinalis adriatica , witli a numoer of oscula, O (after Fr. B. Schulze). Syconidce from the investigations of Fr. Schulze and Barrois. y.--e. o ... Fig. 166.— Development of Sycon rn-phanus (after Fr. E. Schulze). a, Bipe ovum, h. Stage with four segmentation cells, c, Stage of segmentation with sixteen cells, d, Blastosphere with large dark granular cells at the open pole, e. Free-swimming larva, one-half of the body (entodermal) being formed of long ciliated cells, the other (ectodermal) of large granular cells. ^'^^ CCELBNTEIIATA. After the completion of the tolerably regular segmentation (fig. 166, a—c),SyGbn {Sycandra) rcqjhanus passes through a. blastosphere stage, during which the greater half of the ovum consists of clear cylindrical cells, and the smaller half at the still open pole of large dark granular cells (fig. 166, d). The cylindrical cells of the larger half develop cilia, and the embyro passes out of the body cavity and becomes a free-swimming larva, which attaches itself and alters its shape in such a manner that the dark cells grow over the ciliated portion of the globe, which is meanwhile invaginating. The ecto- derm and mesoderm are derived from the dark granular cells, and the ciliated cells give rise to the entoderm of the gastric cavity. Later on the body of the sponge be- comes cylindrical, the osculum makes its appearance, and calcareous needles appear in the wall, which becomes pierced by pores (fig. 167). With the excep- tion of Sjiongilla, the sponges are marine, and are met with under very diSerent con- ditions, and covering a wide area of distribution. The horny sponges live in shallow seas, as also the Myxospongice and Chalinece, or siliciceratous Sponges ; while the Hexactinellidce inhabit very considerable depths. Petrified remains of sponges are also found preserved in various formations, for instance in the chalk ; and these remains differ much from the greater number of those living. On the other hand, the glassy sponges of the deep sea agree so fully with the ancient forms that they seem to be the direct descendants of the latter. Finally, many of the principal groups extend back into the palreozoic age, in which Lithistidce and Hexactinellidce especially are met with in the most ancient Silurian Fig. 167.— Young Sycon (after Fr. E. Scliulze). O, Osculum or exhalent apertui-e ; P, pores of the wall. POBIFERA. ■^^J- strata. Hence paljeontology affords us no facts for determining the phylogenetic development of the Porifera. CLASS I.— SPONGIA. [With the characteristics of the Groui^). Order 1 ,— Myxospongia (gelatinous sponges). Soft, fleshy sponges, without any skeleton, Avith a hyaline gelatinous mesoderm, often contaming fibrous cords. The ectoderm cells are fairly elongated, and bear flagella. Fam. Halisarcidae. Hulimrca Duj. H. loMdarix 0. S., colour dark violet ; encrusts stones ; Sebenico. H. Dujardinii Johnst., forms a white encrustmeut on the Laminaria of the North Sea. Order 2. — Ceraospongia (horny sponges). For the most part branched or massive sponge stocks, with a framework of horny fibres, in which grains of silex and sand are present as foreign bodies. Fam. Spongiadae. Busponijia 0. S., with very elastic fibrous fi'amework, of equal strength throughout, mostly capable of being used for bath and washing sponges. E. adriatica 0. S., equina 0. S., zimocca 0. S., in the Greek Ai'ohi- pelago, violisslma 0. S., Levantine sponge, cup-shaped. Sjwnf/elia elegans Nardo. Order 3. — HALiCHONDRiiE (siliciceratous sponges). Sponges of very various shapes, with usually uniaxal silicious needles, simple sihcious spicula, which are connected by delicate or firmer plasmatic structures, disposed in networks or enclosed in sponge fibres. Of the numerous families the following may be mentioned : — Fam. Chrondrosidae. (^Ghimminece), Coriaceous sponges. Cliroiulrosia reni- furmis Nardo. Fam. Suheritidae. Sponges of massive form, with knobbed silcx spicules, which, as a rule, are arranged in network. Sitherites Mardo. S. doniuncvla Nardo, Adriatic, Mediterranean. Fam. Spongillidae. Massive or branched with simple spicules, connected by investments of sarcode. Sponrjilla _ffvriatilis Lk., Sj?k lacntstria Lk. 07-der 4. — Hyalospongia. Sponges with a firm, often hyaline lattice-work of silex spicviles, which present the most perfect form of six-rayed spicules {Hexactinellidce), and may be cemented together by a stratified silicious substance. Fam. Hexactinellidae (glassy sponges). With connected silicious framework and network of stratified silicious fibres, which join the six-rayed silicious bodies, frequently with isolated spicules and tufts of silex hairs, which serve to attach the sponge. They live for the most part at considerable depths, and are allied to the fossil Vvntriculitidfe. Dactyliwalyx Bbk. Eujileetclla Owen. ^■^•^ C(KLENTBKATA. B. asprvdillum Owen, Philii)pine8. In the body cavity of the glassy sponge are found Arga .ymui/ijma, and a small Palremon.. Hynloiwvm SiehoMii ■Gray, Japan. //. hornile Lov6n, North Sea. Order 5.— Calcispongi^, Calcareous sponges. Usually colour- less, sometimes red-coloured sponges and sponge stocks, the skeletons of which consist of calcareous spicules. These are either simple needles (as they first appear in the embryonic form) or three or four- .armed cross spicules. Very often, however, we meet with two or all three forms of spicules in the same sponge. Fani. AsconidsB (Li-vrosok'nides, Ascons). Calcareous sponges, the walls of which are pierced by simple canals. Grantia Lk. {Leueosolcnia Bbk.), these •are divided by E. Haeckel into seven genera, Ascysm, Ascctta, Ascilla, Ascortu, Ascidmis, Axcaltin, Asoandra, according to the form of the calcareous needles or spicula. Or. hotvy aides Lk. {Ascandra complieata E. Haeck), Heligoland, nearly allied to Gr- LieherkuJmii. 0. S. from the MediteiTanean and Adriatic. Fam. Leuconidse ( Grantiida;, Leucons), calcareous sponges, with thick wall, which is pierced by branched channels. Lcuconia Grt., divided by E. Haeckel into seven genera, according to the form of the calcareous spicules — Leucysm, Lencetta, LenciUa, LericortU, Lcucuhnix, LeucoMis, Leucandra. L. {Leucctta) ■primifjenia E. Haeck. Fam. Sycondiae (Sycons). Mostly solitary calcareous sponges, with thick walls, which are pierced by straight radial tubes. The latter project on the surface as conical prominences of the wall. Sycon Eisso, divided by E. Haeckel into ■seven genera — Sycyssa, Sycetta, Sy cilia, SyGorti.% Syculnw, SycaltiK, Syr amir a. Sub-group II. — CNIDARIA (Ccelenterata, s. str.) Ccdenterata, with consistent tissues not 2Jiercecl by a system of ^^ores ; •the oscxdum is replaced by a mouth ; loith thread cells in the ejyithelial ■tissues. The Cnidaria represent the Ccelenterata in a more restricted sense ; and in their structure the ludial symmetry appears more strongly mai'ked. In them the amceboid cell, as an independent tissue vmit, loses its importance for the functions of locomotion and nourishment, although the entoderm cells often possess the power of absorbing solid particles, after the manner of the amoebfe. The gastrovascular iipparatus, on the contrary, functions distinctly as a digestive and circulatory body cavity. Pore systems in the skin are not required for the introduction of nourishment, since the mouth, which corre- sponds to the osculum, provides for the reception of food. Nemato- .cysts are very commonly found as productions of the epithelial cells, ACTINOZOA. 223 Fig. 168. — Group of nematocysts at the end of the tentacle of a Sci/phistoma principally of the ectoderm, but also of the entoderm. Each Cnido- hlast, from the contents of which a nematocyst is developed, possesses a fine superficial plasmatic process {Cnidocil), which is probably very sensitive to mechanical stimuli, and occasions the bursting of the capsule. Very frequently the Cnidoblasts are found thickly grouped to- gether at dertain places, and form wart-like swellings or batteries (fig. 168). The differentiation of tissues and organs also appears to have leached a higher stage in the Cniclaria, in comparison with the Porifera, in which cnidoblasts have not hitherto been discovered. Sense cells, in particular, are found in the ectoderm, and these are not seldom grouped together as specific sense organs. Nerve cells and fibres are also present; the latter often form a deeper layer of fibrous tracts beneath the superficial layer of the ec- toderm, with \^}^j2ijiif?_^_i®^'i )®;®;*j©jQto./A7i'j0 jq)(?\qjM'tj^)0^1-_/<; which they stand Fig. 169.— Longitudinal section through the nerve ring of Charyhdea. i n connection ^^'^^^ "^^^^ ectoderm ; 6z, ganglion cells ; Nf, nerve - fibres ; iSSG7/2)homedusce). Both Polyps and Medusa? frequently remain at a lower grade of morphological differentiation, the former becoming pol}^oid appen- dages, the latter medusoid buds enclosing the generative products. In either case they are situated on the stem or on some part of the Polyp. The individuality of such appendages appears limited ; the medusoid or polypoid animal sinks, physiologically speaking, to the value of a portion of the body or of an organ, while the entire stock L. Agassiz, " Contribution to the Natural History of the United States, Aca- lephffi vo. iii., 1860, vol. iv., 18(>2. E. Haeokel, "(System der Medusen." Tom. I. and II., Jena, 1880 and 1881. Fig. 180 i.— Free Medusa of Olielia gelatinosa, as yet without generative organs ; g, auditory vesicles. HYDEOZOA. 235 approaches more nearly to a single organism. The more completely polymorphism and division of labour are impressed upon the polypoid and medusoid appendages, so much higher becomes the unity of the whole which is morphologically a colony of animals. In these cases it is often difficult to distinguish between budding and simple growth. For a long time it was considered as a remarkable circumstance, hardly admitting of a satisfactory explanation, that organisms which differed so widely as Polyps and Medusje— they had, indeed, been systematically separated as different classes— should only form dif ferent stages in the life-history of a single cycle of development and thus be united in the closest genetic connection. The theory of "Alternation of Generations" contained only a description of the matter, and offered no explanation. The discovery of the mode of origin of the Medusa as a bud on the body of the Polyp first clearly demonstrated the direct relation of the two forms, for it proved that the Medusa is a flattened, disc-shajyed Polyp with a shallow hut loide gastric cavity, the peripheral part of which has, hy the fusion of its U2}per and lower walls along four, six, or eight radiating areas, become divided into the vascular pouches {gastric pouches), or, as they are called, radial canals, which correspond to the gastrovascular pouches of the Anthozoa. The differences consist, in connection with the discoidal form, mainly in the position of the gastric tube as an external appendage, the manu- brium, and in the great reduction in height of the radially extended septa (mesenteries), which are traversed by a layer of endoderm cells, the vascular or endoderm lamella. This layer is derived from the fusion mentioned above of the aboral with the oral layer of the endoderm of the peripheral part of the gastro-vascular cavity. At the same time the oral disc becomes enlarged and concave to form the cavity of the bell, the ectodermal lining of which gives rise to the muscles of the subumbrella. The supporting SLibstance of the arched (after it is freed from its attachment) aboral surface of the disc becomes very much thickened and gives rise to the gelatinovis substance (mesodei-mic), which sometimes contains cells ; while that of the oral surface keeps the character of a thin but firm lamella, and serves as a support for the muscles on the under surface of the disc. The tentacles accordingly ai-ise near the edge of the disc, and become the marginal tentacles of the Medusa. In addition to these, four simple or branched oral appendages appear as outgrowths from the manubrium. In addition to the sexual reproduction, asexual multiplication is COSLiJNTEEATA. Widely distnbuted, especially ainongst the polypoid forms, in which It leads to the formation of polymorphous animal stocks. The two torms of reproduction alternate for the most part in regular order so as to produce different generations. There are, however, Meclusc^ {Aeyznopsis, Pelacjia) which proceed without alternation of genera- tions and develop directly from the ovum by continuous development with metamorphosis; but, as a general rule, the egg of the Medusa (pliaiiero-codonic gonophore) or the medusoid generative bud (adelo- codonic gonophore) produces a Polyp, and this Polyp either at once by transverse fission {Scyphomedusoe), or later, after a longer period of growth, in which a sessile or free-swimming polyp stock is pro- duced, gives rise to a generation of free-swimming Medusae, or of medusoid buds which never become separate from the polyp 'stock. The Hydromedusse feed entirely on animal substances, and for the most part ai-e inhabitants of the warmer seas. The free-moving Medusce and Siphonophora are phosphorescent. Order 1. — Hydromedus^.='= Colonml forms, the individual Polyjis of which are without msojihageal tube or mesenteric folds. The sexual generation has the form either of small free-swimming Medusce provided with a velum {Crasjjedote Medusce) or of medusoid generative buds {rudimentary Medusoi) which remain attached to the hyclroid colony. The Polyps and polypoid forms are the asexual individuals. They form small moss- or tree-like stocks which are frequently surrounded by chitinous or horny tubes (cuticular skeleton). These exoskeletal structures may become extended into cup-like hydrothecse surrounding the individual Polyps. The stem and ramified branches [ccenosark] contain a central canal which communicates with the gastric space of each individual Polyp and polypoid appen&age and contains the common nourishing fluid. The Polyps have no oesophageal tube, and the ciKated gastric cavity is undivided by mesenteries. As a rule, the ectoderm and entoderm remain simple, and are only separated by a thin interposed supporting lamella which does not contain cells. The presence of elongated muscle fibres as processes of the ectodermal epithelial cells is very general {Hydra,.Podocoryne). These muscles may, however, * L. Agassiz, " Contributions to the Natural History of the United States of America," vol. ii. — iv., 1860—1862. G. J. Allnian, "A Moro.ffraph of the Gymnoblastic or Tubularian Hydroids," vol. i. and ii., London, 1871 aud 1872. N. Kleinenberg, " Hydra," Leipzig, 1872. 0. and E. Hertwig, " Das Nerven- system und die Sinnesorgane der Medusen," Leipzig, 1878. HTDBOZOA. 237 be separated as an independent layer of nucleated fibre cells below the epitheliixm. The Polyps are not invariably alike, proliferous Polyps (or Blastostyles) being frequently found as well as the nutritive ones.. The proliferous Polyps develop generative buds on their walls. The sterile Polyps may differ from one another in the number of tentacles- and in their entire form, so that different kinds of individuals may be found on a single stock. Thus we find the polymorphism of the- Siphonophora foreshadowed amongst the Flydroidea {Fodocoryne^ Phbmularia). The generative products are only exceptionally developed in th& Pol}'^ body itself, in which case they are produced in the ecto- derm [Hydra). This exception is probably to be looked upon as an extreme case of degeneration of a medusoid bud. As a rule the generative products are de- veloped in special medusoid buds [gono- phores] formed from both cell-layers. In the most simple cases the budding in- dividuals of the sexual generation contain a Fig. XSl.—Podoeoryne caniea (after C. Grobben). P, Polyp y. M, Medusa bud on the proliferating polyp ; S, spiral- zooid; Sk, skeleton Polyp (compare the free Medusa, fig. 154). diverticulum of the gastric cavity of the polyp-shaped parent or of the axial cavity of the hydroid stock. The generative products become accumulated around this diverticiilum {Hydractinia echinata, Clava squamata). In a more advanced stage we find a mantle-like envelope enclosing the bud, and con- stituting the rudiment of the umbrella, with a continuous vascular lamella or with more or less developed radial vessels {Tuhularia coronata, Eudendrmm ramosum, Van Ben.) Finally, at the highest stage, the buds develop into small Medusfe {Campamdaria gelatinosa van Ben., Sarsia tuhidosa), which become free, and sooner or later, 238 COiLENTEEATA. often only after a long period of free life, in which they become much larger and undergo a metamoi-phosis, reach sexual maturity. The Medusie belonging to the order Hydromedusie are, with but few exceptions, distinguished from the Acalephce (Scyphomedus*) by their smaller size— although certain forms, for example Aequorea, may attain such a size as to have a diameter of more than a foot— and by their simpler organization. The number of their radial vessels is smaller (4, 6, or 8), their sense organs (marginal bodies) are not covered by folds of membrane (hence GymnojMhalmata Forbes), and they have a muscular velum (hence Craspedota Gegenbaur) (fig. 182). The generative products are always formed fi'om the ectoderm and originate on the wn.lls of the radial canals or of the manubrium, but never, as in the AcalepJui, in diverticula of the gastric cavity. The hyaline gelatinous substance of our Medusaj is, as a rule, structureless, and contains no cellular elements ; there may, how- ever, be fibres running per- pendicularly through it {Lirio2)e). These fibres are probably derived from cell processes of the ecto- derm and entoderm, and have arisen contemporane- ously with the gelatinous disc, which is itself to be looked upon as an excretion product of the adjoining ectoderm and entoderm epithehum. The nerve-ring is placed at the edge of the disc at the point of insertion of the velum. It is covered by a sense epithelium com- posed of small cells bearing sense hairs, and has the form of a double fibrous cord containing ganglion cells. The larger upper nerve-ring runs above the velum, Avhile the weaker nerve-ring, on the other hand, is placed below it. The lower nerve-ring is composed of larger fibres and larger ganglion cells ; bundles of fibrillfe pass off from it to supply the muscles of the velum and subumbrella, where they form a sub-epithelial plexus interspersed with ganglion cells, between Ov Ob Pig. 182. — Plduladiuni cajvai/ie rejireseuted from the underside of the umbrella. T', Velum ; O, mouth ; Oo, ovary ; Oh, auditory vesicle ; Rf, tentacles on the margin of the disc ; Rw, marginal .swellings. HTDBOZOA. 239 the muscular epithelium and the fibrous layer. The ganglion cells in the upper nerve-ring are smaller, and the fibrillte given off from it pass to the tentacles. The fibrill* of the sense nerves may be derived from both rings. The margined bodies have long been recognised as sense organs, and are either eye spots (ocelli) or auditory vesicles ; hence the Hydroviedusce may be divided into two groups, the Ocellata or Vesiculata. In the Vesiculata the auditory vesicles are situated at the edge of the under side of the umbrella, and contain one or more concretions {otolith) which are formed in the interior of cells. Peculiar sense cells surround each vesicle-like cell containing a concretion. The curved hairs of these sense cells (auditory hairs) are in contact with the con- cretion vesicle. A nerve fibrilla enters the basis of the auditory cells (fig. 183). The audi- ^ -^^ tory organs of the Tra- chymedusce are placed above the velum, and are in con- nection with the upper nerve ring ; they have the form of small projecting tentacles furnished with otoliths and auditory hairs. The tentacle may either project freely on the surface {Trachynema), or, as in Geryonia, it may be placed in a vesicle (fig. 184) which lies in the gelatinous substance of, the disc and close to the edge of the latter. Separate sexes are almost invariably the rule, but it is rare to find that the colonies are dioecious, i.e., that male and female medusoids are developed in difierent colonies (Tubularia). Gemma- tion has occasionally been observed among the Medusoi {Sarsia prolifera) and division {StomobracMum mirabile). The larvje of Cunina, which are parasitic on the Geryonidce, may also there give rise to a cluster of buds. Fig. 183.— Sense organ on the nerve-ring and circular vessel of Octorchis (after O. and R. Hertwig). Sb, Sense organ; O, O', two otoliths ; Hh, audi- tory cilia ; Hz, auditory cells ; upper nerve-ring ; Rg, cir- cular vessel. (T^^De of the audi- tory organ of the Vesimilata.) Fig. 184. — Auditoiy vesicle of Gery- onia (Car marina), seen from the sm-face (after O. andR. Hertwig). -ZV" and iV', The auditory nerves ; Ot, otolith ; Sz, auditory cells ; Hh, auditory cilia (type of the auditory organ of the Trachy- meduscB) . 240 CaSLENTKBATA. The development of the ovum, which is, as a rule, naked {i.e., with- out a Wtelline membrane), has hitherto only been completely followed out in a few cases. In every case the segmentation seems to Ije com- plete, and leads to the formation of a segmentation cavity and a single-layered blastoderm [a single-layered Ijlastosphere]. The latter gives rise to a second endodermal layer of cells, which lines the segmentation cavity. The segmentation cavity thus becomes converted into the gastric cavity of the future polyp. The spherical or oval larva now either attaches itself and gives rise by budding to a small hydroid stock, or swims freely and develojjs directly into a small Medusa {Trachymedusce). The Medusa, after becoming free, usually undergoes a more or less fundamental change of form, which concerns not only the alteration caused by the enlai-gement of the umbrella and manubrium, but also the increase, according to definite laws, of the marginal tentacles, sense, organs {Tima), and the radial canals {Aequorea). We must remark, however, that the sexually complete Meduste exhibit very considerable variations in size, number of sense organs and tentacles {Phyalidimn variahile, Clythia voluhilis). The difficulty of systematic arrangement is augmented by the fact that closely allied Polyp stocks can produce different sexual forms. Thus, for example, Monocaulus gives rise to sessile generative buds and Corymorpha to free Medusce (Steenstrupia). Medusae of identical structure also, which one would place in the same genus, may form the sexual generations of hydroid stocks belonging to different families {isogonism). Thei-e are also cases in which we find IfeduscB of closely allied genera, some developed from hydroid stocks by an alternation of generations, and others developed directly. Hence it appears just as little satisfactory to found a classification entii'ely upon the sexual generations as to pay attention to the asexual generation alone. (1) Sub-order: Eleutherohlasteai. Simple hydroid Polyps without medusoid buds ; both generative products are developed in the body- wall of the Polyp. Fam. Hydroidae. Hijdra, the fresh- water Polyp. H. riridis L.. H.fvsca L.. remarkable for great powers of reproduction. (2) Sub-order : Hydrocorallice. Coral-like hydroid stocks with cal- careous coenenchyma and tubular hydrothecse opening to the exterior by pores. Some of these contain the larger nutritive animals, while others contain animals without a mouth and beset with tentacles. HTDEOZOA — HTDROMEDUSiE. 241 The latter are arranged usually in the form of a circle round each of the nutritive animals. The polyparia are found in the fossil state. Fam. Milleporidae. Millt'pora L. M. alcicovmn L. Faiu. Stylasteridse. (3) Sub-order: Tuhularice (Ocellata). Polyp stocks which are either naked or clothed by a chitinous periderm without cup-shaped hydrothecfB surrounding the polyp head. The generative buds arise on the body of the Polyp or on the stock. The Medusce which are set free belong to the genera Oceania, Sarsia, etc., and have ocelli. Fam. Clavidae. Polyp stocks with a chitinous periderm. Polyp club-shaped, with scattered, simple, filiform tentacles. The generative buds arise on the Polyp body and for the most part remain sessile. Cordylopliora AUm. The stock is branched ; there are stolons which grow over external objects. Oval gonophores covered by the perisarc. The animals are dioecious. In fresh water — C. lacustri^ A\\m. Alhieola Kirchp., Elbe, Sclileswig. The foUovdng are marine genera — Clara, 0. Fr. Miiller. Allied are the EiulcndrklcB with Elide ndrlwrn Tamomm. L. Fam. Hydractinidse. Polyp stocks with iiat extended coenenchyma and firm encrusted skeletal excretions. The Polyjis are club-shaped, with a circle of simple tentacles. In addition to the latter there are large tentacle-shaped Polypoids (Spiralzooids). HtjdraGtinia van. Ben. The medusoid buds sessile on the proliferous animals, which are without tentacles. II. Ecli'mata Flem. Podocoryiic Sars. (fig. 181). The generative buds are freed as OemnidcB. P. oarnea Sars. Fam. Tuhularidse. Polyp stocks clothed with a chitinous periderm. The polyps possess a circle of filiform tentacles on the proboscis inside the external circle of tentacles. The generative buds arise between the two circles of tentacles. Tulmlaria L. The hydroid stocks form creeping root-like branches at the bottom, from which arise simple or branched twigs with the terminal .polyp heads ; the generative buds are sessile. T. (^ThamnocnuUa Ag.) coronata Abilg. dioecious. Cornmorplm Sars. The stalk of the solitary polyp is clothed with a gelatinous periderm, attaches itself by root-like processes, and con- tains radial canals which lead into the wide digestive cavity of the Polyp- head. The fi-eed Mi'dnm. is bell-shaped, wdth one mai'^inal tentacle, and bulbous swellings at the end of the other radial canals. C. nutans Sars., C. nana Alder. (4) Sub-order: Cam/xmMfeWce (Vesiculata). The chitinous skeletal tubes widen out round the Polyp-head to form cup-like hydrothecte. The Polyp-head, the oral cone (proboscis), and tentacles can be in most cases completely retracted into these hydrothecte. The generative buds arise almost regularly on the walls of the proliferous individuals, which have neither mouth nor tentacles. The buds are sometimes sessile, and sometimes become separated off 16 242 CCELENTERATA. as small vesiculate Medusce, with generative organs on the radial canals {Eucojmlce, Geryonojjsidce, Aefjuoridcn). Fam. Plumularidae. The liydrotliccas of the branched hydroid-stocks are avrangod in siiinle rows ; those of the nutritive Polyp have small accessory calyces filled with nematocysts (nematocalyces), Plumularia orixtafa Lam.. Antcnnular 'Hi, (tiitcniiinu. Lam. Fam. SertularidsB. Branched Polyp stocks, the Polyps of which pr.jject in flask-shaped hydrotheciu on opposite sides of the stem. BijiMmmui puinila L.. Sertularia ahicttiut, i-vjrrrxs i /m L. Fam. Campanularidse-Eucopidae. The cup-shaped hydrothecse are placed at the end of ringed stalks. The Polyps possess a circle of tentacles below their conical proboscis. Campa/iularia Lam, The proliferous individuals are situated on the branches and ffive rise to free MrduHoi, bell-shaped, with a short^ manubrium with four lips, four radial canals, the same number of marginal tentacles, and eight inter-radial marginal vesicles. After separation the inter-radial tentacles are formed. C. ( (Jlijthia) Johndoni = volubiUs Johnst., probably with Eiuuqn: variahilu Cls. Ohelia P6r. Les., is distinguished from Campanularia by its Medusta;. These are flat, disc-shaped Medusce with numerous marginal tentacles, but with eight inter-radial vesicles. 0. dichotoma L. = CC}.imjmm/laria f/elatimsa van Ben.), C. r/enictdata L., Laomt-dm L&mx. The generative buds remain sessile in the hydrotheca of the proliferous polyps. X. calicidata Hincks. Fam. Aequoridae. Meduftce with numerous radial vessels and marginal tentacles. Aequorea Forsk. The Gcrijoiiopsidrs are allied here. OctorchiK E, Haeck. Tima. (5) Sub-order : Trachymedusce. Meditsce with firm, gelatinous umbrella, supported by cartilaginous ridges with stiff tentacles filled with solid rows of cells ; these may be confined to the young stage (larvse of Geryonidm). Development by metamorphosis without hydroid asexual individual. Fam. Trachynemidae, with stiflE marginal tentacles, which ai'e scarcely capable of motion. The genital organs are developed on vesicle-like swellings of the eight radial canals. Tracliynema ciliatum Ggbr. I{Jiopulo)u:ma vclatum Ggbr., Messina. Fam. Aeginidae. The hard cartilaginous umbrella has a flat, discoid shape. The extended digestive cavity has pouch-like enlargements in place of the radial vessels. The circular vessel is usually reduced to a row of cells. Cunina albescens Ggbr., Naples. Acgineta flavescens Ggbr. Fam. Geryonidse, Umbrella with cartilaginous mantle ridges and f om* or six hollow tube-shaped marginal tentacles. The manubrium is long, cylindrical, or conical, with a proboscis-like oral portion, and four or six canals which lead into the radial canal. The generative organs lie on the radial canals ; eight or twelve marginal vesicles. Liriope Less., with foui' radial canals, four or eight tentacles and eight vesicles. L, tctraphijlla Cham., Indian Ocean. Gcryonm P6r. Les., with six radial canals without lingual cone. G. nvihrJhi E. Haeck., Carmarina E. Haeck., with six radial canals and a lingual cone, E. Haeck. C. hastata, Nice. HTDBOZOA — SIPHOjS'OPHOBA. 243 Order 2. — Siphonophora.* Free-sioimming polymorphous hydroid-stocJcs with contractile stem, luith polypoid nutritive indi- viduals and medusoid huds, usually also toith nectocaly- ces, hyrophyllia and dactylo- zooids. Morphologi- cally the Si2)ho- nophora are dii-ectly allied to the hy- droid-stocks ; but they possess to a much greater extent than the latter the characters of individuals, in consequence of the highly developed poly- morphism of their polypoid and medusoid appendag es. The functions of the latter seem so inti- inately con- nected and are so essential for the preserva- FiG. 185.— Diagi-am of a colony of Physophorkla. Si, Stem; Ek, ectoderm; En, entoderm; Pn, Pneumatophor ; Sk, nectoc'alyx being budded off; S, nectocalyx ; D, hydrophyllium ; G, gono- phore ; T, dactylozooid ; 8f, tentacle ; P, polyp ; O, mouth of the latter ; JV^-, battery of nematocysts. tion of the entire colony that we may regard each colony of Sipho- - I*p?wlf Kollikei-, C Vogt, Hnxley and others, compare C. Gesenbaur LeobachtuDge.i uber Siphonophoren," ^idtxohrift flir wh.. ZoolTlSl C. 244 COiLENTEllATA. nophora pliysiologically as an organism and its appendages as organs. In this connection we may mention that the sexual medu- soid generation is so little independent that it only exceptionally {Velellidce) reaches the morphological grade of the free-swimming Medusa. In place of the attached and ramified hydroid -stocks we find in the iSiphonophora a free-swimming con- tractile unbranched stem (hydrosoma), which is rarely provided with simple kteral branches. The upper end of the hydro- soma is frequently dilated to the form of a flask (pneumatophore), and contains an air chamber [pneumatocyst] (fig. 185). In every case there is a central space in the axis of the stem in which the nutritive fluids are kept in constant motion by the contractility of the walls and by the move- ments of the cilia. The air sac or pneu- matocyst at the apex of the hydrosoma is connected to the chamber which contains it by radial septa, and in many cases attains a considei'able size (Physcdia). It func tions as a hydrostatic apparatus, and in those forms, which have a long spiral hydrosoma (Physojjhoridce), serves to keep the body in an upright position. In some cases the gaseous contents can escape freely by one or more openings. The appendages which are attached to the spirally twisted bilaterally symmetrical stem and whose cavities communicate vnth. that of the stem are of at least two kinds — (1) The polypoid nutritive animals mth their tentacles ; (2) the medusoid sexual buds. The nutritive Polyps (hydranths) are simple tubes provided with a mouth, and never Gegenbaur, " Neue Beitriige zur Kenntniss der Siphonophoren," Nova Acta.. Tom. XXVII., IS.'jg. R. Leuckart. •• Zoologische Untersuchungeu," I., Giessen. 1853. R. Leuckart. •■ Zur naheren Kenntniss der Siphonophoren von Nizza," ArcJih: fur Natvvqcxrh, 1854. C. Clans. "Ueber Halistemma tergestinum n. s. nebst Bemerkiingcn iiber den feineren Bau der Physophoriden. ' Arhritrn am (Inn Zonhu/isHini Instihtt. der Uvir. Wicn. etc., Tom. I.. 1878. E. Met- schnikofe, " Stiidien iiber die Entwickehmg der Medusen und Siphonophoren. ZciUrh. fiir wixx. Zool, Tom. XXIV., 1874. Fig. 186. — A portion of the stem and appendages of HiMstemmu terffestinum. St, Stem ; D, hy- dropliyllium ; T, dactylozooid; Sf, tentacle of the latter ; Wg, female, Mg, male, gonophores. HTDE.OZOA — SIPHONOPHOEA. 245 possess a circle of tentacles. They always, however, have a long tentacle arising from their base. This tentacle can be extended to a considerable length, and be retmcted into a spiral coil. It rarely has a simple foi-m, but, as a rule, it bears a number of unbranched lateral twigs, which are also very contrac- tile. These tentacles are invariably beset with a great number of nema- tocysts, which in many places are closely packed and have a regular arrangement. These aggregations of thread-cells are especially found on the lateral branches of the tentacles, and give rise to large, brightly-coloured swellings, the batteries of nematocysts. The batteries show considerable variations Fig. 187. — Group of buds of a, Physophor at the bottom of the pneumatophore. C, Central cavity ; Sk, nectocalyx bud with the ectodermal ingrowth. (C d Fig. 188.— Development of AgalmopU Saraii (after Metschnikofl). a, Ciliated larva, h. Stage with developing hydrophyllium (D). c, Stage with cap-shaped hydrophyllium (D) and developing pneumatophore (i/'). d, Stage with thi-ee hydrophyllia, (Z>, B', D"), polyp (P), and tentacle. in form in the various species, genera, and families, and such varia- tions afford valuable characters for systematic classification. 246 COSLENTEEATA. The second form of appendage, the gono2)hore8, usually possess a bell-shaped mantle containing circular and radial vessels, and surround- ing the central stalk or clapper (manubrium), which is filled with ova or spermatozoa. They usually arise in clusters at the base of the tentacles, more rarely from the nutritive Polyps themselves {e.g. in Velella). The male and female generative products always arise separately in differently shaped buds, but are usually found closely approximated on the same stock (fig. 186). There are, however, also dioecious Hipho- nophora, or if tlie medusoid buds or gonophores be regarded as generative organs, Hiphono- phora of distinct sexes, e.y., Apolemia uvaria and Diphyes acuminata. The ripe sexual Medusoids frequently become separated from the stock, i.e. after the development of the generative products, and only rarely become liberated as small Medusoi {Chrysomitra in the Velellidce), which produce generative products during their free life. Besides the constant nutri- tive Polyps and medusoid gonophores, there are incon- stant appendages, which are also modified Polypoids or Medusoids. These are the mouthless worm-like dactylo- zoids (fig. 186), which, like the Polyps, are provided with a tentacle, which is, however, shorter and simpler, and has no lateral branches or aggregations of nematocysts ; also the leaf -shaped hard cartilaginous liydrophyUia, which serve to protect the polyps, dactylozoids, and gonopliores ; and finally the appendages known as nectocalyces, which are placed beneath the pneumatophore. The nectocalyces have a structure similar to that of the Medusae, though their bilateral symmetry is apparent ; Fig. 189. — Small larval stock of Agalmopais after the type of Athoryhia, Lf, Pueumatophore ; D, hydrophyllium ; Nk, gi-oups of nemato- cysts ; P, polyp. HYDROZOA — SIPITOlSrOPHOIlA. 247 they are, liowevei-, without manubrium, mouth, tentacles, and sense organs. The deeply concave sub-umbrella surface of the nectocalyx is largely developed and has a very powerful muscular covering in rela- tion to its exclusively locomotive function. All the appendages are developed as buds formed of ectodei-m, entoderm, and containing a central cavity which communi- cates with the central space of the stem. In the nectocalyces and gonophores an ecto- dermal ingrowth gives rise to the covering of the sub-umbrella and to the generative products respectively (fig. 187). The ova, of which there is often only one in each female gono- phore, are large, and have no vitelline mem- brane, and, after im- pregnation, undergo a complete and regular mi segmentation. A nectocalyx [Di2)hyes) is the first structure formed in the free-swim- ming lai-va, or the upper part of the body of the larva gives rise to a cap- shaped protective cover or hydrophyllium as well as a pneumato- phore, and the under part becomes the primary nutritive polyp {AgalmojJsis, fig. 188). Since new buds give rise to leaf -shaped hydrophyllia, a small stock with Fig. 190. — Physophora hydrostatica . Pn, Pneumatophore ; S, nectocalyces arranged in double rows on the swim- ming column ; T, dactylozoid ; P, polyp (nutritive individual) with tentacles, Sf- Nk, groups of nemato- cysts on the latter ; G, clusters of generative buds. 248 COJLKNTEEATA. provisional appendages is formed which allows us to regard the develop- ment of the Hvpliono- phora as a metamorphosis (fig. 188 and 189). The crown of hydro- phyllia, which is com- pleted by the addition of fresh hydi-ophyllia after the appeai-ance of a tentacle with provisional groups of nematocysts, persists only in Athory- bia, where a swimming column with nectocalyces is never formed. In Agalvio2ms and Fhyso2Jhora the primary hydrophyllia of the larva fall off" as the stem be- comes larger, and are replaced by nectocalyces. (1) Sub-order: Physo- plioriclcti. Stem short, extended in the form of a sac (fig. 190), or elongated spirally (fig. 191), with a pneumato- phore, usually nectocaly- ces, which are arranged in two or more rows on a swimming column below the pueumatophore. Hydiophyllia and dacty- lozooids are usually present, and alternate with the pohps and gonophoi-es in i-egular order. The body of the larva usualh' develops Y\G,.\^\.Sali»temmaierge«tiimm. P», pueumatophore ; -S. Nectocalyx ; P, polyp ; D, hydrophyllium ; Nk, proups of ucmatocysts HTDROZOA. — SIPHONOPHORA. 249 first a polyp with pneumatophore and tentacle beneath an apical hydrophyllium. The female gonophore has only one egg. Fani. Athorybiadse. With a bunch of hj'drophyllia in place of the swim- ming column ; resembling a persistent larval stage. Atlioryhia rosacea Esch., Mediterranean. Fam. PhysophoridsB, s. str. Stem short and enlarged to a spiral sac beneath the swimming column with its double row of nectocalyces. No hydrophyllia but instead two outer bunches of dactylozooids with gonoblastidia. nutritive polyps and tentacles lying beneath them. Pity svjjh ova Forsk., Ph. hydrostatlea Forsk., Mediterranean (fig. 190). Fam. Agalmidse. Stem imusually elongated and spirally twisted. ■ Swimming column with two or more rows of nectocalyces. There are both hydrophyllia and tentacles. ForshaUa contorta M. Edw., Ilalhtcmma. ^ Dactylozooids and hydrophyllia directly connected with the stem. In the ciliated larva a pneumatophore is first developed at the upper pole. H. ruhrum Vogt, Mediterranean. H. teryestinvm Cls. (fig. 191). Ayal- I mo^jsis Sarali Koll., A^^oleviia itvaria Less., Mediter- ranean. Dioecious. (2) Sub-order : Physalidce. — Stem dilated to form a large chamber, the pneumatophore lying almost hoi'izontally, containing a very large pneumatocyst opening to the exterior. Necto- calyces and hydrophyllia absent. On the ventral line of the sac are situated lai'ge and small nutritive polyps with strong and long tentacles. There are also clusters of gonophores attached to the tentacle-like polyps. The female buds seem to become free-swimming Mechosce. Fam. Physalidae. With the characteristics of the group Physalia Lam., P. carardla Esch. {Arcthitm Til.), iKlayica, utriculus Esch., Atlantic Ocean. (3) Sub-order : Calycoiolwridce. Stem long and without pneumatophore. Swimming column with double row of nectocalyces (Hippopodidfe) or with two large opposed nectocalyces, more rarely with only one nectocalyx. There are no dactylozooids. The appendages arise in groups arranged regularly, and can be retracted into a cavity of the nectocalyx (fig. 192). Each group of individuals consists of a small nutritive polyp, a tentacle with naked kidney-shaped groups of nematocysts, and gonophores. Pig. 192. — Diphyes acu- minata, ma^ifled about 8 times. Sb, Fluid reservoir in the upper nectocalyx (somatocyst). 250 COiLKNTJiRATA, To these is usually added a funnel or umbrella-.slmpeouch-liJce form. The velum has a smooth margin, and contains vessels [j)rolongations of the g astro-vascular system']. On the margin of tloe disc there are four vertically placed lobe-like appen- dages. There are four covered sense, organs, and the same number of vascular pouches sejxtrated by nar- row partition toalls. The Charybdem are distinguished by the deep bell shape of their body, and were formerly reckoned as " Craspedota " among the Hydro- medusa}, with which they certainly have some characteristics in com- mon. Amongst these character- istics the most striking is the possession of a smooth-edged velum, which, however, contains vessels. Fig. \m.-CharyMca marsnpialis, natural Qn the Other hand, the presence of size. T, Tentacles ; Etc, marginal bodies . '■ (sense organs) ; Ov, ovaries. the gastric filaments and of the large sense organs enclosed in niches points to a relationship with the Acalejjha ; and this view is supported by the character of their whole structure, in which the peculiarities of the Lucernaridce are perceptible, though greatly SCTPHOMEDUSiE MABSUPIALIDA. 259 • moditied. As in Ltccei^naridce, the vascular spaces are wide pouches divided from each other by four narrow septa (figs. 198, 199). The nervous system is allied to that of the Hydromedusce by the presence of a sharply defined nerve-ring. This nerve-ring is placed on the sub-umbrella side of the bell, and, since at the bases of the four sense organs it lies further from the margin than it does at the corners of the bell, it has a sharply mai'ked, zig-zag course. The nerve fibrillfe given ofi" from it mostly supply the muscular system of the sub-umbrella, and there give rise to numerous reticula of fibrillse connected with large ganglion cells. Large bundles of fibrillaj com- parable to nerves have only been found in the four radii of the mar- ginal bodies. The latter attain a high degree of development, since the knob-like swelling in which they terminate possesses, in addition to the lithocyst, a complicated visual apparatus consisting of two large unpaired median eyes and four small paired lateral eyes. The generative organs have a very peculiar form. They are separated from the gastric filaments and as thin, rather broad plates attached in paii's to the four pai-tition walls, length of the Unf ortunately known of the reach the whole vascular pouches, nothing is as yet development. Fam. Charybdeidae. Charyhdm inar- supialia P&-. Les. {Marmpialis Planci Les.) Mediterranean. Fig. 199.— The apicalhalf of a Charyhdea divided transversely, seen from the sub-utnbreUa side. The foiu- oral arms are visible. Ov, Ovaries on the four septa, S ; Ost, ostia of the gas- tric pouches ; Qf, gastric filaments. (3) Sub-order : Discophora [Acra- speda), E2')hyra-medusce. BisG-shaped Accdepha, the, margin of ivhose disc is divided into eight lobes. They have at least eight sub-marginal sense organs contained in niches, and toith the same number of ocular lobes. As a rule there are four great cavities in the umbrella for the generative orgam. The I)isco2}hora, which are generally known simply as Acalepha, can at once be distinguished from the Calycozoa and the Charybdeidoi by the disc-shaped lobed umbrella and usually by the large size of the oral tentacles. The lobes of the umbrella, however much they may differ in detail, can always be reduced to the eight pairs of lobes of the Ephyra, which, as the common starting-point of the Discophora, presents most clearly the eight-rayed symmetry char- COiLENTEBATA. acteristic of the group. Tlie striped mus-cles of the sub-umbrella are strongly developed to coi-respond with the great size of the body ; and beneath them the supporting lamella is usually thrown into a number of closely aggregated cii-cular folds, thus causing a consider- able increase in the surface on which the muscular epithelium with its circularly arranged fibres are placed. The generative organs have the form of horse-shoe shaped frills which project into four widely open cavities in the sub-umbi-ella, the sub-genitcd pits. These cavities are not developed in sonie ex- ceptional cases (lYausithoe, Discomedusa). The germinal epithelium, Fig. '2.f)0.—AurMa (nirita, seen from the oral surface. 3IA, The foiu' oral arms with the mouth in the centre ; GJr, The genital frills ; GH, Openings of the sub-genital cavities ; Bk, Marginal bodies ; RG, Radial vessels ; T, Tentacles on the margin of the disc. which is always embedded in the gelatinous substance, is covered with an entodermal layer, and is probably itself an entodermal product (tig. 200). Development takes place by alternation of gene- rations. In rare cases {Pelagia) the development is simpHfied, and the larva passes directly into the Eph^jra, missing out the attached Scyphistoma and the Strobila stage (Kro/m). 1. Semceostoinece. Discophora with large central mouth sur- rounded by four large often multi-lobed oral arms. The form of the SCTPHOMEDTJS.13 — BIIIZOSTOME.TS. 261 umbrella edge, the number of lobes and marginal tentacles present great variations. Fam. Ephyropsidffi, Ejihryop^U, Ggbr. {XoAisUhoe Koll). Disc small and like that of J^phi/ra, with simple o-astric sacs, without oral arms, but with eight marginal tentacles. The genital organs (in four pairs) do not lie m umbreUa cavities. E. pdaq'u-ii Koll., Mediterranean and Adriatic. Fam. Pelagidffi. Pelaqm P^r. Les. With wide gastric pouches and eight long marginal tentacles in the intcrradii. No alternation of generations. P. ■mcUhtca'^tx. Les., Mediterranean. Chrysaora Per. Les., with twenty-four long marginal tentacles. The radial and intermediate gastric pouches are per- ceptibly different. dir. hyso.se/4la Esch. Hermaphrodite, North Sea and Adriatic. Fam. Cyaneidee. Cyiinea P6r. Les. The tentacles are united in bundles on the under surface of the deeply lobed thick disc. There are sixteen (eight radial and eight intermediate) more or less wide gastric pouches, which break up near the end of the marginal lobes into small ramified vessels. C. cfipillata Esch. Fam. Aurelidse. Biscomedum Cls. With large oral arms, with branched vessels and two marginal tentacles. Subgenital pits present. JD. lohata Cls., Adriatic. Aurdia Per. Les., with branched radial vessels and edge of disc fi'inged with small tentacles. A. awlta L. QMedu-m aurita L.), Baltic, North Sea, Adriatic, etc. A.fiavidula Ag., coast of North America. 2. Rhizostomece. No central mouth, funnel-shaped slits in the eight oral arms and eight, rarely twelve, marginal bodies on the lobed margin of the disc. There are no marginal tentacles. The central mouth, which is at first present, becomes closed during the larval development by the fusion of the edges of the lips. Funnel-like splits are formed on the folded edges of the four pairs of arms, the so- called suctorial mouths, by means of which microscopic bodies are received into the canal system of the oral arms (fig. 195). Rliho-stoma Cuv. The arms end in simple tubular prolongations, and bear accessory tufts at their bases. Rli. Cuvieri Per. Les., Ccpliea Per. Les. The branched oral arms have gi'oups of nematocysts and long filaments between the terminal tufts. Cepliea Per. Les. (C'assiojjm^ horhoiiica Delle Ch., Medi- terranean and Adriatic. CLASS III.— CTENOPHORA.* Medusce of sjjherical or cylindrical, rarely hand-slm'ped form ; with eight meridional rows of vibratile 2ylates formed of fttsed cilia. They * C. Gegenbaur, " Studien liber Organisation und Systematik der Cteno- phoren," Arehiv. fur Naturffexelt., 18.56. L. Agassiz, " Contributions to the Natural History of the United States of America," vol. iii., Boston, 1860. A. Kowalevski, "Entwickelungsgeschichte der Rippenquallen," Petersburg, 1866. H. Fol, " Ein Beitrag zur Anatomic und EntwicklungsgescHchte einiger Rip. penquallen," Inaugural dissertation, Jena, 1869. A. Agassiz, •• Embryology of the Ctenophor£e," Cambridge, U.S., 1871:. C. Chun, "Die Ctenophoren des Golfes von Neapel," Leipzig, 1880. 2G2 CaCLENTEBATA. 2)ossess an oesojihayeal tiibe and a >rhnltz>a mrdafa Kol] Mediterranean. "' Fam. Cestidse. liody elongated to the form of a band in the direction of the sagittal plane. Two tentacles. Vrxlllum pavalldvm Fol., Canary Isles Crstiim. ]'rnerh J.css., Venus' Girdle, Mediterranean. Fam. LobatBB. The laterally compi'essed l)ody possesses two umbrella-like lobes near the mouth, and has relatively small tentacles. Eurhamyhaca vcvAU ligvra Ggbr., Mediterranean and Atlantic Ocean. CMaja papilloxa, M. Edw. {Alcinoc papillom Delle Ch. = .VrapulU/ma Less.), Mediterranean. Fam. Beroidse. Characterised by the laterally compressed body with fringe- like appendages on the periphery of the polar spaces ; without tentacles. Jicrdr For.'ilialii M. Edw. (alhrsmis and ritfcscem Forsk.), Idyiojms Clarlti Ag., Pcuidora Mcmmi/if/ii, Esch. CHAPTER VIII. E CHINODE EM ATA . * Animals with a radial, usually pentamerous arrangement. They 2}ossess a skin bearing spicules and indurated by calcareous deposits, a digestive canal, a water-vascidar a2)p)aratus, and a true vascular system. The radial arrangement of the Echinoderms was for a long time held to be a character of typical valae, and was the principal reason why, smce the time of Cuvier, the Echinoderms were included in one group, the Radiata, with the Meduste and Polyps. It is only in recent times that R. Leuckart has effected the separation of the Echinoderms from the Ccelenterates. The organization of the Echinoderms does in fact appear so different from that of the Ccelenterates, and seems to belong to a so much higher grade of development, that the combination of the two groups * Fr. Tiedcmann, " Anatomie der Eohrenholothurie, des poraeranzfarbenen Heesternes und des iStein-Seeigels," Heidelberg, 1820. Job. Miiller, " Uber den Bau der Echinodermen," Abb. der Berl. Akad, 1853. Job. Mliller, " Sieben Abliandlungen iiber die Larven und die Entwickeluug der Echinodermen." Abb. der Berl. Akad, ISiC), 1848, 18 49, 1850, 1851, 1852, 185-1. A. Agassiz. Embryo- logy of the Starfish." Contributions, etc., Vol., V. 1864. "E. Metschnikoff, " Studien iiber die Entwickelungsgeschichte der Echinodermen und Nemer- tinen, " St. Petersburg. 1869, H. Ludwig, " Morphologische Studien an Echinodermen," Leipzig 1877 and 1878. SYMMETRY. 267 as Racliata is inadmissible, and so much the more so since the i-adial airangement of the structure exhibits some ti^ansitions towards a bilatei-al symmetry. The EcJimodermata are separated from the Ccelenterata by the possession of a separate aHmentary canal and vascular system, and also by a number of peculiar features both of organization and of development. The arrangement of the parts round the axis of the body is ttsually pentamerous. Nevertheless when the rays are more numerous, irre- gularities in the repetition of the similar organs are met with. If we take as the fundamental form of the Echinoderm type a spheroid with the principal axis somewhat shortened and the poles flattened and dissimilar', the long axis of the i-adial body will be this chief axis, and the mouth and anus the two poles (oral and anal poles). We can imagine five planes pass- ing through the long axis of this spheriod, each of which will divide the body into two symmetrical halves. The perfect correspondence of these halves is, in the body of Echinoderms, disturbed by the dif- ferent forms and significance of the two poles, so that our representation is not an exact one. The ten meri- dians, which are separated from one another by equal intervals and fall in these five planes, are differently related to one another, inasmuch as five alternate ones, which ai-e called the chief rays, or radii, contain the most important organs, the nerves, the vascular trunks, the ambu- lacral feet, etc., while the other five meridians constitute the intermediate rays or inter-radii, and also contain certain organs (fig. 206). It is only in cases of complete equivalence of the radii and inter-radii that the echinoderm body presents a pentamerous radial arrangement [regidar EcJdnoderms). It is, however, easy to show that this regular radial symmetry never occurs in its perfect form. Since one organ or another, e.g., the madreporic plate, the stone canal, heart, etc., always remains single, and does not fall in the axis of the body, it will be only those planes, in the radius or inter-radius of which the unpaired organs fall, which can fulfil the R J riG. 206.— The shell of a regular Sea- urchin seen from above. R, Radius with double row of perforated plates ; J, inter-radius with the genital organs and their pores. In the right ante- rior inter-radius is the madrepcric plate. 268 ECniNODEEMA.TA. conditions which admit of the body being divided into two exactly symmetrical halves. Even these planes do not exactly fulfil these con- ditions, since the re- maining organs are not strictly symmetrical in regard to such a plane. Very frequently one of the rays differs in size from the others, and then we have an irreg-w- larily in the external form of the Echinoderm, which I'enders the bi- lateral symmetry visible even from the exterior. The pentamerous body of the Echinoderm may become bilateral, the plane of the unpaired ray forming a median plane, on each side of which two pairs of equal rays are repeated. "We can distingui?h an upper sur- face (apical pole) and an under (oral pole), a right and left side (the two paired rays and their inter -radii), an anterior end (un- paired radius) and a posterior (unpaired inter-radius). In the irregular Sea-urchins, the bilate- rally symmetrical form is still more strongly marked. Not only is the unpaired radius of abnormal size and form, and not only are the angles at which the principal ray and the accessory rays cut each other equal only in pairs, but Fig. 208.— Sc/nrasfer {Spatangidoe), fi-om tiie 1 • J.1 /~n J • 7 /n ctr\n\ ventral side. O, mouth: A, anus; P, also m the Glypecmtridea (fig. 207), ^,^^.^3 „f .^^^ ambuiacrai feet, the a,nus is removed from the dorsal pole to the ventral half of the body in the unpaired inter-radiu-, Fig. 207.-— CI ypeaiiter rogacenii, from the dorsal side. The madreporic plate is situate in the centre and is sur- rounded by five genital pores and by the five-leaved rosette. The unpaired radms is directed forwards. At the side is the median portion of the ventral sur- face. O, mouth ; A, anus. is BIVIUM — TRIA'IUM. 209 while, in Spatangidce, both poles, or only the oral pole, are shifted in the direction of the unpaired radius, and become eccentric (%. 208). Only a few of the regular Echinodermata have the means of loco- motion on all the five rays, and seldom then along the whole length of their meridians; far more frequently the area surrounding the oral pole becomes with regard to the position during movement the ventral sui-face ; it is flatteued and mainly or entirely possesses the organs of locomotion {com- bulaci'cd surface). These re- lations always obtain among the irregular Echinoder- mata which do not move indifferently in the direction of all five rays, but princi- pally in that of the unpaired one. In these animals the mouth, and therewith the oral pole, being pushed to- Avards the anterior edge, the two posteiior radii (biviiom) seem principally concerned in the formation of the ventral surface [Spa- tangidoi). It is otherwise in the case of the cylindrical Holothurians. Their mouth and anus preserve the nor- mal position at the poles of the elongated axis, and the body is not unfrequently compressed in the direction of the axis in such a manner that three radii {trivium) with their organs of locomotion are placed on the foot-Uke ventral surface. On the body of these Holothurians one unpaired and two paired radii can be distinguished, only in this case the unpaired radius Avith its inter-radius marks, not the anterior and posterior, but the dorsal and ventral surfaces. In many Echinoderms (Echinoidea) the oblate spheroidal form is the prevalent one. The principal axis appears shortened, the apical pole may be either pointed or flattened, and the ventral half is Fig. 209. — Cum mar la witli esteiided dendritically branched tentacles {TJ. Af, ambulacral feet. 270 ECIIINODEIIMATA. Fig. 210.— Calcareous bodies from the integument of Holothu- rians. a, calcareous wheels of Chirodota- h, anchor vnfh supporting plate of Synapta ■ c, chair-like bodies ; d, plates of Holothtria Impatietis ; e, hooks of Chirodota. flattened out to form a more or less extended surface. The cylin >nQ «^ {lIololkuroLa) (fig. 209), the round form by a shortening of the same and the penta- gonal disc by the latter process combined with a simultaneous elonL..- tion of the radii. If the radii are elongated till they are two or more times the length '^^^ of the inter-radii, the form takes the shape of a star (Asteroidea), which may be either flat or arched. The arms of the star maybe simple processes of the disc, and en- close a part of the body cavity (Stel- leridea, Star-fish), or they may be more independent moveable organs sharply marked off from the disc, and as a rule simple {02ohiurid(e), but sometimes branched {Euryalidce), or they may even bear simple jointed side twigs, the pinnulcn (Crinoidea). An important characteristic of the Echinodermata is the indura- tion by calca- reous deposits of the deeper layers of the integument (dermal con- nective tis- sue), so as to give rise to a solid more or less moveable or even im- moveable ar- mour. In the leathery iTo^o- thuroidea (fig. 210) alone these skeletal structures are confined to isolated calcareous bodies, which are embedded in the integument, and have a definite form of latticed plates, wheels, or anchors. In these Fig. 211. — Skeletal plates of Astropecten Hemprichtii (after J. Hiiller). BR, dorsal marginal ossicles ; VJt, ventral marginal ossicles ; Ap, ambulacral ossicles ; Jp, intermediate interambulacral ossicles ; Adp, anterior adambulacral ossicles projecting into the mouth. EXOSKELETON. 271 cases the clermal muscular system is strongly developed, and has the form of five pairs of bundles of longitudinal muscles, external to which is a continuous layer of circular muscular fibres covering the internal surface of the integument. In the Star-fishes and Brittle-stars a moveable dermal skeleton is formed on the arms consisting of calcare- ous masses {amhulacral ossicles), connected together like vertebrte, while the integu- ment of the dorsal surface is filled with calcareous plates, and bears projecting processes and spicules (fig. 211). The exoskeleton in the Sea-urchins is immoveable. It consists of twenty meri- dional rows of solid calcareous plates immoveably connected together by their edges so as to form a firm shell, which is continuous except at the two poles, where it is interrupted by membranous structures. The rows of plates are ar- i"anged in two groups, each with five pairs ; of which the one group is radial in position and consists of plates pierced by the pores for the exit of the ambulacral feet {ctmhulacral 2)lates, fig. 212) ; the other be- longs to the inter-radii, and the plates are unpierced {the interambidacral plates, fig. 206, B, J). Near the apical pole, which in the Crinoidea and the embryonic Echinoidea is occupied by a single plate (central plate), there is, in the Sea-urchins, a small area covered with minute calcareous plates and containing the anus. Around this area the five ambulacral and the five interambulacral rows terminate, each in a pentagonal plate; the foi^mer ending in the smaller radial ocular plates (fig. 206), the latter in the larger inter-radial genital plates. The Crinoidea, in addition to the dermal skeleton of the disc, possess a stalk, which is composed of pentagonal calcareous masses, arises from the dorsal side of the body, and becomes attached to firm sur- rounding objects. Amongst the appendages of the dermal armour, the numerous and variously shaped spines and the pedicellarise must be mentioned. Pig. 212. — Third ambulacrum of a youno: Toxopneustes droe- backensis of 3 mm (after Loven). Opj Ocular plate ; P, primary plates and tentacle pores. The sutures of the primary plates are visible on the plates ; Sw, the tubercles to which the spines are articulated. Fig. 213.- Pedicella- ria of a Lnioddang (after Per- rier). 272 l£ClIINODEnMA.TA. The former are moveably articulated to the knobbed tuljercles on the shell of the Sea-urchin, and are raised and moved laterally l)y special muscles develo})e-8ha'pe,d Echinodermata with segmented arms fi- nished with jnnnulce. They are ttsually attached hy a seyrmnted calcareous stalk. The skin upon tlie ahoral side is provided with plates, the ambulacral appendages Imve the form of tentacles, a'nd are situated in the ambulacral furrows of the calyx and of the segmented arms. The greater number of Crinoidea are cha- racterised by the pre- sence of a segmented stalk bearing cirri. This stalk arises from the apical (dorsal) pole of the calyx, and is attached at the in- ferior end to surround- ing objects (fig. 2.31). In some few Hving genera, as Comatida (fig, 232) and Actino- metra, this stalk is only present in the young form. The body with the contained viscera appears, therefore, as the calyx at the upper end of the stalk, and only in exceptional cases is directly * J. S. Miller, " A Natural History of the Crinoidea or Lily-shaped Animals." Bristol, 1821. J. V. Thompson, " Snr le Pentacriniis Europfeus, I'etat de jeunesse du genre Comatula," L'institut, 1835. J. Miiller, " Ueber den Bau von Pentacrinus caput Medusfe," Ahltaiidl. der Bcrl. Aliad., 1841. J. Miiller, "Ueber die Gattung Comatula und ihre Arten," Abliandl. der Berl. Aliad., 1847. Leop. v. Buch, "Ueber Cystideen," Abhandl. der Bcrl. AMd., 1844. Ferd. Eomer, " Mouograpbie der fossilen Crinoideen familie der Blastoideen," Fi&. 231. — Pentacrinus caput Meiusce (after J. Miiller). O, mouth ; A, anus, of the disc, which is represented, from the oral side. CBLNOIDEA. 287 attached by its dorsal apex. The segments of the stalk, which are mostly pentagonal, are connected by bands of tissue, and are pierced by a central canal, which serves for nutrition, and contains a central and five peripheral blood vessels ; at certain distances they bear hollow and segmented ciiii, which are arranged in whorls. The dorsal surface of the calyx is covered externally by regularly arranged calcareovis plates, while the upper (ventral) surface, on which the mouth and anus are situate, is clothed with a leathery Fig. 232— Comatula mediterranea represented from the ventral side. O, mouth ; A, anus. The pinnulse are filled with the generative products. skm. At the margin of the cup there arise movable, simple or forked, and often branched arms, which are supported by a solid , framework consisting of dorsally placed calcareous plates, which are movable upon one another by special muscles. In almost ' f^'f^i-/'"' ^^aturgesch, 1851. W.Thompson, "On the Embryology of the Antedon rosaceus," PM. Trans. Ron. Soc, Tom 155, 1865. W. B. Carpe^lter Re.search(^ on the Structure, Physiology and Development of Antedon Pn^fl' li-: ^'"^ "^^'"gl- Entwickelungsgeschichte der Oomatula Mediterranea," Archlv. fiir mickrosli. Aimtomic, Tom XII H l^udwig, " Morphol. S^tudien an Echinodermen," Leipzig, la77. I 288 ECHINODEEMATA. every case the arms bear, either- on their main stems or on their branches, lateral appendages, the j)innule8, which have an alternate arrangement on each side, one being attached to each segment of the arms. Essentially the pinnules represent the ultimate ramifications of the arms. • The mouth, as a rule, lies in the centre of the cup. From it certain furrows, the amhulacral grooves, traverse the disc (fig. 231) c Fig. 233.— Developmental stages of Comatula (Antedon), much enlarged, a, free-swimming larva vnth tuft and rings of cilia {Wr), also \vith rudimentary calcareous plates, h, At- tached Fentacrmoid form of the same animal. O, OraUa; R, Radialia ; B, BasaUa; Cd, Centrodorsal plate, c, Older stage described as Pentacrimis euvopaev.» with arms and cirri (after Thomson). and pass on to the arms, and their branches and pinnules; they are lined by soft skin, and carry the tentacle-like ambulacral appendages. The anus, when it is present, lies excentrically on the ambulacral (ventral) surface of the disc. The development of the living genus Comatula, which begins with a barrel-shaped larva with four bands of cilia and leads to the fixed stage of the Pen- CRI>'OIDEA. 289 tacriniis form (P. Europcetbs) (lig. 233), consists of a complicated metamorphosis. The greater number of Ci'inoids belong to the oldest periods of the history of the earth (the Cambrian, Silurian, Devonian, a,nd the Carboniferous formations). Existing forms live mostly at considerable depths. AVe distinguish two orders, the Tesselata and the Articulata. The latter is represented by numerous fossil forms, but by only a few living genera as Penta- crimts, IIolopus, and Gomatida (fig. 234). The cup is always less completely provided with plates than in the fossil Tesselata. Articulata. Fain. Pentacrinidae. Crinoids with ten arms, several times bifurcated. There is a pentagonal stalk with whorled cirri. PejLtacrinus caput Mfdusce, Mill, from the Antilles. P. Miillcri Oerst., West Indian Ocean. The fossil forms are : Encrinits liUifornm Schl. (fig. 234) fi-om the Muschelkalk ; also Ajnocrinus, allied to the existing Rhtzocrinns lofotensis Sars, and to Batltycrinus gracilis, and aldHcJiiauns W. Th., from the deep sea. Allied to this group is the third existing genus Bolojms, from the West Indies, with calyx attached by a short unjointed prolongation of its apex. H. Rangii d'Orb. Fam. Comatulidae. Stalked only in the young state. The adult animal is free. There are usually ten arms at the margin of the flattened body ; mouth and anus are present. tulidxe possess the power of striking their arms towards the ventral surface and so of propelling themselves amidst the sea-weeds. The vermiform lai-va, with its four ciliated girdles, makes its appearance within the egg-membranes. It acquires a mouth and anus, also a tuft of cilia at the posterior end of the body, and swims about freely. It passes later, by the formation of cal- careous rings and rows of plates, into the stage of the stalked Pentacrimis , from which the Comatula is produced by the separation of the cup from the stalk. Comatula meditcrranea Lam., Antedoti rosacea Link., known in the young attached stage as Pentacrimis Phironaem. Actinometra J. Miill. To the Crinoids are allied the fossil Cjixtidca and Blastoidea. The Cystidea were provided with short stalks and slightly developed arms. Their generative organs were enclosed in the calyx, whence their products escaped through a genital opening capable of being closed by movable valves. They are found as fossils in the Cambrian, Silurian, and Devonian formations and the Carboni- ferous limestone. To this group belong the genera SpUerowitcs, Carvocrims Apiocijstites. ' The Blastoidea have no arms, and only possess ambulacral areas on the calyx, which is attached by a segmented column. Pentatronatites. 39 Fig. %'34i.—Encrinus Ulii- formis from the Mus- chelkalk. The Coma- 290 ECHrNOBEEMATA, CLASS II.— ASTEROIDEA (STARFISHES).* Echinoderms with dorso-ventrally comjyreased 2J6ntayonal or star- shcq^ed body. The amhulacral feet are confined to the ventral mrface. Internal skeletal jneces in the ambulacra articulated toyether like vertebra}. The Star-fishes are chiefly characterised by the ])redominating pentagonal or star-like discoidal shape of the body, to the ventral Fig. 235. EcUnatter senfns, from the oral surface (after A. Agassiz). O, mouth: Af, ambulacra! feet. surface of which the amUilacral feet are confined (fig. 235). The radii are long in comparison with the inter-radii, which are very short in consequence of the divergence of the interambulacral rows of plates; they constitute more or less projecting movable arms, with movable skeletal structures. These latter consist of trans'versely arranged, paired calcareous plates (ambulacral ossicles), * J Miiller and Troschel, System cLer Asteiiden," Brunswick, 1841 Com- pare besides the numerous papers of Krohn, Sars, Llitken, L. Agassiz, etc. ASTEEOIDEA. 291 which reach from the mouth to the end of the arms, and are articulated toofether like vertebrae. The skeleton of the Aster oidea is distinguished from the globular or flattened shell of the Echinoidea by the fact that the ambulacral and interambulacral plates are confined to the ventral surface, and that on the outer side of the former there is a deep ambulacral groove, which contains, outside the ossicles and beneath the soft skin (which in Ophiurids possesses special calcareous plates), the nerve trunks, the peri- haemal canals with the blood-vessels and the ambulacral trunks. In the Ophiuridea the ambulacral groove is covered by the dermal plates so that the ambulacral feet project at the sides of the arms. Upon the dorsal surface the dermal skeleton appears leathery; it is, however, as a rule, filled with small calcareous plates, on which are placed spines, protuberances, and papillae, constituting a covering of a most varied kind. At the mar- gin in the dorsal integu- ment there is usually a row of larger cal- careous plates (suj)erior mar- ginal plates) (fig. 23 6). Upon the ven- tral surface we can distin- guish, in addition to the internally placed ambulacral ossicles, inferior marginal ossicles (fig. 236, VR), also the adamhulacral plates (Adp), and the intermediate interambulacral plates {Jp). The two last corre- spond to the interambulacral plates of the Echinoidea, ^frhexe they occur as two or more rows, which are united along the whole length of the inter-radius : in the Asteroidea, however, they separate from one another at an angle, and are disposed along the opposed sides of ad- jacent arms. The ambulacral ossicles are calcareous bodies articulated together like vertebrae, with spaces between their lateral processes for the passage of the vessel connecting the ampullte with the radial vessel and the tube feet. The right and left pieces of each double row are either (Ophittridea) immovably connected by a suture, or are Fig. 236.— Skeletal plates of Astropecten Hemprichtii (after J. Miiller). DS, Dorsal marginal plates; VB, ventral marginal plates, Ap, am- bulacral ossicles; Jp, intermediate interambiilacral plates; Adp, anterior adambnlacral plates forming an angle of the month. 292 ECILINODEJIMATA. {Stelleridea) movably articulated by teeth, which iit into one another at the bottom of the ambulacral groove ; the latter only (Stelleridea) possess transverse muscles on the ambulacral ossicles, and are able to bend their arms together towards the ventral surface. The Ophiiiridea are provided with longitudinal muscles only, by means of which they are able to bend their arms to the right and left in a horizontal plane with a serpentine movement. The mouth is always placed in the centre of the ventral surface in a pentagonal or star-shaped depression, the edges of which are usually beset with stili" papillte. The inter- radial angles are marked by the junction of two adambulacral plates, and frequently function as organs of mastication. The anus may be wanting ; when present, it invariably lies at the apical pole. The madreporic plate, of which there may be one or more, is situated inter-i-adially and dorsally [Stelleridea), or on the inner surface of one of the buccal plates [Ophiuridea), on the exterior of which a pore may be present. Development in certain cases takes place without the interposition of a bilateral larval phase with bands of cilia. When such larvae are developed, they have the form of a Pluteus (Ophiurid) or Bipin- naria and Brachiolaria (Stellerid). * The great power of regeneration possessed by Starfishes is not confined to the reproduction of lost arms, but may lead to the new formation of portions of the disc, or even of the entire disc from a single separated arm. This process thus amounts to a species of asexvial reproduction by fission, and has been especially observed in forms with six (OpJdactis) or more than six [Linckia) arms. Fossil star-fishes are found as far back as the lower Silurian strata (Palmaster), where intermediate forms between Stelleridea and Ophiuridea (Protaster) make their appearance. Sub-Class 1. — Stelleridea {Asteridea) Starfishes. Asteroidea whose arms are prolongations of the disc, and contain the hepatic appendages of the alimentary canal, and also the generative organs. They possess a deep, uncovered ambulacral groove running along the ventral surface, in which groove the ambulacral feet are disposed in rows. The Stelleridea usually possess broad arms, and are characterised by the fact that the ambulacral ossicles of the two sides are connected by transverse muscles and are movable upon one another. The amis lies at the aboral pole, but may be wanting in certain genera (Astropecten). The madreporic plate and the genital pores are STBLLEEIDEA. 293 situate inter-radially and upon the dorsal surface. The multilobed branched diverticula of the stomach extend into the cavities of the arms (fig. 218). On the ventral surface of the latter, two or four rows of ambulacra] feet project from the deep ambulacral groove, the edge of which is beset with papillte (fig. 235). Pedicellarice are also found, and dermal gills projecting through the tentacular pores of the dorsal surface. They feed principally upon Mollusca, and, by means of their ambulacral feet, crawl slowly upon the bottom of the sea. Some few of them are developed by a very simple process of metamorphosis within the brood-pouch of the mother ; but the greater number of them pass through the free larval stages of Bipinnaria and Brachiolaria (figs. 224 and 228). Fam. Asteriadae. The cylin- drical ambulacral feet end in broad suctorial discs, and are usually arranged in four rows along each ambulacral groove. Asterias L. QAsteracanthion), A. fjlacialis 0. F. Mliller., He- liaster helianthus Gray. Fam. Solasteridae. The cylin- drical ambulacral feet are dis- posed in two rows. Bays long, often more than five Solaster 2)appoiim Retz., EcMnaster sepositiis Retz., Oj?hidiaste7- Ag., Linchia Nardo. Fam. Astropectinidae. Am- bulacral feet conical, and with- out suctorial disc, arranged in two rows. There is no anus. AstrojJecteii aurantiacus Thil. LvMia Forb., Ctcriodiseits Miill. Tr. Fam. Brisingidae. Body shaped like an Ophiurid. Rays distinct from the disc with only a narrow internal cavity. Brisituja coronuta Sars. Sub-Class 2. — Ophiuridea {Brittle Stars). Asteroidea characterised by the absence df an anus, and by the 2)08- session of long cylindrical arms which are sharply distinct from the disc, and do not contain appendages of the alimentary canal. The ambulacral groove is covered by the dermal plates so that the ambulacral feet p)Tqject at the sides of the arms. The Op>hiuridea can be at once distinguished by the flexible cylindrical arms, which are sharply distinct from the disc, and enclose 294 ECHINODEllMATA. no diverticula of the alimentaiy canal. Tlie movements of the arms are principally in the horizontal plane, and in many cases permit of a cx-eepmg locomotion amongst marine plants. The ambulacral groove is always covered by special dermal plates, and the ambulacral feet project laterally between the spicules and plates on the upper surface (tig. 238). In a few cases the arms are branched, and can be rolled up in the direction of the mouth. In such cases the ventral groove IS closed by a soft skin {Astrophyton). The anus is always wanting, as are pedicellariai. The generative products pass into genital pouches (bursse), and from these directly to the exterior through inter-radial paired slits. The madreporic plate lies upon the ventral sur- face in one of the buccal plates. Some few Ophiu- lids are viviparous, e.g., Am2)hiura squar)iata; these do not undergo metamor- phosis. Most pass through the Phdeihs larval stage, e.g., 02)hiogly2)ha Lym., {Pphioleins) ciliata with larval stage Plutem paradoxus. Fig. 238. — Ophiothrixf raff His. The ends of the rays have been removed. O-S, Slits of the genital pouches ; jE", masticatory ossicles. Fam. Ophiuridae. With simple unbranched arms, and with ventral plates to the ambulacral groove. They are divided into special genera according to the peculiar character of the dermal covering and of the buccal armature. Ophiothrix Miill. Tr. The back is provided with granules, hairs, or spicules. The lateral plates of the arms bear spicules. 0])h. fnujilis 0. Fr. Miiller. Ophiura Lam. QOjihioderma). Two pairs of genital slits in each interbrachial space, 0. lontjicauda Link., Uphwlejns Liitk., Am2)hmra Forb. Fam. Euryalidse. Mostly with branched arms which can be curved towards the mouth and are without plates ; the ventral groove closed -with soft skin. Astropiliyton vernt cumin Lam., Indian Ocean. -1. arhorescoi-s Rond., Mediter- ranean. Asteronijx Loveni Mlili. Tr, CLASS IIL— ECHINOIDEA,* SEA-URCHINS. Spherical, heart-shajied, or disc-shaped Echinodernis with immovable skeleton composed of calcareous plates. The skeleton encloses the body * Besides the works of J. Th. Klein, compare E. Desor, "Synopsis des Echinides fossiles," 18.54 to 1858. S. Lov6n, " Etudes sur les Echinoiddes," Stockholm 1874. Al. Agassiz, "Revision of the Echini," Cambridge, 1872- 1874. ECHINOIDEA. 295 after the manne)' of a shell, and carries movable spines. There is invariably a mouth and anus, and locomotive and often respiratory ainbidaa-al appendages. The dermal skeletal plates are connected together so as to form a firm immovable shell, which has no arm-like prolongations in the dii-ection of the rays, and is sometimes regularly radial, sometimes irregular or symmetrical. With some rare exceptions among the fossil Ferischcechinidce, as Lejndocentrus, the calcareous plates are firmly connected with one another by sutures, and are usually arranged in twenty meridional rows. These rows (fig. 206) are disposed in pairs, and correspond alternately with the radii and the inter-radii. The five radial pairs are the ambulacral plates, and are pierced by rows of fine pores for the exit of tube feet (fig. 212, P), and bear, as do the broad interambulacral plates, spherical promi- nences and tubercles to which the differently shaped spines are movably articulated. The body form of the Sea-urchins, as con- trasted with that of the Star-fish, depends upon the meridional arrangement of the rows of plates, and, at the same time, on the continuity of the interambulacral rows. The position of the nerves and ambulacral vascular trunks beneath the skeleton is the special characteristic of the internal organization. Fedicellarice are found between the spicules, and are especially numerous in the region of the mouth. Some Cidaridea are provided with branched respiratory tubes. The genital pores are disposed inter-radially on the genital plates near the apical pole. One of these genital plates is, as a rule, also the madreporic plate. The ocular plates, which are radial in position, are also pierced (figs. 206, 212). The regular Sea-urchins are often symmetrical, one radius being longer or shorter than the others, which are equal to each other. So we find long oval forms which are laterally symmetrical, having the mouth and anus central, and an anterior unpaired radius [Acrocladia, Echinometra). In the irregular Sea-urchins the anus is thrust away from the apical pole into the unpaired radius {Clypea- stridce). The mouth also often has an eccentric position in front [S2'>atangidce, fig. 208), in which case the masticatory apparatus is always wanting. In many regular forms all the ambulacral feet have the same shape, and are provided with a suctorial disc supported by calcareous bodies; in others the dorsal feet are unprovided with a disc, and are pointed and often have an indented edge. In addition to the ambulacral feet, the irregular Sea-urchins almost all possess ambu- 296 EOHINODBBMATA. lacral branchias upon a rosette formed of large pores on the dorsal surface (fig. 239). The locomotive feet are very small in Clypeaslvido' and are distributed either over the whole surface of the ambulacra', or are confined to branching rows upon the ventral surface. In the i^patamjidce there are peculiar bands upon the upper surfao.^, the faacioles or semitce (fig. 239), upon which, in place of the spicules, knobbed bristles with active cilia {clavulai) are distributed. Develop- ment takes place with a Pluteus lai-val stage, in whicli the larva is provided witli ciliated epaulettes or with an apical rod. The Sea-urchins live, as a rule, near the coast, and feed on molluscs, small marine animals, and Fuci. Some species of Echinus have the power of boring holes in the rocks in which they live. We find many fossil shells, especially in the chalk. Fig. 239. — Brissopsis hjrxfera with the fascicles or Semites surround- ing the rosette. A, aaus. Order 1. — Cidaridea= Regular Sea-urchins. Echinoidea with central mouth and equal hand-like ambulacra ; with teeth and masticatory apparatus ; with sub-central anus in the apical space. Fam. Cidaridae. With very narrow ambulacral and broad interambulacral areas, on both of which are large perforated tubercles and club-shaped spines. There are no oral branchias. Cidaris metnlar ia Lam., Phyllacantlms imperialis Lam., East Indies. Fam. Echinidae, Sea-urchins. The pores are grouped in transverse rows ; there is a round, thin shell, broad ambulacral spaces bearing tubercles and spines, which are mostly short and pear-shaped. Oral branchi^ are present. Tn.roj)- neustes variegatuf', Lam., Echinus melo Lam., Strong nlocentrotus Uvidm Brit. saxatilis Lin., Mediterranean. Fam. Echinometridae. With long oval shell, imperforate tubercles and oral branchiffi. Echimmetra, obloiuja Blainv., Podophora atvata Brdt.. Aeroclndia trigimaria Ag., Pacific. Order 2. — Clype astride a. Irregular Echinoidea comjjressed into the form of a shield. Mouth central and furnished with masticatory apparatus. Very broad ambu- lacra, five-leaved ambidacral rosette round the apical pole, and very HOLOXHUEOIDB A . 297 S7nall tube feet. Five genital pores in the region of the maclreporic plate. Fam. Clypeastridae. The edge of the disc withoiit indentations. Clijijcaitter rosacmx Lam. (fig. 207), EcMmwyannis 2nislllus 0. F. Miiller, Mediterranean. Fam. Scutellidse. Flattened EcMnoidca with a shell often lobed or per- forated, \\'ith rows of pores for the arhbulacral feet. Lolojjliova Infora Ag., Jtotula Rum2)hii Klein, Afriea. Order 3. — Spatangidea. Trregxdar Echinoidea of a more or less heart-sha2Jed form, ivith eccentric onoiUh and anus. There are no teeth or masticatory apixiratus, and there is usually a four-leaved amhulacral rosette and four genital plates. As a rule there are semitfe and four genital pores, but the number of the latter may be reduced to three and two. Fam. Spatangidae. Spataiuins imrimrens 0. Fr. Miill., Mediterranean ; Scliizaifter cancdxferus Ag., Brissvs Klein, CLASS IV.— HOLOTHUEOIDEA.* Wormlike elongated Echinoderms with a leathery body wall, with contractile tentacles surrounding the mouth ; anus terminal. The Holothuria approach the worms in possessing an elongated cylindrical shape and a bilateral symmetry, which is expressed in many ways. In particular they possess so striking a resemblance, so far as their exterior is concerned, to many Gephyrea that formerly they were included in the same group. The body-covering never consists of a firm calcareous shell like that which we find in other Echinoderms, but always remains soft and leathery, the calcareous matter being confined to a few isolated particles of definite form. In rare cases {Citvieria), scales are present in the dorsal skin. These are arranged like the slates on a roof, and may even take the form of spicule-like appendages (Echinocucumis). The bilateral symmetry results not only from the existence of un- paired organs, but from the contrast which is often very distinctly expressed between the dorsal and ventral surfaces. The ambulacral feet are not in all cases regularly arranged in the five meridional * G. J. Jaeger, " De Holothuriis," Dissert, inaug. Turiei, 1833. J. F Brandt ' Prodromus descriptionis animalium ab H. Mertensio in orbis terrarum circumiiavigatione observatorum," Fasc. I. Petropoli. 1835. J. MuUer » Ueber Synapta digatata und liber die Erzeugung von Schnecken in Holothurien " Berlin, 18.52 A. Baur, " Beitrage zur Naturgeschichte der Synapta digitata.'" Dresden, 1864. C. Semper, " Reisen im Archipel der Philippinen " Tom f Leipzig, 1868. ' 298 KCIIINODEIIMATA.. rows from the oral to the anal pole, but may l)e principally or altogether confined to the three rays of the so-called trivium. In this latter case the Holothurid moves upon a more or less foot-like ventral surface. The ambulacral feet may also be distributed uni- formly over the surface of the body, especially on the ventral side. As a rule, the tube-feet have a cylindrical shape, and terminate with a suctorial disc : in other cases tliey are conical, and the suctorial disc is absent. The tentacles, which are in communication with the water-vascular system, and represent specially modified amlnilacral appendages, are simple or pennate, or even dendritic {Dendrochirota) or shield-shaped {Asjndochirota), that is, provided with a disc, which is often divided into many parts. In certain genera {Synafta), the ambulacral feet are altogether wanting, and the tentacles re- main as the sole appendages of the ambulacral system (fig. 240). Locomotion is effected by the strongly developed dermal mus- cular system, the longitudinal fibi'es of which are attached to the calcareous ring surrounding the oesophagus. It is charac- teristic of the water-vascular system that the stone canal, which is usually simple, hangs freely in the body cavity, ending in a calcareous framework com- parable to the madreporic plate. Fig. ^aa.—Synapta inhmrenn (after Quatre- The respiratory trees at the end fages). 0, Mouth ; A, anus : the intestine ^j^g intestine perform the f uuc- can be seen through the skin. . . tion of res2nrah07i, while cei-tam glandular appendages (organs of Cuvier), which open into the rectum, may be regarded as excretory organs : these, as well as the respiratory trees, may be wanting. The generative organs consist of a bundle of branched tubes, the duct of which opens on the dorsal surface in the region of the mouth. The genus Synapta is hermaphrodite. The development is in many Holotlmrians direct (as e.g. in Holothuna tremula according to Koren and Danielssen) ; where there is a com- plicated metamorphosis, the larvaj have the Auricularia form, and pass through a barrel-shaped pupa stage. HOLOTHUKOIDEA. 299 The Holotlmrians are partly nocturnal in their habits, and live at the bottom of the sea, for the most part in shallow places near the coast, where they crawl slowly upon the bottom. The Synrq^tidce, which have no feet, burrow in the sand. They feed on the smaller marine animals, which, in the Dendrochirota, are carried to the mouth by means of the branched, tree-like tentacles. The Asjndo- chirota fill their intestine with sand, which they eject from the anus by means of the current of water from the respiratory trees. It is worthy of notice that they (especially the As2ndochirota) can eject through the anal opening the intestine, which breaks off easily behind the vascular ring, and are able to renew it. The Synaiota, when iri-itated, are able to break their body into several pieces by violent muscvilar contractions. Order 1. — Pedata. Numerous cmxbulacral feet, which are sometimes arranged regularly in the meridians, and sometimes distributed over the whole surface. Fam. AspidocMrotse. With shield-shaped tentacles. Ilolotlmvia L. With scattered ambulacral feet, which are conical on the dorsal side, and are without suckers. H. twhiiloaa Gmel., Adriatic and Mediterranean ; II. edvlu Less., the edible Trepang of the East Indian seas. Fam. Dendrochirotae. With tree-like branched tentacles. Cueumaria Blainv. Ambulacral feet arranged in regular rows. C. frondosa Gr. Psolvs Oken. Ambulacral feet confined to the foot-like ventral surface of the trivium. Ps. ■phantajms. Gr. Order 2. — Apod A. No ambulacral feet ; as a rule without respiratory trees ; the tentacles are usually branched or jnnnate. Fam. Synaptidse. Hermaphrodite and without respiratory trees. In the skin there are wheel-shaped calcareous bodies or projecting masses shaped like anchors, and affixed to calcareous plates. Synajrta dirjituta Mntg. with anchor- shaped calcareous bodies. J. Miiller discovered in their bodies parasitic cylin- drical animals with spermatozoa and ova, which latter develop into small sheU-bearing Gastropods {Entoconclui mlramis). Chirodota Esch. Skin beset with rows of small tubercles bearing calcareous wheel-shaped bodies. The genus M(>lj)adut Guv. is furnished with respiratory trees. ENTEEOPNEUSTA. The remarkable genus Balanoglossus must be placed here. It is the representative of a class, Enteropneusta Gegenb.,* allied to the Echinodermata, but usually classed with the Vermes, and presenting * A. Kowalevski. " Anatomie des Balanoglossus Delle Chiaje," Memoires de I Acad. inij)^r. des ."Sciences de St. Pctcrsbuurg, Tom X., No. 3, 1866. L. Agassiz, 300 ENTEllOPNEUSTA. an affinity to the Tunicata by the mode of respiration. Tliis in- teresting worm was discovered by Delle Cliiaje, and its organization and development have been recently investigated by Al. Agassiz and Kowalevski [more recently by Bateson, Q. J. Mic. Sci. 1884] (fig. 241). The most inte- resting point about this form is the structure of its larva, which renders its relationship to the Echinoderniata probable. The larva was described by J. Miiller as Tornaria, and was legarded by him as an Echi- noderm larva. It Fig. 241.— Young BaUnwglo^xus, strongly magnified. Pr, Pro- (JoeS, in fact, pOSSeSo boscis, the numerous branchial slits are visible. i' i i i a double band of cilia, like Bipinnaria. Of these two rows of cilia, one, the praeoral, forms a ring round the prte-oral lobe, while the other is larger and runs in a more longitudinal direction so as almost to join the former near the apical pole. There is also a transverse pr£e-anal ring of cilia (fig. 242, a, h). Internally a diverti- culum of the ar- chenteron gives rise to an independent sac forming the water-vascular sys- tem, while two Fig. 242, (f,i. — roiviar/V/ larva (after E. MetschnikofE). o, Seen • _ c A' +" 1 from the side ; b. from the dorsal surface. O, mouth ; A, P^^'ll'S Ot dlveitlCUia anus ; S, apex, W, rudiment of water vascular system ; C, furnish the first heart ; P, F', peritoneal sacs. . . , o .-i rudiments or tne body cavity. A pulsating heart is developed from a thickening of "The History of Ealunoglossus and Tornaria," 3Icmoir. ectoparasites on the external surface of the body, and feed on the juices of their hosts. Others live free in damp earth, or in mud ; others, and these are the most highly orga,nized forms, inhabit fresh and salt water. TUBBELLARIA. 309 CLASS I.— PLATYHELMINTHES. Vermes icith a flat, more or less elongated body, loith cerebral gan- ylion. They are often -provided loith suckers and hooks, and are usually hermajyhrodite. The series of forms included under this class are mostly Entozoa, or else live in the mud and beneath stones in the water. In their organization they occupy the lowest place among the worms. Their body is more or less flattened, and is either unsegmented or is divided by transverse constrictions into a number of successive divisions, which, although forming parts of one animal, yet have a strong tendency towards individualisation, and frequently attain to separa- tion and lead an independent life. These segments are products of growth in the direction of the long axis of the body, and stand in a special relation to reproduction. They are by no means to be con- sidered as necessarily indicating a high grade of organization, as does the segmentation of the Annelida. The alimentary canal may be altogether wanting (Cestoda), or, if present, may be without an anus {Trematoda, Turbellaria). The nervous system is usually composed of a double ganglion above the oesophagus, giving off small nerves anteriorly and laterally, and two stems backwards. In many Platyhelminthes simple eye-spots occur, either with or without refractive bodies, and more rarely there is an auditory vesicle. Blood-vessels and organs of respiration are found only in the Nemertinea. The excretory (water vascular) system is everywhere developed. With the excep- tion of the Microstomidoi and Nemertinea, hermaphroditism is the rule. The female generative glands consist of distinct yolk-glands and ovaries. The development very frequently takes place by a very complicated process of metamorphosis connected with alternation of generations. 07'der 1, — Turbellaria.* Free living Platyhelminthes icith oval or leaf-shajjed body, xoith soft skin covered loith cilia. They 2Jossess a mouth and aproctous * Duges, " Rccherches sur rorganisation et les mceiu-s cTe Planaires " imi dex S<} Nat., Ser I., Tom XV. A. S. Oerstedt, " Entvvurf einer systematischen J<.mtheilung unci speciellen Beschreibung der Plattwiii-mern." Copenhagen 1844 De Quatrefages, Memoire sur quelques Planaviees marines," Aim del ;r Schultze, " Beitragc zur Naturgeschichte der Turbellarien " Greifswald, 1851. L. Graff, " Zur Kenntniss der Turbellarien," Zcitsckriif tiir W,ss<. Zool., rem XXIV. L. Graff, " Neue Mittheilungen iiber Turbellarien " ZnUc-h f Ki^s. Zool., XXV., 1875. P. Hallez, "Contributions a I'histoire naturellc des Tm'bcllari6s," Lille, 1879 "i^Lone 310 PLATY1[J5LMINTIIKS. alimentary canal. Hooks and suckers are absent. A cerebral yanglion is prese7it. The Tarbellana usually possess an oval flattened body, and reach only a small ske. The uniform ciliation of the body is connected with their existence in fresh and salt water, beneath stones, in mud, and even in damp earth. Only in exceptional cases do we meet with apparatuses for adhering, viz., small hooks and suckers. The skin consists of a single layer of cells, or of a finely granular layer containing nuclei, which is sup- ported by a stratified basal membrane, and covered externally by a special homogeneous membrane bearing cil" and comparable to a cuticula. Peculiar integumentary structures, which have the form of rods or spindles, and, like the nematocysts in Cvdenterata, take their origin in cells, are not unfre- quently present. Various pigments are also often found embedded in the epi- dermis, and of these pigments the green- coloured vesicles, in Vortex viridis for example, which are identical with chlo- rophyl corpuscles, are specially worthy of remark. Pear-shaped mucous glands are also present. Beneath the conspicu- ous basement membrane which supports the epidermis lies the dermis. It con- tains the strongly developed derma) muscular system embedded in a connec- tive tissue layer formed of round, often branched cells. A body cavity betwee the body wall and the alimentary cana is, as a rule, absent ; it may, however in many cases be recognised as a syste" of lacunje, or as a continuous cavi surrounding the alimentary canal. The nervous system consists of two ganglia connected by a com missure, and giving ofF nerve fibres in various directions; of thes two especially large lateral trunks run backwards, one on either sid (fig 247) The latter are connected at regular intervals by delicat transverse trunks. In a number of dendroccBlous Turbellarians Fig. 247. — Alimentary canal andner- vons system of Mesostomum Ehren- bergii (after Graff). G, the two cerebral ganglia with two eye spots; St, the two lateral nerve trunks ; D, alimentary canal with mouth and pharynx. TUllBELLARIA. 311 diverticulum of the stomach runs forward above the transverse commissure in a groove between the two cerebral lobes {Leim->lana). In some i?enera of Planarians, a ring-shaped double commissure] has been shown to exist in the brain {Polycelis), and ganglion-like swellings, from which nerves are given off, have been observed on the lateral nerve-trunks {Sphyrocephalus, Polycladus). With regard to sense organs, eye spots are tolerably widely distri- buted among the TiMlaria. They either lie in pairs upon the cerebral ganglia or are connected with short nerves given off from the latter. More frequently two larger eyes with refractive struc- tures are developed. Otocysts are but rarely present, 'e.g., in Monocelis among the Rhahdoccela a single one is present lying upon the cerebral o-anelion. The integument is without doubt en- dowed with a highly developed tactile sense ; the large hairs and stiff bristles which project between the cilia may possibly be of importance in this relation. Lateral ciliated pits, which may also be explained as sense organs, are in i-are cases present at the anterior end of the body (compare the Neviertinea). Mouth and digestive apparatus are never wanting^ but the former is frequently removed from the ventral surface of the anterior end of the body to the middle or, indeed, even to the posterior region. According to Metschnikoff and Ulianin, a stomach may in some cases be absent {Convoluta, Schizo- prorco), and be replaced, as in Inftosoria, by a soft internal parenchyma. The mouth leads into a muscular pharynx, which can usually be protruded after the manner of a proboscis. The alimentary canal, of which the intei'nal wall is freqiiently ciliated, is either forked and then simple or branched (^Dendroccela), or rod-shaped (Bhabdo- c(da). An anus is never present. A pecu.liar tube capable of being evaginated as a proboscis, and without connection with the pharynx is sometimes also present (Prostomum). The excretory (water-vascular) system consists of two transparent lateral trunks and innumerable side branches, which begin with closed ciliated funnels, and are furnished with vibratile cilia, which Fig. 248.— Microsto- mum lineare, after Graff. O, Chain produced by fis- sion ; O', mouth openings. 312 PLATYHELMINTHES. project here and there freely into the vessels. As a rule, severtil openings occur on the main trunk of this excretory apparatus. Reproduction may take place asexually by transverse fission, Derostomea (Catenula) and Microatomea (fig. 248). With the exception fof the Microatomea, the Turhellaria are hermaphrodit(^ -. but stops intermediate between the hermaphrodite and the dioecious condition seem by no means to be wanting, for, according to Metschnikolf, in Proatomum lineare the male generative organs are sometimes developed, while the female's remain rudimentaxy ; and sometimes, on the othci' hand, the reverse holds. In Acmoatomum dicecum also the sexes are separate. In the hei- maphrodite forms the male sexual organs consist of testes, which mostly lie as paired tubes at the sides of the body, also of vesi- culfe seminales, and of a protru- sible copulatory organ beset with hooks. The female organs con- sist of ovaries, yolk glands (vitellarium), a seminis, a vagina, (fig. 249). The tory organ and the vagina open as a rule by a common orifice upon the ventral surface. Some- times, as in the Rhabdocoele genus Macrostomum, the vitella- rium (yolk gland) and ovary are united; the ova being pi-oduced at the upper blind end of the ovary, and the yolk at the lower end of the same gland. In the marine Dendrocoela, on the other hand, the vitella.rium is generally absent. After fertilization, a hard, usually reddish-brown shell begins to be formed round the ovum. In such cases, the hard-shelled eggs are laid ; but among the E-habdocoela, in Schizostomum and certain Mesoatomea {M. Ehren- hercjii), transparent eggs furnished with thin, colourless capsules, and undergoing development in the body of the parent, are often produced. According to Schneider, the production of these thin- receptaculum and a uterus male copula- FiG. 249.— Generative apparatus of Mesosto- mum JElirenhergii (combined from Craif and Schneider). S, Pharynx ; Go, sexual openings; Ov, ovary; Uf, uterus, with winter eggs ; Bo, yolk gland ; Dg, duct of yolk gland; T, testis; T'd, vas deferens ; P, penis ; En, receptaculum seminis. TPBBELLAIIIA. 313 shelled or summer eggs invariably precedes that of the hard-shelled or vnnter eggs, and the summer eggs are normally self -fertilized. In rare cases the hemaphrodite generative organs present a segmentation recalling that of the Cestoda [Alaurina composita). The freshwater Turbellaria, as well as many of the marine forms, undergo a simple direct development, and in the young sta,te are often difficult to distinguish from Infusoria. Other marine Denclroccela undergo a metamorphosis, the larvfe being characterised by the possession of iingei'-shaped ciliated lobes (tig. 251). (1) Sub-order : Rhabdocoela. The body is round and more or less flat. The intestine is cylindrical, and there is usually a protrusible pharynx. They ai'e usually hermaphrodite. The Rhabdoccelous Ttxrbellarians are the smallest and most simply organised foi^ms. The intestine is cylindi-ical and elongated, and is sometimes provided with lateral diverticula. The position of the mouth varies exceedingly, and has been employed as a principal characteristic for distinguishing the various families. Sometimes saHvary glands are present, opening into the pharynx. According to Ulianin's discovery, which has been several times confirmed, the alimentary canal may be wanting in many forms, and be replaced by a central cavity, filled with a substance containing numerous vacuoles and rich in oil globules {Convohita, Schizoprora, Nadina). In those Rhabdocoela which possess an alimentary canal, interstices and spaces in the connective tissue parenchyma are often present : these must be related to a body cavity. In some cases (in Frostomiim) the body cavity may be recognised as a continuous space filled with fluid and surrounding the alimentary canal. The Rhabdocoela live on the juices of small worms and of the larvse of Entomostraca and Insecta, which they envelop with a cutaneous secretion containing small rods, and afterwards suck. They are mostly inhabitants of fresh water, and only a few of them are to be met with in the sea or upon the land {G-eocentrophora sphyrocephalcC). Fam. Opisthomidse. The mouth is placed at the posterior end of the body and leads into a tubular pharynx, which can be protruded like a proboscis. Moiiflcelis agilix M. .Sch., OjiMJiumyiii jxiUidiim. 0. S. Fam. Derostomidae. Mouth placed slightly behind the anterior margin ; pharynx barrel-shaped. Berostwmm. Sclimidtianum M. Sch., Vortex viridis, II. Sch., Qitcniila Icmncc Dug. Fam. Mesostomidse. Mouth placed nearly in the middle of the body, pharynx ringlike, cylindrical or resembling a sucker. MrsoHomum Elircnlerqii Oerst., with two eyes. Fam. Convolutidae. (^AcoeUC). Without alimentary canal. The ovaries and 314 PLATYHELMINTHE8, yolk glands arc not separate. Convohita Oerst. C. 2}aradiixa Oerst., North Sea, Baltic. Schizoprora 0. H. Fam. ProBtomidse. The mouth, which is fiituatc on the ventral surface, lead- into a muscular pharynx. At the anterior end tliere is a protrusible tactile- proboscis furnished with papillas. Prostomum Oerst. {Gyratin' Ehrbg:.). I\ llnearc Oerst. With pointed penial spine at the posterior end, imperfectly hermaphrodite, living principally in fresh water. Pr. hflr/dlandiniiii. Kof. . completely hermaphrodite. Fam. MicrostomidaB. i//tY//>r7wcBZ« with separate sexes. The small but vci y extensible mouth lies near the anterior end of the body. There are laterally placed ciliated pits near tli< anterior end of the body. Formation of metamen- and transverse fission tit - quently occur. MicroKtu- wnm lincari; Oerst. (fii:. 248). (2) Sub-order: Den- drocoela. The body Is broad and flat, and the lateral margins are often plicated. There are ten- tacle-like processes at the anterior end. There is a branched alimentary canal and a muscular pharynx which is usu- ally protrusible. They are, as a rule, herma- phrodite. The DendroccBla are mostly marine, but also live in fresh AA-ater and on land. In their ex- ternal appearance they resemble the Trema- todes, and the branching of their straight or forked intestine is a character common to the larger species of the latter. Compared with the RhahcMa, they are distinguished by the greater develop- ment of their bi-lobed cerebral ganglion, as well as by the greater number of theix- eyes (fig. 250). The rows of papilL. or the tentacle-like processes at the anterior end of the body h.ne Fig. 250.— Anatomy of Folyeelh pallida (after Quatre- fages). G, Cerebral gangUon with the nerves given off from it ; O, mouth ; D, branches of intestine ; Ov, ova; Od, oviduct; r, vagina; W.Ooe, female gene- rative opening; T, testes; M.Goc, male generative opening. TUEBELLAEIA — DENDBOCOCLA. 315 probably the function of tactile organs. The mouth usually lies in the middle of the body, and leads into a wide and protrusible pharynx. The skin is often provided with glands, the secretion of which in certain land Plcmaria {Bij)aliic7n, Bhynohodesonus) hardens to a fibrous web. They are almost always hermaphrodite. The fresh-water forms possess a common generative opening, while in the marine forms the geneiutive openings are usu- ally separate (fig. 250). In the latter case a separate vitellarium is absent. In some marine forms development takes place with metamorphosis, as is shown by the larva discovered by J. Miiller, which possessed six provisional finger-like ciliated lobes (fig. 251). In the fresh- water Planarians develop- ment is direct. The cocoon, when laid, contams four to six small eggs. At the close of segmentation there is developed a layer of cells, which is said to split into two layers, an upj)er or animal layer, from which are derived the body wall and muscular system, and a lower or vegetative, from which the alimentary canal is formed. The marine Dendrocoda fre- quently deposit their eggs in the form of broad bands. 1. Monogonopora Stimps. Den- Mk. drocoela with single sexual opening. The land and fresh-water Planaria be- long to this group. Fig. 251.— Larva of JEtiri/lepta atiri ciilata, after Hallez. \ Fain. Planariadse. The body is of a long, oval, flattened shape, and is often provided with lobed processes, more rarely with ten- tacles, and, as a rule, with two eyes, which are provided with lenses. Planaria 0. Fr. Miiller, two eyes, no tentacles. PI. torva. M. Sch. (divided by 0. Kchmidt into lugt/irix, 2>(ihjcJiroa, and torva^ (fig. 2.52). PI. dioica Clap., with separate sexes, Bendrocwlwii Oerst. Distinguished by the possession of lobed processes on the head, also by the presence of a copulatory organ placed in a special sheath. D. lactcnm Oerst., Polijcelh nigra, hrnnnca 0. Fr. Miill. ' Fam. Geoplanidae.* Land Planarians. They are characterised by their * Besides M. Schultze. Stimpson. MetschnikofP, Grube. etc., compare H. N. a Fig, 252.— Planaria pohjcliroa («), lufftibrui (6), torm (c), about twice the natural size (after 0. Schmidt). 316 PLATTHELMINTnES. elongated and llattcned body, which is provided with a foot-lilce ventral surface, Gcoj)la)UL lapidicohi, .Stinips., Jthijiu-hoditginuti tan-entru Gm. {Fasainlti terrcxtrU, O. Fr. Miillor), Earopc. OeodcuinHH hUinetUus, Metsohn., witli thread cells in the integument, found in potter's earth. 2. Digonopora. Z)mfZrocceZa with double sexual opening. Almost all are marine. The proboscis is often folded and lies within a special pouch. When protruded, it spreads out like a lobe, Fam. Stylochidse, The l)ody is flat and rather thick, and is provided with two short tentacles on the head. There are usually numerous eyes on the tentacles or on the head. The genital openings are posterior. Stylorhm viacii- latm Quatr. Fam. LeptoplanidsB. Body flat and broad, usually very delicate. Cephalic region not distinct, withoi;t tentacles. The eyes are more or less numerous. The mouth is visually placed in front of the middle of the bwly. The genital openings lie behind it. Lcpto^platxa trcmvllaru 0. Fr. Mlill., Mediterranean. Fam. Euryleptidae, Body broad, and either smooth or furnished with papilliB. There arc two tentacle-lilce lobes on the anterior region of the head. 'J'he mouth is placed in front of the middle of the body. Numerous eyes are disposed near the anterior margin. Marine, Thymmznon Dic-nngii Gr, Mediterranean, Eurijlcpta auriculata 0, Fr. Miiller, North Sea, Order 2. — Trematoda.* Parasitic Platyhelminthes loith unsegmmtecl, usioally leaf-shajjed, rarely cylindrical body. They j^ossess a mouth a7id ventrally lilaced organ for attachment : the intestine is forked and loithout an anus. The Trematodes are with great probability to be derived from the Turhellaria, with which group, both in form and organization, they show a close relationship. In connection with their parasitic mode of life they develop special organs for adhering, such as suckers and hooks. Cilia are present only in larval life. The mouth is invariably placed at the anterior end of the body, usually in the middle of a small sucker (fig. 253). It leads into a muscular pharynx with a more or less elongated oesophagus, which is prolonged into a forked intestine ending blindly. Moseley, •' Notes on the Structure of Several Forms of Land Planarians," etc. Jovrncd of Micv. Science, vol. xvii. * a; v.' Nordmann. " Mikrographische Beitrage 7.ur Kenntniss der ^virbellasen Thiere " ' Berlin, 1832. G. G. Cams, " Beobachtung liber Leucochloridium parado'xum, etc.." Nov. Act., vol. xvii., 1835. Wagener. " Ueber Gyrodactylus elegans " jVuUcr's ArcJtiv.. 1860. Van Beneden, " Memoire sm- les vers mtes- tinaux," Paris, 18(il. E. Zeller, " Untersuchungen liber die Entwickelung uiid den Bau von Polystoma integerrimum. ZeUxrlir. f. wis.t. Zool., vol. xxii., 1872. E Zeller "Untersuchungen liber die Entwickelung von Diplozoum paradox- nm " did., vol. xxiii., 1873. B. Zeller, " Ueber Leucochloridium paradoxnra unci' die weitere Entwickelung seiner Distomumbrut." Ihid.. Tom XXlV. E, Zeller, " Weiterer Beitragzur Kenntniss der Polystomeen," Ihid., xxvu., 18/6. Compare' also the works of G. Wagener and De Filippi. TEEMATODA. 317 The excretory apparatus consists of two large lateral trunks and a network of fine vessels permeating the tissues and beginning with small ciliated lobules. The two large trunks open into a common contractile vesicle, which opens to the exterior at the posterior end of the body (fig. 353, ^^:>). The excretory system contains a watery fluid with granular concretions. This fluid is probably an excretory product, corresponding to the urine of higher animals. The nerroics system consists of a double ganglion lying above the (Bsophagus, and from it several small nerves and two posteriorly directed lateral trunks are said to be given ofl". Eye sjMs with refractive bodies are sometimes present in the larvae during their mierations. Locomotion is efiected by the dermal muscular system and the organs of attachment, viz., the suckers and hooks, which present numerous modifications in number, form, and arrangement. In general, the size and development of these organs are related to the endo- parasitic or ecto-parasitic mode of life. In the endo-parasitic Trema- todes they are less developed, and usually consist of the oral suckei- and a second larger sucker on the ven- tral surface, either near the mouth, as in Distoimom, or at the opposite pole of the body [Amjjhistomicm). This large sncker may, however, be absent (^Monostomum). The ecto- parasitic Polystomea, on the other hand, are distinguished by a much more powerful armature, for besides two smaller suckers at the sides of the mouth, they possess one or more large suckers at the posterior end of the body (fig. 258), which, moreover, may be supported by rods of chitin. There are often in addition chitinous hooks, and very frequently two larger hooks among the posterior suckers in the middle line [H). The Trematoda are mostly hermaphrodite. As a rule, the male and female generative openings lie !~ide by side, or one behind the other, not far from the middle line of the ventral surface, near the anterior end of the body (fig. 254). The male opening leads into a sac, the Fig. 253.— Younp: Disiomum (after La Valette). Ex, trunk of the excretory (water vascular) system ; JSp, excre- tory pore; O, mouth with sucker; S, sucker in the middle of the ventral surface ; P, pharynx ; B, forked in- testine. 318 PLATYHBLMINTHES. T cirrus sac, which encloses the pvotrusible terminal part (cirrus) of the vas deferens. Tlie vas deferens soon divides into two, which lead back to the two large simple or multilobed testes. The supposed third vas deferens, which, Jiccording to v, Siebold, runs from one testis to the female sexual appai'atus, so as to permit of direct ferti- lization without copulation, has heen recognized as a vagina opening to the exterior on the dorsal surface (canal of Laurer). The female organs consist of a convo- luted uterus and of the glands concerned in the preparation of the egg, viz., an ovary and two yolk glands. There is sometimes in ad- dition a special shell gland. The true ovaiy which produces the pri- mary ova is a round body, and is usually placed in front of the testes. The yolk glands which secrete the yolk are much i-amified tubular glands, and fill the sides of the body (fig, 254). The yolk particles come in contact with the primary ova in the first portion of the uterus, and surround them in greater or less quantities. Subsequently each ovum, with its investment of yolk, is surrounded by a strong shell. The ova in their passage along the uterus become packed together, often in great numbers, and undergo the stages of embryonic development in the body of the parent. Most Trematodes lay their eggs; only a few are viviparous. The just-hatched young either possess (in most Polystomea) the form and organization of the parent; or they present the phenomenon of a complicated alternation of generations (heterogamy) connected with a metamorphosis {Distomea). In the first case, the large eggs become attached in the place where the mother lives; in the last case, the relatively small eggs are deposited in a damp place, usually in the water. After the completion of the segmentation and the em- Fig. 254. — Bistomum hrp, limb of in- testine; S, sucker; T, testes; Bo, vitellarium ; Ov (uterus), oviduct ; Dr, accessory glands. TBEMATODA. 319 biyonic development, the contractile, usually ciliated embryos* (fig. 255, a), Avhicli already possess the first rudiments of an excretory system and more rarely a sucker with a mouth and alimentary canal, leave the egg and wii'nder about independently in search of a new host. The latter is, as a rule, a snail, into the interior of which they pene- trate and there become transfoi'med into simple or branched Sporocysts (without mouth and alimentary canal, fig. 255, c), or into Reclice (with mouth and alimentary canal, fig. 255, d). These give rise, by means of the so-called germs [cells lying in the body cavity of the c d e Fig. 255. — Developmental history of Distomum (partly after E. Leuckart). a, free swimming ciliated embryo of the liver fluke, h, the same in a state of contraction with rudimentary alimentary canal {D) and cell mass (Ov) (rudiments of the genital glands). Ex, ciliated apparatus of the rudimentary excretory system, c, sporocyst developed from a Distomimi embi-yo, filled with Cercariae (C) ; B. Boring spine of a Cercaria. d Redia with pharynx, (Ph), and alimentary canal (D) ; O, mouth ; Ex, Excretory organ ; C, Cercaria inside Redia. e. Free Cercaria ; S, sucker ; D, alimentary canal. sporocyst or redia], which probably correspond to the germinal cells (primitive ova) of the rudimentary ovary, to the generation of the * As E. Leuckart has rightly observed, the B\Gycm 'ula>, which were regarded as Mewzoa by Ed. v. Beneden, as well as the Orthoncctida-, which have recently been especially investigated by Giard and E. Metschnikoff, and which in the reproductive stage do not rise above a form corresponding to the embryos of Trematodes, recall these Distomum larvjB. 320 PLA-TTHELMINTniOS. tailed Cercaricu,, or to another generation of SjwrocyHta or RedicB* which then produce the Cercariae. The Cercarice are nothing else than Distoimini larvae, whicli eventually reach (often only after two migrations, an active and a passive one) the final host, where they become sexually mature. They are furnished with an exceedingly motile caudal appendage, frequently with a buccal spine, and occasionally with eyes, and they present in the rest of their organization great resemblances to the adult JHstomum, excepting that the generative oi-gans are not developed. In this form they leave independently the body of the Redia or Sporocyst and of the host of the latter, and move aljout in the water, partly creeping and pai-tly swimming. Hei-e they soon find a new host (Snail, Worm, Insect larva. Crustacean, Fish, Batrachian), into which they penetrate, aided by the powerful J vibrations of their tail ; they then lose the latter and encyst. The Cercarice from the interior of the snail thus become distributed amongst a number of hosts, and each of them gives rise to an encyste young Bistomum without generative organs. This young Distomum mi- grates passively with the flesh of its host into the stomach of another animal, and thence, freed from its cyst, into the organ (intestine, bladder etc.), in which it becomes sexually mature. There are, then, as a rule, three different hosts in the organs of which the different developmental stages [Redia or S2JoroGyst, encysted form, sexually mature animal) of the Distomum bury themselves. The transitions from one host to another are effected partly by inde- pendent migration (embryos, Cercarise), partly by passive migration (encysted young Distomum). Modifications of the ordinary course of development may, however, take place ; these may be either complications or simpHfications. The embryo at hatching may contain a single Redia (as in Monostomum * In Cercaria cystophora from Playiorlh marfiimtm ; according to G. Wagener, the primafy asexual indiyidual is a Sporocyst, the secondary a hedm. Fig. 256. — a. Embryo of Diplodiscus mbclavatus (after G. Wagener). B, Alimentary canal ; Ex, excre- tory system. 6, Embryo of Mo- nostomum mntahile (after v. Sie- bold). P, Pigment spots ; Ji, redia intho interior of the embryo. TEEMATODA. 321 fiavum and mxdahile), which it carries about until it enters the first host (fig. 256, 6). In other cases the course of development is sim- plified by the omission of the second intermediate host, viz., that which contains the encysted immatui'e Distomum {Cercaria macro- cerca of Distomiwi cygnoides, also Leucochloriclium in the tentacles of Helix succinea). (!) Sub-order : Listomea. Trematodes with at most two suckers, without hooks. They develop with a complicated alternation of generations. The asexual individuals and the larvae live principally in MoUusca, the sexually mature animals in the alimentary canal of Vertebiutes. The sexes are completely separated in Distonmm hcematohium (from the veins of man); individuals of the two sexes being united in pairs (fig. 257). Dimorphic forms are found in certain species of the genera Monostomum and Distomum in connection with the division of labour of the sexual functions ; one individual develops only male sexual organs, and the other only female, the former producing spermatozoa and the latter The rudiment of the functionless ova. generative gland undergoes in these cases a more or less complete degenei-ation. Such Distomea are morphologically hermaphrodite, but practically of separate sexes. The complete biology and developmental history is unfortunately only satisfactorily known for a few species which can be fol- lowed through all the stages of development. Pig. 257 — Dittomum hcemato- hium. Male and female, the latter being in the canaUs gynascophorus of the former. S, sucker. Fam. Monostomidae. Of an oval, elongated, more or less rounded form, with only one siicker, which surrounds the mouth. Monostomum Zeder. Sucker surrounding the mouth ; pharynx poweiful. Sexual openings but slightly removed from the anterior end. M. imitah 'de Zeder, in the body cavity and in the orbit of various water-birds ; viviparous. M. Jiavum Mehlis, in water-birds, develops from Cercaria ephemera of PlanorMs. M. lentis v. Nordm., the young form without generative organs is found in the lens of the human eye. M. hipartitum Wedl., living in pairs enclosed in a common cyst, the one indi- vidual surrounded by the lobed posterior end of the other ; branchiae of Tunny- fish. Fam. Distomidae. Body lancet-shaped, fi-equently spread out, more rarely elon- gated and rounded with a large median sucker ; in fi'ont of which lie the genital openings, usually close together. 21 322 PLATYHELMINTHES. Listoimm. Median sucker approached to the anterior one. D. hr2)aticum I,. Liver Huivc. With conical anterior end, and numerous spine-hke prominenci , on the surface of the broad leaf-shaped body, whicli is about :{0 mm. l(;ij... Lives in the bile-ducts of sheep and other domestic animals, and produce, the liver disease of the sheep. It is occasionally found in Man, and bores iis way into the portal vein and into the system of the vena cava. The elor.gad d embryo only develops after the egg has remained a long time in water ; it Im, a continuous ciliated envelope with an X-shaped eye-spot. E, Leuckarfs i<> searches have rendered it proljable that tlie development is passed througli iu the young Limnceux 2>erc(j(;r and triincatidux, that here the embryo becomes a Sjwrocyxt, and that this produces Rcdla:, in which it is supposed that tailless Distomea arise. [The life-histoiy of the liver-fluke has been completely worked out by A. 1'. Thomas (Q^iart. Journal of Miaroscojrical Sci. 1883, pp. 99—133). He has shown that the ciliated embryo passes into LimncEua truncatulvs, and theie gives rise to a s^jorooyxt which iwoduces redise. The n-dice produce moie rt'dice or Ctircaria;. The CercarioB, which are provided with long tails. lea\ (' the host (^Limnceus trvncatnlvs), swim about for a short time in the water, and encyst on foreign objects, e.g. blades of grass. In this condition they are eaten by the sheep.] D. crassuin Busk., in the alimentary canal of the Chinese, one to two inches in length, and half-inch broad, without spinous prominences, with a simple forked intestine. B. lanceolatum Mehlis. Body elongated into the fonn of a lancet, 8 — 9 m.m. long, lives in the same place with B. licjjaticum. The embryo develops at first in water, is pear-shaped, and only ciliated on the anterior half of the body, bears a styliform spine on the projecting apex. B. o2)htlmlviohium Dies. A doubtful species of which only four specimens have been observed in the lens capsule of a nine-months' child. B. lieterophycs Bilh. v Sieb. 1 — 1"5 mm. long, in the alimentary canal of man in Egypt. B. r/oliath van Ben., 80 mm. long, in Pterohala-na, Numerous species live in the alimentary canal, lungs, and bladder of the fi-og. Bistoviun filicolle Eud. {B. Okeni Kbll) in pairs in the mucous sacs in the branchial cavity of Brama Raji. The one individual is cylindrical and narrow, and produces spermatozoa ; the other is swollen in the middle and posterior region of the body, and is filled with eggs. The dissimilar development of the two individuals is probably due to the fact that copulation only leads to the fertilization of one of them, which alone is able to perform the female sexual functions. B. li(smatoUum. Bilh. v. 'tiieh. {Gijnceco2)ho')m Dies) (fig. 257). Body elongated ; sexes separate. The female is slender and cylindrical. The male has powerful suckers, and the lateral margins of the body are bent round so as to form a groove, the canalis gynscophorus, for the reception of the female. They live in pairs in the portal vein, and in the veins of the intestine and of the bladder of man in Abyssinia. According to Cobbold, the embryos are ciliated, and possess a tolerably well developed excretory system. By the deposition of masses of theii- eggs in the vessels of the mucous membrane of the ureter, bladder, and great intestine, inflammation is set up, which may cause h33maturia. (2) Sub-order: Polystomea. — Trematodes with two small lateral suckers at the anterior end, and one or more posterior suckers, to which two large chitinous hooks are often added. In exceptional TIIEMATODA. 323 cases {Tristomum coccineum) transverse rows of bristles are found. Paii-ed eyes are frequently pi-esent. In some species the elongated body presents a kind of external segmentation. They are for the most part ectoparasitic, to a certain extent like the Ilirudinea, and they develop directly without alternation of generations from eggs which are usually hatched in the locality inhabited by the mother. Sometimes the development is a metamorphosis {Polystomum), and the young lai vte live in another place. The development of Polystomum integerri- mitm from the bladder of the frog is the best known, owing to the researches of E. Zeller Go W (figs. 258, 259). The production of eggs begins in the spring, when the frog awakes from hibernation and proceeds to pair. It lasts from three to four weeks. It ^ is easy then to observe the Polysto- mea in the process of recipro ca 1 copulation. When the eggs are being laid, the parasite forces the anterior end of the body with the genital ^^s— Polystomum inte- xi_ 1 _Li ,1 p gerrimum (after E. Zeller). opening through the mouth of the bladder o, mouth ; ao, genital nearly as far as the anus. The development opening; D, intestine; p , , . W, copnlatory opening ot the embryo takes place m water and occu- (lateral pads) ; Dg, yoik pies a period of many weeks, so that the ^^'^'^'^ '^"'"^ = ^' ^ncker; Oo, ovary ; H, hooks. young larvse are only hatched when the tad- poles have already acquired internal gills. The larvje resemble Gyrodactylus, and possess four eyes, a pharynx and alimentary canal, as well as a posterior disc (for attachment), which is surrounded by sixteen hooks. They possess five transverse rows of cilia ; three are ventral and anterior, two dorsal and posterior. There is also a ciliated cell upon the anterior extremity. The larva? now migrate Fig. 259.— Egg with embryo (a, and hatched larva (J) of Polystomum integerrimum ; Dl; operculum (after E. Zeller). 324 PLATYHELMINTHES. into the branchial cavity of the tadpole, lose their cilia, and are transformed into young Pnlystomea by the formation of the two median hooks and of the three pairs of suckers upon the posterior disc. The young Polystomum, eight weeks after the migration into the branchial cavity, at the time when the latter begins to abort, passes through the stomach and intestine into the bladder, and there Fig. 260. — Young Diplozoon (after B. Zeller). a. Two young Diporpa beginning to attach themselves together, h, After hoth individualB have attached themselves. O, mouth; H, fixing apparatus ; Z, papillse ; G, sucker. only becomes sexually mature after three and more years. In some exceptional cases, and always when the larva has passed on to the gills of a very young tadpole, it becomes sexually mature in the branchial cavity of the latter. The forms then remain very small, are without the copulatory canals and uterus, and die after the production of a single egg, without ever getting to the bladder. Fam. Polystomidae. With seve- ral posterior suckers, which are usually paired and arranged in two lateral rows, and are rein- forced by an armature of hooks. The genital openings are fre- Fio. 26l.-Egg («) and larva (i) of mplozoon (after q^^ently surrounded by hooks E. Zeller). Many species have a length of only a few lines. Polystomum Zed., with four eyes ; with no lateral suckers at the anterior end, but with oral sucker; with six suckers, two large median hooks and sixteen small hooks at the posterior end. P. integerrimnm Rud., in the bladder of Rana temporaria. P. ocellatwn in the pharyngeal cavity of Emys. In the formation of the testis and the absence of the uterus it resembles the adult form of P. integerrimnm from the branchial cavity of the tadpole. Octolothrlum lanceolatvm. Duj. Onchocotyle ap2)endicitMaK\x\m, on the giUs of Elasmobranchs, Diplozoon V. Nordm. The animal is double, two individuals being fused to TEEMATODA. 325 form an X-shaped double animal, the posterior ends of which are provided with two large suckers divided into four pits. In the young state they live solitarily as Dlporpa ; they then possess a ventral sucker and a dorsal papilla (260 a, G and Z\ In the double animals the formation of ova is confined to a definite period of the year, usually the spring. The eggs are laid singly after the forma- tion of the thread by which they are attached, and two weeks later the embryo (fig. 261, &), which only differs from Diporpa in the possession of two eye- spots and a ciliated apparatus upon the sides and on the posterior extremity of the body, is hatched. When an oppor- tunity of fixing itself on the gills of a fresb-water fish occurs, the young animal loses its cilia and becomes a DijJorjM, which possesses, besides the characteristic apparatus for attachment, the alimentary canal, and the two excretory canals with their openings at the anterior part of the body (at the level of the pharynx), and sucks the branchial blood. The junction of the two Diporpa soon follows ; and this does not take place, as was formerly believed, by the fusion of the two ventral suckers, but in such a manner that the ventral sucker of each animal affixes itself to the dorsal papilla of the other, and fuses with it (fig. 260, V). D. iiaradoxnm V. Nordm., on the gills of many fi-esh- water fish. Fam. Gyrodactylidae. Very small Tre- matodes with large terminal caudal disc and powerful hooks. They are viviparous, producing a single young one (first gene- ration) at a time, within which, while still in the body of the parent, another young one (second generation) may be pi'esent, and in this yet another (third generation). V. Siebold believed that he had observed a young animal developing from a germ cell of Oyrodactijhts, and that this became pregnant during its development. He regarded the Gyro- dactijlvs as an asexual form, since he failed to find organs for the production of sperm, G. Wagener, however, showed that the reproduction is sexual, and conceived the idea that the germs from which the second and third generations are fomied are derived from the remains of the fertilized ovum from which the_ first generation is formed. Metschnikoff, too, is of the opinion that the individuals of the first and second generations are formed at the same time from a common mass of similar embryonic cells. Gyrodactylus v. Nordm G. elegans v. Nordm., from the gills of Cyprinoids and fresh-water fish. Fig. 262. — Tcsnia saginata [medioeaneUafa), natural size (after R. Leuckart). 326 PLATYHELMrNTlIBS. Order 3. — Cestoda,* Elongated and usually segmented Platyhelmintlies without iiwuth or alimentary canal, with organs for attachment at the anterior extremity. The tape-wormy, which may easily be recognised by their band- shaped usually segmented bodies, are parasitic in the alimentary canal of Vertebrata, and were formerly taken for single animals. Steenstrupp was the first to introduce a different view, according to which the tape-worm is a colonial animal, a chain of single animals, each segment or 2)roglottis being an individual. There are, however, Cestoda, like Ca7'yophyllaius, which are destitute both of external segmentation and of segmentation of the gene- rative organs ; while in other cases the segments of the body are clearly differentiated, and each is provided with a set of genera- tive organs, but they do not attain individual independence. The proglottides, however, usually become separated off, and in some cases (^Echineibothrium) after their separation from the body of the tape-worm continue to live for a long time independently, and even increase considerably in size ; so that although the individuality of the tape- worm may be justly insisted on, yet the subordLaate and morphologically more restricted degree of individuality of the proglottis must also be admitted. This is the only satisfactory mode of regarding the Cestoda ; especially as the entire tape-worm, and not the proglottis alone, corresponds to the Trematode, and is to be derived from the latter by a simplification of organization and loss of the alimentary canal. The anterior part of the tape-worm is narrow, and presents a terminal swelling by which it attaches itself. This anterior swollen part is distinguished as the head of the tape-worm, but it is only its external form which entitles it to this name. In GaryophylloMS * Besides the older works and papers of Pallas, Zeder, Bremser, Eudolphi, Diesing, and others, compare van Beneden, " Les vers cestoi'des ou acotyles," Brussels 1850. Kiichenmeister, " Ueber Cestoden im Allgemeinen und die des Me'nschen insbesondere," Dresden, 1853. V. Siebold. "Ueber die Baud- und Blasen-wiirmer," Leipzig, 1854. G. Wagener, " Die Eutwicke- lung der Cestoden," Nov. Act. Leop.-Car., Tom XXIV., Suppl., 1854. G Wagener, " Beitrag zur Entwickelungsgeschichte der Eingeweidewurmcr, Haarlem, 1857. R. Leuckart, "Die Blasenbandwiirmer und ihre Kntwicke- lung" Giessen 1856. R. Leuckart, " Die menschlicben Parasiteu," Bd. L. Leipzig 1862. F. Soramer and L. Landois, " Ueber den Bau der geschlecht?- reifen Glieder von Botliriocephalus latus," Zeitschr. f. ivu><. Ziwl., 1872. F Sommer •' Ueber den Bau und die Entwickelungsgeschichte dor Geschlechts- organe von' Taenia mediocanellata und Taenia solium," md.,Tova. XXIV., lf?.l. OESTODA. 327 the head armature is very weak, and consists of a lobed fringed expansion. The apex of the head often ends in a conical projection, the rostellum, which is armed with a double circle of hooks, while the lateral surfaces of the head are furnished with four suckers {Tienia, fig. 263). In other cases only two suckers are present {Bothriocephahis) ; or we find suckers of more compHcated structure and beset with hooks {Acanthohothrium), or four protrusible probosces beset with recurved hooks {Tetrarhyncus) ; while in other genera the head armature presents various special forms. That portion of the animal which follows the head and is dis- tinguished as the neck shows, as a rule, the first traces of com- mencing segmentation. The rings, which are at first faintly marked and very narrow, become more and more distinct and gradually larger the further they are removed from the head. At the pos- terior extremity the segments or pro- glottides are largest, and have the power of becoming detached. After separation they live independently for a long time, and sometimes even in the same medium. The simplicity of the internal or- ganization corresponds with the simple appearance of the external structure. Beneath the delicate external cuticle is a matrix consisting of small cells, 1 1 J 1 n Fig. 263.— Head of Tamia soliwrn, viewed m which are scattered glandular cells. ^^^^ ^-^^ ^.^^^ ^^^^^^^ surface), with Beneath the matrix there is a delicate rosteiimn and double circle of hooks. The four suckers are visible. , superficial layer of longitudinal mus- cular fibres, and next a parenchyma of connective tissue, in which .strongly-developed bundles of longitudinal muscular fibres, as well as an inner layer of circular muscles, are embedded ; both these muscular layers are traversed, principally at the sides of the body, by groups of dorso-ventral muscular fibres. The power which the proglottis possesses of altering its form is due to the interaction of all these muscles. By means of them it is able to shorten itself considerably, at the same time becoming much broader and thicker, or to elongate to double its normal length, becoming much thinner. In the connective tissue parenchyma of the body, not only the muscles, but all the other organs are embedded. In its peripheral portion, especially in the neigh- bourhood of the head, we find small densely packed calcareous concre- ments, which are generally regarded as calcified connective tissue cells. 328 PLATYHELAIINTJtES. Thenervous system consists of two lateral longitudinal cords passing externally to the main trunks of the excretory system. They are somewhat swollen in the head, where they are connected by a trans verse commissure; these anterior swellings and the commissure may represent a cephalic ganglion. Distinct sense organs are wanting, but the tactile sense may be ascribed to the skin, especially to that of the head and the suckers. An alimentary canal is also wanting. The nutritive fluid, already prepared for alisorption, passes endosmotically through the body wall into the parenchyma! The excretory/ apparatus, on the contrary, attains a considei-able development as a system of much ramified canals which are dis- tributed throughout the whole body.* It consists primarily of two longitudinal canals (a dorsal and a ventral), running along each side of the body and connected in the head and in each segment by transverse trunks. According to the state of contraction of the muscular system, these longitudinal trunks and cross branches appear sometimes straight and sometimes bent in a wavy or zigzag manner : theii' breadth also presents consider- able variation, so that the power of contraction has been ascribed to their walls. The longitudinal trunks only serve as the efferent ducts of a system of very fine vessels which ramify throughout the whole paren- chyma and receive nvimerous long tubes : the latter begin in the parenchyma with closed funnels, which contain a vibratile ciliated lappet (fig. 264). In many cases, as in the Ligulidce and Caryo- phyllaius, these longitudinal trunks are broken up into numerous longitudinal vessels, which are connected by transverse anastomoses. In other cases, on the other hand, the two ventral vessels are enlarged at the cost of the two dorsal, which may entirely atrophy. The external opening of the excretory system is, as a rule, placed at the 264.— A portion of the excretory system of Caryophyllmus mntabilis (after Pintner). Wb, Ciliated funnels with the nucleus of the cell belonging to them. * Cempare Th. Pintner, " Untersuchungen iiber deu Bau des Bandwurm- korpers," Wien, 1880. CESTODA. 329 posterior end of the body, i.e., at the hind end of the last segment, in which a small vesicle with an external opening receives the longi- tudinal trunks. According to the observations of Leuckart on Tcenicc cucum&rina, the posterior transverse canals in the segments immediately preceding the last become, by their gradual shortening and the approach of the longitudinal trunks, transformed into the vesicle, which acquires an external opening when the segment behind it is detached. In rare cases the excretory system possesses additional openings in the anterior part of the body behind the suckers. The generative apparatus is also divided into segments which correspond to the proglottides. Each proglottis possesses its own Fig. 265. — Proglottis of Taenia mediocanellata, with male and. female organs (after Sommer). Ov, ovary ; BS, yolk gland (vitellarium) ; ScL, shell glaud ; Tit, uterus ; T, testes ; Vd, vas deferens ; Ch, pcucli of the cirms ; K, generative cloaca; F«., vagina. male and female generative organs, and can therefore, when separated, be considered as a sexual individual of a lower order. The male apparatus consists of numerous pear-shaped vesicles, the testes (fig. 265, T), which are situated upon the dorsal side, and their vasa efFerentia open into a common efferent duct {vas deferens). The coiled end of this duct lies in a muscular pouch {cirrus sheath), whence it can be protruded thi-ough the genital opening as the so-caUed cirrus. This cirrus is frequently beset with spines which are directed back- wards, and serves as a copulatoiy organ. The female generative organs consist of ovary, yolk gland, shell gland, uterus, recep)taGuluvi, and vagina. The vagina and vas deferens usually open into a common 330 PLATTHELMINTHES. Fig. 266.— Eipe proglottides ready to separate. a, of Twnia solium ; b, of TcBnia medioeanellata ; Wc, watervascular (excretory) canal. genital cloaca, which lies eithex- on the ventral surface of the segment {Bothriocepalus), or on the lateral margin (Tcenia) (fig, 265). lu J the last case it Ls placed alter- nately on the right and on the left side. Nevertheless it may happen that the two genital openings are widely separate, the male opening being placed at the side, the female on the surface of the segment. As the segments increase in size and become further removed from the head, the contained generative organs gr-adually reach maturity in such a way that the male generative organs arrive at maturity rather earlier than the female. As soon as the male elements are mature, copulation takes place, and the receptaculum seminis is filled with sperm, and then only do the female generative organs reach maturity. The ova are fertilized and pass into the uterus, which then assumes its characteristic form and size. As the uterus becomes distended, the testes and then the ovaries and vitellaria are more or less completely absorbed (fig, 266). The posterior proglot- tides, viz., those which are ready for separation, have alone under- gone full development, and the eggs in their uterus often contain completely developed embryos. Accordingly we can recognize in a continuous series of the seg- ments the course of development passed through by the sexual organs and products in their origin and gradual progress towards maturity segments between that with the first trace of the generative organs FiG.fl267.— Egg with embryo (a) of Tcetiia solium; (b) of Microtmnia ; (e), of Bothrio- cephalm latus (after R. Leuckart). The number of OESTODA. 331 and the first proglottis with fully developed organs gives us an expression for the number of stages through which each segment has to pass. The tape-worms are oviparous; either the embryo develops within the egg-shell in the body of the mother, or the development takes place outside the proglottis, for example in -water {Bothriocephcdus). The eggs of the Cestoda are round or oval in shape and of small size. Their envelope is either simple or composed of numerous thin membranes, or else forms a thick and strong capsule, which in Tcenia is formed of densely packed rods united by a connecting substance, and presents in consequence a granular appearance. In many cases the development of the embryo coincides with that of the egg- shell, so that the egg at the moment that it is laid contains a It (I Fig. 268. — Stages in the development of Tcenia solium to the Ci/sticercus stage (partly after R. Leuckart). a, Egg with embryo, b. Tree embryo, c, Rudiment of the head as a hollow papilla on the wall of the vesicle, d, Bladder-worm with retracted head, e. The same with protruded head, magnified about fom- times. complete embryo with six, or more rarely, four hooks. In Bothrio- cephalus the development takes place outside the proglottis during the long period that the egg passes in water, and the embryo leaves the egg as a ciliated larva (fig. 267, c). The development of the embryo into the tape-worm probably never takes place directly in the same medium in the intestine of the original host. As a rule there is a complicated metamorphosis, which is sometimes (^Echinococcus, Coenurus) connected with alternation of generations ; the successive stages live in different localities, and usually find the conditions necessary to their development in different species of animals, between which they migrate, partly actively and partly passively. The eggs usually leave the intestine of the host with the proglottis, and are deposited on dunghills, on plants, or in the 332 PLA-TYHELMINTnES. water, and thence pass in the food into the stomach usually of herbivorous or omnivorous animals. As soon as the egg membranes are digested or bui>st by the action of the juices of the stomach of the new host, the embryos which have been thus set free bore thei,. way into the gastric or intestinal vessels by means of their six (rarely four) hooks, the points of which can be approached and removed from one another over the periphery of the small globular embryonic body. When they are once within the vascular systeiii, Fig. 269. — a, Brood-capsule of Echinococcns with developing heads (after R. Leiickart). I, Brood-capsule of ISchinococeua (after G. Wageuer). c, Heads of Echinococcits still connected with the wall of the brood-capsule — one is evaginated ; Vc, excretory canals. they are no doubt carried along passively by the current of blood, and transported by a longer or shorter route into the capillaries of the different organs, as the liver, lungs, muscles, brain, etc. After losing their hooks, they usually become enveloped by a cyst of connective tissue, and grow into large vesicles with liquid contents and a contractile wall (fig. 268). The vesicle gradually becomes a cystic or bladder worm by the formation of one (Cysticercios*) or * Exceptionally two or more heads are found in some Cysticercus forms. OESTODA. 333 several (Ccenunts) hollow buds, which are developed from the walls and project into the interior of the vesicle (fig. 268, c). The armature of the tape-worm head (suckers and double circle of hooks) is formed on the inside and at the bottom of this invagination of the wall of the vesicle (fig. 268, d). When these hollow buds are evaginated so as to form external appendages of the vesicle, they present the form and armature of the Cestode head, as well -as a more or less developed neck, which presents even at this stage traces of segments (fig. 268, e). In some cases [Echinococcus) the iiTegularly shaped maternal vesicle produces from its internal walls one or two generations* of. secondary vesicles which project into it; and the Cestode heads originate in special small brood-capsules on these secondary vesicles (fig. 269, a). In such cases the number of tape-worms which arise from one embryo is naturally enormous, and the parent vesicle may reach a very considerable size, being some- times as large as a man's head. In consequence of this enormovis growth the vesicles frequently obtain an irregular shape ; while on the other hand, the tape- worms which 'are developed from them remain very small, and carry, as a rule, only one ripe proglottis (fig. 270). So long as the tape-worm head (scolex) remains attached to the body of the bladder-worm and in the host of the latter, it never develops into a sexually mature tape-worm ; although in many cases it grows to a considerable length {Cysticercus fasciolaris of the house-mouse). The bladder- worm must enter the a,limentary canal of another animal before the head {scolex) can, after separation from the body of the bladder-worm, develop into the sexually mature tape- worm. This transportation is eflfected passively, the new host eating the flesh or organs of the animal infected with Cysticerci. The tape- worms, therefore, are principally found in the Carnivora, the Insecti- vora, and the Omnivora, which receive the bladder- worms in the flesh of the animals on which they feed. The vesicles are digested in the stomach, and the cestode head becomes free as a scolex. The latter is protected from the too intense action of the gastric juice by its calcareous concretions, and at once enters the small intestine, fastens * In Cysticerci {0. longieoUis, temiicollif) also steille . daughter vesicles are sometimes budded off. Fig. 270.— Tffinia Echin oc occus (after R. Leuc- kart), magni- fied 12 to 15 times. 334 PLATYHELMINTHES. Fig. 271.—Cyaticercoid . of Tmiiia cumtmerina, magni- fiecl 60 times (after R, Iieuckart). itself to the intestinal wall, and grows by gi-adual segmentation intr, a tape-worm. From the Scolex the chain of proglottides proceeds as tlK- result of a growth in length accompanied by segmentation, a process which is to be looked upon as a form of asexual reproduction (bud- ding in the direction of the long axis). Since, however, it is the Iwdy of the Scolex which undergoes gi-owth and segmentation, it seems most natural to assume the individuality of the entire chain, and to subordinate to this the individuality of the proglottides. Th(^ development of the tape-worm is then to l>e explained as a metamorphosis, characterised by the individualization of certain stages of the development. It is only in those cases in which the young form produces a number of Jieads that the development can be ex- plained as a case of alternation of geneni- 'tions. The development of some tape- worms pre- sents considerable simplifications. In tlie cysticercus stage the vesicle frequently dimin- ishes to an excessively small appendage, and the Cysticercus becomes a cysticercoid form, in which one poi-tion bearing the embryonic hooks is distinct from a larger part which represents the scolex (figs. 271, 272). In other cases the embryo becomes a Scolex directly without passing through • a cystic stage, so that the Scolex stage is merely a late stage of the embryo [Bothrio- ce2)halus). The segments produced from the Scolex also show very different degi-ees of individuality, and finally are sometimes not deve- loped at all. In the latter case (Cari/oj}hyllceus) the head and body cannot be sharply distinguished from one another, and represent only one single individual comparable to a Trematode and characterised by its single generative apparatus. Its development is to be looked upon as a metamorphosis completing itself in one individual. Fam. Tseniadse. The armature of the head consists of four muscular sucker?, to which is frequently added a single or double circle of hooks on the rosteUnm. Fig. 272. — EchinococetisAi^b Cyistieercoii from the body cavity of the Earth- worm (after E. Metschnikofl). a. Brood-capsules vrith three Cysticer- coids. h, Cysticercoid with evaginated head. CESTODA. 335 The proglottides have a marginal sexual opening. The vagina is usually long, separated fi'om the uterus, and enlarged at the end to form a receptaculum seniinis (fig. 265). The young stages are Cysticcrci or Cysticercoids, rarely quite without caudal vesicle ; parasitic in warm and cold-blooded animals. Tcenifi L. ( Cystottenia R. Lkt). Development takes place with large vesicles. The heads arise from the embryonic vesicle itself. ^ ' T. solium. L. 2—3 metres long. The double circle of hooks is composed of 26 hooks. The ripe proglottides arc 8—10 mm. long and 6—7 mm. broad ; the uterus has 7—10 dendritic branches. It lives in the human intestine. The Bladder- worms belonging to it ( Ci/stwerctis celluloscB) live principally in the dermal cellular tissue and in the muscles of pigs, but also in the human body (muscles, eyes, brain), in which self-infection with them is possible if a T(enia is present in the digestive canal ; more rarely in the muscles of the Eue-deer, the Dog, and the Cat. In the human brain the Cysticercus acquires an elongated form, and sometimes does not produce a head. T. saginnta Goeze^mediooaneUata Kiichenm., in the intestine of Man, distin- guished by the older helminthologists as a variety of T. solium. Head without circle of hooks orTostellum', but yvith. four more powerful suckers. The Tape- worm reaches a length of four metres, and becomes much stronger and thicker. The mature proglottides are about 18 mm. long and 7 — 9 mm. broad. The uterus forms 20 — 35 dichotomous side branches. The Cysticercns lives in the muscles of the ox (fig. 273). It appears to be principally distributed in the warmer parts of the Old World, but is often found in great numbers in many places in the north. T. serrata Goeze, in the intestinal canal of the dog. The Cysticercus is known as Ci/stice7'C7is j^'isciformis iu the liver of the Hare and Eabbit. T. cvassicolUs Rud. in the Cat, with Custicercns fascwlaris of the common mouse. T. marginata Batsch. of the Dog (butcher's dog) and Wolf with Cysticercus teimicol- lis fi'om Ruminants and Pigs, and occasionally in Man {Cyst, visceral is). T. crassiceps Rud. in the Fox with Cysticercvs longicollis fi'om the thoracic cavity of the Fieldmouse. T. coenurxis v. Sieb. in the intestine of the sheep-dog, -nith Ccsnurns cereiralis in the brain of one year old sheep. The presence of Coemirus in other places has been stated, as for instance in the body cavity of the Rabbit. T. tenuicollis Rud. in the intestine of the "Weasel and the Pole-cat, with a Cysticercus which, according to Klichenmeister, lives in the hepatic ducts of the Field-mouse. Echlnococcifer Weinl. The heads bud on special brood-capsules, in such a way that their invagination is turned towards the lumen of "the vesicle (fig. 2fi9). T. ecliinococciis v. Sieb. (fig. 270) in the intestine of the dog, 3 —4 mm. long, forming but few proglottides. The hooks on the head are numerous but small. Its Bladder-worm is distinguished by the great thickness of the stratified cuticula. It lives as £^6- Amrwocc?/* principally in the liver and the lungs of Man (£". liominis) and of domestic animals {K vettrinoruni). The first form is also distinguished as E. altricipariens on account of the frequent production of primary and secondary vesicles ; it usually reaches a veiy considerable size and Fig. 27^.— Cysticercus of Tainia mediocanellata, magnified about eight times. The head is protruded. 336 VERMES. has a very irregular shape ; while that form which inhaVjits domestic animals, -E scoUciparifm, more frequently retains the form of the simple vesicle. Finally these echinocoecus cysts frequently remain sterile, in which case they are called Acejihalocysts. Another and indeed pathological foi'm is the so- called multilocular UoJiinoaaccits, which was for a long time taken foi- a colloid cancer. It is also found iu Mammalia (in cattle), and here presents a confusing re- semblance to a mass of tubercles. The echinocoecus disease {hydatid 2^l^g'ue') was widely spread in Iceland. This disease likewise seems endemic in many places in Australia. T. {Microtainia). The Oystieercoid torn is small, and has but little fluid in the small portion which corresponds to the vesicle. The head is small, but has a small club- shaped or proboscis-like rostellum, and is furnished with weak hooks. The eggs are provided with several membranes. The embryo is usually furnished with large hooks. The Cysticercoid stages live prin- cipally in Invertebrates (in Slugs, Insects, etc.), and more rarely in cold-l)looded Vertebrates (the Tench). T. cucumerina Bloch, in the intestine of dogs (house dogs). The Cysticercoid is entirely without the caudal vesicle, and lives (according to MelnikoS and R. Leuckart) in the body cavity of the Dog-louse (^2ricliodecte.t canu). The infection with the Oysticercoids takes place when the dog swallows the parasites which are annoying liim, while the para- sites swallow the eggs contained in faeces adherent to the hair of the dog. Nearly allied is T. eWqjtica Batsch. in the intestine of the Cat, occasionally in that of Man. T. nana Bilh. v. Sieb. in the intestine of the Abyssinians, hardly an inch long. T. flavojiunctataWei-al. in the human intestine (North America). The Cysticercoids of the Meal-worm are probably developed into tape-worms in the intestines of Jlice and Eats. In other partially unarmed Tccnias the Fig. 274 a.—Bothriocephalus latua (after generative organs and development are as R. Leuckart). accm-ately known ; such are—T. perfoliata Goeze, and T.^Mcata Eud. in the horse ; T. pectinata Goeze, in the hare ; T. dispar Eud. in the frog ; T. expansa Im. in the ox. Fam. Bothriocephalidffi. With only two suckers, which are weak and flat. The generative organs, as a rule, open upon the surface of the proglottis. The proglottides do not become detached singly, an encysted Scolex. Hydatid stage represented by UJDSTODA. 337 BothriocephulHH Brems. Segmented body. Head with two pits, without hooks. The genital openings are on the middle of the ventral surface. The young stage usually in fishes. B. latn,i Eud. Head not distinct, with two weak suckers and with two pairs of tridentate hooks. The body has no external segmentation. The generative openings are marginal. T. nodulosus Eud. In the intestine of the pike. Asexual encysted form in the liver of Cmivimis. Fam. Jii^vliAsR (^Psc'udo2)hyllidce^, Without real suckers. Hooks are either present or absent. The Cestoid has no segmentation, but the generative oi^ns are repeated. They live in the body cavity of Teleosteans and in Ihe intestine of birds. Ligula Bloch. Body band-shaped and unsegmented. L. siiiijdi- cisswia Eud., in the body cavity of fishes and in the intestine of aquatic birds. L. tuba V. Sieb., in the intestine of the Tench. The families of the Tetrarhynchidae {Tetrarlujnclms li/iffvalis, Cuv., passes its young stages in Soles, and is matured in the intestine of Eays and Dog-fish), and Tetraphyllidse {Echbieiiothrium mwinmm van Ben.) are allied here. Fam. Caryophyllaeidae. Body elongated and unsegmented. The anterior margin is plicated. There are no hooks, and there are eight sinuous longitu- dinal canals of the excretory system. Generative organs single. The develop- ment is a simplified metamorphosis. CaryophjillcBUS viutaUlis Eud., in the intestine of Oyprinoids. The young form possibly lives in Tuhlfcv rh-idonm, if the Helminth observed by d'Udekem was the same. In this worm, however, ttiere lives another parasite, which was observed by Eatzel and has recently been more closely investigated by B. Leuckart, who has shown that it is NJBMEBTINI. 339 Es ; a sexually matui-e Cestoid still fixed by an appendage bearing the embryonio liooks. AroMgetes Sicholdu Lkt. With two weak suckers and a caudal appendage. Order 4. ]SrEMERTINI* = BHYNCHOCCELA. Elongated, frequently band-shaped Platyhelminthes, with straight alimentary canal opening by an anus, and with a separate protrusible proboscis. Usually with two ciliated pits in the cephalic region. The sexes are separate. The Nemertines are distinguished not only by their elongated form, but also by their con- siderable size and high organization. Thick layers of muscles, traversed by connective tissue, are spread beneath the integTiment, which con- tains pigment as well as flask-shaped mucous glands. The external layer of longitudinal muscles, strongly developed in the Anopla, is wanting in the Enopla (Nemertines, the probos- IBiSfMI cis of which is armed with stylets), in which group there is only an outer layer of circular muscles and an inner layer of longitudinal muscles. A long tubular protrusible proboscis, which is sometimes armed with stylet-shaped rods, is always found at the anterior end of the body above the buccal cavity, and projects through a special prseoral opening (fig. 276), and can be retracted into a special muscular sheath separate from the body cavity. At the bottom of the principal portion of the proboscis, there is in many Nemertines (Enopla) a large spine, which is directed forwards, and at its sides numerous small secondary spines in pouches. The posterior glandular portion of the proboscis, to which retractor mviscles are attached, is, according to Claparede, to be regarded as a poison apparatus. When the proboscis is pro- * A. de Quatrefages, " Memoire sur la famille des Nemertines," Ann. dot Sc. Nat.. Ser. 3, Tom. VI., 1846. Fig. 276. — Tetraatemma obsciirum (after M. Schultze). Young specimen about 3 lines in length ; O, mouth j D, intestine ; A, anus ; Bg, blood vessels ; JB, proboscis armed with stylet ; JSx, lateral trunks of the excre- tory system; P, ex- cretory pore; G, ciliated pit ; iVf, nerve centre ; 8s, lateral nerve trunks ; Oc, eyes. Mcintosh, " On the Structure of the British Nemerte aas," Tran.sact. Edinb. Royal Soc, Tom XXV., I .Sc 2, Barrois, " Memoire sur rEmbryolog^ie des Nemertes," Paris, 1877. Hubrecht " Untersuchungen iiber Nemertinen, etc.," Mederl. Archiv., Tom. II. 340 PLATYHELMINTHES. trucled, it is inverted like the finger of a glove, so that the blind end at which the spines are placed becomes the extreme front end of the protruded proboscis. The brain attains a considerable development. Its two halves ai-e connected by a double commissure which embraces the proboscis, and in thein several lobes, usually a dorsal and ventral, may be distin- guished. The two ventral lobes are produced into the two latei'al nerve trunks, which in certain cases (Oerstedtia) may approach each other on the ventral surface. The nerve trunks contain not only fibres but also a superficial layer of ganglion cells, which may give rise to ganglion-like enlargements at the points of exit of the nerve branches. In the embryos of Prosorochmus Claparedii the nerve trunks are said to end in an enlargement. In the cephalic region there are two strongly ciliated depressions known as the cephalic slits, beneath which special lateral organs, supplied with nerves from the brain or it may be posterior lobes of the brain itself, are placed. These structures are probably sense organs. The cephalic slits were formerly erroneously taken for the openings of respiratory organs. Eyes are widely distributed, and usually consist of simple pigment spots which rarely contain refractive bodies. Exceptionally, as in Oerstedtia 2y<^l^'^d'^j otolithic vesicles are found on the hrain. The Nemertines, unlike all other Platyhelminthes, possess a blood- vascular system. This consists of two sinuous lateral vessels in which the blood flows from before backwards, and a straight dorsal vessel in which the blood flows in the reverse direction. This latter is connected with the ventral vessel at the posterior end of the body and in the region of the brain by wide loops, and in the rest of its course by numerous narrower transverse anastomoses. These vessels lie in the body cavity and have contractile walls. The blood is usually colourless, but in some species it is red. In AmpJdponcs splendens, Borlasia splendida, the red colour (hsemoglobin) is con- tained in the oval disc-shaped blood corpuscles. The Nemertines are, with some few exceptions {Borlasia herma- phroditica), dioecious. The two kinds of generative organs have the same structure, and are sacs filled with ova or spermatozoa lying in the lateral portions of the body between the pouches of the intestine, and opening to the exterior by paired openings in the body wall. The ova, when laid, frequently remain connected by a gelatinous substance, and are deposited in irregular masses or in strings, from the middle of which the animal creeps out, like the leech out of its NEMEUTINI. 341 ttmrnttttt" Fig. %77.—Pilidmm (after E. Metschnikofl). a, free swimming larva with invaginated cavity ; b, later stage, helmet-shaped ; -E, E' the two pairs of ectodermal invaginations; D, alimentary canal. cocoon. Some forms, as Prosorochmus Glaparedii and Tetrastemma obscitnoni, are viviparous. Some of the Anopla develop with a metamorphosis. The larva is ciliated and may pass through a free -swimming stage, in which case it is known as the Pili- dium, or it may be without such a stage {TypeofDesor). In both cases the perfect worm is deve- loped within the skin of the ciliated larva. The PiUdium larva is helmet-shaped, and was formerly described as the species of a supposed independent genus, Pilidium, and presents many analogies to the Echinoderm larva. In the case of the Pilidium, the segmentation is regular, and results in the formation of a spherical ciliated em- bryo, which is hatched and becomes a free-swimming larva ; the archenteron is then formed by invagina- tion ; and at the side of the embryo, opposite the blasto- pore, a long flagellum is developed (fig. 277, a). On each side of the mouth a broad lobe grows out, the edges of which are fringed with cilia (fig. 277, h). Two pairs of invaginations of the ectoderm now make their appear- FiG. 278.— Later stage of Filidium, with tuft of cilia and enclosed Nemertine (after BiitschU) ; Oe, oesophagus; B, alimentary canal ; 4m, amnion; R, rudimentary proboscis of the Nemertine ; So, lateral pit. 342 PLATYIIELMINTHES. ance, forming the first rudiment of the Nemertine body. The four discs so foi-med fuse together and give rise to a ventral germinal plate, which gi-adually grows round the alimentary canal of the Pilidium to form the skin of the future Nemertine, The proboscis arises as an invagination of the anterior end of the germinal plate (fig. 278). The young Nemertine subsequently breaks through the larval skin. The Nemertines live principally in the sea, under stones in the mud, but the smaller species swim about freely. There are also forms which live on the land, as well as pelagic forms. Certain species form tubes and passages, which are lined with a slimy secre- tion. The food of the larger species principally consists of tubicolous worms, which they extract from their habitations b}'- means of the proboscis. There are, however, parasitic Nemei-tines which infest Crustacea or live on the mantle and gills of Mollusca. In this case they are, like the Hirudinea, furnished with a posterior sucker i^Malacohdella). The Nemertines are distinguished by their repro- ductive capacity and by their tenacity of life. Mutilated parts are quickly regenerated, and the parts into which certain species readily break are said to have the capacity, under favovirable conditions, of developing into new animals. 1. Sub-order : Enopla. — The proboscis is armed with stylets. The short, often funnel-shaped cephalic slits are connected with lateral organs, which correspond to the posterior cerebral lobes of the Anopla. In the brain the upper lobes are slightly elongated posteriorly leaving the ventral lobes, from which the lateral nerves arise, quite free. Development takes place without metamorphosis. Fam. Amphiporidse. The ganglia are more rounded, the lateral nerve trunks are placed inside the dermal muscles. The mouth is on the ventral surface near the anterior end of the body, in front of the commissures between the ganglia. The lateral organs are separated from the brain and connected with it by fibres : they contain a narrow water canal. Amjih-iporus lartiflorciiii Johnst. Lives under stones, and is distributed from the North (^eas to the Mediterranean, 3— i in. long. A. spectaMm Quatr. Borlasla spUndida Kef., Mediterranean, and Adriatic. TetrmUmimi olscunmi M. Sch. Viviparous : Baltic. T.af/ricola Will. Suhm., terrestrial. Mmertes gracilis Johnst. 2. Svib-order : Anopla. — The proboscis is unarmed. The long cephalic slits occupy the whole side, or the anterior part of the head, and lead into the lateral organs, which are direct processes of the upper lobes of the brain. Development frequently by means of ciliated larvae. NEMATHELMINTHES. 343 Fam. LineidsB. Ganglion elongated. The head has deep slits on either side. L'mcm marhms Mont., L. Icngissimis Sim. (sea long-worm, Borlasia anglica Oerst.. JSVmertes JBorlaxU Cuv.), grows to a length of 15 feet and more. English coast. Ccrelmtulus mar(jinat'us = Mechelia somatotomuft F.S. Lkt., Adriatic and Mediterranean. Mirrura fasoiolata Ehrbg., North Seas to the Adi-iatic. Fam. CephalotricMdse. Cephalic slits and lateral organs are wanting. Head not distinct, very long and pointed. Ce]}halothrix hioeulata Oerst. Sund. Malacohdellii grossa 0. Fr. Mlill. Body broad and flat, with posterior sucker. Is parasitic in the mantle cavity of various MoUusca, as My a, Cgprlna, etc. CLASS XL— NEMATHELMINTHES. Round worvis with tiibular or filiform bodies. The cuticle is fre- quently ringed. The anterior pole is either armed with hooks or provided with papillce. The sexes are separate. The unsegmented body is rounded, more or less elongated, tubular or filiform, and both ends are, as a rule, tapered off. Appendages are always wanting, as are, with few exceptions, movable bristles. On the other hand, special organs for attack and attachment, such as teeth and hooks, are not unfrequently present on the anterior end of the body ; and in some cases small suckers, which serve for attachment during copulation, may be developed on the ventral surface. As a rule, the integument possesses a cuticular layer of relatively considerable thickness, and a well developed muscular layer, which permits not only of the body being knotted, cm'ved, and bent, but, in the thin filiform Nematoda, of undulatory movements. The body cavity is enclosed by the muscular body wall, and con- tains the blood fluid and the digestive and generative organs. Blood vessels and respiratory organs are wanting. A nervous system is, however, always present. Of sense organs simple eyes are not unfrequently present in the free living forms. The sense of touch is probably distributed all over the surface of the body, particularly on the anterior end, especially when papillee and lip-like prominences or bristles are found on it. While in the Acanthocephala mouth and alimentaiy canal are completely absent, the Nematoda possess a mouth placed at the anterior pole of the body, an oesophagus, and an elongated straight digestive canal, which usually opens by the anus on the ventral surface near the pos- terior end of the body. The excretory organs have various forms, and always differ considerably from those of the Platodes. In the Nematoda they consist of paired canals, which open by a common pore and lie in the so-called lateral lines. In the Acanthoce- 344 KEMATHELMrtfTHES. phala they are branching subcutaneous canals. With a few excep- tions the Nemathelminthes have separated sexes, and develop directly without metamorphosis. The larvss and sexual animals are not unfrequently distributed in two different hosts. The majority of the Nemathelmintlm are parasites either during the whole period of their life or at different stages. There are, however, also free living forms which often show the closest relationship to the parasitic members of the group. Order 1. — Nematoda (Thread-worms).* Nemathelminthes, with mouth and ali- mentary canal. They are ininciixdly parasites. The Nematodes possess an extremely elongated thread-like body, which may be provided with papillse at the anterior pole in the i-egion of the mouth, or with hooks and spines within the oral cavity. The mouth leads into a narrow oesophagus, which usually has thick muscular walls, a chitinous lining, and a triangular lumen, and is frequently dilated behind to a muscular bulb (pharynx). In certain genera {Rhabditis, Oxyitris), the chitinous lining of the pharynx is raised into ndges or tooth-like prominences, to which the * Besides the older writings of Eudolphi. Bremser, Cloquet, Dujardin, compare Diesiiig, " Systema helminthum," 2 Bde Wien, -ISoO-ol. Diesing, " Revision der Nematodeu," Wiciu r SitzwigshcriclitL', 1860. Clapar^de, " De la for- mation et de la fecondation des oeufs chez les vers Nematodes," Geneve, IS.'ie. A. Schneider. " Monographic der Nematoden," Berlin. 186(i. R. Leuckart, " Uutei-suchungen tiber Trichina spiralis," Leipzig and Heidelberg, 1866, 2nd edition ; also " Die menschlichen Parasiten." etc., Tom. II., Leipzig and Heidelberg, 1876. C. Claus. " Ueber Leptodera appendicnlata," Marburg. 1868. 0. Biitschli, '• Untersuchungen iiber die beiden Nematoden der Periplaneta orieutali.*.'" Zeitzsclir. fur IT'/s-f. ^wZ., Tom. XXL, 1871. And " Beitrtige zur Kenntnis> des Nervensystems der Nematoden," Archiv. fur Miftr Anatomie, Tom X. Fig. 279. — Oxyuris vermicular is (after R. Leuckart). a, female; 0, mouth ; A, anus ; V, genital opening ; b, male with curved posterior end; e, the latter enlarged; -Sji, spiculum; d, egg with enclosed embryo. NEMATODA. 345 radial muscles converge in the form of conical bundles. Accord- in mm to its function, the oesophagus is essentially a suctorial tube, which pumps in fluids, and by peristaltic action passes them on to the intestine. The intestine follows the pharynx, and opens by the anus not far fi-om the hind end of the body on the ventral surface (fig. 2 79). Its walls are formed of cells and are non-muscular, except behind, where they have a special investment of muscular fibres which render the terminal portion contractile. Muscular fibres passing from the body wall to the wall of the rectum are also frequently present. In certain Nematodes the anus may be want- ing {Mermis) ; and in Gorclius even the alimentary canal undergoes degeneration. Beneath the stiff cuticle, which is often trans- versely ringed, and is composed of several layers, lies a soft granular nucleated sub-cuticular layer {hypodermis), which is to be regarded as the matrix of the former. Beneath this lies the highly deve- loped muscular layer, in which band-shaped or fusi- form longitudinal muscles predominate. The surface of the body may present markings, as for instance polyhedric spaces and longitudinal ribs, also pro- cesses in the form of tubercles, spines,* and hairs. Ecdyses, i.e., shedding the cuticular layer, seem only to occur in the young forms. The muscles are each composed of a single cell, in which two parts are distinguishable, — a clear, sometimes a granular protoplasmic portion (medullary sub- stance), which projects into the body cavity and is often prolonged into processes ; and an external fibrillated layer (fig. 280). The Nematodes may be distinguished as Meromyaria or Polymyaria, according to the arrangement of their muscular system. In the Meromyaria the number of muscle cells (which are arranged according to definite laws) in the cross section is small (eight), while in the Polymyaria their number is considei'able. In the latter the muscle cells are often connected together by transverse processes of the medullary substance, which unite on the so-called median lines to form a longitudinal cord. i Fig. 280. — Muscle- cell of a. Nematode. * There may also be prominences of various Mud?, and even in some cases a complete covering of spines {Cheir acanthus Dies = Gnathosto ma Ow., Cfi. hiKpidiim Fedsch.) NJiMATHELMINTHJSS. In almost every case, with the exception of Gordiua, two later U regions remain free from muscle and form the so-called lateral line, or regions, which may equal in lireadth the neighbouring muscular regions. These lateral regions are formed of a finely gi-anulai- nucleated substance, and enclose a clear vessel containing granules This vessel is connected with that of the opposite side in the anterior part of the body, and the two open by a common transverse slit, the vascular -pore, on the ventral surface in the median line. Vhe lateral lines have the value, both as regards position and sti-ucture, of excretory organs. Median lines {dorsal and ventral), accessoiy median lines (sub-median lines), the latter being placed between the principal median line and the lateral line, are also to be dis- tinguished. The so-called ventral cord of Gordius, which may be compared to the median line and has perhaps the significance of an elastic rod, is very large. Cutaneous glands, in the form of unicel- lular glands, have been observed principally in the region of the cesophagus and in the tail. The nervous system, owing to the difficulty which its investigation offers, has only been satisfactorily recognised in a few forms'. It con- sists of a nerve ring surrounding the oesophagus, and sending off posteriorly two and anteriorly six nerve trunks {Ascaris megalo- cephala). The posterior trunks run in the dorsal and ventral lines (iV. dorsalis, ventralis), to the extremity of the tail ; while of the six anterior nerves, two run in the lateral lines (iV. laterales), four in the interspaces between the lateral and median lines {N. sicb- mediani), and supply the papillte around the mouth. The gangUon cells lie partly near, in front of and behind the nerve ring, partly on the fibrous cords themselves, and are ai-iunged in groups which can be distinguished as ventral, dorsal, and lateral ganglia. There are in addition groups of ganglion cells in the median lines and in the lateral lines in the caudal region. As sense organs we must mention the eyes found in the free- living Nematoda, and the papillae and tactile hairs found principally in the neighbourhood of the mouth. Each papilla is suppHed by one nerve fibre, which is swollen to a knob and forms the axis of the papilla. [The Nematoda possess a' body cavity, but are without anj' trace of a vas- cular system.] Generative organs. The Nematodes are dioecious (mth ex- ception of the liermaphodrite Pelodytes, and of the lihabdonema NEMATODA. 347 {Ascaris) niyrovenosum, which produces first spermatozoa and later ova). The males are characterised by their smaller size, and by the posterior end of the body being generally curved. Both kinds of generative organs consist of single or paired and often much coiled Tubes, at the upper end of which the generative products are de- veloped, the lower ends representing the efferent ducts and recep- tacula of the generative products. The usually paired ovarian tubes, ;it the upper ends of which the ova arise, terminate in a short vagina, which opens on the ventral surface, rarely near the posterior end of the body. The male generative apparatus, which contains hat-shaped spermatozoa, is almost invariably represented by an unpaired tube, and usually opens on the ventral surface near the posterior end of the body in a common opening with the intestine. As a rule, the common cloacal portion contains two pointed chitinous rods, the so-called spicula, in a pouch-like invagination. These spicula can be protruded and retracted by a special muscular ap- paratus, and serve to fasten the male body to the female during copulation. In many cases {Strongyliclce) an umbrella like bursa is added, or the terminal portion of the cloaca can be protruded like a penis {Trichina) ; in this case the cloacal aperture Kes almost at the extreme end but is still ventral {Acroplialli). In the male papillse are almost always present in the region of the posterior end of the body, and their number and arrangement afford important specific characters. Development. The Nematoda for the most part lays eggs ; it is only in rare cases that they bear living yoting. The eggs usually possess a hard shell and may be laid at different stages of the embryonic development or before it has begun. In the viviparous Nematodes the eggs lose their delicate membi-anes in the uterus of the mother (Trichina, Filaria). Fertilization takes place by the entry of a spermatozoon into the ovum, which is still without a mem- brane. The segmentation is equ.al, and leads to the formation of a kind of invaginate gastrula. From the two cell layers are de- veloped the body wall and the alimentary canal. The embryo gradually assumes an elongated cjdindrical form, and comes to lie rolled vip in several coils within the shell. The excretory pore and the rudiments of generative organs, as well as a nerve ring, are present in the embryo, which is also provided with mouth and anvis. The free development is a metamorphosis, usually com- plicated by the circumstance that it is not undergone in the habitat of the mother. The young stages or larvae', probably of most Nema- 348 NKMATJIELMINTHES. todes, have a different habitat to that of the sexual aniuial; the young and the adult Nematode being contained in different organs of the same or even of different animals. The larvse live for the most par-t in parenchymatous organs, either free or encysted in a cohnective tissue capsule ; the adults, on the contrary, live principally in the alimentary canal. The embryo is almost invariably characterised by the special form of the oral and caudal extremities, but sometimes also by the poss(,s- sion of a boring tooth, or of a circle of spines (Gordius). Sooner or later the skin is shed, and the animal enters its second stage, which may often still be considered as a larval stage; repeated ecdyses precede the sexually adult stage. The post-embryonic development of the Nematodes presents numerous modifications. In the simj)lest cases the embryo, while still enveloped in the egg mem- branes, is transported passively in the food {Oxyuris vermiculuris and Trichocephalus). In many Ascaridce— to judge by the species parasitic in the Cat — the em- bryos, which are provided with a boring tooth, first make their way into an intermediate host, by which they are transported Fig, 2Sl.-Sclero.^tomum tetracauthum, en- jj^^g ^]^g intestine of the SeCOnd cysted (after R. Leiickart). host With the food or water. More frequently the young forms encyst within the intermediate host, and, enclosed in the cyst, are transferred into the stomach and intestine of the permanent host (fig. 281). For example, the embryos of Spiroptera ohtusa of the Mouse, while still in the egg membranes, are taken with the food by the Meal-worm, in the body cavity of which they encyst. In the viviparous Trichina sp>iralis there is a modification of this mode of development inasmuch as the migration of the embryos and theii- development to the encysted form found in the muscles (muscle-trichina) take place in the same animal which contains the sexually matui-e intestinal Trichinas. The development of the Nematode larvae often makes a considerable advance within the intermediate host into which they have migrated. Thus, for instance, in Gucullmius elegans, the embryos migrate into the Cyclops, and in the body cavity of these small Crustacea undergo two ecdyses and essential alterations of form, obtaining at this early I NEMATODA. 349 sta-e the characteristic oral capsule of the sexually adult stage, to which they only develop in the intestine of the Perch. According to Fedschenko,* a simHar mode of development occurs in Ftlaria medmemis. The embryos pass.into puddles of water, and migrate thence into the body cavity of the CyclopidcB; and after casting their skin assume a form which, except for the absence of the oral capsule, resembles that of the larva of Cucullanus. After the expiration of two weeks there is another ecdysis, with which is connected the loss of the long tail. The later history is unknown. It has not yet Fig. 282. — a, Shabdonema {Ascarin) nigrovenomm of a,boiit 3'5 mm. in length in the stage of maturity of the male products ; (?, genital glands ; 0, mouth; D, intestine ; A, anus ; iV, nerve-ring; Brz, glandular cells; ^, isolated spermatozoa, i, Male and female iJ/wifZiYis forms from about I'o mm. to 2 mm. long ; Ov, ovary ; T, testis ; V, female genital opening ; Sp, spicula. been discovered whether the migTation of the Filarian larva into the permanent host (Man, see p. 356) takes place with the body of the Cyclops, or independently after copulating in the free state. The embryos of some Nematoda develop in damp muddy earth,- after casting their skin, to small so-called Rhahditis forms with a double * Compare FedsclicnVo, " Ueber den Bau und Entwicklung der Filaria medinensis," in the Beriohtcn der Freunde der Natwiois-ieyischaften in 3/o.sJtaii Tom VIII. and X. IV8 ill in a NKMA.THELMINTHES. enlargement of the esophagus and with a pharynx armed with thr teetli. They lead an independent life in this habitat, and finaU migrate to lead a parasitic life within the permanent host, wher after several ecdyses and alterations of form, they attain the sexually mature condition. This mode of development occurs in Dochmi, trigonocephalus from the intestine of the dog, and very probal)ly the nearly allied D. {Ancylostomum) duodenalis of man, and also Sclerostonmm. The offspring of parasitic Nematodes may, however, attain sexual maturity in damp earth, as free Rhahditis forms, and represent special generation of forms wliose offspring again migrate and become parasites. Such a life history is a case of heterogamy. It occurs in Rhahdonema nigrovenosum, a parasite in the lungs of Batrachians. These parasites, which are about half to three-quarters of an inch long, all have the structure of females, but contain spermatozou, which are produced (as in the viviparous Pelodytes) in the ovarian tubes, but earlier than the ova. They are viviparous. The embryos make their way into the intestine of their host, and accumulate in the rectum, but iinally pass to the exterior in the fjeces, and so reach the damp earth or muddy water, where they develop in a short time into the Rhabditis-like forms, which have separate sexes and are barely 1 mm. in length (fig. 282, a and b). The impregnated females of the latter produce only from two to four embryos, which become free inside the body of the mother, pass into her body cavity, and there feed on her organs, which disintegrate to form a gi-anular detritus. They finally migrate as slender, already tolerably large Nematodes into the lungs of the JBatixichia, passing through the buccal cavity and glottis. The Le2Jtodera appendiculata, which lives in the slug Arion empiricomvi, also presents in its development a like alternation of heteromorphic generations, which, however, are not strictly alternating, inasmuch as numerous generations of the Rhccbditis form may succeed one another. The Leptodera are peculiar in that the form parasitic in the snail is a larva characterised by the absence of a mouth, and by the possession of' two long band-shaped caudal appendages; it quickly attains maturity, but only after a migration into damp earth and after losing the caudal appendages and casting the skin. The Nematoda feed on organic juices, some of them also on blood, and ai-e enabled by their armed mouth to inflict wounds and to gnaw tissues. They move by bending their body with a rapid undulatory movement towards the ventral and dorsal surfaces, which thus seem NKMATODA. 351 to be the lateral surfaces of the moving animal. Most Nematoda are parasitic, but lead an independent life in certain stages of their life history. Numerous small Nematoda, however, are never parasitic, but live freely in fresh and salt water and in the eai-th. Some Nematodes are parasitic in places, for example, Anguilhda tritici, dipsaci, etc. ; some live in dei^fying vegetable matter, e.g., the vinegar worm in fermenting vinegar and paste. Nevertheless very similar forms occur in the contents of the intestine and in the faeces of different animals and of man (xl. intestinalis, stercoralis). The power possessed by small Nematoda of resisting the effects of pro- longed desiccation and of coming to life again on being moistened is very remarkable. Fam. Ascaridse. Body tolerably stout. With three lips furnished with papilla. One of these lips is directed towards the dorsal surface, while the two others meet together in the ventral line. The posterior end of the male is ventially cvirved, and usually furnished with two horny spicula. Fig. 29&.—Aiicans himhricoides (after R. Leuckart). a. Posterior end of a male with the two spicula (Sp). b, Anterior end from the dorsal side, with the dorsal lip fm-nished with two papillse. c, The same from the ventral side with the two lateral ventral lips and the excretory pore (P). d, Egg with the external membrane formed of small clear spherules. Ascar/s L. Polymyarian, with three strongly developed lips, the edges of which are in the larger species provided with teeth. The pharynx is not sepa- rated as a distinct bulb. The caudal extremity is usually short and conical, and in the male sex invariably provided with two spicula (fig. 283, a). A. Inmbricoides Cloquet, the human round worm, a smaller variety in the pig CA. xuilla Duj.) The eggs pass into water or damp earth and remain there some months, until the embryonic development is completed ; they are probably carried into the alimentary canal of their later host by means of an inter- mediate host. A. megaloce^yhalM Cloquet (horse and ox): A. mi/sta-v Zed. (cat and dog), sometimes parasitic in man. 0.ajuris Rud. Meromyarian ; usually with three lips, which bear small papiUae. The posterior end of the oesophagus is enlarged to a spherical bulb provided with a masticatory apparatus. The posterior end of the body of the female is thin and pointed, while that of the male has only two prseanal and few postanal papillae, and a single spiculum (fig. 279). 0 vcrmicularu L., in the large intestine of man, distributed in all countries. The female is about ten mm. long. 0. cnrmda Eud., in the c^cum of the Horse. 352 NEMATnELMINTllES. Fam. StrongylidBB. The male genital opening is placed at the hinder end of the body, at tlie bottom of an umbrella- or bell-whaped l)ursa, the margin of which is furnished with a varying number of papillic. EvfitroHfi ylus Dies. With six projecting oral papillae, and a row of papillse on either lateral line. The bursa is bell-shaped and completely closed, with regular muscular walls and numerous marginal papillae. There is only one spiculum. The female genital opening is far forward. Thelarv^ live encysted in fishes. (^Filaria ri/xtira from Symhranckux'). E. (jirjaK Rud., the body of the female is three feet in length, and only twelve mm. thick. It lives singly in the pelvis of the kidney of the Seal and Otter, and very rarely in Man. Strongylm End. With six oral papillae and small mouth. Two conical cervical papillae upon the lateral lines. The ]j08- terior end of the male has an umbrella-like incom- pletely closed bursa. Two equal spicula, usually with unpaired supporting organ. The female sexual opening is sometimes approached to the posterior end of the body. They live for the most part in the lungs and bronchial tubes. St. lonffcvar/matrot Dies. Body 26 mm. long, 5 to 7 mm. thick. The female sexual opening lies directly in fi-ont of the anus, and leads into a simple ovarian tube. Only once found in the lung of a six-year old boy, in Klausen- burg. St. jjaradoxu-f Mehlis, in the bronchial tubes of the pig. St. Jilarin Rud., in the bronchial tubes of the sheep. St. com.nmtatus Dies., in the trachea and bronchial tubes of the hare and rabbit. St. miriotilaris Rud., in the small intestine of Batraclda. Bochiiius Duj. With wide mouth and horny oral capsule, the edge of which is strongly toothed. Two ventrally placed teeth project at the bottom of the oral capsule, while on the dorsal wall a conical spine projects obliquely forwards. JD. duoclcnalU Dub. {Ancylostoimiti duodenale Dub.), 10 to 18 mm. long, in the small intestine of Man, discovered in Italy ; very widely distributed in the countries of the Nile (Bilharz and Griesinger). By aid of its strongly armed mouth it wounds the intestinal mucous mem- brane, and sucks the blood from the vessels. The frequent hsemorrhages occasioned by these Dochmia are the cause of the illness known by the name of Egyptian chlorosis (fig. 284). It has lately been established that this worm occurs in Brazil, and that, like D. triffonocqjJiahis, it develops in puddles of water (Wucherer). D. tri{/07ioeep7iali(.it Rud., in the Dog. Sclcrostonmm Rud. With characters of Dochmius, but with a different oral capsule, into which two long glanular sacs open. So. equijium Duj. = armatiom Dies. In the intestine and the mesenteric arteries of the horse. Bollinger * has shown that the phenomena of colic in the horse may be referred to embolic processes proceeding from aneurism of the intestinal artery. Each aneurism contains about nine worms. * Bollinger, " Die Kolik der Pferdc und das Wurmaneurysma der Einge- weidearterien," Miinchen, 1870. Fis. 284. — Douchmius dodemilis (after R. Leuckart). a, male; O, mouth ; 3, bursa. 6, Female ; 0, mouth; A, anus; V, vulna. NEMATODA. 353 a Sc. tetvaeantKum Mehlis, also iu the intestine of the horse. The cmbiyos, after mi-rating into the intestine, become encysted in the walls of the rectum and ceecum, assume within the cyst their- definite form, break out from the cyst, and escape again into the intestine. Oiicullamts clcgam Zed., m the Perch. Fam. Trichotrachelidffi, with long neck-Uke thin anterior portion of the body. Mouth small, without papilhe. (Esophagus very long, traversing a peculiar cord of cells. T>-irhocr2M<.s- Goeze. Anterior part (fig. 28.5) of the body elongated and whip-shaped: posterior part cylindrical and sharply distinct, enclosing the 2-euerative organs, in the male it. is coiled up. Lateral lines absent. Mam median lines present. The penis is slender and furnished with a sheath, which is turned inside out when the former is protruded. The hard-shelled, citron-shaped eggs undergo the first part of their development in water. Ti: d/sipar End. In the human colon : these worms do not live fi-ee in the intestine, but bury their filiEorm anterior extremity in the mucous membrane (fig. \ C 285). The eggs pass out of the host with the faeces, as yet without a sign of beginning development, which only takes place after a prolonged sojourn in the water or in a damp place. According to the ex- periments of Leuckart per- formed with Tr. affiiiis of the sheep and Tr. crcitatus of the pig, embryos with the egg membranes, if introduced into the intestine, develop into the adult Tricoccphalv-'i ; and we may therefore conclude that the human Tr. disjjar is iuti'O- duced directly, and without an intermediate host either in the drinking water or in uncleaned food. The young Tr. (lis^iar is at first hair-like, and re- sembles a TricliiHa, and only gradually acquires the considerable thickness of the hind end of the body. TrichosomuM End. Body thin, hair-like, but the posterior end of the body in the female is swollen. Lateral lines and the principal median lines are present. The male caudal extremity has a cutaneous fold and a simple penis (spiculum) and sheath. Tr. muris Creplin., in the large intestine of the house-mouse. Tr. cras-slcmula Bellingh., in the bladder of the rat. According to Leuckart, the dwarfed male lives in the uterus of the female. There are usually two or three, more rarely four or five males in a single female. There is also a second . species of TrioJiosonmm found in the bladder of the rat. Tr. Hchmidtii v. Linst., the larger male of wliich was formerly taken for that of Tr. crax.'iicatida. Trichbia Owen.* Body thin, hair-like. Principal median lines and lateral * Compare the ^v^itingsof E. Leuckart, Zenker, E. Virchow, Pagenstecher, etc. 23 Fig. 285. —Trichocephalus diapar (after E. Leuckart). a. Egg ; b, female ; c, male witli the anterior part of the body buried in the mucous membrane; Sp, spiculum. 354 NEMATILELMINTHES. lines are present. The female generative oi)eiiiiig well foi ward. The posterior end of the body of the male has two tei'niiiial cones between which the cloaca is 'S'la. 286.- - Trichina spiralis, a, Matui-e female Trichina from the alimentary canal; da, since, according to Lewis, knotty swellings on the aorta and oesophagus are iiivariably found with these Filaria. F. papMlom Rud. in the peritoneum of the horse. F. loa Guyot., in the conjuntiva of negroes on the Congo. F. labialis Pane. Only once observed at Naples. An immature Filaria described as Filaria lenPis (^oculi himani) has been found in the human capsula leutis. Fam. Mermithidae. Aproctous Nematodes, with very long filiform body, and six oral papillfe. The male caudal region is broad, and is provided with two spicula and three rows of numerous papilliB. They live in the body cavity of insects, and escape into the damp earth, where they attain sexual maturity and copulate. Ifcrmin nlgrescens Duj., was the occasion of the fable of the rain worm. 31. alhicans v. Sieb. v. Siebold established by experiment the migration of the embryos into the caterpillars of Tinea evonymdla. Sphcerularin ioinM Leon Duf. Fam. Gordiidse. Body elongated and filiform. Without oral papillte and lateral lines, with a ventral cord. The mouth and anterior region of the alimentaiy canal is obliterated in the adult state. The testes and ovaries are paired and open to the exterior with the anus near the hind end of the body. Uterus unpaired, with receptaculum seminis. The male caudal region is forked, and is destitute of spicula. In the young stage they live in the body cavity of predatory insects, and are provided with a mouth. At the pairing time they pass into the water, where they become sexually mature. The embryos, which are provided with a circle of spines, bore through the egg-membranes and migrate into Insect larvae {Chironomm-larva; F2)ltcmeri(lce), and there encyst. Water * Compare H. C. Bastian, "On the Structure and Nature of the Dracunculus," Tram. Linn. Society, vol. xxiv., 1863. Fedschenko I. c. PHiETOGIfATHA. 357 beetles and other aquatic predatory insects eat with the flesh of the Epiiemerid laj-vfP the encysted young forms, which then develop in the body cavity of their new and larger host to J'oung Gorcliida;. Gord 'ms atpiaticns Dvj. Fani. Angttillulidee.* Free living Nematodes of small size. Caudal glands are sometimes present. The lateral canals are often replaced by the so-called ventral glands. Some species eithei- live on or are parasitic in plants ; others live in fermenting or decaying matter. The greater number, however, Uve fi-ee in earth or water. TylcncJms Bast. Buccal cavity small, and con- taining a small spine. The female genital opening lies far back. T. scandens Sc\m. = tritiei Ncedham, in mildewed wheat grains. When the grains of wheat fall the dried embryos grow in the damp earth, bore through the softened membranes, and make their way on to the growing wheat plant. Here they remain some time, perhaps a whole winter without alteration, until the ears begin to be formed. They then pass into the latter, grow, and become sexually mature, while the ear is ripening. They copulate and deposit their eggs, from which the embryos creep out, and at length constitute the sole con- tents of the wheat grains. T. dijjsaci Kuhn, in heads of thistles (Cardius) T. Damitiii Bast, on roots of moss and grass. Heterodera ScMclitii Schmidt. , roots of the beet-root, also of the cabbage, of wheat, barley, etc. Rhaiditis Duj., divided by Schneider into Lcj)todcra Duj. and Pelodera Schn. Rh. iieHlis Duj., head very sharply pointed, mouth with two lips, in the saKvary' glands of Limax cinereus. Rli. anf/iostoma Bnj. RJi. apj^endicvlata Schn., in damp earth, 3 mm. long. The larva, which is without a mouth, and has two caudal bands, is found in Arion empiricorwni. AngniUula aceti = ghitinis 0. Fr. Miill.. known as the vinegar worm and pasteworm, 1 to 2 mm. long. Of the many marine AngtiilluUdcs {Enoflldce), we must mention Bory- lalmm maximm Biitschli, D. stafjnaUs Duj., found in mud everywhere in Europe. Enclielklmm marinwn Ehrbg., Enoplus tr'ulentatm Duj. The abberant families Dcsmoscolccklce and Clientosomidm are allied to the Kcniatoda. The Ch^tognatha, The Chcetognatha, f containing only the genus Sagitta, are allied to the Nematodes. • They are elongated ronnd worms, with a pecu- liarly armed mouth and laterally placed horizontal fins, the mem- branous edges of which are supported by rays. The anterior portion of the body is sharply separated ofi" as a head, and bears in * Davaine, '• Eecherches sur I'Anguilliile du bl6 nielle," Paris, 1857. Kiihn, " Ueber das Vorkommen von Anguillulcn in erkrankten Bliithenkopfen von Dipsacus fuUonum," Zcltschr. f ur wiss Zool., Tom IX., 18.59. Bastian, " Mono- graph of the AnguillulidEe or free Nematoids, marine, land, and fresh water," London, 186-i. 0. Biitschli, " Beitrage zm- Kentniss der freilebenden Nema- toden," Nov. Acta, Tom XXXVL. lS73. Lad. Oeiiey, "Monographic der Anguilluliden,-' Buda-Pest., 18H0. f Compare A. Krohn, " Anatomisch-physiologische Beobachtungen iiber die Sagitta bi punctata, ' Hamburg. 18-14. R. Wilms, " De Sagitta mare germani- cum circa iusulam Helgoland incolente," Beroliui. 1846. Kowalevski, " Em- brj'ologische Studien an Wurmern und Arthropoden," Mem. de VAead. St. Peter. ^hourg, Tom XVI. 0. Hertwig, '• Die Chtetognatha, cine Mono- graphic," Jena, 1880. 358 NEM ATHELMINTHES. the region of tlie mouth two lateral groups of hooks which function as jaws. The nervous system consists, according to Krohn, of a cerebral ganglion on which the eyes are situated, and a ven- tral ganglion placed in about the middle of the body length. There are in addition two ganglia near the mouth, which may be considered as the subcesophageal gan- glia, a,nd are connected witli each other and with the cephalic ganglion by oeso- phageal commissures. Ot k9 01 9i Sb- r<<:< Fig. 288— Sagitta (Spadella) cephaloptera, magnified 30 times, viewed from the dorsal side (after O. Hertwig). F, posterior fin ; G, supra- oesophageal ganglion ; 2V, ten- . tacles; R, olfactory organs; Ov, ovary; Od, oviduct; T, testis; Vd, vas deferens; Sb^ vesicula seminalis. [The common view now is that the large ventral ganglion of the middle of the body, which is connected with the cerebral by com- miss?urcs, is homologous with the subcesophageal ganglia of other types.] The straight alimentary canal is at- tached to the body wall by a dorsal and ventral mesentery from the oesophagus backwards, and opens to the exterior at the base of the long tail, which terminates in a horizontal fin (fig, 288). [The body cavity is well developed, and divided by the dorsal and ventral mesenteries into two parts, and agaiii by two transverse verti- cal septa into a cephalic section, a section in the body, and finally a caudal section. Vas- cular and excretory organs are absent.] Reproduction. The Chcetognatha are hermaphrodite, and possess paii-ed ovaries, which open by two apertures at the base of the tail and are connected with seminal pouches. The testes also are paired, and situated posteriorly to the ovaries in the tail; their products pass to the exterior by openings at the sides of the tail. Segmentation is complete, and leads to the formation of a blastospheie. One side of this becomes invaginated so that the segmentation cavity is obliterated and a gastrula is formed, in the entoderm CH-^ITOGNATHA. 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the lattei- becomes folded in such a way that the archenteron is divided into a median and two lateral cavities. The layer of ceUs lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the body cavity, Avhile that of the middle cavity gives rise to the wall of the mesenteron or alimentary canal. The permanent mouth is formed at the end opposite to that at which the blastopore, which is now closed, was situated. There is but one genus, Sagitta Slab., of which several species, e.g., Sagitta Mjnmctata Krohn, S. germanica Lkt. Pag. from the Euro- pean seas have been more accurately described. Orde7- 2. — Acanthocephala.* Elongated round loorms with jjrotrusible 2>'>'ohoscis furnished with hooks ; without mouth and alimentary canal. The saccular, often transversely wrinkled body begins with a proboscis, which is furnished with recurved hooks and can be retracted into a tube projecting into the body cavity (sheath of the proboscis) (fig. 289, R and Rs). The posterior end of this sheath is fas- tened to the body wall by a ligament, and by retractor muscles. The nervous system (fig. 289, G) is placed at the base of the proboscis, and consists of a simple ganglion formed of large cells. Nerves are given off from the ganglion anteriorly to the proboscis, and through the lateral retractors (^retinacida) to the body wall (fig. 289, R). The latter supply partly the muscular system of the body, and partly the genital apparatus, in which there are, princi- pally in the male animal, special nerve centres Fig. 289— Anterior part. consisting of ganglionic enlargements. °* ^r, Echinorhyncht,.. o _ o _ R, Proboscis ; Ms, Sense organs are entirely wanting, as also are sheath of proboscis ; mouth, alimentary canal, and anus. ganglion ; Le, lem. ' _ _ ' msci ; R, retinacnla. The nutritive juices are taken in through the whole outer surface of the body. In the soft granular subcuticular * Besides Dujardin, Diesing, 1. c., compare : R. Leuckart, " Parasiten des Mensnhen," Tom II., 1876. GvecfE, " Untei-suchungen iiber Echinorhynchtis miliaris," Arch, fiir NaturgeHoh, 1864. A .Schneider, " Ueber den Ban der Acaiithocephalen," Milller'n ArnJdv., 1868. Also the Sitzungsherichtc der Oherhessiwhen GesdUohaft fiir Natur- nnd Heillamdc, 1871. 360 NEMATIIBLMINTHES. layer of the integument lies a complicated system of canals, filled with a clear fluid containing granules. Beneath the internal layer of the integument, which layer is often very extensive and of a yellow colour, is placed the powerful muscular tunic ; it is composed of external transverse and internal longitudinal fibres, and bounds the body cavity. The complicated x-amified system of dermal canals, of which two principal longitu- dinal trunks may be recog- nised, is filled with juices, and probably functions as a nutritive apparatus. The portion of this system which extends into two bodies (the lemnisci, fig. 289, Le) project- ing behind the proboscis through the muscular tunic into the body cavity, probably acts as an excretory organ, since the contents of the fre- quently anastomising canals of these lemnisci is usually of a brown colour, and consists of a cellular mass rich in concretions. According to Schneider, the vessels of the lemnisci open into a circular vessel in the integviment, and only communicate with the network of canals in the cephalic region, while the other dermal vessels (nutritive apparatus), the contents of which differs from that of the vessels of the lemnisci, are com- pletely shut off from the latter. Generative organs. The body cavity through which fluids circulate encloses the greatly developed generative organs, which are attached to the end of the sheath of the proboscis by a ligament (figs. 290 Fig. 290.— Male of Echi- norhyiKnis angustatus (after E. Leuckart) . a, iDroboscis ; Us, slieath of the probos- cis ; Li, ligament ; Gr, ganglion ; Le, lem- nisci ; T, testes ; Td, vasa deferentia; Pr, prostatic sacs ; De, ductus ejaculatorius ; P, penis ; B, retracted bursa. Fig. 291. — Generative ducts of a female Mchinorhynchus gigas (after A. Andres). Li, ligament ; F, d i s c- shaped flocculi; F', F", appendages of the same; U, uterus; I', vagina ; B, lateral pouches of the bell ; Gd, dorsal cells at the base of the bell ; 01, lateral cells. ACANTHOCEPHALA. 361 Fig. 292.— Embryo of Eehin- orhynehiia gigax enclosed in the egg membranes (after Leuckart) . and 291, Li). The sexes are separate. The male (fig. 290) has two testes (T), and the same number of efferent ducts {Vd). The latter unite behind to form a ductus ejaculatorius (De), which is often fur nished with six or eight glandular sacs (Pr), and a conical penis (P), at the bottom of a bell-shaped protrusible biu-sa {B), situated at the posterior pole of the body (fig. 290). The generative organs of the larger females (fig. 291) consist of the ovary developed in the ligament ; of a complicated uterine bell, beginning -with a free opening iato the body cavity ; of the oviduct and the short vagina, which is divided into several portions and opens at the posterior end of the body (fig. 291). It is only in the young stage that the ovary is a simple body en- closed by the membrane of the above-men- tioned ligament. As the animal increases in size, the ovary grows, and becomes di%'ided into numerous spherical masses of eggs, the pressure of which bursts the membrane of the ligament ; the masses of ova as well as the ripe elliptical eggs, which gradually become free from them, fall into the body cavity. The egg membranes are not formed till after seg- mentation, and ought perhapsto be interpre ted as embryo- nic mem- branes. The eggs, which already con- tain em- bryos, pass out of the body cavity into the Fig. 293. — Larvse of Echinorhgnchus profeus from Gammariis (after Leuckart). a, Free embryo ; Ek, embryonic nucleus, b, Older stage, uterine bell, with more differentiated embryonic nucleus, e, Young female worm ; which is Ov, ovary, d, A young male worm ; T, testes ; Le, lenmisci. continually dilating and contracting, thence into the o\dduct, and through the genital opening to the exterior. a 362 ANNELIDA. Development. Segmentation is irregular and complete, and results in the formation of an embryo, which is enclosed in three egg-mem- bi-anes. The embryo has a small, somewhat long body, ai-med with small spines at the anterior pole, and containing a central granular mass (embryonic nucleus) (fig. 292). It passes into the intestine of Am- phipqds [Ech. proteus, j)olymor2^1mH), or of Isopods (/iWi. anymtatus), and there becomes free, bores through the wall of the intestine, and after losing the embryonic spines, develops to a small elongated larva, Avhich, like a pupa, lies in the body cavity of the small Crustacean with its proboscis retracted and surrounded by its firm external skin as by a cyst (fig. 293). The skin of the larva gives rise only to the integument, the vessels and the lemnisci of the adult ; while all the other organs enclosed within the dermal muscular envelope, viz., the nervous system, the sheath of the proboscis, and the gene- rative organs, are developed from the so-called embryonic nucleus. It is only after their introduction into the intestine of fishes [Ech. lyrotms) or of aquatic birds {Ech. polymorphus), which feed on these Co'ustacea, that the larvte attain to sexvial maturity, copulate, and i-each their full size. The numerous species of the genus Echbuirlujncna 0. F. Miiller live prin- cipally in the alimentary canal of different Vertebrata ; the gut wall may be as it were sown with these animals. Ecli. pohjmorplms Brems., in the intestine of the duck and other birds, aho in the crayfish. Ecli. protem Westrumb., Ech. amjuntatun Rud„ in fi-esh-water fish. Ecli. gujas Goeze, as large as an Ascaru hmhricoides, in the small intestine of the pig. According t i A. Schneider, the embryo completes its development in the maggot. Lambl found a small sexually immature Echinorhynchus in the small intestine of a child which died of leukgemia. CLASS III.— ANNBIJDA. Segmented Vermes with brain, circum-o&sophageal ring, ventral nerve cord, and vascular system. The larva of Loven and its development seems to throw light upon the organization of the Annelidci and their relations to the lower worms and to the Rotifer a ; and further makes evident the relationship of the Annelida to the Gephyrea, a group of worms which possess an elongated body devoid alike of external and internal segmentation, and, as an equivalent of the ganglionic chain, a ventral nerve trunk, which is usually uniformly covered -vntli ganglion cells. The body of Lov6n's larva, from which we must derive the bod} of Annelids, is unsegmented, and represents mainly the Annelid head. LOTilN'S LARVA. 363 Behind it is continued into an indifferent terminal portion equivalent to the whole body of the adult. At the apical region of the larva (fig. 294, Sp) there is a thickening of the ectoderm, which is called the apical plate. This i-epresents the rudiment of the cerebral ganglion (apical ganglion), and gives off nerves to either side. The wide mouth (0) has a e HWK Fig. 294.— Development of Pnlygordius (after B. Hatsclaek). a, Toung larva ; Sp, apical plate with pigment spot ; Frw, pra-oral circle of cilia ; O, mouth ; Bow, post-oral circle of cilia; A, anus; M^, mesoderm; KN, head kidney. 6, Older larva with commencino; segmentation of the body, a second limb is developed in the head kidney, c, Older stage. The body is elongated to the fonn of a wonn, and divided into a number of metameres ; HWk, posterior circle of cilia; Af, eye spot; F, tentacle. ventral position, and leads into an alimentary canal, which opens at the posterior end of the body {A). In front of the mouth there is a strongly developed circle (prteoral) of cilia {Prw) ; and behind 364 ANNELIDA. the moiith a Aveaker (postoral) circle {Pow) ■ to the right and left there is an excretory canal (head kidney), whicli Ijegins with a ciliated funnel. By the differentiiition of the cephalic region of the larva into prasstoinial lobe and oral segment, and by the gi-adual growth in length of the posterior part of the body and the segmentation of the latter into a numljer of successive metameres, the originally un- segmented larva is transformed into an Annelid (fig. 294, a—d). There is, therefore, between the segmented adult and the larva a morphological relation sunilar to that be- tween the cestoid and the simple scolex, from the posterior end of which the proglottides are developed. The body of the Annelida is sometimes flattened, sometimes completely rounded and cylindrical. It is composed of a number of successive segments, which are usually sepa- rated from each other externally by trans- verse constrictions. The segmentation is generally homonomous, in that the segments following the head resemble each other not only in external appearance, but also in internal structure, i.e., they repeat similar sections of the internal organization. The terminal segment with the anus, however, has a special structure inasmuch as it retains the primitive, more indifferent char- acter of the posterior end of the body of the larva, and diiring the development of the worm gives origin to new segments anterior to itself. The homonomy of the preceding segments of the body is, how- ever, never complete, since certain organs are confined to definite segments. The internal segments, which are separated by dissejnments, either correspond with the external segmentation as marked by the annular constrictions of the integument {Chcetopoda), or each internal segment corresponds to a definite number (3, 4, 5, etc.) of the external rings {Hh-udinea). Fia. 294, tZ. — The young Polygordius ; G, cerebral ganglion ; Wg, ciliated pit ; D, alimentary canal. 1 ANNELIDA. 365 Organs of locomotion. Special organs of locomotion may either have the form of bristle-bearing unjointecl appendages (parapodia) on each ring of the body {ChcatojMcla), or of terminal suckers {Hiricclmea). In the first case each segment may possess a dorsal and ventral pair of appendages (the neicropodia and not023odia), which, however, are sometimes replaced by simple seta3 embedded in dermal pits. Alimentary canal. The month is placed on the ventral surface at the anterior end of the body, and leads into a muscular pharynx, which is often provided with a powerful armature and can be protruded like a proboscis. This is followed by the gastric region of the gut, which occupies the greatest portion of the length of the body, and is either regiilarly constricted in coi-respondence with the segments, or possesses lateral diverticula ; it is only coiled in excep- tional cases. The amis is usually dorsal at the hinder end of the body. The nervous system consists of a cerebral or supra - oesophageal ganglion, which is derived from the apical plate of the larval prse-oral lobe, of an oesophageal ring, and of a ventral cord or ganglionic chain, the two halves of which lie more or less approached to each other in the median line. The venti-al cord arises from two lateral nerve cords, which probably correspond to the latei'al nerve trunks of the Ne- mertines. These two cords are continuous with the cesophageal commissures, and, like the latter, are uniformly covered with ganglionic cells. This form of the nervous system may persist, as may also its ectodermal position (Archiannelida, Protodrilus) (fig. 295). In most Annelida, however, this is only a transitory condition J for at a later stage the lateral cords become separated from the ectoderm, come together in the median line, and acquii-e a segmentation corresponding to the metameres of the body. The nerves of the sense organs arise from the cerebral ganglion ; the other nerves pass out from the parts of the ventral cord or, as the case may be, from the ganglia of the ventral chain and from the longitudinal commissures between the latter. There is in Fig. 295. — Transverse section through the body of Protodrilus (after B. Hat- schek). S S, The two lateral trunks of the nervous system; G, ganglionic layer of the same ; D, alimentary canal ; iV, nephridium ; M, muscles ; Ov, ova. 366 ANNELIDA, xlie almost all oases a visceral nervous system {sympathetic). . following sense organs are found: paired eye spots vvdth refraeti.,- structures, or largei- more complicated eyes; also auditory vesicles upon the oesophageal ring (lirancliiate worms), and tactile organs The latter have, hi the Ghaitopoda, the form of tentacles and tentacular cirri on the head and of cirri on the parapodia. When tentacle^ and cirri are absent, the anterior end of the body and the region ..i the mouth seem to function as tactile organs. Vascular system. A blood vascular sy.stem is very commonly present; in many cases, however, it seems not to be complete]}- closed, but to communicate with the body cavity, which contain! blood. Two main vascular trunks, a dorsal and a ventral, connected with one another by numerous transverse anastomoses, are generally present. The blood is usually coloured (green or red), and its cir- culation is effected by the contractility of the walls of certain vessels : sometimes the dorsal vessel, sometimes the ventral, and sometimes the transverse connecting vessels are contractile. Lateral longi- tudinal vessels are often present in addition to the above. In the Hirudinea these, as well as the median contractile blood sinus, are probably to be regarded as isolated parts of the body cavity. Special respiratory organs are found amongst the Chcetopoda in the branchiate worms. The excretory organs, corresponding to the water-vascular or excretory system of the Platyhelminthes, have the form of coiled canals (segmental organs or nephridia), which are repeated in pair.- in each segment. Each nephridium usually begins with a ciliated, funnel-shaped opening into the body cavity, and opens to the exterior by a lateral pore (fig. 70). These may assume in certain segments the function of generative ducts, e.g., the nephridia of the Gepliyrea, which, however, are much reduced in number. In the cephahc segment or head there is also a segmental organ (head kidney), which in the larva functions as a kidney and later disappears. Reproduction. — Considering the independence of the segments, to which we ascribe the value of a subordinate (morphological) inch- viduality, the occurrence of asexual reproduction by fission and gemmation in the long axis {Chcetopoda) is not surprising. Nume- rous Annelida {Oligochaita, Hirudinea) are hermaphrodite ; the marine Chcetopoda, on the contrary, are for the most part of sepai'ate sexes. Many lay then- eggs in special sacs and cocoons, in which case development is direct, without metamorphosis. The marine worms, on the contrary, undergo a more or less compliciited CHiETOPODA. 367 metamorphosis. The Annelida comprise terrestrial and aquatic animals, and they eat, for the most part, animal food. Many of them [Hiruclinea) ai-e occasionally parasitic. In the group of the Annelida three principal divisions may be distinguished, — the Clicntopoda, the unsegmented Gejihyrea, and the Hirudinea which are adapted for parasitism. The Hirudinea are not in any degree to be regarded as Annelida of a lower grade of organization, but they rather present, at least in the case of some organs, as alimen- tary canal, circulatoiy and generative organs, a more comphcated structure, and agree most closely vdih. the Oligochseta, from which they may be deiived. Sub-class 1. — Ch^topoda.* Free living Annelida, with joairecZ tufts of setce on the segments, frequently with distinct head, also with tentacles, cirri, and branchice. The Chjetopoda are divided externally into segments, which cori'espond with the metameres of the internal organs, and are, with the excep- tion of the anterior region, which is .distinguished as the head, usually tolerably alike (fig. 296). with setas are very frequently present on the segments ; their prin cipal function is that of locomotion, but their va- rious appendages, the branchice and cirrz, also discharge tactile and respi- f — ratory functions (fig. 297). * Besides the older works of Savigny, Audomn et Milne Edwards, and Quatrefages, compare E. Grube, " Die Fami- lien der Auneliden," ArcMv fur JVatnrgesclt, 1850 and 1851. E. ClaparMe, " Rechevclies anatomique sur les Ann61ides, etc.," Geneve, 1861. E. Cla- parMe, " Les Amielides cheto- podes du golfe de Naples." , . ,o-n ■■ , Geneve et Bale, 1868. also Sup- plement, 18/0, and "Eecherches sui- la structure des Annelides sedentaircs," Geneve, 1873. Fr. Leydig, 1. c, also Tafelu zur vergl. Anatomie," 1864 Fl&. 296.— ffraitfff fusi- fera (after Quatre- fages). Ph. pharynx -D, alimentary canal ; C, cirri; F, tentacles. Parapodia provided Pis. 297.— Dorsal (BP) and ventral (FP) Para- podium with bundles of setaj of Nereis (after Quatrefages). Ac, Aciculum ; Mc, dorsal ciiTus ; Sc, ventral cirrus. 368 ANNELIiUA. Tlie form of the movable setae varies extremely, and affords a good character for the classification of families and genera. According to the strength, form, and mode of ending (fig. 298), the following a, i\c d e / ^^ff Fig 298. — Setffi of different Foh/chceta (after Malmgren and Claparede). a, Hooked seta of Sabella crassicoruis ; h, of Terehella BanieUseni ; c, seta with piral ridge from iS^AeneZai* ; d, lance-shaped seta of PhyllochiEtopterus ; e, of Sabella crassieornis ; f, of Sabella pavonit ; g, Composite sickle-shaped seta of Nereis cultrifera. forms can be distinguished : hair-setae, hooked-setae, fiat-setae {jxdeoi), lance-setae, sickle-shaped setae, etc. When the parapodia and their appendages are com- pletely wanting, the setae are embedded in pits in the integument, and are arranged either in one or two rows on either side, that is, in a lateral ventral row on either side, or in a ven- tral row and a dorsal row on either side. In such cases the number of setae is small {Oligo- chceta). The setae may, on the contrary, be pre- sent in great number, so that the integument on either side seems to be covei'ed with long hairs and setae, and a thick felt of hairs shining with a metallic lustre is distributed over the whole dorsal Fig. 299. — Anterior end of Folynoe extenuata, the first elytron on the left hand being removed (after Cla- pai-ede). The two seta of the oral segment are visible ; El, Elytra. CH.Il'rOPODA. 369 surface {A'phrodite). The appendages of the parapodia present an equally great variety of form and not unfrequently vaiy in the tlifterent parts of the body. They are either simple or ringed tenta- cle-like processes, the cviTi, which are disting-uishable into dorsal and \-entral cirri. The cirri are for the most part filiform, and sometimes jointed or conical, and then are often provided with a special basal joint. In some cases the dorsal cii-ri are flattened out as broad scales and leaves, the elytra, which constitute a protective covering {A2)hro- dite) (fig. 299). In addition to the cirri, branchiaj which may be filiform branched and antler-like, comb- or shaped or in the form of tufts, are fi-equently present ; sometimes they ai-e confined to the middle region of the body, or are extended over almost the whole dorsal surface; sometimes they are confined to the head or to the anterior segments immediately following the oral segment (cephalic branchise). The two anterior segments may be i-egarded as foi-ming the head ; they are fused together, and are, with regard to theii- appendages, different from the following segments (fig. 245). The anterior segment projects beyond the mouth as the fi'ontal lobe, and bears the tentacles and palps Yvalps ai-e ten- tacular structures arising from the ventro-lateral sides of the head, vide p. 379] and also the eyes ; the posterior cephalic segment or oral segment bears the tentacular cii-ri. The last segment (anal segment) beai-s the anal cirri. The alimentary canal is usually sti-aight, and extends from the mouth to the anus, which is terminal and rarely "dorsal; it is divided into oesophagus, intestine, and rectum (fig. 300). There is in most cases a dilated muscular phai-yngeal bulb which is armed with papilhe or with movable teeth and can be pi-otruded as a proboscis. The intestine usually preserves the same structui-e in its entrre length and is divided by regular constrictions into a number of 24 Fig. 300. — Alimentary canal of Aphrodite aculeata (after M. Ed- wards). Ph, pharynx ; D, intes- tine ; L, hepatic appendages. CH^TOPODA. divisions or chambers, wliich correspond to the segments and dilate again into lateral diverticula and cajca. The constrictions are due to filamentous or membranous septa (dissepimenta), which divide tlie body cavity into tlie same unmljer of chambers lying one behind another. The vascular system appears to be closed, so that the clear nutri- tive fluid found in the body cavity, which, like the blood, contains amoeboid corpuscles, does not communicate with the usually coloured contents of the vessels. The dorsal and ventral vessels are not only connected at each end by lateral loops, but also in each segment ; and from these connecting vessels proceed peripheral networks, which extend into the integument, the wall of the alimentary canal, and the hrcmcMoi. Special organs of respiration are wanting in almost all the Oligo- chceta. In the marine Worms, on the contrary, branchiae are very genei'ally present, usually as appendages of the parapodia. These branchiae are either simple cirri which have delicate ciliated walls and contain blood-vessels, or are branched {Amjihinome) or in some cases are pectinate structures {Eunice) which co-.exist with special cin-i on the notopodia (fig. 246). The branchiae are sometimes confined to the middle segments {Arenicola), and are sometimes develoi^ed on almost all the segments on the dorsal siirface, being simplified towards the posterior end of the body {Dorsihrcmchiata). In the Tubicolce the branchiae are confined to the two {Pectinaria Sabellidce) or three {Terehella) anterior segments. The respii-atory function is, however, also shared (Gajntibranchiata) by a number of elongated tentacles which ai-e gi-ouped in tufts on the head. These are, in the Sabellidce, supported by a special cartilaginous skeleton, and may have secondary twigs developed upon them. They are either simply arranged in a circle round the mouth, or in two fan-Kke lateral groups {Serpulidce), the base of which is not unfi-equently drawn out into a spiral plate. Such bi'anchial sti-uctures, however, also function as organs of touch, as oi-gans for procuring nutriment, and even foi' building the tubes and shells. Excretory organs. — There are usually in all the segments paii-ed segmental organs, which serve as excretory organs. They begin, as a rule, with a ciliated funnel in the body ca,vity ; they possess a glandu- lar wall, are several times coiled upon themselves, and open to the exterior in each segment by a lateral pore. These glandular passages serve in general for the removal of matters' from the body cavity, and in the marine .Chcetopoda are used during ihe NEllVOUS SYSTEM. 371 breeding season as oviducts, or vasa deferentia, and ijermit of the passage outwards of the generative products, lohich have been $etfree in the body cavity. Amongst the special glands in the body of the Ghcatopoda, those cutaneous glands of the Oligochmta which give rise to the thickening (extending over several segments) knoAvn as the clitellus or girdle, are especially worthy of remark. The secretion 'of these glands perhaps assists the intimate connection of the Worms during copula- tion. In the Serpulidce there are present two large glands, which open upon the dorsal surface of the anterior portion of the body and furnish a secretion used in the formation of the tubes in which the animals live. e Pig. 301— Brain ami anterior portion of the ganglionic chain, a, of Serptda ■ h, of Neveis, (after Quatrefages) ; O, eyes ; G, cerebral ganglion ; c, oesophageal commissure ; T7q, suboesophageal gangUon ; e e, nerves to the tentacular cirri and the mouth segment. Nervous system.— The longitudinal trunks of the ventral cord are often so closely approached that they seem to form a single cord {Oligochoita). In the TuUcolm (fig. 301), on the contrary, they are very mdely separated from one another, especially in the anterior part of the ganglionic chain {Serpula). The visceral nervous system consists of paired and unpaired gangha, which supply the oral region and especially the protrusible proboscis. Sense organs.— Paired eyes upon the surface of the frontal {i.e. 372 OHj^ETOPODA. prsBOi-al or cephiilic) lobe are widely distributed. E}'e-.spots may also be present upon the posterior end of the body (Fahricia), or may Ije regularly repeated upon the sides of each segment {PolyojMhalmus). In species of Sabella, pigment-spots with refractive bodies are found even upon the branchial filaments. The large cephalic eyes of the genus xlfcio^;e* are the most highly developed, Joeing provided with a large lens and a complicated retina. The presence of auditors- organs seems less frequent. They appear as paired otolithic vesicles upon the oesophageal ring of Arenicola, Fahricia, some Sabellidm and young Terebellidce, etc. Besides the tentacles, cirri and elytra, other portions of tlie sui-face of the body may be sensi- tive to tactile sensations. On such parts there are either stiff hairs and tactile setse, or, as ui Biolicerodorum, special tactile warts with ner\'e terminations. Reproduction.— In the smaller Chaitopoda asexual genei-ation by fission and gemmation may occui-. Either (fissiparous reproduction) a large numbei- of segments of the parent be- come separate and give rise to the body of the new worm, as for example in Syllis inolifera, whei-e a series of the posterior segments, which are filled with ova, become separated by a simple transverse fission, after the formation of a head pro^dded with eyes; or (gemmiparous repro- duction) a single segment only, usually the last, becomes the stai'ting-point for the foi-mation of a new indi\T.dual. In this way Autolyti'.s f ro- lifer, one of the Syllidce, asexually repi'oduces itself, giving rise to a male and female sexual form, knowTi respectively as Folybostrichus Miillerif (male) and Sacconereis helgolandica (female). This is a case of altei'nation of gene- I'ations, for the asexual form, Axitolytus, gives lise by budding in the long axis to the sexual forms (fig. 302). In this case a whole series of segments are developed Fig. S02. —Autoh/tns cor- nnhis, with the male animal PolybonMchns (after A. Agassiz). F, Tentacles ; CT, tenta- cular cirri : /, tenta- cles ; ct, tentacular cirri of the male. , " Ueber das Auge der Alciopiden, etc.," Marburg. 187f) ;and " Unter- iiber die Alciopiden," Kor. Art. der K. Lcop. Akad., etc.. Tom * Greefe suchiuigeu XXXIX., Nro. 2 t Compare besides the Avorks of 0. Fr. Miiller, Quatrefagcs, Leuckart, and Krohii. especially A. Agassiz, " On alternate Generation of Annelids and the embryology of Autolytus cornutus," Jio-fton Jonrn. Naf. Hist., vol. iii., 18()3. GENEBATTVB ORGANS. 373 ill front of the last segment of the asexual form, and these segments, after the formation of a head, constitute a new individual. As this process is repeated, a chain of connected individuals is formed, and these, as soon as they are separated, represent the sexual individuals. Among the freshwater Naidfe, in CJmtogaster, a regular and continued budding in the long axis leads to the foi-mation of chains, consisting of not less than 12 to 16 zooids, each having only four segments, while the sexual individuals consist of a greater number of segments. A similar process occurs in the mode of reproduction observed by 0. Fi'. Miiller in JSFcds prohoscidea, from the last segment of which a new zooid is produced. Both generations of Neds, however, become sexually mature. [For a more complete account of the asexual reproduction of Cliaetopoda, vide Balfour, "Comparative Embr^'ology," vol. i., pp. 283, 284.] The Clmtopoda are, with the exception of the her- maphrodite Oligochceta and certain Seiyididce {e.g., S2n- rorhis spirillum, Protula Dysteri) of separate sexes. Male and female individuals seem occasionally so strikingly different in the structure of Fig. 303.— a pai-apodium of Tomopteris with a their organs of sense and lo- ^^^^ °^ '^'^'^^ "^'^ ^''^^ ^^^^^"^ ° Gegenbaiir). comotion that they have even been taken for species of distinct genera. Besides the above- mentioned Sacconereis and Polyhostrichus, the asexual genei-ation of which is Autolyttts, a similar sexual dimorphism has been shown by Malmgren for Heteronereis, a genus of the Lycoridce, in which the males and females differ both in external form and in the number of their segments. A remarkable case of heterogamy is also afforded by this genus, in that a generation of smaller animals swmming upon the surface alternates with a generation of larger forms living upon the bottom. The generative apparatus of the Oligochceta is very highly deve- loped. The ovaries and testes lie in definite segments, and empty their contents by dehiscence of their walls into the body cavity. Special generative ducts often co-exist with segmental organs in the same segment {0. ferricolce), while in other cases the segmen- tal organs are wanting in the generative segments ((9. limicolce). In 374 CHtETOPOBA. the marine Chcetopoda, the ova or spermatozoa originate on the body wall (fig. 303) from cells of the peritoneal membrane, either in the anterior segments alone or along the whole length of the body. Tlie generative products then become free in the body cavity, attain maturity, and pass through the segmental organs to the exterior. Only a few Chcetopoda, as Eunice and Syllis vivipara, are viviparous, all the rest are oviparous ; many lay their eggs in connected groups, and carry them about with them, while the Oligochceta lay theirs in cocoons. Development. — The segmentation is unequal. A primitive streak is very generally developed, though sometimes not until the embiyo has left the egg. It arises on the ventral side in consequence of the development of a middle layer and from neutral plates of the uppei- layer. Excepting in the Oligochceta, the young forms undergo a metamor- phosis and after leaving the egg appear as ciliated larvae, which are provided with mouth and alimentary canal, and essentially resemble, with some modifications, Loven's larva. The capability of renewing lost portions of the body, more espe- cially the posterior part of the body and different appendages, seems to be generally distributed. The Lumhricince and certain marine Worms [jDiopatra, Lycaretus) are even able to replace the head and the anterior segments, with the brain, oesophageal ring, and sense apparatus. Fossil remains of Chcetopoda are found from the Silurian onwards in the most diiferent formations. Order 1. — PoLYCHiETA.* Marine Chcetopoda, toith numerous setce embedded in the 2yarap>odia, usually toith distinct head, tentcccles, cirri, and hranchicB. They are for the most part dioecious, and develoji tvith metamorphosis. The marine Chcetopodct must be considered as belonging to a higher grade of life, on account of the sharp distinction of the head which is composed of the prfestomium (prteoral lobe) and oral segment (in the Amjjhinomidce several succeeding segments are also included), and of the presence of the tentacles, tentacular cirri and * Aiidouin et Milne Edwards, " Classification des Annelides et dcscrijjtion des cellos qui habitent les cotes de la France," A/males dm Sc. Nat., Tom XXVII. to XXX., 1832-88. Delle Chiajc, '' Descrizioni e notomia degli animali senza vertebre della Ricilia citeriore," Napoli, 1841. Quatrefages. " Histoire naturelle des Aiiueles," Tom. I. and II., I860. Also the numerous ■vrritings of E. Grube and E. Claparede. POLTOII.IJTA. 375 gills, and also of the setfe embedded in prominent parapodia, which serve as aids to swimming. The internal organization, however, is in no way more complicated than that of the Oligochceta. Neverthe- less all these- distinctive characters may be less and less marked, and, indeed, so "completely vanish that it is difficult to draw a sharp Une' between the Oligochceta and the Polychceta. The parapodia {Gapitellidce) and also the set£e {Toino2Jteridce) may be wanting. In rare cases, bundles of setffi are present on all the segments behind the head ; they are however arranged in a single row and embedded in a single pair of ventral retractile parapodia in each segment. Tio. 301. — Head and anterior body segments of Nereis Dumerilii (after E. Clapar^de). O, Byes ; F, palps ; Ct, tentacular cirri ; S^, pharyngeal jaws. This arrangement, which is found in Saccocirrus and its allies, pro- bably represents the primitive state, especially as in these animals the character of the nervous system, which lies in the ectoderm external to the dermal muscular envelope, and of the sense organs, which are reduced to two simple tentacles upon the cephalic lobe and to ciliated pits, indicates lower and more primitive conditions. In another and very remai-kable type, Polygorcliibs Schn. and Protodrilus Hatsch., not only parapodia and setse but also the external segmentation are wanting. The segmentation of this achsetous and externally unsegmented worm is entirely confined 376 ClIiETOroDA. to the internal oi-ganization and is, as compared with that of all other Annelida, to a certain extent completely hotnonomous, inasmuch as the esophagus is confined to the cephalic segment and does not extend into the anter-ior segments of the body. Further, the nervous system is connected with tlie ectoderm along its whole length, and the cerebral ganglion maintains its primitive position at the anterior end, corresponding to the apical plate of the larva ; and the ventral cord is without ganglionic swellings. In all the above points these forms seem to have preserved the primitive An- nelidan structure, and they have therefore b e e n united by Hat- schek into a special class, the Archian- nelida. In the Poly- cliceta the vascular system is compli- cated by the ap- pearance of bran- chiae, which are provided with blood-vessels. In the forms with dorsal branchiae the branchial blood is derived from the dorsal trunk and re- turned to the ven- tral by special vessels. When, on the other hand, as in the tubicolous capito-branchiate forms, the respiratory apparatus is concentrated on a few segments, the vascular system of that part undergoes greater modifications. In the Tere- hellidce (fig. 305), the dorsal trunk dilates above the pharynx to a branchial heart from which lateral branches are given off to the branchise. In the same region the transverse loops connecting the Fig. 305. — Tereb^lla nehulom, opened from the dorsal side (after M. Edwards). T, Tentacles ; A', Branchia? ; Di;, dorsal vessel or heart. POLYCHjETA. 377 dorsal and ventral trunks may perform the function of hearts, as is also frequently the case in the Olicjochcuta. Finally the vascular system is in many cases considerably reduced, and, according to Claparede, is entirely wanting in Glycera and Ccqntella, in which the blood is represented by the perivisceral fluid. The generative organs, unlike those of the hermaphrodite Olicjo- chceta, are usually placed in difierent individuals ; and the males and females are sometimes of very different forms. A number of herma- phrodite Polychceta are, however, known ; such principally belong to genera of the Serjndidce, e.g., Spirorhis, Protula. The development, unlike that of the Oligochceta, is invariably con- nected with a metamorphosis. Segmentation is, as in the Hirudi- nea, usually un- equal, and even the first two seg- mentation spheres are of unequal size. The smaller (animal) half, which segments more quickly, gives rise to smaller segments, which grow round and envelope the lai'ger segments proceeding from the segmentation of the larger half. In the subsequent development a piimitive streak makes its appearance in all embryos of PolychcBta, sometimes, how- ever, not until the embryo has begun to lead a free life as larva. The ganglia become differentiated later into the ventral chain. In the free-swimming larvfe the cilia are rarely distributed over the whole surface of the body (Atrocha*). They are usually confined to special rows (ciliated rings) ; sometimes, as in Loven's larva, there is one row placed in front of the mouth at some distance from the * Compare E. Claparede and E. Metschnikofi, '• Beitraa:c znr Entwickclungs- geschichtc der Chretopoden," Zeitschr. fur wiss. Zool., Tom. XIX., 1869. A Fig. 306. — Larvte of Polychoeta (after Busch). a. Larva of iVerttn F, tentacle ; Oc, eyes ; FrW, prworal circle of cilia : O, mouth; A, anus. 6, Mesotrocbal, larva of Chatopferut ; IVp, circle of cilia. 378 CH^TJTOPODA. anterior end of the body {Cephalotrocha, e.(j., lar\-a of Folynoe). Sometimes tliere are two rows, one at each end of the body, con- stituting a pra>oi-al and perianal ring {Telotrocha, e.g., Hpio-NeiMhyn- larva). In ad(Htion to these two rings of ciHa, incomplete rings may also be present on the ventral surface (Gastrotrocha), or both ventrally and dorsally (Aviphitrocha). In other cases one or more rows of cilia surround the middle of the body (Mesotrocha), while the terminal rings (progoral and perianal) are absent {Telepsavus-Chcatop. terus larva) (fig. 306). Many larv£e are provided with long pro- visional setaj, which are later replaced by the permanent structur< > (Metachceta). In spite of their great diversity of form the Chaitopo.l lai'vje can in their later development also be reduced to the type of the larva of Loven. Relatively few forms, as for instance the transparent Alciojnda , live at the surface (pelagic animals) ; most of them live near the coast. Numerous foi-ms descend into the deep sea. i\Iany have the power of emitting an intense light, especially species of the genus C'Ace- topterus which emit light from their an- tennte and appendages. The el}-tra of Polynoe, the tentacles of Polycirrus, and the integument of certain Syllicloe, are „ „„- „ . also phosphorescent. Panceri* has shoA\ii r IG. o07. — Jyereit miirgaritacea. ^ Head with protruded jaw that the Seat of the pliosphoresccnce is SThTdorLi'SrfaSe; unicellular cutaneous glands, which, \ii M. Edwards). K, Jaws; F, Polynoe, were proved to be in communi- tentacles ; P, palps; Fc, ten- ..i tacuiar cirri. cation With nerves. Sub-order 1. Errantia. Free-swim- ming, predacious Polychceta. The prfestomium always remains in- dependent and forms, with the oral segment, a well-marked head which bears eyes, tentacles, and usually tentacular ciri-i. The parapodia are much more developed than in the Tuhicolce, and, together with theii- very variously shaped setie, serve as oars. The anterior portion of the pharpix can be protruded as a probosci> and is divided into several portions ; it is either beset with papilla or contains a powerful masticatory apparatus, which appears at its extremity when protruded (fig. 307). Branchice may be wanting: when present, they usually appear as comb -shaped or dendritic * Pauceri, " La Ince e gli organi luminosi cli alcuni annelidi," Atti dclla E. Acad, sciensz fi. e mat. di Napoli, 1875. ^ POLYCHjETA, ebbantia. 379 tubes on the parapodia {DorsihrancMata). The Errantia are pre- datoxy in theii- habits {Rcqxtcia) and swim freely in the sea; but they may also inhabit temporarily thin membranous tubes. Fam. Aphroditidffi. Brofid scales idntra) on the riotopodia. These are usually placed on alternate segments, often only on the anterior part of the body/ Prffistomium, with eyes, with one unpaired and usually two lateral tentacles, to which may be added two stronger lateral ventrally placed tentacles (palps). Proboscis cylindrical, protrusible, \\ath two upper and two under jaws. 4.pkrodHe aculeata Lin. dlli/stri.v marina Eedi.) The back has a thick felt of hairs. Eyes sessile. Numerous setre on the neuropodia. Polyno'e xcfllopcndrina Sav. Ocean and Mediterranean. Fam. Eunicidee. Body very long, composed of numerous segments. Prfesto- mium vdth. several tentacles. Parapodia usually uniramous, rarely biramous, usually with ventral and dorsal cii-ri as well as branchiEB. One upper javv composed of several pieces, and a lower consisting of two plates ; both lie in a sac, the jaw-sack, on the dorsal surface of which runs the pharyngeal tube. Stauroceyhalus vittatvs Gr., Ilalla (^Lysidioe) pao^thenopeki Delle Ch., Naples. Biopatra neapoUtana Delle Ch., Naples. Eumce Ilavami And. Edw. Fam. Nereidae = iy("('?'Mte.* The elongated body is composed of numerous segments. The prsestomium has two tentacles, two palps, and four eyes. The parapodia are either uni- or bi-ramous, and are furnished with dorsal and ventral cirri and with composite setfe. Proboscis usually possesses spines, and always two jaws. Nereis DumeriUi And. Edw., French and English coasts, to which belongs Hctermeveis fuGioola Oerst. N. cultrifera Gr.. Mediterranean N. fucata Sav., North Sea. The form formerly distinguished as Hcteroncreis Oerst.* differs from Nereis in the great size of the prajstomium and of the eyes, also in the extraordinary development of the parapodia, and in the abnormal formation of the hinder end of the body. It belongs, however, to the same cycle of development as Nereis and Nereilejuis. Fam. Glyceridae. Body slender, composed of numerous ringed segments. The prfestomium is conical and. ringed, with four small tentacles at its point and two palps at its base. The proboscis can be protruded to a great length, and is provided with four strong teeth. The hremal fluid, coloured by red corpuscles, is contained in the body cavity and the branchial sinuses. There is no special vascular 'system. Glijcera cajritata Oerst., North Sea. Fam. Syllidae. Body elongated and flattened, head usually with three tentacles and two to four tentacular cirri. The protrusible proboscis consists of a short proboscis tube, a pharyngeal tube lined by stiff cuticular formations, and a portion characterised by annular rows of points. Sexual and asexual individuals, differing in form, are sometimes found in th6 same species. Many carry their eggs about with them until the young are hatched. Syllis rittata Gr., Mediterranean. Odontosyllis gihha Clap., Normandy, Aittolytus pvolifcr 0. Fr. Mlill., asexual form. The male has been described as Polyhostriclms Miilleri Kef., the female as bacconercis helfjolandica Mlill. S2Jha'rodorvrii 2)einjMtii.s Gr., Mediterranean. Fam. Alciopidae (Alciopea). With two large hemispherical projecting eyes. Ventral and dorsal cirri leaf-like. The proboscis is protrusible, the tube of the proboscis being thin walled and its terminal portion' thick walled. At * Compare E. Grube, " Die Familie der Lycorideen," Jahresher. der ScMvsis- elien Gesellscliaft, 1873. 380 CHiETOPODA. its fiperture aro two hook-sliaped papilbe. The lavvaj are in part parasitic in tl,,- Cy (I Ijqn (!(/■. Alriojia C/intrninU Delle Oh., Nai)les, Fam. Tomopteridee {Gymuocopa). Head well marked, two e^'us. hifi.i pnt'stomiiim, and four tentacles, of which two in many species are only presei;i in the young. The mouth segment has two long tentacular cirri which an supported by a strong internal seta. The moutli is without proljoscis a.,.] jaws. The segments are provided with large bi-lobed i)arapodia without seta- r supporting setie. As a rule, double as many cirri or short wart-like protube- rances arc found on the margin of the body. M. glairum, cirrifermn F. S. Lkt, Sub-order 2. Seden- taria = Tubicolae. * Fig. ZOS.-Sph-orln Imrh (after Claparede). a, The With illdistuictly Sepa- animal removed from its tube, strongly magnified ; rated head and short b, tube ; T, tentacles ; Bs, brood-pouch with oper- culum ; Br, glands, Ov, ova; Oe, oesophagus; M, usually not protrUSlble istomach; 2), intestine. proboScis, without jaWS. The branchife may be entirely absent and in many cases are confined to the two or three anterior segments following the head. In exceptional cases they are placed on the dorsal part of the middle of the body {Arenicolidca). As a rule, however, they are represented by numerous filiform tentacles and ten- * E. Claparede, " Recherches sur la structure des Annelides sedentairc.-^. Gen6ve, 1873. POLtCH.'ETA, TUBICOLyE. 381 tacular cirri upon the head (CajniibrancMata), of which one or more may bear an opercuhim at its apex to close the tube (fig. 308). The parapodia are short, and are never used in swimming ; the notopodia usually carry haii'-like sette ; the neuropodia are trans- verse ridges with hooked set^e or plates. Eyes are very frequently absent ; in other cases they are present in pairs upon the head or on the terminal segment, sometimes even on the branchial tentacles; in the latter case they are very numerous. The body is often cUWded into two (thorax and abdomen) or three regions, the seg- ments of which are distinguished by their unequal size. The Tuhicolce live in more or less fix-m tubes which they construct for themselves, and feed on vegetable matter which they procure by means of theii- tentacular apparatus. In the consti-uction of theii- tubes the animals are assisted in various ways by the long tentacles or branchial filaments of the head ; thus, for example, the Sahellidce are said to accumulate fine ooze at the funnel-shaped base of the branchial ajDpaiutus by means of the cilia of their tentacles, to mix it ^\'ith a cement secreted by lai-ge glands, and then to transfer it to the edge of the tube ; while the Terehelliclce procu^re the gi-ains of sand for the construction of their tubes by their long and very extensible tentacles. There are also boring Annelids, which pierce limestone and mussel shells, like the horny Molluscs; e.g., Sahella saxicola, etc. The development is simplest when the mother possesses a kind of brood-pouch for the development of the young, e.g., Sjnrorbis spirillum Pag., the eggs and larvte of "vhich remain within a dilatation of the opercular stalk until the young animals ai e able to construct a tube for themselves. The free-swimming larvje of most Tuhicolce, on assuming the form of the worm, lose the ciliary apparatus, while tentacles and parapodia make their appearance. In this condition and sometimes surrounded by delicate membranes, they swim about for some time longei-, and, having lost their eyes and aviditory vesicles, gradually assume the structure and mode of life of the sexual animal {Terehella). Fam. Saccocirridae. With two tentacles on the prfestomium, two eyes and the same number of ciliated, pits. A single row of retractile parapodia, furnished with simple setae, on either side of the segments of the body. Sacco- cirrm 2m])'dlocrrc.u.s Bobr., Black Sea and Mediterranean (Marseilles). Fam. Arenicolidse. Pr^stomium small and without tentacles. The pro- boscis is beset with papillfe. There are branched gills on the median and posterior segments. The animals burrow in sand. Avrnicohi niavhia Lin. (^1. pi-s-eafin-vm Lam.), North Sea and Mediterranean. Fam. Spionidae {S^noderf'). The small prtestomium sometimes witii tentacu- 382 OHtETOPODA. Ifir processes, usually with small eyes. The oral segment mostly with two loiKr t_entacular cirri, which are usually frrooved. Cirriform Ijraucliite are presejir Polydora (intcnnafn Clap., Naples. ^p\o nctirornis Fabr., north seas. Fam. Chaetopteridae. Body elongated and separated into several dissimilai' regions. Usually two or four very long tentacular cirri. Dorsal ajjijenda-rs of the middle segments have the shape of wings and are often lobed They live in parchment-liice tubes. TelcpmvvH Coxtanm. Clap., Naples. I'lueUptcrm poujamrnfucnix (Uiv.. West Indies. Fam. Terebellidse. Body vermiform and thicker anterioi-ly, 'J'he thinu(r posterior i)ortion is sometimes distinctly marked off as an appendage destitme of setfe. The prrestomium is indistinctly separate from the mouth segment^ There is frequently a lip above the mouth. Numeious filiform tentacles, usually alTanged in two tufts. There are pectinate or branched, rarely filamentous, gills on a fcAV of the anterior segments. Dorsal prominences (notopodia) fur- nished with simple seta3, and ventral transverse ridges (neuropofai:)lefi. Sahclla jH'nicilhi'S Lin., North Seas. S. Kollikeri Clap., Mediterranean. ProUda Rndolplii Eisso, Mediten-a- nean. Filiijrana hnplcxct Berk., Norwegian and English coasts. Sevpnla nor- vegi-ca Gunu., North Sea and Mediterranean. Sjnrorhis sjm'illunb Lin., Ocean . Order 2.— OligochjEta.* Heymiaiyhroclite Choetopoda loitliout pharyngeal armature UTid paru- podia. There are no tentacles, cirri, or hranchiai. The development is direct. The cephalic region is composed of the prasstomium, which projects as an upper lip, and the mouth segment. It does not essentially differ from the following segments so as to form a special region (fig. 309). Tentacles, palps, and tentacular cirri are never found on it, but tactile papillae are present in great numbei-, as are also peculiar sense organs which resemble taste buds. Eyes either fail or are present as simple pigment spots. Besides the small gland cells of the * Besides the works of W. Hoffmeister, D'Ddekem, and others, compare : E. Claparede, " Eecherches anatomiques sur les Annelides, etc., observes dans les Hebrides," Geneve, 1860. E. Claparede, " Recherches anatomiques sur Ics Oligochretes," Geneve, 1862. A. Kowalevski. '• Embryo! ogisclie Studien an "VVlirmern iind Arthropoden QTAimhricus, Ettaxcs),'' Petersburg, 1861. _ l'>. Hatschek, " Studien iiber Entwicklungsgeschichte der Anneliden," Wien. 1878. Fr. Vejdovsky, " Bcitriige zur vergleichcndeu Morphologic der Anneliden. L Monographic der Enchytrreidcn," 1879. OLIGOCHyTSTA. 383 hypodermis there is present in the cliteUus a deeper glandular layer {Sdulenschioht Clap.), which consists of finely granular cells embedded in a framework of pigmented and vascular connective tissue and situated between the hypodermis and the external muscular layer. There, are but few set^e present, and they are never disposed on special parapodia, but always in simple pits in the integument, by the cells of which they are secreted. There are small secondary bristles which serve as a reserve. The blood is usually red, as in the Hiruclinea. The alimentary canal is often divided into several regions, the relations of which are most complicated in the Lumhricidce. In Liomhricihs, the buccal cavity leads into a muscular phai-ynx, which is probably used for sucking. This is followed by a long oesophagus extending to the 13th segment, and furnished with a thick layer of glandular cells and several glandular dilated ap- pendages (calcareous sacs). The oeso- phagus is succeeded by a crop, a muscular gizzard, and finally by the intestine itself, the dorsal wall of which is pushed inwards so as to form a longi- tudinal fold, the typhlosoU (comparable to a spiral valve). In the Limicolce the alimentary canal is simpler by the absence of a muscular stomach; a pharynx and oesophagus are, however, always present. Reproduction.— The OUgochceta are hermaphrodite ; they lay their eggs either singly or united in greater num- ber in a capsule; and they develop without a metamorphosis. The testes and ovaries are paired and placed in definite segments, usually near the an- terior end of the body ; they dehisce their products into the body cavity. The generative ducts possess funnel-shaped openings into the body cavity through which the generative products pass, and may Pig. 309. — Lumbriciis rubellus (after G.' Eisen) . a, The whole woim ; CI, Clitelhis. h, Anterior end of the body ft-om the ventral side, c, Isolated seta. 384 ClIiETOl'ODA, co-exist in the same segment witli segmental organs {Lumhricidoi). In the earth-worm, whose generative organs were first accurately described by E. Hering, the female apparatus consists of two ovaries in the 13th segment,* and two oviducts, which begin with trumpet- shaped openings into the body cavity, contain several eggs in a dila- tation and open to the exterior on either side on the ventral surface of the 14th segment. There are in addition in the 9th and 10th segments two pairs of receptacula seminis, which open at the junction of the 9th and 10th and 10th and 11 th segment respectively. They The male genital organs con.sist of two pairs of testes in the 10th and 11th segmentf^, and two vasa defe- rent ia, each of which opens inter- nally by two fun- nels and to the extei-ior in the 15th segment. Copulation takes place in June and July on the sur- face of the earth at night. The Fig. 310. — Generative organs of Lumbricus in segments VIII. to . , . XV. (after E. Hering). T, Testes ; St, the two funnels of the WOrms apply their vas deferens on either side ; Vd, vas deferens ; Ov, ovary ; Od, ventl'al surfaces to oviduct ; -ffic, receptacula seminis. one another and lie in opposite directions, in such a manner that the openings of the re- ceptacula seminis of one worm are opposite the clitellus of the other. During copulation sperm flows out from the openings of the sperm duct and passes backwards in a longitudinal groove to the cHteDus, and thence into the receptaculum seminis of the other worm. In Tuhifex and Enchytrceus the ovaries may break up into groups of ova which float free in the body cavity. Special albumen glands and also glands which secrete the substance of the shell of the cocoon are often pi-esent. In the breeding season the above-mentioned * The head (prfEstomium and buccal region) being reckoned as the f^r^; segment. are filled with sperm in copulation (fig. 310). OLIGOCHjETA. 385 girdle or clitellus, which is formed of a thick glandular layer, is almost always present. The embryonic development of the OKgochceta presents many relations to that of the Hirudinea. The unequal segmentation, which is very much alike in the two groups, and the similarity in the method of origin of the mesoderm, from two large cells near the blastopore at the posterior end of the embryo, point to a close relation- ship between these two groups of Annelids. A few Olicjochceta, as for example Chcetogaster, are parasitic on aquatic animals ; the rest of them live, some free in the earth, some in fresh water-, and some in the. sea. Sub-order 1. Terricolse. Oligochjeta which live principally in the earth. They have segmental organs in the genital segments. Fam. Lumbricidae. Large earthworms with compact skin and red blood. Without eyes. Tufts of vessels surround the segmental organs. Their activity in boring into the earth is of the greatest importance, loosening and exposing the soil to the action of the weather. Lviiibriciis L., Earthworm. Pr^stomium distinct fi'om the mouth segment. The clitellus includes a series of segments, and is situated nearly at the end of the anterior quarter of the body /a?' bi'hi?id the fiemtal ojumifir/s. Sette elongated, hook-shaped, arranged in four groups in each segment, each group containing two setffi. The earthworm lays its eggs in capsules, into each of which several small ova, with sperm from the recep- tacula seminis, are emptied ; as a rule, however, only one or but a few embryos are developed. The developing embryo takes up with its large ciliated mouth not only the common mass of albumen, but also the other eggs. L. ar/ricolu, 'RoSm. = tc'rrcstris Lin., L. fastidus SaY., L. amcricaiivs E. Perr. Criodrilus lacvvni HofEm. Sub-order 2. LimicolsB. Oligochfeta which live principally in water. Without segmental organs in the genital segments. Fam. Phreoryctidse. Long filiform worms, with thick skin and two rows of slightly curved seta3 on each side. Pkrcoryctes MenJieamiS HofEm. Found in deep springs and wells ; they seem to feed on the roots of plants. Fam. Tubificidse. Aquatic worms, provided with four rows of simple or divided, hooked seta^. Hair-like setae may also be present. The receptacula are in the 9th, 10th, or 11th segment. They live in mud tubes, from which they protrude the posterior end of the body. Tuiifex rividorvm Lam. The heart is in the 7th, the receptacula in the 9th segment. T. Bonncti Clap. (Seenwis varierjata HofEm.) The heart in the 8th, receptacula in the 10th segment ; both species live in fi-esh water. LiDuuidrilm Hoffmeistcri Clap., L. Udekem lanm Clap. Is distinguished from Tuhifcx hj t\xQ presence of hair-like setae in the upper row of sette. Lnvihvicnlvs variegatns 0. Fr. Miill. Every segmeiit is provided with a contractile vascular loop and saccular contractile appendages of the dorsal vessel. Fam. Naideae. Small Limirolce with dehcate thin skin and clear, almost colourless, blood. The prsestomium is often elongated like a proboscis and 25 386 ANNELIDA. fused with the mouth negment. .Xais {Stularia) proho.rUlea 0. Fr. Miill A. jmranta Schm. Botli species have filiform pn^ston.ium. Cha-toaa,tcr vermicularls O.Yi-.msXi. Sub-class 2, — Gephyrea.* IVorms with cylindrical body, loithout external seyrmniation, with terminal or ventral mouth; with cerebral yamjlion, a^opha.,jeal rimj and ventral cord. Setui are sometimes pi-esent. The Gephyrea possess an elongated cylindrical body and live, as do the Ilolothuria, in sand and ooze in the sea. The characters which distinguish them as Annelids are the possession of an oesophageal ring connected with a cerebral ganglion and of a ventral cord par- tially surrounded by ganglion cells. The larvae of the Chai- tifera present traces of se_g- . mentation (see below, p. 301), while in the Achceta the body cavity remains simple. Of s'ense organs, eye spots have been observed; these in certain Sipunculidce lie directly upon the brain ; there are also dermal papillae, into which nerves enter. The structure of the integu- ment is similar to that of the Annelida; the thick upper „- cuticular layer rests upon a cellular matrix, and is not un- FiG. 311.— Young Echiurus f rorajthe ^ventral frequently wrinkled. There is side (after Hatschek). 0, Mouth at the base i - j.- mi of the proboscis; sc, oesophageal commis- external segmentation. The sure ; -BS, ventral cord. ; ^, anus ; s^, hooks, connective tissue dermis is of considerable thickness and en- closes numerous glandular tubes, which open to the exterior by pores in the epidermis. Below this is the strongly developed dermal muscular tunic, which is regularly composed of an outer layer of circular fibres * Quatrefages," Memoire sur I'Bchiure," Ann. dcs Sc. JVat., 3 Ser., Tom VII. Lacaze-Duthiers, " Eecherches sur le Bonellia," Ann. dcs Sc. 37/f.. 18.58. W. Keferstein, " Beitriige zur auatomischen und sj^stematischen Kcimtniss der Sipunculideii," Zeitsehr fur wiss. Zoulogic, Tom XV.. 1865. B. Hatschek, " Ueber Entwickelungsgeschichte des Echiurus," etc. Wien, 1880. J. W. Sperigel, "Beitrage zur Kemitniss der Gephyreen. I. MittJu-il. avs der zoolo- gisclwn station zu Neaj^iel, 1879 ; II. Zeitsehr, fur n-iss. Zoul,, Tom XIV., 1881. GEPHYfiEA. 387 and an inner layer , of longitudinal fibres. The latter are connected with the former and also amongst themselves by net-like anastomoses. These dermal muscles cause the folds of the cuticle. Internally to the longitudinal muscles there is another layer of circular muscles. In the Chcetifera two hooked setse are present near the genital opening (fig. 311); these assist locomotion. There may also- be present one or two circles of sette at the posterior end of the body (^Echiiorus). In the Chcetifera (fig. 311), the ante- rior- pai't of the body is elongated to form a' kind of proboscis, which projects im- movably and cor- responds to the prfeoral lobe (prse- stomium) of the Annelida. The mouth is placed ventrally at the base of the probos- cis. In the Achceta [Sipunculidce) this proboscis is want- ing ; the mouth is placed at the ex- tremity of the an- terior region of the body, which is sur- rounded with cili- ated tentacles, and can be i-etracted by means of retractor muscles (fig. 312). Alimentary canal. — The mouth opens into a pharynx, which is sometimes furnished with teeth ; this is followed by a ciliated intes- tinal canal, which is usually longer than the body and disposed in coils in the body cavity. The terminal portion of the intestine is Fig. 312. — Sipiniciihis iiKdns, \fuCi open from the side (after W. Kefei-steiu). Te. Tentacles ; G, cerebral ganglion ; !■'(?, ven- tral nerve cord ; D, intestine ; A, anus ; BD brown tubes (ventral glands) . ANNELIDA. muscular and opens to tl.e exterior l,y a termi,ial or dorsally phu^ed anus (fig. 312). The vascular system is probably in communication with the body cavity; it consists of a dorsal vessel, which, as in the Annelvla accompanies the alimentary canal, and of a ventral vessel runnin-' along the body wall. There are also branches on the alimentary canal and m the tentacles. The blood is either colourless or red, and moves in the same direction as in the Annelids, the current being maintained both by the contraction of certain parts of the vessels and by the cilia which line the walls of the vessels. Tlie corpusculated fluid of the body cavity differs from this vascular blood. Excretory organs.— There are two sets of organs, both of which may be interpreted as segmental organs. One kind, the anal vesicles (fig. 314c, Ab), are only present in the Chceti/era ; they have the form of a pair of tufted tubes, which open, on the one hand, into the body cavity by numerous ciliated funnels and, on the other, into the rectum. The other kind, known as the brow7i tubes (fig. 312, Bd) or ventral glands, are placed (one or more pairs) in the anterior part of the body ; they also open into the body cavity by a ciliated funnel, and to the exterior on the ventral surface. The latter, like the seg- mental organs of Annelids, assume the function of seminal vesicles and of oviducts. Generative organs.— The Gephyrea are of separate sexes. There are, however, remarkable variations both in the generative glands and their ducts. In Phascolosoma amongst the Achceta (according to Theel) the generative glands lie at the root of the ventral retractor muscles of the proboscis, and form a ridge from which the generative products are set free. Spermatozoa or ova in various stages of development are found in the body cavity, and thence are carried to the exterior through the two brown tubes (segmental organs) which open on the ventral side. In Bonellia among the Ghcetifera the ovary, which has the form of a thin cord (fold of the body wall) in the posterior half of the body, is attached by a short mesentery to the nerve cord. From the ovaiy the ova fall into the body cavity, and thence pass into the neigh- bouring single uterus (fig. 314, b, TJ), which is provided at its base with a trumpet-shaped opening (Tr) and opens to the exterior on the ventral surface behind the mouth. This uterus ought probably to be considered morphologically as a segmental oi-gan, which has only been developed on one side. The generative organs of the small Tui'bellai'iau-like males which are met mth in the uterus of GEPHTBEA. CH>T;TIFERA. 389 the female of Bonellia have the same relations (fig. 313). These nuliinentavy males are furnished (in many species) with two ventral hooks, in front of Avhich in the anterior region is placed the external opening of the vas deferens. The vas deferens corresponds to the uterus of the female, and is in like manner provided with an internal opening into the body cavity. In Echiurm there are two pairs of hvovm tubes, which function as generative ducts and reservoirs. In Thalassema there are, according to Kowalevski, three pairs of such tubes. The development shows many points of similarity with that of the Annelida. Be- tween the Achceta and Chcetifera, however, there are considerable differences. In both cases a metamorphosis foUows the embryonic development. The larvae resemble Loven's larva "(larva of Polygordius) ; but in the Achceta they are characterised by a great de- generation of the apical region (prseoral lobe) and the absence of a prseoral band of cilia. The remarkable larva known as Actino- trocha, which is the you.ng stage of the tubicolous genus Phoronis* is distinguished by the possession of a contractile prfeoral lobe, behind which there is a circle of ciliated ten- tacles forming a collar. The Gephyrea are aU marine. Some of them live in sand and ooze at considei-able depths, also in holes in the rocks and in crevices between stones and corals, and in the shells of snails. Their food is similar to that of Holothurians and many tubicolous Annelids. Order 1. — Ch.etifera = Echiuroidea. Fig. 313. — Planarian - like male of Bonellia (after Spengel). D. Intestine; WT, ciliated funnel of the vas deferens (Fd), which is flUed with sperm. Gephyrea characterised hy the 2>i'esenGe of two strong hooked setce on the ventral side and hy a terminal anus. The mouth is placed at the ha.se of the prmoral lohe, lohich is develop)ed into a prohoscis. The Echiuroidea or chfetiferous Gephyrea present no external segmentation of their elongated and contractile body; they have, however, in the young state the rudiments of 15 metameres. This * There should be a third order of Gcpliiirea for these animals. 390 ANNELIDA. fact, as well as the formation of the pra^oral lobe and tJie develop- nient of the ventral hooked set^e, points to a close relationship with the Chcetopoda. In the adult animal, however, the internal segmen- tation IS very little maiked. The dissepiments, with the exception of the first, which forms a partition between the head and the body, are lost, and the segmentation of tlie ventral cord is only indicated by the distribution of the nerves. The supra-Q3sophageal ganglion remains at the apical region of the pneoral lobe (proboscis) ; hence the oesophageal commissures are exti-aordinarily long. The strongly developed prajoral lobe forms a proboscis -like Fig. 314. — a, female of Bonellhi viricHs (after Lacaze-Duthiers). h, Interment and generative organs after the intestine has been removed. Sd, Cutaneous glands ; Ab, anal vesicle'; Ad, rectum ; Ov, ovary ; Tr, ciliated funnel of the uterus (U). c, Anatomy of Bonellia viridin (after Lacaze-Duthiers) . D, alimentary canal with anal vesicles (Ab) ; M, mesen- tery ; V, uteras ; R, proboscis. appendage which may develop to a considerable length and become forked [Bonellia) (fig. 314 a). A pair of hooked seta? (with reserve setfB in the sheath of each seta) are always present on the first segment of the body. In Echiurus there are also one or two circles of setfe at the posterior end of the body. There are from one to thi-ee pairs of anterior segmental organs (so-called brown tubes or ventral glands), which open on the ventral surface and are used for the passjige outwards of the geneiative products. Besides these there is also a pair of GEPHYKEA.. CHiETIFEEA. 391 posterior segmental organs (anal vesicles, tig. 314, Ab) m the terminal segment, each of which has a number of peritonea) funnels and opens into the rectum. In Bonellia the segmental organ which performs the function of uterus is, like the ovary, single (tig. 314 ft). Development.— The development of the o^n.im begins with an unequal segmentation. In Bonellia the small cells of the animal pole grow round the four large yolk spheres, which give rise to the entoderm, leavmg a small aperture, the blastopore (tig. 110). The Echiurus larva? (fig. 315) are the most accurately known. They present the type of Loven's larva and possess a strongly developed Fig. 315.— a, Lai-va of JEchiuruK from tlie ventral side (after Hatschek). SP, apical plate; Prw, pra?oral circle of cilia ; Fow, postoral circle of cilia; i'/j, head-kidney ; Tff, ventral ganglionic cord connected with the a]iical plate by the long oesophageal commissures ; AS, anal vesicle, b. Ventral region of the Echiurus larva with segmented mesodermal hands ; SC, oesophageal cnmmissm-e ; Dsp, dissepiments of the anterior body segments ; MS, mesodermal bands ; A, anns. prseoral circle of cilia [Frw), in addition to which there is also a deUcate post-oral circle of cilia {Foiv). Early in lai-val life a seg- mental organ, the head kidney or pronephros (^Kxi), is developed, one on either side ; and behind it a pair of mesoblastic bands makes its appearance and gives rise in the subsequent development to the rudiments of 15 segments (tig. 315 b). In the terminal segment, which is surrounded by a circle of cilia, there appear segmental 392 ANNELIDA. organs, which give vise to the anal vesicles (fig. 315 a jLS) The rudiments botli of the cei-ebral ganglion and of the ventral cord are derived from growths of the ectoderm,-the former from the apical plate, the latter as a. paired thickening of the ventral ectoderm. The two are connected by the esophageal ring, which is also provided with ganglion cells. In older stages, after the disappearance of the segments, the ciliary apparatus begins to degenerate and finally vanishes; after which two strong hooked seta3 make their appear- ance at the sides of the nerve cord not far from the mouth, and two circles of shorter setai are formed at the hind end of the body (fig. 316). The prfeoral lobe of the larva becomes the proboscis of the young Echiurus (fig. 311). Fam. Echiuridae. The anterior end of the body above the mouth is elontrated into a pro- boscis, the under surface of wliich is grooved. The long oesophageal commissures lie in the pro- boscis, and meet in front without anj' cerebral enlargement. Anteriorly and on the ventral side are two setEe for attachment, and on the postf- rior end of the body there are sometimes circles of setaj. The anus is terminal. Erhiimix Pal- Imii Gu6rin {Gacrtneri Quatref., St. Vaast), coast of Belgium and England. Thalasaema fiif/us M. Miill., Italian coast. BoncUta riridiit Rolando, Mediterranean. The males are small and rudimentary, and resemble Planarians. They live in the efferent ducts of the female generative organs. Order 2. — AcHiETA= Sipunculoidea. Gepliyrea loith terminal mouth, dorsally placed anus, and toithout setoi. The ante- rior region of the body is retractile. The Sipuncidoidea ditfer from the chsetiferous Gephyrea in their entii-e want of all traces of metameric segmentation in the degenei-ation of the prasoml lobe and in the position of the mouth and anus. The elongated body is destitute of a projecting pra3oral lobe, so thft the mouth, which is frequently surrounded by a circle of tentacles, Fig. 316.— Older Eekiuras larva seen from the side. The head kidney is atrophied. O, mouth ; 3f, stomach ; A, anus ; BK, circles of setse ; SC, (Eso- phageal commissure ; AS, anal vesicles ; G, cerebral ganglion, developed from the apical plate ; T//, ventral nerve cord ; IT, ventral hooks. &EPHYKEA. AClIiETA. 393 The cerebral ganglion, cesophageal ring and ventral cord run inside the dermal muscular tunic. Only one pair of segmental organs, known as brown tubes or ventral glands, is present. The blood vascular system is w^ell developed. Development. — The segmentation is com- plete and is followed by the formation of a gastrula by invagination. The blastopore marks the ventral side. The two posterior marginal cells* of the entoderm move in- wards as primitive mesoderm cells, and give rise to the mesoblastic bands which do not undergo segmentation. Invaginations of the ectoderm of the animal pole and ventral sur- face of the em- bryo give rise to cepha- lic and ventral plates r e spec- ti vely, while the remain- der o f the ecto- d e r m cells grow round thet-e and form an external envelope for the embryo of the nature of a serous mem- brane (serosa). Cilia project from the latter through the pores of the vitelline membrane and are Fig. 318.— Larva of SijnincHinn (nfter Hats- employed by the embryo in chek). 0, Mouth ; Sp, apical plate ; A, anus ; PoW. T)ostoral circle of nilla- V. Viflnp-ir SWimmiUff. Pig. 317.— Quite young Si- puncnlits still without ten- tacles (after B. Hatschek). O, mouth ; A, anus ; BS, ventral cord ; iV, nephri- dium (brown tube) ; G, cei ebral ganglion ; Bff, blood vessel. QQ4 ANTNELIUA. splanchnic layers, and give rise to the rudiments of the two seg- mental organs ; while the c«sophagus arises as an invagination of the ectoderm, and a postoral circle of cilia is formed around its opening (fag. 318). The serous membrane is cast off with the eg^r membrane and the larva then contains all the essential organs of the adult Sipunculus except the ventral cord and the blood-vessels At a later stage, during the growth of the larva, the ventral cord IS developed from the ectoderm, the circle of cilia disappears, the first tentacles sprout out at the edge of the mouth, and the metamor- phosis of the free-swimming larva into the creeping young Sipun- culus is completed. Fain. Sipunculidae. Body elon-atecl and cylindrical, the anterior part re- tractile, i he mouth is surrounded with tentacles, and the anas is dorsal. The intestine is coiled spirally. Slimmulu.^ nudm L., Mediterranean. Pka.vwlosoma La^ve Ivef., Me literranean. Ph. chmriatwn Kef. St. Yaast. Fam. Priapulidae. Anterior part of the body xvithout circle of tentacles Pharynx armed with papilhe aad rows of teeth. Anus at the posterior end of the body and slightly dorsal, above it there usually projects a caudal appen- dage which l^ears papilla-like tubes (branchiie). The intestine is strai-ht Pnajmlus moudatns 0. Fr. MiiUer. MaUcryptu^ ^rbmlosm v. Sieb , Baltic Spitzbergen. ' Suh-class 3.- -HlRUDINEA* = DlSCOPHORA, LeECHES. Body either with short rings or not ringed, without parapodia, with terminal ventrcd sucker, hermap)hrodite. The body of the Hirudinea, so far as its external form is con- cerned, recalls that of the Tremaloda, with which group the Hirudinea have often been incorrectly connected. Externally the body is marked by a number of transverse rings, which are short and may be more or less indistinct or even entirely absent. These rings correspond in no way with the internal segments, which are separated by transverse partitions or dissepiments; but they constitute much shorter portions of the body, four or five of them corresponding to one internal segment. The large sucker at the posterior end of the body serves as an organ of adhesion ; and there may be in addition a second smaller sucker, either in front of or * Brandt and Ratzeburg, " Medicinische Zoolopic." 1^!29. Moquin-Tandon. " Moiiogi-aphie de la famille des Hirudinees,"' 2iid. (idit.. Paris, 1846. Fr. Leydig, '• Zur Anatomic von Piscicola gcometrica," Zriti'paratus at the anterior end of the body, with cerebral ganglion and excretory canals; without heart or tme vascular system. The sexes are separate. The Rotifera are Worms which can be derived from Loven's larva and have nothing to do with the Arthropoda, since they are mthout limbs and do not develop metameres. The body of the Rotifera is certainly externally segmented and divided into more or less sharply * Ehrenberg, " Die Inf usionsthierchen als vollkommene Orgaiiismen," Leipzig. 1838. Dujardin, " Histoire uaturcUe des Infusoires," Paris. 1841. Dalrymple, Fliil. Traii.-<. Roil, Soc. 1844. Fr. Leydig, Ueber den Pau unddic .systematische StcUung dcr lihdcrthiere," ZeitxrJir. fiir iri.sx. Zoul... Pd. VI.. 18.54. F. Cohu, "Ueber liMevthieve," Zctiehr. fur wiss. Zool.. Ed. VJl.. 1S(')6. Pd. IX., 18.58. Bd. XII., 18()2. Gosse, "Onthe Structure. FuTictions and Homologies of the Manducatory Organs of the class Eotifera," PIril. 'Jniiix., 1856. W. Salensky, " Peitriige zur Entwickelungsgeschichte des Brachionus urceolaris," ZcHxchr. fur wUx. Ziwl, Tom. XXII., 1872. KOTIFERA. 401 defined and very dissimilar regions, but the internal organs show no trace of any corresponding segmentation. There is therefore no true segmentation, i.e., division of the body into metameres. It is usually possible to distinguish an anterior region of the body, in which" the whole of the viscera are situated, and a posterior movable foot-like region, which terminates in two opposed pincer-like styles and is used both in locomotion and for attachment. The broad anterior portion of the body, as weU as the narrow posterior region, is often divided by transverse constrictions into several rings, which can be drawn into one another like the rings of a telescope and can be bent more or less freely upon one another. The anterior cili- ated and usually re- tractile apparatus which projects at the anterior end, and is termed the trochcd disc, or from its like- ness to a rotating wheel, the wheel or- gan, is an important characteristic of the Rotifera. Very fre- quently, especially in the parasitic forms, this trochal disc is re- diiced, and in certain cases entirely aborted {A2Jsilus). In Notom- mata tardigrada the trochal disc is reduced to a small ciliated a Fig 324!.- Ifi/datina senta (after F. Cohn). a. Female; b, male. Sor, Trochal disc ; CBl ; contractile vesicle ; Wtr, ciliated funnel of the excretory apparatus (Ex) ; K, jaws ; Di; salvary glands ; Md, stomach, Ov, ovary ; T, testis ; P, penis. lip round the mouth; in HydaLina (fig. 324) to the margin of the head, the whole circumference of which is ciliated. In other cases the ciliated edge projects ovei' the head and forms the so- called double wheel, e.g., PMlodina, BracJdonus, or becomes a ciliated cephalic shield, e.g., Megalotrocha, Tuhicolaria. Finally, it may be produced into ciliated processes of various form [Floscularia, Stephanoceros). As a rule, the cilia form a continuous border, starting from the mouth and returning to it. The cilia are chiefly 2(5 402 BOTIFEBA.. concerned in locomotion, but in addition they play an impoi-tant part in attracting small particles of food. There is also a second row of delicate vibratile cilia, extending on either side from the doi-sjil edge of the trochal disc to the mouth [parts of the continuous border of cilia just mentioned as starting from the mouth], which is placed on the ventral side of the trochal disc. These cilia serve to guide the small food particles which are captured by the trochal disc into the mouth. Alimentary canal.— The mouth leads into a dilated phaiynx (fig. 324), provided with a special armature. The pai-ts of the armature are in continual movement, and serve for mastication. Following the pharynx there is a short oesophageal tube ; this leads into the digestive sac, which is lined with large ciliated cells. The anterior or gastric part of this cavity is wide, and receives two large glandular tubes, which may sometimes be resolved into unicellular glands. They may be explained from theii' function as salivary or pancreatic glands* The posterior narrow intestinal part visually opens into a cloacal chamber, which is likewise ciliated and opens on the dorsal surface at the point where the foot-like posterior region joins the anterior part of the body. In some Rotifera, as for example Asco- morpha, Asjylanchna, the intestine ends blindly. A blood-vascular system is always wanting, and the body cavity is filled with a clear vascular fluid. The structures, erroneously described by Ehrenberg as vessels, are in ideality the transversely striped muscles and muscular networks beneath the integument. Respiration is carried on by the general surface of the body; special organs of respiration are wanting. Excretory organs. — The so-called respiratory canals are excretory, and correspond to segmental organs. They consist of two sinuous longitudinal canals with cellular walls and with fluid contents, and they communicate with the body cavity by ciliated funnel-shaped openings placed at the end of short ciliated lateral branches (vibratile organs). They open into the cloaca either du-ectly or by means of a contractile vesicle (respiratory vesicle). The nervous system is allied to that of the Platyhelminthes. The central part of it consists of a simple or bi-lobed cerebral ganglion placed above the oesophagus, and giving ofi" nerves to peculiar cuta- neous sense organs and to the muscles. Eyes are often present, and lie upon the brain either as an x-shaped unpaired pigment body or as paired pigment spots provided with refractile spheres. The above- mentioned cutaneous sense organs, which are probably tactile, have EOTIFERA. 403 the form of prominences beset with hairs and setae, or even of tubular elongated processes of the skin (respiratory organs of the neck), beneath which the sensory nerves end in ganglionic swellings. Generative organs. — The sexes are separate, and are distinguished by a strongly marked dimorphism. The very small males have neither oesophagus nor intestinal canal, which are reduced to a string- like rudiment ; and they leave the egg completely developed. Their generative organs are reduced to a testicular sac filled with spermatozoa, the muscular duct of which opens at the hinder end of the body, sometimes on a papillif orm protuberance. The generative organs of the females, which are far larger than the males, consist of a roundish ovary filled with developing ova, and of a short oviduct which contains one or but few ripe ova, and iisually opens into the cloaca. Almost all Rotifera are oviparous ; and their eggs are distinguishable into thin-shelled summer eggs and thick-shelled winter eggs. They carry both kinds of eggs about on their body, but the summer eggs not unfrequently undergo their embryonic development in the oviduct. The summer eggs probably develop parthogenetically, since at the season of the year when they appear the males are not to be found. The thick-shelled winter eggs, which are often dark coloured, are produced in the autumn and fertilized. Development. — As far as the embryonic development is known, it shows a great agreement with that of many Gasteropoda (Calyptroia). The ova undergo an irregular segmentation. The cells proceeding from the smaller segmentation spheres become accumulated at one pole, and finally enclose the darker coloured yolk cells completely, so that a two-layered embryo is formed. The cells of the outer layer are much poorer in granules than are those of the central entoderm layer, and form the ectoderm. A depression of the ectoderm is formed on the (later) ventral surface, from the side walls of which the two lobes of the trochal disc grow out (like the oral lobes of mollusc embryos). The hinder portion of the depression becomes the posterior part of the body, at the base of which a pit forming the first rudiment of the cloaca makes its appearance. The mouth and the anterior part of the alimentary canal are developed anteriorly at the bottom of the depression. The ganglion arises from the ectoderm in the cephalic region. There are no reliable observations on the formation of the mesoblast. In the male embryo the development takes a different course, the alimentary canal not being completely developed. The free development takes place either 404 BOTIFJiUA.. without or witli an inconsiderable and sometimes retrogressive metamorphosis. This hitter is most striking in the Floscularidm, which are fixed in the adult state. The Rotifera principally inhabit fresh water, in which they swim about by means of the trochal disc, and sometimes they attsich them- selves to foreign objects by means of the forked glandular foot. When thus attached, they extend the anterior part of the body, and the cilia begin to move. The currents set up by the latter convey to the mouth food material, such as small Infusoria, Algae, Diatoms. Some species live in gelatinous sheaths and delicate tubes, othei's {Conochilus) are fixed by theii- foot in a common gelatinous mass, and are united to form a free-swimming colony, A relatively small number are parasitic. It seems that many species are able to endure drying, if it be not too prolonged. Fam. Floscularidee. Fixed Rotifera with a long transversely rijigecl foot, usually surrounded by gelatinous coverings and tubes. The margin of the head has a lobed or deeply cleft wheel-organ. Floscvlaria prohoxcidea Ehrbg., Stepha)iuc('ros Mchhornii Ehrbg., Tiibicolaria najas Ehrbg., McUcerta i-i/u/em L., Conochilus volvox, Ehrbg. Fam. Philodinidae. Free, often creeping (in a looping manner) Rotifera ; mth double-wheeled rotatory organ, and jointed, telescopically retractile foot, without gelatinous investment. Callidina rlt-f/ans Ehrbg., Rotifer vulgaris Oken (i?. redivivits Cuv.), Pliilodina erytliropldlialma Ehrbg. Fam. Brachionidae. Rotifera with bifid or multifid wheel-organ ; with broad, shield-shaped armoured body ; and foot ringed, or with short segments. BracMomis Baiter i 0. Fr. Mlill.. B. militar/it Ehrbg.. EneJdanis triqnetra Ehrbg, Fam. Hydatinidae. Edge of ^\•heel-organ prolonged into numerous processes (multitid) or only sinuous ; skin delicate, often ringed ; foot short, usually forked, with two setse or pincer-shaped. Hydatina Ehrbg., II. scnta 0. Fr, Miill. with Enteroplca. lindatintv Ehrbg., as male. Xotoimnata tardigrada Ldg., X. Braeliionvs Ehrbg., N. inirasita Ehrbg. Fam. Asplanclinidae. The sac-like uuarmoured body is destitute of rectum and anus. Axplanclma Sieloldii Ldg., A. myrnieleo Ehrbg., Ascomorjiha germanica Ldg. Two groups of small animals are allied to the Rotifera : — (1) the Echinoderidse which Dujardin and Greef regarded as connecting links between Vermes and Arthropoda (Behinoderes Dvjardinii Clap., E, xetigera Greef) ; and (2) the Gastrotricha * or Ichthydina [Ch^tonotus). ♦ Compare E. Metschnikoff, " Ueber einige wenig bekannte niedere Thier- formen." Zeitsehr. fiir fvisx. Zool., Tom. XV., 186i5. Alfo the works of H. Ludwig and 0. Biitschli. AETHEOPODA, 405 CHAPTER X. ARTHROPODA. Laterally symmetrical animals with heteronomously segmented body and jointed segmental api^endages ; with brain {sxqwao&sophageal ganglia) and ventral nerve cord (^ganglionic chain). The most important characteristic which distinguishes the Arthro- poda from the closely allied segmented worms, and is an essential condition of a higher organization and grade of life, is the possession of jointed segmental appendages which serve as organs of locomotion. In place of the unjointed parapodia of the Chcetopoda, jointed appendages more adapted for locomotion and confined to the ventral surface, are present. Every segment may possess a ventral pair of appendages which, in the simplest case, are short and consist of only a few joints (^Perijyatus) (fig. 325). While in the Annelida loco- PiG. 325. — Fei-iputus capeiuis (after Moseley). motion is effected by the movements of the segments and undulatory movements of the whole body, in the Arlhropoda the function of locomotion is removed from the chief axis of the body to the secondary axes, i.e., to the paired appendages, with the result of the possibility of a much more efiicient discharge of the function. The appendages enable the Arthrojyoda not only to swim and creep with much greater ease and speed, but also to execute various kinds of more complicated movement, e.g., running, climbing, springing, and flying. The Arthropoda are, therefore, true terrestrial and aerial animals. The high development of the organs of locomotion as paired appendages leads of necessity to a second essential property, viz., to the heteronomy of the segmentation, and in connection with this to the hardening of the outer layer of the skin to form a fii-m exo-skeleton. If the function of the limbs is to be perfectly discharged, there will be need of a considerable mass of muscle, the points of attachment of which can only be furnished by the integument of the body. The insertions of the appendages and their muscles, therefore, require 406 ABTnnOPODA. rigid surfaces, which are obtained partly by tlie development of internal chitinoiis tendons and plates, and partly by the hardening of the integument and the fusion of several segments to form larger armoured regions. It is only when the movements are simpler and resemble those of Annelids, that all the segments remain independent and bear similar appendages along the whole length of the body (larvae, Myriapoda). In general, three regions of the body can be distinguished, the head, the thorax, and the abdomen, the appendages of which possess respectively a different structure and fvinction(fig.326). The head constitutes the short and compact anterior region of the body, is covered by a hard integument, encloses the brain and bears the sense organs and mouth-parts (jaws). The appendages of this region are modified to form the antennce and jaws. The head of Arthropods, as compared Avith that of Annelids, contains, besides the frontal (prteoral) or antennal segment and the oral segment, in Pig. 326. — Head, thorax and abdomen of an AcridAMtiit seen from the side. St, Stigmata ; T, tympanum. Fig. 327. — Sqnilhi vianfis. A', A" Antennm; Kf, Kf" the anterior maxillipeds on the cephalo-thorax ; B', B", B'", the three pairs of bii-amous feet. addition at least one jaw segment, the appendages of which may, in larval life {]Vau2)litos), still function as legs. Usually, however, several of the succeeding segments whose appendages function as jaws form part of the head. The middle portion of the body, or thorax, is Hke'ndse distinguished by a relatively intimate fusion of some or all of its segments, as well as by the hardness of its integument. It is sometimes sharply marked off from the head, sometimes fused with the head to foi-m a INTEGUMENT, NEBTOUS SYSTEM. 407 region of the body called the cephalotlwrax (fig. 327). The thorax beai's the appendages which are of most importance in locomotion. The posterior portion of the body, or abdomen, ' is composed of distinctly separate rings, and is, as a rule, without appendages. When the latter are present, they serve partly as aids to locomotion {abdominal feet), partly for respiration, or for carrying the eggs and for copulation. More rarely, as for example in the scorpions, the abdomen is divided into a broad anterior region, the pr(Bahclomen, and a narrow movable posterior region, the postahdomen. The skin, as in the Annelida, consists of two different layers, — an external firm, usually homogeneous chitinous layer, and an internal soft layer, which is composed of polygonal cells {matrix, hypodermis) and secretes in layers the at first soft chitinous cuticle (fig. 22). The latter usually becomes hardened by the deposition of calcareous salts in the chitinous basis, so as to form the firm exoskeletal armour, which, however, is interrupted between each segment by thin connecting membranes. The various cuticular appendages of the skin (fig. 22, a, h, c), which may have the form of simple or pennate haii'S, of filaments, setse, spines and hooks, originate as processes and outgrowths of the cellular matrix. The chitinous cuticle together with its appendages is from time to time, principally in the young stage during the period of growth, renewed, the old cuticle being cast off as a continuous membrane (ecdysis, oi- moult). The musculai- system never constitutes a continuous envelope but the muscles are usually broken up into segments which corre- spond with the segmentation of the animal. The muscles of the body are arranged in longitudinal and transverse bundles in the different segments, and are frequently interrupted. There are in addition large groups of muscles, which move the appendages. The muscular fibres are always cross-striped. The internal organization is allied to that of the Annelida, but does not present such a well-marked internal segmentation. The nervous system consists of brain, oesophageal commissures and a ventral cord. The latter usually has the form of a ganglionic chain (fig. 328), and is placed beneath the alimentary canal. Some- times, however, it exhibits great concentration, and may have the form of an unsegmented ganglionic mass beneath the oesophagus. The segmentation of the ventral ganglionic chain presents in details the greatest variations; in general, however, it corresponds to the heteronomous segmentation of the animal, in that in the larger regions of the body, which have arisen by fusion of several segments, 408 AllTJIHOrODA, au approximation or fusion of the corresponding ganglia has taken place. In one case only, viz., in the Fentastomidce, which in form and grade of life resemble the intestinal worms, the dorsal part of the oesophageal commissure is not swollen out to form a cerebi-al ganglion, and the central parts of the nervous system are com- pressed together into a common gangli- onic mass beneath the ojsophagus. In all other cases the brain is a large gangli- onic mass lying above the oesophagus, and connected by means of the oesophageal ring with the antei-ior ganglion of the ventral chain, which is usually placed in the head and is known as the suboeso- phageal ganglion (fig. 328). The .sense nerves arise from the brain, while the ganglia of the ventral chain send nerves to the muscles, organs of locomotion and the body covering. Visceral nervous system. — In addition to the brain and ventral ganglionic chain, which are compai^able to the cerebro-spinal system of Vertebrata, we can distinguish in the larger and more highly organised Arthropoda a visceral nervous system {symjxithetic), which consists of special ganglia and plexuses connected with the other system and speciall}' distributed to the alimentary canal. In the higher Ar- tliropoda, paii-ed and unpaired viscercd nerves are very generally present, both of which have their origin in the brain. Sense organs. — Eyes are most generally distributed, and are only absent in a few parasitic forms. In their simplest form they are paired or unpaired .structures placed upon the brain, provided with re- fractive bodies, and with or without a simple lens (stemmata, or simple eyes). The compound eyes, which are always paired, are much more complicated. They are distinguished by the presence of nervous rods and crystalline cones, and may be divided into faceted eyes Fig. 328. — Nervous system of the larva of Coccinella (after Ed. Brandt;. Gfr, Frontal ganglion ; (?, brain ; iSVjr, sub- cesophageal ganglion ; & to G", ganglia of the ventral chain in the thorax and abdomen. ALIMENTABT CAKAL. EXCBETORT ORGANS. 409 and eyes with smooth cornea {Cladocera). The former possess numerous lenses, and are sometimes placed on movable stalks {Decapoda). Occasionally accessory eyes are found on other parts of the body, on the jaws and between the legs of the abdomen (Ev.phausia). Auditory organs are found most frequently in the Crustacea as auditory vesicles with otoliths in the basal joint of the anterior antenna?, or rarely in the appendage of the abdomen known as the fan (tail of Mysis). In Insecta, auditory organs of a very different structure have been discovered. Olfactory organs are also widely distributed. They are situated on the surface of the antennae, and consist of delicate tubes or peculiar conical projections, beneath which the sense nerves end in ganglionic swellings. Tactile organs. The antennse and palps of the oral appendages and the ends of the limbs have a tactile function. These parts are provided with peculiar hairs and setse, beneath which nerves end in ganglionic swellings. Alimentary canal. — An independent digestive apparatus is always present, but its structiu'e and degree of development are very various. The ahmentary canal is only exceptionally degenerated and absent {J^hizocepliala). The mouth is placed on the ventral surface of the head. It is furnished with a projecting vipper hp, and usually with paired appendages, which are used either for masticating or for piercing and sucking. A narrow or wide oesophagus leads into the intestine, which either simply traverses the axis of the body or is disposed in several coils. The oesophagus and midgut (chyle stomach) may even be divided into several regions, and may possess saUvary glands and hepatic appendages of various size. Excretory organs. — Urinary organs are widely distributed. In the simplest form they appear as cells on the surface of the intestine (lower Crustacea), in a more highly developed sta,te as tubular filiform diverticula of the hindgut {Malpighian tubes) (fig. 329). In the Crustacea, glands are present in the shell {shell glands) and in the base of the posterior antennae; they are regarded as the morphological equivalents of segmental oi-gans. The circulatory and respiratory organs present the greatest differences in the various groups of the Arthropoda. In the simplest case the clear, more rarely coloured blood fluid, which is often corpusculated, fills the body cavity and the interstices of all 410 ARTlinOPODA. the organs, and is circulated in an irregular manner by the move- ments of the different parts of the body. Not unfrequently {Achtheres and Cyclops) the cii-culation is effected by the regularly repeated movements of certain organs (intestine, vibratile plates, etc.); in other cases, a short saccular heart is present dorsally above the intestine ; or a long vascular tube (the dorsal vessel), divided into chambers, serves as a propelling organ. From this, vessels (arteries) may arise, which conduct the blood in definite directions. Vessels for retm-ning the blood (veins) may also be present. These either begin in the body cavity, or are connected with the ends of the arteries by capillary vessels. The vascular system seems never to be completely closed, since even when the circulation is most complete, lacunar spaces of the body cavity are fo\ind inserted in the course of the vessels. Respiration is very frequently effected, especially in the smaller and more deli- cate species of Arthro2}oda, by means of the entire surface of the body. In the larger aquatic forms, the function of respi- ration is assumed by special tubular, usually bi-anched appendages of the limbs (branchice) ; while in the air-breathing Insects, Centipedes, Scorpions, and Spiders, respii'ation is performed hy means of in- ternal branched tubes filled with air [trachecti) or by pulmonary sacs {fan trachece). The reproduction of the Arthrojjoda is usually sexual, but sometimes takes place by the development of unfertilized ova (jyarthenogenesis). Ovaries and testes are in their origin paired, as are also the gene- rative ducts, which often have a common terminal portion and open by a median generative aperture (Insecta, Arac/moidea). With a few exceptions {Cirripedia, Tardigrada), the sexes are sepamte. Males and females frequently differ essentially in their entire form and organization. In rare cases, for example in the parasitic Fig. 329.— Alimentary canal of Pontia brassicfB (after iVew- port). R, Proboscis (Maxillse) ; Sp, salivary glands ; Oe, oeso- phagus ; S, sucking stomacti ; My, Malpighian tubes ; Ad, rectum. OBUSTAOEA. 411 Crustacea, there is such a luai'ked sexual dimorphism that the males remain small and dwarfed, and are attached like parasites to the body of the female. During the act of copulation, which is often limited to the external union of the two sexes, the spermatophores are fastened to the female genital segment or thrust into the vagina by the organ of copulation, whence they sometimes pass into a special receptaculum seminis. Most Arthropoda are oviparous, but in almost every group there are viviparous forms. The eggs are frequently carried about by the mother, or deposited in protected places where food may easily be obtained. The embryonic development [i.e., development within the egg) is characterised, except in the case of the small stout embryos of the Cyclopidce, Pentastomidce and Acarina, by the presence of a ventrally placed primitive streak, from which especially the ganglionic chain and the ventral parts of the segments proceed. The more or less complex embryonic development is usually followed by a complicated metamorphosis, during which the young form as larva undei-goes several ecdyses. Numerous seg- ments and parts present in the adult are not unfrequently wanting in the just-hatched larva ; in other cases, all the segments of the adult are indeed present, but are not as yet fused together to form regions. In such cases, the larvis resemble the Annelida in their homonomous segmentation, and in their locomotion and mode of life. The meta- morphosis may however be retrogressive ; the larvte are hatched with sense organs and appendages, but in the further course of develop- ment they become parasitic, lose their eyes and organs of locomotion, and develop into strange unsegmented (Lernoice) or entozoon-like (Fentastomidce) forms. The Arthropoda are no exception to the general rule that the aquatic forms which breathe by gills are lower and, from a genetic point of view, older than the air-breathing members of the same group, inasmuch as the Branchiata or Crustacea are the older, the Tracheata the younger types. CLASS I. -CRUSTACEA.* Aquatic Arthrojwda, which breathe by means of gills. They have two pairs of antennce ; numerous paired legs on the thorax, and usually also on the abdomen. * Milne Edwards, " Histoire naturelle des Crustaces," 3 vol. and atlas, 1838- 1840. C. Clans, " Untersuchungen zur Erforscbiing der genealomsclien Grund- lage des Crustacecnsystema," Wien, 187(;. 412 ARTHBOPODA.. The Crustacea, whose name is derived fi-om the body covering (which is often hardened), are principally aquatic animals. Some forms, however, can live on land, and possess respiratory organs adapted for breathing air. An important character of the group is the great number of paired appendages. The appendages of all the segments, even those of the head, may be used in locomotion (fig. 330). As a rule, the head fuses with the thorax, or at any rate with one or more of the thoracic segments, to form a cejMothorax ; which is followed by the remaining free thoracic segments. Some- times, however, these two regions of the body remain distinct. The head and thorax are seldom so sharply marked off from one another as, for example, in the Insecta : usually certain appendages, the so-called maxillipeds, occupy an intermediate position between legs and jaws, and being placed at the boundary between the two regions may be rec- koned either as be- longing to the head or the thorax. The fusion of the seg- ments may be very extensive ; not only may the head and thorax be united, but the boundary be- tween thorax and abdomen may vanish, and the segmentation may even disappear. As a general rule, the form of the body presents extraordinary differences in the various groups. A reduj)li- cature of the skin arching over the thorax and covering the body as a shell is frequently present. This fold of the integument constitutes, in exti'eme cases, a mantle-like investment, which may develop calcareous plates and occasion a certain I'esemblance to Lamelli- branchs {Cirripedia). In other cases the body has quite lost its segmentation, and the animal resembles a worm {^Lerna'ce, Saccidina). On the head there are usually two pairs of antenn:>3, which function as sense organs and sometimes also as organs of locomotion or of prehension. There is a pair of large jaws (the mandibles), one on each side of the mouth, over which a small plate, known as the upper Up, often projects. The mandibles are simple but very x'igid and hard masticating plates, which are usually toothed and correspond Fig. iZO.—Gaminariis neglectus (after G. O. Sars). A', A", The two autennaj ; Kf, maxilliped ; P F~, first to seventh thoracic feet ; Sf, anterior swimming feet. CBU8TACEA. 413 morphologically to the coxal joint of a limb, the following joints developing into a palp-like appendage {mandibular palp). Then follow one or more pairs of weaker jaws [maxillce), and one or more paii-s of maxillipeds, which more or less resemble the legs and, in parasitic forms, are often used for adhering (fig. 331). In parasitic forms, the upper and under lips not unfrequently give rise to a suctorial probos^cis, in which the stylif orm mandibles are placed. The appendages of the thorax, of which at least three pairs are present (Ostracocla), present an extremely various structure, in accordance with the mode of life and the use made of them. They are either broad leaf - shaped swimming feet [PhyllojJoda), or bi- ramous appendages ((Jopepoda) ; they may serve to produce currents in the water like the feet of the Cir7~ipedia, or they may be used for crawling, walking, and running {Isopoda, Deca- poda). In the latter case, some of them end with hooks or chelae. Finally the appendages of the abdomen, which frequently itself moves in toto and assists in locomotion, are either exclusively locomotory as jumping or swim- ming feet {Amphi2)oda), in which case they usually differ from the appendages of the thorax ; or they serve with their appendages for respiration, as well as for carrying the eggs, and for copulation [Decajwda). The internal organization is not less vaiied than is the external form. In the lower forms, the nervous system often consists of a ganglionic mass, which surrounds the oesophagus and is not further Pig. 331. — Young stage Qarva) of the Lobster (after G" O. Sars). a. The larva seeu from the side ; M, ros- trum ; A!, A", antennifi ; Kf" third maxilliped ; F', anterior ambulatory leg. h, mandible with palp ; c. anterior maxilla with two blades and palp ; d, pos- terior maxilla with vibratile plate (scaphognathite) ; e, first, /, second maxilliped. 414 ARTimOPODA, segmented. This ganglionic mass corresponds to the brain and ventral cord and gives off all the nerves. In the higher Crustacea, a distnict brain and ventral ganglionic chain, which is usuiilly elongated and of very varied form, as well as a rich plexus of visceral nerves and ganglia of the sympathetic system are always present. Of sense organs, eyes are the most widely distributed. They may have the form either of simple eyes (paired or unpaired), or compound eyes with smooth or faceted cornea; in the latter case they are often placed on movable stalks, which are attached to the lateral regions of the head. Auditory organs are also present usually in the basal joint of the anterior antenna, rarely in the caudal plate at the posterior end of the body {Mysis). The deHcate hairs and filaments of the anterior antenna are probably olfactory organs. The digestive canal is, as a rule, straight, extending from the mouth to the anus at the posterior end of the body. In the higher forms the oesophagus is usually dilated in front of the mesenteron (midgut) into a stomach or crop, which is armed with chitinous plates. The mesenteron is provided with simple or ramified hepatic caeca. Excretory organs.— The so-called shell glands of the lower Crustacea are regarded as urinary organs, as are also the glands opening at the base of the posterior antenna in the Malacostraca. In the Entomostraca the latter are only preserved during larval life. Short tubes, which correspond to the Malpighian tubes of the Tracheata, may also be present on the rectum i^Ampldiyoda). The circulatory organs present every possible degree of perfection, from the greatest simplicity to the highest comiDlication of an almost closed system of arterial and venous vessels. The blood is usually colourless, but is sometimes green or even red, and as a rule contains cellular blood corpuscles. Respiratory organs are either entirely wanting, or are repre- sented by branchial tubes on the thoracic or abdominal appendages. In the first case they are often contained in a special branchial cavity at the sides of the cephalothorax. Generative organs. — With the exception of the Cirriimlia and cei-tain Isofoda, all Crustacea are of separate sexes. The male and female generative organs usually open on the boundary of the thorax and abdomen, either on the last or the antepenultimate thoracic ring, or on the first abdominal segment. The two sexes are very often distinguished by a number of external characteristics. CEIJSTACEA. 415 The males are smallei-, sometimes even dwarfed, and then attached to the females like parasites. They almost always possess appa- ratuses for holding the females and for transferring the spermato- phores during copulation. The larger females, on the other hand, frequently carr'y the eggs about with them in sacs, the membranes of which are secreted by the so-called cement glands. Development takes place eithei- directly or by metamorphosis. The metamorphosis is sometimes retrogressive. When the develop- ment is direct, the young animals, on leaving the egg, already have the body form of the adult. The larva known as the Nawplius (fig. 332) is of great importance as a point of departure. This larva possesses an oval body, on the ventral side of which are present three pairs of appendages for the sense of taste, the prehension of food, and for locomotion. These appendages correspond to the two pairs of antenn?e and mandibles respectively. Parthenogenesis is said to occur in certain groups {Phyllo- poda). Almost all Crustacea are carnivorous. Some of them suck the juices of living animals on which they are parasitic. For the systematic review of tliis heterogeneous group, it is convenient to divide the numerous orders into t^^'o sei'ies. 1. The small simply organized Crus- tacea, the nvimber and form of whose appendages is very various, will be in- cluded as Entomostraca (O. Fr. Mliller). To this group belong the orders Phyllo- j)oda, Ostracoda, Copepoda, and Cirripedia. 2. The higher Crustacea, characterised by a definite number of segments and appendages, may be grouped together as Malacostraea (Aristotle). In this group are included the orders of Arthrostraca {Ampldpoda and Isopoda), and Thoracostraca {Gumacea Stomatop)oda, Schizopodcb, and Decapodci). In addition there is the genus Nebalia, which has been hitherto erroneously placed with the Phyllopoda, but which is to be regarded as the representative of an ancient group connecting the Phyllojjoda with the Malacostraea, and may be opposed to the latter as Lept- ostraca. Finally, in addition to these chief divisions, there is a number A' Fig. 332.— Naiiplius larva of Sal anus, seen from the side. A' First appendage (first an- tenna); second appendage (second antenna) ; Mdf, third appendage (mandible) ; Ob, upper lip ; D, intestine. 416 AUTIIROPODA. ENTOMOSTBACA. of Crustacean orders, for the most part fossil and belonging to the oldest formations, which present in their development no cei-tain trace of the Nauplius form so characteristic of the true Crustacea, and are in all probability related to the Arachnoidea. These orders, which may be grouped together as the Gigantostraca, are the Merostomata and Xiphositra, to which the Trilohita are possibly allied. 1 .— ENTOMOSTRACA. Order 1 . — Ph yllo poda. * Crustacea toith elongated and often distinctly segmented body ; ViSually with a flat, shield-like carajyace, or laterally compressed bivalve shell, formed by a rediqjlicature of the skin. There are, at least, four pairs of leaf-like lobed swimming feet. The animals belonging to this order differ • very considerably in form and size, in the number of their segments and appendages, as well as in theii- internal structure. They all, however, agree in the structure of their lobed, leaf -like feet. In their form, internal organization and development they appear to be the most primitive of Crustacea, and may be regarded as the least modified descendants of ancient types. The body is either cylindrical, elongated and clearly segmented, mthout free reduplicature of the skin, e.g. Branchijms (fig. 333), or it may be covered by a broad and flattened shield, which only allows the posterior part of the body to project uncovered, e.g. Aptts. In other cases the body is laterally compressed and is enclosed by a bivalve shell, from which the anterior part of the head projects (Cladocera) ; or finally the laterally compressed body is completely covered by a bivalve shell {Estheridca). Sometimes the head is more sharply distinct, while the thorax and abdomen are not so clearly distinguishable from each other. As a rule, the posterior segments only are without appendages. The hind end of the abdomen is very often curved ventralwards and forwards, and bears two rows of posteriorly directed claws, the two last of which arise at the point of the caudal appendage, and are by far the * Besides the works of O. Fr. Miiller, Juri.ie. M. Edwards. Dana, comiare Zaddach, '• Dc Apcdis cancriformis a.iatomo et historia evolulionis. Bonn^, 1841. E. Grube. Bemcrknngcn iibcr die Pliyil.)poff'^' 1853 and 18o5. Fr. Leydig. •' Monographic dor Daphniden. lubuigen. 1S60. PHYLLOPODA. 417 strongest. In other cases a pair of fin-like appendages are present constiti^ting the caudal fork [Branchipus). Appendages. — On the head there are two pairs of antennas, which however, in the adult animal, may be rudimentary or peculiarly modified. The anterior antennse are small, and bear the delicate olfactory hairs. The posterior antennae frequently have the form of large biramous swimming appendages, but in the male may also have a prehensile function, e.g., Branchipus. In other cases {Ap)us) they are rudi- mentaiy and may even be entu'ely absent. Two large mandibles are always present beneath the well developed upper lip; they possess a toothed, biting edge, and in the fully developed condition are invariably destitute of palps. The mandibles are followed by one or two pairs of shghtly developed maxillae. A kind of under- lip is in many cases present, in the form of two promi- nencesbehind the mandibles. The legs, which ar-e placed on the thorax, are usually • very numerous, and are smaller towards the poste- rior end of the body. They are lobed, leaf-like, bira- mous structures, and func- tion as swimming feet ; they also assist in procuring food. They consist of the f ollomng parts : a short basal portion, which is usually provided with a masticatory process and is followed by a long foliaceous stem with set£e on its inner edge ; this is continued into the multilobed internal branch [endopodite] of the bii-amous limb, AvhHe it bears on its outer side the external ramus [exopodite] with marginal seta3, and nearer 27 Fig. 333.— Male of BrancUpws sfagnaUs. Rff, Heart or dorsal vessel with a pair of slit-like openings in each segment ; B, intestine ; M, mandible ; Sd, shell gland; Si; branchial appendages of the eleven pairs of legs ; T, testis. 418 CRUSTACEA. its base a vesicular branchial appendage. The anterior, or even all the legs (^Leptodora) may have the form of prehensile feet, and be destitute of branchial appendages. The Phyllopods possess a large pair of eyes, which are sometimes fused together in the median line. In addition a small median simple eye (Entomostracan eye) may persist. They have a saccular or chambered heart, which controls the regular circulation. Coiled excretory organs, known as shell glands, are sometimes present ; they open to the exterior by a special aperture on the posterior maxilla. The function of respiration is performed by the entire surface of the body, the area of which is much increased by the I'eduplicature of the skin forming the carapace; also by the folia- ceous swimming feet, and especially by the surface of the branchial appendages. Reproduction. — The Phyllopoda are of separate sexes. The males are distinguished from the females by the structure of the first pair of antennee. which are larger and more richly provided with olfactory haii-s, and also by their anterior swimming feet which are armed with prehensile hooks. In general the males are less fre- quently met with than are the females, and, as a rule, only at definite seasons of the year. The females of the smaller Phyllopoda {Clado- cera) are able to produce eggs without copulation and fertilization ; and these eggs, the so-called summer eggs, develop spontaneously and produce generations containing no males. In certain genera of the Branchiopoda, e.g., Artemia and Ajms, parthenogenesis is the rule ; the males, indeed, have only been known a few years. The females usually carry the eggs about mth them on special appendages, or in a brood pouch beneath the shell on the dorsal surface. The just hatched young either possess the form of the sexually mature animal {Cladocera), or undergo a complicated metamorphosis, leaving the egg membranes as a nauplius larva with three pairs of appendages {Bran- chio2}oda). A few of the Phyllopoda live in the sea, the greater number inhabit stagnant freshwater ; some of them are found in brine pools. Sub-order 1. Branchiopoda.* Phyllopoda, ^ith clearly seg- mented body, often enclosed in a flat, shield-shaped, or laterally compressed bivalved shell, with from ten to about thu-ty or more pairs of foHaceous swimming feet. * Schaffer, " Der krebsavtige Kieferfuss," etc. Regensbin^. 1756. A Koz^^^ von Branchipus und Apus," etc., Gbttingcn, 1873. PIIYLLOPODA. BEANCHIOPODA. 419 The alimentary canal is provided with two lateral hepatic appen- dages, which I'ule, branched and racemose and only excep- tionally short and simple. The heart appears as an extended dorsal vessel with numerous paired lateral slits, and may extend throughoxit the whole length of the thorax and abdomen (^Byxmchipus). The genital organs, which are always paired, are placed by the side of the alimentary canal, and open at the boundary between the thorax and abdomen. In the females the genital openings are small slits ; in the male there may be protrusible copulatory organs at the openings [Branchipus). The males are distinguished from the females principally by the fact that the anterior, or two anterior pairs of legs, are armed with hooks {Estheridce), or by the modification of the posterior antennje to form a prehensile apparatus {Branchipus). Remarkable is the rare occurrence of the males ; they seem only to appear under certain conditions and in definite generations, which alternate with parthe- nogenetic generations. The eggs during development are generally protected within the body of the mother, and are carried about either in a saccular brood-pouch of the abdomen or between the valves of the shell on filiform [Estheria, BranoMpus), or in vesicular {Apus) appendages of diiferent pairs of legs (9th to 11th). The eggs, so far as is known, undergo a complete segmentation. When hatched, the young animal has the form of a Nauplius larva with three pairs of appendages, of which the anterior (which become the anterior antennae) are in the Estheridce only represented by slightly de- veloped setigerous prominences. On the other hand, in Apus the thii-d pair is small and rudimentary. Almost all the Branchiopoda belong to inland waters, and prin- cipally inhabit shallow fresh-water pools. When the latter dry up, the eggs, preserved in dry mud, remain capable of development. Some species, as Artemia salina, are found in brine pools. Braiichipios piscifovmis SchafE = B. .stagnaUs L.. without a shell, found in the lakes of Germany, together with Apus cancriformis. B. diaplianus Pr«v. Irance. Artemm salina L., in salt pools, near Trieste, Montpellier. They sometimes lay eggs with a hard shell, sometimes they are viviparous Ipus cancrifovvm Schaff, with shield-shaped shell, Germany. The males, which are rare, can be recognized by the normal formation of the eleventh pair of appen- dages. _ They live in puddles and fresh-water lakes, together with BrancUpus EMieria eycladoides Joly L.. with perfect shell. Sub-order 2. Cladocera.* Water-fleas. Small laterally com- * Besides the works already quoted, compare H. E. Strauss, "M^moire sur les Daphnia de la classe des Crastacds," du Mus. d%lsi nat., Tom V and 420 CnUSTACEA.. pressed Phyllopoda, whose body, with the exception of the head, which projects freely, is usually enclosed in a bivalve shell. They have two large anteniiJB, which are used in swunming, and four to six pairs of swimming feet. The Cladocera are small simply organized Phyllopods, whose resemblance to the larvse of the shelled Branchiopoda, particularly to the larva of Estheria with its six pairs of legs, gives the best indica- tion of the probable origin of the group. Unlike the anterior antennie, which are short, the posterior are modified to form biramous swimming appendages beset with numerous long set*. The four to six paii-s of legs are not always foUaceous swimming feet, but in many cases have the form of cylindrical ambulatory or prehensile appendages. The abdomen, which is ventrally flexed, develops on its dorsal side several prominences, which serve to close the brood pouch. It usually consists of three free segments as well as the terminal anal portion, which is beset with rows of hooks. The anal portion begins with two dorsal tactHe setae and ends with two hooks or styles, representing the caudal fork (fig. 334). . - ^. The internal organization is simple in correspondence with the small size of the body. The compound eyes fuse together m the middle line to form a large, continuaUy tremblmg, frontal eye, be- neath which the unpaii-ed simple eye usually remams. A special sense apparatus, whose function is not quite clear, appears in the region of the neck, m the form of an aggregation of ganglion cgIIs The heart has the form of an oval sac, with two transverse lateral venous ostia and an anterior arterial opening. Its pulsations are rhythmic, and succeed one another quickly. In spite of the want of arteries and veins, the circulation of the blood, which contams amoeboid cells is completed in definite tracts marked out by lacuna and spaces in the body. The looped and coHed shell gland is always present. The cervical gland, which'^functions as an organ of attachment, is less widely distributed. Thei sexual glands lie in the thorax as paired YI, 1S19 and 1820. Leydig. •• Nainv^scl^ vexel hos Lcptodova." \ >dcn.h -^^^^^^^^l ' ^ ; jg^T. C. Clans. " Znr znr Kenntiss der Daplnunden. ' Znt.f. n-us. II l^and. Wien, 1879. PHYLLOPODA . CLADOCEEA. 421 tubes by the side of the alimentary canal. In the ovaries groups of four cells are separated ; one cell of each group becomes an ovum, while the rest are employed as nutritive cells for the nourishment of the ovum, which increases in size and absorbs fat globules. The ovary is du-ectly continuous with the oviduct, which opens dorsally beneath the shell into the brood-pouch. The testes, like the ovaries, lie at the sides of the intestine and ai-e continuous with the vasa deferentia, Fig. S34i.—Daphnia. C, Heart— the slit-like opening of one side is visible ; B, alimentary- canal ; Z, hepatic diverticulum ; A, anus ; (?, cerebral ganglion ; O, eye ; 8d, shell gland ; Br, brood-pouch beneath the dorsal reduplicature of the shell. which open to the exterior ventrally behind the last pair of appen- dages or at the extreme end of the body, the openings being some- times situated on small slightly protrusible prominences. The smaller males usually appear in the autumn ; they may, however, also be present at any other time of the year, and, as recant investi- gations have proved in a tolerably satisfactory manner, always when 422 CBUSTACKA.. the conditions of life and nourishment are unfavourable. Before the appearance of the males, hermaphi-odite forms * sometimes make their appearance with an organization which is half male and half female. At the season when males are not present, normally in the spring and summer, the females produce the so-called summer eggs, which contain a large quantity of oil globules and are surrounded by a delicate vitelline membrane. They develoj) lupidly within the brood- pouch between the shell and the dorsal surface of the mother, and after the space of only a few days give rise to a fresh generation of young Cladocera, which escape from the brood-pouch. The embryonic development takes place accordingly under extremely favourable conditions, which depend upon the rich supply of food yolk in the large eggs, and ai'e sometimes favoured by the secretion of additional food material within the brood-pouch. At the season when the males appear, the females, under the like influence of unfavourable nourishment and independently of copu- lation, begin to produce so-called winter eggs, which are incapable of developing without fertilization. The number of these hard-shelled winter eggs is always relatively small. They are, therefore, distin- guished from the summer eggs by their larger size and the gi-eater quantity of food yolk ; and their origin in the ovary is accompanied by much more extensive processes of absorption. The Dcq^hnidce live for the most part in fresh water. Certain species inhabit deep inland lakes, brackish water, and the sea. They swim quickly, and usually with a jumping movement. Some of them attach themselves to solid surrounding objects by means of a dorsally placed organ of attachment, the cervical gland. When the body is thus fixed, the swimming feet seem to be able by their vibrations to set up currents in which small food particles are swept towards the animal. Sida crystalUna 0. Fr. Miiller. The six pairs of lamellar legs beset mth long swimming seta.\ The rami of the swimming antennre two- to three- jointed. Bnitlinia puUx De Geer. D. nma Liev. Five pairs of legs, of which the anterior arc more or less adapted for prehension. One ramus of the s^xnmmmg antenna is three-jointed, the other four-jointed. Polyphemus prdn;d».-< Dc Geer. In the lakes of Swtzerland, Austria, and Scandinavia. Erad^ir .\ordmanni Lov6n, North Sea and Mediterranean. Lcptodora hyalina Lillj., in lakes. * Compare especially W. Kurz, " Ucl>er androgyne Missbildung bei Clado- ^^x^n:' ktzimglher der Akad. dev Wis-mh. 11 18/4. Also Schmanke- witsch. OSTEACODA. 423 Order 2. — Ostracoda * Small, usually laterally compressed Entomostraca, with a hivalve shell and seven pairs of appendages, which function as antennce, jaws, creeping and swimming legs. There is a pediform mandibular palp, and a short abdomen. The body of these small Crustacea is unsegmented and is completely enclosed in a bivalve shell, which gives the animal a resemblance to a mussel. The two valves of the shell join together in the middle line, and are fastened together by an elastic ligament along the middle third of the back. The action of this ligament is opposed by a two- headed adductor muscle, which passes from one valve of the shell to the other and causes impressions discernible from without. The common tendon of the two heads of this muscle lies nearly in the Fig. 335. — Female Cypris before sexual maturity ; the rislit valve of the shell has been removed, A', A", first and second pair of antennse ; Ol>, upper lip ; Md, mandible with pediform palp ; &, cerebral ganglion with unpaired eye ; S3I, adductor muscle ; Mx', Mx", first and second pair of maxillse ; F\ F", first and second pair of feet ; Fu, caudal fork ; M, stomach ; D, intestine ; Z, hepatic tube ; Ge, rudimentary genital organs. middle of the body. The edges of the valves are free at both ends and along the ventral side. In the marine C'yjorifZmttiog there is.a deep indentation in the edges of the valves, to allow the antennfe to pass out. When the valves of the shell are open, several pediform appendages can be protruded on the ventral side, which enable the animal to move in the water either by crawling or by swimming. * H. E. .Strauss-Dlirkheim, "M6moirc sur les Cypris de la classe des Crus- ' taces," 3Iem. du Mux (VliiKt. iint., Tom VII., 1821. W. Zenker, " Monographie der Ostracoden," Avchiv.fiir yatun/esoJt.. Tom. XX., 1864. C. Glaus, " Beitrage zur Kentiiiss der Ostracoden. Entvvickelungsgeschichte von Cypris." Marbui^. 18f)8.. C. Glaus. " Neuc Beobachtungen liber Cypridinen," Zeitschr. fiir iviss. Zool., Tom XXIII. «'. Glaus, "Die Familie der Halocypriden." Schriften zodlof/lirhen InhalU.\\\&n, 187'i. G. S. Brady, "A Monograph of tlic Recent British Ostracoda," Tranmct. of the Lin. Soc, Vol. XXVI. 424 CRUSTACEA. Tlie abdomen can also be protruded ; it either ends in a caudal fork {Cypris and Cylhere), or has the form of a plate armed with spines and hooks on its posterior margin {Cypridina). Appendages. -The two pairs of antennae are placed on the anterior region of the body (fig. 336, A', A"), and are used as creep- ing and swinuning legs. In Cyp)ridina, however, the anterior pair is provided with olfactory hairs. The antennte of the second pair in Cypris and Cythere resemble legs, and end with sti-ong hooked bristles, by help of which the animal can attach itself to surrounding objects. In the exclusively marine Cyj)ridinidce and Halocypridai this pair of appendages has the form of biramous swimming feet, which consist of a broad triangular basal plate, a many-jointed endopodite beset with long swimming setae, and a rudimentary exopodite, which, however, is stronger in the male and furnished with hooks of a considerable size. In the region of the mouth beneath and to the side of a tolerably large upper lip there are two powerful mandibles with a broad and strongly toothed biting edge. The mandibular palps, which are leg-like and elongated, are usually three- jointed and can be used as legs [Mdf). In exceptional cases {Paradoxostoma), the mandibles are styliform and are enclosed in a suctorial proboscis formed from the upper and under lips. The mandibles are followed by the first paii- of maxillae, which are in all cases distinguished by the great development of their basal portion and by the reduction of the palp. In the Cypridca and Cytheridce the basal joint of the first maxilla bears a lai-ge comb-like setose plate, which by its springing movements aids the function of respiration, but does not itself function as a gill. A similar branchial plate may also occur on the two following appen- dages (the 5th and 6th pair), which sometimes have the form of jaws, sometimes of legs. The anterior of these appendages (maxilla of the second pair or better maxilliped, fig. 336, Mx') functions, in Cypris, chiefly as a jaw, but bears, besides the rudimentary bran- chial appendage, a short, backwardly directed, usually two- jointed palp, which, however, in certain genera and in Halocypris becomes a short, three-jointed or even four-jointed leg. In Cythere it acts ex- clusively as a leg, and represents the first of the three pairs of legs present in this animal. In the Cypridina, however, it has completely the form of a jaw, and is provided with an enormously developed branchial plate (fig. 336 a, Mx"). The appendage of the sixth pair is usually modified to an elongated, many-jointed, creeping and ad- 08TBACODA. 425 hering foot. The appendage of the seventh pair is always elongated to the form of a leg ; in Cythera it is formed like the preceding one, Fig. 336.—Cyj)ridinamediterranea. a, Female; fi, male. M, Stomach; if. heart; SM, adductor muscle ; O, eye ; O', unpaired eye ; 6, brain ; Stz, frontal organ ; T, testis '; P, copulatory organ; Mdf, mandibular palp; Mx', first maxilla; iVx'', second maxiH a ; Ftt, caudal fork. but in Cypris it is curved upwards, and is furnished with a short claw and terminal setse. It has probably the same function (Putzf uss) 426 CRUSTACEA. as the long cylindrical appendage of Cyjyridina, wliicli arises in place of the seventh pair of legs, almost on the back of this animal. The nervous system consists of a bilobed cerebral ganglion and a ventral chain with closely approximated pairs of ganglia, Ashich may unite to form a single ganglionic mass. Sense organs. — In addition to the already mentioned olfactory hair> there is a median eye {Cyfris, Cy there), composed of two (often separated) halves; or thei-e are, in addition to a small unpaired eye, two larger compound and movable lateral eyes {Gy2)ridina). In Ualocyiyris and Cyimdina there is a frontal appendage, which probably functions as a sense organ. Alimentary canal. — The mouth, which is frequently {Cyjyris) armed with toothed latei-al bands, leads through a narrow oesophagus into a dilated crop-like portion of the alimentary canal. This is followed by a broad and long stomach, provided with two long lateral hepatic tubes, which project into the lamellfe of the shell. The anus opens at the base of the abdomen (fig. 337). Of special glands a club-shaped, dilated glandular tube (poison-glands X) found in Cythere must be mentioned, the duct of which opens to the exterior through a spinous appendage of the posterior antennae. A heart is present in Cyjyri- dina and Halocyi:)ris on the dorsal surface, where the shell is con- nected to the animal. The function of respiration is performed by the whole surface of the body, over which an uninterrupted current of water is maintained by the swinging movements of the leaf -shaped setose branchial appendages. In many Cy2yridinid(e (Asterope) there is a double row of branchial tubes on the back, near the last pair of appendages. Generative organs.— The sexes are always separate and are dis- tinguished by well marked differences in their entire structure. The males, in addition to the greater development of the organs of sense, possess apparatuses on different appendages— in Cypridina on the second antennas in Cy2oris on the maxilliped— for holding the females ; or a pah- of legs may be completely modified for this pur- Fig. 337.— Alimentary canal and generative organs of a female Cy^n-is (after W. Zenker). Oe, oesophagus ; P r, crop ; V, stomach ; D, intestine ; L, liver ; Ov, ovary ; SM, adductor muscle ; -B receptaculum ; Vu, vulva ; Fu, caudal fork. OSTRACODA. 427 pose. In addition a large copulatory organ, which may be derived from a modified pair of appendages and often possesses a very compli- cated stmcture, is always present. The male genital organs consist on either side of several elongated or globular testes, of a vas deferens and the copulatory organ ; the presence in Gypris of a very peculiar paii-ed mucous gland and the size and form of the spermatozoa seem to be worthy of notice (Zenker). The female of Cypris possesses two ovarian tubes which project into the reduplicature of the carapace, two receptacula seminis, and the same number of genital openings at the base of the abdomen. Development.— The greater number of Ostracoda lays eggs, which they either attach to water-plants {Cypris), or, as in Cypridina, carry about with them between the shell valves until the young are hatched. The free development of Cypris consists of a complicated metamorphosis. The larvae, when hatched, possess, like the Nauplius form, only three pairs of appendages, but are strongly compressed laterally, and are already enclosed in a thin bivalve shell (fig. 338). In the marine Ostracoda the development is simplified, so that the metamorphosis is entirely absent. The Ostracoda feed altogether on ani- mal matter, as it seems especially on the carcasses of different aquatic animals. Numerous fossil forms are known from almost all formations, but, unfortunately, only the remains of their shells are pre- served. Cypridina. With heart and large movable paired eye. With deep excava- tion in the edges of the shell for the passage of the antennae. The anterior antennfe are bent, furnished with strong sette, and have olfactory hairs at their extremity. The posterior antcnnss are biramous swimming feet. The biting part of the mandible is weak or entirely aborted ; palp is five-jointed, pediform, and of considerable length. The seventh pair of appendages is represented by a cylindrical ringed appendage (Putzfuss). Cypridina mediterranea Costa. Astcrope ohlonga Gv., Trieste. Ilalocypris Dana. Cytliere 0. Fr. Miill. Without heart. The anterior antennsB are bent at their base and beset with short setfe. The posterior antenna are strongly developed, with hooks on the terminal joint. Three pairs of legs, of which the last is the most strongly developed. The abdomen has only the caudal fork, of which the two branches are small and lobc-like. The testes and ovaries do not project between the lamellfE of the carapace. The male genital apparatus has no mucous gland. They arc all marine animals. The females often carry the Fig. 338.— Very yoiino: lai*va of Cypris. Nauplius stage, with, three pairs of appendages. M, stomach ; D, intestine ; SM, shell muscle ; A', A!', first and second antenna3; Mdf, mandible. 428 CRUSTACEA.. eggs and embryos about between the valves of the shell. Cijtlicre lutea 0. Fr. -Miiller, North Seas and Mediterranean. 0. viridw 0. Fr. Miill., North Seas. Cyjn'ix 0. Fr. Mlill. With median eye, but no heart. The sliell valve« arc light but strong, the anterior antennae have, usually seven joints and are beset with long setae. The antenna of the second pair is simple and pediform, with usually six joints. There are two pairs of le'^n, of which the i)osterior smaller pair is bent upwards towards the dorsal surface. The caudal fork is very narrow and elongated, and is provided with hooked setae at the point. The testes and ovaries i)roject between the lamellae of the shell. The male genital appa- ratus has a peculiar mucous gland. Most of them inhabit fresh water. Oi/jprisfi(.scaStv., C.j^;?iJem 0. Fr, Miill., C./w«mto Jur., and others. Xotudronmg monaehns 0. Fr. Miill. 07xle7- 3, — CoPEPODA. Entomostraca with elongated, usiudly loell segme.nted body, without shell-forminy reduplicatiire of the skin, with biramous swimviing feet ; the abdomen is without ajjpendages. The group of the Gopejioda inckides a number of very different forms. The non-parasitic members of the groups are distinguished by a constant number of segments and paired appendages. The numerous parasitic forms differ in various degrees from those which lead an independent life ; in extreme cases some of them are so modified, that without a knowledge of their development and the peculiarities of their structure, they would rather be taken for parasitic Worms than for Arthropods. The characteristic swimming feet are, however, usually retained, though often reduced in number, as rudimentary or modified appendages. When they are absent, the developmental history gives a certain indication of the Copepod nature. Appendages. — The head seems as a rule to fuse with the first thoracic segment ; and the cephalothorax so formed bears two pairs of antennfe, a pan- of mandibles, the same number of maxillfe, and four maxillipeds, which last are only the external and internal branches of a single pair of appendages (fig. 341) ; and finally the first pair of swimming feet, which are not unfrequently modified in form. Then come four free thoracic segments, each with a pair of swimming feet, of which the last pair is frequently reduced and in the male may be modified to assist in copulation. Finally, the fifth pair of feet and * 0 Fr Miiller, " Entomostraca sen Insecta testacca. quaj in aquis Danije et NorvegifB reperit. descrii^sit," Lipsias, 1785. Jurine. " Histoire dcs Monocles. Geneve 1820. W. Lilljeborg. "Crustacea ex oi-dinibus tribus : Cladocera. Ostracoda et Copcpoda, in Scania occurrentibus,- Lund.. Ip:-^- £ Claus " Zur Morphologic dcr Copepoden," Wilrzh. natitriHss. Znt.irhr., Ib60. L. Olaus., " Die freilebenden Copepoden," Leipzig, 18(53. COPEPODA. 429 the corresponding thoracic segment may be entirely absent. The abdomen as well as the thorax consists of five segments, but is with- out appendages and ends in a caudal fork, the branches of which are furnished at their points with several long caudal seta? (fig. 339). In the female, the two first abdominal segments usually unite to form a double genital segment, on which the genital openings are placed. The abdomen, especially in the parasitic forms, very fre- quently undergoes a considerable reduction. Fig. 339.— Female of Cyclops coronatus, seen Fig. 310.— An antenna of the male of from the dorsal surface. D, intestine ; OoS, Ci/clops serrulatns. Sp, olfactory hairs . ovisacs ; A', A'', antennse. muscles. ' The anterior antenna?, which are usually, many- jointed, bear olfac- tory hairs, but serve in the free-swimming forms foi- locomotion, and in the male as prehensile arms for catching and holding the female during copulation (fig. 340). The posterior antennte are always shorter, and not unfrequently bifurcated and adapted for clinging to surrounding objects. With regard to the oral appendages 430 CIIUSTACEA. (fig. 341), two toothed, usually pulped mandibles ave placed be- neath the upper lip. These function in the free-living Copepoda as masticatory organs, but in the parasitic forms are usually trans- formed into pointed styliform rods, which are used for piercing. In this case they ai-e frequently placed in a suctorial tube formed by the junction of the upper and under lips. The two jaws which follow the mandibles are weaker biting plates, and in the parasitic Co2Jepoda are reduced to small palp-like protuberances. The maxil- lipeds, on the contrary, are much longer ; they are used to procure food and, especially in the parasitic forms, to attach the body. The thoracic s-wimming feet consist of a two-jointed basal portion, and two three- jointed setigerous swimming rami, which are comparable to broad swimming plates. In the Argulidm these rami are much elongated, and by their numerous joints approximate to the legs of the Cirrij^edia. Nervous System. — In all cases there is a brain giving off sensoiy nerves, and also a ventral cord, which either develops some ganglia in its course or is concen- trated to a common suboesophageal gan- glionic mass. Of sense organs the median frontal eye, divided into three parts {Cy- dops eye), is pretty generally present. The tactile sense is specially localized in the setje of the anterior antennre, but is probably also present in many other parts of the body. Olfactory hairs are pre- sent as delicate appendages of the an- terior antenna?, principally in the male sex. The alimentary canal is divided into a short nai-row oesophagus, a wide sto- mach which often has two blind diverticula near its commence- ment, and a narrow rectum which opens on the dorsal surface of the last abdominal segment. The surface of the intestine often seems to perform the function of a urinary organ. We find, however, at the same time a shell gland in the cephalo-thorax at the sides of the maxillipeds. In all cases the whole surface of the body performs If Fig. 341. — Mouth parts of Cyclops. M, Mandibles ; Mx, maxilla; Kf, internal; iTf", external maxilUped. OOPEPODA. 431 the respiratory function. Circulatory organs are either replaced by the regular oscillations of the intestinal canal {Cydo'ps, Achtheres), or there is present in the anterior part of the thorax above the intes- tine (Calanidce) a short saccular heart, which may even be continued into a cephalic artery {Ccdanella) (fig. 53). Generative organs. — The Gopepoda are of separate sexes. Both kinds of genital organs lie in the cephalothorax and in the thoracic segments, and open right and left on the basal segment of the abdomen. Sexual differences in the form and structure of the different parts of the body are almost uniformly found. These lead Fi&. 342. — Metamorphosis of Cyclops, a, Nauplius larva of Cyclops serrulatus after hatching. b, Older stage strongly magnified, e, Very young Cyclops form. AD, antennal glands ; 01, upper lip ; Mf, mandibular foot ; Md, mandible ; Mx, maxilla, Mxf, maxilliped ; J", F", first and second swimming feet; He, urinary concretions; D, intestine; Ad, rectimi ; A, anus ; G, rudimentary genital organs. in certain parasitic Copejyoda (Chondracanthidce, Lemceopodidce) to an extremely striking dimorphism. The males are smaller and move with greater facility ; the anterior antennae and the last pair of feet become accessory copulatory organs, the former serving to hold the female, the latter to affix the spermatophores. The sper- matophores are formed in the vas deferens by a mucous secretion Avhich surrounds the seminal mass and hardens to a tough mem- brane. The females are larger than the males and often move 432 CRUSTACKA. more clumsily ; they cany the eggs about with them in sacs, placed to the right and left on the abdomen. Many of them possess a cement gland at the end of the oviduct ; the secretion of this gland passes out with the eggs and gives rise to the stiff covei-ing of the ovisiics. Dui'ing copulation, which is only an external appi-oximation of the two sexes, the male fastens one or more spermatophores on to the genital segment of the female, and, indeed, on to special openings through which the spei-matozoa pass into the receptaculum seminis, and fertilize the ova either within the body of the mother, or as they pass out into the developing ovisacs. Development takes place by means of a complicated metamorphosis, which, ill many parasitic forms, is a retrograde one. The larvae, when hatched, have the Nauplius form, with an unpaii-ed frontal eye and thi-ee pairs of appendages. Hooked setse on the second and tliiixl pau's of appendages serve to conduct the fond into the mouth, which is covered by a large upper' lip (fig. 342, a). The posterior region of the body is destitute of appendages, and terminates with two setae at the sides of the anus ; it corresponds to the thorax and abdomen, which are as yet nndiffei'entiated. The altei'ations undergone by the young larvae in the course of their further gi-owth are connected with a number of successive moults, and consist principally in an elongation of the body and the appearance of fresh appendages. Even in the next larval stage (fig 342, h), a fourth pair of appendages, the future maxiUa?, makes^ its ap- pearance behind the three original pairs, which develop into the antenna and mandibles. In a later stage three fresh pairs of appendages are formed. Of these the first corresponds to the maxillipeds, while the two last pairs represent the first rudiments of the anterior swimming feet. In this stage (Metanaupli^cs) (fig. Fig. 343.— Metanauplius of Cyclopsine. O, eye; G, rudimentary genital organs ; SD, antennal gland ; A', A'', antennae ; Md, mandible ; Mx, max- illa ; Mf, maxilliped. COPEPODA. 433 343), the larva still resembles a Nauplivis, and it is only after another moult that it is transformed into the first Ci/clops-\ike form. It then resembles the adult animal in the structure of the antennae and mouth parts, although the number of the appendages and the body rings is smaller (fig. 342, c). The two last pairs of appendages already have the form of short biramous swimming feet, and the rudiments of the third and fouth pairs of swimming feet have made their appearance as projections beset with setae. The body consists in this stage of the oval cephalothorax ; the second, third and fourth thoracic segments ; and an elongated termiaal portion, which gives rise to the last thoracic segment, and to all the abdominal segments by a pro- gressive segmentation, and already terminates in the caudal fork. ^'""■If-jf^'-^^vercarum.-a, Nauplius form, h. Larva in the youngest Cyclops stage ; ti'^f '^""^f "^'n • ^f""^^^ ^^^^^"^^ Ovaries ; KB, cement . glands, d. The smaller male seen from the side ; Mxf, Mxf, maxillipeds. Many forms of parasitic Gopepoda, for example Lerncmthrojms and Chondracantlms, do not get beyond this stage of body segmenta- tion, and obtain neither the swimming feet of the third and fourth pairs, nor a fifth thoracic segment separate from the stump-like abdomen; others, for example Achtheres, by the loss of the two anterior pairs of .s^vimming feet, sink back to a still lower stage (fig. 344) _ AU the non^parasitic and many of the parasitic Copepoda pass m the successive moults_ through a larger or smaller number of de- velopmental stages, in which the still undeveloped segaaents and appendages make their appearance, and the appendages already 28 CRUSTACEA. present undergo further segmentation. Man}- parasitic Co'pe'poda, however, pass over the series of Nauplius forms, and the larva, as soon as hatched, undergoes a moult, and appears at once in the youngest Cyclops form, with antennae adapted for adhering and mouth parts for piercing (fig. 344). From this stage they undergo a retrogressive metamorpliosLs, in which they become attached to a host, lose more or less com- pletely the segmentation of the l)ody which grows irregular in shape, cast off their swim- ming feet, and even lose the eye, which was originally pre- sent [Lernceopoda). The males, however, in such cases often remain small and dwarfed, and adhere (fre- quently more than one) firmly to the body of the female in the region of the genital open- ing (fig. 345). In the Lerncea (fig. 346) such pigmy males were tor a long time vainly sought for upon the very peculiarly shaped body of the large female (fig. 346, c, d) which carries tubes. At last it was discovered that the small cyclops-like males (fig. 346, a), lead an independent life, and swim about freely by means of their four pau-s of svnm- ming feet; and that the fe- males (fig. 436, b), in the copulatory stage resemble the males, and that it is only after copulation that they (the females) become parasitic and undergo the considerable Fig 345 —The two sexual animals of C?iondra. cantlms ffihboxns magnified about six diameters. o, Female seen from the side; b, from the ventral surface with adhering male ; c, male strongly magnified. An' Anterior anteni.^; An", antennre for attachment; F', , the two pairs of feet; A, eye; Ov. egg-tuhes • Oe, oesophagus; D, intestme parts; T, testis; T'd, vas 3f, mouth deferens ; Sp, spermatophore. increase in size and modification of form which characterises the female with egg-tubes. COPEPODA. 435 1. Sub-order : Eucopepoda. CojJepoda with swimming feet, the rami of which are two or three jointed. They liave biting or piercing and sucking mouth parts. 1. Gnathostomata. For the most part non -parasitic; oral aj)j)aratus adapted for mastication ; fully segmented body. Fam. Cyelopidse. Mostly fresh-water animals, without a heart, and with a simple eye. The second pair of antennse are four-jointed and never biramous. The feet of the tifth pair are rudi- mentary in both sexes. The male employs the anterior antenna for prehension. Oi/clops coronatus Cls., Cantlwcamptus minutxis Cls., Marpaoticihs clielifer 0. Fr. Mtill., jSTorfch Sea. Fam. Calanidae. The anterior antennfe are very long, only one of them is modified for prehension. The posterior antennas are bira- mous. Heart always present. The feet of the fifth pair are, in the male, modified to assist in copula- tion. Cctocliilus septen trionalin Goods., Liaptomus castor Jur. Irenmis Patersmii Tempi. Fam. Notodelphyidae, Structure of body Uke that of the CyclopldK. The posterior antennae modified for attachment. The two last tho- racic segments are fused in the female and form a brood cavity for the reception of the eggs. They live in the branchial cavity of As- oidians. Xotodclplvys afjilis Thor. 2. Parasita* (Siphonosto- mata). Mouth parts adapted for piercing and sucking, usually with incomplete seg- mentation of the body and reduced abdomen. The posterior antenna? and maxillipeds end with hooks for attachment. Some of Fift. 3J,6.— iccKiffatu>! Cls., Sajjphirina fitlgens Thomps. Fam. Chondracanthidse, Body elongated, often without distinct segmenta- tion, and furnished with pointed outgrowths. Abdomen stump-like. I'he two anterior pair of swimming feet are represented by bifid lobes, the others are wanting. There is no suctorial proboscis, the mandibles are sickle-shaped. The pear-shaped males are small and dwarfed, and attached, often in pairs, to the body of the female. Chondracanthus gihhoxun Ki-. (on Lophius). Ch. cornutvs O. Ft. Miill., on flat fish [PleuroncctUlcp) (fig. 345), Fam. Caligidse. Body flat, with shield-like cepluilothovax, and very large genital segment which in the female is especially swollen. Abdomen, on the contrary, is small and more or less reduced. There is a suctorial tube and styliform mandibles. Four paired biramous swimming feet enable the animal to swim rapidly. They live on the gills and the skin of marine fish, and the females have long string-like egg tubes. Calir/us rajjax Edw., C^crojJS Latreillii Leach. Fam. LernseidEe. The body of the female vermiform or rod-shaped ; unseg- mented, with outgrowths and processes on the head. Mouth parts piercing with suctorial tube. There are four pairs of small swimming feet. The females become attached to fishes, in which the anterior part of theii- body is buried. Lernceocera cyprinacea L., Penella mgitta L,, Lerncpa hranchialis L, (fig. 346). Fam. Lernaeopodidee. Body separated into head and thorax, abdomen rudimentary. Mouth parts piercing with suctorial tube. The external maxilli- peds attain a considerable size, and in the female unite at their points so as to form a single organ of attachment, by means of which the animal adheres permanently. Swimming feet completely absent. The males, which are more or less dwarfed, have large free clasping feet, and are, like the females, without swimming feet. AcMhcre>^ percarum Nordm. (fig. 344). Anchorella wicinata O. Fr. Miill. (on species of Gadm). 2. Sub-order: BrancMura,* Carp-lice. With large compound eyes, and long protrusible spine in front of the suctorial tube of the mouth ; with four paii'S of elon- gated biramous swimming feet, ^erkrebse," Nova acta Ac. Cces. Loop.. Tom XVII., 1835 C. Claus, " Ueber den Bau und die Bntwickelung von Achtheres percarum, Ze/^^r/^;^/•^.r vm^. ZooL, 1861 C Claus, " Beobachtungen liber Lerna30cera, etc.. Marburg, l&bb. * Jurine. " Memoire sur I'Argule foliacd," Annates duJImn^m dh.^ ,ujf Tom VII 1806. Fr. Leydig, '• Ueber Argulus-toJiaceus. ^'*^.'tr/»-/<« « w5. Toll Tom II 1850 E. CornaUa, " Sopra una nuova specie di crostocei sifonos- S^mi':" Sila/.o, 1860. C. Clans', " Ueber die Entwickelung, O.|anuation und systematische Stellung der Arguliden," Zeitschrfurwiss. M, lorn XXV., 18/5. COPEPODA. BRANCHIUIIA. 437 The Branoliiura are often placed near the Galigidce, but they differ from them and from the true Copepoda in several essential par- ticulars. In the general body form they certainly resemble the Ccdigidce except in the abdomen, which is split into two plates (caudal fins). Their internal structure, however, and the structure of the appendages distinguish them from the above-mentioned parasitic Crustacea. A large suctorial tube projects above the mouth, and in it are concealed finely serrated mandibles and styliform maxillfe. A little above this proboscis there is inserted a long cylindiical tube, which terminates in a retractile styliform spine, and contains the ducts of a pair of glandular tubes said to be poison glands. Powerful organs of attach- ment are placed on each side of and beneath the mouth ; they consist of two parts — (1) of an an- terior pail- of appendages which correspond to the anterior maxillipeds and are in Argidus modified into large siicking discs, the hook-bearing terminal por- tion being reduced ; and (2) of a posterior pair, which corresponds to the second pair of maxillipeds, and is pro\n.ded with numerovis spines on its broad basal portion, a tactile protube- rance and two curved termi- nal claws at its extremity. Next to these come the four paired swimming feet of the thoracic gion, which, with the exception of the last, are, as a rule, covered by the sides of the cephalo-thoracic shield. Each of these consists of a large many-jointed basal portion, and two much narrower rami, which are beset ivith long swimming setse and in their form and setigerous investment are not unlike the biramous appendages of the Cirrijjedia, being like them derived from the Copepod-like feet of the larvl (fig. 347). Fig. 347.— TouBg male of Argulug foliaceus. A', Anterior autennre ; Sg, sucker (anterior maxilli- ped); Kf", maxilliped ; Sf, swimming feet, S, rostrum ; St, spine ; B, intestine ; T, testes. re- 438 CRUSTACEA. The internal organization i-ecalls that of the Fhyllopoda. The nervous system is diKtinguLshed by the great size of the cerebral ganglion, and by the ventral chain composed of six closely approxi- mated ganglia. In addition to two large compound lateral eyes, there is present an impaired tri-lobed median eye. The alimentary canal consists of a short arched ascending (Ksophagixs, a wide stonjach Avith two lateral ramified appendages, and a rectum which runs directly backwards and opens to the exterior in the median indenta- tion of the caudal fin above the two plates, which correspond to the caudal fork. There are two lateral slit-like apertures in the heart, and a long aorta. The entire surface of the cephalothorax functions as a respiratory oi-gan. There seems, however, always to be a specially strong current of blood in the caudal fin, so that this part of the body may be regarded as a sort of gill. Reproduction. — The small, moi e agile male possesses peculiar copu- latory appendages on the posterior swimming feet. The females do not carry their eggs about in sacs in the t}^ical Copepod manner, but fasten them to surrounding objects. The vitelline membrane of the deposited eggs acquires a vesicular consistence. The young are hatched as larvfe, and undergo a metamorphosis. Fam. Argulidae, Carp-lice. Anjnhis 0. Fr. Miill. The anterior pair of maxillipeds modified into large suckers. There is a styliform spine apparatus. A.foliaceufi L. (Pou de poissons, Baklner) parasitic on Carps and Sticklebacks. A. coregoni Thor., A giga/iteus Luc, GyrojK-ltis Hell. The maxillipeds end in a claw ; styliform spine absent. G. Kollari Hell, parasitic on the liranchiae of Hydrucyim, Brazil. G. Doradls Corn. Order 4. — Cirripedia.* Fixed, and for the most part herriiaphrodite Crustacea with indis- tinctly segmented body enclosed by a reduplication of the skin, and a calcareous valved shell. As a ride, there are six 2Jairs of biramous thoracic appendages. On account of the resemblance of their shell to that of the nuissels, the Cirripedia were held to be Molluscs until Thompson and Burmeister, by the discovery of their larvae, satisfactorily proved that they belong to the E7itomostraca. They are enclosed in a mussel- * Compare S. V. Thomp.son, -'Zoological researches," Tom. L, 1829. H. Burmeister, '' Beitrage zur Naturgcschiehte der Rankenfussler \832 Ch. Darwin, A monograph of the Sub-Class Cirripedia," 2 vol., London, 18ol-18.54. A Krohn '• Bcobachtungcn iiber die Entwickelung der Cirnpedien,- ArrMv fiir NuMmicxch 1860. C. Clans. " Die Cypris-ahnliche Larve der Cirnpedien, etc," Marburg, 186'J. R. Kossmanii, " Suctoria nnd Lcpadina, Wurzburg, 1873. CIERIPEDIA, 439 like shell composed of several (4, 5 or more) pieces. These pieces, which originate by the deposition of calcareous matter in the chi- tinous covering- of a large reduplicature of the skin (mantle), are distinguished as scuta, terga, and carina. The animal is invariably- fixed by the anteiior end of the head, which in the Lepadidce (fig. 348, a) may be drawn out into a long stalk projecting freely from the shell. In the Balanidce, which are without the stalk (fig. 348, h), the body is siirrounded by an external calcareous tube, usvially com- posed of six pieces ; the aperture of the tube is closed by a sort of operculum formed of calcareous plates lying inside (fig. 348, h). In l-'io. 348. — a, Lepax after removal'of the right shell. A', Anterior antennas at the end of the stalk ; C, carina ; Te, tergum ; 8c, scutum ; Mk, oral cone ; F, caudal fork ; P, cirrus or penis ; M, muscle, h, Balanus tintlnnahidim (after Ch. Danvin), one-half of the shell has been removed; Tu, Section of the outer shell ; Oc, ovary; OrZ, oviduct; Oe, opening of oviduct ; Ad, adductor muscle ; Sc, scutum ; Te, tergum ; A', anterior antennje. both cases the attachment is effected pi-incipally by the hardening of the secretion of the so-called cement gland, which opens on the penultimate joint of the small and delicate anterior antenna; this joint being dilated to form a sort of sucker. The body, which is surrounded by the mantle and its shell-plates, lies with its hinder region .stretched upwards so that the appendages, which are used to cause currents in the water, may be protruded from the slit-like space left on the ventral side between the paired scuta and terga. Appendages and external features.— A head with antenna and 440 CRUSTACEA. jaws can be distinguished from the region of the body (thorax) bearing the bii-amous appendages, but there is no distinct boundary between these two regions. The anus is situated at the extremity of the small stunip-like abdomen, which succeeds the thorax and is often only indicated by two caudal appendages. Posterior antennae are in- variably absent, while the antei-ior pair pei-sists, even in the adult, as small organs of attachment. The oral apparatus is situated on a ventral prominence of the cephalic region, and consists of an uppei- lip with palps, two mandibles and four maxillae, of which the two last unite to form a sort of under lip. On the thorax there are usually six pau\s of many- jointed biramous appendages, the elongated cu-riform rami of which are richly beset with haii-s and setae and serve to set up currents in the water in which the particles of food are brought to the animal. The stump -shaped abdomen bears an elongated cirrus, which is bent to- wards the ventral surface between the thoracic appendages, and con- stitutes the male copulatory organ. There are numerous and very pecu- liar variations in the shape of the whole body. Not only may the de- position of calcareous matter in the mantle be wanting, and the bira- mous thoracic appendages be reduced in number or even absent, but the mouth parts and the appendages may also be lost [Feltogastridce), and the body may be reduced to the form of an unsegmented tube, sac, or lobed disc. Nervous system and sense organs. — The Cirripedia possess a paired cerebral ganglion and a ventral chain of ganglia, of which there are usually five pau-s, but which are sometimes fused to a common ganglion mass {Balanidce). There is a double eye, which, although rudimentary, corresponds to the unpaired Nauplius eye, .An alimentary canal is absent only in the Rhizocephala. In the Fig. 349.— The organization of Lepas, after removal of the integument. Cd, Cement gland and duct; L, liver ; T, testis ; Vd, vas deferens ; Ov, ovary; Od, oviduct; Cf, thoracic appendages. Other letters as in fig. 348. CIRRIPEDIA. 441 Lepadidce and the Balanidce, the alimentaiy canal consists of a narrow oesophagus, a saccular dilated stomach provided with several cfecal (hepatic) diverticula, an elongated chyle-forming intestine, and a short rectum, which is only sometimes clearly marked off from the intestine (fig. 349). The RhizoGe2)hala (fig. 354, a), which are with- out an alimentary canal, possess root-like processes of the paren- chyma, which ramify in the viscera, especially the liver of Decapods, and absorb from them endosmotically the nutritive juices (as in Anelasma). Special glandular organs, the so-called cement glands (pecuKar to the Cirrijjedia), open on the sucker of the persistent (anterior) antennae : the animal is fixed by then- secretion, and the Rhizoceiohala alone seem to be en- tu-ely without them. A heart and vascular sys- tem, seem to be wanting in all cases. The tubes which are present on seve- ral thoracic ap- pendages o f many Lepadidce, are regarded as branchiae, as are also two plicated lamel- lae on the inte- rior of the mantle of the Fig. ZhQ.—Alciji-pe lampoD (after Ch. Darwin.) a, Male, very strongly magnified ; A', antenna? ; T, testis ; Vs, seminal vesicle ; D, redu- plicature of the skin ; 0, eye ; P, penis, b, Longitudinal section through female; F, maxiUiped; Qf, the three pairs of legs; Ov, ovary. Balanidce. Generative organs.— The Cirripedia are, with a few exceptions, hermaphrodite. The testes are branched glandular tubes, and lie at the sides of the ahmentary canal (fig. 349, T). The vasa deferentia which chlate into vesiculse seminales reach to the base of the cirri- form penis, in which they unite to form a common ductus ejacula- torius opening at the point of the penis (Vd). The ovaries in the Balanidce lie in the basal part of the body cavity (fig. 348, Ov) ; in the Lepadido} (fig. 349) they are moved into the prolongation of 'the head, which is known as the stalk. The oviducts, according to 442 CRUSTACEA.. Fig. 351 .-a. Larer Nauplius larva. A, anus ; 01, proboscis with moiith ; //, frontal liorns ; D, intestine ; A', A", Ist and 2nd antennte; Mdf, mandibular foot (third pair of appendages). b, Metanauplius larva of Balanus hefoie the moult. Beneath the skin are the rudiments of the lateral eyes (O) and all the appendages F' to P-' of the Cypris stage ; Ff, frontal hlament ; 0' unpaired eye ; Dr, gland cells of the anterior horns ; A , the antennro with suctorial di.sc ; Mx rudiment of maxilla. Krohn, open on a prominence on the basal joint of the anterior pair of thoracic appen- (Liges. The egg.s accumulate in the cavity between the mantle and the body in large thin - walled flat- tened .sacs, which, in the Lepaduhe, are attached to a fold of the mantle and are packed to- gether on the doi- sal surface of the animal. In spite of the hermaphrodit ism, theie are, accord- ing to Darwiii, in certain genera (Ibla, Scaljjellum) very simply orga- nised dwarfed males of peculiar form, the so-called comp lemental males, which are attached like para- sites to the body of the hermaphro- dite. There are also dioecious Cii'- ripedes with a strongly marked dimorphism of the sexes. This is the case mth Scaljjel- OIBEIPEDIA, 443 htm o')matuni -AXiii Ibla Cumingii; also with the remarkable genera Cryptophiahts and Alcippe (fig. 350). The males of these forms are not only small and dwarfed, but also, according to Darwin, have neither mouth, digestive canal, nor thoracic appendages. As a rule, two or sometimes more attach themselves to the body of the female. Development. — The eggs, while still within the brood-pouch, undergo an ii-regular segmentation. The clear cells arrange them- selves around the food yolk in the form of a blastoderm, the ventral side of which soon becomes ' considerably thickened in consequence of the appearance of the mesodermic layer. The larva? leave the egg as Nauplii (fig. 351, a, b), of oval or pear-shaped form, with unpaired frontal eye, lateral frontal horns, and three pairs of appendages, of which the anterior is simple, the two next biramous and, closely beset with swimming setse. Aftei- several moults, the larva, which has grown to a considerable size, enters on a new stage of de- velopment, the so-called Cypris stage (pupa) (fig. 352). The reduplica- ture of the skin has the form of a bivalve mussel-like shell, through the gaping ventral edges of which the appendages can be protruded. While the form of the shell recalls that of the Ostracoda, the structure of the body, so far as the segmenta- tion and form of the appendages are concerned, approximates to that of the Cojjepoda. The anterior ap- pendage of the Nauplius larva has given rise to a four-jointed antenna, the penultimate joint of Avhich' has become large and disc-shaped and contains the opening of the cement gland, while the terminal joint bears in addition to tactile setje one or two dehcate lancet-shaped olfactory hairs. The frontal horns are transformed into two conical prominences near the an- terior margin. Of the two pairs of biramous appendages, those which correspond to the second pair of • antennae are cast ofF, while Fig. 352. — Median section through a pupa of Lr'pas. A' Attaching antenna C, carina; 2'e, tergum; Sc, scutum Ol\ ovary ; G, cerebral ganglion &!/, ganghonic chain ; B, alimentary canal ; Cd, cement gland ; ML; oral cone ; Ab, abdomen ; P, rudiment of the penis ; M, muscle. 444 OBUSTACEA. the posterior pair becomes the rudiment of the anterior jaws (mandibles) of the oral cone, which is still closed and on which the first rudiments of the maxillae and under lip are already visible. The oral cone is followed by the thoracic region with six pairs of biramous Copepod-like swimming feet, and a minute three-jointed abdomen, which terminates in two caudal appendages and caudal sette. The pupa has a lai-ge paii- of compound eyes at the sides of the un- pau-ed eye-spot, and swims about by means of its swimming feet. It appears not to take in food. The material necessary for its further changes is stored up principally in the cephalic and dorsal regions in the form of a largely-developed fat body. After swimming about for a longer or shorter time, the pupa fixes itself by the suctorial disc of its bent antennae to some foreign body. The parts of the adult Ciii'ipede are now visible beneath the skin, and the cement gland begins to secrete a cement, which hardens and so brings abovit the permanent attachment of the young animal. In the Lejmdidce the region of the head above and be- tween the antennte grows so much that it projects from the pupal integument, beneath which the calcareous pieces of the shell of the Cirripede can be seen, and after the moulting of the chitinous skin of the piipa constitutes the fleshy peduncle by which the animal is attached, and into which the rudiments of the ova- ries project (fig. 353). The paired eyes of the free-swimming Cypris larva disappear, while the unpaired pigment spot remains. The mouth parts become fully differen- tiated, and the biramous swimming feet become short, many-jomted cirriform appendages. The Cirripedia are marine animals. They attach themselves to various foreign objects. They are found fixed, usually in groups, to logs of wood^ rocks, mussel shells, Crustacea, the skin of whales, etc. Some, as Lithotrya, Alcippe, and the Gryptopialidoi, are able to bore into Lammellibranch shells and Corals, while the RMzocephaU are parasitic on Crustacea. In the RUzoceplmla the bod}- is saccular. Pig. 353.— Young liepas after disappearance of the two horny valves of the shell and the straightening of the anterior part of the head (stalk) , which in the pupa stage is bent. CIEEIPEDIA.. 445 and the animal loses all its appendages and its alimentary canal, and extracts the juices of its host {pecapoda) by means of root -like processes (fig. 354). 1. Pedunculata. There is a peduncle and six pairs of biramous feet ; the mantle has usually carina, scuta, and terga. Fam. Lepadidae. Peduncle well marked, and not provided with calcareous plates. There is a membranous mantle, which, as a rule, is provided with five shell plates, of which the scuta and terga lie behind one another (fig. 348, ci). Lepm L. (Anatifa Brug.), L./ascictdaris Ellis, (ritrea Lam.) Found from the Northern Seas to the South Sea. L. anatifera L., cosmopolitan. Conchoclcrma KiG. 354.-a, SaectiUna picrptirea (aftjr Fr. Miiller). Or, Apertui-e of the mantle sac ; W, root-like processes ; X, genital aperture, b, Naupliiis larva of Sacadina. A', A", Mdf, appendages, c, Pupa of Lemceodiscns porcellarKE (after Fr. Miiller). F, The six pairs of legs ; Ab, abdomen ; A', attaching antennaj ; O, eye. Olf. {Otion, Cineraa Leach.), C. virr/ata Spengl., frequently attached to ships. a auHta L., Anelanma Darwin. The st^ilk is provided with root-like processes, which grow into the skin of Sqvalidce. A. f^qvalicola Lovdn, Fam. Pollicipedidae. Peduncle not sharply distinct, scaly or hairy. The shell plates very strong, numerous. The scuta and terga lie close to one another. There are sometimes complemental males. PoJUHpcs cornurojna Leach., Ocean and Mediten-anean. Scalprllnm vulgare Leach., North Sea and Mediterranean. Sc. ornatum Gray, South Africa. Ibla quadrivalvis Cuv., South Australia. ./. Cwmingii Darw,, Philippines. 446 OEUSTACiCA. 2. Operculata. The peduncle is absent or rudimentary. The body is surrounded by an external ring of plates at the extremity of which the scuta and terga form an operculum, wliich is usually freely movable and provided with depressor muscles (fig. 348, b). Fam. Balanidae. Scuta and terga freely movable and articulating with one another, 'i'he gills are formed each of a fold. Balanm tinthmahvhim L. Widely distributed and found in a fossil form. B. iiiiprov/.sii.s Darw. F;- traca and 3Mw-ostram. The palieozoic fossil genera II,ivu')wcar,.% Peltocans. etc., would have to be placed in such a group. ABTHROSTBAOA. 449 dages closely resemble the typical Phyllopod limb. As a rule, how- ever, some of the anterioi- thoracic legs take part in preparing the food and have a form intermediate between maxillae and thoracic legs. Such are called foot- jaws or maxillipeds. In the Arthrostraca the anterior pair of thoracic appendages only are so modified, and the segment bearing them joins the head ; the thorax is, therefore, in this group composed of seven segments, each with its pair of appen- dages. In other groups of Malacostraca the next or two next pairs of thoracic legs have the form of maxillipeds, so that there is no sharp division between the head and thorax. The latter is, at least partially, covered by a shield-like reduplicature of the skin, which morphologically corresponds to the Phyllopod shell and forms a more or less extensive carapace, which fuses with the back of the thorax, and under which the posterior, rarely all the thoracic seg- ments may remain separate as free rings. Order 1. — Arthrostraca.* Malacostraca loithlateral sessile eyes, usually with seven, more rarely with six or fewer separate thoracic segments, and the same number of pairs of legs. Withoict a reduplicature of the skin. The head bears four antennje, the two mandibles, four maxillse, and a pair of maxillipeds ; in all six pairs of appendages. A small bilobed plate, distinguished as the under-lip, behind the pair of mandibles, marks the boundary of the primary region of the head. The two pairs of maxillae as well as the maxillipeds are secondary cephalic appendages derived from the thoracic region of the body. Behind the head there are usually seven free thoracic rings with the same number of pairs of appendages, which are adapted for creeping or swimming. The number of distinct thoracic segments is in rare cases reduced to six (Tanais) or five (Anceus), the anterior or the two anterior segments of the thorax becoming intimately con- nected with the head. In the latter case a more or less extensive cephalothor-acic carapace is formed. The abdomen which follows the thorax includes, as a rule, six segments bearing limbs, and a simple or split plate without appendages and representing the terminal segment. The number of the abdominal segments and appendages may, however, be reduced {Isopoda), and the entire abdomen may * Besides the works of Latreille, M. Edwards nnnn ot,^ Spence Bate and .J. 0. Westwood, "A iSow of the Britif ''■T^''''^ Crustacea/' Tom. I. and II., London, ISOS-I^GS 0^0 " Lr naturelle des Crustacos d'eau douce de Noryeo-e," Christiania, 1^7. ""^ 29 450 CEUSTACEA. even be reduced to an unsegmented stump-shaped appendage (^Lcemocli2)oda). The nervous system consists of a cerebral ganglion and a ventral gan- glionic chain, which is most distinctly composed of two lateral halves. In the Isojyocta there is also an unpaired visceral nerve. The two eyes are always sessile, compound eyes, with smooth or facetted cornea; they are never stalked. Delicate olfactory fibres are often present on the anterior antennae, and are especially numerous in the male sex. The alimentary canal begins with a short oesophagus, which passes upwards to open into a wide crop, supported by firm horny bands and often armed -with strong chitinous plates. The crop leads into a long intestine pro\Hded with two or three pairs of tubular liepatic glands. The rectum, which may possess one or two tubular appen- dages (probably urinary), opens at the posterior end of the body. The antennal gland opens on the basal segment of the posterior antenna, often vipon a conical protuberance. Vascular system. — A heart is always present as the central organ of the circulation. It may either have the form of a tube extending along the whole length of the thorax {Am2)M2)oda) ; or it may be saccular and placed in the abdomen (Isopoda). In the first case the gills are placed on the thoracic feet as tubular appendages : in the latter, on the other hand, they are placed on the abdomen. From the heart the blood passes through an anterior and posterior aorta, and usually through lateral arteries. The vessels conduct the blood into the body cavity, whence it returns in regular streams to the lateral paired slits of the heart. Generative organs. — The Arthrostraca are of separate sexes. The males are frequently distinguished from the females by the modifica- tion of certain parts of the appendages to form prehensile organs, by a greater development of olfactory hairs on the anterior antenna?, and by the position of the sexual and copulatory organs. It is rare to find a strongly marked dimorphism of the sexes {Bopyr^s, Praniza). The generative organs open either at the posterior part of the thorax or at the base of the abdomen ; the female always on the ante- penultimate pah', the male on the last pair of the thoracic appen- daoes or between the fii-st of the abdomen {Isopoda). The ovaries are two simple or branched tubes with the same number of oviducts. The testes similarly seem to be composed of one {Amphipoda) or more (3) pairs of tubes (Isopoda), the efferent ducts of which (^asa oda The two pairs of antennfe usually consist of a short strong shaft * Besides the older works of De Geer, Rosel M Fdwirrl^ «fr. Spence Bate "0. the Morphology of some AmKoda oT Kvis on EK"?' ina ' Ann. of ^ at. Hist.. Ser. 2. vol. xix.. 1857. C. SDence filf p - n mdification of Crustacea," Ann. of Nat. Ilht Ser 3 vol ? r- \ ' ^'l ^^^^ " Catalogue of the specimens of Amphipodous Crustace'i in VL .^ii^r'" ?'''t^' British Museum," Loudon. 1862. E. ?an BeiSeTet Z ^-^^^^ sur la formation du Blastoderme chez les aZwoIs ' /^^'^"""e C. Claus. Der Organismus d<3r PhronimMeu ^2?^ ?' Bruxelles. 1868. 452 OnUSTACBA. and a long niultiarticulate fiagellum, which, however, may be more or less riidinientary. Tlie anterior antennte, which are always longer in the male, often bear a short accessory liagellum and present numerous modifications in their special form. In the llyperina they are very shoii in the female ; while in the male they are of consider- able length and are closely beset with olfactory liairs. The posterior anterinfe are frecpiently longer than the anterior : in the male Typhidm they are folded in a zigzag fashion, and in the Corophiidce G. UT.-Phronima sedenfaria, a, female; b, male. O, eyes; A; A", the two pau'S of an tennffi ; Kf, jaws ; D, intestine ; I£, heart and aorta; X, gills ; Ov ovary; N, nervous system ; Dr, glands in the chela of the fifth paii- of legs ; G, gemtal openmg. are modified to form strong pediform appendages. In the female, on the contrary, they may be degenerated and represented only by the basal joint [phronima) (fig. 357, a and h). The mandibles are powerful biting plates with a sharp, usually toothed edge and a lower masticating process. They usually possess a three-jointed palp, which is occasionally reduced. The anterior bi- AMPHIPODA. 453 lobed maxillse also have as a rule a short, two-joiiited palp, while the maxilliB of the second pair are reduced to two lamellae of considerable size attached to a common base. The maxillipeds fuse to form a sort of underlip, which is either tri-lobed {Hyperina) or bears upon a com- mon basal portion an internal and external pair of lamellae, of which the latter may be considered as the basal joint of a lai-ge multiai-- ticulate and frequently pediform palp [Crevettina and Lcemodipoda). Delicate lamellae or tubes, which are attached to the coxal joints of the thoracic legs, function as gills; the active movements of the abdominal swimming feet cause a constant renewal of the water around them. In the female there are in addition to the sills lamellar plates (postegites), which are applied together under the thorax to form a brood-pouch. The males are distinguished from the females not only by the absence of the oostegites, but chiefly by the stronger development of the prehensile hooks on the anterior thoracic feet and the different formation of the antennae. The eggs pass into the brood-pouch and there develop. The yolk sometimes {G. locusta and other marine species) undergoes a com- plete segmentation. Sometimes {G. index), after a superficial seg mentation, a peripheral ceU layer is separated, which develops into a delicate blastoderm beneath the egg membrane. A ventral primitive streak is then formed, and on the dorsal side, beneath a differentiation which has been erroneously taken for a micropyle, a peculiar globular organ makes its appearance : this is the first rudi- ment of the cerxdcal gland {dorsal organ), which is confined to em- bryonic life. The appendages are developed from before backwards on the ventrally flexed body of the embryo. The young animals usually possess at hatching all their appendages and in all essential pomts have the structure of the adult animal, but- the number of joints of the antennae and the special form of the legs stHl present differences. In the Hyperina alone the just hatched young may be Avithout abdominal feet and differ so much in their form ^from' the adult that they may be said to undergo a metamorphosis. The Amphipoda for the most part live in fresh and salt water and lead an independent life (the presence of Arctic species in the Swedish and Norwegian seas is very interesting). Some, however live m tubes {Cerapus), others in holes gnawed in wood (Chelura) The large size of the deep-sea forms is of special interest ; amongst these ^ Gc^nmarid, allied to the genus Iphimedia, and Cystosoma Neptum (Hyperidm) become several inches in length. The Hyperina 454 CRUSTACEA. live principally in transparent marine animals, especially in Medv^ce, and may, as tlie female Phronima sedentaria, take up their abode with their entire brood in transparent Pijrosoma, whose internal parts they eat up. The Gyamidm among the LcBuiodijioda are parasitic on the skin of whales. Tribe 1. — Laemodipoda. Amphipoda xoith cervically placed anterior legs and rv/limentary ajjodal abdomen. The anterior thoracic segment is moi-e or less closely fused with the head and the anterior pair of legs shifted on to the neck. The maxillipeds are modified to form a quadripartite under-lip with long palps. The branchite are usually confined to the third and fourth thoracic segments, the legs of which are often rudimentary or are altogether wanting. The feet end with hooks for attachment. The abdomen is small and reduced to a shoit protuberance destitute of appendages. Caj/rdla linearis L. Body elongated and thin. They aie parasitic on Hydroids and colonies of Bryozoa. Cijamus ceti L. Body broad and fiat ; abdomen quite rudimentaiy ; parasitic on the skin of Cetacea. Tribe 2. — Crevettina. Amphipoda with small head, small eyes, and multiai'ticiolate jyediform maxillipeds. Both pairs of antennse are long and multiarticulate ; in the male they are larger than in the female. The upper or anterior antennae are usually, as in Gammarus, the longer ; their shaft is composed of several joints and bears a small accessory flagellum as well as the principal one. The contrary may, however, occur, as in Corox>hium, where the posterior antennae are elongated and pediform. The maxillipeds in all cases fuse together at their base and form a large under-lip, usually with four lamellae and two jointed pediform palps. The coxal joints of the thoracic legs have the form of broad and large epimeral plates. The abdomen has always the full number of segments. The three posterior pairs of abdominal feet {uropoda) are well developed and often much elongated. This group, which includes an astonishing variety of forms, is principally distributed in the colder seas. Fam Corophiidse. The body is not laterally compressed, liie posterior antcnniX! are more or less i^ediforra. The coxal joints of the legs are frequently very small. They move rather l )y walking. CoropMuvi long home Fabr.. dig AMPHIPODA. 455 passages in mud. Cerapm tnbularis Say., lives in tubes. Pvdocerus variegatus Leach., English coast. Chditra terebrans Phil, is allied here, gnaws, with Limnorin Ihfnnrnm, wood-work in the sea. North Sea and Mediterranean. Fam. Orciiestiidse. Anterior antennae usually short, always without accessory ramus. The posterior pair of uropoda are unbranched and are shorter than the preceding pairs. They live on the shore, especially on sandy beaches, and move by springing. Talitrus xaltator Mont. = T. lomsta Latr. On the sandy coasts of 15urope. Orcliestia littoren Mont., North Sea. Fam. Gammaridse. The anterior antenna often have a second ramus, which is always longer than the shaft of the posterior. The cosal plates of the four anterior pairs of legs are very broad. They move more by sh imming than by springing. Gammarus jndex L., G. flm-iatilu Eos., G. marinnx Leach. In the blind yvpliargus Schiodte the crystalline cones and eye pigment are wanting. N. jntteanns Koch., in deep springs aiid lakes (Lake of Geneva). Lysianassa Costce Edw., Mediterranean. L. atlantica Edw. L. magellanica Lillj, Tribe 3. — Hyperina. Amjjhipoda with large swollen head and large eyes, usually divided into frontal and lateral eyes. They have a pair of rudimentary maxiUip)eds functioning as underli2J. The antennfe are sometimes short and rudimentary, sometimes of considerable size, and in the male are elongated into a multiarticnlate flagellum {Hyperidoi), The posterior antennte may in the female be reduced to the basal joint enclosing the glandular tube {Phromina^ ; in the male, on the contrary, they are folded in a zigzag, after the manner of a carpenter's rule {Platyscelince). A paired auditory vesicle may be present above the brain [Oxycephalus, Rhabdosoma). The maxillipeds form a small bi- or tri-lobed under-lip. The paired legs end in some cases in a powerful chela. The caudal styles are sometimes lamellar and fin-like, sometimes styliform. Development takes place by metamorphosis. They live principally in jelly-fish, and s\vim very rapidly. Fam. Hyperidse. Head globular, almost entirely occupied by the eyes. The two pairs of antennae have a multiarticnlate shaft ; the flagellum longer in the male. The mandible has a three-jointed palp. The fifth pair of feet is gener- ally formed like the sixth and seventh, with claw-like terminal joint. Hyperia [Lestrujonus Edw.) viedusarnin 0. Fr. Miill. (77". galha Mont. = 77". Latreilli Edw.) with Zentrif/om/s exnlans Kr. as male. North Seas. Fam. Phronimidae. Head large, with projecting rostrum and large divided eye. The anterior antcnnfe arc short in the female, with only two or three joints, in the male with long multiarticnlate flagellum and a shaft closely beset with olfactoiy hairs. The thoracic limbs have in some cases powerful chelae. Phrosina niceeensis Edw., Phronima sedentaria Forsk. The female lives with its offspring in Pyromma and Bipliyidee, Mediterranean. Fam. Platyscelidae. Both pairs of antennae hidden beneath the head ; the anterior arc small ; in the male with much swollen bushy shaft, and short, 456 CKUSTACJiA, slender Hagelluiii composed of few joints. The posterior antennae are in tlie male very long and folded three to four times together in a zigzag fashion ; in the female they are short and sti'aight, sometimes quite reduced. The basal joints of the fifth and sixth paii's of legs are usually enlarged into great lamellaa, which cover the thorax. The seventh pair is generally rudimentary. Euty2>his (Ti/jj/iis Kisso) ovoulitx llisso (Plati/sctdus xerratug Hp. Bate), Mediter- ranean. Oxj/ceji/mlux jHscatar Edw., Indian Ocean. 2. Sub-order: — Isopoda."'' Ai'throstraca with usually broad, more or less arched body, with seven free tho- racic rings, with lamellar legs function- ing as branchice on the short-ringed, often redvAied abdomen. The structure of the body, which is flat in shape and covered by a hard, usually encrusted integviment, presents a great agreement with that of the Amphipoda, to which the in many respects peculiar Tanaidcn are most nearly allied. The abdomen of the Isopods is, however, usually much short- ened and composed of six short seg- ments, which are often fused with one another ; it terminates with a large caudal lamella. The abdominal legs are only exceptionally (Tanaidai) swimming feet ; as a rule they have the form of branchial lamellae. The sixth pair may be fin-like or styliform. The anterior antennse are, with a few exceptions, shorter than the posterior and external antennte ; in rare cases {Oniscido}) they become so much reduced that they are hidden beneath the cephalic carapace. In exceptional cases only [Ajyseudes) * H Rathke, " Untersuchungen iiber die Bildung und Eutwickeluiig der Wasserassel," Leipzig, 1832. Lereboullet, "Sur les Crustac6s de la famille des Cloportides, etc," Mem. dv. 3hisenm (Vhixt. nat. de Strmhovrg, Tom. n ., 1850 N. Wagner. " Recherches sur le systeme circulatoire et les oi'ganes de la respiration chcz le Porcellion elargi," Ann. drs .ic. nat., Ser. 5, Tom. IV., ]8fio. A Dohrn, " Die Emliryonalentwickclung des Asellus aquaticus, Zcitxchr jnr w/W Zool., Tom. XVIL, 1867. N. Bobretzky, '^Zur Embryologie des Oniscus murarius," /^cittchr. fiir ivisx. Zool., Tom. XXIV., 1874. Pig. 858. — Asellus aqnaticiis (after Gr. O. Sars). Female with brood pouch, seen fi'om the ventral side. ISOPODA. 457 they bear two flagella. As in the Amphipoda, pale, plumous setse and olfactory cones are present on the antennse. Thg mouth parts are in some parasitic Isopoda modified for piercing and sucking. The mandibles (except in Bopyridce, and Oniscidce) often bear a three- jointed palp. On the other hand, the two paii-s of maxillae, Avhich are usually bi- or tri-lobed, are in general without the palpiform appendage. The maxillipeds form a sort of underlip, but present great differences in the arrangement of their parts (fig. 358). As a rule the seven pairs of thoracic legs are adapted for walking or attachment, and in the female some of them are provided -with delicate membranous plates (oostegites) which form a brood pouch. They never bear gills. The branchial function is dis- charged by the delicate inter- nal rami or endopodites of the abdominal limbs (pleo- pods), the anterior pan- of which is frequently modified to form a lai-ge operculum overlying the following pairs. In certain of the terrestrial Isopods [Forcellio and Arma- dillo) the opercular plates of the two anterior paii's of abdominal limbs contain a system of air spaces which ap- pear to assist respiration. The heart, unlike that in Amphi- pods, lies (except in Taoididce) in the posterior thoracic seg- ments or in the abdomen. The sexes are (except in Cymothoidce) separate, and the position and arrangement of the generative organs correspond in general with those of the Art^Mjmla. The sexes are distinguished by external sexual characters, which in some cases {Bopyridoi) may lead to a strongly-marked dimorphism (fig. 359, a, I). In the male three tubular testes unite on either side to form a dilated seminal vesicle, from which the vasa deferentia are given off. The latter are frequently separate along their whole length and, at the end of the last thoracic segment, each of them enters a cylindrical appendage Fig. 359.— hrancliialU (after Comalia and Pauceri). a. Female seen from the ventral side; Brl, oostegite ; branchlse. h. Abdomen of the same strongly magnified, with atlhering male. 458 CnUSTACEA. CO {Asellus) or they unite together into a common median penis wliicli lies at tlie base of the abdomen {Oniscidai). A pair of styliform or complicated, hook-bearing appendages of the anterior abdominal feet are to be looked upon as accessory copulatory organs ; in addition to these a pair of outwardly turned chitinous rods on the inner side of the second pair of feet may also be present {Oniscidui). The Cymothoiclce are hermaphrodite* (Bullar), but the sexual organs become ripe at different times. In the young stage these animals function as males, and possess three pairs of testes, two rudimentary ovaries internal to the testes, and a paired copulatory organ into which the two vasa deferentia open (fig. 360). After a subse- quent ecdysis and after the fe- male glands have developed at the expense of the gradually diminishing male glands, the oostegites, which in the meantime have been developed, become free on the thoi-acic legs and the copu- latory organs are thro-mi off. Hencefoiward the animal func- tions only as a female. The embryonic development begins after the entry of the eggs into the brood pouch and is in- ti'oduced by a centro-lecithal seg- mentation, the central part of the egg (food yolk) remaining at first unsegmented. The blasto- derm soon consists of a periphe- ral layer of naked nucleated cells and produces by a rapid growth of its constituent cells the ventrally placed germinal bands, at the anterior end of which the cephalic lobes are first marked off. The rudiments of the trifoliate appendages (dorsal organ) of the Isopod embryos are next formed as two prominences on the cephalic lobes. The physiological and morphological meaning of these structures has not yet been explained. Of the appendages the two pairs of antennaj * J Bullar, " The generative organs of the Parasitic IsopoHa," Jovrn. AnuL Fh iisioL. 1876. P. Mayer, " Ucber den Hcrmaphroditismus einiger Isopotlcn, Miitheii atts der Zool. Stat. NeapcL 1879. Fig. 3G(\ — Female of Cymothoa Baiik^i (after M. Edwards). Brl, oostegite. 6, Sexual organs from a Ct/muthoa resti-ides, 13 mm. in length (after P. Mayer). T, The three testes ; Oo, ovary ; Od, oviduct ; Vd, vas deferens ; P, penis. ISOPODA. 459 are the first formed. After these have made their appearance, a new cuticle, the larval skin corresponding to the Nanplius stage, is formed (as also is the case in Ligia according to Fr. Miiller). While the other appendages are successively developed, the caudal region of the embryo becomes bent towards the dorsal surface. Of the embryonic membranes the chorion is the first to disappear, then the cuticle of the blastoderm, and finally, when the embryo is fully developed, the Nauplius skin. The young animals, when they become free in the brood-chamber (fig. 361), are still without the last pair of thoracic legs; in the Tandidce the abdominal feet are also wanting. They undei-go not inconsidei-able changes in the form of the appendages until the attainment of sexual maturity. The Isopoda may therefore be said to undergo a metamorphosis which is most complete in Ta^ nais, Praniza [Anceus) and the £o2}i/rid(e. The Isopoda live some in the sea, some in fresh waters, and some on land {Oniscidce). They nourish themselves . on animal matters ; many of them are paiu- sitic (seldom complete endopara- sites, EntonisGus) piincipally on the skin and in the buccal and branchial' cavities of fishes {Cy- mothoidce) or in the branchial cavity of prawns {Bopy7-idce). Tribe 1. — Anisopoda.* Body more or less resembling that of a7i Am2)hi2}od. The abdomen vnth biramous sivimming feet (Tanais), lohich do notficnction as gills, or with fin-like feet (Anceus). Fam. Tanaidae. Tamifi (Uhhix Kv., Brazil. Two kinds of males, "smellers and claspers." T. fp-aciUit Kr., Spitzbergen. Fam. Pranizidse, Anceidre. Ancenx m axillaris Mont. (P?-. cceruleata Desm.), North and West coasts of Europe. * Compare Spence Bate, "On Praniza and Anceiis, etc," A7in. of Xat. Hixt , 8er. 3 Vol. IT 1858. Hes.se, " Memoire sur les Pranizes et les Ancees " A7m. (1. li(n<-n 2^at., Ser. IV., Tom IX., 1864. Fr. Miiller, " Ueber den Bau der Scheerenasseln, ArcMv. fiiv Katimicu-li, Tom XXX., 1864. A. Dohrn, ILiitwickclung und Organisation von Praniza maxillaris sowie zur Kenntniss des Baues von Paranthura costana " Zcitwhr. fur Zool, Tom XX 187 0 Fig. 361. — Larva of Bopyrtis virbii, with six pairs of thoracic leR-s (after R. Walz)- Ul, Under lip ; Ahs, first abdominal seg- ment ; A', A", two pairs of antennae ; Mdb. mandible. 4G0 OEUSTACEA. Tribe 2, — Euispoda. Body with seven free thoracic segments cmd as many pairs of aj^eiidages. Abdomen relatively short ami broad, with abdominal feet modified to form branchial lamellce. ¥iim. Cymothoidse, Witli l)itins and sucking mouth parts, broad abdomen with short segments and shield-hke caudal plate. The last maxillipeds in the form of an operculum. They live partly as parasites on fish, and partly as free-living animals. C'pnotkoa cestrnvt Leach., C. wxtroideK Risso, Mediter- ranean. Anilocra mcdltcrrama Leach., ^ga hicarinata Leach., Serolit parado.ra Fabr. Fam. Sphaeromidae. Free-living Impoda with broad head and short, very convex body, which can often be rolled up in a ball towards the ventral side. Sphceroma fosmmm Mont., in the Pontine marshes ; nearly allied is the S. granulatiini of the Mediterranean. S.serratum Fabr., Ocean and Mediterranean. It also lives in brackish water. Fam. Idoteidae. Free-living Isojjoda with elongated body, biting mouth parts, and a long caudal shield formed of several segments fused together. The last pair of abdominal feet is modified to form a wing-shaped operculum for the protection of the preceding branchial feet. Idotea entomon L., Baltic. Fam. Asellidse. Body flattened ; the last pair of abdominal feet (pleopods) are styliform (not shaped like an operculum). Jcera alhifrom Mont.. British seas. Aselht!< arpmticnfi L., fresh water form. A. cavatieiis Schiodte, in deep springs. Limnoria tereiratu Leach. L. Iiff/i07"uiii, gnaws wood-work in the sea. Fam. Bopyridae. Parasitic in the bi-anchial chamber of prawns ; the body of the female is disc-shaped, unsymmetrical, and without eyes. The males are very small and elongated, with distinctly separated segments and eyes. Boj^jrus squillarum Batr., on Paloemon squilla. Here are allied the Entoniscidoe, which are parasitic in the body cavitj^ of other Crustacea (^Cirj'ijwdia, Pagiirkles, and Crabs), Cri/jjioniscux jtlanarioides Fr. Miill., parasitic on Saonulina pur^mvea of a Pagurus, Brazil. Cr. jv/r/iiKsun Rathke, parasitic on Peltogastcr. Entoniscun Porccllance Fr. Miill., lives between the heart and the intestine of a species of Porccllana in Brazil. Fam. Oniscidae. Land Isopods. Only the internal lamella (endopodites) of the iibdominal feet are modified to form delicate branchise, the exopodites constituting firm opercula. The two anterior abdominal feet are sometimes provided with air chambers. The mandibles are without palps. They live mostly in damp places on land. Ligiii occanica L.,on stones and rocks on the sea coast. Onisms murariu-t Guv., PorceUio scahcr Leach., Armadillo vulgaris Latr., A. officinarvm Brdt. Order 2. — Thoracostraca.* Malacostraca with compound eyes tohich are usually 2)l(i'Ced on movable stalks, with a dorsal shield which connects all or at least the anterior thoracic segments with the head. * Besides the larger works of Herbst, M. Edwards, Dana, and the essays of Duvernoy, Audouin and M. Edwards, Joly, Couch, etc. compare Leach, THOEACOSTEACA. 461 The Thoracostraca, like the Ai-throsti-aca, possess a cephalo-thorax composed of thirteen segments and an abdomen composed of six segments, as well as a caudal plate (telson) ; but the body is stouter and adapted to a more perfect locomotion and a higher grade of life. The thorax, instead of being composed of seven distinctly separate segments, is covered by a dorsal carapace which eifects a fii-m and intimate fusion between the head and thorax. The degrees of development of this dorsal carapace are various. When most highly developed, it forms the dorsal integument of the anterior or of almost all the thoracic segments ; and its lateral portions only, which have the form of wings and are bent towards the ventral surface, consist of a free reduplicature. The application of the appendag^es differs from that in the Arthrostraca, and, indeed, varies in the different groups of the Thoracostraca. The cephalothorax has thirteen pairs, and the abdomen seven. The facetted eyes are born on two movably separated stalks. These were for a long time considered as the anterior pair of appendages, while in fact they are merely lateral portions of the head which have become jointed. Both pairs of antennje belong to the anterior region of the head. TJie anterior antennfe or antennules as a rule bear on a common shaft two or three Jlagella—B,s the peripheral multiarticulate filaments are called— and are pre-eminently sense organs. In the Deccqjoda the auditory vesicles are placed in the basal joint, and on one of the flagella there are delicate hairs and fibres, which are in connection with nerves and are to be looked on as olfactory organs. The second antennaj are attached externally to and somewhat beneath the antennules. They bear a long flagellum and in the macrurous Decapoda are often provided with a more or less considerable scale. A gland (the green or antennal gland) usually opens on a conical process of their basal joint. The following three pairs of appendages function as jaws; the powerful mandibles, which are furnished with palps, lie at the' side of the upper lip; further backwards are the two paii-s of lobed maxillse, in front of which and behind the mouth is the small bilobed underlip. The following eight pairs of appendages present a very Decapoden," Leipzig, 1863. ' ^ehororgaii der 462 CRUSTACEA. different form and Adaptation in tlie various groups. As a rule, the anterior pairs are modified to assist in tfiking up food and are moved nearer the mouth ; these are the maxillipeds, which, with regard to their structure, hold an intermediate position between jaws and feet. In the JJecapoda (fig. 362) three pairs of appendages have the form Fig 362 -Male aurl female of Adaeus fltmatilh seen from the ventral side. In the male the ambulatory and abdominal feet of the left side have been removed ; ni the female the am- bulatory feet of the rit?ht side and the maxillipeds of both sides. A antennales; A antenn^ ; P/, scale of antenna ; Md, mandible ^^■ith palp ; MA M.'\ first and second maxi lse Mxr to 2Ixf \ the three pairs of maxUlipeds ; Goc, geuital openm- ; Doe, openmg of the green gland ; F', r', first and second abdominal foot ; On, eggs ; A, anus. of maxillipeds, so that there are only five pairs of legs left on the thorax. In the Stomatopoda the first five pairs of thoracic append- ages are modified to form maxillipeds and there are only three pairs THOEACOSTBACA, 463 of bil-amous swimming feet, which arise from the three posterior free segments of the thorax. The thoracic legs are either, at least in part, biramous (A\dth swimming ramus), or as in the Decapods the exopodite is absent and the legs have the form of ambulatory appendages. They then terminate with simple claws ; the anterioi- frequently with large - chelfe. The terminal joints may however be broad plates, in which case they can be used as SAvimming feet. The biramous legs of the sixth abdominal segment are, as a rule, broad and fin-like and form, together with the last abdominal segment which is transformed into a large plate (telson), the caudal fin. The feet of the five anterior abdominal segments, on the other hand, are sometimes swnim-m'ng feet {Stomato2)oda), sometimes serve to carry the eggs, or the anterioi- may assist in copulation (in the male). They may however be more or less rudimentary and some of them absent. With rare excep- tions [Myaidce) all the Thoracostraca possess gills, which are either tufted or composed of regular lancet-shaped leaves. The gills are appen- dages of the limbs: in the 8tomatoiyoda they are attached to the abdominal feet, in the ScJiizopoda and Decajmda to the maxillipeds and ambulatory feet. The Cuviacea are without gills, except for a single pair on the second pair of maxil- lipeds. In the Decapods they are contained in a special branchial chamber beneath lateral expansions of the carapace (branchiosteoite^ (fig. 363). / The organs of circulation also attain a high degree of development, the highest not only among the Crustacea, but in general amongst all Arthropods. A heart and vessels are always present. In the Stomatopoda the heart has the form of an elongated tube, which extends through the thorax and abdomen, possesses numerous paired sUts, and m addition to an anterior and a posterior aorta gives off to the right and left several branching arterial trunks. In the Gumacea, Schizopoda and Becapoda the heart has a saccular form and lies in the posterior region of the cephalo-thorax. More rarelv Fig. 363.— C'ephalothornx f)f AdocKsfliivhifilU, after removal of the branchiostegite (after.Huxley). K, Gills ; H, ros trum ; O, stalked eye ; Mp, scaphognathite (of the second maxilla) ; Mxf", third maxilliped. 464 CRUSTACEA. as in the youngest larvte of the Deca2Joda, only one pair of slits is present and the arterial system has but few branches. In the fully- developed Decapoda the number of paired slits is increased by the addition of a doi-sal and a ventral pair, and the vascular system is considerably perfected. An anterior cephalic aorta supplies the bi-ain, the antennae and eyes. Two lateral pairs of arteries send branches to the stomach, liver and generative organs. The posterior abdominal aorta usually divides into a dorsal and a ventral artery, of which the first supplies the muscles of the tail, the latter (known as sternal artery) sends branches to the appendages of the thorax and abdomen (fig. 364). From the ramifications (often capillary-like) the blood flows into larger or smaller canals with connective tissue walls which may be regarded as veins, and from thence into a wide blood space situated at the base of the gills. It thence passes through B Aa U \ T r I 0 F" F' ¥^a. 364.— Longituclinal section through Astacus fluviatUis (after Huxley). C, Heart; Ac, cephalic aorta; Aa, abdominal aorta, the sternal artery (Sta) is given off close to its origin; Km, masticatory stomach; X>, intestine; L, liver; T, testis; Yd, vas deferens; Go, genital opening ; G, brain ; N, gangUonic cord ; Sf, lateral plate of the caudal fin. the gills and, having become arterial, passes into other vascular tracts (branchial veins containing arterial blood), which conduct it to a receptacle surrounding the heart, the pericardial sinus : from the latter the blood enters' the heart through the slits which ai-e pro\dded with valves. The alimentary canal consists of a short ciesophagus, a wide saccular crop and an elongated intestine which opens by the anus beneath the median plate (telson) of the caudal fin. The wide crop or masticatory stomach is supported by a firm chitinous framework, to which are afiixed several pairs of masticatory plates (derived from thickenings of the chitinous lining). In the Decapoda two round concretions of carbonate of lime (Cray-fish) may be deposited in the walls of the masticatory stomach beneath the chitinous lining ; these are the so-called " ejjes" and are found in the spring and summer. THOBACOSTBAOA. 465 The ducts of the very numerous, multilobed hepatic cjeca open into the anterior part of the elongated intestine. A simple or looped glandular tube (the green gland) opens on the basal joint of the posterior antenna. A shell gland is not developed. The nervous system is distinguished by the size of the brain, which is placed far forwai'ds find gives off nerves to the eyes and antennee. The ventral cord, which is connected with the supra- cesophageal ganglion (brain) by very long commissures, presents very different degrees of concentration. In the brachyurous Decapods this concentration reaches its highest point, all the gangKa being fused together to form one great thoracic ganglionic mass. The system of visceral nerves is also very highly developed. Sense organs.— The eyes are large and facetted. Except in the Fig 365. -Generative organs of .Maces, a, Female ; h, male. Oc. ovaries ; Od, oviduct; J' a, vulva on the basal joint of the third pan- of ambulatory legs {F'") ■ T testis • Vd Tegs'JJo^''^' "^^''^^^ openings on the basal joint of the fifth pair'of amloulatory Cumacea, in which the eyes are sessHe, they are borne on movable stalks, which morphologically are to be regarded as the lateral parts of the anterior region of the head which have been segmented off. In the larva a median simple eye, equivalent to the unpaired Ento- mostracan eye, may appear between the stalked facetted eyes. In exceptional cases the adult animal may have paii-ed eyes at the sides of the thoracic appendages, and unpaired eyes between the abdominal feet {Buphausia). Auditory organs are wanting in the Cumacea and StomMopoda. In the Decapoda they are present as vesicles contanimg otoliths in the basal joint of the anterior antenna, and in mfxny Schizopoda in the lame]]a>, of the ca,udal fin. The delicate 30 466 UnUSTACEA.. tiliuueiiis and hairs on the surface of the anterior antennae have the vahie of olfactory organs; the antennae function as tactile organs, as do also the palps of the jaws, the maxillipeds and the legs. The generative organs are paired and lie in the thorax or in the abdomen [>StotiuUoj)oda), and, as a rule, are connected across the middle line by a median portion. The female organs consist of two ovaries and two oviducts, which open on the basal joint of the antepen- ultimate pair of ambulatory legs or on the sternal region between these appendages (fig, 365, a). The testes (fig, 365, b) are composed of numeroxis sacs and blind tubes, and, like the ovaries, are connected by a median portion ; there are two vasa deferentia, often much coiled, which open on the basal joint of the last pair of ambulatoiy legs, more rarely on the sternum, and occasionally on a special copulatory organ {Schi- zojwda). The fii-st, or the first and second, pair of abdominal feet act as intromittent or- gans. The eggs either pass into a brood-pouch formed by lamellar ap- pendages of the thoracic legs (Cuonacea, Schizo- 2)oda), or become at- tached by means of the cementing secretion of special glands to the Iv.iixy abdominal feet of the female, where they remain until they are hatched {Becajwda). Development. -Most of the Thoracostraca undergo a metamor- phosis which may be more or less complicated. The Cwmicea, some Schizopoda {Mysidea) and the fresh-water Decapoda (Astacus) leave the egg membranes with the full number of segments and appen- dages. All the Stomatopoda, on the contrary, as well as most of the Decapoda, are hatched as larvae ; the latter in the so-called Zomi form with only seven pairs of appendages in the anterior region of the body (there are two pairs of antennae, mancUbles, two paii*s of maxillfe," and two pairs of maxillipeds), mthout the last six thoracic segments and with a long abdomen destitute of appendages (fig, 366). The two pairs of antennfe of the Zocea are short and destitute of flagella. The mandibles are without a palp ; the maxillje are already Fig. 366.— Crab zosea {Thin), after the first moult. ZS, Zosea spine on the back ; Kf, Kf, the two pairs of biramous appendages corresponding to the fli-st and second pau-s of maxUlipeds. THOEAOOSTEACA. 467 lobed and used as jaws : the four anterior- maxillipeds are biramous and act as biramous swimming feet ; and behind them, in the macru- rous Decapods, the maxilliped of the third pair also appears as a biramous swimming foot. Gills are as yet wanting, being repre- terior maxUte; Gl, sixth ard serntWatVn/^ maxilUpeda. c.Zoaa stage; O.eyes appendages or first and second •sented by the thin surfaces of the sides of fl.. u , 1 ip- ^ snort heart with one or two pairs 468 CaUSTACEA. of slits is present. The facetted eyes are of considerable size, but are not stalked. Between the facetted eyes there is in afldition an unpaired simple eye, the Entomostracan eye. The Zoma larvae of the short-tailed Decapoda (Crabs) are, as a rule, armed with spinous processes. They visually have one frontal spine, a long, curved dorsal spine, and two lateral spinous processes of the cephalo-thoracic shield. The Zosea, however, is not by any means always the earliest larval stage. Passing over those cases in which the larva has the Zo8e.'i form but is without the middle maxillipeds, there are Podo'pUthal- mata (Penceus), which leave the egg as Nauplii (fi§. 367). Thus ¥m. 3m.- Zoma of Ina^hus in advanced stage with rudiments of the thu-d masilUped (Xf ) and the five pairs of ambulatory feet {oBp) ; C, heart; Z, liver, b, Megalopa stage of Portimus; Ab, abdomen. F' to F- first to fifth ambulatory legs. the developmental history proves that the series of forms of Ento- mostraca and Malacostraca are continuous. During the growth of the Zotea, the subsequent metamorphosis of which is quite gradual and always different, the six (five) pairs of thoracic legs, which are as yet absent, sprout out beneath the cephalo-thoracic shield. The abdominal feet also make then- appear- ance on the abdomen, and the larvae finally enter the Schizopod-like stage, from which the adult form proceeds. The Crab Zoa^a, how- ever, after a later ecdysis, enters upon a new larval stage, that of the Megcdopa (fig. 368, b) ; in this stage it already presents the cha- racters of the Brachyura, but still possesses a large abdomen, which is indeed ventrally flexed, but provided mth a caudal fin. CUMACEA. 469 The Thoracostraca are for the most part marine, and feed on dead animal matter or capture living prey. Most of them are good swimmers ; others, e.g. numerous species of crabs, walk and run and sometimes move sideways or backwards with great agility. The chelfe of the first pair of ambulatory legs (fourth thoracic appendages) constitute powerful weapons of defence. Besides the frequent ecdyses of the larval stages, the sexually adult animals cast their shell once or several times in the year {Decapoda). They then live with the new and still soft skin for some time in protected hiding-places. Some Brachyura are able to live for a long tithe in holes in the earth away from the sea. These land crabs undertake, usually at the breeding season, common migrations to the sea and return later to the land with theii- fully developed offspring {Gecarcinibs ruricola). The most ancient fossil PodojMhalmia hitherto known are the mac- rurous Decapoda and Schizojioda, from the carboniferous formations {Pcd(Bocrango7i, Paloeocarabus, Pygocephalus). (1) Sub-order: Cumacea.* Thoracostraca with a small cephalo-thoracic shield, {four to) five free thoracic segments, two pairs of maxillipeds, and six pairs of legs, of which at least the two anterior 2iairs have the hiramous Schizopod form. The abdomen is elongated and com2yosed of six segments, and hears, in the male, tioo, three or fve pairs of sioimmingfeet in addition to the camlal appendages. The Cumacea, the systematic position of which was formerly very cUfFerently estunated, have a superficial i-esemblance to Decapod larvfe, which they also recall m the simplicity of theu^ organization ; while in many of then- characters, such as the formation of the brood-pouch and.their embyronic development, they approach the Arth-ostraca. A cephalo-thoracic shield is always present and includes, besides the segments of the head, the anterior thoracic segments and theii^ appendages ; the four or five posterior thoracic segments, however, remain free. ' • The anterior antennae are small and consist of a three-jointed basal portion, to the end of which, especially in the male, tufts of olfactory hau-s are attached, and of a short flagellum and secondary flagellum. iR^i^' S'T" '"^^1;^ ^'-tev af sV^gten Cuma," Xaturh. Tkhshr., Tom III lit I'Sv. p'^o' ; Cumac6ernes FamUie," Xaturh. Tlrlsshr. N. Z Tom HI 184( . G. 0. Sars, Besknvelse af de paa Fregatten Josephines EvnT fundne Cumaceer," Stockhohxi, 1871. A Dohvn °l]Xlv Tr^^ illT -"^^l":?- Entwickelung der Cvunaceen Jen. naiu,ts^tusoki^TZ ^^S7o' ' 470 CnUSTACEA. In the female the posterior antennae are short and rudimentaxy, while in the adnlt male they, together with their multiarticulate fiagellnni, may be as long as the body (tis in Nehalia). The upper-lip is usually small, while the deeply cleft under-lip is of considei-jible size. The mandibles are without pulps, and possess a comb of bristles and a powerful masticatory process below their strongly toothed extremity. The anterior maxillie consist of two toothed blades and a cylindrical, flagellate appendage directed backwards. The unpalped maxilla of the second pair is composed of several pairs of masticatory plates lying one above another. The two following paii-s of appendages may be distinguished as maxillipeds. The anterior, which corresponds to the palped under-lip of the Isox>oda, is five- jointed and may be recognised by the process of the basal joint ; the posterior, which is also usually five-jointed, is of considerable length and the basal joint is cylindrical and elongated. They also bear the large pinnate gill and a peculiar plate. Of the remaining six paii'S of thoracic appendages, the two anterior are always formed like the feet of the Sclnzopoda ; they consist of a six-jointed leg, the basal joint being strongly developed and lamellar, and of a multiarticulate accessory ramus (exopodite) beset with long swimming setse. The four last pairs of appendages are also six-jointed, but are shorter ; they bear in many cases, with the invariable exception of the last pair, a larger or smaller swimming appendage as exopodite. The very narrow and elongated abdomen is, in the female, entirely without swimming feet, but bears on the large sixth segment at the sides of the caudal plate long-stalked biramous caudal styles ; while in the male two, three or five pairs of swimming feet may in addition be present on the preceding segments. Fam. Diastylidse. Dlastijlh Ilathldl Kr., North Sea. D. Edward fit Kr. Leucu?i nuMCiis Ki-. , Norway. (2) Sub-order : Stomatopoda. " Elongated Thoracostraca ivith short ceplmlo-thoracic shield which does not cover the thoracic segments. There are five pair of maxilli- peds and three pair of biramous thoracic feet. The swimming feet on the strongly developed abdomen bear branchial tufts. * Besides Dana, M. Edwards and others, compare 0. Fr. Miillcr. " Bruch- stiick aus der Eutwickehingsoeschiduc der Maulfiisscr," I. and 11., Arch iv fur KatnrffcKcJi., Tom XXVIII.. 1862, and Tom XXIX., 1803. C. Clans, "Die Metamorphose der Squillidcn," Ahhandl. dcr Gottimicr /-ocirfdt, 1872. C. Grobbcn, " Die Geschleohtsorgane von Squilla mantis," Sit:un /T. ^atunJesck., Tom XIX., 1803. Fr. Milller, Fiir Daz.vTn" ' Ldpzig; 1861. ^ 476 CEUSTACEA. are usujilly inserted externally and somewhat ventrally to the first pair on a fiat plate placed in front of the mouth (ejnstom or oral shield) : they frequently possess a scale-like lamellar appendage. At their base there is always a protuberance with a pore at its end, through which the duct of the antennal gland (green gland) opens. The mandibles vary considerably in shape in the different forms, but have, as a rule, a two or three-jointed palp, which, however, Ls absent in many prawns (Caiididse). They are either straight and sti'ongly toothed on their thickened antei'ior edge (Bracht/ura), or are slendei- and much bent {Crangon), or else forked at the ends {Palcemonidm and Al2)lieidce). The anterior maxillae always consist of two lamellae and a palp, which is usually simple. The posterior maxillfe, on which there are usually four lamellae (two double lamellae) as well as palps, bear a large respiratory plate with setose edges (scaphognathite). These ai^e followed by three pairs of maxillipeds, which, as a rule, have a flagellate appendage. There remain, therefore, only five pairs of thoracic appendages for use as legs ; of these the two last are sometimes re- duced or may even be entirely absent [Leuci- /er) as the result of Fig. 372. -Young form (larva) of the lobster (after G. retrogressive changes. O. Sars). JE, rostrum; antemiEe; X'". third rj^^^^ thoracic segments maxilliped ; I" anterior ambulatory leg. ° to which the ambulatory legs belong are, as a rule, all or all -but the last fused together and form on the ventral side a continuous plate, which in all the Brachyura is broad. The legs consist of seven joints, which corre- spond to those of the Arthrostraca, and frequently end with a chela or prehensile hand. Development. — The gi-eater number of marine Decapoda leave the egg membranes in the zoaea form ; in Homarus, amongst the Macrwra, the metamorphosis is much reduced and the just-hatched young possesses all the thoracic legs, which are, however, pro\-ided with external swimming rami, but it is still without the abdominal feet (fig. 372). Embryonic development. — In addition to the classical researches of Rathke * on the crayfish, more recent works, especially those of * Besides Rathke 1. c. and LerebouUet 1. c., and a Russian paper of Bobretzky, DECAPODA — MACBUEA, 477 Bobretzky (prawiis and cray-fish) and Reichenbach (cray fish) bave yielded important results. The segmentation seems (in all cases?) to be superficial (centrolecithal), that is, to be confined to the peripheral yolk (formative yolk). This divides successively into two, four, eight, and an increasing number of segmentation cells, while the central granular food yolk, which is rich in oil globules, remains unsegmented. The young of Astacus, when hatched, resemble the adult animal, excepting that the caudal fin is still rudimentary. I. — Macrura. The abdomen is strongly developed and is at least as long as the anterior part of the body ; there are four or five pairs of abdominal feet and a broad, well-developed caudal fin. The antennules bear two or three flagella, the antennte have one simple flagellum and frequently bear a scale at the base. The maxillipeds of the third pah- are long and pediform and do not completely cover the pre- ceding ones. The Zocea larva, when hatched, is elongated and has usually three pair of biramous feet. Fam. Carididae. Prawns. Body laterally compressed, with a thin shell, which, is often provided with a median ridge and prolonged into a saw-like frontal process. The posterior (external) antennaj are inserted beneath the anterior (internal) and have a large scale projecting over the stalk. The long and slender anterior pairs of ambulatory legs fi-equently end in chelaj. They live in shoals near the coast. Some genera QPenceits) possess a rudimentary swimming ramus. PalcBmon sqiiilla L., Crangon mdgaris Fabr., Pontonia trjrrliena Eisso, lives between the shells of bivalves. Scrgcstcs atlantimis Edw. Fam. Astacidae. Tolerably large, usually with a hard shell. The cephalo- thorax is slightly compressed, the abdomen flattened. The anteunje are attached near the antennules, and bear a small or quite reduced scale at their base. The first pair of ambulatory feet ends with large chela;, as do in many cases the weaker and smaller second and third pairs. Some soft-skinned forms bury themselves in the mud or sand. Axtacns Jiuviatilis ^oti(\.., Crayfish. Hovmrvs vulgar ii! Bel., Lobster. jYcj?Jiro]Js norwcgicus L., Gchia Leach., Thala-mna Latr., CalUanassa suhterranea Mont., buries itself in sand on the sea-shore. Fam. Loricata. With very hard, rough armour, and large broad abdomen. The antennules end with two short flagella ; all five pairs of ambulatory feet with simple claws. The larvfe are described as species of Phyllosuma. Palimrm qvadriccrnh Latr. Scyllarus latus Latr. Fam. Galatheidse. With broad, rather large abdomen, and well-develpped caudal fin. The first pair of legs is chelate, the last is weak and reduced. Galathea strigosa L. Fam. Hippidae. Cephalo-thoracic shield long ; end of the abdomen curved. The first pair of legs usually with a finger-shaped terminal joint ; the last is Kiew 1873 compare H. Reiclienbach. " Die Embryonalanlage und erste Ent- wickelung des Ilusskrebses," ZciUchr.fiir Whx.Zuol., Tom XXIX., 1877. 478 CEUSTACKA, weak. Ilippa rrmiita L., lives l^uried in the sea sand, Brazil. Alhnu-a sijiiinixtd Fill)]-., Mediterranean. Fani. Paguridse. Hermit crabs. Ahdonien lonp, usually covered with a soft skin and distorted, with narj-ow anal fin and rudimentary abdominal feet. The first pair of feet ends with jjowerful chelie, the two last are reduced. Some of them seek shelter in empty snail shells, to jn-otect their soft -skinned abdo- niinal region. Pa(jHvnx Brrnliardtix L., CoanohiUt riKjima Edw., JHrfiiiK Intro Herbst, said to climb palm-trees. II. — Braciiyura, With pits for the reception of the short internal (anterior) antenna and so-called orbits, i.e., cavities for the reception of the stalked eye.s. Abdomen short and reduced, without caudal fin, curved round against the excavated ventral surface of the thorax ; in the male narrow and pointed, with only one, more rarely two pairs of abdominal feet ; in the female broad, with four pairs of abdominal feet. In the female each oviduct dilates to form a bursa copulatrix. The thii-d pair of maxillipeds have broad flat joints and completely cover the anterior mouth parts. The just-hatched Zocea larvae of stout shape, with only two pairs of biramous feet and a dorsal spine ; later they assume the Megaloixi form. Many Brachyura live on land, Fam. Notopoda. Transitional between the Bracliijura and Macrvra. The two or four posterior thoracic feet are articulated higher up than the four or three posterior pairs, and shifted on to the back. The fii-st pair of feet has large chelee, the last is often modified to swimming feet. Porcellana platycJi^tlf^s Penn, Droiiii/t rnhiarix Edw., LithHlcx. Latr. Fam. Oxystomata. With rounded cephalo-thorax. The frontal region does not project. The buccal frame is triangular. The male genital openings are on the basal joint of the last pair of thoracic legs. Calappa (jvaimlata L., Ilia nnclcusi Herbst, Mediterranean. Fam. Oxyrhyncha. Cephalo-thorax usually triangular, with projecting pointed rostrum. There are nine gills on either side. The male genital opening is on the basal joint of the last pair of thoracic legs. The thoracic ganglia are united iiito one mass. They do not swim but crawl. Inachvs scorxno Fabr., 3Iaja squinado Eond., Pim, armata Latr., Stenorhynelmn Lam. Fam. Cyclometopa. With broad, short cephalo-thorax, roimded anteriorly. Without projecting fi'ontal rostrum. There are nine gills on either side. The male genital opening is on the basal jointof the last pair of thoracic legs. Some of them are good swimmers. Cancer pafjuvuii L., J^CLiitlio I'ivuloxiDs Eisso, Mediterranean. CavcinvK mccnas L., Furtwius puhcr L. Fam. Catometopa. Quadrilatera. Cephalo-thorax quadrilatei-al. Frontal region is curved downwards. There are fewer than nuie gills. The male genital openings usually lie on the sternum. Some of them live for a long time away from the water. Home live in holes in the earth, as land crabs. Piiinnthcron jn-'^iim L,, in the shells of Mytilus. P. vctcrvm Bosc, in the shells of Pinna ; known to the ancients, who thought that there was a relation of mutual assistance between the crab and the moUusk. Oci/poda curxor Bel., GIGAJS'XOSTKACA — MEROSTOMA.TA. 479 Gelasimus forceps Latr., Gnqwis variuit Latr., Gt'cavchms ruricnla L., Land Crabs. Water is retaiued for a long time in the branchial cavities, owing to the presence of secondary spaces around the branchial plates, which are thus pre- vented from sticking together. They live in holes in the earth in the Antilles. III.— GIGANTOSTEACA. The Xiphosura or Pcecilopoda, represented by the living genus Limulus and the orders of the fossil Merostomata, may be united under this head, as opposed to the Entomostraca and Malacostraca. They are principally characterised by the possession of a single pail- of appendages placed in front of the mouth and innervated from the cerebral ganglion, also by the presence of four or five pairs of legs, which are placed round the mouth and whose basal joints are modified to foim large mandible-like masticatory organs. Behind the last pair of legs there is a simple or cleft pi'ominence, forming a sort of underlip. The region of the body which bears these appen- dages is to be considered as an unsegmented cephalo -thorax ; it is shield-shaped and may be drawn out into projecting wing-shaped lateral portions. On its upper surface two small median frontal eyes as well as two lar-ge lateral eyes can be distinguished. Following the ceplialo-thorax there is an abdomen, which is usually elongated and comjjosed of a greater number of segments. The abdomen tapers posteriorly and terminates in a telson, which may be flat or drawn out into the form of a spine. Order 1. — Merostomata.* Gigantostraca with five j^cdrs of appendages on the cephalo-thorax lahich is relatively short ; with an elongated apodal abdomen, usually Gomjyosed of twelve segments and ending in a flat or styliform telson. The powei-ful body of the Eurypteridm (included with the Pmcilo- poda by Woodward), as the most important family of the Merostomata is named after the genus Eurypterus, consists of a cephalo -thora;cic shield with median ocelli as well as large projecting marginal eyes, also of an abdomen with numerous (usually twelve) segments which become longer posteriorly, and of a caudal shield, which is prolonged into a spine. Round the mouth on the underside of the cephalo-thorax * Woodward, " Monograph of the Brit, fossil Crustacea belono-ing to the order of Merostomata." P. 1.. II., Pulwout. Soc. of London, 1866-1869. Wood- ward, " On some points in the structui-e of the Xiphosura, having reference to their relationship with the Eurypteridm," Quarterly Jom-n. Gcol. Soc. of London 480 CJIUSTAOEA. there are five pairs of long spiny legs, of which the last is much the largest and ends in a broad swimming-fin. Some of the anterior appendages may be armed with a chela. The resemblance of the true Eiiryjjteridm (in tlie general shape of their body) to the Scorpionidai is very striking, while the genus Jlemiasjns presents affinities to the Piecilojioda. The most important forms are : Eurypterua pyyriioiua Salt., Devonian strata, Pterygotios anglicus Ag., four feet long, from the upper Silurian (fig. 373). Fig. d73.—Suri/pterus remipex after Nieszkowski. a, Dorsal view ; b, ventral view ; O, eyee ; St. caudal spiue ; M, hypostome. Order 2. — Xiphosura.* Gigantostraca lohose body is divided into three p)Ct,rts, lohich are movably articulated together ; a large shield-shajied ceplmlo-thorax, an abdomen with Jive pairs of lamellar feet and a long movable caiuLal spi^ie. The large body of these Crustacea is covered with a strong chiti- • C GegenbauT, " Anatomische Untersucliung eincs LimuluP, mit bcsonderer Beriicksichtigung der Gevvebe," AhJuuull. ,Ur naturforxch Gesdhrhaft zu Ilallc \y 18r.8. Packard. " The Development of Limnlus Polyphemus, ,Vv. ' ~lst 1870. A. M. Edwards. •' Reclicrclies sur I'anatomie des Limules. ■' - _ T, T '•Limulus lwl"t5«irV°"Seir'fom7 XVII.' 187i [E. R. Lanko-stcr an Arachnid," Qvart. Jnurn. Mio. Soc, vol. xxi.] I XIPHOSUBA. 481 nous armour and is divided into an arched cephalo-thorax and a flat, almost hexagonal abdomen, which ends in a movable sword-like caudal spine. The cephalo-thorax (fig. 374) forms by far the larger part of the body; it bears on its arched dorsal surface two large compound eyes, and further forwards, nearer the middle line, two smaller simple eyes ; while on its venti-al surface there ai-e six pairs of appendages, of which the anterior pair is slender and may, on account of its position in front of the mouth, be re- garded as a pair of antennae, although it ends, like the others, with a chela. The latter ai-e placed to the right and left of the mouth, and their coxal joints serve as organs for the mastication of the food. At the end of the cephalo-thorax thei-e is a pair of lamellar appendages, which are connected in the middle line and form a kind of operculum for the branchial ap- pendages of the abdomen. It seems of interest that the form of this branchial operculum in the Asiatic and American species presents constant differences, in that the median portion in the foi-mer is undivided, and in the latter consists of two joints. The shield-shaped abdomen which, by means of a transverse joint, is movable on the cephalic shield in a dorso- ventral direction, is armed on either side with movable spines, and bears on its ven- tral surface five pairs of lamellar feet, which are almost completely covered by the operculum. These abdominal feet assist both in swimming and in respira- tion, since the respiratory lamellje are placed on them (fig, .374, a, h). The internal organization attains a re- FiG. 374. — a Limulus inolticcanus, seen from the dorsal side (after Huxley). O, eyes; St, caudal spine, b, L. rotundi- Cauda (after M. Edwards), seen from the venti-al side. A Antennae; B, the feet with their coxal jaws ; X, gills ; Op, operculum. latively high development in correspondence with the large size of the body. In the nervous system the following parts can be distinguished oesophageal ring, the anterior part of which constitutes and gives off the optic nerves, while from the lateral ; — a broad the brain parts the 31 482 CEUSTACJSA. SIX pairs of nerves to the antennae and legs take tlieir origin ; a suba^sophiigeal ganglionic mass with three transverse comnii>isures : and a double ganglionic cord, which gives off branches to the venti-al feet and ends with a. doul)le ganglion in the abdomen. The alimen- tary canal consists of (usophagus, masticatory stomacli, and a straight intestine communicating with a liver and opening ))y the anus, whicli is placed immediately in front of the base of the caudal spine. The heart is elongated and tubular, and is pierced by eight pairs of slits, which can be closed with valves; it is also provided with arteries, which, after a short course, pass into lacunar blood paths. From the base of the gills, two spaces, returning the blood, extend to the pericai-dial sinus. Five paii's of appendages of the abdominal feet function as gills. These are composed of a vei-_>' large number of delicate lamellae, l} ing one on another like the leaves of a book. Generative organs. — The branched ovaries unite to form two oWducts, which open by separate openings on the under side of the operculum (first pair of abdominal limbs) ; in the male the openings of the two seminal ducts are placed in the same position. In the male, the anterior thoracic feet end in simple claws. Development. — It is known that the young leave the egg without the caudal spine and often without the three posterior paii-s of gill- bearing feet. This stage has been suitably named the Tiilobite stage, on account of the i-esemblance which the larva presents to a Trilobite (fig. 375). On the cephalic shield there is a median keel- like ridge, which is also found on the abdominal segments. The last abdominal segment includes between its lateral portions the short i-udiment of the caudal spine. In the next stage the segmen- tation of the abdomen becomes less obvious (the caudal shield becomes consolidated) and the caudal spine developed. The adult animals reach a length of several feet, and live exclusively in the warm seas, in the Indian Archipelago and on the east coast of America. They exist at a depth of two to six fathoms and move about in the mud by the alternating bending and straightening of the cephalic and abdominal shields and the caudal spine. Their food .consists chiefly of Nereids. They are found in a Fig. 375. — Embryo of Limulus in the Trilobite stage (after A. Dohin). TBILOBITA. 483 fossil state, especially iii the Sohlenhofen lithographic slate, but also ill older formations as far back as the Uebergangsgebii-ge (Cambrian, Silurian, etc.) formation. Linmlus moluooannx L.itr., East Indies. L. pohj2^Jifi9}i.v.s L., East Coast of North America. TRILOBITA.* In connection with the Merostomata and the Xiphosura, the Trilobites may be considered. Their systematic position cannot as yet be defined with certainty. They lived only in the most an- cient periods of the earth's his- tory, and their fossil remains are found in great numbers and are excellently preserved; but, un- fortunately, the conditions under which they wei-e fossilised wei-e such that the under side of the body, and, consequently, the structure of the appendages, that is the very characters which would enable us to decide theii- affinities, i-emain unknown to us. We may probably infer from this absence of any trace of appendages * in the fossils, that the legs were soft and delicate ; but Burmeister's conclusion that they resembled the legs of the Phyllopoda is not justified. The body, which is frequently found rolled up, is covered with Fig. 376.— Diagram of Dalmatites (after Pictet). Gl, Glabellum ; Sf, great suture (ocular suture); 0, eyes; Ge, separable gena (cheeks) ; JRh, rhachis (tergum) ; PI, jjleu- ron ; Pff, pygidium. a thick shell, which is di^dded by two parallel longitudinal furrows into an elevated median portion (rhachis) and two latera;! portions {pleura): it rarely attains any considerable size. There is an * Bm-meister, " Die Orgaiiisation der Trilobiten," etc. , Berlin, 1843. Beyrich Uutcrsuchuugen uber Trilobiten," Berlin, 1845,1846 J. Bai'rande "SySme silurien du centre de la Boherae," Prague, 1852. W Saltt 'TmonSlnnl! of the British Trilobites," London, 1864-1866 monogiaph of *an°'l3^/v^r^-'^n?5'' ^T' .""'^^^^^ "^'^^'^^^ ^^^tral surface ot ^ Amphvs ( Notes on some Specimens of Lower Silurian Trilobites " bv H S '^'^ n"*' r}^"" ^^"^1^"^ '-^"^ appendages of 1 apZ?' etc^ by H Woodward, Qjuivt. Jonrn. of tlie Grolo,,. Soc . London, 1870) which are said to point to the affinity of TriloMtrs with the J.sojmla ' 484 TEILOBITA — AUACHNIDA. anterior arclied, semicircular region, which may be regarded a.s head or perliaps as cephalo-thorax, and a number of sharply dis- tinct segments, which belong partly to the thorax and paitly to the abdomen and are tei-minated by a larger shield-shaped caudal portion, the pygidium (fig. 376). At the edge of the pygidium, the armour of the upper surface is folded round on to the ventral side and leaves only the middle part of the latter uncovei-ed. The lateiul regions of the head, the median part of which especially projects as the " glabellum" bear usually upon two piotuberances large compound facetted eyes, and are often prolonged into two very long backwardly directed spines; they are also folded inwaixls on to the ventral sui-face. With the exception of a plate (hypostoma) comparable to the under-lip of Ajous, no trace of mouth parts has been observed for certain on the ventral surface of the head. The number of thoracic (trunk) segments varies considerably, but is tolerably definite for the adults of each species. Their lateral portions are likemse folded inwai-ds on to the ventral sui-face, and present variously shaped wing-like processes and long pointed spines. The Trilobites lived in the sea, probably in shoals in shallow water near the coast. Their fossils are amongst the most ancient remains of animal life, and are found principally in Bohemia, Russia, Sweden etc., in the lowest strata of the Uebergangsgebirge (Cambrian, Silurian, etc) They have been divided into numerous families according to the structure of the head (especially of the glabellum), the form of the pygidium and the number of segments. The most important genera are Calymene Blumenhachii Brogn ; Olenus gihhosus Wahlb., Ellipsocephcdus Hoffii Schlotth. Class II.— ARACHNIDA.- Air-breathing Arthropoda with fused head ami thorax, with two pairs of jaws, four pairs of ambulatory legs and apodal abdomen. The Ai-achnida include animals of extraordinarily different form. The head and thorax are almost invariably, fused to form a short cepholo-thorax ; but the condition of the abdomen presents very great variations. * C A Walokenaer et P. Gervais, " Histoire naturelle des Insectes Apteres," 3 Vols Paris, 18.37-1 H44. Halin und Koch, " Die Arachniden, getreu nach der Natur abgebildet und beschneben." Niirnberg, 1831-1849. E. Blanchaitl, " Organisation du r^gne animal. Arachnides," Paris, 18G0. AEACHNIDA. 485 In the Spiders (Araneida) the abdomen is swollen and is joined to the cephalo-thoi-ax by a shoi't stalk. In the Sco^yionidce, on the contrary, the long abdomen is joined to the cephalo-thorax by its whole breadth, and is divided into a broad segmented prse-abdomen and a narrow, veiy movable post-abdomen, which is also seg- mented. In the Mites I^Acarina) the abdomen is unsegmented and fused with the cephalo-thorax. In the Pentastomida the entii'e body is elongated, ringed and vermiform, with four (two pairs of) hooks in place of the appendages ; these animals ai-e known as Linguatidida, and might be placed, on account of theii- parasitism, amongst the intestinal worms. The marked i-eduction of the cephalic region, which is without true antennae and possesses only two pair of oral appendages, is chai^acteristic of the Arachnida, The anteiior pair of cephalic appendages (chelicerfe), which are used as jaws, have been regarded as modified antennte ; but it is perhaps more natural to regard them as morphologically equivalent to the mandibles of Crustaceans and Insects. These anterior jaws or chelicerse are either chelate, in which case the claw-Kke terminal joint can be moved against a process of the preceding joint (Scorpions, many Acarina), or sub- chelate, when the last joint is folded down upon the next like the blade of a pocket-knife upon the handle (Spiders). The chelicerse may also have the form of stylets, which are enclosed in a tube formed by the second pair of jaws (Mites). The latter, which constitute the second pair of appendages of the head or the pedipalpi, consist of a stout basal joint and a palp, which has fre- quently the form and segmentation of a leg. This either ends with or without a claw or with a chela (Scorpions). In the true Spiders there is an unpaired plate, the lowei- lip, between the basal joints of the two pedipalpi and belonging to the same segment as the latter. The four following pairs of appendages of the thorax are ambulatory legs. The first of them is sometimes modified in form and elongated like a palp ; its basal joint may function as a jaw. The legs consist of six or seven joints, which, in the higher forms, have been called by the same names as the analogous regions' of the Insect leg. The internal organization of the Arachnida shows hardly fewer differences than does that of the Crustacea. The nervous system may have the form of a common ganglionic mass around the oeso- phagus (Mites), and may even possess only a simple commissure above the oesophagus {Pentastomida). As a rule, however, there is a distmct separation between brain and ventral cord, the latter showino- 486 AEACHNIDA. very different grades of development. Visceral nei-ves have been shown to exist in the Spiders and Scorpions. The sense organs ai-e, as a rule, not so highly developed as in the Crustacea, and, puttijjg on one side the tactile function of the extremities, are confined to eyes. . The eyes are simple and immovable, and never possess a facetted cornea ; they are from two to twelve in number, and are sym- metrically arranged on the anterior surface of the cephalo-thoracic shield. Auditory organs have not yet been discovered, but there are tactile and olfactory organs. The alimentary canal runs straight from the mouth to the hind end of the body, and is divided into a narrow oesophagus and a wide intestine, which is, as a rule, provided with lateral cfeca. The intes- tine is, in the Spiders and Scorpions, divided into an anterior dilated portion — the so-called stomach — and the rntestme pi'oper. The glandular appendages of the digestive canal are salivary glands ; in Spiders and Scorpions, a liver, composed of a number of branched canals ; and, with a few exceptions, Malpighian tubes, which function as urinary organs and open into the hind end of the intestine. The organs of circulation and respiration also show very different degrees of development and are onl}' absent in the lowest Mites. The heart Hes in the abdomen, and is a long, many-chambered doi-sal vessel with lateral slits through which the blood enters. It is fre- quently continued into an anteiior and posterior aorta, and in Scoi'pions gives olf in addition lateral branching arteries. The organs of respiration are internal air chambers, which have the form either of ramified' tubes (trachece), or of hollow lamellae {fan-trachea', lungs) placed upon one another in great number like the leaves of a book and connected together by trabeculfe so as to have the form of a sac. The air chambers are always kept open by a firm internal chitinous membi'ane, so that the air can enter by the paired openings {stig- mata) of the tracheae or lungs at the beginning of the abdomen, and be distributed to the finest ramifications. The chitinous lining may become thickened so as to give rise to a spiral fibre. Generative organs. — With the exception of the hermaphrodite Tardigrada, all the Arachnida are of separate sexes. The males are frequently distingiiished by external characteristics, as for example by theii- smaller size, by the possession of organs of attachment (Mites), or by the modification of certain appendages. Their genera- tive organs consist of paired testicular tubes, and the vasa deferentia . often receive the contents of accessory glands before opening to the exterior by a single or double aperture at the base (anterior end) of LINGUATULIDA. 487 the abdomen. Special copulatory organs in the region of the genital openings ai-e, as a rule, wanting, but appendages far removed from the genital openings {e.g., pedipalpi of Spiders) often serve to transfer the sperm from the male to the female. The female sexual organs ai-e also paired, usiially i-acemose glands, with two oviducts, which usually dilate to a i-eceptaculum seminis befoi-e their single or double opening at the beginning of the abdomen. They are also connected with accessory glands. Rai'ely {Phalangium) there is a long pro- trusible ovipositoi-. Only a few of the Arachnicla are vivipai-ous (Scorpions and some Mites) ; the greater number lay eggs, which they sometimes cairy about with them in sacs till the young are hatched. As a i-ule, the just-hatched young have the foi-m of the adult ; but in most Mites two or more rarely four legs ai-e wanting, and appear only with the succeeding moults. The development of the Pygnogonida Pentas- tomida and Hydrachnea (water-mites) (which latter pass through a pupa-like inactive stage) consists of a complicated metamorphosis. Almost all Arachnida Kve on animals, a few on vegetable juices. The lowest forms are parasitic. The larger and more highly orga- nised foi-ms prey on living animals, principally on Insects and Spiders, and are usually fui-nished with poison weapons, with which they kill their prey. Many of them, by means of the seci-etion of spinning glands, spin webs, in which theii- prey becomes entangled. Most of them remain during the daytime beneath stones and in hiding-places, and come out to catch their prey only in the evening and at night. Order 1. — Linguatulida,* Pjentastomida. Parasitic Arachnida xoith ringed, elongated, vermiform body, toith two pairs of hooks in the neighbourhood of the jatoless mouth. The vermiform I'inged body of these parasites, which were for a long time taken for intestinal worms, is to be reeurded as beinji- principally formed of the extremely enlarged and elongated abdomen, the cephalo-thorax being much reduced ; an interpretation which the form of the body of the Dermato2Mli seems to support. In the adult, jaws are completely wanting, but there are four curved hooks (two on each side of the mouth, fig. 377), which can be protruded from pouches in the skin and are attached to special chitinous rods. These may correspond to the terminal claws of the two posterior pairs of legs, since the two paii-s of legs of the larva, which are to * K. Leuckart, "Bau und Entwickelungsgcschichte cler Pentastoraiden." Leipzig und Heidelberg, 1860. 488 AUACIINIDA. be regarded as the Fig. 377. — Fentantomum denticidatum. Young form of P. tcenioides. O, Mouth; Sf, the four hooks ; D, intes- tine ; A, anus. anterior appendages, are lost in the coui-se of development. The nervous system is confined to a simple subcesophageal nervous mass, witli oesophageal ring and giving off numerons ner- vous trunks. Eyes and organs of respiration and circulation are wanting. The alimentary tract is a simple canal in the middle of the body, which opens by an anus at the posterior end. Special cutaneous glands are present in great numbers and strongly developed. Male and female are distinguished by considei-able differences in size and by the different position of the genital openings. While the genital opening of the surprisingly small male lies not far behind the mouth, that of the female is situ- ated near the anus, at the hinder end of the body. The Linguatulida, when sexually adult, in- habit the air chambers of warm-blooded animals and Amphibia. The developmental history of Peiitastomum tcenioides, which lives in the nasal cavities and in the frontal sinuses of dogs and wolves, is known from the researches of Leuck- art. The embryos of this species, while still enveloped in the egg-membi-anes, pass out with the nasal mucus on to plants, and thence into the stomach of Rabbits and Hai-es, more rarely into that of Man. When freed from the egg- membranes, they piei'ce the walls of the in- d Fig 378.— Young forms of Peiitatfomum tcenioidea (after E. Leuckart). a. Egg with embryo. b, Embryo with two pairs of hooked feet, Sf and Hf". c, Larva from liver of rabbit. &, Ganglion ; D, iutestuie ; JM, skin glands, d, Older \ax\a. 0, mouth ; A, anus ; Gd, genital glands. LINGUAXULIDA — ACAEINA. 489 testine and reach the liver. There they surround themselves with a cyst, in which they pass through a series of changes of form, ac- companied, as in insect larvse, by repeated ecdyses (fig. 378). When six months have elapsed, they have attained a considerable size and have acquired the four oral hooks, as well as a number of finely serrated superficial rings. They have now reached the stage formei'ly described as P. denticulatum (fig. 377), hi which they break through theii' capsules and begin a fresh migration. They traverse the liver, and if present in great numbers, occasion the death of their host. In other cases, on the other hand, they soon become enveloped in T A P'^G. 379.— Ripe male of Atax Bonzi, seen from the dorsal surface (after E. Claparede). Kt, Pedipalpus; G, brain; Oc, eyes; T. testis; iV, Y-shaped gland; D, intestine; a', anus ; Sd, cutaneous glands. another cyst. If they now pass with the flesh of the Hare or Rabbit into the buccal cavity of the Dog, they penetrate into the neighbouring air-chambers, and in two or three months become sexually mature. Pentmtomum tcenioides Rud., 80-85 mm., Male only 18-20 mm. long. P. muUiemcttim Harl., in the liver of Kaja hajc. P. eomtrictum v. Sieb. Encysted in the liver of negroes in Egypt. Order 2. — Acarina,* Mites. Arachnida with stout body. The abdomen is unsegmented and * 0. Fr. Miillcr, " Hydrachnee," etc., 1781. A. Duges, " Recherches sur ARACHNIDA. fiised with the thorax. The oral ax>paratm is adax>ted for bithig or piercing and suokimj,. Re82nration, as a rule, by means of tracheoi. The body of the Acarina is genei-ally small and possesses a stout and unsegmented form. The head, thorax, and abdomen are fused into a connnon mass (fig. 379). The form of the oral apparatus varies exceedingly, and may be adapted either for biting or for piercing and sucking. The cheliceriB are accordingly some- times retractile styles, and are sometimes furnished with claws or chelae. In the fii-st case, the bases of the pedipalpi form a sheath which surrounds the styliform chelicerae and serves as a suctorial rostrum, while the peripheral part of the pedipalpus or palp frequently projects laterally, and ends with a claw or chela. The structure of the four pairs of legs is not less various, in- asmuch as they may serve for crawling, attachment, mnning and swimming. They usually end with two claws, sometimes in parasitic forms with stalked suctorial discs. The nervous system is re- duced to a common ganglionic mass representing the brain and ventral cord. Eyes may be absent or may be present, as one or two parrs of simple eyes. The alimentary canal is frequently provided with sali- vary glands, and gives off on either side a number of blind saccular diverticula which may be forked (fig. 380). Heart and blood vessels are invariably absent, but respiratory organs ai'e frequently pi-esent in the form of trachew, which arise I'ordre des Acariens en general et les families des Trombidios, Hydi-achnes on part," Ann. des Sr. Xat., II. Ser.. Tom. I. and II. H. Nicolet. Histoire naturclle des Acariens. etc. Oribatides." Archircs dii turner '0vided with claws, and tlie anterior pair of the legs elongated, resembling antennce. The abdomen has eleven or tioelve segments, and is clearly marked off from the rest of the body. The Scoi'pion-spiders (fig. 409) are allied both to the Spiders and the Scorpions. The abdomen, which is always separated from the cephalo-thorax by a constricted portion, is divided into a considerable number of segments, but presents no distinction into a broad pra?- abdomen and a thin styliform post-abdomen as in the Scorpions. * H. Lucas, " Essai sur une monogi-aphie du genre Thelyphonus," Magax. do Zool., 1835. J. V. d. Hoeveii, " Bijdrageu tot de kennis van het geslacht Phrynus," Tijdschr. voor nat. Gexchicd. IX., 1842. PEDIPALPI. 507 In the genus Tlielyplionus, however, which is most closely allied to the Scorpions, the three last segments of the abdomen are narrowed to the form of a short tube, the end of which is prolonged into a long-jointed appendage. The chelicerJB ai'e always provided Avith claws, and probably, as in the spiders, contain a poison gland, since the bite of these animals is much feared. The Pedipalpi, on the other hand, are sometimes of considerable strength and aimed with a claw and several spines [Phrynus). Sometimes (Thelyplwnus) they are, as in the Scorpions, chelate. The legs of the anterior pair are always very long and thin, and end with a flagellifoi-m ringed portion. There are eight eyes, of which the two largest are placed Fig. 409.— Phrynns veniformis (regne animal). Xt, Pedipalpi ; Gh, flagellifoim anterior leg. in the middle of the cephalo-thoracic shield, while the three smaller pairs are situated on each side behind the frontal margin. They breathe by means of four lung sacs, composed of a very large number of lamellar tubes. The slit-like openings of the lung sacs lie on either side of the posterior margin of the second and third abdominal segments. In the structure of the alimentary canal they resemble the Scorpions, in that of the nervous system the Spiders. The genus Phrynus is viviparous. All the Pedipalpi live in the tropics of the Old and New World. Fam. Phrynidae. With the characters of the order. Plirynvs OMy. The large broad pedipalpi are armed with a luimber of spines and end with a claw The masticating blades are free. The abdomen is flat and relatively short and has 508 ABA.CHNIDA.. eleven segments and no jointed anal filament. Ph. rrmformis Latr,, in Brazil T/u-Ii/phoms Latr. The clieiiccriE are sliort and end in a ckela, their masti- cating blades fuse in the middle line. The elongated twelve-ringed abdomen with segmented anal filament. T. cavdatvx Fabr., in Java. Order 6. — Scorpionidea* (Scorpions). Arachnida with chelate chelicerce, and elongated, j)^d%form chelate 'ped%2xdin, with a incB-ahdomm composed of seven segments, and an elongated 2)ost-ahdomen of six segments, with poison sjnne at the hind e,nd ; loith four 2)airs of lungs. The Scorpion.s have a cer- tain resemblance to the De- ca2)od Crustacea in their powerful chelate pedipalpi and firm armour (fig. 410). The .stout cephalo-thorax is joined to an elongated abdomen, which is di\dded into a cylin- drical prae-abdomen, composed of seven segments, and a very narrow six-.segmented post- abdomen, which is curved dorsalwards. The po.st-abdo- men ends with a curved poison spine, which is provided with two poison glands. The cheU- cerje are three-jointed and chelate; the pedipalpi end with a swollen terminal chela, while the basal joint sei'ves ^^^^th its broad gi-indiug surface as a jaw. The four pairs of legs are strongly de- veloped and end yvith. double claws. In their iaternal oi-ganization the Scorpions reach the highe.st * P. Gervais, " Kemavques sur la famillc des Scorpions et description de plusieurs esp6ces nouvoUcs, etc." ArcJi. dn mvitcc d'kixt. naf., IV. Newport, " On the structure, relations, and development of the nervous and circulatory- Systems in Myriapoda and macrourous Arachnida." Phil. Trann. 184.S. L. Dufour, " Histoire anatomique et phj^siologique des Scorpions," ^f6v^. pres a Vacad. des sciences, XIV., 1856. E. Metechnikoff, " Embryologic des Scor- pions," Zeits^clir fii/r tmss., ZooL, 1870. Fig. 410. — Cephalo-thorax and. prse-abdomen of Scorpio afneanus (regne animal). Kf, Cheli- certe ; Kt, pedipaljsi ; K, pectines ; St, stigmata. SCOEPIONIDEA. 509 grade of all the Arachnida. The nervous system is composed of a bUobed braia, a large oval ganglionic mass in the thorax, and seven to eight smaller ganglionic swellings in the abdomen, of which the last four belong to the post-abdomen. The visceral nervous system is i-epresented by a small ganglion, which is placed at the beginning of the cesophagus, connected with the bi-ain by fibres and gives oif nei'ves to the alimentary canal. The principal oi-gans of sense are the simple eyes. Of these there are from three to six pairs, which are so distributed that the largest pair is situated on the middle of the cephalo-thorax, and the others right and left at the sides of the frontal region. The alimentary canal is a narrow straight tube, which is sur- rounded in the prse-abdomen by the large multilobed liver, and opens on the penultimate ling of the ab- domen. Two Malpighian vessels function as excretory organs. The circulation is the most com- phcated in the whole class, but, as in the Decapoda, special blood si- nuses of the body cavity are inserted into the vascular system. The elongated dorsal-vessel, which is divided into eight chambers and is attached by alary muscles, is surrounded by a pericardial sinus, from which it receives the blood through eight pairs of slit-like open- ings, which are capable of being closed. From the heart the blood is diiven through an anterior and posterior artery, and through lateral arteries to the organs. The finer ends of the arteries seem to be connected with the commencing veins by capillaries. From the veins the blood is collected in a receptacle on the ventral surface. Thence the blood passes to the respiratory organs, whence it passes by special veins into the pericardial sinus, and so back to the heart. Respiration is effected by means of four pairs of lung sacs, which open to the exteiior by four pairs of stigmata on the thn-d to the sixth abdominal segments and are composed of a rela- tively small number of flat tubes. The male and female generative organs open on the ventral face of the first abdominal segment [the meclia,n opening being covered Fig. 411.— Embryo of a Scorpion (after B. Metschnikoff). Kf, Chelicer^ ; JO, pedipalpi ; JS' to , the four pairs of thoracic legs. There are rudimentary limbs on the abdomen. 510 ahacjinida. by a small valve-like flap, the genital operculum] ; on the second abdominal segment are attached two peculiar comb-shaped structures, known as pectines. The latter are probably the remains of the appen- dages of the segment, and serve as tactOe organs. The males are distinguished from the females by their broader chelje and longer post-abdomen. The females are viviparous. The development of the ovum takes place in the ovary, and the embryos have the rudiments of appendages on the prse-abdomen (fig. 411). The Scorpions live in warm couniries, and leave their hiding-places at dusk. When they run, the post-abdomen is bent upwards over the back. They seize theii- prey, i.e., principally spiders and large insects, with their large chelate pedi^jalps, and sting them to death with their caudal poison-spine. Some species attain a veiy consider- able size, and their sting may even prove fatal to man. Fam. Scorpionidse. Smrjno em'02)fsvs Schr., of small size and with only six eyes, iu Italy. Androctonm ocoitanva Am., Bnthus afer L. Order 7. — Pseudoscorpionidea.* Arachnida of small size and resembling scorjnons, but without caudal sjyine or jooison gland. They breathe by means of trachece. . , .J The Pseudoscorpions are far smaller and Fig, 412. — Obisiwn rrombidioiaes ^ (regne animal). Kt, Pedi- more simply organised than the scorpions, ^^^P*^^' They bear much the same relation to the true scorpions that the mites do to the spiders. In their form and the structure of their chelicerje and chelate pedipalpi they resemble the scorpions. On the other hand, the hind end of the segmented abdomen does not become narrow so as to form a post-abdomen, and is without a caudal spine and poison gland (fig. 412). They all possess spinning glands, the openings of which lie near the genital openings on the second abdominal ring. They possess only two 01- four ocelli, and respire by means of tracheae, which open by two pairs of stigmata on the two first abdominal rings. They live beneath the bark of ti^ees, in moss, between the leaves of old books, * W. E. Leach, " On the characters of Scorpionidea with description of the Nurnberg, 187:?. V SOLirUGiB. 511 etc. ; they run rapidly laterally and backwards, and live on mites and small insects. Fam. Chernetidae. Chel'tfer cancroides L. Book-scorpion with two eyes. Ohmum isclinoaccles Herm,, -svith four eyes. Clifhonins troviMclioides Latr. (fig. 412). Order 8. — Solifug^.* Simler-lihe animals loith se2Xirated head and thorax, with elongated, segmented abdomen; suh-chelate chelicero} and 2^Qdiform, pedijialpi. Respiration is effected hy means of trachece. The Solifugce ap- proach insects in the segmentation of the body. The cephalo -tho- rax is divided into two regions of which the an- terior is comparable to the insect head, the pos- terior (composed of thi-ee segments) to the insect thorax. The long cylin- drical abdominal region, which is composed of nine to ten segments, is quite distinct (fig. 413). The body is closely covered with hairs. The oiul aj)paratus consists of powerful chelicerfe, which end in a large veitically placed chela, the lower arm of which can be moved perpendicularly against the upper. The pedipalpi serve as ambulatory legs, but are with- out claws, which are found only on the three posterior pairs of legs. The latter arise from the three free thoracic rings, and bear pecuKar cutaneous lamella? at their base. The anterior pair of legs belongs to the head and may be considered as a second pair of pedipalpi (maxillary palps). The Solifugm pos- sess two large projecting simple eyes, and respire like insects by * L Dufour, " Anatomie, physiologic et histoire naturelle des Galeodes " Comptesrendus d I acad. dc,scienecs,-X^Yl.,m,S. Th. Hutton, " ObservSs Xnll'lsS ^ ^'-^^ge «P^«ie« of Galeodes," Ann. and Mag. o/ Tt mZ Fig. 413.— Qaleoden oraneoides (regne animal). 512 ONYCHOPHOEA. means of tnieheoe, which open to the exterior by foui- slit-like openings between the first and second pair of thoracic appendages and on the ventral surface of the abdomen. They live in warm, sandy localities, especially of the Old World. They are nocturnal in their habits, and are feared on account of their bite. Fam. Solpugidae. Snlptuja QGalcodrs) arancoidcs Pall., found on the steppes of the A^olga and in ISouth Russia. Other larger species are found in Africa, and some forms are known in America. Class III.—ONYCHOPHORA * (PROTOTRACHEATA). Tracheata with elongated vermiform body, two antennce, and short paired imperfectly-jointed legs armed with claws. Fig. 414. — Peripatus capemU (after Moseley). The Onychophora, which are represented by the single genus Peripatus, have a moderately elongated body, which is provided mth paired legs (from fourteen to more than thirty pau-s), each armed with two small claws (fig. 414). The head is distinct, and bears a pair of antennfe and two simple eyes. On its under surface the mouth is placed beneath a large projecting suctorial lip, and is fur- nished -ttdth a pail* of jaws armed with chitinous claws. On each side of the mouth short, indistinctly jointed oral papillae are attached to the sides of the head. The nervous system is distinguished by the re- markable separation of its two halves. The paired cerebral g-anglion gives off two nerve trunks, which indeed approach each other closely * E. Grube, " Ueber den Ban des Peripatus Edwardsii." Jliiller'x Archiv.. 1853. Moseley, " On the Structure and Development of Peripatus capensis." Phil. Tram., 1875. [F. M. Balfour, " On the Structure and Develoi)ment of Peri- patus Capensis," Quart. Jovrn. of Mic. Science, 1888.] Fig. 415. — Head of a Peripatus embryo (after Moseley). An, Antennfe; K. jaws, anterior to which are the ecto- derm thickenings, which will form the brain. PERIPATUS. 513 beneath the oesophagus, but, soon diverging, remain widely separate for the rest of their course. They are without ganglionic swellings ; are connected together in their whole length by fine transverse commissures, and finally unite with each other over the rectum at the end of the body (fig. 416). The alimentary canal begins with a muscular pharynx, and runs in a straight course. The anus is terminal. A dorsal longitudinal vessel probably functions as heart. [A pan- of elongated unbranched glandvdar tubes, the salivary glands, open into the buccal cavity,] Moseley discovered a well-developed tracheal system, the stigmata of which are distributed ovei- the whole surface of the body. The tracheal trunks are delicate tubes, which are distributed upon the viscera in fine tufts. Long slime glands (considered as testes by Grube) open on the oral papillae ; they produce an exceedingly sticky fluid, which the ani- mal ejects when irritated. The Onycliophora are, ac- cording to Moseley, of separate sexes. The ova- ries are united to form one structure placed in the middle line on the dorsal side of the intestine, near the hind end of the body. There are two long ovi- ducts, which function as uterus and open by a common aperture on the ventral surface close to the hind end of the body (fig 416) The testes are paix-ed and egg-shaped, and lie towards the hind end of the body. The vasa deferentia are coiled and unite to for common duct which opens at the same place as do the female (fig 417). The eggs develop in the uterus. t --^^^tomy of a female Teripatm (after Moseley). -P, Antenna; G, brain with the ventral nei-ve cords (J'c); Ph, pharynx - D intestine; A, anus; 8d, slime Rland-'o«' ovaries ; Od, oviduct ; V, uterus ■m a organs 33 514 ONYOIIOPHORA — MYEIAPODA. [Segmental organs or nephridia, resembling those of Annelids, are found one pair in each segment. They open externally at the base of the legs and internally into the body cavity. The body cavity is divided into four parts by three septa — (1) into a dorsal section containing the dorsal vessel, (2) a main central division containing the alimentary canal, slime glands and generative oi-gans ; and (3) two lateral compartments, wliich are continued into the legs and con- tain the salivary glands, segmental organs and ventral nerve cords.] The animals live in damp places beneath decaying wood, [They are viviparous; in Peripatus Ca- pensis the period of gestation is 'Prf Jill 1? eleven or twelve months, the young being born in April and May.] Fam. Peripatidae. Peripatus Edward- sii Blanch., P. cajjemis Gr. ff Class IV.— MYRIAPODA.* Tracheata with separated head and numerous fairly similar seg- ments. They have one pair of an- tenwe, three pairs of jaws, and numerous pairs of legs. The Myaiapoda of all the Arthro- 2}oda present the greatest resem- blance to the Annelids, in the serial similarity of the segments, in the possession of an elongated, some- times cylindrical, sometimes flat- tened body, and in the mode of locomotion. In fact, they bear much the same relation to the An- nelids that the Snakes do to the vermiform fishes amongst the Yertebrata. * J F Brandt, "Recueil des memoires relatifs a I'ordre des Insectcs Myria- nnclps"" St Petersbourg, 1841. G. Newport. " On the organs of reproduction Kd the de;eloiSent of' the Myriapoda,^ Phil. Tram., 1841. Koch " System der Myriapoden, Regensburg, 1847. M. Fabre ; j"^ Z r organs reproducteiu'S ct sur developpment des Myriapodes Ann.dr.j. kc. .\at ?V Ser., Tom. III. Fr. Meinert, " Danmarks Clnlognather ' Natnrh fuh- skrift S R., Tom, V. ; and " Scolopcndrer og Lithobier, Ihid., Tom. V IbW. Latzei, " Die Myi-iopoden dor ostcrreichisch-ungarischen Monarchie f ]^ Chilopoden,'' Wien, 1880. Erich Haase, " Schlcsiens Chilopodcn, Breslau, 1880, 1881. Fig. 417.— Hind end of a male Peri- patus (after Moseley). T, Testes; Pr, prostate gland ; Vd, vasa defer- entia; Be, ductus ejaculatorin 8 ; D, rectum ; Vc, venti-al nerve trunks. MYBIAPODA. 515 The head of the Myi-iapods corresponds closely with that of the Insects, and, like the latter, bears a pair of antennte, the eyes, and two or in the Chilopoda three pairs of jaws. The antennse are placed on the frontal region, and are usually filiform or setiform. The strongly-toothed mandibles resemble those of Insects, and, like the latter, are without palps. The maxillfe in the Chilognatha have the form of a complicated lobed oral valve (fig. 427 h), the pai'ts of which were formerly supposed to represent two pairs of maxillfe fused together ; while in the Chilopoda they consist of a single blade bearing a short palp (fig. 425). In rare cases the mouth parts are transformed into a suctorial apparatus [Polyzoniiom). The body is composed of similar and distinctly separated segments, the number of which varies considerably in different species, but is usually constant for the same species. The segments bear paired appendages, and a strong dorsal and ventral plate (tergum and sternum) may often be distinguished. Although the segments of the body are so much alike that it is impos- sible to fix a limit between thorax and abdomen, fig. still certain features of the internal organisation, especially the fusion of the three first ganglia of the ventral chain, show that we must regard the three anterior body segments at least of the Chilognatha as constituting a thorax. In the Chilognatha a single pair of legs is attached to each of the first three to five body segments ; each of the following segments, on the other hand, bears 418. — Scolopendra morsitans. Fig. 419.— lulus terrestrit (after C. L. Koch). almost invariably two pairs, so that they may be regarded as double segments, formed by the fusion of two somites. The legs may be attached to the sides of the somites (Chilopoda), or nearer the middle line of the ventral surface (Chilognatha), and are usuaUy short with from SIX to seven joints, and terminate with claws (figs. 418 and 419) In their internal structure the Myriapods closely resemble the MYRIAPODA. Insects. The nervous system is distinguLslied by the gi-eat elongation of the ventral ganglionic cord, which runs along the whole length of the body and is swollen in each segment to foi-m a ganglion. According to Newport, there is a system of paired and unpaii-ed visceral nerves, like those of Insects. Eyes are only rarely wanting, and are usually present as ocelli which are sometimes closely packed together, oi- rarely (Smtigera) as peculiarly-formed facetted eyes. The alimentary canal, vnth rare exceptions (Glomeris), takes a straight course through the entire length of the body, and opens by the anus in the last segment. The following parts can be distin- guished : — a narrow cesojjhagus beginning with the buccal ca\dty and, as in Insects, receiving the contents of two to six tubular salivary glands ; a wide, very long mesenteron, the surface of which is closely beset with short hepatic tubes projecting into the body cavity; a hind gut, which receives two or four Mal- pighian tubules, the latter being coiled round the intestine ; and finally a short and wide rectum. The central organ of the cii-- culation is a long pulsating dor- sal vessel, which extends through all the segments of the body (fig. 420). It is divided into a gi*eat number of chambers, which cor- respond to the segmentation and, in Scolopendra, are attached to the dorsal wall by alary muscles to the right and left (fig. 420, If). The blood passes from the body cavity through lateral paired slits' into the chambers of the heart, and is tlience driven, partly through paired lateral arteries and partly through an anterior cephalic aort-a which divides into three branches, to the organs of the body CiiA-ity, from which a blood sinus, embracing the ventral ganglionic chain, is separated off. All Myria2)ods breathe by means of tracheae. These, as in Insects, receive the air from the exterior through paired slits, which are found in almost every segment (sometimes beneath the basjil jomts Fig. 420. — Head and anterior segments of Scolopendra (after Newport). 6. brain ; 0, eyes ; A, antenna; ; Kf, max- illiped (poison-claw) ; C, heart : M, alai-y muscles of the heart; Av, arteries. MYEIAPODA. 517 of the limbs, sometimes in the connecting membranes be- tween the sterna and terga) ; and they give off bunches of tracheae, which bi-anch and are distributed to all the organs. Generative organs. — ^The Myriapocla are dioecious. The ovaries and testes usually have the form of long unpaired tubes, while their ducts are often paii-ed and are always connected with accessory glands, and in the female are sometimes provided with a double receptaculum seminis (fig. 421). The genital open- ings lie on either side on the coxal joints of the second pair of legs, or behind this pair of appendages {Chilognatha), or, as in the Ghilopoda, there is an unpaired genital opening at the posterior end of the body (fig. 422). In the male sex amongst the Chilognatha there are often external copulatory or- gans* on the 7th segment, remote fi-om the genital open- ings. These become full of sperm before copulation, and during the coitus introduce it into the female genital open- ing. Development.— The fe- males are usually larger than the males, and lay * Besides Fabre I.e., compare Voges, "Beitra^e zur Kenntiiiss dcr Juliden," Zcitschr. filr mss. Zool, Tom XXXI., 1878. Fig. 421. — Generative organs of Glomeris marginata (after Fabre). T, Testis; Oo, ovaries; Od, Oviduct. IG. 422.— Generative organs of Scolopendra com- planata (after Fabre). T, Testis; Vd, vas Oeferens ; Br, accessory glands ; Sb, loop of tlie vesicula seminalis ; Ov, ovary. 518 MYETAPODA.. their eggs in earth. The just-hatched young often pass through a metamorphosis, having at first only three or seven pairs of legs in addition to the antennae, and a few somites without Umbs (fig. 423). The young animals undergo numerous moults, and gi-adually increase in size; the extremities sprout out on the somites, which are already present. New somites are constricted off" from the termi- nal one until the full number is completed; the number of ocelli and of the joints of the antennae is increased, and the resemblance to the sexual animal is gi-adually perfected. In othei- cases [iScolo2)e7icha, GeojMliclce) the embryo already possesses the full number of appendages. Fig. 423. — Embryo of Sfrongylosoma (after E Metsclmikoff) . Order 1. — Chilopoda.* Myria'poda of usucdly flattened form, with long many-jointed antennae, and mouth imrts adapted for predatory habits, with only one pair of appendages to each segment. The body is long and usually flat- tened. The chitinous exoskeleton is hardened on the dorsal and ven- tral surface of each somite, consti- tuting the tergal and sternal plates, while on the sides of the somites it remains soft. In certain forms some of the terga develop to large shields, which over-lap the smaller terga of the intermediate somites (fig. 424). The number of legs is never gi-eater than that of the sepa- rate segments, a smgle pair only being developed on each segment. The antennae are long and many- jointed, and are inserted bene;xth the L. Koch). Kf, Poisou claws. frontal margin. The eyes are simple or aggi'egated ocelli, except in the genus Scutigera which has facetted * Newport, " Monograph of the Class Myriapoda, order Chiloi)oda," Linnaan Transactions, XIX. Fig. 424. — Litliobius forficatus (after C. CHILOPODA. 519 eyes. There are always two pairs of jaws (fig. 425) ; the mandibles {Md) and one pair of maxillae (ITcc'), the latter bearing a short palp. In addition, the first paii- of (thoracic) legs {M^') forms a kind of underlip which often bears two long palps. The next paii^ of legs always approaches the head as a kind of maxilliped, and forms by the gi-owing together of its basal' parts a consider- able median plate, on the right and left of which great four-jointed poison claws {Mf) project. The remaining ap- pendages arise from the sides of the body segments, the last pair being frequently elon- gated so as to project back- wards far behind the last seg- ment. The generative organs open by a single aperture at the hind end of the body. There is no male copulatory appai-atus. The young, when hatched, have seven pau-s (Lithohius) or the entire number of appen- dages {Scolopenclra). The ChUopoda feed entirely on animals, which they bite with the poison claws and kill by the secretion of the poison gland which flows into the wound. Certain tropical species of large size are able to inflict wounds which are dangerous even to man. Fam. Scolopendridae. Antennae long and thin with a relatively small number of joints, only a few ocelli. The seg- ments of the body are sometimes equal, sometimes unequal. Scolopenclra (with' nine pairs of stigmata) gigantea L., found in the East Indies. Sc. morsi- tans, from South Europe. Geojyliilvs ■mhtervanevs, eleetricvs L. Fam. Lithobiidae. With long, many- jointed antennjB and numerous ocelli. Some of the terga are greatly developed, and partially over-lap those of Fig. 425. — Oral apparatus of Scolopendra mtitica (after Stein). Oh, Upper- lip; Md, mandibles; Mx', maxilla ; Mx'', first paii- of legs or second maxillas ; Mf, poison claws (maxilliped) ; Ta, pulp. Fifi. 426. — Mouth parts of acophihts (Carus, Icones). K, Maxilte; Mf, maxilliped. 520 MTRIAPODA. Lithohivs forficatuH L., witli fifteen pairs of the intermediate segments, legs. Fam. Scutigerid*. The antenna; are at least as long as the ho