ZOOLOGY FOR MEDICAL STUDENTS MACMILLAN AND CO., LIMITED LONDON • BOMBAY • CALCUTTA • MADRAS MELBOURNE TI1K MACMILLAN COMPANY NKW YOKK • BOSTON • CHICAGO DALLAS • SAN FRANCISCO THK MACMILLAN CO. OF CANADA, LTD. TORONTO ZOOLOGY FOR MEDICAL STUDENTS BY J. GRAHAM KERR REGIUS PROFESSOR OF ZOOLOGY IN THE UNIVF.RSITY OF GLASGOW MACMILLAN AND CO., LIMITED ST. MARTIN'S STREET, LONDON 1921 COPYRIGHT PREFACE THIS volume represents in book form the Lecture course in Zoology for medical students as it has evolved during recent years in the University of Glasgow. The task of designing and conducting a course in Zoology that will play its proper part in the education of the graduate in medicine is no light one. It is made immensely heavier by certain conditioning factors, above all in the case of our Scottish universities, by the compulsory limitation of the course within a period of ten weeks. This renders it necessary for the teacher to confine himself rigidly to those parts of the subject which can really justify the expenditure upon them of the necessary time from the short period available. In deciding what portions of the vast science of Zoology satisfy this condition, the teacher has to be guided by certain governing principles. Above all he must have clearly denned in his mind what he regards as the main objects of his course. So far as the present writer is concerned he has kept before him three objects which he believes to be of pre- eminent importance. I. To awaken and develop, so far as the animal kingdom is concerned, interest in biological science. Medical students are training themselves to be efficient practitioners of a particular department of applied biology. It is of vital importance that they should become inspired at the earliest possible stage in their curriculum with a living interest in the study of the animal body. One of the first endeavours then of the teacher of Zoology should be to cast over the minds of his pupils some of the fascination of the most fascinating of sciences, so that they may pass on their way quickened and inspired by the stimulus of its interest. II. To lay an adequate foundation for the superstructure of detailed knowledge of the animal body imparted in the courses of Anatomy, vi ZOOLOGY FOR MEDICAL STUDENTS Physiology, and Pathology, with their clinical applications in the later parts of the curriculum. 111. To provide a reasonably up-to-date account of the more im- portant animal parasites, more especially of the pathogenic microbes of animal nature, and of the ways in which they are carried or harboured by members of the animal kingdom other than man. Of these objects the last mentioned, though seeming at first sight to be very important, is actually of far less importance than the first two; for specialized knowledge of the type indicated can readily be added at a later stage, provided always that a sound foundation of general zoological knowledge has been laid. Many teachers indeed favour its relegation to a special course at a later stage of the medical curriculum. The present writer is on the whole in favour of retaining it in the general course in Zoology: (i) because the later parts of the curriculum are already much complicated by the multiplicity of subjects, and tend to be- come more and more so with increase of specialization ; (2) because various parasitic forms of life are best studied along with their free-living allies; and (3) because many of the animal organisms that would naturally come into such a specialized course can quite well be made use of in the general course. The course represented by this book is preponderatingly morphological, and this for two reasons. Firstly, because our knowledge of the morpho- logical features of the lower types of animal is much more advanced than our knowledge of their physiology. Secondly, because morphological study affords an intellectual discipline better adapted to the needs of the elementary student than that afforded by physiology. The student observes structural features in the laboratory, and he records his observa- tions in the form of drawings. He receives valuable training in observa- tion and in the interpretation of observations ; and he is able to compare what he observes with what he is told or reads. When, on the other hand, he tries to make physiological observations he finds even in the <>! the simplest phenomena that behind these phenomena are at work unseen powers and factors, unobservable and yet perhaps all- important . He is taught that particular phenomena are due to metabol- ism or to some obscure process of a physical or chemical character — but lie is concerned, transcendental, and as far beyond his powers of comprehension or criticism as if they were the direct result ncy. PREFACE vii The general lecture course in Glasgow is accompanied by laboratory work extending over a hundred hours. In great part this follows the usual lines such as are laid down in Marshall and Hurst's Text-book, but a special feature is made of the study of a valuable series of demonstra- tion specimens. This includes the study, under high-power immersion objectives, of such organisms as Trypanosomes, Malarial Parasites, Leish- manias, and Spirochaetes. Experience has shown that students fully appreciate the privilege of being able to examine such preparations for themselves, and that they take the greatest care not to do damage. Op- portunities are also given for seeing Trypanosomes, Miracidia, Cercariae, and so on, in the living condition. This demonstration part of the course is regarded as being of special value in arousing and gripping the interest of the student. Time limitations have made it necessary to exclude the anatomy of the higher vertebrates from the course in Zoology. This has been done with regret for, apart from the intrinsic interest of the comparative anatomy of the higher vertebrates, it is of advantage to the student to obtain a superficial view of mammalian structure — such as he gains by the dissection of a rabbit or rat — before he submerges himself in the .minute detail of human anatomy. But time limitations are inexorable, and the time available for the regulation curriculum in medicine being what it is, the indications seem to the present writer clearly to point to the necessity of restricting the anatomical study of the higher verte- brates practically to the anatomy of man. If the student of medicine desires, as he ought, to broaden his anatomical outlook by excursions into the comparative anatomy of the higher vertebrates, these will come most profitably after he has spent some time at human anatomy. In Glasgow special lectures on Vertebrate Morphology for such students are given during the summer term. Another short supplementary course of lectures deals with the Theory of Evolution — The Evidences of Evolution, Inheritance, Variation, Natural Selection. Attendance at this course is voluntary, and its subject matter is not included in the medical degree examinations, but a large proportion of students attend it after going through the regulation course. The illustrations in this book have been drawn by my friend, Mr. A. Kirkpatrick Maxwell, to whose artistic skill I am again under a deep debt of gratitude, " Line " has been employed rather than " half- viii ZOOLOGY FOR MEDICAL STUDENTS tone/' for the practical reason that line diagrams are more easily copied. So long as examinations retain the place they hold at the present time in university curricula, it may be assumed safely that students will continue to use this method of impressing anatomical facts upon the memory. In conclusion, I have to express my indebtedness to three friends and colleagues — to Mr. James Chumley and Dr. M. Taylor., who have read the whole volume in proof, and to Dr. J. S. Dunkerly, who has given me the advantage of his advice and criticism as regards the chapter on the Protozoa. J. GRAHAM KERR. 28th June 1921. CONTENTS CHAPTER I PA(iH PROTOZOA i CHAPTER II METAZOA — INTRODUCTORY REMARKS : COELENTERATA . . 84 CHAPTER III PORIFERA ... . . Il8 CHAPTER IV ANNELIDA . . . . . . .129 CHAPTER V THE PARASITIC WORMS . . 159 CHAPTER VI ARTHROPODA ... . . 209 CHAPTER VII MOLLUSCA . . . . . . .267 ix x ZOOLOGY FOR MEDICAL STUDENTS CHAPTER VIII PAGE Kl'HINODKKMATA ....... 282 CHAPTER IX INTRODUCTION TO THE VERTEBRATA : DESCRIPTION OF THE DOGFISH AS ILLUSTRATING THE GENERAL STRUCTURE OF A VERTEBRATE . . . . . .291 CHAPTER X KISIIKS . ... . 347 CHAPTER XI INTRODUCTION TO TETRAPODA : AMPHIBIA . . . 402 CHAPTER XII AMNIOTA : REPTILIA, AYES . . . . -419 CHAPTER XIII MAMMALIA ...... 436 CHAPTER XIV KI.I.MKXTS OF VERTEBRATE EMBRYOLOGY . 449 INDEX . 477 CHAPTER I PROTOZOA IN commencing the study of Zoology it is advisable to do so by making a study in some detail of one particular type of animal, so as to obtain definite and precise data to serve as a foundation for further knowledge. It is also desirable to select as the object of this study some animal possessing the minimum complexity of structure so that the beginner may easily grasp from it what are the fundamental features of animal organization. Such a creature is the little animal called Amoeba proteus which is to be found still surviving on the Earth to-day,, as a not uncommon inhabitant of freshwater pools and slowly moving streams, although it represents a relatively very early stage in the evolution of animal life. AMOEBA PROTEUS The Amoeba consists of a minute particle just visible to the naked eye — measuring up to about -6 mm. in diameter — of that substance or mixture of substances which we call protoplasm and which Huxley used to speak of as the " physical basis of life " — because the condition which we call life occurs, so far as we know, only as an attribute of this substance. Wherever you find life there you find protoplasm. It would be of course of tremendous interest to obtain exact knowledge of the chemical and physical constitution of protoplasm because once this knowledge was obtained it would be but a comparatively small step farther to produce protoplasm — to make it in the laboratory. But unfortunately this knowledge has eluded — and probably must for ever elude — discovery, for the very first steps in the investigation — the first steps of chemical analysis — are such that they deprive the protoplasm of its all-important characteristic — that of living — the property which marks it off from all other substances. It is naturally of interest, though of relatively minor interest, to i B ZOOLOGY FOR MEDICAL STUDENTS CHAP. ps. inquire as to the composition of the remains of the protoplasm which are left when the life has departed from it. Analysis shows that these consist of a mixture of those complicated substances known to the chemist as proteins — compounds of Carbon, Oxygen, Hydrogen, Nitrogen and Sulphur in about the following percentages : €52, 023, Hy, Ni6, 82. Chemists are not yet able to tell us precisely how the atoms of these various elements are united to form the very complex molecules of the various kinds of proteins. The student of living things is, for the reason indicated above, to a great extent debarred from using what to the Chemist or Physicist is his most powerful instrument of in- vestigation, the method of analysis, whereby the complicated subject of investigation is- split up into its simpler components and these studied individually. He is therefore driven to make his main stand-by the method of mere observation— in the case of small creatures like Amoeba with the aid of the microscope. When the living Amoeba is observed under the microscope it is seen (Fig. i) to form an irregularly shaped blob of protoplasm — greyish white when seen against a dark background, clouded and finely granular when seen against a light background. The irregularity of form is characteristic and no two specimens are exactly alike. Examination under a high magnification shows that the blob of protoplasm is not homogeneous but consists of a main larger portion known as the cytoplasm, and embedded in this a rounded denser portion —the nucleus (Fig. i, n). The cytoplasm is further seen to consist of a main portion known as the endoplasm (Fig. i, en) and a thin superficial layer — the ectoplasm , ec). 01; these the endoplasm is fluid in its nature and is laden with minute particles which give it its very characteristic granular appearance. By the use of very high magnifications and the application of appropriate tests it can be determined that these granules differ in character. Some are droplets of fat, some are crystals of waste c.v. FIG. .\iimcba proteus. c.v, Contractile vacuole ; ec, ectoplasm ; en, endoplasm ; /.;', food-vac- uolc ; n, nucleus ; ps, pseudopodium. i AMOEBA 3 material (such as Calcium phosphate), some are drops of watery fluid and these when they are of considerable size are known as fluid vacuoles. Small bubbles of Carbon dioxide (gas vacuoles) are sometimes to be recognized in other kinds of Amoeba but they are exceedingly rare, if they occur at all, in Amoeba proteus. The ectoplasm differs from the endoplasm in that it is less fluid, more highly refracting, without enclosed granules and of a clear glassy appearance. It is usually very thin and its outer surface normally covers itself with a thin coating of slime which is of importance in the creature's movements. The nucleus is a dense, fairly solid, highly refracting body — colourless like the rest of the protoplasm and showing a characteristic uniformly mottled appearance. This mottled appearance is due to its being com- posed of two different substances the one denser and more highly refracting than the other. The former, consisting of a complicated mixture of proteins which are specially rich in Phosphorus, is characterized by a special affinity for certain stains or dyes. When the dead Amoeba is subjected to the action of these stains this portion of the nuclear material becomes stained specially deeply and the substance of which it is com- posed has consequently been given the name chromatin to distinguish it from the less deeply staining achromatin. Continued observation of living specimens of Amoeba brings out many other points besides those already mentioned. The living Amoeba shows almost constant movement — movement of a kind so characteristic that although it occurs in various other animals it is known technically as amoeboid movement. This movement consists in the pushing out of portions of the body surface into projections known as pseudopodia — usually with blunt rounded ends.1 The pseudopodia are pushed out from any part of the surface indifferently and their tips end freely, never fusing with one another. If a single pseudopodium is carefully watched during its protrusion it is seen that its centre is occupied by an outwardly rushing stream of endoplasm. The ectoplasm which bounds it may often be observed not to reach quite to the tip — the extending tip being formed of granular endoplasm. Such endoplasm, however, exposed on the surface of the pseudopodium to contact with the water, is seen gradually to lose its granular character and take on the appearance of ectoplasm. In some kinds of Amoeba other than Amoeba proteus what are termed eruptive lobopods are formed. When an eruptive lobopod is protruded what 1 Pseudopodia with blunt rounded ends are sometimes called lobopods to distinguish them from other types of pseudopodia such as will be mentioned later, 4 ZOOLOGY FOR MEDICAL STUDENTS . CHAP. happens is that the ectoplasm ruptures and a quantity of endoplasm wells out at the perforation. This extrusion forms then an extension of the body of the Amoeba composed at first entirely of endoplasm (Fig. 2, A). Very soon however the superficial layer loses its granularity, be- comes highly refracting and assumes the characters of typical ectoplasm. Portions of the original ectoplasm covered in and sheltered from contact with the water by the newly formed lobopod gradually fade away, losing the characters of ectoplasm and taking on those of endoplasm (Fig. 2, B). We are taught by such observations as these that ectoplasm and endo- plasm are not fundamentally distinct substances, and that the characters which normally differentiate ectoplasm from endoplasm are merely temporary modifications taken on by the surface layer of the cytoplasm as a reaction to its contact with the surrounding water. The pseudopodia are not permanent organs : they are pushed out A B FIG. 2. Illustrating the formation of an eruptive lobopod. as has been described and they may be drawn in, apparently by the shrinkage of their containing layer of ectoplasm. Although they may be pushed out from any part of the surface indifferently, their formation is as a rule at any one moment of time taking place more actively on one particular portion of the surface. The result is that, as the pseudo- podia are pushed out more actively in one particular direction, the Amoeba undergoes change of position towards that direction, pseudopodia pointing in other directions shrinking in and disappearing. There are few sights more impressive to the eye that has a brain behind it than that of a large healthy Amoeba with its protoplasm actively streaming out into its pseudopodia, carrying on that old world type of movement, just as it in all probability took place at the dawn of evolution when living substance first began to move. Although the Amoeba can push out its pseudopodia when floating freely suspended in the water it can creep along as a whole only upon a i AMOEBA 5 solid surface. Further the coating of slime over the surface of the ectoplasm is apparently also essential to creeping movement. The actual rate of creeping appears to be anything up to about one to three milli- metres in an hour. Besides the ordinary fluid vacuoles which may be present in the endoplasm there exists a contractile vacuole. This makes its appearance — usually towards the part of the Amoeba whrch is hindmost when it moves — in the form of a small droplet of water which slowly and regularly increases in size till it is about as large as the nucleus. When it reaches its limit of size it is suddenly obliterated and if the water surrounding the Amoeba contains solid particles, e.g. of indian ink or carmine, these can be seen to be pushed aside, the water of the contractile vacuole passing out to the exterior. Presently a new droplet appears in the position of the vacuole, and this increases and collapses just as before. There goes on in fact a regular rhythm of expansion (diastole) and con- traction (systole) the complete cycle occupying commonly from *$ to 8 minutes. The contractile vacuole is constantly pumping water from the protoplasm of the Amoeba to the exterior — water no doubt being absorbed by the general surface of the creature to make good the amount withdrawn. The living protoplasm is thus constantly being flushed with watery fluid by the activities of the contractile vacuole. The purpose of this seems to be mainly in connexion with two great functions of living matter — respiration or breathing and excretion or the getting rid of waste products. The incoming water brings with it supplies of the Oxygen which is essential to all living activity : the outgoing water carries with it the Carbon dioxide and other more complicated waste substances which are constantly being produced by this living activity. It is said that the fluid from the contractile vacuole exhibits a curious property outside the body of the Amoeba — that of causing bacteria living in the water to agglutinate or become clumped together in solid masses, and so rendering them more easily taken in by the Amoeba as food. The actual process of feeding is difficult to observe for it takes place preferably in the dark or in very faint light. The Amoeba feeds on solid particles of suitable food material such as small plant or animal cells. If the Amoeba comes up to one of these it seems as it were to flow round it (Fig. 3,/) sending out extensions of its protoplasm on all sides of the food particle which meet beyond it and enclose it with a small quantity of the surrounding water. The food particle comes thus to be shut up in a drop of water — the food-vacuole (Fig. 3, f.v) — within the endoplasm of the Amoeba. ZOOLOGY FOR MEDICAL STUDENTS CHAP. n. Sometimes the food-organism is in the form of a long slender thread as is the case with so many of the simpler water weeds or algae. In such a case the Amoeba goes to work in rather a different fashion. Surrounding the filament it spreads along it and gradually draws it inwards so that eventually the filament is coiled up into a mass within the endoplasm. Following on the process of ingestion in which it is taken into the body of the Amoeba the food particle undergoes a series of changes. If an actively moving organism its movements are seen to continue for some little time within the food-vacuole but presently they cease and the food- organism is evidently dead. Chemical testing at this stage shows that the watery fluid within the food- vacuole has become strongly acid. Acid has been formed, or secreted as the technical expression is, by the surrounding protoplasm of the Amoeba and poured into the food- vacuole. This acid secretion has apparently for its sole function the killing of the food-organism. There now ensues the process of digestion. The green vegetable cell, as we assume the food-organism to be, gradually changes colour becoming dark green, yellow, yellowish red, brown, brownish red. The cellulose wall breaks down and the whole body of the food-organism disintegrates. The protoplasm of the Amoeba has secreted into the food-varuole digestive ferments. Ferments or enzymes are a remarkable class of substances produced by the living activity of animals or plants, which have the mysterious power (" catalytic " power) of inducing or hurrying up specific chemical changes in substances with which they are in contact. In this particular case the main ferment at work is one which causes the dead protoplasm of the food-organism to break up into simpler • Iicmical compounds which the living substance of the Amoeba is able to absorb and make use of in building up new protoplasm of its own, for it appears to be the case that living protoplasm is never able simply to add to itself living protoplasm directly. The latter must be killed and ted, i.e. broken down into simpler substances, before it can be made FIG. 3. i' MI of a siualJ animal by an Amoeba. c.v. Contractile vacuole ; /, food-organism ingestion; f.v, food-vacuole; H, imc.lrllS. I AMOKIJA 7 use of for building up additional protoplasm for the animal that feeds on it. An important detail to notice in the case of the Amoeba is that the fluid in the food-vacuole when this digestive ferment is at work is found to have lost its acid reaction and to have become distinctly alkaline. In the fact of its working in an alkaline medium the ferment resembles what is known in the higher animals as the tryptic type of digestive ferment. Each type of ferment has an absolutely restricted and specific type of action and it is believed that the breaking up of the cellulose wall of the vegetable cell is brought about by a second ferment quite different from that which digests the protoplasm. As the process of digestion goes on the contents of the food-vacuole become completely disintegrated. All that can be made use of for building up new protoplasm is absorbed by the living protoplasm of the Amoeba, and there eventually remain in the vacuole only particles of useless debris — faecal matter. Finally the food-vacuole is seen gradually to approach the surface : it touches the surface film of protoplasm and bursts, and the Amoeba proceeds on its way leaving the little heap of faecal matter behind. Before leaving the process of digestion it may be well to accentuate the fact that the killing of the food-organism is an essential preliminary to the process, for it is a remarkable fact that living protoplasm is com- pletely immune to the action of the digestive ferment. Thus the proto- plasmic walls of the food-vacuole though in immediate contact with the digestive ferments are completely unaffected by them, because they are alive. As the Amoeba feeds and keeps on building up new protoplasm it naturally increases in bulk : it grows. Now it is fairly clear that, the organization of Amoeba being what it is, prolonged growth would neces- sarily lead to an impossible state of affairs, for the Amoeba must have a sufficiently great area of surface by means of which it can feed and absorb water and oxygen and get rid of waste products. But the more it increases in bulk the smaller in proportion would become its surface, until soon it would be quite inadequate for the animal's needs. Again, the fluid cytoplasm of the Amoeba would obviously lose all cohesion and be quite unable to retain its form were it to be otherwise than of very small dimensions. Consequently there has to exist in nature some corrective to the process of growth which will ensure the Amoeba's not reaching too large a size for efficiency. This corrective is found in the process known as fission, by which the body of the Amoeba becomes nipped across so as to form two individuals. The process is inaugurated by the nucleus dividing into two and ZOOLOGY FOR MEDICAL STUDENTS CHAP. this process is of the complicated kind known as mitosis or karyokinesis, characterized above all by the chromatin becoming condensed in the region of the equator of the nucleus into a definite number of rounded or rod-shaped pieces called chromosomes, each of which divides into two luilvi-s. these being distributed to the two daughter nuclei. The details of the process are obscure in Amoeba and its allies and therefore tlu-ir description will be held over until we are dealing with a group of animals in which they are more distinct (Chap. V.). The process of mitosis having been completed (Fig. 4, A-D) the D FIG. 4. 1 -i-Mon of Amoeba (nuclear detail after observations by L. A. Carter). A, Amoeba before the onset of fission ; B, most of the chromatin has become concentrated in chromosomes about the equator of the nucleus ; C, each chromosome has divided into two, the two sets of daughter chromosomes are moving apart and the boundary of the nucleus is becoming indented between them ; D, the nucleus has heroine completely divided into two daughter nuclei; E, the two nuclei have moved apart and the cyt<.]>lasniie. body of the Amoeba is undergoing constriction ; F, the process has been il. Amoeba takes on an elongated form and its cytoplasm becomes con- st ri< led across between the two daughter nuclei (Fg. 4, E). As the • •onstriVtion deepens the isthmus connecting the two masses of cytoplasm l>eromes narrower and narrower until finally it snaps across, leaving in place of the original Amoeba two Amoebae of half its size which gradually move apart and lead their own lives (Fig. 4, F). In this process of fission which serves in Amoeba as a corrective to the process <>l growth we see a good example of the simplest of all types of reproduction, in which increase in the number of individuals is brought i AMOEBA 9 about by a parent individual simply resolving itself into two daughter individuals. This simple process of fission is not the only method of reproduction occurring in Amoeba proteus. During the winter season when conditions are unfavourable to the continued activity of the Amoebae they hibernate in the seclusion of a protective cyst which they form round themselves and during this period of seclusion they reproduce by ;i method other than that of simple fission. At the commencement of this period of encystment the Amoeba is seen to become sluggish in its movements and there accumulates over its surface a thick mass of sticky slime mixed with particles of debris — derived partly from the mud outside, partly from solid faecal matter which the Amoeba extrudes completely at this time. The Amoeba assumes a spherical shape and now secretes all over its surface a tough membrane, usually in two distinct layers with a space between. The three layers indicated — the outer slimy and the two inner membranous layers — constitute the protective cyst in which the Amoeba hibernates. During the period of encystment the nucleus of the Amoeba under- goes mitosis repeatedly until there is a large number of nuclei — commonly from 75 to 100 — and the cytoplasm now breaks up into as many pieces, each containing a nucleus. When conditions become favourable, in the early spring, these issue from the cyst as so many small Amoebae, which proceed to lead their normal life and gradually grow to the full size. What has been said up till now regarding Amoeba has been ascertained, by the method of simple observation, but it is possible to add considerably to the knowledge so obtained by making use of the method of experiment, i.e. by introducing some sudden change into the conditions under which the Amoeba is living and ascertaining how it reacts to the particular change of conditions. Various kinds of change may be made use of to serve as a stimulus to the Amoeba — mechanical, chemical, thermal, electrical — and so on. Of such it may be said in general that the Amoeba reacts to a really strong stimulus of almost any kind by retracting its pseudopodia and assuming a spherical shape. On the other hand a comparatively slight stimulus produces a reaction more or less specific to the particular type of stimulus. As a mechanical stimulus for example one may touch with a needle the edge of the Amoeba on the side towards which it is creeping : the Amoeba reacts by turning aside — altering the direction of its movement. Chemical change has been tried on a kind of Amoeba known as Amoeba Umax, characterized by the possession of a single blunt pseudopodium (Fig. 5, A). In this case making the water slightly alkaline was found to be followed by a change in the whole form of the Amoeba which protruded a number of slender tapering 10 ZOOLOGY FOR MEDICAL STUDENTS CHAP. pseudopodia and took on the appearance of what was supposed to be a quite distinct kind of Amoeba known as Amoeba radiosa (Fig. 5, B). A slight rise in temperature is reacted to by increased activity of move- ment but if the rise be continued up to about 30° C. the increased activity is only temporary and is followed by gradual slowing and eventually by death. Exposure to a temperature of 35° C. is followed rapidly by retraction of the pseudopodia and death. Reduction of temperature below the normal is followed by a slowing of the movement without any other apparent ill effects. Exposure to intense light is followed by rounding off and death. Even ordinary daylight produces a reaction. If the Amoeba is in a shadow it has been observed to draw back when it comes to the edge of the shadow,, and it is only in comparative darkness as a rule that such events of normal life as feeding or fission take place. The Amoeba shows interesting reactions to electrical con- ditions. If a very weak current of electricity is caused to pass through water containing indi- viduals of Amoeba Umax they are observed to creep towards the cathode or negative pole. Sudden re- versal 'of the current is followed by reversal of the movement of the Amoebae.1 Exposure to the emanations of Radium is soon followed by rounding off and death. An instructive experiment which has been made on Amoeba is the division of the living creature into two portions (merotomy). When the experiment is successfully carried out it is usually found that the smooth slippery nucleus slips to one side of the cutting edge and passes completely into one of the two portions of the cytoplasm. The result of the experi- ment then is the production of two Amoebas which may be almost •ly alike as regards size but which differ in this respect that one 1 Such movements of an organism or of parts of an organism in response to -p.-ridc stimuli are designated in technical language by special names ending in in 01 -taxis, Thus chemiotaxis (chemical stimulus), plwtutaxis or hdiotropism (light), galvanotaxis (electrical), etc. FIG. 5. Reaction of Amoeba Umax to slight alkalinity of water. A, before adding alkali (creeping type) ; B, after adding alkali (floating type). I AMOEBA ii consists entirely of cytoplasm while the other possesses in addition a nucleus. Subsequent observation shows that this difference as regards the possession of a nucleus is accompanied by striking differences in behaviour. The nucleated portion continues its existence as a perfectly normal Amoeba, performing all the vital functions — moving, feeding, growing, exactly as did the original individual. The non-nucleated portion may for a little seem to behave much as the other. It may push out pseudopodia ; if it does not contain the original contractile vacuole it may form a new one. But its movements are sluggish. It is unable to creep along a solid surface and investigation shows this to be correlated with its being no longer able to form the thin coating of slime which normally covers its surface. Again it may ingest food particles but these are no longer properly digested — the power of secreting the digestive ferments is apparently no longer present. Gradu- ally its activities diminish and within a week or ten days they come to an end in death. GENERAL REMARKS ON AMOEBA Having summarized the main facts regarding Amoeba it will now be convenient to glance back at these facts and see what general lessons are to be learned from them. Firstly we see that the facts fall into two main categories — facts of structure and facts of living activity or function. These two categories constitute the two primary subdivisions of zoological science : Anatomy, which deals with all details of structure, form, etc. — such details in fact as can be gathered from the investigation of the dead body — and Physiology, which concerns itself with all the various manifestations of life. Secondly Amoeba provides us with a good example illustrating the conception of what is known as a cell — a conception which permeates all biological science. A cell is " a mass of protoplasm containing a nucleus." An Amoeba consists of a single cell while the body of any of the larger animals is composed of vast numbers of cells derived by fission repeated over and over again from a single original cell — the egg. The cell does not merely enclose the nucleus but while alive its activities are to a great extent controlled by the nucleus. This again is illustrated by Amoeba for we have seen how the removal of the nucleus not only stops some of the striking activities of the Amoeba but after a short time renders life itself impossible. A further point of importance emerges from the experiment in which the Amoeba is cut in two. Careful and continued scrutiny of the nucleated portion shows that its nucleus undergoes a gradual diminution 12 ZOOLOGY FOR MEDICAL STUDENTS CHAP. in size until at last it bears just about the same proportion to the cyto- plasm containing it as it did to the original full amount of cytoplasm before any had been cut away. And this appears to illustrate a general principle namely that in any particular type of cell there is a fairly definite normal proportion in size between nucleus and cytoplasm, and it is suspected that the disturbance of this proportion may play an important part in the production of certain abnormal conditions met with in disease. We have seen that the movements of the Amoeba are of a characteristic type. Now it is an interesting fact that these movements can be produced artificially in non-living substance. It is possible by using special methods to produce extremely fine froth or foam composed of a mixture of slightly rancid oil and watery fluid. Minute droplets of this foam placed in water are seen when watched under the microscope to change their form, " pseudopodia " being pushed out from their surface and the whole droplet changing its position just as a live Amoeba would do. But in the case of these oil droplets the movement is capable of physical explanation. Any drop of fluid immersed in another kind of fluid with which it does not mix is subject to the laws of " surface tension." Its surface layer acts as if it were an elastic membrane always tending to shrink in area : the tendency of the drop therefore is to assume a spherical form — the form in which the proportion of surface to bulk is at its minimum. If a portion of the surface begins to bulge out and form a projecting " pseudopodium " the meaning of the phenomenon is that that particular part has had its surface tension reduced so that it gives way to the internal pressure due to the tension of the surface layer as a whole. Now there is no reason to doubt that the pushing out of the pseudo- podium of a living Amoeba is due similarly to weakening of the surface tension over that particular portion of the Amoeba's surface. But it must be borne in mind that this explanation of pseudopodium-formation while quite satisfactory so far as it goes is not by any means a complete explanation, for when we ask the question why should the surface tension in some particular region undergo the diminution which leads to the pushing out of a pseudopodium the only answer is that it is due to some process involved in the living activity of the cytoplasm regarding the nature of which we are quite ignorant. It is not merely the extrusion of pseudopodia which can be produced in non-living substance : the same applies to the gradual ingestion of a slender filament. If a drop of chloroform is brought into contact with a fine filament of glass-wool coated with shellac it is found that the surface ion of the chloroform gradually draws the glass filament into the i AMOEBA 13 interior of the drop of chloroform forcing it into a coil. There is again no reason to doubt that the ingestion of the vegetable filament by the Amoeba is brought about by the action of surface tension. The Amoeba, as we have seen, feeds and the products of digestion are absorbed and built up into new living protoplasm. Although normally this results in increase of bulk, growth, it does not do so neces- sarily. An Amoeba or other animal may go on absorbing food material and yet show no increase in size. This indicates that in the living body there goes on not merely a process of building up new protoplasm but also a breaking down of the existing protoplasm. While on the one hand relatively simpler substances derived from the food are constantly being built up into the enormously complex living protoplasm/at the same time the living protoplasm is undergoing a process of breaking down into less complicated non-living substances. The sum total of these processes constitutes what is known technically as metabolism. It is further customary to distinguish the building up processes by the name anabolism and the breaking down processes by the name catabolism. These names are of practical convenience but the student should guard from the commencement against the fallacy that attaching a long technical name to a phenomenon necessarily implies any increase in our knowledge concerning it. As a matter of fact very little is known regarding the intimate nature of metabolic processes. One of their characteristic features is their accompaniment by oxidation. The catabolic processes in fact are as it were accompanied by a slow process of combustion. In many of the larger animals with active metabolism the temperature of the body is actually raised considerably above that of the surroundings by this process of combustion. In a tiny creature like Amoeba this is not perceptible, but, no doubt, could we test its temperature by a sufficiently delicate and reliable method, we should find that even it is slightly warmed up by its oxidation processes. In the living creature at any one particular time there may be com- plete metabolic balance — the anabolic and the catabolic processes simply counteracting one another — or one of them or the other may preponderate. If anabolism preponderates the creature increases in bulk, if catabolism preponderates diminution of the living substance takes place. In the latter event there may be no visible shrinkage in bulk, for the diminution of living substance may be made up for by the accumulation within the body of bulky non-living substance formed by its breaking down. An important detail in connexion with the metabolism of Amoeba has to do with the nature of the food. This must contain ready-made proteins for the Amoeba is quite unable to build up its protein out of ,4 ZOOLOGY FOR MEDICAL STUDENTS CHAP. chemically simpler materials. And here we see one of the most striking differences between the animal and the vegetable kingdoms. The typical plant by the aid of its green colouring matter or chlorophyll is able during exposure to daylight to take comparatively simple materials such as carbon dioxide and nitrates and build them up into more and more complex substances culminating in living protoplasm. The typical animal is quite unable to do this : it must have its proteins ready provided for it ; and it will be realized from this that the animal world as it exists to-day is dependent for its continued existence upon the vegetable world —for upon the latter depends ultimately the supply of protein food material. We have given a description of a particular kind or species of Amoeba — Amoeba proteus—but it will have been gathered from such names as Amoeba Umax and Amoeba radiosa that there are other kinds. It is customary in scientific writing when referring to any particular kind of animal to give it two names — to use what is called the binomial system of nomenclature developed by the great Swedish naturalist Linnaeus (1707-1778). We may conveniently illustrate this system by the case of animals larger and more familiar than Amoeba such as for example the various " cats " large and small which exist as wild animals. Each of the kinds or species— such as the lion, the tiger, the leopard, the jaguar, the puma, the tiger-cat, the lynx, the wild cat — is given a special or specific name, while they are also given a common name, indicating that the several species belong to a group corresponding to the idea conveyed by the English word Cat when used as above in the broad sense. Such a group, of higher order than a species, is termed a genus, and is given a special generic name. In the particular case under discussion the generic name is Felis, while of the various species mentioned the lion is known as Felts leo, the tiger as F. tigris, the leopard as F. pardus, the jaguar as F. on fa, the puma as F. concolor, the tiger-cat as F. pardalis, the lynx as F. lynx, the wild cat as F. catus, and so on with other species. It is usual to print such scientific names, which are regarded as Latin substantive and adjective, in italics, and \\hcn it. is quite clear what is meant the generic name is commonly con- ted down to its initial letter. The use of such names is not due to mere pedantry : it is rendered necessary for purposes of precision by the fact that popular names are liable to give rise to confusion. Thus such a name as " Crow " is applied in different English-speaking regions of the world to totally different kinds of birds. Of tin Amoebae a number of species arc recognized, differing from i AMOEBA 15 A. proteus in such characters as size, shape of pseudopodia, and so on, but the only ones that call for special mention are a group of species which have taken to a parasitic existence and are therefore of practical interest to medical men. Of these parasitic Amoebae — which are usually now set apart as a separate genus with the name Entamoeba — there are three species which are well-known parasites of man. Two of these appear to do no harm, playing the part of scavengers and devouring bacteria, etc. — E. gingivalis, a small Amoeba, commonly found creeping about in the mouth, especially in and about teeth which are not kept properly cleansed, and E. coli (Fig. 6, B), a sluggishly moving Amoeba common in the large intestine. The third species — E. histolytica (Fig. 6, A) — burrows in the wall of the large intestine, devouring and destroying its cells and causing ulceration. In the great majority of cases this ulceration is not sufficient to produce obvious disease, but in other cases where it goes farther it causes dysentery — "Amoebic" dysentery as it is termed to distinguish it from " bacillary " dysentery caused by bacteria — or, it may be, localized abscesses in the liver or, more rarely, in the lung or brain. E. histolytica is a smallish Amoeba, measuring as a rule when rounded about 20 //, to 30 /u, in diameter.1 Under normal conditions in the body it has a limax-like form, without clear distinction of ectoplasm from endoplasm, and glides along with great rapidity. If examined on a glass slide outside the body at ordinary room temperature it, on the other hand, remains in one spot, pushing out broad flat pseudopodia of clear transparent ectoplasm (Fig. 6, A, i). The endoplasm of E. histolytica is finely granular in appearance and usually contains ingested food material — remains of cells of the intestinal wall and, more especially, red blood-corpuscles (Fig. 6, A, i, e). The presence of these latter is a reliable diagnostic feature distinguishing E. histolytica from the harmless E. coli which feeds mainly on bacteria. As was the case with A. proteus, reproduction is carried out normally by a simple process of fission. From time to time, however, the Amoebae leave the intestinal wall and make their way into the cavity of the in- testine as a preliminary to encystment. They are now much smaller in size : their nucleus is relatively larger : the endoplasm is full of vacuoles and is without blood-corpuscles. As the time of encystment approaches, reserve food material is stored up in the form of one or more 1 The unit of length commonly used in biological science for small dimensions is the one-thousandth part of a millimetre, usually designated by the Greek letter ^. A convenient rough gauge always at hand is afforded by the red blood-corpuscle of man which is a circular disc measuring normally from 7 /j. to 8 fj. in diameter. i6 ZOOLOGY FOR MEDICAL STUDENTS. CHAP. vacuoles containing the starchlike substance glycogen (Fig. 6, A; 2, g) and rodlike masses of chromatin-like material (Fig. 6, A, 2, ch). The Amoeba now rounds itself off and secretes over its surface a thin mem- branous protective cyst. The cyst is most frequently between n ft, and 14 n in diameter, but there is a wide range of variation in size, for races ch. n. 9- 2 FIG. 6. Amoebae from the intestine of man. A, Entamoeba histolytica ; B, E. coli. (From drawings by M. W. Jepps.) /;, i nested vegetable cell (Blastocystis) ; b. ingested bacteria ; ch, chromatoid bodies ; e, ingested blood-corpuscles ; g , glycogen vacuole ; n, nucleus. of the parasite have been found in which the diameter is as small as 5 //, and others in which it is as large as 20 /z.1 1 In the case of E. coli there exist similar differences so that, although there is a tendency for the cysts of E. coli to be larger than those of E. histolytica, mere size does not afford a trustworthy criterion for distinguishing between the cysts of the two species. I AMOKIJA 17 The encysted phase is essentially a resting phase in the life-history during which the living activities are slowed down. Apart from the gradual using up of the reserve food-material — first the glycogen and then the chromatoid substance — the only conspicuous activity is division of the nucleus which takes place normally twice in succession so that the encysted amoeba contains 4 nuclei (Fig. 6, A, 2, n).1 The encysted stage serves for the infection of new individuals. The cysts pass away to the exterior. Under conditions of drought the encysted Amoebae very soon die but under cool and moist conditions they retain their vitality for some time, it may be as long as five weeks although as a rule not longer than a fortnight. If swallowed, e.g. in drinking water, the cyst is dissolved under the influence of the digestive ferments and the contents set free in the intestine to start a new infection. It does not appear to be definitely established whether the division into four separate uni-nucleate Amoebae takes place before or after being set free from the cyst. E. histolytica appears to be widely distributed over the earth's surface as a parasite of man, although its presence is more conspicuous in warm climates. When it gains a footing in the human body it is apt to persist for prolonged periods, probably throughout life, unless special measures are taken to get rid of it. And a point of great practical importance is the fact that only a comparatively small proportion of infected individuals — perhaps under ten per cent — betray their infection by recognizable symptoms of disease : for the others, while not recognizable as invalids, yet serve all the while as animated reservoirs or carriers of the parasite, disseminating it in the encysted phase and in this way spreading infection. Apart from the causation of disease it would appear that Amoebae are of practical importance to man in relation to agriculture. Many green plants are dependent upon the existence in the soil of bacteria which prepare for them a supply of nitrates from which they can obtain their supplies of nitrogen. Now these nitrifying bacteria have active enemies in the form of amoeboid and allied creatures which creep about in the soil. Sometimes a rich soil becomes " sick " i.e. its fertility becomes greatly diminished although chemical analysis fails to show any evidence of diminished richness. By baking such " sick " soil for a few hours, or by treating it with chloroform vapour it has been found that a marvellous recovery can be induced and it has been suggested that the 1 It should be noted for purposes of diagnosis that in the case of the harmless E. coli an additional division normally takes place so that there are eight nuclei within the cyst. C i8 ZOOLOGY FOR MEDICAL STUDENTS CHAP. " sickness " is caused by an abnormal increase in number of the amoeboid and other organisms which prey upon the nitrifying bacteria, and that the " cure " is brought about by the killing off in turn of these organisms so that the more highly resistant bacteria are able to multiply till they again reach their normal numbers. PROTOZOA SCHEME OF CLASSIFICATION I. SARCODINA. A. Rhizopoda. i, Amoebaea. 2, Foraminifera. B. Actinopoda. 3, Heliozoa. 4, Radiolaria. II. FLAGELLATA. III. SPOROZOA. A. Telosporidia. i, Gregarinida. 2, Coccidia. 3, Haemospondia. B. Neosporidia. 4, Cnidosporidia. 5, Sarcosporidia. 6, Haplosporidia. IV. CILIATA. i, Holotricha. 2, Heterotricha. 3, Hypotricha. 4, Peri- tricha , Appendix to Ciliata — ACINETARIA. I. SARCODINA Amoeba is a characteristic example of the main sub-division or phylum of the Animal Kingdom called Protozoa because its members come first in order of simplicity of obvious structure. This phylum includes a vast number of different genera which when compared together and classified are found to fall naturally into certain sub-groups the more important of which are mentioned in the scheme given above. Amoeba itself is placed with a number of its allies in the small group AMOEBAEA,, character- ized above all by the blunt pseudopodia (lobopods) which do not show any tendency to fuse with one another. Two interesting examples of this group arc tin.' genera Arcella and Dtfflugia, both common in fresh- water pools. These, unlike Amoeba which is naked, shelter their bodies within a portable house or shell, only the pseudopodia projecting beyond its opening. In Arcella the shell is composed entirely of i PROTOZOA 19 secreted material, somewhat hornlike in appearance, shaped like a concavo-convex lens, and possessing a circular opening in the middle of the concave side. In the case of Difflugia the shell or house is on the other hand built up of foreign particles, it may be small grains of sand, or particles of the skeletons of freshwater animals, fitted closely together over its surface and held together by a cement secreted by the animal's protoplasm. An important physiological peculiarity of Arcella is that it habitually produces in its protoplasm gas-vacuoles 1 which counteract the weight of the shell. It is said that when the Arcella is capsized on to its convex side it is able to right itself by active formation of gas vacuoles which buoy it up and enable it to recover its normal position. POLYSTOMELLA Of the FORAMINIFERA it will be convenient to study in the first instance a special example namely Polystomella which can be easily investigated in the laboratory. If handfuls of seaweed are plucked from rocks near low-water mark and washed to and fro in a dish of clean sea-water it will be found that numerous grains of " sand " are washed out of the seaweed and sink to the bottom of the vessel. If now the vessel be left undisturbed it will be found after a few hours that some of the apparent grains of sand have crept up the sides of the vessel and are attached to it. They are really living Foraminifera and amongst them one can often recognize Polystomella by the characteristic brown colour of its cytoplasm. Examined with a low power of the microscope the Polystomella is seen (Fig. 7) to have its body coiled in a flat spiral. It is supported by a shell or skeleton composed mainly of calcium carbonate, and divided up into successive chambers which increase in size from the centre outwards — each chamber fitting spoonwise ' over the chamber on its inner side. Each chamber is filled by a mass of protoplasm the form of which is readily seen on dissolving away the opaque calcium carbonate by means of acid. A characteristic detail which enables one to distinguish Poly- stomella from other and somewhat similar Foraminifera is that the edge of each protoplasmic mass, except those nearest the centre, projects backwards in a row of peglike extensions over its predecessor (Fig. 8, i and 7, p). The successive masses of protoplasm are not completely isolated from one another : each mass is continuous with its two neigh- bours by one or more bridges of protoplasm towards its central end. The outer wall of each chamber is perforated by numerous minute pores or 1 The gas has generally been regarded as carbon dioxide but more recent investi- gations (Bles) suggest that it is oxygen. C i 20 ZOOLOGY FOR MEDICAL STUDENTS CHAP. foramina and through these the protoplasm is continued outwards to form a thin layer of external protoplasm which ensheathes the whole" shell. The shell is therefore not an external shell like that of a snail but an internal skeleton completely embedded in living substance. The external protoplasm extends outwards — when the creature is active — into pseudo- FlG. /. Polystomella, alive. Certain of the pseudopodia towards the lower side of the figure are flowing round a particle of food. podia (Fig. 7) which are however in appearance totally unlike those of Amoeba. They are very long slender threads of protoplasm which show a marked tendency to fuse in places with their neighbours. With a hi-h magnification it may be seen that there is a constant slow circulation in the extruded pseudopodium — the cytoplasm streaming outwards along j POLYSTOMELLA 21 one side and inwards along the other. While the Polystomella can creep slowly along by means of its pseudopodia the most conspicuous function f I'olystomella according to Lister, i, Megalospheric individual ; za and 26, gametes derived from two different megalospheric individuals ; 3 and 4, syngamy ; 5, zygote (central proto- plasm of new mirrospheric individual) ; 6, early stage in growth of microspheric individual ; 7, micro- individual ; 8 and 9, young megalospheric individuals derived from 7. b, Protoplasmic bridge; chr, rhniinidia ; M, central protoplasm of megalospheric individual; m, central protoplasm of microspheric individual ; n, nucleus ; p, peg-like projections of cytoplasm. i POLYSTOMELLA 23 i.e. division not into two but into a greater number of new individuals. The nuclei become all broken down into chromidia and the protoplasm streams out into the pseudopodia forming a kind of halo round the now empty shell. New round nuclei, probably formed from chromidia, now make their appearance and the protoplasm divides up into rounded masses each containing a nucleus and chromidia (Fig. 8, 8). These secrete over their surface a thin spherical shell and if measured they will be found to be from 60 /x to 100 ^ in diameter. Each of these is a young megalosphere. It creeps away by means of its pseudopodia. As it increases in size the protoplasm bulges out and forms the character- istic horn-shaped piece that fills the second chamber (Fig. 8, 9). As growth continues other pieces are added on by a similar process and the spiral form of the complete megalospheric individual is built up chamber by chamber. During the process of growth the nucleus — the principal nucleus — migrates onwards passing from chamber to chamber so as to retain throughout its position about the middle of the whole mass of protoplasm. As the megalospheric individual approaches its reproductive period the principal nucleus disintegrates entirely into chromidia and then the nuclear material concentrates secondarily to form very numerous small round nuclei. The protoplasm becomes segregated round each of these to form a small cell and this undergoes fission twice in succession so that each original cell is represented by four. A curious quivering movement may be seen in the interior of the shell and presently the contents issue forth as a swarm of minute rounded bodies (Fig. 8, 20) each provided with a nucleus and possessing at one end a pair of fine protoplasmic threads or flagella by the lashing movements of which the cell swims actively through the water. Each of these flagellate cells simply dies after a short time unless it happens to come across a similar cell (Fig. 8, 26) derived from the breaking up of another megalospheric individual. If this happens the two cells come into contact by their non-flagellate ends (Fig. 8, 3 and 4), the flagella disappear and the two cells gradually undergo complete fusion, not merely the cytoplasm but also the two nuclei becoming indistinguishably fused together (Fig. 8, 5). The single rounded cell so produced is found by measurement to be about 10 /* in diameter. It is a young microsphere and gradually grows into a typical microspheric individual. In this reproduction by the megalospheric individuals we have an example of a sexual process — of what is known technically as syngamy. This process consists essentially of the fusion together, and more especially the fusion of the nuclei, of two cells termed gametes to form a single cell- 24 ZOOLOGY FOR MEDICAL STUDENTS CHAP. individual known as the zygote. In this particular instance the biflagellate cells are gametes, the young microsphere is a zygote. It is customary to use the expression sexual "reproduction " but it should be realized from the beginning that the sexual act — syngamy— is not in itself reproduction : it is in fact the opposite for it involves not an in- crease but a decrease in the number of cell-individuals. At the same time it is usual to find the sexual act intimately associated with an increase in the number of individuals. In the case of Polystomella as in many others this increase takes place immediately before the act of syngamy - in the production of the very numerous gametes. Looking back upon the life-history of Polystomella we see in it a good example of what is called alternation of generations— generations of individuals which reproduce sexually (megalospheric) alternating with others (microspheric) in which the reproduction is asexual i.e. unac- companied by syngamy. We also see why it is that the sexual individuals are much more numerous (about 30 to i) than the asexual — because in their case enormous wastage takes place owing to the act of syngamy depending on the small chance of (i) two individuals in the same neigh- bourhood producing gametes at the same time and (2) the gamete from the one happening to come into immediate proximity with a gamete from the other. The Foraminifera in general are characterized especially by (i) the slender thread-like pseudopodia with their tendency to fuse together into a network, (2) the presence of a shell or skeleton which may be composed of horny, or siliceous ( = flinty) or, as is .much more usual, calcareous secreted material, or may on the other hand be built up of foreign particles such as grains of sand or fragments of the skeletons of other minute animals, and (3) by the differentiation of the individuals into two types — asexual microspheric and sexual megalospheric. They are typically marine. Many of them creep about on the bottom or on seaweeds, like Polystomella ; others are pelagic (i.e.. inhabiting the open ocean), forming an important part of what is known as the plankton i.e. the drifting population of organisms too small or too feeble to travel about by the activity of their own movements. In the pelagic Foramini- icni the cytoplasm commonly contains large numbers of vacuoles filled with a watery jelly of slightly less specific gravity than the sea-water, which serves to keep the specific gravity of the creature approximately the same as that of the surrounding water in spite of its heavy skeleton. These pelagic Foraminifera play an important part in the formation of submarine deposits. They exist in countless myriads in the upper layers i FORAMINIFERA 25 of the ocean water in the warmer regions of the globe and consequently there is as it were a continuous rain of their dead bodies down into the depths. Sinking down with extreme slowness their shells are all the while exposed to the solvent action of the sea-water and in the deepest oceans they have been completely dissolved long before the bottom is reached. In waters of more moderate depth however they reach the bottom and, being protected from further solution by the bottom layer of water becom- ing saturated with calcium carbonate, they accumulate in the form of a FIG. 9. Skeletons of Foraminifera. characteristic greyish foraminiferal ooze— called Globigerina ooze from the genus Globigerina (see Fig. 9, uppermost figure) which is one of the commonest of these pelagic foraminifera. Foraminiferal ooze has played an important part in the building up of rocks in past geological ages. If a piece of natural chalk be examined by appropriate methods it is found to consist in great part of foraminiferal shells, though these frequently resemble the shells of shallow-water species of the present time rather than pelagic types. Another rock composed of foraminiferal remains is the remarkable Nummulite limestone of Eocene age which stretches in a belt across the Old World from Southern Europe to Japan. This is 26 ZOOLOGY FOR MEDICAL STUDENTS CHAP. composed in great part of coin-shaped discs or nummulites ranging up to more than two inches in diameter, each disc being really a huge foraminiferal shell with successive chambers arranged in a spiral, and showing the usual differentiation into microspheric and megalospheric individuals. There are two groups of organisms — the Mycetozoa and the Proteomyxa — which are not specifically mentioned in the scheme of classification on p. 1 8 but which must be briefly referred to now. The MYCETOZOA (Myxomycetes of the botanists) constitute a group which shows such a mixture of animal and vegetable characters that it cannot be said definitely to have taken up either the one line of evolution or the other. Mycetozoa are frequently seen in damp woods forming a beautiful lace-work, often of a bright orange colour, on the surface of fallen and decaying trees. There is no sign of life obvious to the naked eye, except that continued observation shows that the patch of lace-work slowly changes its position. If the network is allowed to spread on to a moist glass slide and examined microscopically it is seen to consist of actively streaming cytoplasm, showing a differentiation into ectoplasm and endoplasm and containing numerous nuclei. Whereas a portion of protoplasm containing a single nucleus is termed a cell, such an undivided mass of protoplasm as this containing a number of nuclei is termed a syncytium or plasmodium. It has become advisable to use the former term rather than the latter to avoid confusion owing to the word plasmodium being used also in another sense (see p. 55). The Mycetozoa are organisms which have given up aquatic existence and become more or less terrestrial, and, correlated with this, they have developed special arrangements for protection against desiccation. When subjected to drought the protoplasm segments up into numerous pieces each surrounded by a protective cyst — the whole forming a hard brittle mass. When conditions again become favourable the cysts soften and the protoplasmic masses creeping out flow together again. When the reproductive period comes on the protoplasm clumps together into compact masses usually supported by secreted stalks attached to bark or other solid substance. These " sporangia " are of characteristic shape and colour in the different genera and species. The protoplasm in their interior gives rise to numerous small reproductive bodies or spores each enclosed in a hard protective cyst. These spores are blown about by the wind and under favourable moist conditions the protoplasm in their interior issues forth as a minute cell which swims for a time by the movements of a single flagellum but presently takes on an amoeboid character, and undergoes fission a few times. The amoeboid cells so MV( KTOZOA AND PkoTI <)MY.\ A 27 arising an gametes and the zygotes formed by their iusion give rise to the conspicuous syncytial stage — partly by simple growth accompanied by mitosis, partly by numbers of them crowding together and undergoing fusion. Under the name PROTEOMYXA are frequently grouped together a numlu-r <>l interesting genera of rather uncertain affinities. Two members of the group may be mentioned as being of special intm-st. Pseudospora is i'reijurmly encountered in the laboratory during the examination of I'obox (sec below, p. 38). It has the appearance of a small Amoeba (Kig. 10, A), and is to be seen creeping about in the Volvox colonies and devouring the cell-individuals. In a bad epidemic of Pseudospora the Volvox colonies may be almost entirely destroyed by its ravages. The FIG. 10. Pseudospora (from M. Robertson). A, creeping ; B, swimming ; C, floating phase, c.v, Con- tractile vacuole ; /, food ; f.v, food-vacuole ; n, nucleus. special feature, apart from its frequent occurrence, which makes it worthy of mention here is its remarkable polymorphism. On occasion it will take on an elongated form of body with two flagella at one end by the movements of which it swims actively through the water (Fig. 10, B). At other times (Fig. 10, C) it will become spherical with fine tapering pseudopodia radiating from it, and superficially placed .contractile vacuoles, looking precisely as if it were a member of the group Heliozoa to be described below. Pseudospora thus illustrates particularly clearly, within the compass of its own life-history, three different physiological types of body which occur independently of one another very commonly in the Protozoa and which are associated respectively with a creeping, a swimming, and a floating habit of life. Plasmodiophora the other genus is of interest as being the cause of the important disease of Cabbages and Turnips called Finger and Toe 28 ZOOLOGY FOR MEDICAL STUDENTS CHAP. disease from the projecting tumours which develop on the roots of plants affected by it. The life-history begins with a small flagellate, somewhat spindle-shaped, stage which hatches out of a spore. This makes its way into one of the cells of the plant and there grows enormously, forming a syncytium and causing the cell to swell up to an immense size. Eventually the syncytium breaks up into numerous small spores which surround themselves with a protective cyst and which when set free by the rotting of the plant are scattered about and serve to infect other plants. The Protozoa mentioned up till now are linked together by the character of their pseudopodia, which are soft and frequently branch. This community is expressed by placing them in a group by themselves — the Rhizopoda — separated from the Actinopoda in which the thin tapering pseudopodia radiate stiffly from the body all round. Of the first section of Actinopoda — the HELIOZOA — -we will consider first Actino- sphaerium which is common in freshwater pools and excellently suited for laboratory study. ACTINOSPHAERIUM A well-developed Actinosphaerium is clearly visible to the naked eye as a round greyish-white spot when seen against a dark background. Under a low magnification it is seen to float freely in the water, spherical in form with stiff pseudopodia radiating out all round making it resemble the conventional figure of the sun and justifying its popular name " Sun animalcule." As in the floating types of Foraminifera the cytoplasm (Fig. u) is highly vacuolated, there being a superficial layer of vacuoles of specially large size marking the ectoplasm, while in the endoplasm the vacuoles are much smaller so that it has a more opaque appearance than the ectoplasm. Certain of the vacuoles of the ectoplasm — as many as 15 there may be in a large specimen — are contractile and when at the height of distension (diastole) they may be observed to bulge conspicuously beyond the general surface (Fig. n, c.v). The pseudopodia are extensions of the ectoplasm and examination with a very high power shows that each pseudopodium is supported by a stiff axial filament running up its centre and ensheathed in more fluid cytoplasm which shows slow streaming movement. By careful focusing on the extreme edge of the specimen the axial filament may be seen to extend as far as the outer limit of the endoplasm. The pseudopodia are used in feeding. If a small food-organism touches one it is paralysed and killed almost instantly. It seems to adhere to the pseudopodium which is reinforced by the neighbouring pseudopodia bending over to HELIOZOA 29 its assistance. The pseudopodia then shorten and thicken and the food- particle is drawn down to the surface and finally into a food-vacuole, the ectoplasm closing over it. As the pseudopodium thickens and shortens the axial filament becomes resolved into ordinary fluid cytoplasm. It is therefore not a permanent structure but merely cytoplasm— possibly an extension of the endoplasm— temporarily modified for a special function. The food-vacuoles pass down into the endoplasm where the digestive processes take place (Fig. u,f.v). In the endoplasm is also contained the nuclear apparatus which consists not of a single nucleus but of a large number — several hundred there may be in a large specimen. The Actinosphaerium is therefore a syncytium rather than a cell. The life -history of Actinosphaerium shows some important features. Under ordinary circum- stances the Actino- sphaerium reproduces by a simple process of fission. At certain periods however,, occa- sioned apparently by the onset of unfavourable conditions, a much more complicated process of reproduction, of a sexual type, takes place. The pseudopodia are withdrawn, the axial filaments disappearing, and the Actino- sphaerium surrounds itself with a thick protective jelly-like cyst. The vacuoles practically disappear, and the protoplasm becomes uni- formly granular and opaque, laden with minute particles some of which are apparently reserve food-material while others are particles of silica, afterwards used to strengthen the zygote cyst. About nineteen out of every twenty nuclei disappear and the protoplasm segregates round the remaining nuclei to form rounded uninucleate masses which are termed the primary cystospores. Each of these secretes a special jelly-like cyst and then within this divides with mitosis into two secondary cystospores. The nucleus of each of these undergoes mitosis without however this Actinosphaerium. c.v, Contractile vacuole ; ect, ectoplasm ; f.v, food-vacuole ; n, nucleus. 3o ZOOLOGY FOR MEDICAL STUDENTS CHAP. being followed by cell-division. One of the two daughter nuclei in each case is simply pushed outside the protoplasm and got rid of. The nucleus which remains behind again undergoes mitosis and again one of the two daughter nuclei is extruded. In the place of each secondary cystospore there is no\v therefore a uninucleate cell with two extruded nuclei lying outside it. These extruded nuclei simply degenerate and play no further part. The two uninucleate cells on the other hand are soon seen to be gametes,, for they undergo syngamy with one another to form a /ygote. In the place formerly occupied by a pair of secondary cysto- spores and at an earlier period by a primary cystospore there is now therefore a zygote. This may be distinguished from a primary cystospore by its smaller size and denser protoplasm and by the cyst round it being thicker and reinforced by minute particles of silica extruded from its cytoplasm. After a prolonged rest,, lasting normally for several weeks or months,, the zygote — conditions being again favourable — makes its way out and resumes the form of a typical small Actinosphaerium. This sexual reproduction of Actinosphaerium is of special interest and importance in two respects, (i) The extrusion of nuclei seen here as an essential integral part of the preparation or maturation of the gametes exemplifies a phenomenon very widely distributed throughout the whole Animal Kingdom. (2) On the other hand a glaring exception is seen here to a very general rule in sexual reproduction namely that the two gametes which conjugate together must not be close " blood- relations." We have here a striking example of extreme " inbreeding. ': The general features of the Heliozoa are well exemplified by Actino- sphaerium : the, with rare exceptions, freshwater habitat and the stiff radiat- ing pseudopodia supported by axial filaments are particularly characteristic. The power of secreting particles of silica seen in Actinosphaerium during the reproductive period is much more highly developed in other members of the group, some of which form a definite supporting skeleton of silica. Apart from Actinosphaerium the commonest Lleliozoan in our fresh- water pools is Actinophrys which is distinguished by its smaller size, its single centrally-placed nucleus and by the extension of the axial filaments inwards right up to the nucleus. The group RADIOLARIA will be dealt with only briefly, not because they are not of great interest but because on the one hand specimens are not readily available for direct study and on the other hand they arc not intimately linked up with medical or other studies of immediate importance. RADIOLARLA s.c. Radiolarians are typically planktonic— floating in the waters of the open ocean. They present a superficial resemblance to Heliozoa through the presence of the slender, radiating, pseudopodia associated with the floating habit but in this case the pseudopodia are usually devoid of any axial filament. The protoplasm is as a rule divided into an outer and an inner portion by a membranous central capsule (Fig. 12, c.c) with wide perforations the arrangement of which differs in different members of the group. The internal protoplasm is dense and granular and contains the nucleus. The external protoplasm on the other hand is reduced to a sparse network between large vacuoles containing watery fluid which as in the pelagic Foraminifera serve to bring the creature to the same specific gravity as the sea-water so that it hangs suspended with- out any tendency to rise or sink. In stormy weather the vacuoles shrink, the specific gravity is increased, and the creature sub- sides to below the limit of dangerous wave- action. A most charac- teristic feature of the Radiolarian is its skele- ton, composed as a rule of glassy silica. This may take the form of radiating rods of silica or, in addition to or instead of these, a lattice-work of silica is laid down in the external protoplasm (Fig. 12, sk) just below its surface. As growth continues the surface of the external protoplasm becomes further removed from the lattice-work and after a time another lattice-work is laid down, and then another and so on until there may be a number of clear flinty lattice-work shells arranged concentrically one within the other (cf. Fig. 13, left-hand figure). These skeletons of Radiolaria (Fig. 13) are in their marvellous variety of form and pattern amongst the most beautiful of microscopic objects. There remains to be mentioned a remarkable feature which, though FJG- I2- Diagram illustrating the structure of a Radiolarian. c.c Central capsule ; N, nucleus ; ps, pseudopodium ; s.c, symbiotic cells ; sk, skeleton; vac, vacuole. ZOOLOGY FOR MEDICAL STUDENTS CHAP. not an essential element in Radiolarian organization, is yet very generally to be observed, namely the presence, in the strands of external protoplasm, of numerous little rounded yellowish or greenish cells (Fig. 12, s.c) apparently of extraneous origin which live and multiply sheltered within the body of the Radiolarian. Here we have an excellent example of what is known as symbiosis, in contradistinction to parasitism. In both cases we have two different types of organism living in close association e.g. the smaller living within or attached to the body of the larger. When the benefit arising from this association is .entirely one-sided the biologist FIG. 13. Skeletons of Radiolarians. terms the beneficiary organism a parasite, when on the other hand the benefit is mutual, when the smaller organism makes some return for the hospitality it receives, the two organisms are said to be symbiotic. The small cells living within the body of the Radiolarian receive from it shelter and possibly a small amount of food-material, but on the other hand they contain a colouring matter like that of green plants which enables them to appropriate the carbon dioxide produced in the metabolism of the Radiolarian, retaining the carbon for their own use but setting free the oxygen to be made use of by the Radiolarian, so that we are clearly justified in speaking of these small cells as symbiotic. Like the pelagic Foraminifera the Radiolaria play an important part i PROTOZOA 33 in forming submarine deposits. In their case too the dead and disin- tegrating bodies rain slowly down through the depths of the ocean. But the silica of their skeleton is much less soluble than the calcium carbonate of the Foraminifera. Consequently in deep regions of the ocean where both Foraminifera and Radiolaria are constantly sinking down from the surface layers the skeletons of the former are removed by solution before they reach the bottom wfiile those of the latter persist and are deposited to form Radiolarian ooze. Deposits of fossil radiolarian ooze are found amongst the rocks of various parts of the world, as for example the comparatively modern deposits in Barbados and some of the flinty " cherts " which occur here and there amongst the more ancient rocks. Glancing back over the Protozoa so far described we note that they possess this great characteristic in common that the outer layer of their protoplasm is soft and can be pushed out to form pseudopodia, and further, that they ingest food in the form of solid particles. These features unite them together in a group — the SARCODINA. II. FLAGELLATA In contrast with the Sarcodina the members of the next group — the FLAGELLATA — have the surface layer of their protoplasm more or less condensed to form a smooth bounding layer which is incapable of being pushed out into pseudopodia. We will first review the characters of three different genera of Flagellata which are easily obtained for laboratory study and thereafter consider certain genera which have been discovered within recent years to be of great practical importance as causes of disease in man and domestic animals. Two of the three genera first mentioned are characterized by their bright green colour due to their containing chlorophyll, the colouring matter which occurs in ordinary green plants, and the first of them often exists in freshwater puddles in such numbers that the water is throughout quite green and opaque* EUGLENA Euglena (Fig. 14) is a comparatively small creature (the commonest species E. viridis about 55 //. by 15 /A), somewhat spindle-shaped with one end blunt and the other pointed. Its smooth surface dips inwards close to the blunt end to form a conical funnel which expands at its apex into a spherical space — the reservoir (Fig. 14, r). From the funnel there D ZOOLOGY FOR MEDICAL STUDENTS CHAP. c.v. issues a long flagellum originating by a double root from the lining of the reservoir. In the centre or slightly nearer the pointed end is a large rounded nucleus, and close to the reservoir is a contractile vacuole (c.v) which expels its water into the reser- / voir. The animal is bright green in colour, the chlorophyll being situated in rounded discs — the ehromatophores or chloroplasts (chr}~ which are arranged in a single layer just below the surface. The chlorophyll is not the only colour- ing matter. Close to the contractile vacuole and just beneath the surface is a shining orange-red spot — the stigma (s) — which is physiologically an eye — not an eye which can " see " in the ordinary sense but merely a portion of the proto- plasm sensitive to light and enabling the creature to detect from which direction the light is coming. As regards the life-history one of the chief peculiarities is a negative feature — the absence so far as is known of any process of syngamy. Ordinarily reproduction takes place by fission, the Euglena splitting longitudinally from the flagellar end. As a rule the Euglena is encysted in a clear jelly during the process and the young individuals surround themselves also with a cyst and then divide again. In this way great masses of jelly are formed with the dividing Euglenas contained in it. These, buoyed up by bubbles of gas, often form a conspicuous green scum on the surface of pools. As regards the physiology of Euglena features of special interest are presented by its movements and by its mode of nutrition. The Euglena swims by means of its flagellum which is directed forwards and describes a conical or spiral kind of movement in such a way that the body of the Euglena is as it were towed after it. Such a type of flagellum is some- chr N: FIG. 14. Eugleni. c.v, Contractile vacuole; ch> chromatophore ; fl, flagellum ; N, nucleus p, paramylum ; r, reservoir ; s, stigma. i EUGLENA 35 times given the special name tractellum. When not swimming the Euglena may often be seen performing curious writhing movements so characteristic in appearance that such movement is termed euglenoid when met with elsewhere. The Euglena is seen to widen itself out about the middle of its length and then the swollen part gradually narrows again towards the two ends. The main nutrition of the Euglena is like that of a green plant (" holophytic "), i.e. in the presence of daylight and by means of the green chlorophyll carbon dioxide is split up, the oxygen being set free in the form of bubbles while the carbon is elaborated into a substance termed paramylum almost identical in chemical and physical characters with starch. In Euglenas which have been exposed to light for some time the cytoplasm is crowded with bright shining rods of this substance (Fig. 14, p). In correlation with this mode of nutrition the living Euglena shows a tendency to swim towards the light (" positive heliotropism ") provided it is not too bright. A culture of Euglena in the laboratory contained in a glass trough will gradually concentrate towards the end of the trough nearest the window where the light conditions are at their optimum. The Euglena is not entirely restricted to the holophytic type of nutrition. A laboratory culture is found to be benefited by the addition of a little organic food-material in solution and this would appear to indicate that a certain amount of absorption of such food-material can take place through the general surface of the body. COPROMONAS Copromonas (Fig. 15) is a small pear-shaped inhabitant of fresh water measuring about 16 /* by about 7 ^ or 8 /u. It may usually be obtained in quantity by keeping the contents of the large intestine of a frog or toad in a little water for six or seven days at ordinary room temperature. The shape of the creature is fixed owing to the outer layer of cytoplasm being more condensed and stiffer than is the case with Euglena. Close to the narrower end the surface layer is turned inwards to line a long gradually tapering funnel which as it is used for the ingestion of food we may call the oesophagus or gullet. The single long flagellum (/) projects from the oesophagus and may be traced inwards along its wall to its origin in a deeply staining dot known as the basal granule. Close to this latter there is usually to be seen a reservoir (r) with a small contractile vacuole (c.v). Embedded in the cytoplasm, rather towards the broad end of the creature, is the nucleus (N) — ZOOLOGY FOR MEDICAL STUDENTS CHAP. rounded in form and having its chromatin concentrated in a large central mass. In ordinary movement the flagellum acts like that of Euglena — as a tractellum by which the animal is towed along. When moving slowly the spiral or conical movement may be seen to be confined to a small portion of the flagellum near its tip while in more rapid movement the whole flagellum is thrown into action. The Copromonas feeds on bacteria and other small solid particles of food- material which pass down the oeso- phagus and are ingested by the soft exposed cytoplasm at its inner end. The ingested food-particles soon become surrounded by fluid secreted by the protoplasm and in the food-vacuoles so formed, which usually remain in the neighbourhood of the broad end, the food-material gradually undergoes the process of digestion. When a Copromonas is kept under favourable conditions it feeds actively and grows and when the limit of size has been reached it proceeds to reproduce, resolving itself into two individuals by a process of longitudinal fission (Fig. 1 6). The flagellum is drawn in ; the nucleus becomes transversely elongated ; the basal granule divides into two — a new flagellum sprouting out from each. A constriction now cuts into the creature from the flagellar end (Fig. 16, 3) ; the reservoir and the nucleus divide into two and as the constriction deepens the Copromonas becomes completely divided into two new individuals which presently separate and swim away, the whole process taking about twenty minutes. This reproduction by fission does not go on indefinitely : at a variable period (two to six days) it is interrupted by the converse process — that of syngamy. c. v. FIG. 15. Copromonas (after Dobell, from Lec- tures on Sex and Heredity, by Bower, Graham Kerr and Agar). c.v, Contractile vacuole ; /, flagellum ; f.v, food-vacuole ; AT, nucleus : r, reservoir. COPROMONAS 37 The gametes (Fig. 16, 6) are to all appearance ordinary individuals : they may happen to be slightly different in size (e.g. as in Fig. 16, 7) but this appears to be purely a matter of chance. They become attached by their flagellar ends, one flagellum is drawn in and the bodies of the two individuals gradu- ally undergo a process of complete fusion (Fig. 16, 7 to 10). While this is taking place the nucleus of each individual divides into two (Fig. i6; 7) one of the two daughter nuclei degenerating : here we have clearly a matura- tion process comparable with that of Actino- sphaerium (p. 30). In the case of Actlno- sphaerium a second nuclear division of this kind took place and in Copromonas this is apparently represented by the extrusion of one or more small granules of nuclear material from the nucleus (Fig. i6; 8). The two nuclei having thus prepared them- selves they approach one another and fuse together (Fig. 16, 10 to n) to form the single nucleus of the zygote. The process of syngamy is followed by encystment, either directly (Fig. 16, n) or after a period during which the Copromonas swims about and undergoes fission in the ordinary manner (Fig. 16, short circuit from 9 to 5). The encysted stage here as in other Protozoa is a device for getting through unfavourable conditions. During it the Copromonas FIG. 1 6. Diagram illustrating the life-history of Copromonas (after Dobell, from Lectures on Sex and Heredity by Bower, Graham Kerr and Agar). The upper circle of figures (i to 5) illustrates the process of reproduction, the lower circle (5 to 12) that of syngamy. 38 ZOOLOGY FOR MEDICAL STUDENTS CHAP. is able to withstand such vicissitudes as drying up for a considerable period. The Copromonas is no doubt swallowed in this stage by the frog or toad and passing through the alimentary canal is eventually voided in the faeces. VOLVOX The genus Volvox owes its most conspicuous difference from the flagellates hitherto described to a peculiarity in its method of fission. When the cell-individual undergoes fission the resulting individuals instead of separating and leading an independent existence remain attached together by slender threads of cytoplasm so as to form a cell- community or colony. The cell-community (Fig. 17, A) is spherical or slightly ellipsoidal in shape. It consists of many, it may be several thousand, cell-indivicluals and is large enough (up to nearly i mm. in diameter) to be distinctly visible to the naked eye. Each cell-individual (Fig 17, B) is ellipsoidal in form and is surrounded by a thick jelly-like envelope,, the envelopes of the various individuals taking a prismatic form owing to their mutual pressure and together constituting the thick wall of the colony in which the cell-individuals are embedded. Each cell-individual possesses a pair of flagella which project beyond the surface of the jelly into the surrounding water, a pair of contractile vacuoles (Fig. 17, B, c.v) which contract alternately, and a rounded nucleus (n). The greater part of the cell, all except its outer end, is ensheathed in a thin green chromato- phore and at one point embedded in the chromatophore near its edge is a bright orange stigma (Fig. 17, B, st}. Finally each cell-individual is connected with its neighbours by extremely delicate thread-like bridges of cytoplasm (b). During life the projecting portions of the flagella perform active lashing movements by which the colony is propelled along in a charac- teristic manner which led an examination candidate to describe Volvox, correctly if not very clearly, as a creature which Amoves in a direction at right angles to that in which it goes." What happens is that the Volvox colony rotates and at the same time advances along a line which is roughly the axis of rotation. In other words the movement is somewhat similar to that of a rifle bullet although not so regular. This peculiarity in the movement of the Volvox community is correlated with peculiarity of structure in the cell-individuals, for stigmata are present only in the individuals situated in that hemisphere of the colony which is in front as the colony moves, and further the stigma is situated on the side of each individual which is in front. In other words the • VOL VOX 39 stigmata or eye-spots are concentrated both in the colony as a whole and in the cell-individuals towards that pole of the colony which is in n. B FIG. 17. Volvox. A, View of a whole colony containing a single daughter-co'ony ; B, a single cell-individual ery highly magnified ; C, part of colony as seen in surface view under a high power ; D, a single microgamete very highly magnified. b, Protoplasmic bridge ; c.v, contractile vacuoles ; chr, chromatophore of microgamete ; M , macrogametocyte ; n, nucleus ; st, stigma. front during movement. Putting it crudely and not quite accurately " the eyes look forward." The features that make Volvox a creature of very special scientific interest and importance are connected with the reproductive processes 40 ZOOLOGY FOR MEDICAL STUDENTS CHAP. by which new communities are produced. The first of these processes is an asexual one. Certain cell-individuals from 20 to 35 in number and situated usually in the hinder hemisphere of the colony become dis- tinguished from their neighbours by their larger size. These are special reproductive individuals. As a rule only eight of these proceed to carry out their function. 'In this event the reproductive cell-individual con- tinues to increase in size and undergoes repeated fission, dividing into two, four, eight, sixteen, and so on, forming a plate of cells which as the process continues curves into the form of a saucer. With increased curvature the saucer deepens to form a cup and finally the mouth of the cup becomes narrowed and eventually obliterated so that it takes the form of a complete sphere. This is a young daughter-colony. It stands out conspicuously in the substance of the mother-colony by its deeper green colour, the green cell-individuals composing the daughter-colony being still in close contact while those of the mother-colony are spaced out by the intervening colourless jelly. The daughter-colony presently makes its way from the wall of the mother-colony into its cavity which is occupied by a very watery jelly, almost pure water, and here it may be seen for some time gradually increasing in size, performing the ordi- nary movements, and eventually bursting its way out and leading an independent existence. This is the normal mode of increase of the Volvox communities but it does not go on indefinitely. After a time a new, sexual, type of reproduction takes place. This is again inaugurated by the appearance of special reproductive individuals which are in this case gametocytes i.e. cells which are destined to give rise to gametes. And when the further development of these is watched they are seen to belong to two different types — male or micro-gametocytes and female or macro-gameto- cytes — for here for the first time we find a differentiation of two sexes. The macrogametocytes (Fig. 17, C, M) are at first very much like the asexual reproductive individuals being like them distinguished from the ordinary cell-individuals by their greater size. They are without flagella. They increase greatly in size, becoming about as large as a daughter- colony is before its cells become pushed apart by the secretion of jelly. They are spherical in shape and are easily recognizable by their dark green colour and their very dense granular cytoplasm which is laden with particles of yolk or reserve food-material. The macrogametocyte undergoes a process of maturation, involving divisions of its nucleus, and thereafter it is capable of syngamy and we speak of it as a macrogamete <>r cjrg. There are commonly about 30 macrogametes in all developed in a single colony. i VOLVOX 41 The microgametocytes are found in very young colonies and may form a large proportion of all the cell-individuals. Each microgametocyte divides repeatedly by fission, forming eventually a slightly curved disc composed of numerous elongated cells — the microgametes. Each micro- ^aincte (Fig. 17, D) possesses the same details of structure as the ordinary individual of the colony only the proportion and arrangement of these are different. In shape it is much more slender, one end being drawn out to a fine point and the other rounded. The pointed end is prolonged into the two flagella which are long and powerful. The nucleus is elongated,, the stigma well developed, and the chromatophore is situated at the rounded end. When the flagella of the microgametes become active the whole mass is moved about and it may make its way out of the colony. Sooner or later however the microgametes separate and swim away through the water as independent cells. If one of these comes into the neighbourhood of a colony containing a macrogamete it bores its way in and syngamy takes place. The zygote immediately proceeds to surround itself with a clear transparent cyst or shell and the presence of this affords a conspicuous character by which the zygote can be at once recognized and distinguished from a macrogamete. When the colony as a whole dies and disintegrates the zygotes fall down into the mud and remain there within their protective shell or envelope until conditions are favourable for their development into adult colonies. The study of Volvox introduces us to several matters of general biological importance. While the Volvox consists of ordinary cell- individuals these are not independent but are linked together into a community which constitutes itself an individual of a higher order. And the cells which form this individual are sharply differentiated into two sets — the ordinary cells and the reproductive cells. The importance of this lies in the fact that here for the first time we find an arrangement that is universal among the higher animals, the body of which invariably consists of a great mass of non-reproductive cells known as the soma, and a special set of purely reproductive cells known as the gonad (see p. 85). We see also in Volvox a -differentiation of two sexes. This consists fundamentally of a specialization of the gametes in two different directions. Essential features of the gametes are (i) that they shall have the power of uniting in syngamy and (2) that the resulting zygote shall possess, stored up in its cytoplasm, a sufficiency of yolk or food-material to start the new colony on its way. To facilitate the achievement of the first the gametes are differentiated into (i) a stationary type without flagella, and (2) a highly active type with greatly developed flagella which is 42 ZOOLOGY FOR MEDICAL STUDENTS CHAP. able to swim rapidly. The first type is the macrogamete, female gamete or egg : the second is the microgamete, male gamete, or spermatozoon. The yolk or food-material is concentrated in the non-motile female gamete and this leads to these gametes being large in size and there- fore few in number ; while on the other hand the male gametes are small in size and developed in great numbers. To prevent inbreeding micro- and macrogametes are not developed in the same colony at one time. They are either developed in separate male and female colonies or the sexual colony is at first male — producing microgametes — and then becomes female later on. Having passed in review the main features of three illustrate e genera of flagellates as seen from the point of view of pure science we will now proceed to study certain genera which are of special practical importance from their having adopted a parasitic mode of life and being thereby in some cases seriously harmful to the welfare of the host in whose body they have taken up their abode. TRYPANOSOMA Animals belonging to the genus Trypanosoma seem to have been observed as far back as the year 1843 when specimens were found in the blood of the frog by Gruby. It is only within the last two decades however that trypanosomes have excited very special interest owing to the fact that a number of important diseases have been traced to their agency. A typical trypanosome (Fig. 18) is elongated and somewhat spindle- shaped,, one end being rather more pointed than the other. The pointed end is prolonged into a single flagellum (/) which is continued back along the side of the body for some distance, it may be throughout almost its whole length. The flagellum from its basal end forwards to the tip of the body lies at some little distance from the surface of the latter, being connected with it by a thin protoplasmic membrane (Fig. 18, m). This membrane looks like a kind of fin running along the body, the flagellum forming its thickened free edge : the membrane has a frilled appearance, the flagellum along its edge following a somewhat sinuous course. The cytoplasm of the trypanosome is finely granular : and superficially it is slightly stiffened to form a bounding layer. The nuclear apparatus is very characteristic. Somewhere about the middle of the creature is a con- spicuous nucleus (Kig. 1 8. /), rounded or oval in form, with a dense central mass of rhromatin (karyosome) and smaller masses round the periphery. TRYPANOSOMA I ides this main nucleus known as the trophonucleus there is present null rounded or rod-like particle regarded by some as being also corn- ed of nuclear material and believed to have to do with controlling the movements of the creature. This particle, commonly termed the kinetonucleus (Fig. 18, k), is in typical cases situated near the " posterior '' (non-flagellar) end of the body although its position varies at different stages in the life-history. The flagellum originates close to the kineto- nucleus, usually in a small granule (basal granule— Fig. 18, b.g). The trypanosome multiplies by a characteristic process of longitudinal fission in which basal granule (and ? flagellum), kinetonucleus, tropho- nucleus and protoplasmic body divide in the order named. Whether during this process the flagellum undergoes longitudinal splitting, commencing at its basal end, or whether on the other hand only the basal granule splits and a new flagellum sprouts out from one of the two resulting granules, seems to be still a matter of doubt. As is the case with Euglena there appears to be no process of syngamy in the life- history of Trypanosoma. The Trypanosome is typically a parasite of the blood of a vertebrate. Its convey- ance from one host to another is typically carried out through the agency of some blood-sucking animal, such as a blood- sucking fly or flea in the case of terrestrial animals or a leech in the case of those inhabiting water, and, in accordance with this, part of the life-history is specially adapted to existence in the body of this intermediate host. As an example of a trypanosome life-history we will take that of Trypanosoma gambiense — the parasite of sleeping sickness. This is essentially a parasite of mammalian blood, in which when abundant it may be seen readily with a high power of the microscope, wriggling through the fluid and knocking about the corpuscles in its course. When infection takes place the few trypanosomes inoculated into the blood multiply rapidly and they may become very numerous. The numbers in the blood do not remain constant at any maximum but at varying intervals undergo great reduction, all except comparatively few dying off. When the blood is examined at this time, before the trypanosomes again increase FIG. 1 8: Trypanosoma gambiense. b.g, Basal granule ; /, flagellum ; k, kinetonucleus ; m, membrane t, trophonucleus. 44 ZOOLOGY FOR MEDICAL STUDENTS CHAP, i in number, it is found that the trypanosomes which survive through this " period of depression " are characterized by their short stumpy form (Fig. 19, A). These short stumpy trypanosomes,, which are apparently endowed with special powers of resisting unfavourable conditions, are of peculiar practical importance for when a meal of infected blood is taken in by the transmitting insect — the blood-sucking fly Glossina palpalis — they alone survive and serve to infect the fly, all the other trypanosomes being killed and digested. It follows that the blood of a particular host is infective only when trypanosomes of this short stumpy type are •present. When blood containing them is swallowed by the Glossina the trypano- somes multiply actively in the fly's alimentary canal until they fill the whole intestine as a seething mass. Much variation is seen in both size and shape (Fig. 19, B, C, D) but at a period varying from about the eighth to the eighteenth day there begin to make their appearance trypanosomes which are conspicuous by their peculiarly long and slender form (Fig. 19, E). These gradually work their way forwards in the cavity of the aliment- ary canal and eventually (sixteenth to thirtieth day) make their way into the salivary glands (see p. 231). Here they attach themselves by their flagella to the lining of the gland and sway about within the cavity. They multiply actively, they become shorter in form and many are found to have the kinetonucleus on the flagellar side of the trophonucleus (Fig. 19, G). This is known as the Crithidial type from the name of a genus Crithidia which is distinguished from Trypanosoma by the position of its kinetonucleus. Amongst the others there also appear stumpy trypanosomes (Fig. 19, H) resembling those of the blood and it is apparently these which, injected into the blood when the Glossina bites, serve to start a new infection in the mammalian host. Ordinarily these infective trypanosomes make their appearance in the salivary glands from about the twentieth day after the fly has taken in the infected blood. It will be noticed from the above life-history that the Glossina which serves to carry the trypanosome infection from one mammalian host to another is rendered capable of successfully inoculating the latter by the presence in its salivary glands of the short stumpy type of trypano- some which marks the final stage of the cycle within the fly. Here we have to do with what is known as the cyclical type of infection — dependent upon the completion of a definite life-cycle within the intermediate host. This contrasts strongly with what is known as direct infection in which a microbe is conveyed directly from one host to another by simple transference. Infection would be direct were individual H U FIG. 19. Trypanosoma gambiense. Stages in life-history passed in Glossina according to M. Robertson. A, Blood-type as ingested by Glossina ; B, from intestine of fly 48 hours after ingestion ; C, from intestine of fly on fifth day ; D, from intestine of fly at twentieth day ; E, from proventriculus ; F, newly arrived at salivary gland ; G, crithidial type from salivary gland ; H, final type from salivary gland, k, Kinetonucleus. 4,> ZOOLOGY FOR MEDICAL STUDENTS CHAP. trypanosomes to be sucked up in the infected blood and injected bodily into the blood of the new individual to produce infection therein. All the detailed knowledge of Trypanosomes is of comparatively recent date. Many species have been described but our knowledge of most of these is very incomplete. In the following list we will confine ourselves to species which are of special interest and importance either practical or scientific. T. brucii is of practical interest as the cause of Tsetse fly disease of domesticated animals in various parts of Africa, and of special scientific interest as being the species of Trypanosoma in which the method of transmission was first discovered. The disease (a Nagana ") has long been known as occurring over large districts in Africa and as being invariably fatal to Horses, Asses and Dogs, and usually so to imported Cattle. These districts were known as " fly belts," the disease being associated with the bite of a particular species of fly (Glossina morsitans — " Tsetse "). It was suggested by David Livingstone that the disease was due to a living microbe injected into the bite, but the actual proof of this was first given by the experiments of David Bruce, which showed that any individual fly was powerless to cause the disease unless it had previously bitten a diseased animal. This clearly pointed to the poison being not something inherent to the fly but rather something which the fly merely transported from an animal already diseased. The fact that the infectivity of the fly appeared from these early experiments to last only for a period of about 48 hours after it had bitten the diseased animal pointed to the "poison" being really some living organism which was able to survive about the fly's proboscis for a period not longer than 48 hours. This suspicion led to the examination of the blood of diseased animals and in it Bruce duly discovered the trypanosome which we now know as T. brucii. Subsequent research has fully borne out Bruce's discovery, with however an important amplification namely that a fly which has lost its primary or direct infectivity becomes again infective after a period of about 18 days, and apparently now remains so for the rest of its life. In other words there occurs here in addition to the direct infection first observed a cyclical infection which in all probability is the important one. T. evansi causes a disease ("Surra"), somewhat similar to Nagana, affecting especially Horses and Camels. It probably is transmitted also by a biting fly — possibly a Horse-fly (Tabanus) or Stable-fly i TRYPANOSOMA 47 (Stomoxys) — though this has not so far been absolutely determined. The disease occurs from India to the Malay Archipelago and the Philippines, and in various parts of Africa, while destructive epi- demics have been caused by its being carried to Mauritius and Australia. T. equinum causes the disease known as ;'Mal de caderas " which occurs in the form of very destructive epidemics amongst the horses of Paraguay and adjoining regions of South America. Dogs are also affected, as well as Tapirs, Carpinchos (Capybaras) and other wild animals. Nothing is known as to the mode of transmission. T. equiperdum, the cause of the disease known as "Dourine" among breeding horses round the shores of the Mediterranean, differs from the trypanosomes hitherto described in being transmitted directly from one individual to another by sexual contact. T. lewisi is of interest as being the trypanosome most easily obtained for purposes of study in most civilized countries. It is practically world-wide in its distribution and is to be found in the blood of Rats, especially of young individuals. The intermediate hosts are parasitic insects — especially fleas of various species. T. gambiense was first observed in 1901 by Forde in the blood of a patient supposed to be suffering from malarial fever in the neighbourhood of the River Gambia. To this new trypanosome the name T. gambiense was given by Button. Other cases were observed and it was recognized that there existed a definite disease — " Trypanosome fever " — distinct from ordinary malaria. Just about the same time a deadly epidemic had been ravaging the native population of Uganda — a peculiar disease which had long been known on the West Coast as sleeping sickness from the drowsy lethargic symptoms of its later stages. A Commission was sent out by the Royal Society to try and find out the cause of the disease as a preliminary to the devising of means for its prevention or cure. In April 1903 a member of this Commission — Castellani — found a trypano- some in the cerebro-spinal fluid of a sleeping-sickness patient and at once suspected that this was the microbe which caused the disease. Bruce and his colleagues on the Commission confirmed Castellani's discovery and amplified it by discovering that the trypanosomes were invariably present in the blood of sleeping-sickness patients, while in the later and more typical stages of the disease they were present also in the cerebro-spinal fluid. The earlier stages of the disease were found in fact to be identical with " Trypanosome fever." Bruce, the discoverer of the cause and means of transmission of Nagana, being a member of the Commission the suspicion naturally 48 ZOOLOGY FOR MEDICAL STUDENTS CHAP. came into his mind that the trypanosome of sleeping sickness might similarly be transmitted by a Tsetse or other blood-sucking fly. Blood- sucking flies were collected from all over the region of Africa under investigation and the localities in which each species occurred were plotted out on a map. When the distribution of the various species of biting flies was gone into, it was found that the distribution of one species — Glossina palpalis a close ally of the Tsetse — corre- sponded practically exactly with the distribution of cases of sleeping sickness. This was strong presumptive evidence of the correctness of Bruce's suspicion that sleeping sickness was a kind of human " Tsetse-fly disease." Experiment soon demonstrated the truth of this. It was found that a Glossina palpalis which made a meal of infected blood was able to inoculate a monkey with the disease if allowed to bite it within a period of a few days. Thus, as in the case of G. morsitans with T. brucii, the possibility of direct infection was proved. Just as in the case of Nagana, however, further investigation showed direct infection to be of relatively minor importance. After a period of 20-30 days the fly was found in a certain small percentage of cases to recover its infectivity, the infectivity now lasting for several months if not for the whole life of the fly. This prolonged cyclical infectivity of the fly is associated with the trypanosome undergoing in its alimentary canal the various changes described on p. 44. Our knowledge of these changes is almost entirely due to the work of Miss Muriel Robertson. It seems fairly clear that the cyclical transmission by G. palpalis is that which is of real practical importance in spreading epidemics of sleeping sickness. Direct transmission by the insect no doubt occurs occasionally but probably much more rarely. It must not be forgotten that T. equiperdum is conveyed by sexual contact and it is at least a possibility that this may happen occasionally also in the case of sleeping sickness — a possibility the less to be ignored in view of experiments (Hindle) which have shown that T. ga&biense is able to make its way through thin skin. For purposes of combating the spread of sleeping sickness three methods at once suggest themselves : — (1) The segregation of sleeping-sickness patients within fly-proof houses so as to prevent new flies from becoming infected. This method is unfortunately made ineffective by the occurrence of T. gambiense as a natural parasite of the Sitatunga antelope (Limnotragus] which consequently acts as a persistent reservoir of the trypanosome. (2) Destruction of the wild antelopes in the neighbourhood of human 49 settlements. The effectiveness of this will be conditioned by the extent, at present undetermined, to which animals other than antelopes act as natural carriers of the trypanosome. (3) The local extermination or at least reduction in numbers of the transmitting Glossina (see p. 253). The most effective measure to this end is probably the clearing away of shade-giving brush and trees along the margins of rivers and lakes. Low thatched shelters may be constructed over loose dry soil near the water's margin so as to attract the flies of the neighbourhood, and induce them to deposit their pupae where they can readily be collected and destroyed. A further palliative may be found in the encouragement of fowls, Francolins and other scratching birds which are useful for unearthing and destroying the buried pupae. (4) The withdrawal of the human population from the fly-infested zone along the margin of the fresh water. This, the most practical method, has actually been carried out on a large scale in the Uganda region, and has resulted in a reduction in the number of sleeping- sickness cases to comparatively small dimensions. Of recent years (1909) "sleeping sickness," of a particularly virulent form, has made its appearance in Rhodesia and East Africa. In this region the transmitting agent is apparently Glossina morsitans and it is suspected that the trypanosome concerned is not T. gambiense but either a separate species (" T. rhodesiense ") or a local strain of T. brucii which has developed the capacity of living in the blood of man. In either case the outlook is an anxious one owing to the wide distribution of the transmitting insect upon the continent of Africa. T.1 cruzi. In 1907 a new species of human trypanosome — T. cruzi — was discovered in Brazil. An interesting feature of the discovery was that the trypanosome was first observed as a parasite in the alimentary canal of a large bug (Conorhinus). By experiment it was determined that monkeys became infected with the trypanosome when bitten by the bugs, and the discoverer — Chagas — then proceeded to examine the blood of the human inhabitants of the district (Matto grosso) from which the infected bugs had been obtained. He found that the trypanosome was regularly present in the blood in cases of a severe illness particularly prevalent amongst children of the district. He also was able to make out that the transmission is cyclical, the bug not becoming infective until 10-25 days after sucking infected blood. A striking characteristic of T. cruzi is that the process of fission takes place normally not in the blood stream but in the substance of the muscles and other organs, the 1 Often called Schizotrypanum instead of Trypanosoma. E ZOOLOGY FOR MEDICAL STUDENTS CHAP. trypanosomes drawing in their flagella and assuming a rounded or spindle shape as a preliminary to dividing. LEISHMANIA Leishmania. In Assam, Lower Bengal, and occasionally in other parts of India there occurs a somewhat malaria-like fever known as " Kala-azar." In 1900 Leishman discovered the parasitic cause of the disease — in the form of small rounded or oval bodies measuring about 4 fj. by 3 fj, or less, which are to be found embedded in the cytoplasm of the large amoeboid cells of the spleen (Fig. 20, A, L). The true nature K. B FIG. 20. Leishmania. A, Large cell from human spleen containing nine parasites ; B, two parasites more highly magnified; C, D. stages taken from artificial cultures. K, Kinetonucleus ; L, Leishmania , M, host cell ; n, nucleus of host cell ; t, trophonucleus. of these " Leishman-Donovan bodies " is hinted at by the fact that each when stained with a chromatin stain is seen to have within it two deeply stained structures — a larger rounded and a smaller rod-shaped (Fig. 20, B, t and K) — which clearly recall the trophonucleus and the kineto- nucleus of a trypanosome, and Rogers was able to show eventually that when inoculated into certain culture media containing blood, especially if slightly acid, these bodies lengthen out, develop a flagellum and swim about as creatures resembling the genus Leptomonas (Fig. 20, D). Nothing definite is known regarding the transmission of the parasite but it has been found to develop into the flagellate form when taken into the alimentary canal of bugs and this suggests that these insects may be the normal intermediate hosts. i FLAGELLATA 51 Three species of Leishmania have been clearly recognized so far : — (1) L. donovani, the parasite of Kala-azar. This species has been found to occur in Dogs and possibly this animal is the normal host of the parasite. (2) L. in/antum, which produces an enlargement of the spleen, something like that of Kala-azar, in children in Algiers, Tunis and South Italy. (3) L. tropica, which is found in superficial sores in the skin (Tropical Ulcer, Delhi Boil). As these occur always on exposed parts of the body transmission may in this case be effected by some flying insect. L. tropica occurs in Northern Africa and Asia, and appears also to be responsible for an ulcerative disease prevalent in some districts of Paraguay and Brazil and often mistaken for Syphilis or Yaws. The Flagellata are Protozoa of usually comparatively small size and possessing in the normal adult phase of their life-history one or more flagella by the movements of which they swim. They show a remarkable variety in their form, while of even greater interest is the variety in their mode of nutrition. In attempts to draw a boundary between the animal and the vegetable kingdom a principal factor made use of is the difference in mode of nutrition — an animal typically nourishing itself by the ingestion of complex organic food material (holozoic nutrition) while a plant either builds up its complex organic material out of simpler components as in the case of green plants (holophytic nutrition) or else absorbs products of metabolism or decay by its general surface (saprophytic nutrition). The invalidity of any such general distinction is at once clear from the study of the Flagellata, for here we have an assemblage of creatures undoubtedly closely related together and yet making use of all three types of nutrition. The great majority of flagellates live free lives in water but many members of the group have taken on a parasitic mode of existence. They are particularly common in the alimentary canal of various animals. Insects such as flies are very prone to harbour them (Crithidia, Lepto- monas, etc.) and it seems probable that such forms as Trypanosomes are to be regarded as primitively insect parasites which with the develop- ment of the blood-sucking habit have spread to vertebrates. III. SPOROZOA We will commence the study of the Sporozoa by going over in some detail the life -history of two illustrative genera — Monocystis, chosen 54 ZOOLOGY FOR MEDICAL STUDENTS CHAP. because it is more easily obtained for laboratory study than any other Sporozoan, and Plasmodium, chosen on account of its great practical interest as being the cause of one of the most destructive of all diseases — Malaria. MONOCYSTIS Monocystis is a very common parasite of the ordinary earthworm Lumbricus. If the body of a freshly killed earthworm be slit open and the body wall pinned out flat so as to display the internal organs there will be seen towards the head end a clump of irregular yellowish- white organs known as the seminal vesicles (Fig. 67, p. 138). If a piece of one of these be pulled off with a pair of forceps and dabbed up and down in a drop of normal saline solution1 the latter will be made milky by the whitish contents of the seminal vesicle. Examination with the micro- scope shows these to consist of various stages of the developing micro- gametes or spermatozoa of the worm. Amongst these a very conspicuous stage is that known as the sperm-morula, from its resemblance to a microscopic raspberry or mulberry (tnorula), spherical in shape and having its surface covered by a layer of little rounded bodies destined to lengthen out and become microgametes. The young Monocystis is to be found as a small spherical or ellipsoidal cell embedded within the central protoplasm of the sperm-morula (Fig. 21, A). Within this it grows rapidly, absorbing nourishment from the surrounding protoplasm which becomes stretched out by the growing body of the parasite (Fig. 21, B) until eventually it forms merely a thin film (Fig. 21, C). In the meantime the microgametes of the worm have been going on with their development, each little rounded body becoming first pointed at its outer end and finally drawn out into a fine thread. When this stage has been reached the Monocystis is enclosed in a thick furry coat, each hair of which represents a spermatozoon of the worm. Eventually the Monocystis becomes freed from its furry coat and presents the appearance shown in Fig. 21, D. It contains a single round nucleus. Its protoplasm is bounded by contractile ectoplasm the surface layer of which is stiffened to form a distinct pellicle. The endoplasm is laden with stored-up food material in the form of highly refracting granules of paraglycogen, a substance allied to glycogen or animal starch, which give the Monocystis a snowy white appearance when seen against a black background by reflected light. The portion of the life-history so far described is above all characterized by the active absorption of food, which finds its expression first in growth and later in the storing up 1 '75% Common Salt (NaCl) in water. MONOCYSTIS 53 reserve food material. This phase in the life-history is known Before as the trophozoite phase. The mature trophozoite lives for a time free in the cavity of the B FIG. 21. Mcnocyslis. A-D, Stages in growth of trophozoite. In D the " adult " trophozoite is seen as it is found free in the cavity of the seminal vesicle ; in A, B and C it is seen to be contained in the interior of the sperm-morula ; E, gametocytes in cyst ; F, formation of gametes ; /i-?, details of process of syngamy ; G, cyst containing pseudo-navicellae ; H, pseudo-navicella free from cyst ; I, group of sporozoites free from pseudo-navicella. [The details of syngamy are such as have been actually observed in Monocystis. Probably the whole process resembles that shown in Fig. 22 which is taken from an allied genus, Stylorhynchus.] 54 ZOOLOGY FOR MEDICAL STUDENTS CHAP. seminal vesicle and then becomes a gametocyte. Two individuals enter into association, rounding themselves off, coming into intimate contact and surrounding themselves with a spherical protective cyst (Fig. 21, E). Each of the two individuals so associated together is a gametocyte. The nucleus of each individual undergoes repeated mitosis, giving rise eventually to a great number of small nuclei which take up their position in the superficial layer of the protoplasm. The latter takes on an irregular lobed shape and eventually each nucleus collects a small quantity of cytoplasm round itself to form a gamete. The gametocyte FIG. 22. Details of process of syngamy in Stylorhynchus (after Leger). i, Motile ( (5 ) and stationary ( 9 ) gametes have come in contact ; 2 and 3, stages of fusion ; 4, zygote. thus is resolved into (i) an immense number of gametes and (2) a mass of residual protoplasm which is left over and disintegrates. A noteworthy feature is that the gametes derived from the two gametocytes differ in shape, the one lot being rounded the other pointed (Fig. 21, F), and may differ very obviously also in size. There now takes place an active quivering movement, the pointed gametes dashing wildly about — an indication that they are provided with a flagellum as is the case with the somewhat different species illustrated in Fig. 22. Syngamy now takes place between pairs of gametes one pointed and one rounded (cf. Fig. 22). In other words — of the two gametes which fuse together one is derived from each gametocyte. In this way (Fig. 2i,fi-f$) numerous zygotes i MONOCYSTIS 55 are formed each of which takes on a spindle shape and surrounds itself with a characteristic boat-shaped cyst (" Pseudo-navicella " of micro- scopists). Within this cyst (Fig. 21, /6) the zygote divides with mitosis three times, becoming resolved into eight sausage-shaped bodies, each with a nucleus near its centre, the sporozoites (Fig. 21, /;). So far as is known nothing more happens during the life of the worm but if the worm dies and its body disintegrates or if it is digested by a bird the cysts are set free, their walls break down, and the " pseudo-navicellae " become distributed through the soil. Presumably wtien swallowed by an Earth- worm the boat-shaped cyst is dissolved, the eight sporozoites are set free (Fig. 21, I) and make their way through the tissues of the worm to the seminal vesicle where they bore into sperm-morulae and start the life-cycle afresh. PLASMODIUM There exist probably a number of different species of malarial parasite and of these three have had their life-histories fully worked out. The relatively small differences in detail which mark off the species from one another will be indicated after a general sketch of the life-history has been given (Fig. 23). As is well known malarial fevers are characterized by the recurrence of febrile attacks at definite intervals such as 48 hours or 72 hours. If a drop of blood taken from a patient at the end of one of his febrile attacks be examined microscopically the parasite will be found in the amoebula stage — a minute amoeba-like creature which creeps about slowly in the substance of a red blood -corpuscle (Fig. 23, A). The amoebula nourishes itself at the expense of the corpuscle and increases gradually in size (Fig. 23, B). As it does so a characteristic feature is the appearance within its cytoplasm of minute particles of a dark brown, almost black, pigment — one of the iron-containing pigments known to the chemist as melanins, a product of the digestion by the parasite of the red iron-containing pigment of the blood (Haemoglobin — see p. 141). It is also very usual for fluid to accumulate within the amoebula as a conspicuous vacuole which gradually attains to such a size that the parasite assumes the appearance of a signet-ring — the nucleus being pushed to one side (Fig. 23, C). With further growth the vacuole dis- appears and the parasite occupies the whole of the interior of the corpuscle. The portion of the life-history so far described is the trophozoite phase. The full-grown trophozoite now becomes a schizont, i.e. a stage which reproduces by schizogony. Its nucleus divides several times (Fig. 23, D) and the cytoplasm segments into a number of fragments MOSQUITO MAN IAP. I PLASMODIUM 57 Fie. 23. Life-history of Plasmodium. A-E, Stages of growth and division of schizont within red blood- corpuscles ; A, B, young amoebuhe ; C, " ring " stage ; D, schizont with divided nucleus ; E, group of merozoites formed by subdivision of schizont ; F, group of merozoites set free by rupture of corpuscle — the granules of melanin are left behind in a loose heap : F-A, the dotted line indicates the repetition of the schizogony cycle ; G, amoebulae which will become male ( £ ) or female ( 9 ) gametocytes (H) ; I, fully developed gametocytes lying free in the blood ; J, maturation of the gametes in stomach of mosquito — the upper (9) is extruding its polar body, the lower (Q*) isjgiving off six slender microgametes . K, syngamy ; L, actively moving zygote (ookinete) ; M, zygote has burrowed into one of the cells of the stomach-wall ; N, it has rounded itself into a sphere on the outer surface of the stomach ; O, it is increasing in size and dividing into sporoblasts ; P, each sporoblast is developing slender sporozoites over its surface ; Q, large round mass of sporozoites with residual protoplasm ; R, sporozoites set free in blood of the mosquito by rupture of the mass ; S, lobe of salivary gland, sporozoites are seen embedded in the cells of the gland while others have penetrated into the central duct ; T, free sporozoites as injected into man. [The schizogony part of the life-cycle (A-F) takes 48 hours in P. vivax, 72 hours in P. malaria^ . In P. falciparum it is less regular, occupying from 36 to 48 hours. The sporogony part of the life- cycle commonly occupies about 10-12 days.] 58 ZOOLOGY FOR MEDICAL STUDENTS CHAP. (merozoites) each containing a nucleus (Fig. 23, E). The corpuscle now bursts and in its place there remains the group of merozoites (Fig. 23, F) with the melanin granules collected in a little heap, having been extruded from the living protoplasm during the process of schizogony. The merozoites now creep away through the blood as little amoebulae which eventually bore their way into new corpuscles and start the cycle afresh (Fig. 23, F->A). The whole of this part of the life-cycle ending in the process of schizogony takes, in at least two of the species of Plasm odium, a definite period (48 and 72 hours respectively) for its completion,, and its completion, the setting free of the merozoites, is punctuated by the onset of a febrile attack (Fig. 24) due apparently to some virulent poison or toxin, possibly the excretory material of the parasite, being set free in the blood along with the melanin when the corpuscles rupture. The schizogony cycle goes on being repeated over and over again in the blood of the patient so long as the disease lasts. Its result is auto- infection, i.e. the spreading of the infection in the blood-corpuscles of the same individual host. Eventually however certain of the amoebulae which have entered blood-corpuscles are seen to be behaving rather differently from those destined to become schizonts : they are inaugurat- ing a new and very complicated part of the life-history known as the sporogony cycle, characterized by the occurrence of a sexual process of syngamy, culminating in the production of sporozoites, and having as its special function the conveyance of the parasite to new host individuals. The amoebulae which start the sporogony (Fig. 23, G) increase in size within the corpuscles but when they reach the limit of their growth are not schizonts but gametocytes which become free from the corpuscle and may be seen as spherical cells (Fig. 23, I) lying free in the fluid of the blood. Two distinct types may be recognized — female macro- gametocytes, rather larger, the cytoplasm more deeply staining and laden with particles of stored-up reserve food material, the nucleus situated on one side close to the surface— and male microgametocytes, rather smaller, the cytoplasm staining less deeply, and the nucleus large and central in position. If blood containing fully developed gametocytes is drawn from the body and allowed to cool upon a slide under the microscope, more especially if moistened by being breathed upon, the gametocytes may be observed within a period of half an hour or so to give rise to gametes (I<"i,u. 23, J). In the case of the macrogametocyte the nucleus becomes constricted across into two parts, one of which is extruded : the macro- gametocyte is by this process of maturation converted into a macro- gamete. In the case of the microgametocyte the single nucleus becomes PLASMODIUM 59 divided so as to produce small nuclei normally about six in number. The cytoplasm now becomes rapidly extended outwards into about six long slender threads into each of which there passes one of the small nuclei also elongated, almost threadlike, in form. These protoplasmic threads lash about violently like so many flagella, eventually tear them- selves free from the central mass of cytoplasm— which then degenerates — and swim off actively through the fluid of the blood as so many micro- gametes. If it comes into proximity with a macrogamete, the micro- gamete dashes towards it, fuses with it (Fig. 23, K), and becomes drawn in and completely merged with its substance. Complete nuclear fusion A B FIG. 24. Curve showing the fluctuation in body temperature in a person suffering from Malaria caused by Plasmodium vivax. The temperature scale on the left is in degrees Centigrade, that on the right in degrees Fahrenheit. The horizontal line indicates normal temperature. The vertical lines are drawn at 24-hour intervals. Parasites (A, schizont ; B, group of merozoites) are drawn below the curve to indicate the point about which schizogony takes place. takes place and there now exists a zygote in place of the two separate gametes. The fact that these processes of maturation and syngamy are induced by cooling the blood is correlated with the fact that this part of the life- history takes place normally in the body of a cold-blooded insect — a mosquito of the genus Anopheles or of one of the genera allied to it. When such a mosquito takes in a meal of malarial blood all the stages of the parasite except the fully developed gametocytes are promptly killed and digested. The gametocytes on the other hand give rise to gametes in the way described,, and zygotes are formed by the process of syngamy. The zygote — the superficial layer of whose protoplasm becomes modified to form a distinct thin membrane — soon loses its spherical form, becoming somewhat pointed at each end, and becomes an actively motile zygote or ookinete (Fig. 23, L). This creeps about 60 ZOOLOGY FOR MEDICAL STUDENTS CHAP. within the cavity of the stomach of the mosquito; burrows through its wall and just outside the layer of cells which form the greater part of the thickness of the wall again rounds itself off into a sphere (Fig. 23, N). Its nucleus divides several times in succession and the cytoplasm segments into a number of sporoblasts each containing a nucleus (Fig. 23, 0). Each nucleus now undergoes division a great many times in succession, the small nuclei making their way towards the surface of the sporoblast and a little mass of cytoplasm segregating round each to form a sporozoite (Fig. 23,, P). These sporozoites are at first rounded but become spindle- shaped and later much elongated and shaped almost like the microgametes. During the process of sporozoite-formation the spherical mass of sporo- blasts undergoes a great increase in size (Fig. 23, Q) and the number of sporozoites into which, with the exception of a certain amount of residual protoplasm, it is ultimately resolved is very vast. Eventually the delicate membrane enclosing the mass of sporozoites ruptures, the sporozoites are set free in the blood of the mosquito (Fig. 23, R), they bore their way through the cells of the salivary glands (Fig. 23, S) into its duct and when the mosquito next bites are injected with its saliva into the blood of the animal bitten (Fig. 23, T). If this be a human being or other suitable creature the sporozoite attaches itself to a red corpuscle, burrows into it, and becoming an amoebula starts the whole life-cycle afresh (Fig. 23, A). . Of the various types of malarial fever in man there are three which are particularly well marked and which have been investigated particu- larly completely. These are associated with three different species of parasite — Plasmodium vivax, P. malariae, P.falciparum. P. vivax is the parasite of ordinary Tertian fever or Tertian ague as it used to be called. The period occupied by the schizogony is about 48 hours so that the fever attack occurs every other day (Fig. 24). A distinctive characteristic is the number of merozoites (15-20) composing the group derived from a single schizont (Fig. 25, B). P. malariae is the parasite of Quartan fever or Quartan ague in which the schizogony cycle occupies 72 hours so that two successive fever attacks with the intervening period occupy four days. In it the number of merozoites arising from a single schizont is usually 6-12, and they often are arranged in a very regular rosette (Fig. 25, A). P.falciparum is the parasite of "Tropical fever." In this case it is difficult to make certain of the exact period occupied by the process of schizogony owing to the fact that the breaking up of the schizonts takes place usually in the capillary blood-vessels of the brain, spleen and other PLASMODIUM 61 internal organs and not in the more accessible vessels of the skin. The period is most probably 48 hours (" malignant tertian fever ") but it may be some other period or even irregular. The merozoites are usually 8-15 in a group and a marked diagnostic feature is the form of the fully developed gametocytes which until they become free from the corpuscle are sausage-shaped (" crescents," Fig. 25, D). One of the unpleasantly interesting characteristics of malaria is its liability to recur in an individual who may have been apparently free from the disease for a prolonged period — several years — and who has not been exposed to any possibility of re-infection. It is clear that this must be due to some of the parasites lurking on within the body after the great majority have died off. It is most probable that during the periods of apparent health a few parasites are all the while going on B F.G. 25. Malarial parasites showing characteristic differences between different species. A-C, Schizogony in Plasmodium malariae (A), P. vivax (B), and P. falciparum (C) ; D, gametocyte of P. falciparum enclosed within remains of blood-corpuscle. with the normal cycle of schizogony — the total numbers never becoming large enough to cause obvious symptoms — although on the other hand it has been suggested that it is one particular phase of the parasite, namely the macrogametocyte, that is endowed with special powers of resistance and is capable of remaining for long periods dormant, awaiting the onset of favourable conditions, when it bursts into activity, behaves as if it were a schizont, and starts off the infection of large numbers of corpuscles. It will be of interest before leaving the subject of the malarial parasites to note the names of the chief workers by whom our present knowledge has been built up. The foundation of this modern knowledge may be said to be the discovery by Lankester in 1871 of the first protozoan parasite of a blood-corpuscle — the genus Drepanidium or Lankesterella as it is now called — in the blood of the frog. The special foundations of our knowledge of the malarial parasites of man were 62 ZOOLOGY FOR MEDICAL STUDENTS CHAP. laid by Laveran (1880) who observed for the first time quite a number of the stages in the life-history — amoebula, merozoites, gametocytes. He even observed the formation of microgametes though he interpreted them as flagella and looked on their development by the " flagellated body " as an abnormal process. Laveran definitely held that these various appearances which he observed were stages in the life-history of a parasitic organism and that that organism was the actual cause of malarial disease,, but for a long period his views met with little acceptance. In 1895 Ronald Ross fed mosquitos on the blood of a malarial patient containing " crescents " and observed the formation of the " flagellated body " within the insect's stomach. In 1897 a most important step was made by MacCallum, who observing a parasite of the malarial type in the blood of a " crow " saw the process of syngamy take place before his eyes and consequently rendered clear the meaning of Laveran's " flagellated body." Meanwhile Ross was continuing his investigations in India. On feeding mosquitos (1897) with malarial blood containing crescents he as a rule got no result, but in one kind of mosquito with dappled wings he observed the parasites after 4 to 5 days lying embedded in the wall of the stomach, in the form of round cells containing the characteristic melanin pigment. Ross concluded that he had now found the normal insect host of the parasite, and although he does not name the mosquito it is clear from his description that it belonged to the genus Anopheles. Circumstances interfering at this point with his work on human malaria Ross carried on his experiments with a malarial parasite (Proteosoma) of Birds and was able (i) to show that in this case the transmitting insects were mosquitos of the genus Culex and (2) to work out practically the whole sporogony cycle. The working out of the corresponding details in the parasite of human malaria within the body of the Anopheles is due in great part to Grassi and his Italian colleagues (1898) and the final completion of the life-history may be said to have been achieved by Schaudinn (1902) who was able to observe the sporozoite actually attacking the blood-corpuscle. The group Sporozoa includes a great variety of Protozoa which are linked together by certain common features. They always live as parasites within the bodies of other animals. They are, in the full- grown condition, without cilia or flagella. Their surface protoplasm is condensed to form a thin pellicle without any openings and correlated with this they feed by simply absorbing nourishment through the general surface of the body. They fall naturally into two main groups according as to whether the reproductive processes are distributed through the i SPOROZOA 63 trophic part of the life-history, i.e. through the period of active feeding, or are concentrated into a special period at the end of the trophic stage. These two main groups are known as the Neosporidia and the Telosporidia. A. TELOSPORIDIA (1) GREGARINIDA. This group includes Monocystis. In it the tropho- zoite is at first intracellular, living embedded in the protoplasm of the host, but it is to be noted that the cell containing it is never a blood- corpuscle. The parasite becomes free from the host-cell before sporogony takes place and the number of sporozoites enclosed within one cyst or capsule is usually eight. The gregarines occur as parasites in most of the main groups of invertebrate animals except possibly the Mollusca. (2) COCCIDIA. These occur as parasites in Arthropods (e.g. Centipedes), in Molluscs (especially Gasteropods and Siphonopods) and in Vertebrates (e.g. Rabbit). The trophozoite in this case remains throughout a more or less spherical intracellular parasite, growing within the host-cell and gradually destroying it. When fully grown it becomes a schizont and divides into merozoites which infect new cells and in this way great destruction of tissue may take place, resulting sometimes in the death of the host. Even without this a limit is reached in the activity of the schizogony process and sporogony takes place (also intracellular), gametes being formed which conjugate to form zygotes. The spherical zygote surrounds itself with a stout cyst which shelters it when it passes away from the protection of the host's body. Within this cyst the zygote divides into sporozoites the number of which differs in different members of the group. The sporozoites are set free when the cyst is swallowed by a suitable host and burrowing into host-cells start the life-cycle afresh as young trophozoites. (3) HAEMOSPORIDIA. In this group — exemplified by Plasmodium — the trophozoite is for a time at least amoeboid and intracellular — the host-cell being usually the red blood-corpuscle of a Vertebrate. Repro- duction takes place by schizogony, followed after a time by sporogony — the zygote giving rise to sporozoites which become free instead of remaining shut up within a cyst. Typically the sporogony or sexual cycle is gone through in the body of an intermediate host such as some species of blood-sucking insect. Parasites closely allied to those which cause malaria in man occur in various mammals and other vertebrates. In Birds there occur commonly species of Plasmodium — sometimes separated off as a different genus under the name Proteosoma — and Haemoproteus in which latter 64 ZOOLOGY FOR MEDICAL STUDENTS CHAP. the gametocyte has a very markedly crescentic form. These species of Proteosoma and Haemoproteus are of historical interest for as already indicated it was in them that the process of syngamy was first observed (Haemoproteus} and the details of the sporogony cycle first worked out (Proteosoma of Indian Birds). A group of parasites of great practical importance are those which are grouped under the generic name Babesia or Piroplasma (Fig. 26). These are small amoeboid parasites of red blood-corpuscles, commonly rounded or oval in form. Within the corpuscle they reproduce by fission so that commonly two occur together within a single corpuscle and sometimes four or eight. Whether or not a sexual or sporogony cycle occurs and whether or not there is an actively swimming flagellate phase are questions not as yet satisfactorily answered. The best-known species of Babesia (B. bovis or B. bigeminum) is that which causes the very destructive disease of Caltle known by such names as Texas Fever — North America, Tristeza — Spanish America, Redwater Fever (i.e. Haemoglobinuria) — Australia. The life- history of this parasite was first blocked out by Smith and Kilborne (1893) in North FlG- 26- America. TWO red blood- The disease is endemic in various regions corpuscles of a mammal infected with Babesia. in the corpuscle in the Southern States and Mexico. Cattle on the left the parasite has under- dri northwards during the warm season gone fission into two. from these infected areas were found to infect pastures through which they were driven. Two important peculiarities were observed in this infection, (i) The herds of cattle were found gradually to lose their power of infecting new pastures as they were driven further and further northwards. (2) It was found that a newly infected pasture did not communicate the disease until at least 30 days had elapsed since the passage of the infective herd. Both of these puzzling peculiarities were explained when the mode of trans- mission of the parasite was worked out. Transmission is carried out through the agency of intermediate hosts — Ticks (see p. 257) of the genus Rhipicephalus or Boophilus, the precise species being different in different parts of the world. If, and only if, infected blood is taken in by an adult female Tick certain pear-shaped individuals of the Babesia leave the blood-corpuscles, put out long slender radiating pseudopodia, and wander away through the tissues of the Tick's body, some of them reaching the ovary and creeping into the substance of the eggs which become thus infected. i SPOROZOA 65 Tin- tick at length drops off the animal whose blood it ha> U-rn sucking and proceeds to deposit its eggs amongst thi . I -.already infected while in the body of the parent, develop into infected ticks and these creep up on to grass blades and patiently wait for the oppor- tunity of attaching themselves to an animal. Should thi> happen the animal bitten is inoculated with the Babesia, which occurs in the salivary -lands of the infected tick in the form of swarms of " sporozoites." The details of the life-cycle within the body of the tick which culmin- ates in the formation of these sporozoites is not yet completely worked out. Besides the parasite of Texas Fever a number of other closely allied species are known. In various parts of Europe a similar parasite has been found in the blood of cattle and may be the same species. In South Africa practically all cattle have in their blood B. mutans, while occasion- ally a somewhat similar parasite — Theileria parva — causes destructive epidemics [" East Coast Fever " — South and East Africa, Central Asia, Japan, etc.]. Other species of Babesia occur in Dogs, Sheep, Horses, Mice, and other mammals. A conspicuous feature of the disease (Piro- plasmosis or Babesiosis) is the destruction of red blood-corpuscles and the consequent passing away of the red colouring matter of the blood in the urine (Haemoglobinuria), and it is further a general characteristic that the transmission of the parasite is carried out through the agency of Ticks, in the body of which the parasite goes through a complicated and as yet not completely worked out cycle of developmental changes. B. NEOSPORIDIA The remaining members of the Sporozoa differ from those hitherto mentioned in the feature that reproductive processes go on throughout the period of active feeding and growth instead of being relegated to a point in the life-history subsequent to this. They are hence grouped together under the special heading Neosporidia. (4) CNIDOSPORIDIA. Under this heading are grouped together a number of Sporozoa characterized by the fact that their spores possess peculiar bodies known as polar capsules (Fig. 27). A polar capsule is a pear-shaped hollow structure containing in its interior a spirally coiled hollow filament which can be instantaneously shot out so as to perforate any soft surface with which the spore is in contact and in this way anchor it in position. The extrusion of the filament commonly takes place in the alimentary canal of some animal that has swallowed the spore, and the spore thus is held in position attached to the lining of the alimentary F 66 ZOOLOGY FOR MEDICAL STUDENTS CHAP, canal should it happen to have been properly situated in relation to this lining at the moment extrusion occurred. A. Myxosporidia. This, the first sub-section of the Cnidosporidia, includes a number of common parasites of fish — usually harmless but occasionally causing destructive epidemics. They are to be found creeping about amongst the tissues of the body or within its cavities such as the urinary bladder or the gall bladder. They are more or less Amoeba-like organisms which creep by lobopods but which do not feed by them — nourishment being absorbed in solution by the general external surface. There is a distinct clear transparent ectoplasm and a granular endoplasm containing numerous nuclei. Within the endoplasm are produced the spores — by complicated processes, accompanied by sexual fusion of nuclei, which need not be described in detail. Normally the process of spore- formation goes on continuously but it may be concentrated in particular seasons. Thus in Myxidium — a bright orange -coloured parasite found creeping about on the lining of the urinary bladder of the Pike — the process is almost confined to the summer « P months while in winter the creature re- . produces actively by separating off bud-like FlG. 27 outgrowths from its surface. A, Spore with B. Microsporidia. This group includes its two polar capsules ; B, a a number of very small intracelluliir para- separate polar capsule ; C, a polar capsule with its tube extruded. sites occurring occasionally in Fishes but far more usually in Arthropods. Some of them are of practical interest as causing destructive epidemics in animals of economic importance (Nosema bombycis — Silkworm disease, N. /.v — Bee disease). They are distinguished from Myxosporidia by their possessing only one polar capsule and filament. (5) SARCOSPORIDIA. This group includes a number of intracellular parasites of the higher vertebrates, especially mammals, which are ex- tremely common but the life-history of which is still very imperfectly known. The young parasite in the form of an amoebula makes its way into the interior of a muscle-fibre and absorbing nourish- ment grows actively, becoming eventually free from the muscle-- fibre and taking the form of a long tendinous-looking thread or tube, reaching a length of it may be 16 mm. (Slurp) or even 50 mm. (Roe-deer). The parasite becomes enclosed in a distinct envelope which extends into its interior dividing it up into numerous chambers. ! NKOSPORIDIA 67 \\'ithin these there are formed enormous numbers of crescent-shaped spores. These threads or tubes (" Miescher's tubes ") are distinctly visible to the naked eye, and are common in butcher-meat. Sheep and I are nearly always more or less infected, the parasites being particularly frequent in the muscular wall of the alimentary canal especially the oesophagus. A virulent toxin is commonly formed in the substance of the parasite but it is only in exceptional cases, as in the species found in the Mouse, that marked pathological symptoms are produced. Nothing is known definitely as to the normal means of transmission though mice can be infected experimentally by feeding them on infected muscle. The fact that spores sometimes — possibly by bursting of the cyst — find their way into the circulating blood indicates at least the possibility of there being a blood -sucking intermediate host. (6) HAPLOSPORIDIA. In this group are included a number of com- paratively simple but still very insufficiently known parasites. They typically begin their existence as an amoebula which may reproduce repeatedly by fission but which eventually increases greatly in size, becomes multinucleate, and breaks up into numerous spores. These spores are simple in structure and are without the polar capsules which are so characteristic of the Cnidosporidia. Some cause disease, sometimes very destructive epidemics, in Fish ; one is found in a rare type of tumour of the nose in Man (Rhinosporidium), while others live within the bodies of various kinds of animals without producing any obvious pathological disturbance. IV. CILIATA The last great division of the Protozoa the CILIATA or INFUSORIA includes a vast assemblage of species showing wonderful variety in form and in the details of their structure. Included amongst them are the most complex and highly organized of all the Protozoa : some indeed — being unicellular — may be said to be the most complicated and highly organized individual cells known. We shall commence their study by considering in detail a very common member of the group, Paramecium, specimens of which are commonly to be seen in the form of minute white specks, just visible to the naked eye, gliding slowly about in fresh water in the neighbourhood of decaying organic matter. Observed under the microscope a Paramecium is seen to have the form shown in Fig. 28. The body is limited by ectoplasm of remarkable complexity in which four layers can be distinguished. The outermost FI 68 ZOOLOGY FOR MEDICAL STUDENTS CHAP. c.v. ex. oes. of these — the pellicle — forms a tough membrane which bounds the surface of the creature and gives it its definite form. In a Paramecium which has been allowed simply to dry up on the microscope slide it may be seen that the surface of the pellicle is not absolutely smooth but is sculptured into a minute honey- comb pattern. Beneath the pellicle is a layer of protoplasm which shows a fine striation in a direction perpendicular to the surface, and beneath it in turn is a layer characterized by its containing numerous shining spindle - shaped bodies also arranged with their long axes perpendicular to the surface. These bodies — the trichocysts (Fig. 28, tr) have a remarkable function. If the surface of the Paramecium be irritated the trichocysts suddenly explode, each losing its spindle form and taking that of a fine needle or filament. The exploding trichocysts, shooting out into the water all round, surround the Paramecium with an impenetrable entanglement which effectually keeps off the attacks of assailant organisms. The fourth or inner- most layer of the ectoplasm con- sists of a spongy protoplasm in the meshes of which water collects from the fluid endoplasm. As the water in this layer increases in quantity it collects especially in two groups of radiating tubu- lar channels situated respectively about half-way between the centre and each end of the Paramecium. The water in the radiating channels collects towards their inner ends, and then slowly drains out of the channels into a central drop (Fig. 28, c.v). When this has attained to its full size it suddenly discharges to the exterior, showing itself thereby to be a contractile vacuole. The process of expansion (diastole) and n. c.v: FIG. 28. Paramecium. c.v, Contractile vacuole sur- rounded by star of tributary vacuoles ; ex, excretory crystals ; f.v, food vacuoles : m, mouth ; N, macronucleus ; n, micronucleus ; oes, oesophagus ; p, peristome ; tr, trichocysts (some at the upper end of the figure have exploded). i PARAMECIUM 69 mut ruction (systole) of the contractile vacuole is repeated rhythmically at intervals — commonly about five minutes, hut \ar\in- with circum- stances and with the individual. Tin- complexities of the ectoplasm are not exhuuMrd by tin leatun •> already mentioned : there remains one of its most characteristic feature namely that it projects outwards in the form of innumerable little protoplasmic hairs or cilia. Each cilium is actively movable, being able to bend suddenly in a definite direction and then recover slowly. The cilia are present in enormous numbers, one arising from the centre ol each of the little dimples that give the ectoplasm its honeycomb surface. The cilia work in unison, the rapid bending taking place in the same direction towards one end of the body. The result of the movements of the cilia, like thousands of little paddles, is to cause the Paramecium to glide slowly through the water, one definite end — that which is above in Fig. 28 — being normally in front, although the move- ment is capable of reversal. The endoplasm of the Paramecium is, so far as visible structure is concerned, comparatively simple. It is very fluid ; it is granular in appearance — the granularity being due to the presence of minute particles of various kinds, such as excretory matter or reserve food-material. In the living Paramecium the endoplasm shows a slow circulation within its bounding layer of ectoplasm. Embedded in the endoplasm lies the very characteristic nuclear apparatus. This consists of a large maeronucleus (or meganucleus, Fig. 28, N) — kidney-shaped, and serving to carry out the ordinary nuclear function of controlling the general processes of metabolism — and a small rounded micronucleus l (Fig. 28, n) lying usually in the concavity of the surface of the maeronucleus. So far as is known the micronucleus becomes functionally active only at the period of repro- duction,, in which process it plays a very important part. The fact that the fluid endoplasm is enclosed within a comparatively stiff ectoplasm gives the Paramecium its definite form — rather elongated, slightly pointed towards the ends, the front end rather less pointed than the hinder one, and somewhat flattened. A broad shallow valley — the peristome (Fig. 28, p) — starts from one edge of the Paramecium near its front end and extends somewhat diagonally backwards across about one-half of its width. The somewhat oblique position of the peristome and especially of its hinder boundary causes the water pressure on the surface of the Paramecium as it glides along to give it a movement of 1 In P. caudatum. P. aurelia a very similar species, also common, possesses two micronuclei. F2 7o ZOOLOGY FOR MEDICAL STUDENTS CHAP. rotation about its long axis which is very characteristic. At the hinder limit of the peristome is an opening — the mouth (Fig. 28, m) — at the edges of which the pellicle is turned inwards to form a wide somewhat curved and tapering tube — the oesophagus or gullet — which dips down into the endoplasm and is cut off sharp at its inner open end. During the life of the Paramecium a rapid flickering may be observed within the gullet. This is caused by the movements of the undulating membrane — a thin protoplasmic curtain (formed of a row of large cilia adhering together side by side) down which there pass in rapid succession waves of movement just like those produced on a large scale by moving one end of a curtain backwards and forwards. The undulating membrane performs an important part in the feeding of the Paramecium. Minute food particles such as Bacteria are whirled round by a vortex produced by the cilia of the peristome into the neighbourhood of the mouth where they are caught by the indraught due to the movements of the undulating membrane. They are carried down the gullet, collecting at its lower end and being forced into a drop of water which bulges from the inner end of the gullet into the fluid endoplasm. This drop (Fig. 28, f.v) increases in size as more and more water is forced into it until at length it detaches itself like a soap-bubble from the end of a pipe and passes away into the endoplasm as a typical food-vacuole. As it is carried round in the slow circulation of the endoplasm, much of the water of the vacuole is absorbed by the surrounding endoplasm, so that the vacuole diminishes in size. Acid is secreted into the vacuole to kill the food- organisms and this acid phase is succeeded by an alkaline one in which the food-material is attacked by the digestive ferments, the products of digestion being absorbed while the indigestible detritus is left as faecal material. Finally the vacuole approaches the surface and bursts to the exterior at a point between the mouth and the hinder end at which there is a definite small opening (anus) in the pellicle, the faecal matter being in this way got rid off. The life-history of Paramecium, while of much less complexity than that of some of the Protozoa already described, is of great importance especially in relation to its reproductive processes. A healthy Para- mecium, isolated and provided with abundant food, grows rapidly and as in the case of Amoeba this increase in size finds its corrective in a process of fission, the Paramecium becoming gradually constricted across into two individuals which for a time remain connected together end to end but eventually separate. Preparatory to this constriction the two nuclei divide — the macronucleus by a simple process of constriction, the micronucleus by a mitotic process — so that the young individuals are i PARAMECIUM 71 provided each with a macro- and micronucleus like the parent. The two parental contractile vacuoles become the anterior vacuoles of the two in w individuals. The mouths of the young individuals arise by division of that of the parent. The original gullet persists as the gullet of the anterior young individual, while an outgrowth from it forms that of the posterior. The two new individuals so arising proceed with their growth and presently repeat the process of fission. Under favourable conditions fission takes place at more or less regular intervals (e.g. about once in twenty-four hours) until it may be several hundred generations have been produced. In time however the interval between successive fissions becomes prolonged and eventually fission fails completely to take place. This loss of the power to divide is accompanied by general enfeeblement o! the vital processes. There has come about a condition of senility or depression which unless counteracted will lead to the death of the whole culture. The process may be counteracted and new vigour given to the culture by various stimuli — such as changes of food or other factors in the environment — and if this corrective is applied in time the Paramecia go on dividing with renewed vigour and the onset of senility is deferred. Amongst such antidotes to senescence the most important perhaps in nature is the process of syngamy. At a period before the onset of senescence the Paramecia develop a tendency to conjugate. Syngamy does not take place readily between individuals of an isolated culture such as has been described, in other words between closely related indivi- duals. But if two broods are mixed together there comes about an epidemic of syngamy when the individuals are ripe. After this has taken place an isolated individual kept under favourable conditions will be found to be rejuvenated and to have regained its full powers of fission. The minute details of the process of syngamy in Paramecium are of much interest owing to the striking peculiarity that the division of the gametocyte into separate gametes has become suppressed except in so far as the nuclei are concerned and the process of syngamy is no longer a process of fusion of complete cell-individuals but only of nuclei. Two individuals ( = gametocytes) become attached together by their oral surfaces and in this position may be seen swimming about in a normal manner. The micronucleus of each individual undergoes mitosis twice. Of the four nuclei so arising three degenerate and no longer function. The fourth on the other hand divides once again by mitosis and the two nuclei so arising are the functional gamete nuclei. We may take it that probably all four nuclei were once functional, each 72 ZOOLOGY FOR MEDICAL STUDENTS CHAP. dividing into two gamete nuclei, and that the whole individual (gametocyte) gave rise to eight gametes. In Paramecium as we now know it however three of the four nuclei have ceased to function and the division into distinct gametes has also disappeared from the life-history. Of the two functional gamete nuclei in each individual one passes across into the other individual— the pellicle disappearing temporarily over part of the surfaces in contact to allow of this interchange taking place. The two nuclei now in each individual undergo fusion to form a zygote-nucleus, the gap in the pellicle is repaired and the two individuals usually spoken of as exconjugants — separate. During the processes so far described the macronucleus has remained without change hut it now begins to degenerate, it gradually breaks up into fragments which are digested by the cytoplasm, its role having come to an end with the process of syngamy. Each exconjugant starts life provided with a single nucleus the zygote-nucleus which has arisen in the way described. In some allies of Paramecium the zygote-nucleus divides into two, one of which becomes the definitive micronucleus while the other rapidly increases in size to become the macronucleus. In Paramecium itself the process while essentially similar is obscured by repeated nuclear divisions followed by the degenera- tion of certain nuclei and is consequently unsuited for description in detail in an elementary book. The group Ciliata contains a vast assemblage of different kinds of 1'rotozoa which are classified into four sub-groups the scientific names of which are indicative of characteristic differences in regard to the cilia. I. MOLOTRICHA. This section is characterized by the body being covered with a fairly uniform coating of short cilia. It is exemplified by the genus Paramecium. II. HETEROTRICHA. Here also there are cilia distributed over most of the body surface but in this case the cilia round the edge of the peristome are greatly enlarged and concentrated into groups the cilia in each of which are fused together to form a ciliary plate. A good example of this section is the trumpet-shaped Stentor common in fresh waler (Fig. 29, A). Interesting peculiarities of Stentor in detail are the presence of pseudopodia at its narrow end (ps) by which it attaches itself to the surface of water-plants or stones, the curious beaded (" moniliform ") macronucleus (N) and numerous small micronuclei (n), and the power of contracting itself with great rapidity into a rounded form. This lust peculiarity is associated with the fact that special strands of ectoplasm have IK (dine highly specialized for the function of contraction (myonemes). CILIATA 73 III. HYPOTRICHA. In these (Fig. 29, C) which usually resemble Paramecinin in their general form, the cilia have disappeared except on tin- oral surface and here they have become restricted to localized tufts and fused together. At the edge of the peristome they form ciliary plates like those of Stentor but in addition to these there are a number <>! stout pointed structures — each formed of a group of fused cilia — on the tips of which the creature runs about as on so many legs. The movements of the Hypotricha as they run hither and thither on some solid surface in an apparently purposeful manner form perhaps their N. art. ps: Examples of Ciliata. A, Stentor; B, Vorticella : C, Stylonychia. an, Anus ; c.v, contractile vacuole ; N, macronucleus ; n, micronucleus ; PS, pseudopodia ; t.v, tributary vacuole. most striking characteristic. Two common members of the group are Stylonychia (Fig. 29, C) with its dumb-bell-shaped macronucleus and two micronuclei and Kerona which may often be observed running up and down on the surface of Hydra (see p. 87). IV. PERITRICHA. Here the ciliary coating is still more reduced — there being only a spiral of ciliary plates round the edge of the peristome with sometimes a circle of small cilia in addition. A well-known example is Vorticella (Fig. 29, B) — the Bell animalcule — which may often be found attached to freshwater plants. The general shape is that of a bell — the handle prolonged into a slender stalk by which the creature is attached 74 ZOOLOGY FOR MEDICAL STUDENTS CHAP. and the mouth occupied by a slightly convex disc, surrounded by a groove deepened at one point into a funnel-shaped recess (the vestibule) which is continued down into the gullet and into which opens the anus. Disc, groove and vestibule make up the peristome, and the row of cilia runs round the disc and down into the vestibule round which it twists in the form of a continuous undulating membrane like that in the gullet of Paramecium. The macronucleus of Vorticella (Fig. 29, B, N) is elongated and is usually bent into a more or less horse-shoe shape, and there is a single small micronucleus (n). A contractile vacuole is present (c.v) which empties itself into a little pocket-like recess in the wall of the vestibule — the reservoir. The Vorticella is extremely sensitive and at the slightest shock the body contracts to a spherical shape the disc being drawn down and the outer lip of the surrounding groove contracting over it while the stalk becomes coiled into a close spiral. As in the case of Stentor contraction is brought about through the agency of highly developed myonemes. In the stalk when extended a single myonemic band may be seen running throughout in a spiral course ; when this contracts it tends to straighten and the stalk containing it assumes on the contrary a spiral twist. Another member of the Peritricha should be mentioned as it may be observed by the student during his work on Hydra. This is TricJiodiua, a curious disc-shaped ciliate which occasionally may be seen gliding about on the surface of a Hydra. Like Kerona it is found only as a parasite of Hydra and other aquatic animals. ACINETARIA As an appendix to the Ciliata may be mentioned the group ACINETARIA or Suctoria. comprising creatures which have given up the actively moving habit and have undergone characteristic modifications in corre- lation with their sedentary mode of life. The typical Acinetarian (Fig. 30, A) consists of a mass of protoplasm, commonly pear-shaped or in the form of a somewhat triangular disc, attached to the solid substratum by the narrow end which is more or less prolonged to form a stalk. The complex structure of the ectoplasm seen in the typical actively moving ciliate has disappeared. At tin- attached end the ectoplasm is connected with the substratum by secreted material — which may be looked on as ectoplasm that has lost its organi/ed structure— and it is this which may take the form of a long stalk. In some cases the material of the stalk is continued upwards so as to \< l\l.l \KI.\ 75 I'onn a kind of cup enclosing and protecting tin- greater part <>f the body of the creature. In the endoplasm are the nuclear apparatus (macro- and micronucleus) and contractile vacuole as in ordinary Cilia tes. In the sedentary Acinetarian the cilia have disappeared but there an- pit sent other projections of the protoplasm which constitute its most characteristic feature: these are the sucking tubes. Each of these, which looks like an extremely slender pin projecting from the body, is really a very fine straight tube with a slightly expanded trumpet- shaped end (Fig. 30, /). The wall of the tube is a prolongation of the stiff ectoplasm while the interior contains fluid. The tubes vary in I ' N. B FIG. 30. Acinetaria. A, Tokopnrya ; B, Acineta, showing young ciliated stage, c.v, Contractile vacuole ; emb, embryo ; N, macronucleus ; n, micronucleus ; /, suctorial tube. number in different Acinetarians from one up to a large number, when many are present they may be either scattered irregularly or as in the genus Acineta collected into clumps. The function of the sueking tubes is seen if a small Ciliate knocks up against them. It adheres to the end of the tube and its protoplasm is gradually sucked down through the tube into the endoplasm of the Acinetarian where a food-vacuole forms round it and it is gradually digested. That the conclusion which may be drawn from the presence of the characteristic Ciliate type of nucleus that the Acinetaria are modified Ciliates is correct is shown by the study of their life-history — for the young stage is provided with cilia and swims about like an ordinary Ciliate. The young creature arises as a kind of bud. as a projecting piece 76 ZOOLOGY FOR MEDICAL STUDENTS CHAP. of the parental body which projects either outward from the free surface or inwards into a special brood-cavity in which it remains for some time before being set free (Fig. 30, B). Again the process of syngamy in the life-history of the Acinetaria agrees closely with that of the typical Ciliata. It is interesting to note that some of the Acinetarians have taken on a parasitic mode of life : sometimes they are parasitic in the adult condition on Hydroids (Ophryodendron on Marine Hydroids) ; more commonly the young stage alone is parasitic, burrowing in some cases into the body of other Acinetarians (Tachyblaston in Ephelota). An intermediate step towards the parasitic condition is shown by numerous genera whose habit is epizoic, i.e. which live attached simply to the surface of other aquatic animals such as Hydroids, Crustacea, etc., without actually absorbing nourishment from them. Before leaving the subdivisions of Protozoa it is necessary to glance at two groups of organisms the relationships of which are rather uncertain but which have come into prominence of recent years in connexion with the causation of disease. THE SPIROCHAETES The first of these includes a variety of creatures which may be called by the general name Spirochaetes. These resemble somewhat in general appearance very slender Trypanosomes and although formerly grouped together as one genus Spirochaeta are now usually subdivided among a number of separate genera. For the sake of simplicity we shall here adhere to the older practice — mentioning parenthetically the newer generic names. A spirochaete is as already indicated somewhat like a very slender and usually very minute trypanosome. The thread-like body shows in the dead specimen a number of undulations from side to side but owing to the minute size it is difficult to be certain as to whether these are merely bends from side to side as in a trypanosome or, as is more probable, the turns of a cork-screw spiral. When alive the movements are very characteristic, the spirochaete swimming rapidly in the direction of its length for a short distance and then reversing its movement. It is again difficult to make sure whether this movement consists, like that of a trypanosome or an eel, of movements of flexure from side to side, or merely of a rotatory movement of the spiral about its long axis. SPIROQIAETES 77 There does not appear to be any concentration of the particles of material, which are scattered throughout the body, to form a definite nucleus. Reproduction takes place so far as is known simply liv transverse fission. The first Spirochaetes to be discovered and described — so far back as 1833 (Ehrenberg)— were free living creatures but the species of greatest practical interest have taken on a parasitic mode of life. One group of these (Cristispira) are found in the alimentary canal of Oysters and other Pelecypoda (p. 267) and being of relatively large size and easily obtainable are especially convenient for laboratory study. They may be nearly always found in the " crystalline style " of freshly collected specimens of the common fresh- water mussel (Anodontd). Of special human interest are the spirochaetes — of very minute size — which live as parasites in the body of man and other vertebrates. Certain of these (Spiroschandinnia) inhabit the blood and cause fever, e.g. in fowls and geese or in man. In man the disease produced is the well-known Relapsing or Intermittent Fever which occurs in various parts of the world. The common relapsing fever of tropical Africa is associated with the presence, in the blood, of S. duttoni — a spirochaete measuring about 14 fj. in length. Infection is carried by a species of Tick — Ornithodorus moubata — common in huts and camping grounds. When infected blood is swallowed by this animal the spirochaetes instead of being digested multiply rapidly and spread throughout its body. The infected tick apparently does not inoculate the spirochaetes into a new individual directly by its bite as one might expect. What happens is that excretory material containing spirochaetes exudes from its anal opening and, spreading over the surface of the skin, gets into the wound. Amongst other tissues the eggs of the tick are penetrated by spiro- chaetes, with the result that the young ticks of the next generation are infected and are capable both of causing infection themselves and of passing on the infectivity to their progeny. The type of Relapsing Fever once common in Western and still occurring in Eastern Europe is brought about by the presence of a different species of spirochaete (S. recurrentis or obermeieri) which may be dis- tinguished from S. duttoni by its smaller size (7-10 /*). Observations carried out in Northern Africa on what appears to be the European type of Relapsing Fever have shown that the infective agents here are not Ticks but Lice. When the louse has ingested infected blood the spiro- chaetes gradually disappear from its alimentary canal and at the end 78 ZOOLOGY FOR MEDICAL STUDENTS CHAP. of 24 hours are no longer to be detected. However they again make their appearance after an interval of about eight days and are to be found in abundance up till about the nineteenth day all through the haemocoele or body cavity of the louse. From about the nineteenth day the spiro- chaetes undergo a gradual reduction in numbers until apparently they eventually disappear completely and finally. It is during the period mentioned (eighth to nineteenth day) that infection is liable to be con- veyed— not by the bite of the louse,, but only in the event of its body being crushed and its infected blood rubbed into a scratch or coming in contact with some part of the skin, such as that covering the surface of the eyeball, through which the spirochaetes are able to make their way. Spiroschaudinnia icier ohaemorrhagiae is the microbe of epidemic jaundice. It is apparently normally a parasite of the Rat, passing away in the urine and infecting wet soil from which it gains access to the human body either directly through the skin or by being swallowed. The last type of spirochaete to be mentioned (Treponema) is that which is the causative agent of the human diseases Syphilis and Yaws. The parasite of Syphilis (T. pallida) was discovered by Schaudinn in 1905 and is a minute spirochaete averaging about 7 /* in length, the body tapering off at each end into a delicate flagellum-like extension. It is transmitted directly from one individual to another by intimate contact without the intervention of any intermediate host. It multiplies rapidly and invades all parts of the body including the reproductive cells, or embryo if present in the uterus, so that the offspring when born is already infected. The tropical skin disease known as Yaws or " Framboesia tropica " is believed also to be due to a spirochaete (T. pertenue, discovered by Castellani) closely resembling the spirochaete of syphilis. Infection takes place by direct contact, the parasite gaining entrance to the body through any small abrasion or wound of the skin. Spirochaetes are by no means restricted in man to the specific diseases that have been mentioned. They are common inhabitants of the mouth and in the septic condition of the gums known as Pyorrhoea alveolaris are found in enormous numbers and in great variety, as may be gathered from Fig. 31. Various of the spirochaetes are of very minute size, and in this con- nexion an important observation was made by Schaudinn the discoverer of the parasite of syphilis. He found in the excretory tubes of mosquitos spirochaetes which reproduced so rapidly by fission that the ultimate SPIROCHAETES 79 product- were no longer individually visible even under tin- i powers oi the microscope. The practical interest of this is in connexion with diseases due to " invisible germs." Thus Yellow Fever has been pm\rundt d dot-like objects which may be observed within the cell- That thoe are living organisms is indicated by their frequently oi division into two, going through a dumb-bell- Odies tend to become enclosed in a sheath (the •i given the name CHI.AMYDO/OA and they are i PROTOZOA 81 regarded by many as being the actual causal agents of the disease. Regarding the details of their structure and life -history we are still ignorant. PROTOZOA AND DISEASE In the course of this chapter various protozoan parasites have been mentioned as the causative agents of harmful diseases of man and other animals, and the student is apt to get into the habit of thinking of such parasitic protozoa as being necessarily and naturally associated with disease. As a matter of fact however there is reason to believe that the production of disease is by no means a natural characteristic of such protozoan parasites : disease is caused rather when the parasite finds itself in some host other than that which is normal to it. When some particular species of protozoan parasite is introduced into the body of a host animal this latter may behave towards the parasite in any one of three different ways. I. It may show itself to be repellent : the intruding organism is rapidly killed and destroyed. II. It may show itself to be tolerant towards the particular type of parasite : the latter continues to live and reproduce but without increasing to such an extent as to cause perceptible interference with the health of the host. III. It may show itself to be susceptible : the parasite not only lives and multiplies but it increases to such an extent as to upset to a less or greater extent the normal living activities of the host — in other words it produces disease. Now under natural conditions, individuals or strains of individuals which are in any degree susceptible towards such protozoa as are liable to be introduced into their blood, are through this handicap being constantly eliminated in the intense struggle for existence. Consequently we find in Nature that the vast majority of individual animals in any particular locality show little or no susceptibility towards such protozoa of that neighbourhood as are likely to gain access to their bodies : they are either tolerant or repellent towards them. It will be readily understood that the development of tolerance on the part of the host will be helped by Nature acting on the parasite — for it is clearly to the advantage of the parasite that it should be able to live within the body of the host without causing its disease or death. Strains of parasites will tend to flourish and increase in proportion as they are readily tolerated by the host, while strains of parasites towards which a particular species of host animal is markedly susceptible will stand a G 8a ZOOLOGY FOR MEDICAL STUDENTS CHAP. much poon : <>t" persisting as a parasite of that particular host anin re thus comes about in Nature a kind of equilibrium between the ho- .mil the parasites of a particular region that equilibrium :o to be upset by (a) the introduction of new host animals into that or (b) the introduction of new parasites. There are then brought my with one another hosts and parasites between which has had no time to bring about the elimination of susceptibility .m«l tlu-rc are now liable to occur violent outbreaks of disease, lasting until tin- particular species of host animal is completely exterminated or on the other hand, a condition of tolerance or repellence has been gradually brought about by the weeding out of the more susceptible and the survival of the less susceptible strains. Such disturbances of equilibrium occur doubtless frequently in Nature but particularly striking examples have come about through the action of man. white man colonizes parts of the world infected by the parasite iria and he suffers greatly from the attacks of the malarial parasite towards which the aboriginal negro inhabitants have become tolerant, while continuing to act as carriers or reservoirs of the disease. Again :<>duces domesticated animals into regions in which they fall ! to epidemics of Trypanosomiasis or Piroplasmosis caused by bich spread to them from the native animals in which they thout causing any obvious disease. it may be the parasite rather than the host which is trans- mtu unaccustomed regions. Such seems to have been the case with .slrrpini: >ickness, which has long existed on the West Coast of Africa but which, conveyed up the course of the Congo by carriers, l>ed among the natives of Uganda into an epidemic of the greatest compared with the comparatively feeble outbursts on the West Coast where susceptibility had been in the course of time gradually dimini I'.OOKS R)k I'VKTHER STUDY I. GENERAL TEXT-BOOKS Minchin. Introduction to the Study of the Protozoa. Doflein. Lchrbuch der Protozoenkunde. PROTOZOA 83 II. SYSTEMATIC WORK FOR THE IDENTIFICATION OF FREE-LIVING PROTOZOA Kent, Saville. Manual of the Infusoria. III. BOOKS DEALING WITH PARASITIC PROTOZOA Brumpt. Precis de Parasitologie. Manson. Tropical Diseases. Castellan! and Chalmers. Manual of Tropical Medicine. CHAPTER II METAZOA— INTRODUCTORY REMARKS the phylum Protozoa we had to do with single cells living inde- pendently. The individual cell multiplies from time to time, by fission or otherwise, but the cells so arising separate and lead an independent ,-nce like the parent. In all the members of the animal kingdom 1'rotozoa— commonly grouped together under the name XZOA — the life-history commences with a stage in which the indivi- le cell — a zygote — which as in the case of the Protozoa mul; a process of fission repeated over and over again, but in case the successive generations of cells produced by the process of fission do not break apart and lead an independent existence. On the remain as a coherent mass of cells which, in correlation with fission of its component cells, exhibits growth in size. Just i the case of the Protozoa, the process of fission slackens off in due • that the cell-mass does not increase in size indefinitely but K to a more or less definite full-grown size. This mass of cells forms the body of the Metazoon, an individual of a higher order ndividual seen in the Protozoa, for it is composed of a mass of cells which cohere together and have their individualities merged in »! tin- whole. In mple Protozoon the cell-individuals are unspecialized ; earl like its forebears. In the body of the Metazoon on the i«l the successive generations of cells which come into existence dun iiMing up of the fully developed body become more and m(>rc S|M : they gradually lose the primitive unspecialized character of their zygote ancestor and, with the loss of the unspecialized r. they lose for the most part their capacity for con- JUK process of syngamy with other cell-individuals. At one the Ix.dy however there remain nests of cells which do not become side-tracked on any path of specialization for particular 84 CHAP, ii METAZOA 85 functions but which, as they go on multiplying by fission, retain the ancestral unspecialized character and with it the capacity for conjugation. These cells, whose function it is to provide the living substance for subsequent individuals, form collectively what is known as the gonad, while the remaining, much larger, portion of the body forms what is termed the soma. A highly important point to realize about the living substance of the gonad is that so far as we know it is without that great characteristic of most living substance that in due course it dies a natural death. Any piece of the gonad may in the process of syngamy be passed on to a new individual, and this may be repeated so far as we know through an unlimited number of generations, so that the substance of the gonad is potentially immortal. Of course by far the greater part of it is not in practice actually immortal, for it is dependent for its existence upon the soma in which it lives and when this dies it suffers what may be called an accidental death. The body of the Metazoon, composed as it is of myriads of cells, reaches a relatively enormous size, and the specialization characteristic of the somatic cells is intimately linked up with needs imposed by this great increase in size. To support the soft semi-fluid living protoplasm, and prevent it from collapsing into a shapeless mass, portions of the cells, or masses of whole cells, are specialized to form hard supporting substance or skeleton. To enable the individual to move, certain tracts of cells in immediate relation to the parts of the skeleton are specialized for contractility, forming the muscles. To deal with impressions from the outer world, to bring about appropriate movements through the muscular system, and in general to control the various living activities, the several regions of the body are linked together by the nervous system. The outer surface of the body — through which the minute Protozoon takes in its nourishment, gets rid of its waste products, and carries out its respiratory exchange of gas with the surrounding medium — becomes hopelessly inadequate to carry out these indispensable functions in the bulky Meta- zoon, and in consequence we find three other important developments. An increase of surface for the taking in of nourishment (and the getting rid of faecal material) is obtained by a part of the surface being pro- longed into the interior of the body in the form of a more or less tubular alimentary canal. A system of finer tubes or vessels arises, through which nourishment and oxygen are distributed to the various tissues of the body and carbon dioxide and other waste products carried from them — the blood system or vascular system. Another system of tubular channels are developed, the walls of which have for their special function the extracting of the poisonous waste products from the blood and the 86 ZOOLOGY FOR MEDICAL STUDENTS CHAP. passing thi to the exterior— the renal or nephridial system. And fi : . t in 1 he bodies of most Metazoa hosts of permanently .-i-d amoebocytes— cells which retain a more or less amoeboid i-hara. • ! >le to creep about actively and to attack, and either -port to a position in which they are harmless, noxious lound their way into the body such as for example »bes. individual cell of the Metazoon, just as the whole individual Protozoon, may be said to be aquatic in its habit, for in order to live it has to be in contact with watery fluid. The whole body then of the permeated, all its intercellular chinks are filled, by watery fluid, fonnin- an internal medium just as the water forms the external medium !«>r a live-living Protozoon. This internal medium however is < (.>mple\ in chemical composition than ordinary water, for into it are «!IM har-ed the various products of the metabolism of the living .\>m. Just as the myriads of cells which constitute the body bvious specializations of form and structure, so also there exist less obvious peculiarities in their metabolism. Consequently the substances which find their way into the internal medium from the \.irious types of cell are by no means identical but have their own special nties. These various substances, contributed each in its normal proportion, constitute, with the water into which they are discharged, •niniusly complicated internal medium of the body. Every living • ell in a particular species of animal is adapted to life in an internal medium ••omposition, and if any particular organ or tissue fails to contribute its quota, or contributes it in abnormal proportion, then the ML,' abnormality in composition of the internal medium is apt to .irmful— it may be disastrous— effects on the health of the whole body. COELENTERATA SCHEME OF CLASSIFICATION I. HYDROZOA. A. Hydrida. 15. II\dromedll.sac. (1) (iymnohlastea (Anthomedusae). (2) Calyptoblastea (Leptomedusae). C. At .drpliac. II . ACTINOZOA. A. A!< \ ' : ;-. ect, Ectoderm ; E, egg ; end, endoderm ; i.c, interstitial cells of ectoderm ; m, moogl'x .1 : m.t, muscular tail of ectoderm cell, lying in close contact with mesogloea and seen i section ; ov, ovary ; /, t. an inner endoderm (end), separated by an apparently structureless i he mesogloea (Fig. 33, A, m). The ectoderm (Fig. 33, A; ect) real part made up of tall columnar-shaped cells which taper off towards their inner ends and there pass into two tail-like prolongations in line with one another resemble the crosspiece of a letter T '. This crosspiece is in great part composed of protoplasm like the inyoneme of a Protozoon, is specialized in the direction of extreme contractility. HYDRA 89 -EL MY. A layer of cells arranged side by side is what is technically known as an epithelium. A cell of the type just mentioned is termed a myo- epithelial cell,, for not only does it form with its neighbours an epithelium, but in the presence of its contractile tail it represents an early stage in the evolution of a muscle-cell. In the case of the ectoderm the myo- epithelial cells are so arranged that the contractile tails run lengthwise, so that when they contract the Hydra shrinks up into a short squat form (Fig. 32, B). The tails lie in close contact with the outer surface of the mesogloea and in transverse sections under a very high magnification they look like irregular tags projecting from the mesogloea (Fig. 33, m.t). The myo-epithelial cell of the ectoderm (Fig. 34, A) is normally almost filled by a large fluid vacuole, the protoplasm being so distended as to form merely a thin wall surrounding the vacuole and containing the nucleus embedded in its .substance. At the extreme outer end of the cells the protoplasm is slightly condensed so as to form a protective cuticle over the external surface of the Hydra. On the flattened base of the Hydra the myo-epithelial cells are without cuticle and in place of a large fluid vacuole their protoplasm contains numerous droplets of secreted material — a sticky cement which is extruded at the outer end of the cell and helps to attach the Hydra to the substratum. Here we for the first time meet with a gland-cell — a cell specialized for the formation of some par- ticular substance or secretion which is passed out from the body of the cell to serve some particular function. The spaces between the tapering inner ends of the myo-epithelial cells are occupied by comparatively undifferentiated rounded interstitial cells (Fig. 33, i.c). These are cells which have remained as it were in a young condition, not having developed the various peculiarities characteristic of the myo-epithelial cell. Certain of these cells are destined give rise to very remarkable cells termed cnidoblasts which play an iportant part both in defence and in the capture of food. The fully developed cnidoblasts are most numerous in the tentacles. Each (Fig. 35, A) is a somewhat oval-shaped cell prolonged into a stiff protoplasmic hair called a cnidocil (en) which projects freely beyond FIG. 34. Isolated myo - epithelial cells of Hydra. A, From the ectoderm ; B, from the endoderm. (From Graham Kerr's Primer of Zoology.) FL, Flagellum; MY, con- tractile strand. ZOOLOGY FOR MEDICAL STUDENTS CHAP. tentacle into the surrounding water. The most remark- the rnidoblast is the nematocyst (nem) which fills up a interior. This is a hollow flask or bulb the neck of which :i extremely fine tube open at its free end (Fig. 35, C). k of the bulb are arranged three sharp blades. .is observed within the cnidoblast the fine lu[K I!R- neck with its blades, is turned outside in and lies in the interior of the bulb, the tube being coiled up int.. a spiral. The rest of the cavity of the bulb is filled with fluid, apparently of a virulently poisonous kind. lea the nmloblasts such as that which has been. nheil there exist others, more numerous, smaller in size en. nem n. B FIG. 35. Cniclobbst a i. -ts. A, Unextrucled ; B, early stage of extrusion; C, extrusion complete, en, Cnidocil ; n, nucleus ; nem, nrm.it aematocysts of slightly different shape and unprovided with I.: • nil 1« (blasts are originally interstitial cells which gradually form n tln-ir interior. They then creep away from their : ually though not always into the tentacle, burrow their mce i>f an ordinary ectoderm cell and there settle their .nidocil beyond the surface of the host cell into tfater. In a normal ectoderm cell of the tentacle •:ip or battery of cnidoblasts, a larger one in the i -mailer ones round it. ii HYDRA 91 The nematocyst is a powerful offensive organ. If any small organism swim m ing through the water blunders up against the cnidocil this acts like a trigger and causes the nematocyst to discharge, the thin tube k'ing violently everted so as to pierce the body of the organism, an <)]H'ninL!, being made for it by the three blades as they swing outwards. The movements of the animal pierced by the nematocyst, at least if it be small, are paralysed — and it is assumed that a virulent poison is injected through the tube. As regards the mechanism by which the explosion is brought about, a hint is got from the fact that nematocysts which have been freed from their cnidoblast commonly explode instantly when they come in contact with water. This suggests that the fluid within the nematocyst is of such a nature as to cause very rapid diffusion of water inwards through the nematocyst wall if this is in contact with water, the increased pressure so set up bringing about the discharge of the nematocyst. Possibly what happens is that on the cnidocil being touched the proto- plasm of the cnidoblast shrinks back and exposes the outer surface of the nematocyst to the action of the water. The cells which constitute the gonad of Hydra are also derived from interstitial cells. The gametes show well-marked sexual differentiation ; the portions of gonad which give rise to the small actively motile micro- gametes are termed testes, those which give rise to the large non-motile macrogametes or eggs are termed ovaries. The testes when present are in the form of conical or rounded thickenings of the ectoderm, varying in number and most usually situated towards the upper end of the Hydra : the ovaries, fewer in number, are spherical in shape and are situated rather towards the basal end. Hydra is hermaphrodite, i.e. the same individual may develop both ovaries and testes, but as a rule the ovaries develop later than the testes. The study of sections shows that the young gonad is formed by a heap of actively multiplying interstitial cells. In the case of the testes (Fig. 33, C) they keep on multiplying until there are formed enormous numbers of minute round cells each of which becomes a slender micro- gamete with a long vibratile tail and a small rounded " head." The earliest stages in the development of the ovary are precisely like those of the testis but presently a few cells begin to grow rapidly in size at the expense of the others. One of these (Fig. 33, B, E) eventually shoots ahead of the others, ingesting the bodies of its neighbours in Amoeba fashion, and not merely grows to a relatively enormous size but in later stages loads up its cytoplasm with reserve food material or yolk. This cell after it has undergone a process of maturation by the 9a ZOOLOGY FOR MEDICAL STUDENTS CHAP. r bodies (see below, p. 185) constitutes the macro- derm of the Hydra, even in the living specimen examined mder a low power of the microscope, shows a striking difference :-m in that it is distinctly coloured, green or brownish to the species, whereas the ectoderm is colourless. The endoderm (Fig. 33, A, end) is composed mainly of a layer of myoepithelial :>iderably larger than those of the ectoderm and •11 them also in other details. The muscular tails are arranged iinally but circularly, so that when they contract they cause • Ira to assume an attenuated threadlike form, increasing greatly in length. The end of the cell next the coelenteron is rounded and in '•arries flagella (Fig. 34, B) which by their constant lashing •"i»l and other particles to be swirled about in the fluid of the . In a Hydra which has been starved the endoderm cells .•Holes. In a well-fed specimen on the other hand these onspicuous while there occur scattered about in the protoplasm < ply staining protein-spheres — composed of stored-up food d while there are also present clumps or isolated granules of brown excretory substance. Amongst the ordinary endoderm cells there occur, especially in the • i" t In- oral cone, occasional gland cells — squat-shaped cells, without ir tails, and containing droplets of secretion in their cytoplasm. As regards the physiology of the Hydra we have to notice first its I of feeding. A small food organism such as a Water-flea caught ralvsed by the nematocysts of the tentacles is drawn to the mouth, whirh opens to receive it, and slowly passed into the coelenteron. In ' s of digestion taking place not within the substance of a «ell (mtracellular). as was the case in the Protozoa, but in a space bounded ' ' T< -ellular), the dissolved products of digestion being absorbed '•IK bounding the space. The process of digestion throughout i- for the most part intercellular. In the case of Hydra till persists a certain amount of intracellular digestion, disintegrated food are ingested bodily by the inner ends •ihyll-comaining Flagellates to which the bright green ii HYDRA 93 colour is due, and in correlation with this the animal seeks the light necessary to the functioning of the chlorophyll, while the brown Hydras seek rather the shade. The movement of the Hydra from place to place may be a slow gliding movement carried out by small pseudopodia pushed out by the ectoderm cells of the base, or a more rapid movement in which the Hydra attaches itself by similar pseudopodial extensions of the ectoderm cells of its tentacles while it temporarily detaches its basal end from the substratum to re-attach itself elsewhere. When living under favourable circumstances and well supplied with food the Hydra multiplies actively by an asexual process of budding. A little pocket-like outgrowth comes to project from the body, it increases in size and gradually takes on the form of a small Hydra, a circle of tentacles sprouting out from its end and a mouth perforation developing between them. Finally it becomes constricted off at its base as an independent Hydra just like the parent except that it is smaller in size. When the budding process is very active a number of buds may be present on the parent at one time, and the buds may have secondary buds sprouting out from them so that there is formed a continuous mass of Hydra individuals forming a sort of colony. Such a condition however is only temporary and before long the mass separates into its constituent individuals which proceed to lead a free independent existence. Budding is not the only method by which the Hydra is capable of multiplying asexually. Occasionally — though very rarely — it may be observed to reproduce by fission, the Hydra dividing lengthwise into two, the process commencing at the oral end and slowly spreading downwards towards the base. After a more or less prolonged period during which the multiplication of the Hydra is entirely asexual there comes a time when gonads make their appearance — the season differing in different species of Hydra. Probably we may safely say that the appearance of the gonads is associated with the onset of conditions in some way unfavourable to the life of the rticular species. The fully developed ovary forms a conspicuous rounded mass pro- jecting from the body of the Hydra, its interior filled by the large spherical egg or macrogamete. The testis forms a rather more pointed projection of a whitish colour. When fully developed, examination with the high power of the microscope shows a wild commotion going on in its interior, due to the active movements of the microgametes. Eventually the wall of the testis ruptures and the microgametes disperse through the water. The overwhelming majority are wasted — this is a general characteristic * 94 ZOOLOGY FOR MEDICAL STUDENTS CHAP. luit it one reaches an ovary containing a ripe macro- ;his and syngamy takes place. fertilized egg now proceeds to undergo the process of segmentation— consisting of fission repeated over and over again. This :n tin- formation of a blastula— a mass of cells forming a sphere 1 in a sinule layer round a central cavity. As development Us derived from the cells of the wall drop into the cavity and eventually fill it. The mass of cells, or the embryo * as we now call solid and consists of two distinct layers of cells — those xvhirh formed the wall of the blastula and those which fill its cavity. These are tin- two primary cell-layers of the individual — the ectoderm .md the endoderm. The embryo now comes to be enclosed in a protective chitinous shell, i l»v the ectoderm and differing in appearance in different species ol 1 1 \-dr a. It drops off the parent and remains in the mud at the bottom .if the water tor it may be a prolonged period until conditions again become Mr. When this happens the embryo, apparently by the secretion e ferment to soften the shell, makes its way out and gradually pa into a typical small Hydra. HYDROMEDUSAE nst ruet ive to compare with Hydra those animals grouped together under the name HYDROMEDUSAE, in which the life-history is somewhat implicated than it is in Hydra. It is also a characteristic feature ip that while asexual reproduction by budding takes place !i\iduals so arising do not as a rule become separate but remain 'tout life connected together in the form of a community or colony. OB ELI A A good illustrative example of the Hydromedusae is the common ' : 'lia. To the naked eye a colony of Obelia looks like a •Ahitish thread creeping over the surface of a seaweed or stone or snr11 ;r "ft at intervals little branches which project freely into of these branches can be seen with a magnifying lens to be ben; in a characteristic /ig/a- manner and to give off from the outer 1 An embryo i- .1 v<,un- developing individual, which is contained within the body s"hln •> l"<>t'-etr other envelope. A larva is on the other '••pirn: individual, diifi-rinj,' in form from tin- adult, but not con- Utard within th.. i,,,dv ,,f the parent or other protective envelope, HYDROMEDUSAE side of each bend what looks like a tiny conical sherry-glass mounted on a stalk (Fig. 36, h). In this conical receptacle there resides an individual of the colony — a polyp or hydroid individual, so called because in the main features of its organization it agrees with Hydra. The body wall is com- posed of the same layers as in Hydra • there is a large oral cone with a wide mouth (Fig. 36, o.c), and a ring of tentacles like those of Hydra except that they have a solid core of much vacuolated endoderm cells, the coelenteron not extending into them. The lx)dy of the polyp is continued downwards as a tube composed of the same layers and this in turn joins the thread-like stolon — similar in structure — which meanders over the surface of the stone or seaweed. The cuticle is greatly developed in Obelia forming a thick horny protective layer — the 9 bl o.c. perisarc (Fig. 36, ps). This covers the whole surface of the colony and its branches. At the base of the polyp the perisarc loses its intimate contact with the cells of the ecto- derm and expands to form a wide cup — the fdrotheea — which sur- mnds the polyp (Fig. h). Here and icre in the angle between a hydrotheca and the stem of the )lony there is present a vase-shaped gonotheca (Fig. 36, g). This, like the ordinary hydrotheca, contains an individual of the colony but one rery different from the ordinary hydroid polyp. This individual — the lastostyle (Fig. 36, bl} — is somewhat piston-shaped, its end being in the >rm of a flattened disc without trace of tentacles or mouth opening. tts special function is that of reproducing by budding, and numerous ids may commonly be seen projecting from its surface. In their young ige, as may be seen towards the lower end of the blastostyle, the ids are just like those of Hydra but the older buds (Fig. 36, M), visible FIG. 36. Obelia. Portion of hydroid colony, x 25. I>1, Blastostyle ; g, gonotheca ; h, hydrotheca ; M, medusa bud ; o.c, oral cone ; ps, perisarc. ZOOLOGY FOR MEDICAL STUDENTS CHAP. ,n of the gonotheca, and is now seen to be not a hydroid HYDROMEDUSAE 97 luit a small jelly-fish or medusa (Figs. 37 and 41, D). This swims about, Uvds and grows, and when fully developed has the following structure. The larger part of the body is concavo-convex, somewhat umbrella- like, in form and is technically known as the umbrella. In the centre of the concave surface, in place of the umbrella handle, is a short thick projection— the manubrium (Figs. 37 and 41, D, m). From the edge of the umbrella extends a fringe of fine threadlike tentacles. Study of the minute structure of the medusa shows it to be composed of the same layers as the hydroid, only the mesogloea is greatly thickened forming in any ordinary jelly-fish the mass of clear jelly from which the creature gets its popular name. The ectoderm covers the whole external surface and it shows a distinct advance in evolution from that of Hydra. More especially on the lower concave surface of the umbrella the myo-epithelial cells have their con- tractile tails much more strongly developed so as to form powerful muscles, arranged concentrically with the edge of the umbrella. By means of these the medusa makes the familiar pulsations, opening and shutting, by which it swims through the water. Further we find distinct rudiments of a nervous system. Here and there scattered through the ectoderm are sensory cells (Fig. 38, s), tall and slender in shape, bearing at their outer end a fine sensory hair of protoplasm which projects freely into the surrounding water and serves to receive impressions from the outer world. The sensory cells are prolonged at their inner end into a proto- plasmic thread — a nerve-fibre (a.f) which joins with many others to form a plexus or network lying near the inner limit of the ectoderm all over the creature's body. Here and there such a fibre may be traced to a cell lying in the deep layers of the ectoderm (Fig. 38, g.c), or beneath it entirely — a ganglion-cell, probably to be interpreted as a sensory cell which has become withdrawn from the surface. Sometimes it is possible to trace another nerve fibre (Fig. 38, e.f) passing away from the ganglion cell and leading to a muscle cell (m). Here we have an excellent example of a nervous mechanism of the simplest possible type, consisting of a nerve centre — in this case a single ganglion cell — and in relation with it a sensory or afferent path along which come messages or impulses from m FIG. 38. Illustrating the nervous mechanism of Medusae, a.f, Afferent (sensory) nerve fibre ; e.f, efferent (motor) nerve fibre ; g.c, ganglion cell ; m, muscle cell ; s, sensory cell. ^ >UK;\ FOR MEDICAL STUDENTS CHAP. . ptivc apparatus, and a motor or efferent path along 5ent from the nerve-centre towards a muscle to make -In point* on the margin of the umbrella there is present a flection of sensory irlls forming a sense-organ, in this case an \st or primitive ear, an organ not for hearing but for performing ,rr ancient function o! otocysts, that of perceiving change of tion in relation to the- vertical. The otocyst (Figs. 37, ot, and 39) rounded sac situated on the- lower side of the rounded swollen base tentacle. Tin- wall of the otocyst is very thin consisting of two • i . flattened cells, a covering layer of ectoderm, and a lining ' ii the lining cells some have not the flattened form characteristic The most conspicuous of these is a large club-shaped cell which projects into the cavity and •vr the end of which is weighted by a large spherical mass of very dense calcium car- oL bonate. the otolith, secreted within its cyto- plasm. The otolith (Fig. 39, oT) in its containing cell (o.c) rests lightly upon sensory hairs which project from a patch of 4-7 sensory cells (s.c) into the cavity of the otocyst. It is clear that the action of gravity adifferentiated _ J >l upon the dense, relatively heavy, otolith ;;:,'•::""" wai cause it to bear down upon the sensory hairs which support it. It is irther that the strain upon the sensory hairs will be altered if the position of the medusa be changed, e.g. if it be tilted up on edge, .uparrntly in this way that the otocyst conveys to the medusa the mtormation that its position has become abnormal. The i nelcnteric cavity is small in comparison with the bulk of the : .ha\ in- been lor the most part obliterated by the great development i. l\i-ht in the centre of the medusa a portion of the cavity Ltent, lorming what is usually termed the stomach since in it tion of the food takes place (Figs. 37 and 41, s). This ill the exterior by a wide four-rayed mouth at the end of tin- munuhrium. It also extends outwards towards the edge of the umbrella u tour tubes, the radial canals (Figs. 37 and 41, r.c). re the ribs of an ordinary umbrella by a thin ; >K M nting coalesced portions of the endodermal roof and floor of the coelenteron. Around its extreme outer margin HYDROMEDUSAE 99 O.C. the coelenteric cavity is again patent forming the ring canal into which the radial canals open at their outer ends (Fig. 37, r). The medusa is the sexual phase in the life-history of Obelia. It -onad (Figs. 37 and 41, #) in the form of testes or ovaries which form four conspicuous pear-shaped or rounded masses hanging down from the lower (concave) surface of the umbrella, immediately beneath tlu- radial canals. Each is situated, as in the case of Hydra, in tin thickness of the ectoderm. Obelia affords an excellent example of alternation of generations — rations of sexual individuals (in this case Medusae) being intercalated amongst others which are not sexual (in this case the individuals of the colony). TUBULARIA It is interesting to compare with Obelia another common marine Hydrozoon — Tubularia. Here again colonies are formed by a process of budding, consisting of hydroid individuals which however differ from those of Obelia in various details, some unimportant, some important. Amongst the former are the much greater size of the polyps, and the fact that a second set of smaller tentacles are present situated close to the tip of the oral cone and immediately surrounding the mouth (Fig. 40). The most important fact is that in Tubularia there is no hydrotheca surrounding and protecting the body the polyp, the horny risarc (ps) being re- stricted to its cylindrical stalk and stopping short of the swollen body of the polyp. ? FIG. 40. Tubularia, upper portion of a single polyp, x 17. M, young medusa bud ; o.c, oral cone ; ps, perisarc. Such H I * 100 ZOOLOGY FOR MEDICAL STUDENTS CHAP. type of hydnml without a hydrotlura is said to be gymnoblastic, in con- ihc calyptoblastic type in which a hydrotheca is present. l belongs to a group in which the Medusae (Fig. 41, B) tnportant differences from tliose of Obelia. (i) They are deep ied instead ot saucer-shaped; (2) the gonad (g) is situated not re. Fin. 41. Diagram In illiistr.it.' tin- distiiiLjuisliiiii; features of A, Gymnoblastic polyp; B, Anthomedusa ; |inlv|>; I >, I.i-ptninrdiis.i. K, (ionad; h, hydrotheca ; m, mouth; ps, pcrisarc ; . "iin. [In Figs. B and D the right half cf the figure is in the plane : ili.- 1. ft half is in a plane lirtwr. n two ra into dose proximity to the gonad the lower of the umbrella is tucked inwards to form a subgenital pit *hich remains freely open to the sea-water by a wide A hiU- the roof, separating it from the stomach and giving : ,m its upper or gastral surface, is comparatively thin. on as already mentioned a bright purple colour and the 77C. m. Aurtln men viewed iVc.in l>rl<>\v. • \(->. g.f, Group of gastral filaments; m, manu- liriutn with iiioiitli npi nin- .it tip; r, \-\\\\i canal ; rr, radial canal ; s.t, sensory tentacle ; st, stomach. ..I' tin- iii.mubrinm have not yet grown out into long arms; ..( tin' st.. in. nh and the (,'onads have not yet appeared; and the branching iplrx a-, it is in the adult.] '1 I rum it pass into the cavity of the stomach and thence to r by the month opening. Within the curve of the gonad i Iniin the floor of the stomach a row of somewhat tentacle- like gastral filaments (l-'i.ns- -\~ ;m'l 44, I, g.f). The endoderm covering >\\ilrt itutes a characteristic feature of the ii AURELIA io.? The sense-organs (Fig. 42, s.t) are highly Hiur;i< -trri.stir : they are right in nuinbrr, situated round the margin «.t tin- umbrelhi prr-radial and inter-radial in position, and are really tentacles which have become much shortened and modified to form sense-organs. Each is traversed by a tubular cavity continuous with the ring-canal and lined with endo- derm. The somewhat swollen end of the tentacle is occupied by a solid mass of endoderm cells continuous with the lining of the tube already mentioned and the cells forming this secrete numerous masses of calcium carbonate -so that the end of the tentacle is heavily weighted. In Aurelia the sensory tentacle projects freely, although it is sheltered 1 >\ a hood-like arrangement, but it is interesting to note that in some other medusae similar sense-tentacles come to be surrounded by a wall-like B FIG. 43. Illustrating the enclosure of a sensory tentacle within an otocyst in a Medusa — Rhopalonema. fter O. & R. Hertwig.) In the younger stages (A and B) the tentacle projects freely. In the • stage shown in C it is in process of being enclosed. In still later stages the opening of the otocyst completely obliterated. growth which eventually arches over and causes them to be completely iclosed in an otocyst (Fig. 43). As in the Hydromedusae so also in the Acalephae a polyp phase :urs in the life-history, but it is small and inconspicuous as compared rith the medusoid phase. The eggs are fertilized within the stomach microgametes which have come in from the exterior and the zygotes formed pass out through the mouth opening and along the groove extending from it along the arm-like prolongations of the angles of the mouth. Here they become lodged in pocket-like outgrowths of the groove in which they proceed with their development. The zygote under- goes a process of segmentation which results in a spherical blastula. One wall of this becomes tucked within the other very much as one hemisphere of a child's indiarubber ball may be pushed within the other (Fig. 44, A). In this way is reached a very important stage of development known as 104 ZOOLOGY FOR MEDICAL STUDENTS CHAP. ;. B). The characteristic features of the gastrula >hupe is cup-like, (2) that it has a simple internal cavity 3 T3 H-s irchenteron which romiminirates with the exterior by a single primitive mouth or protostoma, and (3) that its wall consists ii AURELIA 105 of two layers of cells, an outer ectoderm and an inner endoderm. It will be seen that the gastrula in its fundamental characteristics agrees with Hydra : the differences are rather differences in detail — the cup-like body being in Hydra deepened to form a tube, the mouth being narrowed into a minute pore, the body-wall being prolonged to form the tentacles, and the component cells being specialized for different functions. The fundamental similarity which underlies these superficial differences justifies the statement that the gastrula phase in the development of an animal is in fact simply a temporary hydroid phase. In the case of Aurelia the gastrula undergoes a series of modifications which culminate in a condition very much like that of Hydra even in detail. The gastrula assumes a tubular form and the mouth becomes narrowed into a minute pore (Fig. 44, C). It makes its way out of the shelter in which it has developed so far and swims away through the water by means of powerful cilia which have developed from its ectoderm. The free-swimming larva presently attaches itself by its closed end to some solid body, very often the frond of the large Tangle or Oar-weed (Laminaria}, and gradually takes the form of a little greyish-white creature which was supposed by the older naturalists to be simply a marine species of Hydra and given the name Hydra tuba (Fig. 44, D). Features which distinguish it from the true Hydra are the larger number of tentacles and the fact that the endoderm undergoes an increase in area by forming four folds which project into the coelenteron as -four longitudinal ridges. This hydroid stage in the development of Aurelia is known as the scyphistoma stage. The scyphistomas may sometimes be observed during the autumn in untold myriads, dotted about on the fronds of Laminaria, in our quiet sea-lochs. The scyphistomas feed actively, grow, and multiply by budding during the autumn months but during the early winter a change begins to come over them. Ring-like constrictions encircle the body and gradu- ally deepening divide it into a pile of saucer-shaped structures one over the other (strobila stage — Fig. 44, E and F). The margins of these saucer-shaped bodies grow out each into eight lobes, while the scyphi- stoma tentacles upon the uppermost one degenerate and disappear. Finally the saucer-shaped pieces break off one by one and swim away as little star-shaped medusae (ephyra stage — Fig. 44, G) each with eight lobes radiating outwards. Careful examination of the ephyra shows it to be, notwithstanding its star shape, a young Aurelia medusa, for in a little recess at the end of each of the eight arms there may be recognized the characteristic sensory tentacle just like that of the adult (Fig. 44, H, s.t.). The ephyrae grow rapidly, the spaces between the rays becoming gradually I06 ZOOLOGY FOR MEDICAL STUDENTS CHAP. H ,-ially active outward growth so that the nearly circular ,,utlin< ih is attained. Inrt'lia the group Acalephae includes a number of other • ornmon and conspicuous Medusae. Cyanea is the common stinging irlly-lish familiar to 1 fathers. In this case the threadlike tentacles margin of the umbrella are very long— it may be several feet !i. and they are richly provided with large and powerful nemato- e di>i -har-e of which into the skin produces the stinging sensation. The more important of the features which mark off the Acalephae irom the Hvdromedusae are (i) the greater size and conspicuousness i tin- lite-history; (2) the endodermal position of ,,id : (:;) the presence of gastral filaments; and (4) the presence in the polyp stage of four longitudinal folds of the endoderm. This :itioned feature is of special interest from its foreshadowing a ( oiiditiun >ren in a much higher degree of development in the Actinozoa. The three tvpes of ( 'oelenterate so far dealt with — the Hydra-like forms (llvdrida) without any medusoid phase in their life history, the Hydro- ie, and the Acalephae — are included in the first of the two main >ul>di\isions of the phylum Coelenterata, the HYDROZOA. Apart from irrence of the Medusa type of structure, the two special Hurt's of the Hydrozoa are that in the polyp stage the mouth opening is situated at the outer end of the projecting oral cone and the nu-lrntrrnn is a continuous cavity throughout, although it may litlv encroached on b\ inwardly projecting folds of endoderm tiistoma). Tin- remaining ( 'oelenterates are grouped together as the ACTINOZOA. ()l tli- II take as our first example the genus Alcyonhim. AtLCYONIUM :>.ilc yellow or pale llesh coloured colonies of this animal, il.irlv lobed shape which has suggested the popular name " |(l'"1 ^' ' . ' arc to be found attached to rocks and stones • <•' r mark downwards. A specimen removed from the \) shows no obvious sign of life, but if placed in fresh 1 ill •jradnallv protrude from its surface numerous semi- - ' 11 i" I"- B colonial organism (Fig. 45, B). •M of the polyp (Fig. 46) is somewhat cylindrical ii ALCYONIUM 107 in shape and at its free end has a slit-like " mouth " surrounded by a circle of eight tentacles each of which is pinnate in form. The mouth opening leads into a flattened tube, the stomodaeum (Fig. 46, s/), which 1 mugs down into the coelenteron and opens into the latter at its truncutcd lower end. This stomodaeum corresponds with the oral cone of the Ilvdrozoa, but here, instead of projecting outwards, it has become as it were inverted into the interior of the polyp. It is therefore lined with ectoderm. Along one edge the cavity of the stomodaeum dilates to FIG. 45- Alcyonium. x !(. A, Colony as found cast up on the shore (A. digitatum) ; B, colony with polyps extended (A . palmatum). form the ciliated groove (Fig. 47, A, c.g), the cells along the floor of which carry powerful flagella and serve to pump fresh sea-water into the coelen- teron. The stomodaeum does not simply hang freely in the coelenteron but is slung up by eight delicate membranes,, the mesenteries, which radiate out from the stomodaeum to the outer wall (Fig. 47, A, M). These mesenteries are covered, as are all surfaces abutting on the coel- enteric cavity, by endoderm : each is formed of two layers of endoderm cells with an interposed layer of mesogloea. Along the surface which faces the ciliated groove (often called the " ventral " surface) each mesentery possesses a longitudinal thickening of its endoderm, caused ZOOLOGY FOR MEDICAL STUDENTS CHAP. by a si l.»pment of longitudinally running muscles (Fig. 47, «0 = these a, on the pair of mesenteries next the ciliated groove and .1 less conspicuous on those furthest removed from it. .,rc prolonged downwards beyond the truncated rn,l ()t tlu- stomodaeum uml are then attached only by one— the outer dy wall, the opposite margin projecting freely into the coel: Tries. end. FIG. 46. Alcycmium. Vertical section through part of colony, coel, Coelenteron ; end, endodermal tube Bten "t I\\M neighbouring polyps; m.f, inosenterial ii lament ; m.f*, dorsal, uncoiled, mocntrri il fil inn nt , nu-s, nn-v.Kloe.i ; sf>, spic.ulc ; st, stomodaeum. 47, IJ). The free edge is thickened to form a con- ioua thread the mesenterial filament (M.f). The rorlriurrir. caxity of the ])roje(liiig portion of the polyp is pro- tlnwnwanls into the substance of the colony, the mesenteries d downwards. In the case of the two mesenteries • fn»in the ciliated groove ("dorsal" mesenteries) the mesenterial -. is cuntinui'd right down into the basal part of the ••vcrcd with cells bearing powerful cilia which beat ALCYONIUM 109 in such a way as to produce an upward current of water towards the stomodaeum. In the case of the six other mesenteries the mesenterial eg. B coel. t FIG. 47. Alcyonium : transverse sections. A, through stomodaeum ; B, slightly below stomodaeum. Ciliated groove ; M, mesenteries ; m, muscle ; A/./, mesenterial filament ; s, stomodaeum. lent is much folded, forming a little skein-like mass ; it extends ily for a short distance down the free edge of the mesentery and epithelial covering is >wded with gland-cells, secrete the diges- i ferment. Below level of the mesen- •ial filaments these six jnteries show during winter months little mnded swellings near free edge. These sellings are the young ovaries or testes, which are consequently endo- dermal in position as they are in the Acalephae. The ectoderm covers FlG- 48' Portion of section through Alcyonium colony parallel to the OUter Surface Of the surface, coel, Coelenteron with the eight mesenteries, the two dorsal ones showing mesenterial filaments (black) ; sp, spicules ; sp. polyp and of the colony in general : the endo- derm lines the coelen- teric spaces. Between the two layers of cells are extensive regions filled st, strands and t, tubes of endoderm connecting the walls of the various coelentera. The dotted groundwork of the figure repre- sents the matrix of mesogloea. IIO ZOOLOGY FOR MEDICAL STUDENTS CHAP. with stiff translucent mesogloea which gives solidity to the colony as a whole. This mesogloea forms a kind of packing between the various g. 48) and it is further traversed by a coarse network : of tubes (/) and strands (st) of endoderm which link up the .rlentera with one another. The mesogloea is further colonized liv numerous amoeboid or mesenchyme cells which have wandered into it •derm. These immigrant ectoderm cells are functionally scleroblasts, i.e. their function is to produce skeleton. They settle down in the mesogloea and secrete spicules (Fig. 48, sp) of calcium carbonate Characteristic thorny appearance though they vary much in their ,e. These spicules are specially crowded together in the surface Kiver of the colony converting this into a harsh protective rind and I the surface its characteristic colour. ALCYONARIA The group Alcyonaria includes a great variety of marine creatures ing with Alcyoirium in their main features — the number (8) and shape (pinnate) of the tentacles, the eight mesenteries each with a muscular thickening on its ventral face, the presence in the mesogloea of a ;lar skeleton formed by immigrant ectoderm cells,, and as a rule the formation ot a colony by a process of budding. r.etore leaving the group it will be well to notice one or two interesting departures from the condition seen in Alcyonium as regards the skeleton. The well-known " Red Coral " is formed by an Alcyonarian named illiuiii, the colonies of which differ conspicuously from those of . \lt-ytniiiun in their being slender and much branched, and in their bright red colour due to the colour of the spicules. In Alcyonium the spicules .ire sj)tcially crowded together towards the surface of the colony: in Corallinni a similar crowding together takes place along its axis, where lm\\» \< i the spicules become actually cemented together to form a solid r«>d -likr mass which is " Red Coral." Another Alcyuiiurian the colonies of which have slender branches .upported by an axial skeletal rod is Gorgonia, but in this case an interest in- diticrcncr exists in the mode of formation of the skeletal r"d. Tin- young commencing colony secretes a plate of horny material hetuern its l.asr and the substratum to which it is attached. As the more material is added by the secretory activity of the basal « •• tnrlerm, so that the plate becomes converted into a little hillock, >till mure is added till it forms a cylindrical pillar, and this being added to continuously becomes gradually converted into a long rod over ACTINOZOA in which the substance of the colony is stretched like a glove over ii linger. Spicules — often brilliantly coloured — are present us usual but, as will have been gathered, they take, in Gorgonia, no part in the formation of the axial rod. ZOANTHARIA This group includes the ordinary sea-anemones, so commonly seen in rock pools or attached to the piles of piers. While agreeing in their A Sea-anemone (Peachia) bisected transversely. A, Upper ; B, Lower half, c.g, Ciliated groove ; d, directive mesenteries : m, muscle of mesentery ; m.f, mesenterial filament ; s.m, muscles of secondary mesenteries ; st, stomodaeum. general structure with Alcyonarian polyps they show characteristic differences in detail. The individual polyps are normally of much larger size and as a rule they do not form colonies. They may reproduce asexually by budding or by a process of fission from above downwards but the individuals so arising separate from one another and lead an independent existence. The tentacles instead of being pinnate are simply conical ; instead of being eight in number they are numerous, and commonly arranged in several rows. The stomodaeum is usually provided with two ciliated grooves , on opposite sides , although in some of the more primitive anemones there is only a single ventral groove as in the Alcyonarians (Figs. 49 and /OOUKiY FOR MEDICAL STUDENTS CHAP. .»). The stomodacum is slung up to the body-wall by mesenteries ,h<> \\in_ tin- >ame characteristics— muscles (w), mesenterial filaments d as in the Alcyonarians but these mesenteries show charac- diherences in their arrangement. They are more numerous and are distinguishable into different sets— primary mesenteries, which at their inner edge are attached to the wall of the stomodaeum, and others lary, tertiary, etc.— Figs. 49 and 50, s.w) much smaller, which extend imvards only a short distance and do not reach the stomodaeum. Of primary mesenteries the number is from twelve upwards. They an- arranged in couples and in each couple the muscular thickenings FIG. 50. lr.ui-\M-r -rations through a simple Sea-anemone (Peachia). A, Through stomodacum; I'., lirlow ~t<>mi«Ucimi. c.g, Ciliati-il groove; d, directive niescnteries ; ect, ectoderm ; end, cndodtTin : : in./, iiii-i-nti Ti.il filament ; s.m, secondary mesentery ; si, stomodaeum. The mesogloea •i. d by a black line. face toward* one another, except in the case of the directive mesen- ilu- cnuplr which support the ventral ciliated groove and the couple directly opposite, which support the dorsal ciliated groove if one. In the case of these directives the muscles are borne not on tin- inner but on the outer surface, i.e. they face away from one another (l-'i-. 50, < ement is imperceptible it may on the other hand form a dis- tinct horny layer, as happens in Adamsia, an anemone which commonly vmhiotically on shells inhabited by Hermit Crabs. Finally in Mil .division of the Zoantharia the secreted mass, composed of ii ACTINOZOA 113 ( all ium carbonate and bulky in amount, forms a characteristic external skeleton coral. These zoantharian corals show a wonderful variety of form and M/< and complexity. In the simplest type there is first laid down a thin flat plate of calcium carbonate between the base of the polyp and the solid substance, rock or shell, on which it rests (Fig. 51, tti). Along radiating lines, alternating with the mesenteries, the secretion becomes more active so that ridges are formed which increase in height and become thin vertical plates — the septa (cf. Figs. 52 and 53). A circular ridge is formed connecting the septa near their outer ends, and this also increasing in height forms the rim of a cup or theca in which the polyp rests (cf. Fig. 52, A). Very often a mound of calcium carbonate is deposited in the centre from which the septa radiate, and this increasing in height becomes the columella. It is important to bear clearly in mind that all this calcium carbonate is laid down by the outer surface of the ectoderm of the base of the polyp, so that it is strictly speaking entirely outside the living substance ; it is an external skeleton or exoskeleton, although its various parts are closely ensheathed by the polyp floor which is pushed upwards as they increase in height. The simplest type of coral is a simple cup . ';. . A young Coral-polyp (A stroides). Or theca With radiating Septa (Fig. 52, A) (After Lacaze-Duthiers.) th, Cal- but there exist many complications of this careous plat" '°™ing the com- mencement of the theca. simple type, due for the most part to peculiarities in processes of asexual reproduction. Coral polyps, unlike ordinary anemones, very usually form communities or colonies by processes of budding or fission. By budding a tree-like colony may be built up of numerous conical thecae, each containing an isolated polyp, as in the case of Lophohelia (Fig. 52, B), masses of which are sometimes drawn up on fishermen's long lines off our western and northern coasts. In other cases the polyps do not become completely separated but remain in continuity. In this case the layer of living tissue between the polyps (coenenchyme) goes on secreting calcium carbonate on its basal surface and consequently the individual thecae instead of being quite separate are connected together by an intervening solid mass (Fig. 52, C). Again in other cases a process of imperfect fission takes place. The polyp becomes much drawn out so that it is band-shaped instead of circular as seen from above, but there is no attempt at division into separate I II4 ZOOLOGY FOR MEDICAL STUDENTS CHAP. that tin- mouth opening becomes divided up into a row o7 ;, In such a case the individual thecae instead of elongated (Fig. 52, D and E) and in extreme cases omc ver) greatly drawn out and pursuing a tortuous course Of coral an appearance resembling that of the human brain (" Brain Corals "- -Fig. 52. 10- In the Mushroom coral (Fungia—Fig. 53) a reproductive process OUng that seen in the stn.bili/ation of the scyphistoma takes place, the rim of the oip-shapedtheca spreading out trumpet-fashion (Fig. 53, B) FIG. 52. H •'•hyllia; B, Lophohelia ; C, Solcmstraca ; D, Dichocaenia ; and event ually forming a flat disc which with the upper part of the off and becomes independent, the process being repeated again. The adult Fungia, cut off in this fashion, lies »n the sea bottom, the theca being a flat disc with the septa radial in- outwards on its upper surface (Fig. 53, C). ralSj including some of the commonest branching corals or what is termed perforate— the wall of the theca being . numerous openings which give it a spongy character. In '. lie addition of calcium carbonate to the rim of the theca is by tubular connexions between the main wall of the polyp ii ACTINOZOA 115 lying inside the theca and its flap-like extension which overhangs the lip and reaches for some distance down its outer surface. It is the interruptions in which lie these tubular connexions that show as foramina in the dry skeleton. The ACTINOZOA, exemplified by the Alcyonaria and the Zoantharia, arc clearly marked off from the Hydrozoa by a number of features which they possess in common. Their general form of body is of the polyp type: there is no medusoid phase in their life-history. The individual polyp is larger in size and more complex in structure than is that of the Hydrozoon. The part of the body corresponding to the 1 1 nil cone of the Hydrozoon is turned inwards so as to hang down in the • orlenteric cavity as the stomodaeum. The stomodaeal wall is sus- FIG. 53- Mushroom Coral — Fnngia. A and B, young fixed stages ; C, adult condition. (A and B after Bourne.) pended from the body-wall by the mesenteries — thin vertical partitions of mesogloea, the deep recesses between which are lined with endoderm. The endoderm on the face of the mesentery develops prominent longi- tudinal bands of muscle : it also develops the gonad. Finally, when a skeleton is present this is normally not in the form of thickened cuticle (perisarc) but in the form either of isolated spicules secreted in the meso- gloea by immigrant cells from the ectoderm or of a mass of secretion formed underneath the ectoderm of the base. The phylum COELENTERATA, the more important subdivisions of which have been dealt with in this chapter, includes what are on the whole the most nearly primitive members of the Metazoa. They are above all characterized (i) by the possession of a general internal cavity, the coelenteron, which has not yet become subdivided into a separate I i ZOOLOGY FOR MEDICAL STUDENTS CHAP. TOO or alimentary canal and coelome or body -cavity, and (2) by I .at the coelenteron communicates with the exterior by a single og tin- primitive mouth (protostoma). «>dy-\vull of the coelenterate consists of the two primary layers ; , xlerm and endoderm, separated by the jelly-like or membranous i. This latter is primarily a secretion formed by the activity , layers of cells bounding it but where it becomes bulky, as in iinbrrlla of the Medusa or the ground-substance of the colony of the .U vonarian or the body-wall of the Anemone, it usually becomes •ii/rd l>\ cells of the primary cell-layers (most usually of the ectoderm) which have taken on an amoeboid character and wandered into it. e cells, which collectively constitute the mesenchyme, are of import- in ronnexion with the subsequent evolution of the Metazoa, for in many of the more highly evolved groups of animals the mesenchyme forms a large proportion of the entire bulk of the body and becomes iali/.ed to form important tissues. Tlu- two primary cell-layers are especially interesting in the coelen- i e from the fact that they display to us incipient stages in the evolution of the muscular and nervous systems. The myo-cpithelial cell like that of Hydra is apparently the starting- point for the evolution of the muscle fibre as it occurs in the more complex animals. In such a comparatively actively moving coelenterate as a medusa the myo-epithelial cells on the under surface of the umbrella frequently have their cell-body reduced to the nucleus, with a small quantity of cytoplasm round it, lying beside the greatly enlarged con- tractile cross-piece. The myo-epithelial cell has in such a case actually become a muscle fibre. The i oelenterate nervous system consists of the scattered sensory cells, the ganglion-cells which have sunk inwards and lost their sensory hair, and the plexus or network by which sensory and ganglion-cells linked together. As has already been indicated the Medusae show an advance in the evolution of their nervous system in the aggregation of sensory cells into groups so as to form definite sense-organs. Another feature of the Medusae is that they show a tendency for ganglion-cells to be concentrated together in the neighbourhood of the margin of the umbrella to form nerve centres. In the Acalephae such a nerve centre is developed in the neighbourhood of the attached base of each sensory tentacle. From these centres emanate the stimuli that bring about the contraction of the muscle fibres and hence the rhythmic pulsations of the umbrella. If the tentacles with a small adjacent portion of the umbrella are carefully excised from the living A urelia its pulsations ii COELENTERATA 117 stop. If the umbrella is cut into eight sectors each with an uninjured nerve centre the eight sectors go on pulsating but they soon " lose step " with one another owing to the loss of continuity of the nerve plexus which linked them together into a single co-ordinated whole. In the Hydromedusae the nerve centre forms a continuous ring. In the polyp type of coelenterate with its less complex structure we do not find either definite sense organs or definite nerve centres, although in the Anemones an approach to the development of nerve centres is expressed by the tendency for ganglion-cells to be especially numerous in the region of the mouth and tentacles. BOOKS FOR FURTHER STUDY I. GENERAL TEXT-BOOKS Sedgwick. Student's Text-Book of Zoology, Vol. I. Hickson. The Cambridge Natural History, Vol. I. Delage and Herouard. Zoologie concrete, Tome II. II. SYSTEMATIC WORKS FOR THE IDENTIFICATION OF SPECIMENS Hincks. British Hydrozoa. Mayer. Medusae of the World. Gosse. Actinologia britannica. CHAPTER III PORIFERA Tin-, phylum I'ORIKKRA (Sponges) is constituted by a group of organisms comparable in the decree of complexity of their structure with the Coelenterata but which are pretty clearly not Coelenterates. They appear to have arisen as a side branch during the evolution of the animal kingdom and to have sprouted out from the Protozoan stem quite inde- pendently of the Coelenterata. The essential characters of the group are most easily demonstrated by the study of young specimens of the comparatively simple Ascon type of sponge exemplified by the genus • '.fulfiiin. specimens of which may be found attached to overhanging rocks or stones or seaweeds near low-water mark on almost any coast. LEUCOSOLENIA The Leucnsulenia has essentially the character of a tube one or two millimetres in diameter and whitish in colour. The tube is at first simple, io\\n in the illustration (Fig. 54), but it becomes complicated by the lopment of lateral or basal outgrowths which may lead in the first 10 the formation of complex tree-like masses, and in the second to the formation of colonies of " individuals " connected by a stolon which BT the .surface of the substratum. bached end the tube is closed while at its free end it opens by a wide opening the osculum (Fig. 54, os). Tin- mil TOM opjc study of thin sections (Fig. 55) shows the wall of In- ((imposed of two layers— an inner gastral and an outer dermal (Fig, 55. I >). Of these the inner is the simpler for it ingle layer of cells, all alike. These, the choanocytes are highly characteristic. Each is somewhat flask- contains a rounded nucleus rather nearer its free end, and is prolonged into a single powerful flagellum which projects inwards towards 118 CHAP. Ill PORIFERA 119 s.m. the centre of the cavity of the sponge. These flagella beat actively and kcrp up an outwardly flowing current of water through the osculum . Tlu- most peculiar feature of the choanocyte and that which gives it its name is the presence of a thin soft membrane of protoplasm — the collar — which projects from tin- margin of the free end of the cell, forming a kind of tube or funnel, of which the axis is occupied by the flagellum. The protoplasm of the collar shows active streaming movements by which food-particles coming against and adhering to the collar are carried down into the < ell hocly. The collar can be re- tracted completely into the cell- body and as a consequence may invisible in sections not pre- ired very carefully. The choano- :ytes line the whole cavity of the sponge except a region in the leighbourhood of the osculum which is floored in by the sieve- lembrane — a thin perforated mem- >rane stretching straight across the ivity of the sponge some little listance internal to the osculum (Fig. 54, s.m). The dermal layer consists of a itrix of clear jelly, resembling the mesogloea of the Coelenterates, rith which are associated several types of cells. Covering the whole eternal surface and extending iwards at the osculum as far as ic sieve-membrane is the dermal jpithelium (Fig. 55, d.e)— a layer )f closely fitting polygonal cells so flattened out as to appear in tion merely as a fine line with a dot here and there in its course presenting a nucleus. In some of the allied sponges, though not the genus Leucosolenia. these cells are highly contractile and A*1 FIG. 54. A young Ascon sponge, x 60. os, Osculum ; p, pore ; s.m, sieve-membrane ; sp, spicule. 120 ZOOLOGY FOR MEDICAL STUDENTS CHAP. a. d.e. the sponge shrinks up under unfavourable conditions the , about by the contraction of the dermal epithelial about at intervals in the dermal epithelium are modified porocytes (Fig. 55, PC). Each of these, instead of being ,m,l plate like, extends inwards throughout the thickness of the (il.rmal ia merging between the choanocytes, comes into direct on \s ith the water in the cavity. When the sponge is fully expanded the porocyte concentrates its proto- plasm peripherally so that it forms a tubular sheath round an axial cavity which opens on the one hand on the external surface of the sponge and on the other into the gastral cavity. These temporary intracellular openings through the body-wall of the sponge are the pores (Fig. 55, p) and it is through them that there takes place the indraught of water necessary to replace that which passes out through the osculum. Other cells of the dermal epi- thelium become modified as sclero- blasts. These take on an amoeboid character, wander into the jelly, and there settle down to form the spicules which compose the skeleton of the sponge. These spicules are needles of calcium carbonate with an axial core and thin external sheath of organic material. The first formed spicules are simple needles (monaxon spicules — Fig. 58, A, two lower figs.) but later on there are lormed numerous three-rayed (triradiate) spicules each of \\hieh represents a group of monaxon spicules radiating from a point \. upper ti The mode of formation of these compound triradiate spicules is tic, Three scleroHasts approach one another and arrange in trefoil fashion. The nucleus of each cell divides, and •i the two nuclei a fine needle-like spicule makes its appearance. Tin- three spiculex heroine continuous centrally so as to form the three : the •••impound triradiate spicule. Each ray lies between two i ro. 55. :u ilhHtr.itiiiL; tin- structure of an Ascon as MM in .1 transverse section, a, ! '. >lri iii.il l.ivrr ; , pore; pc, porocyte; in PORIFERA . 121 more or less distinct though closely apposed cells, formed by the cytoplasm of the original scleroblast clumping together round the two nuclei. As the two spicule-forming cells keep on depositing more and more calcium carbonate on the spicule they wander apart, one passing towards the tip of the ray, the other remaining at its base. Eventually the apical cell disappears and after a time the basal cell wanders out and lakes its |>l;i< <• ut the tip of the ray (Fig. 55, sc). One of the rays of the triradiate spicule commonly differs in Icn-th from the other two, the whole spicule having a Y -shape. The spicule has a definite orientation, the unpaired ray, forming the stem of the Y, being arranged longitudinally and pointing away from the osculum (Fig. 54). In accordance with the cylindrical form of the wall of the sponge in which they are embedded the two equal rays are not exactly in the same plane and they may be slightly curved. Of the cells which leave the dermal epithelium and wander into the jelly a third set are characterized by their retaining a to-all-appearance undifferentiated amoeboid character. These amoebocytes (Fig. 55, a), which are found scattered irregularly through the jelly, play an important part in the life of the sponge. They serve for the transport of food and excretory material, and it is some of them that function as the gonad. In the latter case the cell rounds itself off and either divides over and over again to form a mass of microgametes, or simply increases in size and stores up reserve food material or yolk, becoming a single egg or macrogamete. The above description has dealt with the genus Leucosolenia but young Ascons collected for practical work will often be found to belong to another genus — Claihrina. While agreeing in its main features with Leucosolenia, Clathrina presents certain differences in detail which serve for its identifi- cation, (i) The individual tubes and their branches tend to undergo fusion together so as to form a kind of network ; (2) the nuclei of the choanocytes are close to their basal or attached end ; and (3) all three angles between the rays of the three-rayed spicules are equal. GRANTIA The Ascon is one of the simplest types of Sponge. A good idea of the way in which sponge structure becomes more complicated is obtained by the study of another very common type of sponge — the Sycon type — exemplified by the genus Grantia (Fig. 56). While built up of precisely the same elements as the Ascon, the Sycon differs in its usually larger size and more complicated arrangement of these elements. Simple ZOOLOGY FOR MEDICAL STUDENTS CHAP. tcosolenia frequently undergo an imperfect kind of budding, ring out into tubular pockets which radiate out at axis of the sponge. Now in a typical Sycon the UJ1(] is u.Set l»v siu-h outgrowths in close contiguity with one 57, B). At their tree ends the outgrowths are fused more ]ir ,, as to present a continuous surface broken by small openings whirl, lead into the spaces between the original outgrowths, »eing m>\\ known us the inhalent canals (Fig. 57, C, i.c). as it were parts of the outer world enclosed between the outgrowths, are lined with ordinary dermal epithelium similar to that covering the rest of the outer surface. The pores in the Sycon are restricted to the walls of the inhalent canals and are consequently not visible when the sponge is viewed from the outside (Fig. 57, C). The interior of the Sycon is divided into a central cavity (c.c) and radiating chambers (r.c), the latter represent- in- the cavities of the outgrowths. Finally the choanocytes are found only in the radiating chambers, the central cavity being lined with a simple thin epithelium like that covering the external surface. Apart from the differences that have been men- tioned the Sycon type of sponge agrees with the Ascon : here again the body-wall is composed of the same elements — gastral layer with its choanocytes, and dermal layer with its jelly, its dermal epithelium, its porocytes, amoebocytes and scleroblasts. The PORIFERA are, with the exception of a few genera, marine in habit. They vary greatly in si/.r and form — the differences in general appearance being due 1 1 part to imperfect processes of budding and fission: e.g. may reach ;l relatively large bulk and numerous oscula us surface betoken so many incompletely separated imlividiiaU. Internally the differences have to do mainly with differences . which are referred to collectively as the canal-system. In •here is just the single gastral cavity (Fig. 57, A); in the the central cavity and the radiating chambers to which the ch» .ire restricted (Fig. 57, B and C) ; in still other sponges • 'itaining the choanocytes become small and rounded i while their communications with inhalent canals and with central me drawn out into more or less complicated tubular channels. i. X6. lllllll. in PORIFKRA 123 Throughout the group we find the two main layers of cells, gastral and dermal, and everywhere we find the various types of cells which occur in the Ascon. There are characteristic differences in the shape of the spicules and their chemical composition as will appear later. A number of different sponges, mostly inhabitants of fresh water, although some of them are marine, possess an interesting adaptive arrangement by which they are enabled to tide over periods of unfavour- able conditions, such as winter in a cold climate, or the dry season in a warm one. This consists in the development of what are known as gemmules and the process is well seen in our common fresh-water sponges. As the period of unfavourable conditions comes on, amoebocytes which have stored up in their cytoplasm large quantities of yolky reserve c.c. FIG. 57- Diagrammatic transverse sections of the Ascon (A) and Sycon (B, C) types of sponge. The layer of choanocytes is represented by the heavy black line, the dermal epithelium by a thin line, and the jelly by fine dots, c.c, Central cavity ; g.c, gastral cavity ; i.c, inhalent canal ; r.c, radiating chamber. food -material congregate together in more or less spherical clumps, usually rather less than half a millimetre in diameter and conspicuous to the naked eye from the yellowish colour due to the yolk. Round these spherical masses which are the gemmules there collect other cells of a glandular nature, and apparently by the activity of these the gemmule becomes surrounded by a tough capsule. External to this spicules are collected together to form a further protective envelope. As conditions become more and more unfavourable the ordinary cells of the sponge die away and eventually there is left simply the sponge skeleton with the gemmules scattered through its meshes. This condition persists until conditions again become favourable when the gemmules hatch out, their cells become distributed through the substance of the sponge, they multiply actively and, becoming differentiated in various directions, ZOOLOGY FOR MEDICAL STUDENTS L HAF. •ha complete new cell-outfit such as it had durin ason. ire classified as follows into three main subdivisions : I. CALCAREA .ubdivision includes the Ascon and Sycon types which have en described. It is characterized above all, as indicated by the tact that the spicules are composed of calcium carbonate. ^hape the triradiate spicules are particularly characteristic idrs these there arc commonly present straight or curved monaxon ^picules. and tour-rayed spicules derived from the triradiate by the addition of a small fourth ray which projects inwards into the gastral :i "f Sponges. A, Calcareous spicules (Lcucosolenia) ; B, siliceous spicules (Hyaloncma; all spcmgine (Pachychalina and Chalina). cavity (Fig. 5-S, A ; Fig. 55). The general plan of structure is primarily ih.it ol tlu- Ascon but complication may take place to a less or greater aloiu; tin- lines indicated on p. 122. II. HEXACTINELLIDA tmrlluU arc lor the most part deep-sea sponges: the plan ol' their .structure is somewhat S\ con-like : their most striking .Mul that which gives them their name is the nature of their latter are composed of clear glassy silica, in the form of ecting one another at right angles so as to give when all re «-,,ii;illy developed a spicule with six equal rays (Fig. 58, B). "nrnonly ' S are not developed equally— e.g. a single ray : or absent as in the right-hand spicule of Fig. 58, \\, or one plane may he reduced so as to produce a spicule which has the (lecept i\ e appearance of being monaxon. Finally particular Ill PORIFERA rays may grow out at their ends or elsewhere into spikes, knobs or other projections as e.g. the middle spicule of Fig. 58, B. Well-known members of the group Hexactinellida are illustrated by Figs. 59 and 60. Fig. 59 represents the " Glass Rope Sponge," characterized especially by the tuft of gigantic spicules, looking like a piece of rope of spun glass, by which the sponge is rooted in the mud (Fig. 59, r.s). Each of these rooting spicules possesses at its free end an anchor-like arrangement of recurved hooks which some- times represent four reduced rays while at other times they appear to be secondary outgrowths. An interesting feature commonly seen in speci- mens of Hyalonema is the presence of small symbiotic anemones, attached to the portion of the root tuft which was not buried in the mud (Fig. 59, an). Fig. 60 illustrates the " Venus's Flower Basket " sponge — Euplectella—oiten brought as a curiosity from the East. Here the body of the sponge forms a cylindrical tube, closed at the top by a sieve-plate. The wall of the sponge is supported by a beautiful trellis-work formed of fused spicules, the strands of the trellis- work — longitudinal, circular, right-handed, and left-handed spirals — being so arranged as to meet the stresses to which the wall of the sponge is subject. When the sponge develops in situations where there is a prevalent current pressure in one direction the cylinder takes on a correlated curvature as in the specimen figured, the concavity of the curve facing the current. Further strengthening of the sponge may be brought about by the development of spiral flanges projecting from the surface as in the specimen figured (Fig. 60). III. DEMOSPONGIAE The Demospongiae comprise a great variety of sponges including most of the common and more conspicuous genera. The canal system rs. FIG. 59. Skeleton of Hyalonema. an Small anemones (Palythoa) growing on portion of root above the surface of the mud ; r.s, root spicules. ZOOUKiY FOR MEDICAL STUDENTS CHAP. d, small rounded chambers containing choanocytes with two sets of tubular channels the one inhalent the lent. >picules are composed of silica and are either tetraxon— with - diverging from a point at equal angles— or monaxon. The tour FIG. 60. Skrlrtnll (if E re lre<|ueiitly united into a continuous framework — not however i.il lu>ion between the siliceous substance of the spicules as is the ith the llexactincllids but by the interposition of a peculiar Niihsiam-r allied to silk in its chemical composition and known as spongine. In various members of the group n,n interesting modifica- -kdrton has come about by the increase in amount of the ' orresponding reduction of the siliceous spicules Ill PORIFERA 127 (Fig. 58, C). The final stage in this process is exemplified by the ordinary Toilet sponges l in which the spicules have completely vanished, leaving behind them the spongine framework. The Toilet sponge as purchased is simply this skeletal framework from which the protoplasmic tissues have been removed by putrefaction or maceration. Amongst the Demospongiae are included the few genera, such as Spongilla and Ephydatia, which have forsaken the sea and live in fresh water. It is more especially these which have developed the power of forming gemmules. Zoologists are very generally inclined to regard the Porifera as a group which has arisen in the course of evolution from the Protozoa inde- pendently of the Coelen- terata and other Metazoa. They take this attitude for two reasons amongst others. (i) The early stages in the development of the few types of sponge in which they have been investigated are peculiar and do not provide any evidence to show that the osculum of a simple sponge corresponds to the primitive mouth of the coelenterate. (2) Perhaps xGOO FIG. 61. Proterospongia. x 600. (From The Cambridge Natural the most Striking cliarac- History — after Saville Kent.) a, Amoebocyte ; b, cell-individual . . r , -r. • r • undergoing fission ; c, cell with small collar ; z, jelly. tenstic of the Ponfera is afforded by the choanocytes — a type of cell which, if it occurs at all, is of the greatest rarity in other Metazoa. Now there is a special section of the Protozoa, known as the Choanoflagellata, in which the cell individual is identical with a free-living choanocyte. Further a special genus of Choano- flagellata is known (Proterospongia — Fig. 61) in which a number of cell individuals remain associated together as a community. Some of these remain typical choanocytes while others, embedded in a jelly-like secretion, become amoebocytes. In Proterospongia we see actually existing a creature which may reasonably be interpreted as representing a first 1 These belong to the genera Euspongia — the finer-textured sponges, and Hippo- spongia — the coarser sponges, in which the body of the sponge is traversed by numerous wide branching channels in addition to the normal canal system. ILOGY FOR .MEDICAL STUDENTS CHAP, m 'lution ot choanoflagellate Protozoa in the direction of tin- !' IJOOK FOR FURTHER STUDY GENERAL TEXT-BOOK Minchin. I'orilVra, in Lankester's Treatise on Zoology. CHAPTER IV ANNELIDA IN the simple metazoan type represented by Hydra we saw that the body consists of the two primary cell-layers or epithelia — the ectoderm and the endoderm. In the slightly more complex type represented by the sea-anemone the body still consists for the most part of the two primary layers but there are now apparent two advances in detail, (i) The endo- derm bulges outwards to form the lining of the recesses or pockets lying between the mesenteries and (2) the mesogloea, the jelly-like material lying between the two primary layers, has become colonized by cells which have wandered into it by amoeboid movement. These immigrant amoebo- cytes constitute a new element in the structure of the body known as the mesenchyme. In the still more complex type of structure characteristic of the great majority of the more highly evolved animals, grouped together under the common name COELOMATA, we find each of the two peculiarities just mentioned showing further development. The endoderm pockets have become separated off to form a body - cavity , known as the coelome, surrounding a central tubular enteron or alimentary canal. The lining of these coelomic cavities, originally part of the endoderm, is now distinguished from the definitive endoderm (lining the enteron) under the name mesoderm : it gives rise to the greater part of the muscles of the body and of the excretory and reproductive organs. The mesenchyme again has become much more abundant. Indeed in the more bulky animals mesoderm and mesenchyme are responsible for by far the greater part of the body. Apart from the nervous system, which may be of considerable bulk, the ectoderm is confined to a thin layer covering the surface of the skin while the endoderm is similarly confined to a thin layer lining the alimentary canal. Not only has the mesenchyme increased greatly in quantity : it 129 K ZOOLOGY FOR MEDICAL STUDENTS CHAP. Ps. dp. a. XIV highly specialized in various directions. Much of it connective tissue and special tracts of this may become hardened and stiffened to constitute the skeleton. Other tracts of it form the blood-system. A large section of the mesenchyme remains on the other hand comparatively unspecialized as a mobile defence force to the body. Its cells retain the character of amoe- bocytes, creeping about among the tissues of the body, performing scavenging and other important func- tions and remaining permanently mobilized, ready to concentrate and attack alien organisms, such as disease - producing microbes, which have found their way into the body. LUMBRICUS The general features of the phylum ANNELIDA are conveniently studied in a large earthworm of the genus Lumbricus which, though not so unspecialized as are some of the marine annelids, has the advantage of being almost everywhere obtain- able and of being thoroughly suitable for dissection. The general appearance of the creature (Fig. 62) is familiar to every one — rounded in section, pointed towards the front or head end, flat- tened from above downwards towards the hinder end. The upper or dorsal side is darker in colour, the lower or ventral side is paler, and is somewhat flattened. Sharply marked on the surface of the body divide it into a •i'lmln-r of segments or somites. The general surface is covered with a thin. tran>lucent, somewhat iridescent, cuticle which is apt 1-1 An K.utlnvorin -Lum- The left - hand uts the dorsal -t hand figure . ntnil side (after r, Primer of Zoology). A, anus; Cl, ,i ; ,//', dorsal pore; .!/, iii'iuth ; /'s, prestomium. ii.d opening ; XIV, ' 'nite. LUMBRICUS to strip off when the dead worm has been for some time submerged under water in the dissecting dish. Projecting slightly from the surface of the body are minute stiff bristles or chaetae. Of these there are in each somite eight, arranged in four pairs in a transverse row round the ventral half of the segment. They are most easily detected in the posterior flattened portion of the body and may be felt am. cLu. .ec. y.c. C.TTV. l.m. end. coel. N~. uu FIG. 63. Lumbricus, transverse section about the middle of the body, am, Amoebocytes; c.m, circular muscles ; coel, coelome ; d.v, dorsal blood-vessel ; ec, epidermis ; end, endoderm ; int, intestine ; l.m, longitudinal muscles ; N, ventral nerve-cord ; n, nephridium ; ty, typhlosole ; v.v, ventral vessel ; y.c, yellow cells. [The cavities of blood-vessels are shewn in black.] by a sensitive finger passed along the surface of the body, or seen with the aid of a lens. At the front end of the earthworm is the wide mouth opening (Fig. 62, M), overhung by a fleshy projecting lobe — the prestomium (Ps). At the extreme posterior end of the body is a vertical slit— the anus ( A) or posterior opening of the alimentary canal. Finally, if the worm be sexually mature, a distinct pale coloured and somewhat saddle-shaped swelling (Fig. 62, Cl) is seen encircling the ec. 'c.m. ZOOLOGY FOR MEDICAL STUDENTS CHAP. on its ventral side, towards its anterior end. This thicken- . Meiium • it varies somewhat in position in different species mites XXXII-XXXVIl in L. he^deus, one of our ,-thworms) .and as will be seen later it performs an ,,n m connexion with reproduction. -wall of the worm is, as may be seen by examining trans- ulth the microscope, of complicated structure (Fig. 63). toderm or epidermis (Figs. 63 and 64, ec) a layer of elium, the individual cells of which are columnar in shape and have rficial layer of their cytoplasm condensed to form the cuticle sses uninterruptedly from cell to cell. Here and there may be seen a gland-cell — its cytoplasm laden with drops of secretion, its nucleus situated deep down towards its inner end, and its outer end tapering off to a com- lm paratively narrow tip which is devoid of cuticle. The cuticle is thus incomplete, or perforated by a minute pore over each gland-cell, so that the secretion passes away readily on to the outer surface of the skin which it serves to loist. Should the section pass longitudinally through a chaeta il may demonstrate the interesting fact that the chaeta i> -imply a small patch of cuticle enormously thickened so that it ts on the one hand outwards beyond the general surface and, • >n the other, downwards into the thickness of the body-wall. The •ing part of the chaeta is ensheathed in an epidermal the chaeta sac— and the chaeta is in fact the cuticle secreted i mis forming the chaeta sac. Chaetae are liable to be worn 1 . and in order to provide for this contingency a young reserve •:med liy an outgrowth of the main chaeta sac (Fig. 64). If •>i, the reserve chaeta commences to grow actively • >on takes its place. IMC,. 64. e section of an earthworm passing through V.-jdovsky). c.e, Coelomic epithelium ; c.m, circular l.m, longitudinal muscles; m, muscle (or moving chaeta. [The cuticle is indicated by a thick : iv LUMBRICUS 133 The greater part of the thickness of the body- wall is occupied by muscle fibres, which are arranged in two sharply defined layers, an outer layer of circular (c.m) and an inner layer of longitudinal fibres (Lin). The latter have a very characteristic appearance in a transverse section, owing to the contractile substance of each fibre being arranged in a curious pinnate pattern. When the longitudinal muscles contract they cause the body of the worm to shorten and thicken, while on the other hand the circular muscles by their contraction cause the body to diminish in diameter and consequently to increase in length. Amongst and around the muscle fibres is a small amount of packing or connective tissue, and finally the inner face of the body-wall is lined with a layer of extremely thin flat cells — the coelomic epithelium (Fig. 64, c.e). The body-wall, the structure of which has been described, forms an outer tube and within this there runs from end to end of the worm an inner tube which is the enteron or alimentary canal (Fig. 63, inf). The wall of this is composed of elements similar to those which constitute the body-wall only arranged in reverse order. Internally is a layer of columnar epithelium — the endoderm (end), outside this is a layer of circular and then a layer of longitudinal muscle fibres — both very thin, and finally a layer of coelomic epithelium (y.c). The tube so constituted is seen on slitting open the body-wall of the worm to consist of several regions of distinctive appearance (Fig. 65, A). The buccal cavity — the cavity of the mouth (b.c) — leads into the somewhat ellipsoidal pharynx (ph) of characteristically furry appearance owing to the presence of numerous slender muscles which radiate out from it to the body-wall. These muscles when they contract serve to dilate the pharynx and in this way produce a sucking action by which food particles are drawn in through the mouth. The pharynx is con- tinued back by a slender tube— the oesophagus or gullet (oes) — which about segment XIV dilates to form the somewhat conical crop (c). This in turn opens into the gizzard (g), about the same size as the crop but differing from it in its walls being thick and hard, due to the exaggerated thickness of the muscular layers. The gizzard stretches through about three mites and then is continued onwards as the intestine (int) which extends ithout further change to the anus at the hind end of the body. The testine is of characteristic appearance, its wall is thin and sacculated, it has a brownish -yellow colour, owing to the peculiar nature of the coelomic epithelium covering it, and along the mid-dorsal line its wall projects inwards as a prominent fold — the typhlosole (Fig. 63, ty). The main functions of the enteron are concerned with the ingestion, digestion, and assimilation, of the food. The food drawn into the /OOLOGY FOR MKDICAL STUDENTS CHAP. s.o.g. n.c. {)h.u onwards by what are termed peristaltic contractions ot- ti wall, waves of constriction— produced by the contraction in succession of the circular muscles — passing tailwards and pushing the contained food in the same direction. In the gizzard, where the muscular coat is specially de- veloped, the food undergoes a process of grinding into pulp. The food during its onward progress is subjected to the action of various secretions. The actual process of diges- tion is mainly carried out in the intestine and the digestive' ferments are produced by gland-cells which are scattered about in the endoderm arid pour their secretions into the intestinal cavity. In the region of ' the oesophagus (about seg- ments X-XII) special collec- tions of gland-cells are found in three pocket-like outpush- ings of the enteric wall (Fig. 65, A, c.g). These are the caleiferous glands, so called from the nature of their secre- tion — calcium carbonate — which gives the glands a very characteristic white chalky FIG 6 appearance. The function of Diction, through these, the protoplasm of the nephridial wall extracts trom it the nitrogenous waste products of metabolism and passes i int<> the tubular cavity. (2) The cilia of the nephrostome cause a slow current of the watery coelomic fluid to set outwards through the funnel and down the cavity of the tube. This current serves to carry • •I' coclomic fluid and small particles of disintegrated yellow • •elU which may be floating about in it. The outgoing stream also incidentally assists the first-mentioned function, inasmuch as it serves to flush out the excretory substances passed into the cavity of the nephridium by the activity of its wall. The other communications between the coelome and the exterior are much simpler than those afforded by the nephridia, being in the form of direct openings in the mid-dorsal line known as the dorsal pores .'. , ovary; sp, spermathecae ; s.v.l., anterior seminal vesicle ; s.v.II., posterior *enin rounded U.dio become pear-shaped, their outer ends becoming pointed. Later on thc\ become drawn out into fine filaments, the inner thicker • 1 of the condensed nuclear material, the outer composed m highly contractile and capable of active flexure from side :iallv they break off as complete microgametes or spermato/oa • ly by the movements of their contractile " tails." matozoa like the eggs lie in the cavity of the coelome but, "n with their small si/.c and active movements, they are iv LUMBRIfUS 139 imprisoned in a special chamber which becomes cut off from the main cavity of the somite. This chamber is the seminal vesicle.1 A seminal vesicle is formed in each of the somites (X and XI) in which testes are present (Fig. 67, s.v.l. and s.v.ll.). Each is like a rectangular box walled in by thin membrane, the anterior and posterior walls being simply portions of septa. As the quantity of developing microgametes within the vesicle increases in amount the walls of the vesicle bulge outwards at their lateral angles forming large pouches, three on each side, which pushing the septa in front of them project into the cavities of neighbouring somites and in a dissection form the most conspicuous parts of the vesicles. Of these lateral pouches the anterior vesicle forms two pairs — one from each angle — while the posterior vesicle forms only a single pair — which are however the largest — from its posterior angles. From the coelomic cavity the gametes find their way to the exterior through the paired genital ducts. These are simplest in the case of the female organs. The duct — oviduct (Fig. 67, od) — is in this case a short somewhat trumpet-shaped tube opening by a wide funnel-like mouth through the septum bounding somite XIII on its posterior side, and passing outwards and tailwards to open to the exterior by a minute slit on the ventral surface of somite XIV. The trumpet-shaped coelomic funnel of the oviduct bulges backwards on its inner side into segment XIV as a rounded pocket — the receptaculum ovorum — in which eggs accumulate and remain for a time before passing to the exterior. The male gametes reach the exterior on each side by a slit-like opening with tumid lips on the ventral surface of somite XV (Fig. 62, (J). The genital duct can be traced forwards from this as a straight slender tube — the vas deferens (Fig. 67, v.d) — very inconspicuous except when filled with spermatozoa — more or less embedded in the inner layers of the body- wall. Into the anterior end of this there opens a vas efferens, a slender contorted tube which comes to it from the anterior seminal vesicle and, one segment further back, a similar vas efferens comes from the posterior vesicle. Each vas efferens communicates with the cavity of the vesicle by a very wide ciliated funnel (Fig. 67, c.f), the wall of which is deeply frilled, folded backwards and forwards, so as to break up the mouth of the funnel into a system of very fine chinks. These chinks are so narrow that the sperm-morulae and other stages in the development of the microgametes are effectively prevented from passing out, whereas the 1 This name has unfortunately come into practically universal use. The earlier naturalists more correctly applied the name testis to the whole cavity full of developing microgametes instead of restricting it to the small mass of germ -cells still attached to the lining of the cavity. J4o ZOOLOGY FOR MEDICAL STUDENTS CHAP. hairlik. microgamete can readily pass down the chinks and so rior. uiin to be mentioned as accessory reproductive organs the ; spermathecae (Fig. 67, sp). These are inpushings of the .til in the grooves between somites IX and X, and X and XI, which I'.Tin spin-nail pockets projecting forwards into the cavity of IX and X respectively, and which functionally serve as in which microgametes received from another worm are mtil needed lor the process of syngamy. When rilled with thev arc conspicuous by the brilliant white appearance reflection of light from the surfaces of the dense nuclear portions innumerable microgametes. The complicated arrangement of organs which has just been described r its object the production of zygotes — new individuals in the mm ellular stage. When the worm is about to lay its eggs the gland- the clitellum become active and produce a liquid secretion which . -; over the surface of the clitellum and there hardens to form an elastic membrane encircling the clitellar region of the body like a piece : indiarubbcr tubing. The worm next proceeds by writhing aents to work itself backwards out of this elastic sheath. As the •es over somite XIV the macrogametes (commonly about four in number) from the two receptacula are passed outwards so that they lie between the sheath and the body. At the level of somites XI to IX mature microgametes from the spermathecae — received it will be remembered from another worm — are squeezed out into the same space. Finally as the worm withdraws its head end from the elastic sheath the ends of the latter close up and it forms a small lemon-shaped cocoon. • •£K or macrogamete within the cocoon is fertilized by a micro- .nul one or more of the zygotes so produced proceeds to develop individual, the remainder degenerating. •arthworm possesses a well-developed blood-system. Of the vessels the two most conspicuous are longitudinal — the dorsal vessel '. more or less hidden amongst the yellow cells on the U e of the alimentary canal, and the ventral vessel (v.v) sus- i by a thin meml.rane underneath the alimentary canal. These !•> lar-e hoop-like vessels in about five somites 1 ^ ' ' ' ";<' inuu end of the worm, which from their function the hearts. Connected with these main blood-vessels -els \\hirh divide into smaller and smaller inr1' "1 eventually into a network of extremely fine, thin- capillary blood-vessels in which the blood is brought into' iv LUMBRICUS 141 extremely close relations with the living protoplasm of the various organs of the body. The capillary network is well seen in the wall of the ali- mentary canal, immediately outside the endoderm, where it is concern^ I with the taking up of the products of digestion, in the wall of the nephridium where it is concerned with excretion, and immediately beneath the epidermis where it is concerned with respiration. From the capillary network the blood drains away into small vessels which uniting together in a branched system return it eventually into the main vessels. The blood is propelled onwards by waves of peristaltic contraction of the walls of the larger vessels. These are particularly accentuated in the hearts, in which the peristaltic waves pass downwards from dorsal to ventral end. The hearts being situated towards the head end of the worm it follows that the blood-stream passes forwards in the dorsal vessel, ventralwards in the hearts, and tailwards in the ventral vessel. The blood itself consists of small irregular or rounded cells floating in a copious fluid or plasma. The latter is coloured " blood-red " owing to its carrying in solution the same iron-containing colouring matter — haemoglobin — as gives the red colour to the blood of Vertebrates. This substance haemoglobin is intimately concerned with the process of respiration. It has a great affinity for oxygen and if brought into relation with it at once combines with it to form oxyhaemoglobin characterized by its bright red colour. The oxygen and the haemo- globin in this compound are combined in a very loose fashion and are readily torn apart. It is this chemical characteristic that enables the haemoglobin to perform its great physiological function, that of acting as a vehicle for the oxygen so necessary for the metabolism of all the living protoplasm of the body. As the blood circulates through the capillary network of the skin the haemoglobin combines with the oxygen which diffuses in from the outer air. The oxyhaemoglobin so formed is then whirled away in the blood -stream until, somewhere in the interior of the body, coming into the neighbourhood of tissues hungry for oxygen, it breaks up, sets free its oxygen, so that it can be appropriated by the tissue, and passes onwards as reduced haemoglobin until it under- goes re-oxygenation on again passing near the surface of the body. There finally remains to be mentioned the nervous system which serves to control the activity of the worm and to knit together its con- stituent parts into a coherent and functional whole. The most important change which we see when we compare higher stages in the evolution of the nervous system with lower consists in the greater centralization of control, correlated with greater concentration of ganglion-cells, so >LOGY FOR MEDICAL STUDENTS CHAP. central part of the nervous system becomes more and more sharply a peripheral portion which, composed mainly of nerve- : vly to convey the nerve impulses to or from the nerve In this respect the nervous system of the earthworm shows marked advance on that of a Coelenterate; inasmuch as the portion of the nervous system is sharply marked off and com- highly developed. It consists firstly of a longitudinal ventral nerve-cord— which runs throughout the length of rm in the mid-ventral line and immediately internal to the body- .-.'.<-. and Fig. 63, AT). In each somite the cord is slightly >U(,llen— these swellings or ganglia being simply portions of the cord in which there is a specially marked aggregation of ganglion-cells. From lion there pass off to each side slender nerves, i.e. bundles of fibres, some of which are motor, connected with the muscles of the bodv-wall. while others are sensory, ending in sensory cells in the rmis. Secondly, in addition to the ganglia of the ventral cord there are present a pair of ganglia (cerebral, or supra-oesophageal ganglia— v.0.£), which lie side by side, dorsal to the pharynx and close front end. These are continuous with one another through a commissure or bridge of nerve-fibres, while each is also continued outer side into a eircum-oesophageal commissure which curves round the side of the alimentary canal and is continued ventrally into the first ganglion of the ventral cord. From the cerebral ganglion on •here passes forwards a conspicuous little nerve consisting mainly of sensory fibres connected with sensory cells in the epidermis of the prestomium, this latter being an extremely sensitive organ by \vhi< h the worm, so to speak, feels its way when burrowing through the earth. « if the Earthworm serves to illustrate a number of important i prim iples of animal structure. The Coelenterates and Sponges Cfl either sessile in habit (i.e. fixed in one spot) or capable •aparauvely sluggish and indeterminate movements. The thcr hand moves about actively and its movements are in relation to the structure of its body— one particular end front under normal circumstances, and one particular side being above. Correlated with this type of movement, the body of undergone adaptive evolution in its general structure. It has become elongated in the line of movement. Its two ends have iated— though not so markedly as in many other worms iv LUMBRICUS 143 — into an anterior or head end, carrying the mouth, the sensitive pre- stomium and the cerebral ganglia, and a posterior end carrying the anus. Again, the side which is normally uppermost (dorsal side) is differentiated from the side which is normally below (ventral). The general symmetry of the worm is bilateral, i.e. with the right and left side alike and equal, in contradistinction to such creatures as Hydroids or Medusae in which there is radial symmetry. Such bilateral symmetry is usual in animals which move actively forwards, while radial symmetry on the other hand is associated with a sessile or drifting habit. Lastly the worm affords a good example of metamerism or metameric segmentation, i.e. the subdivision of the body into successive somites, each a repetition of the others in its main structural features — body-wall with dorsal pore and chaetae, coelomic compartment, pair of nephridia, nerve ganglion, etc. Portions of an animal (or of different animals) which are morphologically equivalent, built up out of the same elements, are said to be homologous. Thus the fore-limb is homologous in the various types of vertebrate — the fore-leg of a lizard, the wing of a bird, the fore-leg of a dog, the wing of a bat, the flipper of a whale, the arm and hand of a man — these, in spite of their dissimilarity in appearance and in function, are homologous, for they correspond in structure and have arisen in evolution from the fore-limb of the common ancestor. This adjective homologous must be carefully distinguished from analogous which is used to express functional not structural correspondence. Thus the wings of a Fly and a Bird are analogous organs for they serve the same function but they are not homologous for there is no structural or evolutionary correspondence. Again, a nephridium of one worm is homologous with that of another. Even within the body of the same animal organs may be homologous, e.g. organs on one side of the body are homologous with their fellows on the other. Or in the case of a metamerically segmented creature they may be homologous with their representatives further forward or back in the series. In this case the expression serial homology (or homodynamy) is used : thus the individual somites of the worm are said to be serially homologous. The last great difference between the worm and the coelenterate is that whereas the latter has, interposed between the two primary layers of cells, a mere structureless mesogloea, with at the most a few scattered immigrant cells, the worm has on the other hand interposed between ectoderm and endoderm the complicated arrangements of tissue consti- tuting the mesoderm and mesenchyme. From . the worm upwards, throughout the animal kingdom, these constitute, as indicated at the M,Y FOR MEDICAL STUDENTS CHAP. : this chapter, by tar the greater part of the hulk of the rerything except the thin layer of endoderm lining the alimentary canal,, the thin ecto- derm or epidermis covering the outer surface, and the nervous system. In the worm and in the large variety of animals grouped together as the Coelomata the mesoderm is excavated to form the coelomic body-cavity. This may be defined as "a body- cavity lined by mesoderm, com- municating with the exterior by nephridia and developing the gonad from its lining epithelium." The phylum ANNELIDA is sub- divided into three sections (i) Polychaeta, (2) Oligochaeta,, (3) Hirudinea. I. POLYCHAETA The general features of the Polychaeta are well illustrated by Nereis — the Ragworm (Fig. 68) — of which several species are common round our coasts under stones or amongst sandy mud. The first striking difference in appearance from Lumbricus is afforded by the presence of numerous rude leg-like projec- tions — the parapodia (pp) — arranged down the sides of the body— a pair to each somite. If a thick transverse section of the worm is made so as to show a parapodium from its anterior side (Fig. 69) it is seen that the parapodium is bilobed— ing the notopodium and the ventral the neuropodium. A i to, Ntrtt •vorin from th<- , I,. ,-,.,! Oi l,,..,,| VMVtt In s!i,,w ; e, eye ; """ ; />/>. p-irapodium ; p», pntlomiiim ; /, prcst iv POLYCHAETA 145 At the base of each of these lobes there projects from the surface of the body a tentacle-like projection or cirrus — known respectively as the notopodial cirrus (d.c) and the neuropodial cirrus (v.c). Further each lobe has embedded in it a bundle of numerous chaetae, larger and projecting further beyond the surface than those of Lumbricus. One of the chaetae near the centre of each bunch is thicker and stouter than the rest. It is known as the aciculum (Fig. 69, ac) and its main function appears to be to act as a support to the parapodium. At the front end of the Nereis there is a more pronounced development of head than is the case in Lumbricus. The prestomium is larger and more highly developed (Fig. 68, B, ps). A pair of prestomial tentacles (t) project from it in front while embedded in its dorsal wall s. dc. FIG. 69. Isolated somite of Nereis, ax, Alimentary canal ; ac, acicula ; ch, chaetae ; d.c, notopodial cirrus ; m, longitudinal muscle ; n, nerve-cord ; s, coelomic septum ; v.c, neuropodial cirrus. are two pairs of eyes (e). On each side there projects a palp (p), a tentacular structure with a large base into which the terminal portion can be retracted telescope-fashion. Immediately behind the prestomium is the peristomium (Fig. 68, A, pe), representing two somites fused together, in correlation with which it bears four cirri on each side instead of only two ; it has no projecting parapodia. There are various features of interest to be noted in connexion with the reproductive phenomena of the Polychaetes. In Nereis itself the adult worm in some species assumes when- sexually mature a peculiar change of form which before the life-history was understood caused it to be regarded as a separate genus to which the name Hetero- nereis was given. In the heteronereid condition the hinder part of the body, in which alone are gametes developed, becomes modified, its parapodia becoming enlarged and flattened, while chaetae of a curious L ZOOLOGY FOR MEDICAL STUDENTS CHAP. liki- term may make their appearance. These modifications of , do with the fact that the Nereis at this stage gives •n-lrequentin- habit and swims about freely so as to distribute Aider area. With the assumption of the pelagic •:ies about another modification very usual in pelagic animals namely a -n-at increase in the size of the eyes. In an allied family of Polychaetes, the Syllidae, to which a number FIG. 70. if M-xual phenomena in the Syllidae ;,ham, Cambridge Natural History). I, Heterosyllid ; hiimata ; III. and IV, Autolytus ;V,A. edwardsii, -.nal individual; B, C, D, E, sexual stage in development of a sexual mion marine worms belong, there occur similar modifications, rurrird in nuidi further than in Nereis. In the simplest . I) what takes place is very much the same as in Nereis, isyllid condition beinu assumed in which the hinder part of the MID- the ^amclrs takes on an appearance markedly different 1 tlu- front part. In exceptional species of Syllis (S. Jianmta) •.•Hid condition has been reached there makes its appear- iv POLYCHAETA 147 ance, just in front of the sexual region, a constriction of the body which gradually deepens so that the whole of this portion of the body becomes completely separated off (Fig. 70, II). This latter retains its vitality for some time, wriggling about and distributing the gametes. Another species of Syttis (S. hyalina) behaves in the same way as 6'. haniata but in this case the separated-off portion after a few days develops a definite head region so that it now forms a complete sexual individual. In various species of Autolytus the sexual individual develops its head before it separates off (Fig. 70, III) and further the process is repeated — new sexual individuals being produced in succession, each after its predecessor has separated off. In certain exceptional cases sexual individual No. 2 (Fig. 70, IV, z) develops before No. i separates off and this leads up to the condition met with in old specimens of A. edwardsii where as many as six new sexual individuals may be recognizable before the first-formed one has become detached (Fig. 70, V). In a closely allied genus, Myrianida, chains of as many as thirty sexual individuals have been observed — the youngest and least developed one in front — attached to the hinder end of the original individual. This latter is of course asexual, developing no gametes, though it may be said to reproduce asexually by the production of the new sexual individuals at its hinder end. These reproductive peculiarities which find their climax in Myrianida are of special interest from their parallelism with phenomena character- istic of one of the great groups of parasitic worms dealt with in the next chapter — the Tapeworms or Cestoda. While Nereis exemplifies satisfactorily a typical member of the Polychaeta there occur within the group many variations in details of structure. A general idea of the kind of variations met with is got by examining a few common marine genera. One of the features which is particularly apt to depart from the normal is the dorsal or notopodial cirrus. In Cirratulus, common amongst mud and sand under stones near low-water mark, the dorsal cirri are long and threadlike, twisting actively about in the live animal and functioning as gills. In Eunice — found in burrows in sand near low-water— the dorsal cirrus also functions as a gill but here it develops side branches so as to have a feathery appearance. In Polynoe — one of the commonest genera of marine worms — the back is covered by overlapping plates or elytra, each of which is really the greatly expanded tip of a notopodial cirrus which has grown out in mushroom fashion all round (Fig. 71, A, d.c). In Aphrodita— the " Sea-mouse " to give it its somewhat absurd popular name — an 148 ZOOLOGY FOR MEDICAL STUDENTS CHAP. unusually bulky worm which attracts attention, when cast up on the y the magnificent iridescence of its long chaetae, there exist quite i-lvtra to those of Polynoe (Fig. 71, B,, d.c), only in this case they ;i>le in surface view, being hidden away under curious tough i'he nature of this is seen by studying a transverse section of the like that shown in the diagram (Fig. 71, B). It is seen that the notopodium does not project from the surface but its position is indicated Of these there are three different sets. Furthest out are the luiu; fine iridescent chaetae (i), which give the creature its rharaeteristir appearance. To the inner side of these are short stiff iiaetae (2). and amongst these are produced the third set of chaetae d.ct FIG. 71. Utatrate tin- parapodium of Polynoe (A) and Aphrodita (B) (from Benham, Cambridge History), ac, Acicula ; d.c, dorsal cirrus ; v.c, ventral cirrus, i, Iridescent chaetae ; 2, stiff . i, f«'lt. whu-h break up into their constituent fibres and become felted together nn a roof (3) which covers over the elytra. In many cases the Polychaete shows peculiarities clearly related to Cities in its mode of life. For example Tomopteris lives a pelagic •wimmintf about in the surface waters of the sea. Its para- "<• I«.n- and paddle-like: their chaetae have disappeared except h side at the head end. As in so many pelagic creatures levdoped a glassy transparency which makes it almost nvisiUe when alive in the sea-water. Many Polychaetes ich as the ordinary Lug-worm (Arenicola) which is responsible fur the numerous heaped-up sand castings so commonly of sandy mud between tide-marks. In it the parapodia as to be quite inconspicuous and the neuropodium •m.I notupu.hum are some distance apart. A number of the notopodia iv POLYCHAETA 149 carry conspicuous branched gills. These may IK- situated near the middle of the body — about a dozen pairs, the exact number differing in different species — or they may extend right back to the hinder end of the body as is the case with two species. A remarkable feature of Arenicola, very unusual amongst Polychaetes, is its possession of a pair of otocysts, lying one on each side close to the circum-oesophageal commissure and consisting of a tubular invagination of the outer surface, dilated at its inner end into a rounded cavity full of fluid and containing little grains of sand which act as otoliths. Another very common inhabitant of sandy shores is Terebella which not only burrows in the sand but gives its burrow a certain degree of permanence by lining it with grains of sand, fragments of shell or even small pebbles, cemented together by secretion produced by conspicuous patches of gland-cells on the ventral surface of the anterior segments. In Terebella there is present a transverse row of long thread-like tentacles projecting from the dorsal surface of the prestomium. Each has a ciliated groove along its ventral side. When building its tube of sand- grains the Terebella stretches out its tentacles over the surface of the sand and grains are caught up and carried by ciliary movement along the groove in towards the head of the worm where they are built on to the edge of the tube. Normally the Terebella extends its tube so that it projects slightly above the surface of the sand, ending off in irregular branched threads of sand which often form conspicuous tufts studding the surface of the sand near low- water mark. In the genus Pectinaria also a tube is built up of sand grains cemented together, in this case with great regularity, so as to form a slightly curved tapering " house " which the animal carries about with it. The tube is open at both ends the hinder opening being plugged by the flattened posterior end of the worm and the wider anterior opening being guarded by stout golden chaetae springing in a row from each side of the second segment. Finally there exist a large series of Polychaetes which are still more intimately adapted to a tube-dwelling habit ; the parapodia are in these greatly reduced, as is the prestomium with its tentacles, while on the other hand the palps are greatly enlarged, forming branched plume- like and often beautifully coloured gills. In Serpula and its allies the tube is composed of calcium carbonate with a slight organic basis of conchiolin (see p. 268) and a branch of one of the gills forms a conical stopper which plugs the mouth of the tube when the animal with- draws, as it does with lightning rapidity. A very common Serpulid is Pomatoceros triqueter, which forms the white tubes, with a longitudinal ZOOLOGY FOR MEDICAL STUDENTS CHAP. ridge p- into a spike over the opening, so frequently seen winding :rface of stones and shells on the seashore. Another is the ^nrorbis, whose tube, coiled into a flat spiral, is so common on ,1 rocks. In this case the stopper is hollowed out to form itv in which the eggs undergo part of their development. 'la and its allies resemble the Serpulids in their general ( )nc of their interesting features is their intense sensitiveness urns— a shadow falling on the expanded worm causing . draw back into its tube. This sensitiveness is due to sensory cells in tin- epidermis of the gills and in some of the allied genera such ,-ells hem me clumped together to form definite and complicated eyes which >ho\v as round black dots on the gill. The genus Sabella itself nbedded in mud. its vertical tube of mud grains projecting upwards ;al surface. Large worms of this genus a foot or so in may be frequently found a little below low-water mark, e.g. in grass in quiet bays and sea lochs. D I. \ ELOPMENT OF POLYGORDIUS in example of the mode of development of the marine annelids \\e will take the case of the little marine worm Polygordius a member of a small group of worms which on account of their very simple and primitive character are usually separated off from the typical Polychaetes as a group by themselves — the Archiannelida. At sexual maturity the body of the Polygordius breaks up and sets •netes, male or female as the case may be. Syngamy takes in the sea-water and the zygote undergoes the usual process of in giving rise to a blastula. The gastrula stage is reached process of invagination similar to that of Amelia except that a relatively much smaller portion of the blastula-wall becomes in- i to form the archenteron. The opening of this, the protostoma, beconv ted, taking on an elliptical shape, and then it narrows in the middle, its outline becoming that of a dumb-bell. Finally the Milr lips of the opening come together and completely fuse so that the M(.\\ represented by two distinct openings some irt. 01 these openings one persists as the mouth while li it closes temporarily for a short time, is represented by the amis. Consequently these two openings in Polygordius are to be regarded simply as the end portions of the original protostoma, and rora hints given us by the study of the develop- "ilier animals that this represents the way in which the IV ANNELIDA 1-1 mouth and anal openings of the more complicated animals in general have arisen in evolution — as the isolated and persisting ends of an elongated primitive mouth or protostoma of the type still exist in- ;it the present day in such an animal as a sea-anemone. The young Polygordius gradually takes on the form of a very charac- teristic type of larva known as a trochosphere (Fig. 72, A). This is rounded in form, slightly pointed at its anal or posterior end, and pro- FIG. 72. Development of Polygordius (A, from Balfour's Embryology ; B and C, after Woltereck). an, Anus ; m, mouth ; n, nephridium ; s, stomach ; s.g, rudiment of supra-oesophageal ganglion ; t, sensory tentacle. jecting slightly round the equator. On one side (ventral) is the mouth opening (m) leading into the ~| -shaped alimentary canal. The middle part of this is dilated to form a stomach (s) while the oesophagus between the stomach and the mouth is lined by an ingrowth of ectoderm and is consequently stomodaeal in nature. The lining of the alimentary canal is ciliated, while on the outer surface there are powerful cilia round the equator, arranged in a double (pre-oral) band in front of the mouth and a row of smaller (post-oral) cilia behind the mouth, by the movements (LOGY I-OR MEDICAL STUDENTS CHAP. of which tin- larva swims. At tin- apex of the dome-shaped pre-oral the lar\a. which subsequent development proves to be the • rid, thm- is a cushion-like thickening of the ectoderm (s.g) the nerve centre or brain of the larva. Certain of the •nm- this project into the water as long sensory hairs,, looking ilia and probably to be regarded as cilia which have lost their tor function and taken on a new sensory one. Between the •derm and ectoderm there is a wide space containing mesoderm cells, iitercd and others forming a compact band on each side. also present on each side a nephridium (n) of the primitive type known as protonephridium which instead of having an open nephrostome ! inner end is provided with the peculiar structures known as " flame- " which will be described in the next chapter (p. 161). • rot h.isphere becomes converted into the fully formed adult in the way indicated by Figs. 72, B and C, the region round the anus growing out rapidly to form the body of the worm, while the main mass of the ,< -sphere becomes the head region. In fact the trochosphere larva might be described as the precociously developed and free-swimming of tin- Poly-ordius ! ipecies of rolygordius which occurs in the North Sea and round the \\est coast of the British Isles shows a curious peculiarity in its lopmcnt in that the trunk portion of the worm remains for a time tolded up in concertina fashion within the body of the trocho- gphere. In the typical I'olyi -hai-ta the course of development is in general much a^ in rolygordiits. There is typically a trochosphere larva although rnav be modified in details such as its shape and more especially rraiv.'cment of its cilia. •nain characteristics which serve collectively to mark off the a distinct group of annelids are (i) the marine habit; )<-e of groups of chaetac usually embedded in distinct 0 the well-developed head region usually provided with other projections; (4) the separate sexes; and (5) • imming larval stage. II. Ol.K.nciIAKTA ochaeta includes the true Earthworms, of which a numl)('r <«nd genera are known, and also a number of iv ANNELIDA 153 genera which are aquatic, living in the mud at the lx>ttom of fresh water or, in a few cases, of the sea in close proximity to the coasts. The general characteristics of the group are well exemplified by Lumbricus — the most important being the reduction in the number of the chaetae, the disappearance of the parapodia, tentacles and cirri, and the hermaphroditism with its correlated complexity of the reproductive organs. It is also characteristic, as is usually the case with groups originally marine which have taken to a fresh-water or terrestrial exist- ence, that the active free-swimming larval stage has been eliminated from the life-history, early development taking place within the cocoon. III. HlRUDINEA As an example of the group Hirudinea it will be convenient to examine the ordinary leech used in medicine — belonging to the genus Hirudo. The leech is a somewhat flattened worm measuring commonly about 3 to 5 inches in length although varying greatly according to the state of extension or contraction of the body. The colour is green or brown with dark mottlings. The whole body is marked off superficially into narrow rings or annul!, about 95 in all. At the hind end of the body is a powerful round posterior sucker, while at the front end the muscular lips of the wide mouth opening form an anterior sucker. There are no traces of parapodia or chaetae. The alimentary canal of the leech is of special interest in its adapta- tions to the peculiar feeding habits — the food being blood, which can be obtained only at long and uncertain intervals. In the buccal cavity arranged in radiating fashion are three small saws (Fig. 73 A, j), each with a curved edge set with numerous small teeth of hard chitinous material. The saws are provided with muscles by which they can be rotated backwards and forwards so as to make a characteristic 3-rayed cut in the skin of the animal attacked. The buccal cavity leads into the pharynx (pK), ellipsoidal in shape and of furry appearance due to the radiating muscles which pass from its surface to the body-wall. These by their contraction cause the cavity of the pharynx to dilate and in this way bring about a sucking action through the mouth. The pharynx leads, with scarcely any intervening oesophagus, into an enormous crop furnished on each side with ten or eleven blindly ending pockets or caeca (c}. These are least clearly marked in front, while on the other hand the hindmost caecum on each side is very large. The crop serves merely as a reservoir for the ingested blood, the actual digestion being carried out in the intestine (ini) — the narrow tubular portion of the ZOOLOliV FOR MEDICAL STUDENTS CHAP. alimcr, 1 which passes from crop to anus. The intestine is at mewhat funnel-shaped, then it becomes slightly dilated, rds as a pair of small rounded caeca (d.g). Behind these [traight narrow tube which finally dilates somewhat to form t. FIG, 73- . \Iinn-ntary canal; B, reproductive organs; C, enlarged view of female organs <».£, Albumen ul.md ; c, caeca of crop ; d.R, digestive caecum ; c.d, ejaculatory • ; n.r, nerve-cord ; od, oviduct ; ov, ovary ; ph, pharynx ; f, rectum ; i, posterior sucker ; /, tcstis ; v, vagina ; v.d, vas dcfcrens : v.e, vas efferens. the rectum (r), terminating in the anal opening, situated on the dorsal surface just in front of the posterior sucker. Mai is as usual provided with definite glandular arrant: ' Projecting from the wall of the pharynx are numerous iv HIRUDINEA 155 slender pear-shaped or club-shaped cells of great size. These are uni- cellular glands which discharge their secretion into the cavity of the pharynx, so that it mixes with the blood as it is being ingested. This secretion has the remarkable property of preventing the coagulation of blood, and owing to its presence the blood taken into the crop remains perfectly fluid and unclotted for weeks or even months, so that it can slowly pass onwards into the intestine to undergo the process of digestion. The actual digestive ferment is apparently secreted by the lining cells of the two rounded intestinal caeca (d.g). A characteristic feature of Hirudo, as of .the majority of le, lies in the fact that the coelome is to a great extent obliterated, the open coelomic body-cavity seen in other annelids being replaced by a dense spongework. Here and there the cavity persists in the form of fluid-containing spaces or sinuses, one of which encloses the nerve- cord ventrally while another runs longitudinally in a dorsal -position. Round the limit of the coelome, immediately underlying the body-wall, is a network of small tubular cavities characterized by the dark pigment in the tissue surrounding them — the botryoidal tissue. A remarkable peculiarity of the coelomic fluid of Hirudo is that it is coloured red by the presence, in solution, of haemoglobin — the red iron-containing pigment which is present in the blood of various animals. It would appear from this that the coelomic fluid has in the leech taken over functions normally pertaining to the blood. The spongework of coelomic spaces with their contained coelomic fluid seem to have replaced the blood system functionally and in correlation with this the original blood-system seems to have to a great extent disappeared. Along each side of the body of the leech there runs a conspicuous lateral " blood-vessel " with contractile muscular walls but this communicates freely with the coelomic spaces and is filled with coelomic fluid and the probability seems to be that even it is not a true blood-vessel but merely a coelomic sinus with specially muscular wall. The leech possesses normally seventeen pairs of nephridial tubes, the internal funnel of which is broken up into a spongework of fine pores and is contained in a special little coelomic cavity. Just before perforating the body-wall the nephridium dilates to form a bladder, spherical in shape when distended. The opening to the exterior is a minute pore situated laterally and on the ventral side of the leech. It can most easily be detected by the drop of fluid which oozes from it when the surface of the leech is carefully dried and the animal subjected to slight pressure. An important point to notice is that the nephridial openings over the greater part of the body occur on each fifth annulus — an indication of ZOOLOGY FOR MEDICAL STUDENTS CHAP. n>iip of five ol these annuli corresponds to a single ,t I'olvchaete or Oligochnete. is hermaphrodite, the gonad being formed from the lining -i'-al roelomic chambers — nine (sometimes ten or eleven) I'.. /) situated vcntrally on each side of the mesial •ingle pair of rather smaller rounded ovaries (ov) situated rd. Ovaries and testes occur in successive somites,, i.e. of live annuli from one another. As regards ducts, each prolonged into a slender oviduct (Fig. 73, C, od} — one or other ;H. h passes under the ventral nerve-cord. The two ducts unite to an unpaired oviduct which winds from side to side in the substance in -looking albumen-gland (a.g) and on emerging from this la i'nr\vard on itself and is continued as the thick-walled muscular I to the median external opening. The eggs before being laid .inulate in the vagina and the inner portion of this organ is conse- Iy sometimes termed the uterus. K.idi test is is continued outwards into a minute vas efiferens (Fig. 73, i these open at right angles into a longitudinal duct — the vas This extends forward to the level of the somite next in front ol tin- ovaries where its lining becomes highly glandular. This glandular portion of the vas deferens is coiled into a compact mass — epididymis (). IJcyoml this its wall becomes thick and muscular • jaculatorius — e.d] : it then is continued towards the middle Imr as ;i very narrow tube and opens into the swollen inner end of a thick muscular tube— the penis— which can be pushed • 1 1 rough the male opening. This like the female opening is unpaired in the mid-ventral line, the two openings being five annuli (--one part. •i the leech, fertilized in the vagina or uterus, are deposited • ocoon. measuring about 25 mm. by 15 mm.; secreted on the surface Mites in the neighbourhood of the female opening, though this portion of the ectoderm is not so thickened as to form a conspicuous < litellum as is the case in the earthworm. A thick outer •I the cocoon is said to be deposited secondarily by the i the lips and to be formed possibly by the pharyngeal glands. i' posited in damp earth near the water margin. nervous system of the leech is arranged on the same general ! nmhricns. the only important difference being that the the two ends of the ventral nerve-cord are crowded together •!• Thus i he rircum oesophageal commissures pass ventrally "presenting the first five ganglia of the ventral iv IIIkfDIM-A 157 cord fused together, while at the hinder end a group of seven ganglia are similarly fused. Sensory cells are scattered through the epidermis ; here and there these are clumped together in definite sensory papillae which are seen regularly arranged on the outer surface of the body and at the front end five pairs of these on the dorsal surface of the following annuli— i, 2, 3, 5, 8— are converted into eyes. These can be clearly seen as black spots if the front end of a pale-coloured specimen is held close to a lamp so that the light shines through it. The Hirudinea as a group are annelids which have become specialized in adaptation to their bloodsucking semiparasitic habits. They are characterized by their suckers — the anterior one for fixing the lips round the incision which they make in the skin of their prey, the posterior for adhering. The development of the posterior sucker at the hinder end, where during the growth of an ordinary annelid new segments are added to the body, serves to bring this increase in the number of somites to an abrupt stop after a more or less definite number (not exceeding 34) has been reached. The somites themselves have become far less distinct than they are in the other annelids — in external view owing to the absence of chaetae and parapodia and to the superficial subdivision of most of the somites into annuli, in internal structure owing to the spacious coelomic cavity with its division into distinct compartments having become converted into a continuous spongework. A characteristic difference in detail is that in the Leeches the original paired genital openings have been shifted into the middle line so as to become unpaired. Included in the group Hirudinea are a number of different genera. One of these — Acanthobdella, found attached to fresh-water fishes in the great lakes of Northern Russia — is of great interest from the evolu- tionary point of view, being as it 'is a link which serves to connect up the Leeches with the other annelids, for it possesses on its first five somites perfectly typical chaetae and its coelome is still a spacious body- cavity divided up by about twenty incomplete transverse septa. Amongst the Leeches more closely allied to the medicinal Leech (Hirudo medicinalis) are the genera Aulostoma, the Horse Leech, which commonly leaves the water to devour earthworms — its favourite food, Haemadipsa, the unpleasant Land Leech of the tropics, and Nephelis one of our commonest small fresh-water Leeches, which feeds on worms and molluscs. Another set of Leeches are characterized by the saws having dis- appeared and a protrusible proboscis having taken their place. Amongst these are Pontobdella — a large marine leech which attacks Skates and ZOOLOGY FOR MEDICAL STUDENTS CHAP, iv lU) which can be retracted and thrust out. A little ehind the proboscis is a distinct nerve-ganglion (g) and embedded in this a characteristic nc-shaped eye. Further back in the body there v FASCIOLA 165 is present on each side a flame-cell (Figs. 74, B, n; and 75, B). Con- spicuous amongst the parenchyma which fills the interior are numerous reproductive cells — the germ-cells. The miracidium swims actively hither and thither but normally dies after a short life of about eight hours unless it comes across a small water-snail of a species (Limnaea truncatuld) common in marshy dis- tricts. The miracidium is able to detect the presence of a snail in its neighbourhood and is indeed able to follow up its track on the mud. It approaches the snail and attaches itself to its soft skin by its proboscis, a Limnaea which has been attacked by many miracidia being given a furry appearance through the miracidia hanging on to it by their probosces. In such a case the snail may be killed, but more usually it is attacked only by one or a few miracidia and does not suffer serious damage. The miracidium after attaching itself bores through the skin of the snail, makes its way into the body-cavity and eventually takes up its position in the large blood spaces in the roof of the lung. Here it loses its shape, the nerve-ganglion and eye de- generate and it becomes a mere bag (sporocyst — Fig. 74, C) containing the germ-cells. The latter undergo repeated division, giving rise to large masses (g.c) at first rounded in form but gradually becoming elongated. Each of these gradually takes on the form of the next stage of the life- history, known as the redia (Fig. 74, D) — a worm-like creature with two short stumps projecting from its body one on each side towards its hinder end. At the front end is the mouth which leads through a muscular pharynx into a short simple blindly-ending intestine (ent). The interior of the body is filled as before with parenchyma containing scattered masses of germ-cells (g.c) and near the head end is a single reproductive opening (b.o). The rediae when fully formed make their way out of the remains of the sporocyst and wander through the tissues of the snail where they are found especially in the liver. The germ-cells within the redia com- monly develop into a new generation of rediae, but the germ-cells of this second generation develop not into rediae but into a type of larva known as a cercaria (Fig. 74, E, c, and F). The cercaria is a somewhat tadpole-shaped creature with an ellipsoidal body and a tail, by the flapping movement of which the cercaria is able to swim. There is a mouth at the front end, surrounded by a sucker and leading into a pharynx which opens into the intestine of characteristic n -shape (Fig. 74, F, ent}. A ganglionic nerve-ring (g) surrounds the alimentary canal in the region of the pharynx. Towards each side of the body is a mass of gland-cells (gt). The space between these organs is filled with paren- chyma. Finally about the middle of the body is a second sucker — the j66 ZOOLOGY FOR MEDICAL STUDENTS CHAP. .,-). If we ignore the tail it is clear that the structure merely that of the adult fluke in a comparatively simple and undeveloped .-(.million. The cercariae, whether produced from the nerati n of mliae as they sometimes are or from a later generation mure usually are, make their way out of the body of the parent :id finally out of the snail, swimming away with a characteristic jerky motion. . ntly they drop off their tails and creep about in leech-like by means of their suckers, shooting out the body to a considerable ,ind then shortening it. Under normal circumstances the cercaria creeps up on to a blade of grass and adhering to this by its ventral sucker proceeds to encyst, surrounding itself with a clear secretion prodiu •<•(! l>y the conspicuous gland on each side of its body. Within the cyst the cercaria goes on slowly developing, the features of the adult fluke becoming more and more distinctly recognizable (Fig. but the development is not completed unless the blade of grass with its adherent cyst is cropped by a sheep. In this event the cyst is 1 and the young fluke set free in the sheep's alimentary canal: it wanders up into the bile-duct and there in due course attains to its adult form and sexual maturity. The TKKMATODA are essentially parasites, and the group is charac- teri/ed by the following combination of structural features — the un- sr- merited body with usually at least a ventral sucker for attachment to the host, the thick cuticle, the forked blindly-ending intestine, and the hermaphrodite reproductive organs. The uroup is divided into two sub-groups, the Monogenea and the •iea. The 1 >I<,I.M;A, or digenetic Trematodes, are given this name from the tact that the parasitic portion of their life-history is divided between two distinct host-animals as is the case with Fasciola — the sexual inhabiting usually the alimentary canal of a Vertebrate, while rations (rediae, cercariae) infest Molluscs. In some cases a second Vertebrate host may be introduced into the life-cycle, the : making its way into the body of a fresh-water fish and there There exist a great variety of Trematodes in the group Digenea, inhabit in- various Mammalian hosts, and many of these are liable, as is the cast- with Im the ovary by way of the oviduct and the resulting zygotes, with yolk-cells from the yolk-duct, become encased in shells by the vity of the shell-gland and then pass forwards into the uterus in which they accumulate. The accumulation of eggs brings about changes in the appearance of the uterus which are very characteristic. Its side walls become gradually distended to form pockets which gradually inerea-e in length, becoming irregularly branched as they do so, until they almost reach the lateral boundary of the proglottis. Eventually tlui greater part of the whole proglottis is occupied by the branches of the uterus ( I-"iur. So, A and B), the other reproductive organs shrivelling up and becoming quite inconspicuous. This condition, in which the proglottis is little more than a packet •ygotes or fertilized eggs, is found in the hindmost proglottides in the chain, i.e. the oldest proglottides,, for the proglottides originate, just as do the sexual individuals of Myrianida, by being cut off from the hinder end of the asexual portion or scolex. Eventually the old proglottis, full of eggs, is dropped off. It passes to the exterior with the faeces of the host : it may show signs of life for some time but before long it dies and disinteii rates and the eggs are scattered abroad, nothing more hap- pening unless the egg is swallowed by the appropriate host animal. CESTODE LIFE-HISTORIES Tai-nia scrrata, one of the commonest tape-worms of the dog, affords an cxrrllent example of the typical cestode life-history. The further development of the egg takes place if, and only if, it be swallowed by a rabbit. In this event there hatches out within the alimentary canal .1 small rounded larva, provided with six sharp blades by means of which it cuts its way into the wall of the alimentary canal. Eventually ! the blood and is carried off in the blood-stream towards the liver. It apparently usually leaves the blood-vessel within that organ and then may migrate for some distance before it finds a suitable spot, Mi«h a> the linin- of the body-cavity, in which to settle down. 1 1 now ^rows into the cysticercus or bladder-worm — a semi-transparent tped vesicle about the size of a pea and full of clear fluid. i.-h the wall of the vesicle can be seen shimmering an elongated whitish body which projects inwards from one pole. In a well-infected ! 'ladder-worms may be seen scattered about in it the bo.ly-cavity. If a fresh bladder- worm be slightly squeezed between the fin-ers the whitish structure within it shoots out, and it, v TAENIA 175 is now seen to be a typical scolex, with suckers and hooks, which had been inverted into the interior of the cysticercus. If a live bladder-worm is swallowed by a dog the scolex becomes everted within its alimentary canal and attaches itself to the lining by means of its hooks and suckers. The vesicle is digested off and the scolex proceeds to grow in length and bud off the chain of proglottides so that it assumes the characters of the typical tape-worm. Taenia caninum, another common tape-worm of the dog and also of the cat, is often separated from Taenia as a distinct genus Dipylidium owing to conspicuous differences. Each proglottis has not the typical rectangular form but rather approaches the elliptical — the lateral bound- aries bulging outwards. Further, each proglottis contains a double set of reproductive organs, each with its own openings situated in a little notch which is conspicuous in the middle of each lateral edge. In this case the cysticercus is of very minute size and is often termed a cysti- cercoid. The minute size is correlated with the fact that this phase of the life-history is passed in the body of a dog-louse (Trichodectes) or flea (Pulex). Its occasional occurrence in the human flea is perhaps responsible for the fact that this tape-worm occurs occasionally, though rarely, in the human being. Taenia coenurus is a tape-worm which occurs not uncommonly in Sheep-dogs. The bladder-worm stage is remarkable for its great size (up to 2 inches, or even more, in diameter) and for the fact that it produces from its lining not a single scolex but very many scolices which are visible as distinct little white grains through the translucent wall. This bladder-worm occurs in the sheep and a favourite situation for it is the brain where it produces the disease known as sturdy or staggers. Among the Cestodes commonly occurring in Man there are four species with which the student should make himself familiar. Of these the commonest in Britain and America and other beef- eating countries is T. saginata (Fig. 80, B), a tape-worm which quite usually reaches a length of thirty feet and sometimes much more. A peculiarity of this species is that the scolex is without hooks, while on the other hand the suckers are unusually large and powerful. The shed proglottides — which are what the medical man is most likely to come across — are readily differentiated from those of the next species by the number (20-35 on eac^ s^e) anc* slenderness of the branches of the uterus. ZOOLOGY FOR MEDICAL STUDENTS CHAP. re (C. bavis) occurs amongst the muscles of the ox . So, A) is a common tape-worm of pork-eating countru-. 1 1 measures from six to nine feet in length, has well-developed I its uterus in the old proglottis possesses on each side only < ven to ten) and relatively stout branches. The ovary f •nit hum, m tap.' -\\-i.nns (A, 'I'.unici snlium ; 15, T. saginata ; C, Bothrioccphalus IOUS f the pig hut has been found in various other animals — dog, < rn in man himself. Taenin eclrinofnccits occurs not uncommonly in the dog in various iie world. It is more frequent than elsewhere in Iceland, itul the stock-raising portions of South America. Its most CESTODA 177 striking feature is its very small, almost microscopic, size — the whole worm, which consists of a scolex and 3 or 4 proglottides, measuring only about 2-5 to 6 mm. in length. In correlation with this minut£size the tape- worm usually occurs in the dog's intestine not singly or only as a few individuals but in enormous numbers together. The bladder-worm phase of the life-history occurs in various animals, especially ox, sheep, and pig and occasionally in man. Whereas the tape-worm stage is extraordinarily small, the bladder-worm on the other hand is relatively enormous, reach- ing sometimes a diameter of 7 inches. As in the case of T. coenurus, the wall of the bladder-worm produces not one merely but a large number of scolices, and in this case pocket-like ingrowths of the wall are formed which become separated off, drop into the cavity as secondary bladders, and go on actively producing crops of scolices. Ordinarily the sur- rounding tissues of the host endeavour to protect themselves by enclosing the bladder-worm in tough connective tissue, the whole forming one variety of what the surgeon terms hydatid cysts — easily identified as a rule when large by drawing off the contained fluid and searching it for hooks or complete scolices. The bladder-worm of Taenia echinococcus is a very dangerous parasite, both from the large size to which it may grow within some important organ such as the liver, and also owing to the small size of the proglottis which renders it 'liable to be swallowed complete with its numerous con- tained eggs, each of which may develop into a bladder-worm. Fortunately it is not common in most localities. Where a dog is infected the shed proglottides or their disintegrated remains are liable to get mixed up with the fur ; thence they get on to the animal's tongue and are then ready to be deposited, when the animal licks a plate or a hand, in a position from which they may readily be transferred to the mouth of a human being. BOTHRIOCEPHALUS B. latus (Fig. 80, C) is again one of the large tape-worms — reaching a length of thirty feet. The genus Bothriocephalus is readily dis- tinguished from Taenia by the fact that the "head" is somewhat lance-shaped, is without hooks, and possesses only two suckers, each in the form of a longitudinal slit along the side of the head. The mature proglottides are much broader in proportion to their length than in Taenia, and the reproductive openings are situated not on one side of the proglottis but in the mid line of its flat surface. The small bladder-worm occurs amongst the muscles of various fresh-water fish, N ZOOLOGY FOR MEDICAL STUDENTS CHAP. more especially the burbot (Lota) and perch (Perca). Consequently it parasite <>1 man in regions where fresh-water fish form a article of diet. Tape worms live in the intestine of the host amongst the digesting food and they nourish themselves by absorbing the products of digestion but although they reach a relatively large size it is difficult to believe that the amount of food which they purloin from the host can in itself be of any appreciable importance. Where actual pathological effects are pro- duced by them, as in the case of the severe anaemia sometimes produced l>v ttothriocephalus latus, this would appear to be due to the metabolism of the tape-worm producing a toxin which is absorbed into the blood of the host. The mode of infection by the various species of Cestoda will have become clear from their life-histories. In general it may be said that in- fection with the larval (bladder-worm) stage is brought about by swallowing the egg, while infection with the adult (tape-worm) stage is brought about by swallowing the scolex contained in the bladder-worm. Consequently the precautions to be taken against infection are in the case of the first mentioned such as are dictated by ordinary cleanliness, in particular the prevention of possibly infected animals from licking the skin or dishes used for food, and in the case of the second care that fish or meat is >uffk iently cooked to destroy any larval cestodes that may be present in it. NEMATODA An excellent example of the nematode worms is afforded by the genus Ascaris which is a common parasite in the intestine of the horse and Pig- ASCARIS The adult Ascaris (Fig. 81) is a cylindrical worm tapering towards its two ends and measuring in the female of A. megalocephala, the species found in the horse, as much as 390 mm. in length : the male is smaller, about 180-200 mm. Apart from its smaller size the male is easily dis- ied by the tail end of the body being curled in a ventral direction. The surface of the body is covered with a smooth cuticle ; the skin is devoid of pigment ; the body shows no trace of division into somites, and there are no para podia or other conspicuous projections. ••"iily -wall ot the Ascaris when examined in microscopic sections ess many interesting features (Fig. 82). The cuticle (c) CESTODA, ASCARIS 179 -n. covering the surface is very thick, and it is underlaid by the epidermis or ectoderm (ep) which is in the form of a syncytium or plasmodium, a continuous sheet of protoplasm containing scattered nuclei. At four points in the transverse section— in the dorsal line (d.l), the ventral line (v.l) and the two lateral lines (LI) — / the epidermis is seen to be greatly thickened; so as to traverse the whole thickness of the body- wall. Of these thickenings of epidermis the dorsal and ventral are comparatively narrow, while the two lateral are broad. In the latter towards their inner end a round opening can be seen (ex) which is the cavity of the excretory tube cut in transverse section. This is a very remarkable organ, quite unlike the nephridium of the annelid. It forms a straight tube consisting of a greatly elongated single cell, hollowed out into tubular form, and traversing nearly the whole length of the body. At its hinder end the tube ends blindly, while in front it unites with its fellow to open by a minute pore in the mid-ventral line near the front end of the body (Fig. 81, n). Also connected with the epidermis is the nervous system of the Ascaris which is of a comparatively simple character. Anteriorly a nervous ring encircles the alimentary canal and from this there pass back six longitudinal nerve strands embedded in the epidermis. Two of these lie in the substance of the dorsal and ventral line respectively while two others lie on each side, just dorsal and just ventral to the outer end of the lateral line. As in parasites generally, there are no eyes or otocysts or other highly developed organs of sense. The space between the inwardly projecting shelves of protoplasm forming the dorsal, ventral, and lateral lines is occupied by the muscular system (Fig. 82, m.e) — of special interest in the Nematoda from its consisting of a single layer of large myo-epithelial cells. Each of these cells is widest towards its inner end where it bulges into the body- cavity, and is narrower towards its outer end where it fits in amongst its neighbours. The surface layer of protoplasm in the outer portion of the cell is modified to form the specially contractile substance. The inner end of the cell tapers off into a protoplasmic tail (t), which may FIG. 81. Female Ascaris as seen from the ventral side, an, Anus ; nt, mouth ; n, excretory opening; v.l, ventral line ; 9 , genital open- ZOOLOGY FOR MEDICAL STUDENTS CHAP. i So B very primitive kind of nerve, as it passes straight to the do- ntral line and forms a direct bridge connecting the •udinal IKTVC trunk with the myo-epithelial cell. axis of the body is the alimentary canal which is very simpk in structure in correlation with the fact that the Ascaris lives ,,1-matcruil which is digested for it by its host. The mouth t int. ex. m.e. tit otr. Ascaris (9), transverse section, c, Cuticle; d.l, dorsal line; ep, epidermis; ex, excretory tube ; : .1 line; m.e, myo-epithelial cell; od, oviduct; ov, ovary; t, tail of myo- :, vnitr.il line. i- a small pore at the tip of the body, surrounded by three roundish lobes arranged like a trefoil, one lobe being dorsal and the other ••ntral. The mouth leads into a short pharynx with thick walls and this in turn kads into the thin-walled intestine which passes without ins — a trans verse slit on the ventral side close to the pos- nd of the body (Fig. 81, an). The wall of the intestine (Fig. 82, int) v ASCARIS 181 consists of a simple layer of tall epithelial cells, bounded both externally and internally by a distinct cuticle-like membrane. There is no layer of coelomic epithelium covmnur the surface of the alimentary canal and it is indeed quite uncertain whether the body- cavity through which the alimentary canal runs is really a true coelome at all. Towards the anterior end of the worm the body-cavity contains four remarkable structures known as the phagocytic organs, which can be distinctly seen as dark shadows if the fresh worm is held close to a strong light. In a dissection they are seen as small fluffy objects, of a pinkish or brownish colour, lying between the alimentary canal and the body-wall. Although arranged in two pairs the members of each pair are not exactly opposite one another but are arranged obliquely. Microscopic examina- tion shows that each organ consists of an enormous cell, the body of which extends into tree-like branches which serve to anchor it to the body-wall and the intestinal wall. Both cytoplasm and nucleus of the cell are con- verted into a stiff material so that the cell and its branches are fairly rigid. This cell serves a mainly supporting function, its branches acting as supports to numerous little blobs of protoplasm which are perched all over them. These protoplasmic blobs are actively phagocytic, i.e. they serve to ingest solid particles of a harmful nature which may come to be in the fluid of the body-cavity. The reproductive organs of Ascaris are of a characteristic type. In the female the external genital opening (Fig. 81, $ ) is a small pore, situated mid-ventrally in a slightly marked shallow groove which encircles the body at a very variable position in the anterior half or third of its length. This opening leads into a vagina and this at its inner end bifurcates to form the two uteri. Each uterus is a thick tube of considerable length, normally packed with eggs (Fig. 82, ut). The eggs are enclosed in thick shells of distinctive appearance (Fig. 90, B) and of extraordinary impermeability so that they may remain alive for many months after the adult is placed in a strong preserving fluid such as formalin. The uterus is continued into the oviduct (Fig. 82, od} a much narrower tube which normally contains scattered eggs without any shell — the eggs not yet having been fertilized — and this in turn is continued into the actual gonad. The gonad of the nematode is of very characteristic appearance, consisting of a greatly elongated thread which tapers off at its end into an extremely fine, freely ending, filament. In trans- verse section (Fig. 82, ov) the gonad shows an equally character- istic arrangement of genital cells radiating out from a solid core of protoplasm known as the rachis. As the gonad merges into the 182 ZOOLOGY FOR MEDICAL STUDENTS CHAP, • this rachis breaks down so that the cells now lie loose in the In tin- male the arrangement is similar as regards the most important points the arrangement of the cells round a rachis and the direct con- tinuity of the thread-like gonad with the tubular duct. But there is le gonad or testis in the male. Its duct serves as a seminal in which the microgametes accumulate, and this opens not v to the exterior but into the floor of the alimentary canal near the anus. The terminal piece of the alimentary canal into which the male duct opens has on its dorsal side two curved forwardly-projecting pockets each of which secretes a strong chaeta. These chaetae can be protruded through the genito-anal opening and are inserted into the external genital opening of the female at the time the microgametes are rred, GAMETOGENESIS AND FERTILIZATION reproductive organs of Ascaris are of special interest and im- portance from the fact that their study provided the foundation for much of our present-day knowledge regarding the origin and develop- ment of the gametes and their union in the process of syngamy or fer- tili/ation. \Ye shall now make use of them in giving a description of processes. It has already been indicated that one of the chief characteristics <>f mitotic nuclear division is the concentrating of the nuclear material or ( hromatin into special little masses named chromosomes. What >t been mentioned, so far, is that the number of these chromosomes in the dividing nucleus is as a rule fixed and definite in the cells of any particular species of animal. This holds even for the period when the individual consists of a single cell or zygote. But seeing that the zygote in s\ iiLamy by the fusion of two gametes it necessarily follows that the gametes must contain each only half the normal number of chromosoi It thu-. (nines about that there are two chromosome numbers char- it of the species (i) the number characteristic of the ordinary i the body— known as the diploid number and (2) the number— as the former— characteristic of the gametes and known as iploid number. "•t eristic (diploid) number of chromosomes in a particular imal may be very large, as many as 168 in a little fresh-water •;/>/), while on the other hand it may be comparatively small. In Ascaris »icgalocephala the number is only four (in the case v ASCARIS 183 of a particular variety of this species only two) and the smallness of this number is one of the main factors which have facilitated the follow- ing out of the details of the processes involved in gametogenesis and fertilization. The study of sections through the long thread-like gonad of A. megalocephala shows its cells to be in a state of active multiplication, and each mitotic nucleus presents the diploid number of chromosomes — four (Fig. 83, A). This holds down to a certain level in the gonad but then there comes a remarkable change, as from now onwards each mitotic nucleus shows only two, i.e. the haploid number of chromosomes. Further, each individual chromosome has undergone a complication of its struc- ture and has now the appearance of a bundle of four beaded rods in close apposition (Fig. 83, B). Each of these complex chromosomes is known as a tetrad from its .quadripartite nature. The reduction of the number of chromosomes from diploid to haploid — meiosis as it is termed — a necessary forerunner to the process of syngamy, is clearly a phenomenon of great import- ance and the interesting question arises — How is the reduction brought about ? . Nuclei from the testis of Ascaris. A possible explanation readily SUg- A> before and B, after the formation of gests itself. It will have been noticed the tetrads- n the left and female on tin- ri^lit (after Braner and •• i meiotie division — the two tetrads are seen at the equator of the spindle — a centt' • nt at each pole of tlie spindle in the male but not in the female. B, The first • ompl.-te, the two daughter cells each containing two dyad chromosomes — • , the nial. hut very unequal in the female. C, Second meiotic division. In the male ' • IK resulting from the preceding division is dividing, the upper cell beintf shown at a later st.i^e than the lower. Kach cell contains two monads and a centrosome. In the female 1 I ••.'. " and lecond polar body). I, first polar body; II, second (in i; left hand fi trosome at each pole of the spindle has pre- • .", in preparation for the second mitosis.) another towards tlu- poles of the spindle; possibly owing tn the rnmrartion «,| sjjindlc fibres attached to them (Fig. 84, B). As • ijiart a ( onstriction appears round the equator of the cell whirl) gradually deepens until the cell-body is completely divided v ASCARIS 185 into two. It will be noted (i) that each of the daughter cells contains two, i.e. the haploid, number of chromosomes and (2) that each individual chromosome is a dyad. Each of the two cells repeats the process of mitosis. A spindle is again developed, having at its poles centrosomes arising by the division of the original centrosome, and the two dyad chromosomes arranur them- selves at its equator (Fig. 84, C, lower half), and become split into their two constituent halves which move apart towards the poles of the spindle as monad chromosomes. The cell-body becomes as before constricted into two cells — each of which again contains the haploid number of chromosomes, these being now monad or single in their nature, together with a centrosome (Fig. 84, C, upper half). Thus the original cell from which we started is now represented by four cells. Each of these cells (spermatids) gradually takes on the form of a functional microgamete or spermatozoon. This (Fig. 86, 4) is quite unlike the spermatozoa of most animals in appearance, being somewhat conical in shape with a rather expanded base of soft protoplasm, by the amoeboid movement of which the spermatozoon creeps. Within this lie the two chromosomes and the centrosome, while the apical portion is filled by a clear glassy body of unknown function. FEMALE (Fig. 84, right-hand figures) In the ovary just as in the testis there is a special level at which the mitotic nuclei show chromosomes tetrad in structure and haploid in number. The cells are larger owing to the cytoplasm being distended by large granules of reserve food-material or yolk. As mitosis com- mences a spindle makes its appearance as in the male with the two tetrad chromosomes at its equator, but this spindle is devoid of centrosomes and it is situated close under the surface of the cell, with its axis in a radial direction, i.e. perpendicular to the surface (Fig. 84, A). Each tetrad splits apart into two dyads and then the cell divides but instead of the two daughter cells being of approxi- mately equal size, one of them is reduced to the smallest dimensions, consisting of hardly more cytoplasm than is just sufficient to contain the two dyads. This tiny cell is known as the first polar body (Fig. 84, B, I). The mitotic process is repeated in the large cell. Each dyad becomes rotated through a right angle so that it takes up a radial position : its constituent monads move apart towards the poles of the spindle. The cell as a whole divides, and again one of the two daughter cells 186 ZOOLOGY FOR MEDICAL STUDENTS CHAP. •my— forming the second polar body (Fig. 84, C, II)— while the .v the macrogamete or mature egg. Occasionally the smaller of the two cells formed by the first division, !ar body, also divides with mitosis into two daughter cells containing two monads — but as a rule this division is suppressed. :umal occurrence is however of importance for in such a case e clearly the fundamental identity of the processes at work in the mali- and female gonad. In each case the cell in which the reduced number of (tetrad) chromosomes makes its appearance gives rise by •nitotic divisions in rapid succession to a set of four cells each con- taining the reduced number of chromosomes monad in character. These two divisions, associated with the reduction in the number of chromosomes, are known as the first and second meiotic or maturation divisions. Tin- conspicuous difference between the two sexes is a comparatively rhcial one, namely that in the male each one of the four cells resulting from the meiotic divisions becomes a functional gamete, while in the female only one does so, the other three being the reduced, functioniess polar bodies. Here \ve have come in touch with the most characteristic difference the gametes of the two sexes throughout the animal kingdom. The female gamete is relatively large in size, frequently containing a of reserve food-material or yolk, and is incapable of active move- ment : whereas the male gamete is relatively small, without stored food-material, and active in its movements. The fact that three out of each four potential macrogametes degenerate is no doubt an adaptive arrangement facilitating the increase in size of the fourth, necessary to enable it to contain a sufficient store of yolk. The act of syngamy between the two gametes, the " fertilization of to use the older name, takes place in the cavity of the uterus, which a supply of microgametes has been passed by the male through the external genital opening.1 A single microgamete attaches itself Toad end to an egg (Fig. 85, A) and the nuclear material s, together with the centrosome, into the cytoplasm of egg. Tin- two nuclei which now are in the cytoplasm — the i< lens (N) and the immigrant sperm nucleus («)— undergo a -ra.lual increase in size, the two chromosomes in each becoming lengthened out into slender meandering filaments. Eventually • ss we describe the processes of maturation and 1 sequence but as a matter of fact in Ascaris the two lap, tin- formation of the polar bodies being delayed, until after entered the egg (cf. Fig. 85, A). ASCARIS 187 the two nuclei are absolutely identical in appearance (Fig. 85, C). The two nuclei gradually approach one another, the centrosome— surrounded -AT-. FIG. 85. Diagram illustrating syngamy in A scaris megalocephala. A, Commencing fusion of microgamete with the egg before formation of polar bodies ; B, formation of second polar body ; C, two gamete nuclei alike, each chromosome drawn out into a long thread ; D, shortening of chromosomes, separation of centrosomes ; E, mitotic spindle with split chromosomes ; F, division of zygote into two daughter cells. M, Macrogamete ; m, microgamete ; N, egg nucleus ; n, sperm nucleus ; II, second polar body. by an area of deeply staining cytoplasm — lying between them. The centrosome divides into two — the two gradually receding from one another. Meanwhile the two chromosomes of each nucleus become greatly shortened i88 ZOOLOGY FOR MEDICAL STUDENTS CHAP. to take tin- form of stout curved rods (Fig. 85, D). The bound- - of tlu- two nuclei disappear so that the chromosomes lie free in the cvtoplasm, and a spindle makes its appearance, fibres passing from the chromosomes which lie about its equator to each centrosome and radiating out from the centrosome into the surrounding cytoplasm. h chromosome splits longitudinally (Fig. 85, E) and its two halves slowly recede towards opposite poles of the spindle (Fig. 85, F). Thus there travel towards each pole four daughter chromosomes,, two of which are <>l paternal origin — derived from the sperm chromosomes, two oi maternal — derived from the egg chromosomes. The egg now imes surrounded by a furrow round its equator which gradually >i us until the egg is completely divided into two daughter cells. This division is the first step in the development of the new individual, the first step in the process known as the segmentation of the egg. The important point to notice is that the chromosomes in each of the two dau-hter cells or blastomeres are diploid in number, and are half of puti-mul and half of maternal origin. Throughout subsequent develop- ment, as the blastomeres divide over and over again to form the immense - of cells constituting the adult body, the process of splitting is repeated at every mitosis, so that each cell in the body contains nuclear material derived equally from the two parents. When eventually, in the gonad of the new individual, the process of -iyndesis takes place there is reason to believe that the two chromosomes that come together are one of paternal origin and one of maternal. The evidence on which this belief is based comes not iromAscaris but from other animals but it is possible to indicate in a few words its nature. It has been possible by detailed study of the chromosomes of various animals to determine that any particular species is characterized not merely by the definite number of its chromosomes but also by definite rs of the individual chromosomes. Thus in the developing the haploid group is constant not merely in its number but in imposition. It is made. up of a definite assemblage of chromosomes mall differences in size and shape which make it possible to them individually and to label them with definite designations, ttCTS of the alphabet. Thus supposing the haploid number is six possible to distinguish a. b, c, d, e, f, each characterized by • lr finite shape and size. In each haploid group we find the same set of recurring— each recognizable by its special peculiarities. It iv there is present in the diploid group a double 2!), 2c, 2d, 26, 2f. The corresponding chromo- ' • hromosomes for example— are known technically v ASCARIS 189 as homologous chromosomes, and it is obvious that in each homologous pair one chromosome is paternal in origin and the other maternal. Now when syndesis occurs it is found in cases such as 1 have described that the two chromosomes that come together are always homologous. We are justified then in defining syndesis in general as the pairing or coming together of the homologous chromosomes. We have seen how, in Ascaris, the process of syndesis is the inaugural phase of the first meiotic division and how in the course of the two meiotic divisions each tetrad becomes resolved into its four constituent monads. It is clear that in this latter process there take place (i) separation of the two chromosomes which came temporarily together in syndesis and (2) separation of the two halves into which these chromo- somes were already split before syndesis took place. In Ascaris it is difficult to decide by actual observation what is the order in which these two separations take place, but — judging by the analogy of many other animals in which the matter has been worked out decisively — we are justified in regarding it as probable that they take place in the order named. In other words the retreat of the homologous chromosomes from one another after their temporary apposition takes place in all probability in the first of the two meiotic divisions. We have described the more conspicuous features of the processes of maturation and syngamy in Ascaris but it is now necessary to say some- thing about a complication in detail — difficult to observe but of great importance — which has been discovered in the course of recent research. The kernel of the discoveries in question consists of the fact that while the macrogametes are all alike, the microgametes on the other hand are divisible into two distinct types, one of which produces a male and the other a female zygote when it fuses with a macrogamete. We are consequently brought directly into touch with one of the great problems of zoological science— the determination of sex. It turns out that the cell of the testis which shows the two tetrad chromosomes possesses in addition to these a small sex chromosome (Fig. 86, i,x). This may be distinct or, as is much more frequently the case, it may be unrecognizable through being fused with one of the large tetrads. Now when this sex chromosome is traced through the two meiotic divisions it is found that in one or other of these divisions it passes bodily over to one of the two daughter cells instead of being divided between them. Thus in Fig. 86, A, 2, it is seen that the sex chromosome has passed bodily over into the upper cell in the first meiotic division. It becomes however shared between the two daughter cells in the second 190 ZOOLOGY FOR MEDICAL STUDENTS CHAP. . so that of the four resulting microgametes two are led with sex chromosomes while two are without. end-result is arrived at in Fig. 86, B, although here it is the ,tic division in which the sex chromosome passes over bodily. A sex clin.inosi.me occurs also in the egg before meiosis but in this ^rikiU'd in the ordinary manner at each division and one is ore present in every ripe macrogamete. ainy taking place at random between large numbers of micro- uametes and marrogametes will clearly result in the formation of two ..f zygote in approximately equal numbers— the one type differing FIG. 86. •AJIIK the behaviour of the sex chromosome (x) during the meiotic divisions in the < hromosome remains undivided in the first meiotic division ; B, sex chromo- some rciii.iin> undivided in the second meiotic division. from the other in having an extra sex chromosome brought in by the iinete. It follows that the adult individuals into which the Inp will similarly be divided into two types the one differing from the other in the fact that each of its cells contains two sex chromo- somes in place of one. The former are the female individuals, the latter le : and the sex of the individual would appear to be the result of pure chance — according as the macrogamete is fertilized by a microgamete Miing the extra sex chromosome or by one which is devoid of it. It will be seen that the recognition of these additional facts con- sex chromosome involves an emendation of the statement on j). is- that in A. megalocephala the diploid number of chromosomes v ASCARIS 191 is four. To make the statement complete we must say the diploid number of chromosomes is in the male 4 + x and in the female 4 + 2x, x being the inconspicuous sex chromosome. DIFFERENTIATION OF SOMA FROM GONAD Another series of phenomena of great general interest which have been worked out more fully in Ascaris megalocephala than in any other animal have to, do with the marking off of those cells which constitute the soma from those of the gonad. In A. megalocephala this has been found to occur at the earliest possible stage of development — when the zygote has divided into its first two blastomeres. As these commence the next mitosis a difference becomes apparent between them. In one the process is perfectly normal, the four chromosomes undergoing longi- tudinal splitting precisely as in the first mitosis. In the other blastomere however before the chromosome splits it undergoes transverse segmen- tation. The swollen club-shaped ends of the chromosome drop off, while the more slender central portion segments up into a number of little pieces (Fig. 87, I, left-hand nucleus). It is only these small pieces which undergo the splitting process and enter into the composition of the nuclei of the two daughter cells. The club-shaped ends are simply left lying in the cytoplasm, by which they are gradually digested and destroyed. The result is that in the four-blastomere stage (Fig. 87, II) two cells have nuclei with the full amount of chromatin while the other two have nuclei with greatly diminished amount of chromatin. With this diminution in the amount of chromatin the last-mentioned two cells have become somatic cells. Throughout the numerous subsequent mitoses during the course of development the descendants of these cells remain with nuclei relatively poor in chromatin. When the two blastomeres (of the 4-cell stage) whose nuclei contain the full amount of chromatin undergo mitosis one of the two repeats the process of eliminating the club-shaped ends of its chromosomes, so that two more somatic cells are produced, while the other cell divides normally so that there are again in the 8-cell stage two cells with the full amount of chromatin. Exactly how often this process of diminution of the chromatin is repeated in normal cases — whether three times as in the figure, or four times or five times — is not absolutely determined, but in any case the end-result, when it has taken place for the last time, is that all the cell- nuclei of the embryo have undergone, or in the case of one nucleus are undergoing, the process of diminution so that one nucleus alone retains ZOOLOGY FOR MEDICAL STUDENTS CHAP. 192 the four unmodified chromosomes. This latter cell (the thick arrow at *7 points to it) is the primordial germ-cell, the ancestor that constitute the- ^onacl of the individual. Diagram t<> illustrate tin- inn 1< -.ir dillercntiation of soma from gonad in Ascaris megalocephala. Z represents the zygote-nucleus with its four chromosomes. I, II, III, IV represent successive M 1. 1 descended from the original zygote-nucleus. I — two nuclei, those of the first 'lli.it «in tlir 1' it i ---im.-itic) shows the commencement of the process of chromatin II i (ill -t.i,:;i'. 'lln'twn nuclei on the left have undergone diminution: the cast- [] nui-.ide the nuclei. Of the two nuclei on the right the lower •-.; diminution. Ill — 8-cell stage. Of the eight nuclei, six (somatic) have undergone • U commencing diminution, and the remaining one retains the four IV id-iell stage. Of the i<> nuclei, i.\ (somatic) have undergone diminu- tion, and of tin- ti-iii.iiiiiiig two (above and to the right) one (somatic) is commencing diminution while the other r.-t.mis tin- n-ii-ni.il four unmodified chromosomes. The last mentioned is the primordial germ-- [ aU the cells d the Konad. During suUsi-quriit drvc-lopment the cells go on dividing with ordinary t nninhcrs of cells poor in chromatin forming the compli- •>t tlic adult : vast numbers of others with the full amount v ASCARIS 193 of chromatin constituting the gonad and eventually developing into functional gametes in the way already indicated. The phenomena that have been described are of much importance in relation to the known facts of heredity. In the first place it may be recalled how the study of the Protozoa brought out the fact that the nuclear material of the cell plays an important part in governing and controlling its vital activities. When we study the phenomena of inheritance in animals in general, one of the most striking features of these phenomena is seen to be that, on the whole, the two parents contribute in equal parts to the characters of the offspring. It is clear in a large proportion of cases that the total inheritance of the young individual is already contained within the zygote, for this may go through the complete course of its development into the new individual under conditions which render it impossible that any moulding influence can be exercised by the parent. Conse- quently the inheritance must have been brought into the zygote, and brought in in equal parts, by the two gametes. Therefore the material basis of inheritance, the special substance whatever it is that carries inheritable qualities, must be contributed to the zygote in equal quan- tities by the two gametes. These two gametes may of course be enor- mously different in their size, for example in a bird the macrogamete may be millions of times the size of the microgamete — but the evidence of Ascaris, and also of many other animals, indicates that, in spite of superficial differences in the size of the two gametes, there is one element contributed in approximately equal parts by the two gametes, namely the chromatin of the nucleus apart from the sex-chromosome. As there is so far no evidence to prove that this holds for any other substance, the presumption is obviously very strong that the chromatin is the vehicle which carries the hereditary qualities. Another striking feature of heredity is the way in which it permeates every portion of the body. In any part of the body, in any tissue, a feature may make its appearance which has clearly been inherited from one or other parent. But we have already seen how the ordinary chromo- somes of the zygote, derived half of them from one parent and half from the other, are at each one of the countless mitoses which take place during the development of the individual accurately split into two halves, one going to the one daughter cell and one to the other, so that the meta- bolism of every resultant cell is controlled by chromosomes half of paternal origin and half of maternal. Clearly this fits in perfectly with the idea that the chromatin is the bearer of heredity. . Still another point. One of the striking features of heredity is that O ZOOLOGY FOR MEDICAL STUDENTS CHAP. i '\pressed in the familiar statement that " Acquired " characters, or to use a preferable expression Impressed characters, are not inherited. By this is meant that characters impressed upon the soma during its life, such as local injuries caused by wounds or disease, or changes in size of an organ brought about by increased or decreased use, are not passed on to the descendants. This fact, at first sight remarkable, becomes less puzxh'ng when we see in Ascaris how the cells of the soma become marked off at the very earliest possible \T/ stage of development as a race of cells apart from those of the gonad and never give rise to true repro- ductive cells. The latter, the cells which will develop into the individuals of succeeding generations, are in fact not derived from the soma upon which the character was impressed, but are rather persisting portions of the am estral reproductive substance or gonad from which that soma itself was derived. There is indeed a continuous streak of gonad — the germ track as it is called — which comes down through the aues in any chain of descent and the somas or bodies of successive individuals are simply shed-off or side-tracked portions of this (Fig. 88). The soma is not the parent of the gonad, it is merely the foster parent or nurse which has charge of its portion of gonad — conveying it about, protecting, and nourish- ing it. In the last few pages we have made use of Ascaris megalocephala for illustrating some of the basic facts of cytology — the department of biology which concerns itself with the detailed study of cells — relating more especially to the reproductive cells. These facts are of the greatest importance and we will therefore emphasize them liy a few additional comments. (i) It will l.e rivalled that the study of the Protozoa t a 111: hi us to regard the nucleus of a cell as that portion of its protoplasm or living substance in which is con- « cut rated control over its living activities. I-IG. 88. king of soma from gonad. The diagram represents a portion k passing through individuals of six successive generations. Each group of three black circle* represents the gonad of an individual, enclosed within its soma (S). v ASCARIS 195 (2) As has already been indicated it is a general characteristi. of heredity that, upon the average, the inherited qualities of the individual are derived equally from the two parents. If therefore inherited qualit i«-s liave a material basis this must consist of a substance derived in approxi- mately equal amounts from the two parents. Ascaris has shown us that there is one such substance (and there is no evidence of any other) namely the substance of the ordinary chromosomes. We are therefore justified in the belief that this substance is the material basis or " vehicle " of heredity. It is necessary that we should guard ourselves from forming too definite a conception of this chromatin as a substance of fixed chemical and physical constitution. We should regard chromosomes primarily as portions of the living substance or protoplasm in which certain living activities are concentrated. The physical and chemical features — staining properties, high refractive index and so on — which make the chromosomes recognizable by the sense of sight are to be regarded as relatively super- ficial accompaniments. (3) Syngamy, involving the union of two nuclei, is necessarily accom- panied by meiosis, to keep the chromosome number constant and prevent it from being doubled at each successive syngamy. (4) Syndesis, the temporary pairing of homologous chromosomes, is apparently an essential part of the process of meiosis. Its primary meaning is probably to make use of the existing mechanism of mitosis for transferring entire chromosomes — instead of split halves — to the two daughter cells, thus ensuring that the latter shall contain only the haploid number. While we have there the probable primary meaning of syndesis it is well to bear in mind that in such a case as Ascaris where the two homologous chromosomes lie in close apposition side by side their proximity to one another may provide the opportunity for possible exchange of substance between the two chromosomes, or for the exercise of mutual influence in more obscure ways. (5) The fact that maternal and paternal chromosomes are alike dis- tributed during the course of development to every cell in the body is an important bit of confirmatory evidence to the conclusion, already stated under (2), that the substance of these chromosomes affords the material basis for heredity. (6) In the process of mitosis, which is the almost universal mode of nuclear division all through the animal kingdom, perhaps the most characteristic feature is the remarkable longitudinal splitting of the chromosomes. In fact we might define the mechanism of mitosis as a mechanism for the accurate splitting of the chromosome into two halves and the distribution of these two halves to the two daughter cells. I96 i LOGY FOR MEDICAL STUDENTS CHAP. ,il on -urrcncc of this longitudinal splitting would appear •v with it a vrry interesting logical conclusion namely that the i tin- chromosome is not homogeneous throughout its extent I »ut differs in quality from point to point along its length. Only on this assumption docs it become clear why the chromosome splits longi- tudinally—namely in order that every quality spaced out along its [iially shared between the two daughter chromosomes. In relation to the phenomena of heredity we appear then to be justified in the belief that not only are hereditary qualities in general carried by tin' chromosomes but that different hereditary qualities are localized in different portions of the individual chromosome. (7) In the description of the mitotic process reference has been made to the centrosomes and to the fibrils which constitute the spindle. The student should guard himself against regarding these as discrete struc- ipart from the cytoplasm. They are to be regarded rather as local modifications of the cytoplasm, its constituent particles t emporary re-arrangement under the stress of some unknown ,il influence; in somewhat similar fashion to that shown by iron- i n a magnetic field. The centrosome would appear to be the centre trom which this influence, whatever it may be, radiates. (8) Sex chromosomes were first observed in insects and even to-day •er number of clearly-worked-out cases belong to this group of animals, most of our knowledge having been accumulated by American cvtolouists. Apart from Insects and Nematodes they have "1 in many other cases scattered through the animal kingdom. 'se of Ascaris shows us how sex chromosomes even when present unrecognizable through being fused with ordinary chromosomes ane\iial differentiation of gametes. In cases where sex chromosomes are definitely known to occur they exhibit differences in detail, in numbers and so on, which need not be into in this book. ons which the sex chromosome phenomena of Ascaris I liai in the animal in question the microgametes consist in equal miml.er.s n| two different classes, differ from one another in the fact that they hen they undergo s\ nuamy, zygotes of opposite sexes, and ••ile- producing type is characterized by the fact that it v ASCARIS.. TRICHINA 197 extrudes the sex chromosome bodily in one or other of the two meiotic divisions. Care must be taken not to attribute necessarily any active sex- determining role to the sex chromosomes : the present state of our know- ledge does not justify any such conclusion. All that we are strictly justified in saying is that the extrusion of the sex-chromosome in one type and its non-extrusion in the other provides us with a kind of label indicative of a deep-seated sex-producing difference between the two sets of microgametes. Whether the existence of two different sex-producing types of micro- gametes is to be regarded as the normal cause of the determination of the sex of the sexually-produced individual throughout the animal kingdom is a question to which a definite answer must wait for future research. Probability appears on the whole to point to this answer being in the affirmative. (9) Lastly we have seen how the " germ track " is continuous from generation to generation — the bodies of the successive individuals in a chain of descent being as it were " side-tracked " from it. Here we are in 'touch with one of the most impressive ideas in biological science namely that the living substance of each creature existing on the earth to-day is continuous right back through the eternal past with the living substance which first came into existence in the dawn of the evolution of living things. Various nematode worms are liable to occur as parasites of man and we will now survey the life-histories of a few of the more important of these. TRICHINA T. spiralis, the female of which measures about 3-4 mm. in length and the male about 1-5 mm., is apparently primarily a parasite of the rat, although it is. liable to spread to many other animals including man. The worms reach maturity in the small intestine. After the act of fertilization the male dies while the female grows rapidly to its full length, bores into the intestinal wall and settles down there in the connective tissue. At about the sixth day the birth of young commences : it con- tinues during several weeks, a single female producing many thousands of young larvae. The larvae, which measure about -i mm. in length, pass along the lymph spaces and a large proportion of them eventually reach the blood-stream and are by it distributed through the body. These larvae are small Z98 /OOLOGY FOR MKDTCAL STUDENTS CHAP. . .isily through the capillary network but when they find •.raversing the capillaries of a muscle — particularly if it be one nt tlu- nit in- act i vi- muscles such as the diaphragm — they bore their way the capillary and pass in amongst the muscle fibres. After spending sonic time, it may be several days, migrating through the . tin- young worm curls itself up into a spiral and settles down in • nditicm between the muscle fibres (Fig. 89), the connective <>uml it reacting to its presence by enclosing it in a lemon-shaped iin.nl -4 mm. in length. Within its cyst tin- young Trichina may retain its vitality for many T. mf. FIG. 89. Larval Trichinae encysted in muscle. X 26. m.f, Muscle fibres ; T, Trichinae. .•in this is unusual and as a rule after a few months or years the calcification. No lurther development takes place unless the muscle containing the Trichina is swallowed by a suitable animal. When this happens the worm is set tree by the digestion of the cyst : it rapidly — within from one to two days— attains to sexual maturity and the life-cycle is •I afresh. particularly liable to become infected, probably through the flesh of an infected rat, and then the infection is liable to be "! tn man by his eating insufficiently cooked pork or ham con- taming the encysted worms. As the infection is liable to be a heavy 'ill in- in the presence of many millions of young worms within re .symptoms of disease (trichinosis) are produced— more h«.l«-ra-likr symptoms during the intestinal stage, and high tem- "Uipanied by severe muscular pains (luring the period of NEMATODE PARASITES 199 An important duty of public health authorities consists in the system- atic inspection of pork and ham, especially when imported from abroad, to make sure that it does not contain encysted Trichinae. ASCARIS A species of Ascaris (A. lumbricoides), resembling that already de- scribed in detail but rather smaller in size ((£15-25 cm., 9 20-40 cm.), is a common parasite of man all over the world. Normally it inhabits the small intestine though it may wander into other parts of the body. Its presence in the alimentary canal may be determined by the eggs B FIG. 90. Eggs of parasitic worms from the intestine of Man. They are drawn as resting on a slide ruled in squares with lines 10 /a (i.e. -01 mm.) apart. A, Trichocephalus (the commonest egg of the four) ; B, A scar is ; C, A ncylostoma ; D, Schistosoma japonicum. in the faeces (Fig. 90, B). These eggs are ellipsoidal and are enclosed in a thick envelope with knobbed surface measuring about 60 //, in length. They do not normally show any signs of developing until they reach the exterior where in moist earth the eggs segment and give rise to embryos. If swallowed by a human being these hatch out in his alimentary canal and in about five weeks are sexually mature and producing eggs. TRICHOCEPHALUS Trichocephalus (Fig. 91) is a very common and as a rule harmless inhabitant of the alimentary canal of man, where it is found most fre- quently in the caecum. It measures about 40-45 mm. in length in the 200 ZOOLOGY FOR MEDICAL STUDENTS CHAP. :ul 45-50 mm. in the female, and it is easily recognized by the fact that the anterior portion of the body forms a fine filament which is threaded h tin- intestinal lining and serves to anchor the worm in position. i an elongated ellipsoidal form and measuring about 52 p. in len-th by 23 //. in width (Fig. 90, A), pass to the exterior and if the go on developing to an advanced embryonic stage. If s \\allo\\ed at this stage — which may last for a prolonged period — the uorms hatch out in the alimentary canal of their new host and in about a month are mature and producing eggs. OXYURIS 0. vermicular is is a small nematode ( £ 3-5 mm., 9 I0 mm.) which occurs in the large intes- tine of man and is the usual cause of the complaint popularly known as " worms " in children. The female when ready to deposit the eggs travels down the intes- tine towards the anal openingand the eggs are deposited either on the skin or in faecal matter. If swallowed there hatches out from the egg a young worm which de- velops into an adult like the parent. FIG. 91. Trichocephalus trichiura 9x6. ax, Alimentary ANCYLOSTOMA canal ; g, uterus ; M, moutli. The Miner's Worm or Hook- worm (Aucylostoma duodenale) although small in size ( £ about 10 mm., is mm.) is yet a very dangerous parasite of man, for it is liable to occur in the small intestine in enormous numbers and cause "1 anaemia, It occurs in practically all the warmer parts of the world and is particularly prevalent in Egypt and some parts of India, and in the Southern States of North America. In colder climates it is be introduced and flourish amongst workmen where the necessary condition-, ot warmth and moisture are present, as in tunnels and mines. In mining districts its appearance should always be borne in mind as a NEMATODE PARASITES 201 Apart from the general features characteristic of a small nematode the most striking characteristics are two. (i) The mouth opening (Fig. 92, A) has become shifted on to the dorsal surface of the head, and there project into its cavity upon each side two strong recurved spines. A little further back there projects upwards from the floor of the cavity a pair of flat cutting blades. (2) In the male (Fig. 93) the edges of the anal opening project into a conspicuous thin flap on each side which serve to grip between them the body of the female. The mature worms are to be found especially in the small intestine, holding on by their hooks and feeding on the lining epithelium and the underlying connective tissue with its capillaries. The eggs (Fig. 90, C), ellipsoidal in shape and enclosed in a delicate shell measuring about 60 //, x 37 //,, are laid in the intestine and pass to the exterior amongst the faeces. If conditions are favourable — the ground being moist and the temperature warm (25°"35° C.) — the egg hatches out within n couple of days as a small larva about a quarter of a millimetre in length. These larvae have pointed tails and show certain resemblances to another genus Rhabditis from which fact they are often spoken of as the Rhabditis-stage. The larva feeds actively on faecal matter, grows to about double its original length, and casts off the outer layer of its cuticle. This process of moult- ing is repeated but this time the shed cuticle remains as a loose membranous sheath round the body of the worm. The worm now — a week to ten days old — has lost the Rhabditis character- istics ; it ceases to feed and becomes sluggish in its movements but is capable of remaining alive for several months so long as it is kept wet. It is this stage, after the second moult, that is alone capable of infecting a new mammalian host. This may take place through the worm simply being swallowed, for example in drinking water or among insufficiently cleaned vegetables. But Looss discovered by a personal accident that the Ancylostoma may find its way to its destination by a much more circuitous route. While carrying on investigations in his laboratory in Cairo he let fall on his hand a FIG. 92. Enlarged view of dorsal side of head end of A, Ancylostoma and B, Necator, to show the buccal cavity. 202 ZOOLOGY 1-oK MKDK'AL STUDENTS CHAP. ph. .inl. drop 0, fttaining numerous Ancylostoma larvae. Within a few nsation and a distinct reddening of the skin became louml that the larvae had disappeared into the skin, mpty articular sheaths behind. An experimental repetition In ,,i a liml) an lu.ur before amputation disclosed the larvae in tin- act of burrowing through the skin, their entry hem- as a rule by way of the hair follicles. The exact route from the skin to the intestine was determined by experiments on puppy-dogs with another species of Ancylostoma (A. caninuni). It was found that the larvae make their way into tin- blood stream either directly through the veins uf the skin or by way of the lymphatics. Carried round the circulation they reach the lung- and there Uore their way out of the blood-vessel into the pul- monary cavity. From this they migrate up the trachea or windpipe and thus reach the oesophagus en route for the intestine. There is no reason to suppose that the Ancylostoma of man differs in these migrations from that of the dog. It is very surprising to find in an intestinal parasite like Ancylostoma the existence side by side of two distinct modes of infection, one of them the simple method which we should expect in the case of a parasite of the alimentary canal by ingestion 'hrough the mouth, the other by the unexpectedly roundabout route through the skin. The suspicion suggests itself that in Ancylostoma we have to do with a parasite which formerly inhabited not the alimentary canal but the connective tissue, obtain- in- access to it by boring through the skin, but which has comparatively recently become a parasite of the alimentary canal, not yet however having had time to get rid entirely of its former habit of entering the body through the skin. .•hing the int-'stine, by whichever route, the Ancylostoma rapidly reaches sexual maturity and the first eggs make their appearance In lorahi: lih-yliiittmnt is prevalent the obvious precautions to 'void drinking or usin- lor kitchen purposes water that tain living larvae, and (2) to avoid letting the skin come 93- Ancylostoma d v NEMATODE PARASITES 203 in contact with moist soil possibly contaminated with faecal matter. In mines in temperate climates, whither the parasite may be brought by infected immigrants, the chief precaution to be taken to prevent it from establishing itself is strict sanitary control, to avoid the possibility of eggs becoming scattered about the ground in faecal matter : keeping the temperature down below 25° C. by efficient ventilation, and efficient drainage are also when practicable of importance. NEGATOR Over many parts of Africa, India, the West Indies and the warmer parts of the American continent (extending into the Southern States of North America) there occurs, either alone or associated with Ancylostoma, another Hook-worm which is placed in a distinct genus — Necator. While agreeing with Ancylostoma in the main features of its structure and life- history Necator can be distinguished by certain details. It is slightly smaller ( <$ 8 mm., $ 10 mm.) : the head is bent more sharply towards the dorsal side than is the case with Ancylostoma : and the two recurved teeth are replaced by a flat cutting plate (Fig. 92, B) which meets its neighbour in the middle line. DRACUNCULUS The Guinea Worm—Dracunculus 1 medinensis — is a well - known parasite of man, as well as of the dog and various other mammals, in suitable localities throughout the warmer parts of Africa and Western Asia. It has also been introduced into Fiji and here and there in tropical America. The parasite lives in the connective tissue of the host, the male being comparatively small (22 mm., Leiper) but the female reaching a length of up to about 1 20 cm. with a thickness of a little under 2 mm. The full-sized female, about a year old, is practically filled by the enormously dilated uterus containing millions of eggs, the alimentary canal degener- ating especially in its hinder portion and the anal opening becoming completely occluded. After fertilization has taken place the male apparently dies. The female, as the embryos within the eggs develop, slowly migrates towards some portion of the host's skin which is liable to be wet — ordinarily towards the foot or ankle, but in the case of water-carriers accustomed to carry water in skins or other vessels upon their backs or heads towards 1 Sometimes called by the older name Filaria medinensis. 204 ZOOLOGY FOR MEDICAL STUDENTS CHAP. i uf the body. The head of the worm bores towards the and the epidermis rises up over it as a blister which presently tor mini; a small ulcer with the opening of the burrow in which •m lies in its centre. If now tin- ulcer comes into contact with cold fresh water the worm ts the muscular wall of its body and forces out a portion of its which immediately bursts and exudes a drop of milky-looking fluid, containing myriads of young worms coiled up in spiral form. larvae measure usually from -5 mm. to -75 mm. in length, are somewhat flattened, and have the hinder end of the body drawn out into a fine point. Contact with the water rouses the larvae to activity. They uncoil themselves and swim away rapidly. They are capable ot remaining alive for a period of up to three days but do not proceed with their development unless within that period the}- find their way into the interior of a small fresh-water crustacean — Cyclops (Fig. 94). According to the older accounts the young Dracunculus bores its way into the body of the Cyclops between the hard plates covering its abdomen. More recent ob- servers state that it has to be swallowed by the Cyclops, and that within a period of from 6 to 24 hours it has bored its way through the wall of the alimentary canal and taken up its position in the blood- spaces (haemocoele) of the crustacean. I"" the body of the Cyclops various changes in detail take place ifn-r alx.ut five weeks the larva is ready for transference to its 94- Cyclops, 9 containing tun host. This takes place apparently by the infected Cyclops Wallowed in drinking water. •bvioui precaution to take against infection with Dracunculus tl'«" all drinking water is either filtered, heated, or otherwise to destroy the Cyclops. Where the individual is already rfidal ulcer should be douched with cold water at in- < 2 to 3 weeks-the uterus is completely emptied. The *onn may then slowly be drawn out by winding it round " •'• OT two at a time, or it may be killed in situ by injecting v DRACUNCULUS, FILARIA 205 into it a little corrosive sublimate solution. In the hitter case the dead remains of the worm are gradually destroyed by the living activities of the surrounding tissue. FILARIA BANCROFTI This is a slender thread-like worm, the female about 100 mm. in length and the male about 40 mm. It occurs as a parasite of man in widely distributed localities throughout the Tropics and inhabits the lymphatic spaces. The young when born pass into the lymph and have a very characteristic appearance. They are about -3 mm. in length and each is enclosed in a loosely-fitting tubular membranous sheath closed at each end in which it lashes actively backwards and forwards. The sheath is simply the egg-shell which has entirely lost its rigid character and become quite soft. The young worms pass from the lymph into the blood in which there may be several millions present without producing any apparent ill-effects on the health of the host. When the blood of an infected person is examined microscopically in the ordinary way — by taking a drop from the skin — the worms are found to present an extraordinary periodicity. During the day they may be completely absent. Towards sundown they begin to make their appearance and during the evening their numbers undergo a rapid in- crease, until between 10 and 12 o'clock the blood is swarming with them. Then they gradually decrease again in numbers until by 7 or 8 A.M. they have almost entirely disappeared. This periodic appearance and disappearance of the worms was a great puzzle until Manson established the fact that during the hours of daylight they do not pass out of existence but merely frequent the deep vessels of the body, congregating in the great arteries and more especially in the lungs, in which organ they collect in the capillaries as well as in the larger vessels. The migration from these deeper vessels into the vessels of the skin turns out to be an adaptive arrangement, correlated with the fact that part of the life-history is passed in the body of night-flying mosquitos of various species. When blood containing the young worms is taken in by a mosquito, the worms continue their active movements within the sheath. The sheath is now restrained in its movements by the viscid contents of the mosquito's alimentary canal and after a while the worm, butting against now one and now the other end of the sheath, breaks its way out of it. It soon (within six to twelve hours) bores through the wall of the ali- mentary canal and takes up its position amongst the muscles of the. 206 ZOOLOGY FOR MEDICAL STUDENTS CHAP. •..mi becoming shorter and somewhat sausage-like. Here it runains tor some time, growing to a length of about 1-6 mm., and its way to tlit- proboscis. Although in many cases no ill effects upon the health of the host :>aivnt as a result ol" infection with Filaria bancrofti it is believed that this parasite is really the cause of those swellings of the lower parts of the body known in medicine as Elephantiasis arabum. This disease '•losrly in its geographical distribution with Filaria bancrofti: its diri-ct cause appears to be obstruction of the flow of lymph and it is believed that this obstruction is due to the presence of the filaria. As Is the precise method by which the obstruction is brought about it is helie\ ed by some to be due to inflammatory change in the lining of tin- hmph-spaces, brought on by the presence of the parasite, while by others it is attributed to actual blocking of the lymph stream either :ps «ii' adult parasites or by their eggs. In exceptional cases the lemale lays eggs ol" which the shells have not become soft and extensible as happens normally, but have on the contrary remained hard and rigid. Whereas the natural slender larva in its soft membranous sheath readily through the narrow chinks of the lymphatic glands such abnormal i-ggs retaining their ellipsoidal form are liable, on account of their much greater diameter, to stick in the lymphatic gland and thus obstruct the lymph-flow (Manson). FILARIA LOA Tin's parasite occurs in tropical West Africa (also Uganda), living •lily in the connective tissue under the skin, in which it creeps activi-ly about. Sometimes it attracts particular attention by traversing :it of the eye-ball. The adult female measures about 45-60 mm. •h. the male about 25-30 mm. The young are born in a sheath by the softened egg-shell, as in the case of F. bancrofti, and they •1 their way by lymph channels into the blood. They are about the young of F. bancrofti (about 250-300 fj, in length) but may be distinguished by their shorter sheath and by the less regular ' the body. They are also at once distinguishable by their for they migrate to the superficial vessels of the skin during the uliirh account they were formerly usually known under the while the young of F. bancrofti were known as F. noc- diffcrence in habit is due to the fact that the transmitting b in this case IH.I a nighl-ilying mosquito but a biting fly of the (Leiper) which is active by dav. NEMATODE PARASITES 207 FlLARIA PERSIANS This is a common para>ite in \\VMrrn Tmpi«-al .\lrna. tin- Congo, Uganda, Algeria, Tunis and in British (uiiana. inhabiting tin- con- nertive tissue, particularly in the mesentery and lining «.i the U)dy- ravity. The female measures 70-80 mm. in length, the malt- aUmt 45 mm. The younu reach the Mood as in the two prrrrdinu ^|)ecies but can at once be distinguished by then smaller size (200-230 /x) and by their being without a sheath. There is no periodicity in their appear- ance in the superficial blood-vessels, and this, as well as the lesser frequency of this parasite in natives who wear no clothes, suggests that possibly the transmitting insect in this case may be a louse or a flea (Christy). STRONGYLOIDES Female individuals of S. stercoralis — small worms slightly over 2 mm. in length — occur as parasites in the lining of the intestine of man in many of the warmer parts of the world. In some cases of intestinal trouble the parasites are apt to be particularly numerous though it is not believed that they are the actual cause of disease. The eggs, which apparently develop without being fertilized (parthenogenesis), give rise to minute larvae which make their way into the cavity of the intestine and after growing to about -75 mm. in length pass out to the exterior in faecal matter. If conditions are favourable (moisture and a tem- perature of 25°-35° C.) they develop into Rhabditis forms with separate sexes. The eggs from these develop into larvae which are at first Rhab- ditis-like but which gradually take on the form of the parasitic Strongy- loides. If now they reach the intestine of man directly through the mouth, or possibly also as in Ancylostoma by boring through the skin, they develop into adult females like those from which the life-eyi le started. The combination of features that we find in the typical Nematodes — the thick cuticle, the absence of colour, the relatively inefficient move- ments, the absence of sense-organs, the simple alimentary canal, the large numbers of eggs — make it clear that they constitute a group which has been evolved as parasites. A survey of the group shows however that many of its members have emancipated themselves to a less or greater extent from simple parasitism within a single type of host. I. Amongst the forms dealt with in this chapter Trichina remains ZOOLOGY FOR MEDICAL STUDENTS CHAP. V ristence completely parasitic, passing from one individual • In T simply by bodily transference in the encysted condition. II. In Ascaris, Trichocephalus, Oxyiiris, the eggs pass to the exterior .(1 fur a time a free existence but they complete their development •i taken in by a second individual host. III. In Aucylostoma not merely the egg stage, but the young worms For a time free-living, but a return to the host is necessary lor them to complete their development. In Dracunculus a similar condition is found, with the additional complication however that the •: uikes its way into a second or intermediate host, of a species ;t from the principal host : while in Filaria bancrofti, F. loa, and /•'. pcrstaus, the whole of the part of the life-history free from the prin- cipal host i> passed within the body of. the secondary host. IV. In Asniris uigrovenosa, a common parasite in the lung of the ordin.ii the young which pass to the exterior become sexually mature and one or more generations of free-living individuals occur the return to the host. A somewhat similar life-history occurs in the human parasite Strongyloides stercoralis. \ . In Angiiillula and its allies — small nematodes which occur com- monly in soil, in flour-paste, in vinegar (exhibited sometimes by showmen under the mi< -roscope as " eels " in paste or vinegar) — the parasitic phase come completely eliminated from the life-history. HOOKS FOR FURTHER STUDY Brumpt. Precis de Parasitologie. Manson. Tropical Diseases. Castellani and Chalmers. Manual of Tropical Medicine. CHAPTER VI ARTHROPOD A A. Terrestrial Arthropods breathing air by tracheal tubes. I. PROTARTHROPODA — Peripatus. II. MYRIAPODA — Centipedes, millipedes. III. INSECTA — Insects. 1. Aptera (Primitively wingless insects) — Machilis, Lepisma. 2. Orthoptera — Cockroaches, Earwigs, Locusts, Grasshoppers, Praying Insects (Mantis}, Stick- and Leaf-Insects (Phas- midae). 3. Hemiptera — Bugs, Green fly (Aphis). 4. Neuroptera — Dragon-flies, May-flies, Termites. 5. Coleoptera — Beetles. 6. Hymenoptera — Bees, Wasps, Ants, Ichneumon-flies. 7. Lepidoptera — Butterflies and Moths. 8. Diptera — Flies. Aphaniptera — Fleas ; Anoplura — Lice ; Mallophaga — " Biting Lice." B. Mainly terrestrial Arthropods possessing book-like breathing organs. IV. ARACHNIDA — King-Crab (Limulus), Scorpions, Spiders, Mites, Ticks. C. Aquatic Arthropods breathing by gills. V. CRUSTACEA — Lobsters, Prawns, Shrimps, Crabs, Sand-hoppers, Slaters (Wood Lice). The phylum Arthropoda includes, as will be gathered from an inspec- tion of the foregoing table, an immense variety of creatures — the known species greatly exceeding in number those of all the other phyla put together. The beauty and variety of their colouring and form, and the 209 p 210 • LOGY FOR MEDICAL STUDENTS CHAP. with which they can be preserved, make them special favourites of collectors, while many members of the group are of directly practical importance to mankind by providing food material, or by destroying manufactured articles, or by causing bodily injury by bites or by acting as carriers of disease-producing microbes. In this . tin- barest outline of the characters of the phylum will be attcm] Tin- ueneral plan of structure of the Arthropod is a further develop- ment of that seen in the Annelid. Here again the body is metamerically 1 and the individual segment carries a pair of appendages : but appendages are longer and more slender and in general much more hjulilv evolved than the stump-like parapodia of the Annelid. Here •i. in correlation with the mode of movement, the front end of the oecialized to form a head : but the head has reached a far .er degree of complexity than that of the Annelid. The central nervous system with its ventral chain of ganglia and its supra- oesophageal ganglionic mass is clearly of the same type as that of the Annelid. But the Arthropoda have diverged from the annelidan type of structure 18 to develop peculiarities of their own. Two of these are of funda- mental importance. (i) The cuticle, which in the Annelid is thin and membranous except where it undergoes local thickening to form a chaeta, has in the Arthropod become greatly exaggerated to form an armour coating, composed of the nitrogen-containing substance chitin, covering the entire surface of the body and forming an admirable protection against the attacks of other .nisms including disease-producing microbes. We may probably take it that the development of this protective coat has been one of the : ors — if not the chief factor — in enabling the Arthropods so suc- uilly t<> hold their own in the struggle for existence. As will become apparent in the course of this chapter, the development of the rigid ton has also brought in its train important secondary results wliirli find their expression in peculiarities of structure, function, or life- (2) The other fundamental feature of arthropodan organization which • alls tor mention ;it this point is that the coelome has shrunk up to . its place as body-cavity being taken by a <>rk lilled with blood and continuous with the cavity of the heart. This spongework represents the network of blood-vessels, • ir definite tubular form and become widened out into indefinite irregular spaces. Sudi a type of body-cavity — formed of VI ARTHROPODA 211 degenerate blood-vessels — is spoken of as a haemocoelic hody-aivity 1 in contradistinction to the coelomic body-cavity such as is present in Annelids. There remains to be mentioned a third — less fundamental but still very characteristic — feature of arthropodan structure, namely, that one or more pairs of the appendages in the neighbourhood of the mouth arc modified as jaws or other organs connected with the act of feeding. This peculiarity, which no doubt found its commencement in evolution in the fact that these appendages, like other parts of the body, are cnsheathed in hard exoskeleton well adapted for crushing the food, is so characteristic that a name expressing it — e.g. GNATHOPODA — would really be preferable to the more commonly used name for the phylum — Arthropoda. As will have been gathered, the Arthropods appear to be descended FIG. 95. Peripatus (from Sedgwick : Cambridge Natural History). from annelid-like ancestors : the primitive form of the body is in con- sequence elongated and worm-like, as may still be seen in Peripatus (Fig. 95), or in the Myriapoda, or in the larvae of various insects (Fig. 97, A). In the more highly developed types of arthropod, on the other hand, the body has become more compact and has also become differentiated into distinct regions. Thus in an insect one may recognize distinct head, thorax and abdomen, marked off from one another by more or less distinct constrictions, while in a typical crustacean, such as a Lobster or Crayfish (Fig. 102), one may similarly distinguish cephalo- thorax and abdomen. In the Arachnids there may be three main body- regions distinguishable — the prosoma, mesosoma and metasoma— as in the Scorpions (Fig. 99), or only two — prosoma and opisthosoma— as in the case of Limulus (Fig. 98) or of the spiders. In the Arachnida and 1 From haemocoele — a term sometimes applied to the whole system of spaces in the animal body containing blood, i.e. the cavities of the blood- vessels. /.OOLOGY FOR MEDICAL STUDENTS CHAP. the Cr lie extreme hinder end of the body may be marked off i nun : > a telson (Figs. 98, 99, 102, t) : in the Scorpion this and encloses a poison gland which opens close to its tip — tin- whole constituting a hypodermic syringe for the injection of venom. Some of the most characteristic features of the Arthropods are .-i-d with the great development of the cuticle over the surface of the body. This has, as already mentioned, become greatly thickened while i me hardened and stiffened by its conversion into chitin,1 which in the case of the Crustacea is rendered still harder and more rigid \ becoming infiltrated by calcium carbonate. This development of the cuticle would naturally tend to interfere with two of the most important vital activities of the animal— (I) move- ment and (II) growth, and some of the most striking secondary charac- teristics of the group have their functional significance in the eliminating or at least counteracting this interference. I. Tin1 increase in hardness and thickness of the cuticle is not con- FIG. 96. Section through joint of an arthropod's exoskeleton. tinuous over the whole surface. Along certain lines it remains thin and flt-xiMr. and these portions of cuticle are folded in below the general level (Ki-. 96). The result of this arrangement is that the rigid armour is divided hy joints at which the edges of the rigid portions can approach "in one another, and in this way flexibility is given to the \\hole and movement rendered possible. The degree to which this jointing «»t the exoskeleton is carried out is directly related to (i) the thi< knc- and hardness of the exoskeleton and (2) the need for flexibility particular j.art of the body. Thus it is specially marked in the tl««- limlis, and the jointed character of the limbs is regarded as 1 • haracteristic of the phylum that it has been made use of hnirul name Arthropoda. II . I hiriiur Hie earlier part of an animal's life, as it progresses towards •n.lition, it as a rule undergoes (i) a gradual increase in size Wfcal change of form. In the typical Arthropod both of interfered with 1,\ the presence of the rigid exoskeleton. M in 01 her groups, the amount of living substance in the lual increase during the earlier stages of the life- 1 See below, p. 216. vi ARTHROPODA 213 history, there is under normal conditions no p<»sil>ility of increase ID the volume of the body, ensheathed as it is in incxtrnsiNc armour. And change of form is similarly impossible. These difficulties are met by the developing Arthropod undergoing periodical moults or ecdyses, at which it sheds its complete suit of cuticle. A new cuticle is developed under- neath the old and this remains for a brief period soft and extensible, so that before it becomes hard and rigid the animal is able to undergo a rapid increase in bulk. The Arthropod therefore grows in a succession of spurts, one at each ecdysis, instead of by a continuous process. So also with the change of form. At each ecdysis a slight change takes place and the sum of these comparatively slight changes makes up the, frequently great, total change in form from the young up to the adult condition. METAMORPHOSIS In some of the most highly evolved Arthropods — e.g. those included in certain orders of Insects — there has come about an interesting further development, in that the two processes — increase in size and change in form — instead of keeping step with one another have become concen- trated in different parts of the life-history, practically the whole of the change in form taking place at the last two ecdyses, while the increase in size takes place in the preceding part of the life-history. In such cases the change in form at the last two moults is so striking in amount that it is given the special name metamorphosis. An excellent example of metamorphosis is afforded by any ordinary Butterfly or Moth (Fig. 97). Here the earlier parts of the life-history are passed as a Caterpillar larva (Fig. 97. A), of elongated worm-like form, which feeds voraciously and grows actively 1 without showing any conspicuous change in form. On the completion of the penultimate moult, however, the creature is found to have undergone an extra- ordinary change in form, having become a resting pupa or chrysalis (Fig. 97, B). In this stage the creature is somewhat spindle-shaped : there are no legs or other appendages projecting beyond the general surface, although careful inspection reveals appendages plastered down to the body and absolutely immovable and useless : there is no mouth or anus. There is, in fact, no obvious sign of life except it may be an occasional slight bending of the body. After a more or less prolonged pupal period the last ecdysis takes place, and again an extraordinary change in form is to be seen — the creature becoming now an imago or 1 The skin has reverted to the soft extensible condition like that of a worm, so that the growth of the caterpillar is no longer restricted to sudden spurts. ZOOLOGY I'XW MEDICAL STUDENTS CHAP. Fig. 97, C), fitted for active movement and provided with and other appendages, /.ion of the details of internal structure of the insect shows fe FIG. 97. .!...#• nuttrrfly (I'ifris). (From Grah.iin Krrr's I 'rimer of Zoology.) A, Larva •'< cir adult. A, antenna ; abd, abdomen ; c, radiate eye ; : aa; 1 h, tlmmx ; W, wing. that ii undergo quite as profound an alteration during as does the external form. It would appear to he a common feature of all animals except the t that there is constantly taking place during the period of active vi METAMORPHOSIS 215 life a certain amount of cell-replacement. Individual cells, or groups of cells, worn out by their activities become moribund, die, and are replaced by substitutes — cells which lag behind in their development and retain their juvenile characteristics until their services are needed, when they rapidly complete their development and take the place of their worn-out predecessors. A conspicuous example of this process has already been alluded to on p. 135— that of the yellow cells of Lumbricus, but it is probable that its occurrence, although in less conspicuous form, is a common characteristic of living tissues. In the process of metamorphosis we have to do not merely with the concentration of change of bulk and change of form in a particular stage of the life-history but also with a similar concentration of this process of cell-replacement. The reinforcing cells are either scattered or in certain parts of the body form definite, easily recognizable patches, to which the old-fashioned name " imaginal discs " is still commonly given. Apart from the blood, the nervous, and the reproductive systems, the process of cell-replacement is to a great extent held back until the time of metamorphosis, when there sets in a process of wholesale dis- integration of the tissues (histolysis) — their cells becoming moribund, dying, and disintegrating, and their remains being devoured by amoebo- cytes. At the same period the replacement cells burst into activity, multiply rapidly and undergo tissue specialization, until by the end of the pupal period they have provided a complete new outfit of tissues and organs, replacing those which have disintegrated and, it may be, differing greatly from them, in correlation with the changed functions which they will have to perform in the new life of the perfect insect. It has been established in the case of some insects that the epidermis behaves in a different fashion at metamorphosis. In its case the cells do not die and become replaced by others : the cells of the larva appear to persist in the adult. This is apparently rendered possible by their possessing the power of recovering their juvenile activity by the extrusion from their inner ends of a quantity of chromatin which is at once ingested by amoebocytes. This process of rejuvenation of cells by the elimination of, presumably effete, chromatin is not at all understood : it appears to be of not infrequent occurrence in the animal kingdom, and it is probably the expression of a phenomenon of very deep biological significance. Apart from the general effects which have been so far alluded to, the chitinous exoskeleton of the arthropod has definite functions of its own. (i) It forms a magnificent protective envelope to the soft living protoplasm of the body, guarding it from mechanical violence, from 2I6 /noi.oCY FOR MEDICAL STUDENTS CHAP. on, from the effects of harmful substances, and from the attacks ofot: -rns. (2) ! is a skeleton in the ordinary sense, supporting the soft tissues and giving firm attachments for the muscles by which movements •lied (Hit. (3) Formed as it is of chitin — a compound of carbon, hydrogen, and nitrogen — it constitutes so much waste, or excretory, nitrogenous material which, instead of being got rid of as soon as formed, is deposited in the cuticle and cast off periodically at the ecdyses. It thu> plays an important part in the function of nitrogenous excretion. While the hard jointed exoskeleton is highly characteristic of the typical arthropod, yet under certain conditions it is liable to revert to a thin flexible condition. Thus in Hermit Crabs this has happened on the hinder, abdominal portion of the body, which is kept tucked snugly in the shelter of a Gasteropod (p. 267) shell. Again, in arthropods which have assumed parasitic habits the same may happen over the whole surface of the body. APPENDAGES The limbs of the Arthropoda are primitively numerous, arranged in pairs, one to each segment of the body, and alike. But it has been a characteristic feature in the evolution of the phylum that the appendages mis the hinder end of the body have tended to become reduced and indeed to disappear entirely. Thus in the prawns and shrimps the men pussrsscs small but comparatively well developed appendages which are u-ed tor swimming : in the lobsters and crayfishes (Fig. 102, B) these have become further reduced in size : in male crabs — in which the '•iien i- carried bent forwards beneath the cephalo-thorax — they have redurrd ID the verge of disappearance, except the front two pairs, which have been preserved owing to their performing a sexual function. In the teinale crab the whole series has been preserved from reduction, m 1( bo owm- to their having a sexual function— for the eggs are carried about cemented on to the bristles which project from these '" ' ••<• division the same tendency is seen. In Peripalus «>5). or in a Myriapod. the series of appendages extends without to the hinder end. In the most primitive insects — as the " Silver fish " (Lepisnm), sometimes imported with sugar boxes, or the little Machilis- which may often be seen running about vi ARTHROPODA 217 actively on rocks and walls near the sea— the atxlominnl appendages are reduced to small simple vestiges. In tin embryo of the higher insects the same vestige.s may be seen, but in the adult they have disappeared completely so that the abdomen is limbless. In the Arachnida we find the same tendency, although some of the abdominal appendages may persist in the Spiders in the form of little finger-like organs for the manipulation of the threads of the web, or, as will l)e shown later, in the form of special breathing organs. The persisting appendages show in each of the three main subdivisions of the Arthropoda modifications in relation to differences in function which afford extraordinarily interesting studies in morphology.1 We will sketch these modifications in outline, keeping the three subdivisions separate and so avoiding the comparison — which the present writer believes to be fallacious — of the individual appendages, and still more of parts of appendages, in one group with those in another. While avoiding such comparisons in detail, it is important to bear in mind that all three groups present the common feature already alluded to, that the jaws used for masticating the food are modified appendages. APPENDAGES OF THE ARACHNIDA Within this group the series of appendages is seen in its least modified form in Limulus (Fig. 98, B). The first pair of appendages, the chelicerae (I), are small nippers placed just in front of the mouth. The nipper at the end of the appendage is formed by the penultimate segment being prolonged into a kind of prong alongside the terminal segment. The terminal segment can be pressed strongly against this prong so as to take a tight grip of any object between them. A nipper constructed in this fashion is known technically as a chela, and a limb possessing it is described as chelate. The next five limbs on each side (Fig. 98, B, II-VI) are large walking legs. In the female these are all chelate except the last : in the male, however, II is without a chela and ends in a stout swollen claw. VI is provided near its tip, in both sexes, with a number of flat plates which the king-crab uses in burrowing into the sand. . These appendages (II-VI) are arranged round the sides of the mouth. 1 The word morphology — invented (1807) by Goethe, poet and naturalist — is used to designate the philosophical side of anatomy. Anatomy deals with the " unmitigated facts " of structure : it becomes morphology as soon as the attempt is made to correlate these facts of structure with underlying prin- ciples— such as, above all, evolutionary history. 218 ZOOLOGY R)K MEDICAL STUDENTS CHAP. The basal segment of each is enlarged to form a stout blade with stiff s projecting from it. This blade-like development is gnathobase. and the development of a gnathobase is one of c.e. • ! view, r.t, ('(Mitral eye ; l.c, Literal eye ; o, opistho- the most cl. ic of the modifications of the arthropod limb for purposes of mastication. The seventh appendages (Fig. 98, B, VII) are a pair of stout rod-like orur;ms railed chilaria, just behind the mouth. The eighth appendages (VIII) are flattened plates, continuous with one VI APPENDAGES OK I.IMULUS another across the mesial plane and forming the genital operculum, the genital openings being situated on their posterior face. Appendages IX-XIII are also plate-like structures very much like the FIG. 98. A King- crab (Limulus), female. B, ventral view. I, Chelicera ; II-VI, walking N^s ; VII, chilarium ; VIII, genital operculum ; IX-XIII, respiratory appendages; a, anus ; t, telson.. culum, differing from it, however, in that each carries on its posterior surface an arrangement of thin flat plates, lying over one another like the leaves of a book (Fig. 101, A). These gill-books are the breathing organs of the Limulus, each leaf containing blood and possessing a very ZOOLOGY R)K MKDICAL STl'DKXTS CHAP. iiiiinous wall which allows ready respiratory exchange of gas the Mood within and tin- sea-water without. PI INS arc Arachnids which have forsaken the sea and taken nee, and their appendages, while agreeing exactly met. 99. • • ; /.,-. I.it.-r..l eyes; mes, niesnsoma ; met, meta- D. I. ( h.'hci-r.i ; 11, petlipalp; III-VI, walking legs. A ill) those ol I.juiulus, show interesting modifications in The first appendages (I), as in Limulus, are small • 1 1 -VI) are walking legs. These are more der t! '' I.i»inlns and, excepting II (pedipalp), 'which is stout and chelate, posse- a ^harply clawed foot. The pedipalps are of special VI APPENDAGES OF SCORPIO interest as showing the very first beginning of the modification oi an appendage for purposes of mastication — the basal segments (Fig. 100. 11* being especially stout and strong, being attached to the body in close proximity to one another, and being movable in such a way as to be squeezed together and crush any food substance between tlu-in. The FIG. 99. A Scorpion. B, ventral view. I, Chelicera ; II, pedipalp ; III-VI, walking legs ; (VII, missing in adult); VIII, genital operculura ; IX, pecten ; X-XIII, openings of lungs, t, Telson. succeeding two pairs of appendages (III and IV) are also adapted for mastication, but here the specialization has gone a step further, the basal segment of each projecting forwards as a stout blade, with a cutting edge on its mesial side, which can be brought against its neighbour like the blade of a pair of bone forceps. The seventh appendages, corresponding to the chilaria of Limulus, are completely absent in the adult, but it is ZOOLOGY FOR MEDICAL STUDENTS i n CHAP. FIG. 100. Scorpion — enlarged view of the bases of appendages II-IV. The reference line points in II, III .•ginent modified for crushing and cutting the food. FIG. 101. •lu.MiKli l.i-r.itliinn oi^.m of (A) I.imulus ; (B) Scorpion. af>r> Appendage ; r.l, respiratory lamellae ; s, stigma. vi ARTHROPODAN APPENDAGES 223 interesting to note that in the young embryo they duly make their appear- ance as small rudiments although they fail to proceed with their develop- ment. The eighth appendages form a genital operculum (Fig. 99, B, VIII) as in Limulus, only greatly reduced in size. The ninth appendages are curious comb-like organs (pectines — Fig. 99, B, IX) the function of which is unknown. Appendages X-XIII appear to be completely absent in the adult, and in about the position where each of them should be there is present a narrow somewhat obliquely-placed slit (stigma — Fig. 99, B, X-XIII) which leads into one of the breathing organs or lungs. Each lung is a small chamber into the cavity of which there project backwards from its anterior wall an arrangement of thin leaves (lung-book) like those of the gill-book of Limulus (Fig. 101, B). A fascinating light is thrown on the evolutionary history of these lungs by the study of their mode of development in the young Scorpion embryo, for it is found that in place of each lung there exists for a time a definite limb rudiment, which develops projecting plates on its posterior surface, agreeing exactly with the rudiments of the gill-book in the young Limulus. In the Scorpion however as development goes on the limb rudiment with its gill-book ceases to project and becomes flush with the neighbouring surface, while the leaves project into a depression of the surface, which gradually deepens to form the lung cavity. APPENDAGES OF CRUSTACEA To illustrate the series of appendages of the Crustacea we may take those of the Fresh-water Crayfish (Astacus).1 The appendage is seen in its least modified form about the middle of the abdomen, say the third or fourth of the six abdominal segments (Fig. 102, B). Shaped like an inverted Y, it consists of a basal portion the protopodite, which bears at its end two diverging branches, an outer, the exopodite, and an inner, the endopodite, each tapering to a point and divided up into numerous segments. At each end of the abdomen the appendages are modified : at the hinder end the last of the series, while showing the same three parts as the typical appendage, has these parts broadened out into flat plates which when spread out in the same plane as the flat plate-like telson (Fig. 102, A, t) form with it an expanded kind of fin, the possession of which enables the animal to shoot rapidly backwards in the water by violently bending its abdomen in a ventral direction. At the front end 1 Failing specimens of this animal the common Lobster (Homarus) or the Norway Lobster (Nephrops) may be studied : they agree with Astacus in all their main features. ZOOT.OCV FOR MEDICAL STUDENTS CHAP. of the abdomen the first two pairs of abdominal appendages (Fig. 102, B, i-inl.in'-s ; An, ami-,; /•,, <-yi: ; /...r,, walking legs; »i/>.3, third walking legs — five pairs. In the- young Norway Lobster, which is a ,ip[)( nda-cs show the same three component •inininal appendage, but as development goes on the the walking le- o! the adult consisting entirely of APPENDAGES OF CRAYFISH 225 protopodite (2 segments) and endopodite (5 segments). The larger basal segment of the protopodite carries attached to its outer surface (i) a flattened plate, the epipodite (Fig. 103, £4, ep}, prolonged into numerous slender processes which constitute a breathing organ or gill, and (2) a tuft of fine chitinous threads (/). The three front pairs of walking legs differ from that which has been described in the fact that they are chelate, and in the case of the front pair (chelipeds) the chela is much enlarged (Fig. 102, A, Lj). The series of appendages in front of the chelipeds and extending to the level of the mouth is constituted by six pairs —the third maxillipeds (Fig. 103, Mp.$), the second maxillipeds (Mp.2), the first maxillipeds (Mp.i), the second maxillae (Mx.2), the maxillulae or first maxillae (Afac.i), and the mandibles (M)— charac- terized by loss of the primitive locomotor function of the limb and specialization in connexion with the act of feeding, specialization which reaches its height in the appendages nearest to the mouth. Thus the endopodite and exopodite are reduced in varying degrees as will be seen in Fig. 103. In the case of the first maxilliped and the two pairs of maxillae, flattened plate-like gnathobases have grown out from the inner side of the protopodite, and in the mandible the limb has come to FIG. 103. Appendages of Crayfish as seen from the ventral side. A i, First antenna ; A 2, second antenna ; en, endo- podite ; ep, epipodite ; ex, exopodite ; £4, fourth walking leg ; M, mandible ; Mp.i, first maxilliped ; Mp.2, second maxilliped ; Mp.3, third maxilliped ; Afar. i, first maxilla ; Af*.2, second maxilla ; n, external opening of nephridium ; p, protopodite ; t, chitinous threads. en. IX. en. Mx.\ en. en. Mx.2 ep, ex. en. ep. Mp.2 Mp.3 ZOOLOGY FOR MEDICAL STUDENTS CHAP. consist almost entirely of a massive gnathobase, the rest of the limb being represented by a small 3-jointed endopodite (Fig. 103, M, en) which acts as a palp or sensory organ. In the appendages anterior to the second maxilliped the gills have disappeared, but the plate-like epipodite is still present on the first maxilliped and the second maxilla. In tin- latter the epipodite is continuous with the exopodite which is also plate-like in character, the two together constituting what is known te scaphognathite (Fig. 103, Mx.z, ep. + ex). The scaphognathite performs an important function in respiration. It lies in the front portion of a chamber into which project the gills, covered in and protected by a flap-like downgrowth of the dorsal body- wall the branchiostegite or gill-cover. During life the scaphognathite performs rhythmic movements of such a kind as to draw a current forwards through the chamber so that the water bathing the respiratory surface of the -ill undergoes constant renewal. In front of the mouth lie two pairs of antennae. These are prolonged into one (second antenna) or two (first antenna or antennule) long, taper- many-jointed filaments which are crowded with sensory cells and dearly function as organs of sense. They naturally recall to memory the sensory tentacles borne by the prestomium of the annelid, but the study of their development shows that they are quite different in their nature. The antennae of arthropods are true appendages which have become shifted forwards, so as to lie in front of the mouth, and taken on a purely sensory function. APPENDAGES OF INSECTA As has already been remarked, the insects are normally without :idagcs on the abdomen. In the thoracic region there are present pairs of highly developed walking legs each terminating in a foot or tarsus consisting of a number (5 or less) of small segments, the terminal one carrying a pair of claws. Between the claws there may be present a cushion-like pad, the surface of which produces a sticky secretion enabling the insect to creep on smooth vertical or inverted es in the neighbourhood of the mouth are modified in i with the act of feeding and — in correlation with the great • 1 1 1 terent types of insects in the nature of their food aanneroi feeding — great differences exist between the mouth 'lilfcrent insects. Thus in the Bees the mouth-appendages are adapted for collecting pollen and nectar from flowers ; in the Mosquitos VI ARTHROPODAN APPENDAGES 227 they form a miniature case of surgical instruments for piercing the skin and obtaining a supply of blood ; in the Butterflies and Moths they form a long tubular siphon which can be lowered into the depths of deep flower corollas to suck up the nectar from their inmost recesses. And yet a careful comparison of these various types of feeding equipment show them to be composed of the same elements, the same series of appendages, modified to varying extents in size and form so as efficiently to perform their particular type of function. A relatively unspecialized set of insect mouth appendages is to be found in the group Orthoptera, of which the Cockroach or " Black mx2 mx\ FIG. 104. Mouth appendages of three different types of Insect. A, Cockroach (Periplaneta) ; B, Bumble- Bee (Bombus) ; C, female Mosquito (Culex). c, Cardo ; g, glossae or ligula ; ga, galea ; h, hypo- pharynx ; /, labrum ; Ic, lacinia ; l.p, labial palp ; m, mandible ; m.p, maxillary palp ; me, mentum ; mx.i, first maxilla ; mx.2, second maxillae (labium) ; p.p, paraglossa ; s.me, submentum ; st, stipes.*" Beetle " is an example (Fig. 104, A). There are here three pairs of such appendages — the mandibles (m), the first maxillae (mx.i) and the second maxillae (mx.2). Of these the mandible is simply a gnathobase, ensheathed in very dense chitin with a sharp serrated inner edge : there is no palp on the mandible in insects. The first maxilla is much more complicated : its basal joint (cardo — c} carries an elongated piece (stipes — st) and this in turn bears at its end two lobes, an inner, claw-like and with stiff bristles— the lacinia (Ic.), and an outer, soft and cushion-like — the galea (go). This appendage is provided on its outer side with a jointed sensory maxillary palp (m.p). The second maxillae can be seen to be composed of the same set of elements as the first 228 ZOOLOGY FOR MEDICAL STUDENTS CHAP. but here the two appendages have undergone fusion across the mesial plane to form the labium. The two cardines are completely fused rm tin- submentum (s.me). The two stipites have fused except just at their tips to form the mentum (me) and each of them carries its inner and outer lobe— here, called respectively glossa (g) and paraglossa (pg). ( )n each side of the labium is a small labial palp (l.p). In addition to the foregoing portions, which can be readily correlated with corresponding portions of the first maxillae, there is present a conspicuous soft tongue- like lingua (sometimes called hypopharynx) which springs from the base of the labium on its anterior side, just behind the slit-like mouth opening. Finally there is present the labrum (I)— a plate-like flap of exoskeleton hanizin^ down in front of the mouth and differing in nature from the mouth parts hitherto described in that it has nothing to do with the true appendages. An examination of the mouth parts of one of the bees, such as a common bumble-bee (Bombus — Fig. 104, B) shows that here again is the same set of mouth parts. The chief differences are in the maxillae. In the second maxillae or labium the glossae are fused together to form a long tongue — the ligula (g) — used for licking up the pollen and traversed along its hinder surface by a deep groove, nearly converted into a tube by its edges meeting, up which nectar is sucked. The paraglossae (p.g) are reduced but the labial palps (Lp) have their two basal joints much elongated and hollowed out along their median surface into a deep \Vhen the two palps are approximated together the soft and delicate ligula lies safely protected in these grooves. In the first maxilla t he ladnia (lc] — shaped like the blade of a scythe — is the most conspicuous part, while a small maxillary palp (m.p) is also recognizable. In the Mosquitos the females — which alone suck blood — possess the array of mouth appendages shown in Fig. 104, C. The labium (mx.i) is elongated and is given a trough-like form by a wide and deep groove which traverses its anterior surface and serves to contain and protect delicate piercing stylets. Of these four represent the two mandibles (tri) and the two first maxillae (mx.i).1 The remaining two are the unpaired labrum (/) and hypopharynx (h). Of these the former is pro- • 1 with a deep, nearly closed-in, groove along its hinder surface so it forms practically a tube up which the blood is sucked. When not in use these piercing organs lie within the groove of the labium. When the mosquito bites the end of the labium is pressed against the skin and it becomes bent upon itself, while the stiff stylets, 1 A in, ixillary palp is present (m.p} which in the females of some genera, •ircil, is imu'h slHirtrr than in others. vi APPENDAGES OF INSECTS 229 remaining straight, pass onwards into the skin guided between two finger-like lobes (labella) at the end of the labium. In the male mosquito the mandibles have disappeared and the hypopharynx has become fused with the labium. In the House-fly (Mused) and its allies, which belong to the same order (Diptera) as the mosquitos, the mouth-parts are adapted entirely for sucking. Mandibles and first maxillae (except the palp) have entirely disappeared, while the labium forms a large conspicuous proboscis, the surface of its broadened-out end (labella) traversed by a system of deep channels by which fluid is drawn in to the mouth when the proboscis is moved, after the manner of a vacuum cleaner, over a surface on which soluble food matter is present. In the Butterfly or Moth the mouth-appendages are practically reduced to the labial palps — conspicuous hairy-looking organs (Fig. 97, C, Lp) — and the first maxillae (mx.T). The latter are greatly elon- gated and are deeply grooved along their mesial surface, so that when fitted together they form a long tubular siphon. When at rest this is coiled up in a spiral under the insect's head, as shown in Fig. 97, C, but it can be uncoiled and lowered into the recesses of a deep tubular flower corolla. In the Insects there is only a single pair of antennae. WINGS In the Insects, and in them alone among the Arthropoda, there occur normally in the adult stage wings. Of these there are usually two pairs, attached to the hinder two of the three segments which make up the thorax. These wings have nothing to do with the series of true appendages : they are flaps of skin rendered stiff by the two layers of stiff cuticle on their upper and lower surfaces, except at their base where the cuticle remains flexible so as to admit of free up and down movements. The two layers of the wing are in close contact except along a branched system of nervures along which blood-spaces and tracheal tubes (see below, p. 232) are interposed between the two layers. The arrangement of the nervures of the wing is very constant within the limits of particular groups, e.g. families or genera, while on the other hand characteristic differences occur between different groups. Consequently the neuration of the wings is made much use of by entomologists in classifying insects. The wings show characteristic differences in their general appearance 230 ZOOLOGY FOR MEDICAL STUDENTS CHAP. in the various orders or main subdivisions of the insects. In the Dragon- Mav-tlirs. Termites and other insects commonly grouped together as ra tlu- two pairs of wings are membranous and much alike. In IKmenoptera (Bees, Wasps, Ants) both pairs of wings are again membranous, but the hinder wings are smaller and are attached to the - by numerous little hooks which project from the front edge of the hind-win- and hook over the hind edge of the fore-wing. Fore- and hind-wing on each side thus act as if they were a single continuous structure. In the Diptera (ordinary flies, Mosquitos, Midges) the fore- win-s are membranous while the hind-wings are converted into curious dub-shaped organs of unknown function called halteres. In the Lepidoptera (Butterflies and Moths) the numerous little bristles scattered over the surface of the wing have taken the form of overlapping which the colours of the wings are due. In the Orthoptera !v roach, Locust, Grasshopper, Earwig) the front wings have lost their function in connexion with flight : they have become thick and horny and serve as wing-covers to protect the delicate membranous hind-wings when these are not in use. In the Hemiptera the two pairs of win^s may be thin and membranous (Aphis — Green fly) but more usually the fore-wings are thickened and protective as in the Orthoptera, only in this case the thickening of the wing is often more conspicuous in its basal portion (many Bugs). The protective modification of the lorc-win-s finds its greatest development in the Coleoptera or Beetles where' they are greatly thickened and are known as elytra. \Vin-s are thoroughly characteristic of the group of Insects. Among those which are without them we have to distinguish between those in which the loss of wings is probably to be regarded as a secondary develop- ment— such as insects inhabiting small islands in the ocean and insects that have adopted a parasitic mode of life — and those which probably represent a primitive wingless stage of insect evolution (Aptera : such as >nn and Machilis). The wings of insects arise in the embryo as flat flaps of skin which grow out from the surface but much more frequently arise in the "I deep pockets of the skin, the " imaginal disc " corresponding ••• h win- having sunk deep below the surface. As regards their ••ury origin we are still pretty much in the dark, but there is •»n to suspect that the extensions of the body-surface which function as wings were primitively respiratory in function. In the of certain insects (May-flies) there are present flat plate- like gills on the abdominal segments which present a striking resemblance ire to wings and there is a tendency to revert to the view, vi ARTHROPODA 231 which was discredited for a time, that wings may have evolved out of such " tracheal gills " on the thoracic segments which have become more and more freely movable so as to function as organs of propulsion. The alimentary canal of the Arthropoda is characterized by the tendency of the cuticle of the outer surface to be prolonged inwards as a protective lining. This is brought about by an ingrowth of ectoderm at the mouth end forming a stomodaeum and a similar ingrowth at the anal end forming a proctodaeum. It is a quite usual feature in animals for the tubular alimentary canal to have such an ectodermal section at each end. But in the Arthropoda these tend to be more highly developed than in other groups — stomodaeum and proctodaeum encroaching more and more, while the original endoderm-lined alimentary canal or mesen- teron undergoes a corresponding shrinkage. This shrinkage is well exemplified by the case of the Crayfish or Lobster, where careless dis- section commonly results in the alimentary canal breaking through the short mesenteric portion owing to this being devoid of the tough cuticular lining elsewhere present. In the Insects there are usually " salivary " glands opening in the neighbourhood of the mouth. No doubt these originally fulfilled a function in connexion with the preparation of the food for digestion, but in special cases they have become specialized for other functions. Thus in blood-sucking insects the irritating secretion helps to promote the flow of blood to the wound. In the larvae of various insects the secretion is silk. The haemocoelic character of the body-cavity has already been alluded to as a highly characteristic feature of the Arthropoda. It may be that the degeneration of the blood-vessels to form such a body-cavity arose in evolution in relation to a peculiar type of respiratory system — that found in the insects. In these arthropods the process of respiratory exchange with the external medium takes place through parts of the cuticle which have not undergone the thickening occurring elsewhere but have remained extremely thin. Wherever in the animal kingdom there are such portions of the surface specially devoted to respiratory exchange it is usual for the progress of evolution to bring about increase in the area of these respiratory surfaces. Most usually this increase is brought about by the surface sprouting out into more or less elaborately branched projections —the gills. In the insects the increase on the other hand has been brought about by growth of the surface not outwards but inwards. Each respiratory patch has come to dip down as a deep pocket into the interior of the body, and this pocket has had its lining enormously ZOOLOGY FOR MEDICAL STUDENTS CHAP. <] by its developing into a complicated tree-like arrangement of Ing tubes. These tubes or tracheae branch all through the body, their very fine terminal twigs ending blindly in immediate relation with the living protoplasm of the various tissues. Except in the case of the fine ultimate twigs the tracheae are kept open by a kind of spiral spring formed by a spirally running thickening of their cuticular lining. Mil performing its breathing movements the insect compresses the contents of its body by muscular action. This compresses the tracheae, the air in their interior out through the external openings or ui situated in a row along the sides of the body (Fig. 97, A, st). When the muscles relax, the spiral thickening of the tracheal lining causes the tracheae to resume their former volume and they are filled by an inrush of fresh air through the stigmata. <*/ Whereas, therefore, in the Annelid worm the necessary supplies of oxygen are conveyed to, and the excreted carbon dioxide conveyed away from, the living tissues by the circulating blood, in the insect on the other hand the conveyance takes place in gaseous form in the tracheal tubes. In other words the tracheal tubes perform what is in the annelid one of the main functions of the blood-vessels, more especially of the capillary blood-vessels, and the suspicion is aroused that here may be the factor which has inaugurated the degeneration of the arthropodan capillaries and finer vessels. This idea goes against the commonly accepted belief that the arthropods were primitively aquatic animals, but it is supported by the fact that Peripatm — by far the most nearly primitive of the existing arthropods — is terrestrial and breathes by means of tracheae. Tlif spongy packing tissue between the spaces of the haemocoele is in the insects charged with droplets of fat which give it a snowy-white colour. This tissue is known as the " fatty body " of the insect. What its special function is, whether it fulfils a purely physical one in protecting the tissues from too great cooling by the constant indraught of air through the tracheae or on the other hand performs some more com- '1 role in metabolism, is not yet certainly determined. In certain insects, such as the beetles known as fire-flies, particular of fatty-body are specialized as light-producing or photogenic organs. The light is apparently produced by the oxidation ! substance formed in the metabolism of the organ. And lation with this the photogenic organs possess a very rich supply of tracheal tubes. The pale-greenish light most commonly produced is remarkable for its economical character, the proportion of the energy non-luminous rays being extraordinarily small as compared vi ARTHROPODA 233 with light produced by artificial means. The emission of the light appears to be under the control of the nervous system, there being probably some special mechanism for controlling the access of air to the light-producing cells. Physiologically the significance of these organs is connected with sexual attraction. In many cases the light is not a continuous glow but is emitted in long or short flashes : as is well known such an intermittent light is more efficient in attracting attention than is a steady glow. Further, the female sex is in some cases distinctly recognizable by the longer duration of the flash. The sexual attraction of this naturally-produced light is merely a special case of the well-known " attraction towards light " shown by many insects. And this in turn rests on the fact that an insect in the presence of light proceeding from one localized source tends to assume an attitude in which its head is directed towards the source of light so that its two eyes receive the impression equally. To put it crudely and not quite accurately it " faces " towards the direction from which the light-impression comes. This tendency is well brought out in Fig. 105, a record made in tropical South America twenty-five years ago. The author sat at work in the evening by the light of a lamp on his left hand. Numerous homopterous insects alighted on the sheet of paper on the table in front of him. Each insect within a few seconds of its flopping down on the table rapidly adjusted its position and remained stationary with its long axis pointing towards the lamp, as indicated by the arrows of the diagram. It is obvious that an insect can fly only in the direction in which its head points : consequently if it adjusts itself during flight so that its head points towards the light it necessarily flies towards the light. As regards the insect's orientating itself towards the light there is in the author's opinion no scientific justification for excluding the action of psychical factors as is done by some physiologists. The excretion of nitrogenous waste products is carried out in Peripatus by numerous pairs of nephridia. In the more typical arthropods the continuous series of nephridia is no longer present but the last survivors of the series are still distinctly recognizable in the Crustacea and the Arachnida. In the more lowly organized Crustacea they form what are known as the shell glands — opening at the base of the second maxilla- while in the more highly developed Crustacea such as the Crayfish, Lobster, or Crab, they form the green glands— opening at the base of the second antenna. In the Arachnida they form what are called coxal glands opening at the base of the fifth appendage. In the Insects they are no longer recognizable, there being here numerous blindly ending ZOOLOGY FOR MEDICAL STUDENTS CHAP. (Malpighian tubes) opening into the alimentary canal be front limit of the proctodaeum. In connexion with the disappearance of the nephridial tubes during the evolution of the Arthropoda it should be remembered that in this phylum the function of excretion is carried out to a great extent by the skin, waste nitrogen being contained in the chitin which covers the •Light \ \ V \, \ < r \ .«-.«- \ T \ FIG. 105. •I ;il.«mt coo homopterous insects which alighted on a sheet of paper and a lamp. IHoD of the I'.MK axes of the insects' bodies : the two fine lines OUtOC of li.^ht. •m«l is -«,t rid of at each ecdysis. Furthermore the coloured 'i' h are often present in the exoskeleton are excretory sub- for example, in the group to which our common Cabbage ' belong the characteristic white pigment is impure •1. while the orange and yellow pigments are derivatives of uric H"1 Arthropod then has developed the habit of depositing its '<<"> material in insoluble form on the surface of its vi ARTHROPODA 235 body and shedding it periodically, instead of passing it off in solution by means of its nephridial tubes. It is probable that the development of this habit in the Arthropoda has had much to do with bringing about the reduction of the primitive nephridial tubes. In the blood-system of the Arthropoda the most important feature is that already alluded to, namely that what would normally be the finer blood-vessels have become greatly dilated, forming a system of irregular spaces which constitute the haemocoelic body-cavity. The blood is driven through these spaces by the beating of the heart, which is here a development of the dorsal blood-vessel — extending it may be through a great part of the entire length of the body or being on the other hand shortened down into a compact organ. Whatever be its shape the heart of the Arthropoda always shows this striking peculiarity that its walls are perforated by slits or ostia by which its cavity is in free communication with the body-cavity round it. The explanation of this peculiar circumstance lies of course in the fact that the body-cavity consists really of blood-spaces instead of being coelomic in its nature — and therefore isolated from the blood-system — as it is in most types of animal. The blood is as a rule practically colourless but sometimes it is of a bluish colour owing to its containing in solution a bluish copper-containing substance Haemocyanin, possessing the same affinity for oxygen as Haemoglobin has (p. 141) and serving like the latter as a " vehicle " for oxygen. The nervous system of the arthropod is constructed on the same general plan as that of the annelid. It has, however, reached a higher stage of evolution : the ganglion cells show a higher degree of con- centration, the ganglia of the ventral cord being more distinct and in particular the supra-oesophageal ganglia or brain having reached a higher level of size and complexity of structure. This latter advance is correlated with (i) the great development of sense-organs in the head region ; (2) the crowding together of the appendages and their associated nerve-centres around and in front of the mouth ; and (3) the general tendency, apparent in the evolution of groups of animals which move actively with one definite end in front, for the control of the activities of the body as a whole to become more and more concentrated in the front portion of the central nervous system. The fact that the general surface of the body consists not of soft living protoplasm but of hard chitin involves necessarily peculiarities in the sensory apparatus by which impressions are received from the outer world. Distributed more or less generally over the surface are sensory 236 ZOOLOGY FOR MEDICAL STUDENTS CHAP. bristles -fine hair-like projections of the cuticle ensheathing a prolonga- tion of one or more underlying sensory cells. The function of these sensory bristles appears in many cases to resemble the sense of touch ; in other cases, more especially those of the antennules of Crustacea and iniennae of insects, they would appear to have to do with a sense which detects differences in chemical composition of the surrounding medium, like our senses of smell and taste ; but in all probability the il kinds of sensation with which they are concerned are such as we :orm no conception of, as they are not included within the limits of human sensory experience. Just as is the case with the sensory cells of Coelenterates, so we find also in the Arthropoda the occasional occurrence of aggregations of these sensory bristles in special, more or less closed-in, depressions of the surface forming otocysts. An excellent example of the arthropodan otocyst is to be seen in the Decapoda (Lobsters, Crayfish, Prawns) where it is situated in the base of the first antenna, and forms a chamber which remains open to the exterior by a narrow slit guarded by bristles. From the floor of the chamber there project into its cavity a group of specially developed ~| -shaped sensory bristles, on the top of which rest a number of otoliths. In cases where the otocysts become completely closed the otoliths are secreted by the otocyst wall, but in the animals now under consideration where the otocyst remains open to the exterior the otoliths are grains of sand which the animal itself inserts into the cavity. At each ecdysis the entire lining of the otocyst with the contained :ns is shed and consequently a new supply of otoliths has to be inserted before the organ can function. This has rendered possible a fascinating experiment to demonstrate the function of the otocyst. r ecdysis had taken place a prawn was provided not with ordinary sand but with fine iron filings — to which it duly helped itself. In ordinary circumstances the iron " otoliths," under the pull of gravity, served exactly as the ordinary grains of sand. When, however, a powerful electro-magnet in the immediate neighbourhood of the animal brought into action, the result was the same as if the direction of gravity had been suddenly altered and the animal immediately heeled over, away from the direction of the magnet. he sense organs of the Arthropoda the most interesting are the eyes uhi. h are normally present in the head region. An eye consists , like any other sense organ, of an aggregation of sensory cells. hlfer from the ordinary type of sensory cell in the fact that they •l'-d with a sensory hair but have a portion of their cyto- vi ARTHROPODA: SENSE-ORGANS 237 plasm highly specialized for carrying out the primary function of the eye, namely the conversion of light waves of the ether into living impulses. These specially modified portions of cytoplasm, characterized by their glassy transparency, are known as rods— from their shape in some of the better-known types of eye. In the Arthropoda there exists great variety in details of eye structure, but amongst this variety there stand out two main types of structure (i) the simple or camera type of eye and (2) the " compound " or, better, radiate type. The simple eye (Fig. 106, A) consists primarily of a biconvex thickening of the cuticle — the lens (1) — clear and transparent and serving to condense the light upon an underlying clump of sensory cells — the retina (r). The latter is usually displaced inwards, so as to be in the neighbourhood of the focus of the lens, by a mass of transparent cells constituting the vitreous body (v). When the eye has its retina displaced inwards in this fashion it no longer serves merely for the detection of light, and thus distinguishing between light and dark, but provides the means for producing a definite sensory picture of the external world. The radiate eye is of a fundamentally different type — characterized above all by the fact that the retina is subdivided up into a number — it may be a vast number, many thousands — of retinulae, commonly arranged in radiating fashion owing to the fact that they are normal to the surface of the head and that this surface is strongly convex. Each retinula (Fig. 106, B) consists of a group or bundle of elongated sensory cells (R), the portion of cytoplasm of each cell next the axis of the bundle forming the rod (r) while the deep end of the cell is prolonged into a nerve fibre (n.f). Between the outer ends of the retinulae and the cuticle with its underlying layer of cells is the vitreous body. This typically undergoes a subdivision similar to that of the retina, there being at the outer end of each retinula a vitrella, a group of (commonly 4) vitreous cells (v). The portions of these cells next the axis of the group are fused together and of a glassy transparency, constituting the crystalline cone (c.c). Each vitrella is separated from its neighbours by cells (/>) containing black or dark brown pigment. These together constitute a tubular channel through which the light rays reach the retinula — each retinula receiving its light only through the one of these tubes which is in line with itself. In the more highly developed radiate eyes the cuticle covering its surface has undergone a subdivision corresponding with that of the underlying structures, the portion overlying each vitrella having become ZOOLOGY FOR MEDICAL STUDENTS CHAP. la and, it may be, inwards as well. The cuticle is thus r R. nf. inn FIG. 1 06. ; tnrc df (lie simple (A) and radiate (B) types of eye of the Arthropoda. niie cone; ep, epidermis ; /, lens ; n.f, nerve fibre ; p, pigment-roll ; R, retinal ( HI ; r, •. up into numerous little lenses the function of which apparently vi EYES OF ARTHROPODA 239 is to ensure that the only rays that reach the individual retinula are those whose direction is approximately that of its own axis. All other rays, coming in at a considerable angle to this axis, are shunted off into the pigment cells and there absorbed. It is clear that in this radiate type of eye we have to do with an organ that carries out the function of vision in a manner totally different from that of the more usual camera type of eye — where an image is formed, as in a photographic camera, by a lens and then cast upon a sensitive screen — the retina. The vision of the radiate eye is of what is called the mosaic type, there being formed not a continuous optical image but a collection of separate light impressions from the outer world, each coming in along the axis of a vitrella. A definite mechanism exists for the weaving together of these separate impressions, for the group of nerve fibres which pass brainwards from each retinula separate from one another and then become collected into new groups, the fibres composing which, although the same in number as those of the original group, are derived not from a single retinula but from a group of neighbouring retinulae. In spite of the existence of this mechanism for weaving together the isolated impressions into a continuous whole it is quite incredible that that impression can, as a representation of the outer world, be otherwise than exceedingly crude compared with that given by the camera eye. In all probability the special efficiency of the radiate eye lies not in forming a picture but in the detection of objects close at hand which are in motion relatively to the eye, either because the object is itself moving or because the arthropod is moving with regard to it. The same effect will be produced if the arthropod while remaining in one spot rotates its head — as a Dragon-fly may be seen to do when on the outlook for mosquitos. ARTHROPODAN TYPES OF SPECIAL INTEREST The main subdivisions of the phylum Arthropoda have been indicated on p. 209. While there is no necessity for the elementary student to go into the details of the classification of the group it is of importance that he should have a general idea of the features of special interest in connexion with various types of arthropod. I. The group PROTARTHROPODA comprises the genus Peripatus — to the morphologist the most important of all arthropods, for it has lingered on until the present day as the sole representative of those annelid-like creatures that were gradually developing the characteristics of arthropods. 24o ZOOLOGY FOR MEDICAL STUDENTS CHAP. (Fig. 95, p. 211) still retains the elongated worm-like form ,ly with simple stump-like appendages similar in character through- i, li-nuih of the body, the soft muscular body-wall without marked thj, •;. lie cuticle, and the nephridia distributed in pairs segmentally throughout tlie length of the body. On the other hand it has developed the typical arthropodan characteristics that a pair of appendages have become converted into jaws and that the coelomic body-cavity has shrunk up and been replaced by a haemocoelic body-cavity filled with blood and in free communication with the interior of the heart by means of ostia. It possesses also the arthropodan character that it breathes by trachea! tubes. Further, although the number of segments in the body ulicated by the appendages differs in different species and even, to ^s extent, in different individuals of the same species, this number remains fixed throughout the life of the individual from an early stage of embryonic development. Peripatus is then on the balance of its structural features to be classed as an Arthropod. That it is a very primitive arthropod is shown — apart from the annelidan features already mentioned — by the relatively feeble elaboration of the head region. Only a single pair of appendages have become modified as jaws ; there is none of that crowding together of appendages in the neighbourhood of the mouth and their modification for different functions that is so striking in the typical arthropod. Nor is there the correlated crowding together of the corresponding nerve- ganglia to form a complex brain. Over fifty species of Peripatus are known, distributed in seven different centres in the warmer parts of the world (Mexico and West Indies to Rio ; Congo ; Malaya ; South Africa ; New Britain ; Austral- ; Chile). Such wide discontinuous geographical distribution is a feature commonly met with in ancient types of animal. rcripatus lives in damp localities and during the day is to be found lurking under logs, bark or stones. It may be recognized by its general shape, its velvety skin, and its habit of ejecting sticky slime from the ends of the appendages next behind the jaws. II. Cn.ler the name MYRIAPODA are included a number of groups of I arthropods which still retain the elongated form of body without differentiation into distinct regions but which in other respects are highly developed arthropods. They are exemplified by the carnivor- ipedes and the vegetarian Millipedes. The former possess poison ' the tip of the fourth pair of mouth appendages which • of claw-like poison-fangs. In the case of the large tropical are grouped together under the name Culicidae. They feed com- monly on plant juices but in the female, which alone sucks blood, a meal of Blood is apparently essential for the complete growth and maturation of the eggs. In marshy districts in the tropics, e.g. in tropical America, mosquitos may exist in such numbers as to make life almost intolerable to freshly arrived human beings, although fortunately immunity is eventually developed to the poison of their bite. The main objectionableness of mosquitos resides, however, not in the irritation caused by their bite2 hut in the fact that they are the transmitters of various disease-producing parasites. In this connexion there are two specially important types to In- distinguished, represented respectively by the genus Culex and the - Anopheles — both of them common in warm and temperate climates. As the anopheline type is responsible for transmitting the parasite of malaria it is important to be able to distinguish it from the culicine type. correct identification of the species of mosquitos is a matter for specialists but there are certain conspicuous peculiarities which usually • nable one to recognize an anopheline mosquito at a glance. The most conspicuous of these is the attitude assumed by the mosquito either M at rest or when about to bite. The culicine mosquito assumes the attitude shown in the right-hand figure at the top of Fig. 107 : the irly parallel to the substratum ; the head and proboscis is the ordinary Spanish word for a small fly, although in English be used in n restrirfrd sense for the long-legged gnats called in do. t.ition caused l>v the. injection of the mosquito's salivary ue not to ;my poison excreted by the mosquito, but to the ,l>ioti< fun.ui t>rjon»ing to the group Eiitomophthorincae which " k«-(-like divertinda of the oesophagus and are passed into the ith tin- Ball lion when the mosquito bites. VI MOSQUITOS 247 bent down at an angle. The anopheline mosquito on the other hand (top left-hand figure) has its abdomen tilted up at an angle to the solid surface so as to be practically in line with the proboscis, the insect HI FIG. 107. Diagram to illustrate the life-history of Anopheles (A) and Culex (B). (Based on a diagram by Nuttall, in Hindle's Blood-sucking Flies.) Note (i) that the adult female Culex has very short maxillary palps whereas in Anopheles of both sexes, as in the male Culex, they are as long as the labium ; (2) That in the pupa of both genera the respiratory openings or stigmata are situated at the ends of paired trumpet-shaped projections from the thorax ; and (3) That in the larva of Culex the stigmata are situated at the tip of a tubular projection from the eighth segment of the abdomen, and further that the natural position of this larva is hanging downwards from the surface-film of the water, while that of the larva of A nopheles is horizontal immediately beneath the surface-film. having consequently the appearance of standing on its head. Other characteristic differences which are of use in distinguishing between anopheline and culicine mosquitos in different phases of their life-history are shown in Fig. 107. ZOOLOGY FOR MEDICAL STUDENTS CHAP. MOSQUITO CONTROL Malaria — one of the most debilitating and widely spread of all .md Yellow Fever — one of the most dangerous, not to mention lesser diseases, being spread entirely by the bites of mosquitos it follows keeping down the numbers of these insects to the minimum, in re- ions where natural conditions favour their existence, is an insistent need. Its successful accomplishment may indeed be an essential condi- tion to the continued existence of a white community in a particular locality. It is in warm climates where mosquitos carry on their life-history throughout the year that their control is especially necessary. In regions preading tropical swamps it is of course out of the question to think of exterminating mosquitos entirely, but even in such places much may he done to diminish their numbers within actual human settle- ments owing to the fact that normally mosquitos do not travel any great distance. What renders the problem of mosquito control a practical one is the fact that the earlier and normally much longer portion of the life-history is passed in the form of a larva (Fig. 107,, I) or pupa (Fig. 107, II,) which lives in water but has to breathe air by means of a pair of stigmata, situated in the pupa of Anopheles and in the larva and pupa of Culex at the end of projections from the body. The line of action indicated there- is to take measures (i) to ensure, if possible, that bodies of water suitable for the larvae and pupae shall not exist in the immediate 'ibourhood of human habitations, and (2) should such be in existence M'lcr them uninhabitable by the mosquito larvae and pupae. The r object may be attained most easily by covering the surface of the r with a thin film of oil which blocks up the stigmata and renders i' impossible for the young mosquito to breathe. The oil1 may be •i to the surface of pools two or three times a week, or allowed to drip from a tin or drum with a small hole plugged with cotton waste. lerely temporary in its effects and in the case of permanent •ore definitive measures are required. All unnecessary wrater which may afford suitable breeding grounds for ; • oil. Its efficiency is said to be much increased by the addition rrd in the following way. Carbolic Acid (150 gallons) is ty Ix'ilm- point and Jor lite to avoid altogether being bitten. But it should be rememU-ivd that only a small proportion of mosquitos are infective and consequently any reduction in the number of bites means a reduction in of infection. The hours of sleep should be passed in the u efficient mosquito net,, properly used, and for persons on ni-ht duty in the midst of swamps it is worth while to smear the exposed • i' the skin with some oil repellent to mosquitos and other biting insects1 although such mixtures are themselves apt to cause much irritation to persons with delicate skins. In the case of regions where malaria is not of normal occurrence the immigration of numerous malarial patients makes it of importance to take measures to reduce the chance of" their infecting the local mosquitos. Such patients should be prevented so far as possible from settling in fen lands and other low-lying districts where mosquitos are more numerous than elsewhere, and patients actually in such districts should during their attacks be kept in the seclusion of mosquito nets or gauze-screened rooms. -in via, the genus concerned in the spread of Yellow Fever and Deniuie Fever, is controlled by the methods already mentioned, but it should In: remembered that mosquitos of this genus are particularly prone (i) to breed in water-butts, old tins and other receptacles, and (2) to haunt the interior of houses and ships. In the latter case they >t cleared out by fumigation,2 care being taken to paper up all chinks, to leave the room or cabin sealed up for three hours, and to burn •arcntly dead mosquitos. The CHIRONOMIDAE include a large variety of Midges. The genus CktfonomuSf one of the commonest, has a worm-like aquatic larva which <:ies is coloured red by haemoglobin (" Blood-worms "). In the genus Culicoides or Ceratopogon the female is blood-sucking and is tii'- commonest type of blood-sucking midge or "sand-fly." In -"in«- re-ions these midges are even more annoying than mosquitos, for mall size ordinary wide-meshed mosquito-nets are no protection against them, and owing to their occurring in swarms, not Might but only occasionally, immunity to their poison is not '|>cd so readily as in the case of mosquitos. i part, Kr >un.l of sulphur per thousand cubic feet of room. Burn in an iron :i"'l with spirit. vi DIPTERA 251 The PSYCHODIDAE include Phlebotomus a small black " Sand-fly " or midge which may be recognized by its sharp almost flea-like darts from side to side, and by the characteristic fashion in which it holds up its wing when biting. It is the transmitting agent of " Sand-fly Fever " or " Three-day Fever " or " Pappataci Fever " (p. 80). The TIPULIDAE include Tipula, the Daddy-long-legs, the underground larva of which (" Leather-jacket ") does much damage to crops. The SIMULIIDAE include the very blood-thirsty midges or sand-flies of the genus Simulium, with large clear wings, which occur occasionally in swarms in Britain and in some warmer climates form a great pest. The TABANIDAE include the many species of Gad-flies, Horse-flies and Clegs. The common noiseless light-footed " Cleg " belongs to the genus Haematopota. The genus Chrysops, in which the eyes are often golden- green and the wings partially dark in colour (Widow Fly or Viuda of Spanish America), is incriminated as the transmitter of Filaria loa (p. 206). The SYRPHIDAE include many of the most abundant kinds of flies. Some of them present a remarkable mimicry of Bees and Wasps, and these have possibly given rise to the ancient legend of Bees coming forth from decaying matter such as carcases. The MUSCIDAE include a number of the most abundant types of fly, some of them of practical importance in connexion with the spreading of disease. They are characterized by their short three-jointed antennae with a jointed, sometimes feathery, bristle projecting from the terminal segment. The common large House-fly (Musca domestica, identified by the four dark lines which run along the dorsal surface of the grey thorax), world- wide in its distribution, does a certain amount of useful work as a scavenger, but any good that it does in this way is far overbalanced by the harm that it does in spreading diseases of the alimentary canal such as Typhoid, and where large numbers of non-immune human beings are crowded together under conditions favourable to the microbes of such diseases its control becomes of the greatest practical importance. The eggs, to the number of about 800 from a single female, are deposited in rotting material or preferably in fresh faeces of horse or man. After about twelve hours x the larva, a slender white grub or maggot rounded at its hinder and pointed at its front end, hatches out. The larva grows rapidly to a length of about 12 mm., and after about 5-8 days crawls into a dry spot and metamorphoses into the pupa, and 1 These time periods are greatly shortened by warmth and lengthened by cold. ZOOLOGY FOR MEDICAL STUDENTS CHAP. •at 5 days in this stage? the adult fly emerges. About 7-10 days the It-mule begins to lay eggs. during the adult phase that the fly becomes a source of danger. ..lie in its tastes it is at one moment wandering about amongst and • a human faeces, at another wading in the jam on a tea-table, nother creeping about food in course of preparation in a kitchen. Human faeces are liable to be crammed with microbes of diseases of the alimentary (anal, such as Diarrhoea, Typhoid, Cholera, Dysentery, and such microbes' taken into the alimentary canal of the fly are apt to remain alive and even in the case of bacteria to multiply within it. The fly when feeding, especially when feeding on sugar or other soluble material, ies from time to time a drop of clear fluid from its mouth and both this and the faeces of the fly are liable to infect the food on which they • ieposiu-d. Portions of faecal material containing microbes are also liable to adhere to the feet or other parts of the fly and to be left behind \\hen it wades in jam or other food-material. Such disease germs lying in wait in food-material are obviously liable to infect with disease human who swallow them. It is clear that House-flies constitute a distinct menace to health, and therefore that their numbers should be kept down to the minimum about human habitations. The adult flies should be destroyed by AperSj "tangle-foot,"1 or poison.2 But here again, as is the case with mosquitos, the most effective measures are those directed against the earlier stages of the life-history. The all-important thing is to prevent the accumulation of garbage, fresh manure, or faecal matter in ••s accessible to flies and in the neighbourhood of habitations. Such trials arc preferably incinerated: if they have to be accumulated, ^cumulations should be buried under at least two feet of earth. The genus I-annia (or Homalomyid) includes the small House-fly (F. •••iniculuns) and the Latrine fly (F. scalaris), distinguishable from Musca by their smaller size, the greater overlap of the wings when at rest, the plain (not leathery) bristle of the antenna, and the fact that two nervures 'her 62 parts of resin, 26 of castor oil, and 12 of honey. Dip this and leave about. When well covered with flies ii .1 llainu ami recoat the wire. "• i lb. (or Cooper's Sheep-dip Powder 3^ Ibs.), sugar 10 iter 10 gals. Owing to its very poisonous character •ur this solution with some distinctive dye. Cloths moistened ung up ; or bottles of it may be left standing about with wick non. ly poison consists of 3 per cent Formaline in sweetened ve about in rooms during the night in saucers, taking care '•i fluid is available for the flies to drink. vi DIPTERA 253 run parallel to the tip of the wing, whereas in Musca they converge so as nearly to meet. These flies are both very common, but they are probably of less importance from the health point of view as they are less apt to interfere with human food. The larvae, recognizable by the pointed projections from the sides of the body, are occasionally found as parasites in the intestine of man. The Blue-bottles or Blow-flies (Calliphora} and the Green-bottle (Lucilid), which normally deposit their eggs on dead meat and fish, are well-known flies, and both are liable on occasion to lay their eggs in uncovered wounds. Among the Muscidae there are also included a number of blood- sucking flies. The common biting Stable-fly, Stomoxys, is usually mis- taken for a House-fly, which it closely resembles in general appearance, apart from the straight pin-like proboscis projecting forwards from its head. Closely allied to Stomoxys is the African genus Glossina (Tsetse), recognizable by the wings when at rest lying flat one over the other and projecting back beyond the tip of the abdomen. As already indicated, the various species of Glossina are of great practical importance from their acting as intermediate hosts for disease-producing trypanosomes. The Glossinas are readily attracted by moving bodies ; they are active during the day ; and like mosquitos they avoid white surfaces — so that white garments are desirable in Tsetse-infested districts. The egg develops within the uterus of the parent and there is eventually brought forth a fully developed larva which is deposited on loose soil into which it at once burrows and pupates. Deep shade and a certain amount of moisture are essential to the well-being of the Glossinas, and consequently something can be done towards keeping down their numbers in the neighbourhood of settlements by clearing the zone along the margins of lakes and rivers of trees and brushwood. The OESTRIDAE are remarkable from the fact that the larvae are parasitic on vertebrates : the adult fly as a rule does not feed, the mouth parts being reduced to functionless vestiges. Gastrophilus deposits its eggs on the fore-parts of horses, and when licked off and swallowed they develop into " bots," peculiar larvae surrounded by rings of hooks which hang on to the lining of the stomach, often in great numbers. Hypoderma similarly deposits its eggs on the body of cattle, but in this case the larva (" warble ") takes up its position under the skin of the back. These warbles when numerous do much damage to hides by the perfora- tions through which they make their way out. Occasionally they occur in man, and in South America they are in places regarded popularly as the larvae of a large moth ! ZOOLOGY FOR MEDICAL STUDENTS CHAP. In the genus Oestrus the egg is retained within the body of the mother until it has developed into a young larva. This is deposited in the •ourhood of the nostrils and finds its way into the frontal sinuses— IT cavities in the skull communicating with the nose — where it its larval existence. In all these Oestridae the larva makes its way out of the body before -limes the pupal condition. The last subdivision of the Diptera calling for special mention is -that til" tlif HIPPOBOSCIDAE, the adults of which are blood-sucking. They >lu>\v a tendency to pass their time creeping about among the feathers or hair of their host instead of alighting on it merely for short periods when feeding. Correlated with this we find in different members of the group less or more marked reduction of the wings, culminating in such forms as the Sheep-ked (Melophagus) , in which they have entirely dis- appeared, the creature spending its whole life among the wool of the Such cases are of interest as illustrating how members of a group <>l animals characterized by great activity and freedom of movement may become transformed in the course of evolution into highly specialized parasites whose life is confined entirely to the body of their host. Having dealt with the eight main orders of insects we now come to three less conspicuous groups, the members of which are entirely parasitic in habit, and in correlation with this are wingless and in other ways modified in structure. The Fleas (APHANIPTERA or Siphonaptera) are especially characterized by the shape of the body, being greatly compressed from side to side, of being depressed dorsiventrally as is the case with other flattened insects. The mandibles are long piercing styles, and between them is an unpaired piercer which may be hypopharynx or labrum. re are many species of flea, each having its favourite host but being often quite ready to bite animals of other species. In the case of imary human flea (Pulex irritans) the small whitish worm-like larva live* in dust, especially under carpets. Where large numbers of deposited and where the resulting fleas have had no nity of being carried away by human beings, as in deserted huts :i