i 2 siaeaty Aare Gornell Mniversity Library Digitized by Microsoft® ‘he date shows when this volume was, tak e renew this book copy the call No. and give | to the librarian. 3 - < <4 Cornell University Library QL 757.L66 TO 003 176 991 sm» ‘il ites of 3 1924 Digitized by Microsoft® This book was digitized by Microsoft Corporation in cooperation with Cornell University Libraries, 2007. You may use and print this copy in limited quantity for your personal purposes, but may not distribute or provide access fo it (or modified or partial versions of if) for revenue-generating or other commercial purposes. Digitized by Microsoft® THE PARASITES OF MAN. Digitized by Microsoft® Digitized by Microsoft® THE PARASITES OF MAN, AND THE DISEASES WHICH PROCEED FROM THEM. A TEXT-BOOK FOR STUDENTS AND PRACTITIONERS. BY RUDOLF LEUCKART, PROFESSOR OF ZOOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LEIPZIG. TRANSLATED FROM THE GERMAN, WITH THE CO-OPERATION OF THE AUTHOR, BY WILLIAM E. HOYLE, M.A. (Oxon.), MRCS. F.RS.E. NATURAL HISTORY OF PARASITES IN GENERAL. SYSTEMATIC ACCOUNT OF THE PARASITES INFESTING MAN. Protozoa—Cestoda. EDINBURGH: YOUNG J. PENTLAND. Digitized b y Migroson® 7977 B66G A. 35680 Pees. S ff CORNELL UNIVERSITY \ LIBRARY y EDINBURGH: PRINTED FOR YOUNG J. PENTLAND BY SCOTT AND FERGUSON AND BURNESS AND COMPANY, PRINTERS TO HER MAJESTY. ry Digitized by Microsoft® AUTHOR’S PREFACE. WHEN my permission was asked to publish a Translation of my Work upon Parasites, which was just then appearing in a Second German Edition, I was the more ready to grant the request, since the branch of Science of which it treats is one which has been cultivated more especially upon German soil and by German investigators, but has by no means found in other countries such wide-spread attention as its great scientific and practical importance render desirable. It is true that English Literature possesses in the Translation of Kiichen- meister’s Work on Human Parasites, and in the Treatises of Cobbold (Entozoa and Parasites of Man), writings which cover the same ground as my own; but Kiichenmeister’s work is entirely out of date, while Cobbold aims at giving a general sketch rather than a complete delineation of the group. This, however, is the aim which I have kept in view in the compilation of my book. I have endeavoured to serve the interests both of the Physician and the Hygienist, as well as of the Zoologist—the interests of practice and of theory, which are by no means so diverse as at first sight might appear. The relations which obtain between Parasites and their hosts are in all respects conditioned by their natural history; and without a detailed knowledge of the organization, the development, and the mode of life of the different species, it is impossible to determine the nature and extent of the Pathological conditions to which they give rise, and to find means of protection against these unwelcome guests. Even small and apparently isolated facts become often of great significance in this connection, and hence the Physician cannot afford Digitized by Microsoft® vi AUTHOR’S PREFACE. to neglect matters which at first sight appear further removed from his department than from that of the Zoologist. But just as little is it permissible for the latter to forget that the knowledge of the life-history of animals, after which he strives, is to be obtained by the investigation not only of their organization and development, but also of the position which each species occupies in the economy of Nature,—in the present instance of the attitude which the Parasite assumes towards its host. But few decades have passed since the full extent and the significance of these relationships have been made clear to us. It was only with the introduction of Helminthological experiment that a new path was opened to the field of knowledge, and we Zoologists gratefully recognise that the first impetus to the brilliant discoveries which our science has to show was the work of a Physician, and we rejoice that at the present day Medicine takes an active part in the prosecution of these studies. This partnership in the work ensures further progress in the future, which is the more important, since our knowledge of the Parasites. of Man in particular has in no respect reached a satisfactory condition. Numerous weighty questions still await their final solution. As to the part which I have personally taken in the cultivation of the science, it may be passed over with the remark that I have devoted my labours to it for a period of more than thirty years. If my efforts have in many respects been crowned with success, I owe it mainly to the long period during which I have followed up the solution of the problems in hand. The number of animals used for Helminthological experiments amounts to many hundreds, and much larger is the sum of the Parasites investigated. What I offer to my readers, then, is the result of a prolonged and minute investigation, and my work contains little which does not rest upon the basis of personal observation. Although my book is devoted mainly to the Entozoa infesting Man, it offers an almost complete survey over the present state of that part of Zoology which treats of Parasites. The first section Digitized by Microsoft® AUTHOR’S PREFACE, vil contains a general natural history of these remarkable animals, intended to give a clear exposition of the phenomena of Parasitic life in its various forms, as well as to narrate the history of our knowledge of them. And similarly there is prefixed to the special account of the various species a general sketch of the structure and life-history of the groups to which they belong. This course was adopted not only for purely scientific reasons, not only in order that the individual facts might be fully treated in connection with related phenomena, but also because by this means alone was it possible to supply, by well-grounded hypothesis and inductive reasoning, the gaps in our experience. The basis of our knowledge must be as extensive and as profound as possible, in order that the origin and nature of Parasites may be treated clearly and satisfactorily. By this mode of dealing with the subject I hope to have met the wants of those who are actuated by no interest in the Parasites of Man in particular. Here I refer chiefly to the Veterinary Surgeon and Cattle Breeder, who, in a summary of all that is known regarding the life-history of Parasites, will find the means of becoming more closely acquainted with those. specially important Entozoa of our Domestic Animals which also infest Man. In leaving out of consideration the Therapeutic treatment of Parasitic Diseases, I have followed the advice of one of our greatest medical authorities, and I did so the more readily, since, owing to the lack of personal experience in this matter, I could only have re- capitulated the works of others. Correspondingly greater prominence has, however, been given to those Hygienic principles which the study of Parasites gives us for the protection of society and its material interests, and which demand the more attention since they have hitherto been insufficiently practised. It is in this connection that the importance of modern Helminthology is most conspicuous ; for nowhere is it more true that “prevention is better than cure,” than in the case of Parasitic Diseases. It is sufficient to point, by way of illustration, to the Hydatid Tumours, Liver-Rot, and Trichinosis. ite of the importance attributed to the medicinal aspects of an see ioit tized by Microsoft® 2 vili AUTHOR’S PREFACE. the question, it was no part of my plan to make the book into a collection of Pathological curiosities by the detailed narration of numerous cases. Those who desire such a record are referred to the pages of Davaine, “ Traité des entozoaires et des maladies vermineuses,” —a work which only partially justifies its title, since the Zoological portion is very incomplete, and by no means up to the level of our present Helminthological knowledge. In conclusion, I must point out that the earlier sheets of the German Edition of this volume have already been published six years, in the course of which investigation has been active, and much has been added to our sum of knowledge. Whilst revising the present translation, I have striven, by the addition of notes and by modifica- tions of the text, to give an account of this progress, and hope that nothing of importance hag been omitted. In the original compilation of this work I thought primarily of German readers, and hence it bears throughout traces of its origin. But the quiet activity of the man of science is everywhere a portion of the universal work of that spirit whence the history of culture took its origin, and so may my book for the profit of the whole pass over the bounds of its home, and win for itself new friends in other lands ! In conclusion, it affords me very great pleasure to express my hearty thanks to Mr. W. E. Hoyle, the Translator of my work, for the conscientious and in every way satisfactory manner in which the English Edition has been prepared. RUDOLF LEUCKART. Lurrzic, September 1886. Digitized by Microsoft® TRANSLATOR’S PREFACE. THE present translation was undertaken, in the first instance, by my friend and former colleague Mr. F. E. Beddard, who found, on his ap- pointment to the Prosectorship of the Zoological Society, that his leisure was insufficient to allow of his completing the work, and therefore made the proposal that I should carry it forward. The manuscript which he had already prepared was handed to me, and contained an admirable rendering of the first half-dozen sheets, which, with few modifications, is here reproduced. As regards my own share of the work, but little needs to be said: not even those reviewers who so persistently, and in many cases so reasonably, decry the translation of German text-books, will require an apology for an attempt to render more widely known in this country a work which has long since attained the rank of a classic in its native land. No pains have been spared to present the English reader with a faithful rendering of the original; and the supervision which the Author has exercised over the proof-sheets not only furnishes a guarantee that he has not been misrepresented, but has also rendered it unnecessary for me to do anything in the way of bringing the work up to the times. A number of passages which in the course of time had become antiquated were cut out by the Author, who also supplied other paragraphs containing the results of more recent researches. These have all been placed in brackets, and are followed by the initials of the Author. The few additional remarks which I have thought it necessary to make ars dnstyapeiiatisans by my own initials. x TRANSLATOR’S PREFACE. Thanks to the writings of Cobbold and others, our language already possesses equivalents for most of the technical terms in this work, but it has always appeared to me that it would be very desirable to distinguish between the transference of a parasite from one host to another, and its movement from one organ to another in the same host. Hitherto the word “ migration ” has been used in both these senses, but in the present work I have confined it entirely to the former signification, and adopted “wandering” to express the passage from one organ to another. The Second Volume of the original is now being revised by the Author, preparatory to the issue of a new edition; he has kindly undertaken to forward the proof sheets of this for translation, so that the English version may pass through the press pari passw with the German, and be published simultaneously with it. In conclusion, I must fulfil the pleasant duty of expressing my great indebtedness to my friend Mr. J. Arthur Thomson, M.A., who has acted as my assistant throughout the progress of the work. Upon him much of the more laborious part of the work has fallen, and without his painstaking and intelligent co-operation the present translation could not possibly have been completed in the time which has elapsed since it was undertaken. WILLIAM E. HOYLE. Digitized by Microsoft® CONTENTS. SECTION TI. NATURAL HISTORY OF PARASITES IN GENERAL. CHAPTER I. NATURE AND ORGANIZATION OF PARASITES. Definition — General scope of the Subject — Pseudoparasites—Degrees and Varieties of Parasitism—Form of the Body—Organs of Fixation and Locomotion—Commensalism, CHAPTER II. OCCURRENCE OF PARASITES. Abundance—Distribution—Respiration and Respiratory Organs—Ectoparasites and Entoparasites—Nutrition and Mouth-Organs—Encystation, . CHAPTER IIL THEORY OF THE ORIGIN OF PARASITES REGARDED HISTORICALLY. Theory of Spontaneous Generation—Heterogeny—Linné and Pallas—Hypothesis of Inheritance—The School of Rudolphi—First Proof of Metamorphosis in Trematodes and Cercaria — Eschricht and Steenstrup — Discoveries of Dujardin, von Siebold, and van Beneden—Introduction of Helminthological Experiment by Kiichenmeister —Its further development, CHAPTER IV. LIFE-HISTORY OF PARASITES. Sexual Maturity—Eges and Embryos—Developmental Stage of Eggs when laid —Migration of the Eggs—Worm-Nests—Continuous development and Reproduction (Rhabditis) — Hamatozoa — Development of the Eggs externally — Influence of Moisture— Constitution of the Egg-Shell— Influence of Temperature—Duration of development, . Migration of the Young Brood—Eggs with contained Wiibryos—ibeaye of the Embryos after digestion the Shel pBesaps fm the Host—Free Embryos PAGE 1-9 10-21 22-41 42-57 Xi CONTENTS. PAGE or Larve—Entrance of Free-living Larve (Active aa Gana Migration (with Food)—Viability of the Germs, ; : Development of the Germs after Migration—Direct ee within the Body of the Host—Development of the Larval or intermediate stage (‘‘Helminths of the Second Developmental Stage”) — Sexually mature Larve, ‘ : A “ : F Change of Host—Development and Migration of the ee of Strongylus—Of Bladder- Worms—Action of the Digestive Juices—Migration of Pentastomum—Parasites with Free Sexual Forms—Intermediate and Definitive Hosts—Law of numerous Embryos, and its significance in regard to Parasitism—Theory of erratic Embryos and of Degeneration— Conditions of development—Duration of Life—Death, . : : 71-88 57-66 66-70 CHAPTER V. THE ORIGIN OF PARASITES, AND THE GRADUAL DEVELOPMENT OF PARASITIC LIFE. Various kinds of Parasitism—Relations to Free-living Animals—Free-living Nematodes — Rhabdonema nigrovenosum — Parasitic Nematodes with Rhabditiform Larve — Loss of the Rhabditic Stage — Cestodes and Trematodes—Relations to the Hirudinea and Turbellaria—Acanthocephali and Nematodes—Origin of the intermediate Host—Of the intermediate stage, : : : : : i> Eons . : 89-119 CHAPTER VI. THE EFFECTS OF PARASITES ON THEIR HOSTS, PARASITIC DISEASES, History of the Subject—Nature of Parasitic Diseases—Loss of Nutritive Material—Consequences of growth and of increase in numbers—Influence of Wandering and Migration—Diagnosis of Helminthiasis—Therapeutics and Prophylaxis —Etiology—Statistics of Human Parasites—Sources of Human Parasites—Their occurrence and distribution, ‘ . 120-170 SHCTION II. SYSTEMATIC ACCOUNT OF THE PARASITES INFESTING MAN. INTRODUCTION. Number of Human Parasites—Larval and Adult Parasites—Entozoa and Epi- zoa—Zovlogical position, . ; ‘ = . . » 178-174 Digitized by Microsoft® CONTENTS. Sus-Kincpom IL— PROTOZOA. Characters and Classification—Unicellular Organisms — Protophyta—Parasites resembling normal] Cells, Crass I.—RHIZOPODA. Organization—Modes of Reproduction—Foraminifera—Radiolaria—History of Parasitic Forms, Ameceba, Ehrenberg, . Amceba coli, Lésch, Organization and Vital Phenomena—Mode of Infection—Patholoyical results—Experimental investigation, Crass II.—SPOROZOA. Organization and Occurrence — Gregarines—Pseudonavicelle — Psorosperms — Coccidia—Miescher’s Tubes, Coccidium, Leuckart, Coccidium oviforme, Leuckart, . Organization—Development—Coccidia and Psorosperms—Pathological sig- nificance, Cuass III.—INFUSORIA. Organization—Life-History—Modes of Reproduction—Nucleus and Nucleolus— Classification, Order I.—FLAGELLATA. Definition— Vital Phenomena—Distribution— Reproduction, Cercomonas, Dujardin, Cercomonas intestinalis, Lambl, Occurrence—Organization—Pathological significance, Trichomonas, Donné, , Trichomonas vaginalis, Donné, Trichomonas intestinalis, Leuckart, Order II.—Cru1ata. Definition—Organization, Family BursaRiz. Balantidium, ClapartlGtiZeddyidicroson® Xl PAGE 175-182 182-185 185 186 186-191 191-202 202 208 203-228 228-237 237-240 240 242 242-246 246 248 250 252-254 254 xiv CONTENTS. Balantidium coli (Malmsten), Stein, ‘ i ‘ - : Definition —History—Occurrence—Structure and Mode of Life—Repro- duction—Infection, . . . . Svus-Kinepom I]—VERMBS. Definition— History —Subdivisions, . é 3 F : i Crass IL.—PLATODES. Definition and general characters, Order I.—CerstTopDa. Definition—Polyzootic nature—Head and Segments, . é . . The Anatomy of Cestodes—Calcareous Corpuscles—Cuticle and its Appendages —Musculature—Nervous System—Excretory System— Generative Organs —Their general Structure—Male Organs—Female Organs—Constitution of the Primitive Eggs—Structure and Development of the Embryo, 2 The Development of Cestodes—Historical—Migrations of the Embryos— Structure and Development of the Bladder-Worms—Cysticercoid Larve of the Tzniade—Of the remaining Cestodes—Development of the Dibothria— Modification into the definitive state—General survey, . " SYSTEMATIC ACCOUNT OF THE CESTODES. Classification—Synopsis of Human Tape- Worms, i Family I.—Tanyianz, Definitions — General Structure — Fixing Apparatus—Malformations — Sub- divisions, . . . ‘i i : ‘ . Division I.—Cystic Targe-Worms (Cystici). Definition—General Characteristics—Rostellum—Specific distinctness of the various forms, Subgenus Cystoteenia, Leuckart, Characters—Number and Distribution of the Species, . . Tenia saginata, Goze, 3 3 3 5 ‘ 3 Definition—Tape-Worms known to the Ancients—Rectification of Nomen- clature, Growth and Gina of ‘fe mane? Were ormation of ihe Head— Reproductive Organs—Development of Reproductive Organs—Unripe and Mature Uterine Eggs—Malformations—Defective and Super- numerary Joints—Prismatic Tape- Worms—Perforated Worms, Development and Structure of the Bladder-Worm —Experimental Rearing— Acute Cestode Tuberculosis, Distribution and Frequency—Modes of Tnfscioa—Waration of Uife— Medicinal signifcaysioitized by Microsoft® PAGE 265-268 269-270 270-279 279-330 330-387 388-390 391-400 400-403 404 404-406 406 406-422 423-458 458-475 476-488 CONTENTS. Teenia solium, Rudolphi, . . : Definition—General Characters, Origin and Development from the Bladder- Mov of the Bie Heian Experiments—Breeding of the Bladder-Worms—Of the Tape-Worm— Occurrence of Cysticercus cellulose, 3 . Development and growth of Tenia stew slog of the Bladder. Worm—Structure of the full-grown Bladder-Worm—Duration of Life —Identity with the Bladder-Worm of Man—Development and Growth of the Tape-Worm—Malformations, . ‘ . : Organization of Tonia solium—Ripe Proglottides—Head and er of Hooks—Development of the Generative Organs—Ripe Eggs, Occurrence and Medical significance —The Adult Tape- Worm — Trans- ference of the Bladder-Worm—Modes of Infection—Medical signifi- cance of the Tape-Worm—Of the Bladder-Worm—Historical Account —Cysticercoid Disease of Swine—Of Man—Mode of Infection—Self- infection—Occurrence in different Organs, in the Muscles, in the Eye, in the Brain—Cysticercus racemosus, Cysticercus turbinatus— Oldest record of ladder Worms in Man—Symptomatology of the Disease, Fi ‘ Fi A . Tenia acanthotrias, Weinland, Definition and History, . ‘ Tenia marginata, Batsch, : : : : ; Definition—Doubtful occurrence of the Bladder-Worm in Man—Distinctions between this and related species—Development of the Bladder-Worm (Cysticercus tenuicollis) — Experimental Breeding and Pathological significance—Full-grown Bladder-Worm, . . . Subgenus Echinococcifer, Weinland, : 9 . F Peculiarity of the Cysticercoid Stage—Specific distinctness—Metamor- phosis of the Hooks—Historical Account of the Zchtnococcus— Acephalocysts, 3 Teenia echinococcus, von Siebold, ‘ ; ‘ ‘ Definition—Description of the Adult Worm—Generative Organs—Duration of development—Supposed occurrence in Man, Development of the Zchinococcus-Bladder — Experimental Breeding — Structure of the Cuticle—Absence of Vascular System, ‘ Structure and Development of the Echinococcus-Heads—Brood-Capsules— Budding of the Heads, ; : The Formation of Daughter-Bladders—Zchinococcus simplex or granosus— Interlamellar Budding—chinococcus hydatidosus—Metamorphosis of Heads into Bladders—Metamorphosis of Brood-Capsules into Bladders —Direct formation of Daughter-Bladders — Echinococcus multi- locularis—Chemical Prikpeerpp the Bladder’ all and Fluid, XV PAGE 488 488-490 490-498 498-518 519-528 521-561 561 561-563 563 568-578 578 578-586 586 586-594 594-603 603-611 611-631 XV1 CONTENTS. PAGE Occurrence and Medical Significance—Multiple Echinococci—Etiology— Distribution and Frequency of the Disease—Zchinococcus in the Icelanders and Pastoral Peoples—Influence of Age and Sex—Growth of the Parasite—Prognosis—Nature and Symptoms of the Disease— Death of the Echinococcus, . ; : : : + 632-652 Division II.—Orpinary TaPe-Worms (Cystoidei). General Characters—Larval Stages—Number of Species, H ; . 652-656 Subgenus Hymenolepis, Weinland, : : ‘ 3 ‘ 657 Teenia nana, von Siebold, . 4 : : ‘ : . 657 Definition—Development—Eggs, . : ‘ ‘ 3 . 657-661 Tenia flavo-punctata, Weinland, ; ‘ : ’ : 881 Definition and Characters, : * A j ; . 661-663 Tania madagascariensis, Davaine, . : F ‘ ‘ 663 Definition and History, . ‘ . : ; - 663-665 Subgenus Dipylidium, Leuckart, . F : ‘ : : 665 Tenia cucumerina, Rudolphi, . : 665 Definition—Historical AneountDénlopment-—etalatlaoss's ieee = Egg-Masses, . 2 ji : F 5 F - 665-673 Family II.—BoruriocepHanipa, Definition—General Characters —Head, Nerves, and Excretory Organs—Generative Organs—Number of Species, - ‘ . : F . 674-682 Bothriocephalus, Rudolphi, F : ; ; ’ 5 682 Bothriocephalus latus, Bremser, : 683 Definition—Historical Sketch — ‘Anstong-—Dilasctee 9? expel Canigetie ’ Organs—Male Organs—Female Organs—Their Development—Abnor- malities, : , ; - 683-714 Occurrence — Historical Siete = Braun’s Teoma analy Btiees of Development—Ciliated Embryos—Metamorphosis, . 714-729 Distribution and Medical Significance—Modes of Intedtina Sects aid Individual Differences, ‘ : : i ; » 729-735 Bothriocephalus cristatus, Davuine, . : ‘ ; : 735 Bothriocephalus cordatus, Leuckart, . é : ‘ . 736 Detinition—Occurrence in Man—Description and Specitic Distinctness— Peculiarities of Young Forms, é . . . » 736-745 Bothriocephalus liguloides, Leuckart, : d ; ; 745 Definition — Historical Account — Occurrence in Man — Anatomy-- Disposition of the Organs—Structure of the Head, . . » 745-751 Digitized by Microsoft® LIST OF ILLUSTRATIONS. Acanthobothrium coronatum, larval state of, after van Beneden, 3 Ameba coli in intestinal mucus, with blood-corpuscles, Schizomycetes and similar bodies (after Lusch), Anthomyia canicularis— Larva of, Larva of, from the intestine of man, Archigetes Sicboldi (x 60), ” a9 ” ” ” ” ” Ascaris Lamnbirlaolien Eggs from, Aspidogaster conchicola (after Led Balantidium coli— Aubert), ” In conjugation (after Wising), In various stages of division, With widely opened peristome (dorsal stent) Bladder-worm — From the brain, with a spirally coiled body (x 12), From the pig, after the digestion of the bladder ( x 20), Head after digestion of the caudal bladder, Head of, from the pike, In the anterior chamber of the — sift: de Wecker (x 3), Longitudinal section through the head process (x 40), . Of the pig, with evaginated head (x 2), > 29 With invaginated head (x 4) a? 2? . . . . Sagittal longitudinal section through the protruded head of, from the pike, . The head and receptacle of, froma eee eel 6 mm. in size (x 25), 2 Transformation of, into a tape-worm (Tenia serrata), With extruded head, aes from the pike, Bladder-worm of the rabbit— with cnotepinwtal heal ’ ” Cephalic end of a young (x 45), F Longitudinal section through the head of (x 60), Metamorphosis of, into the young tape-worm (x 4), Transverse section of the anterior end of, at the level of the suckers (x 40), Bodo saltans (after Stein), Digitized by Microsoft® b . FIG, XVili LIST OF ILLUSTRATIONS. FI. PAGE Bothriocephalus— Diagrammatic representation of the course and connections of the vagina, as seen in longitudinal section, . 371 709 Egg of, with imperfectly onl a co ie expelled by compression, . 384 722 Eggs of, with epee. ‘ , , : ‘ 38 59 Encapsuled larva of a, from the smelt, . . 375 715 Head of a, reared in the cat from bladder-worms fiom the piles (after Braun), ‘ ‘ : i 380 719 Larva of, from the skink oe 20), : : - x 352 674 Larva of, from the smelt, 221 377 Segment of, with yolk chambers andl tisaflave aunts” (after Hechricht}, 370 708 Transverse section through the body of a larva of, 5 7 391 728 Young, from the alimentary canal of the dog, . 376 716 Young, from the intestine of the cat, after feeding with blader. worms from the pike, ; ; é ' j 379 719 Bothriocephalus cordatws— A number of mature joints of, . : : i a 395 737 From man, : : 2 : ‘ : ‘ 350 674 3 7 : , , A : : 397 739 “Head of, from the side and from above (x 8). . : 398 739 Four young specimens of, , ; “ : 2 401 743 Head and anterior portion of (x 5), . i ‘i 140 278 93 ss : F F : 394 737 Three joints of, seen from the dorsal and from the ventral surface ( x 2), 399 739 Transverse sections of the head of ( x 20), é F 3 400 742 Uterus of, F i : , : ‘ . 396 737 Bothriocephalus latus— . , ; 3 : 2 s 137 275 #3 e : ‘ : ‘ 357 684 55 isephalle end) ea 8), i ‘ i 226 389 Ciliated embryo of (x 500), P 7 ; Fi ¢ 177 327 Club-shaped head of, , ss ‘ ‘ ‘ 393 734 Development of the reproductive — in, ‘ 872 712 Diagrammatic representation of the course and eiesean of the vagina, as seen in longitudinal sections, : F : 367 701 Egg of, with embryo, A : : F 36 57 Egg, showing yolk-cells and hall (x 300), . : ; 171 321 Eggs of (x 300), . ‘ : ‘4 5 359 685 Embryo of, escaping from its atiatel gnvelone: ‘ : ‘ 388 725 Embryo of, in the egg, . r 385 723 Female generative organs of, oe the wenteal aunts re x 20), . 366 700 369 705 Female sexsi organs of, diowing the tere, evary, diall-giend, and yolk-gland (x 12), 2 : : ‘ 7 157 305 Free ciliated embryos of (x 500), 4 i . : 386 723 Free embryo of, . : : : . : ‘ 40 60 Free-swimming embryo of, F : ‘ : 70 110 a5 - (x 500), i ; ‘ : 351 674 ” ” (x 600), ‘ 5 374 715 Free-swimming embryo of, with the pavionleanita threads &e. 387 725 Generative organs of (ventral aspect), ‘ : = 141 278 Head of (x 8), . ‘ ‘ : é ‘ f 358 684 Larva of (x 55), . . ; : z 354 676 Larva of, with protruded fiend, ‘ , ; F : 378 718 Larvee of, from the pike, ‘ ‘ 8360 686 o » Digitized by Microsoft® : 377 717 LIST OF ILLUSTRATIONS. FIG, Longitudinal se acta representation of the three generative ducts, 363 Male and female sexual organs of ( x 20), 7 170 Male generative organs uf, seen from the dorsal surface ( x 20), 365 Mature joint of (x 8), 362 Ovum of, with yolk-cells and shell, 381 Ovum of, after Schauinsland, with four Seibiyanis walla ands en- veloping cells on the granular yolk (x 600), 382 Another ovum, with covering cells es 8 the eines body (x 600), : 282 Ripe joint of, with the uterine rosette (x 6), 368 Series of joints with double genital apertures, 373 Sexual organs of, from the ventral side ( x 20), 159 Transverse section through the body of, at the level of the cirrhus-pouch (x 10), E 364 Transverse section through the head of a Suuad ( x 55), 355 Transverse sections through the body of (x 10) . 361 Bothriocephalus liguloides— ‘ : 402 Head of (x 5), 404 Transverse section through the javval sity of, : 403 Bothriocephalus proboscideus, Excretory apparatus of, after Stendenet (x 82), j 155 Boblicibiephatun salmonis, Heteyonte dieydinument of, after " Kolliker (x 300), : 178 Brain of a lamb with wads of Chenavas, 81 ? 9 206 Brood-capsule— Closed and ruptured, showing their connection with the paren- chymal layer (x 40), 325 Development of, and of the appended head (x 90), 328 Metamorphosis of the, into bladders, after Naunyn (x 90), 332 With heads of Echinococcus in the interior (x 40), 7 324 Cercaria— A free, ‘ 50 . A free and an sreapeiled, the lekte without Yel, ‘ 21 and 22 An encysted, without tail, . ; 51 Cercomonas from the liver (after Lambl), 122 Cercomonas intestinalis (after Davaine), 121 Cercomonas musce at different stages (after Sicin), 117 Coccidia— Development of psorosperms in, . 111 Enclosing psorosperms, 112 From the human liver, 7 114 From the intestine of the aneiestie mouse, 100 - a 113 From the kidneys of the garden snail, 101 From the liver, 110 Coccidium-nodule, Cross section ef a, ighity eniawed, 108 Coccidium oviforme— From the liver of the rabbit, i 102 : 3 (x 550), 106 Canurus— Head and body of, in situ ( x 100), : 197 Heads of (x 25), Digitized-by Microsoft® 203 Passages of, in the brain of a lamb, ‘ 181 244 237 213 210 208 198 204 352 356 340 XX LIST OF ILLUSTRATIONS. Cucullanus, Embryo of, 65 Cysticercus— Head rudiment of an adult (x 12), 268 Subretinal, in the eye (after de Wecker), 298 With evaginated head (x 3), 269 Cysticercus acanthotrias— Head and circlet of hooks seen from above, after Weinland (x 60), 302 Hooks of, after Weinland (x 280), 303 Cysticercus arionis— With head retracted and protruded (x 50), 209 ” ” 336 Cysticercus cellulose— Completion of the head formation in (x 15), 283 Head of, with rudiment of the receptacle (x 25), 279 Metamorphosis of the head-process into the head proper (x 20), 282 The beginning of the bending of the head inside its receptacle (x 25), fi : 280 Various stages in the fatale if the head of (x 45), 188 With the formation of the head just beginning (x 19), 278 With the head in the coe ‘ x 2); F 235 Cysticercus fasciolaris, 202 5 236 Eiysticen cus glomeridis— (After Villot) (x 50), 210 35 (x 200), 214 ” (x 50), 337 Cysticercus pisiformis— A piece of liver from the cabbii showing passages made by, 46 Before the development of the head, with granular sheath and cyst (x 60), : 183 Head and body of, in completely evactaatel state (x 19), 199 Head of, with vascular system (x 45), : 190 Head of, just mature (x 40), . 3 F 2 2 191 Metamorphosis of the head of (x 45), . , 2 194-196 Piece of rabbit’s liver with passages of (x 10), 82 ” » 207 With head half evaginated (x 6), 198 Young, é ‘ 12 Cysticercus racemosus— (After Marchand), 300 (After von Siebold), 59 Cysticercus Tenic saginate— Embedded in the muscle, 267 Evaginated head of ( x 30), 270 Head of, with frontal sucker and staseniline ring tx 30), 248 Head-rudiment of, before and after the isla: sas of the suckers (x 25), , ‘ ‘i 265-66 Longitudinal section tteanehi ie head in situ (x 80), . 271 Cysticercus tenebrionis, Development of, after Stein (x about 100), 208 Cysticercus tenuicollis— (After Bremser), 305 Anterior end of, with retracted ade and ribbon- ule appendage, 313 Exit of a young, from the liver, 83 Longitudinal section through the head re an adult (x 20), 312 Digitized by Microsoft® LIST OF ILLUSTRATIONS. Fig, The head process (x 15), 310 Three months old, 311 Young, 310 Young, in situ, 309 Development of an Eelidnmeoceise: like dyeldcannsiA fe the er diy of the earth-worm, after Mecznikoff (x 25), 213 Distomum hematobivm, male and female, 29 Distomum hepaticum— (Natural size), 68 Ciliated embryo of, with an eye spade; 69 Egg of, with embryo, 35 Free embryo of, 39 Distomum luteum (young), with suckers wad viscera (hitter de _ Valette), 1 Dochmius duodenalis, Cephalic extremity of ; profile and front view, 10 Dochmius trigonocephalus— A, Free living young form ; B, Young parasite, 63 Rhabditis-like condition of young stage of, 43 Echeneibothrium minimum— (After van Beneden) (x 8), 226 Chain of joints of, : 134 35 isolated living lend antl ies worm, . 24 and 25 Taclated living head of, Fons the intestine of Zrygon pas- tinaca, 133 Strobila and proplotite of after t: van Pineianj : . 135 and 136 Echinococcus, : 13 Before the Ragioane of the ee ee (x 15), 319 Bladder, eight weeks old (x 20), 321 Brood-capsule of, with adherent heads in various stages of de- velopment (x 36), 204 Brood-capsule of, with retracted head atid with two appended buds at different stages ( x 100), 3 . z 314 Diagrammatic representation of a proliferating, . 205 Head, metamorphosis of an, into a bladder in the re of the brood-capsule, after Naunyn (x 60), 331 Head, with the anterior part of the head invaginated ( x ' 90), 323 Heads, development of the, from those hanging freely into the internal cavity of the brood-capsule, after Wagener (x 90), 327 Hooks (x 600), 315 In its natural size and position, 329 Proliferation of the membrane of an, 330 Young, four weeks old, escaping from the cape (x 50), 320 Echinococcus multilocularis (x 30), d 335 Section through an, 333 Echinococcus raccmosus, 334 Echinococcus veterinorum— : ; 317 Brood-capsule of, with adult and ello’ indiana heads (x 40), 326 Head of (x 90), 322 Echinorhynchus angustatus, male, = . i ll Embryos of ; (A) the profile ; (B) ventral view,. 72 Echinorhynchus gigas, Egg of, with embryo, : 37 Echinorhynchus spirula, natural size (after Westrumb), 71 Eggs of worms found in the alimentary canal of man, . 90 Entozoa in the second stage HidH¢ceahyyp, Mierosok® 45 Xx LIST OF ILLUSTRATIONS. Filaria sanguinis hominis (after Lewis), Flea, Larva of the, Flesh of pig with bladder-worms ‘(naturel in). Flesh of pig with Trichinw (x 45), Gregarines, encapsuled ; (A) after aiapietions 3 (B) ater darmation of pseudonavicelle, (4) Monocystis agilis ; (B) Cepard in, cinmctteny (C) Styloriynehs oligacanthus, Hexamita intestinalis, in the young and adult aban (after Stein), Hirudo medicinatis, Cephalic end of, with the three mandibles at the base of the oral cup, Infusorians with undulating Teagtiudivel cnendinene Boe che: intestine of the hen (after Eberth), Lacerta vivipara, Unarmed cystic worm from the body- caitey ae (x 30), Liver of a rabbit with Coccidium-nodules, Measly is Mieaches's 8 "pila, Betrendey of one of, with its donttente, Monostomum capitellatum— 39 Ciliated entiege of, ‘ Moreton misiabile, Infusorian-like embryos of, with the “necessary parasite,” Musca vomitoria, Larvee of, (Natural size and enlarged), Muscle-7richinw, seven weeks old, in the distended snoecdleeenivien siheutha, Nasua socialis, Kidney of, with Lustrongylus in the distended agi: . Oxyuris ambigua (young), : Oxyuris vermicularis, Eggs of, Paramecium coli (after Malmsten), - Pediculus (Phthirius) pubis, Pentastomida, Lung of rabbit infected wit, Pentastomum, Lung of a rabbit infected with, Pentastomum constrictum (after Aitken), Pentastomum denticulatum, 22 From the diver of man, Piece of a rabbit’s liver with bladder-worm ee (x 10), Piestocystis variabilis, Longitudinal section of an cme cystic worm from the lung of a crow (x 30), Pseudonavicelle, with germinal rods in their inbeiian, Psorosperm-balls and Gregarines on human hair, Lindemann’s, Psorosperm-nodule, The epithelium of a, filled with parasites, Psorosperm-saccule from the urinary bladder of the pike (after Lieberlitihny, Psorospermie from the connective tissue of the human kidney (after Lindemann), é : Psorosperms, (A), from he seca bladder of uthes lota; (B), from the gills of the bream, ‘ ; Pulex penetrans, male and female, Rainey’s bodies, one of, within an isolated mmsoular Abie, dblonged 100 diameters, Rainey’s tubes enlarged about 40 diameters, Rediz, Bojanus’ ‘‘ kingsyellow worms,” ae the pond- stall With brood of Distomes in the interior, (A) with germs ; (B) with Cercaria in the fatertoe 3 (C) — Cercariz, 19 Digitized by Microsoft® LIST OF ILLUSTRATIONS. Rhabditis terricola, e Adult female and satis, Embryo of, Rhabdonema (Ascaris) iisteeeain, Rhabditoid form eo Rhabdonema nigrovenosum, Mature embryo of, Sarcoptes scabiet, 55 2 ‘ ‘ 5 . ‘ . ‘ ‘ Scabies norvegica, Crust of, with mites, their borings, eggs, and excreta, Sclerostomum tetracanthum, encysted, Scoleces (x about 30), Spiroptera murina, Young form of, fiom the ee -wor, Sporocyst and Redia, with Cercariz in the interior, Strongylus filaria, Embryo of, s Tenia, Double joint of, with three sexual eueteus Embryo of (x 100), Tania cenurus it x 10-15), Cephalic end of, with hearamerdua sqaiineiny (x 25), Connection between the different parts of the female generative apparatus in, Form of uterus in, Joint of, with excretory vessels anid aneatinale: organs ( x 10), Larger and smaller hooks of (x 280), Sexual organs of (x 10), 39 : - ‘ : : ‘ Tenia crateriformis, Embryonal hooklet of, from a bird, after v. Siebold (x 700), ; Tenia cucumerina, ‘ Cysticercoid of (x 60), ” Gystinevens of, from the don louse, Head of, with rostellum and hooks in ditverant — of contraction ( x- 140), Proglottides of, in a sexually mature tate (x 20), Rostellum of (x 140), Teenia echinococcus ( x 10), Adult specimen of (x 12), ” Generative organs of ( x 80), Sexual organs of ( x 100), Tenia elliptica, Recently formed egg of (x 600), Sexually mature proglottis of, . Tenia flavo-punctata, after Weinland— . Ripe proglottides of, one barren (x 40), Tenia marginata, Embryonic development of (x 550), Form of uterus in (x 6), Hooks of (x 280), Recently formed egg of ( x 600), Tenia mediocanellata (natural size), Tenia nana (x 18), é 3 Head of, with retewoted eostellant (x 100) ; (A ) an isolated hook (x 600), ” Digitized b y Microsoft® XXiv LIST OF ILLUSTRATIONS. Occurring in man, egg of (x 400), Proglottides of (x 100), Proglottides of, at maturity (x 100), Ripe egg of, with embryo (x 250), Tenia nymphea, Embryo of ( x 400), Tenia perfoliata, Male and female organs of, after Rahante (x 15), Mature joint of, with uterus (x 10), Nervous system of ( x 20), Sexual organs of, from the horse, after iohane ( x 15), Teenia sayinata, . Cephalic end of, in : teteneted and extended state ( x 8), Cross section of a joint of (x 38), j Development of the efferent generative organs in, Development of the germ-producing organs (x 5), Eggs of, after HE. van Beneden (x 550), . Formation of the first lateral branches of the uterus ( x 5), Four last joints of, about to be liberated, Generative organs of ( x 10), Half-ripe joint of (x 2), 3 Head of, in a state of contraction (x 8), Head of, in longitudinal section (x 25), Head of, in contracted and extended condition (x 8), Isolated proglottides of, ” Joint of, Joint of, with dopsvaity simwale rive of the oT ( x 2), Joints of, with two and three genital openings, . . Longitudinal sn of (young chain of joints) (x 25), . . Uonpitidinal section through (a young ‘ahata) (% :25),. « Lower end of the vagina, showing its connection with the re (x 380), E ‘ Mehlis’ body’ in ecnoction with ihe various parts of the Pinay productive organs (x 30), Prismatic proglottides, With double porus genitalis, In transverse section through the porus penis (x 8), Proglottides of, in various conditions of contraction, Proglottides of, in motion, : : s . Ripe segment of (x 2), . ‘ " Series of joints with perforated qroplettines Supernumerary joint of (x 4), Supernumerary joints of, . Tape-worm form of, Var. abietina, Ripe segment of, afte Woinland ( X 2); 3 Young bladder-worms of ( x 30), Young bladder-worms of, with rudimentary bens (x 30) - Teenia serrata— Calcareous corpuscles of, Development of the hooks of, Embryonic development of (x 550), Form of uterus in (x 4), . Head of, with its excretory vessels ( x 24), . Larger and smaller hooks of (x 280), . ; i Digitized by Microsoft® 246 148 252 253 256 255 150 . 249-50 296 244 247 238 132 PAGE 322 659 316 660 321 315 316 296 313 407 433 292 446 447 449 448 294 439-43 528 431 435 408 271 423 316 427 450 291 293 431 447 444 453 456 455 65 425 408 457 329 452 271 479 345 460 282 287 326 568 298 566 LIST OF ILLUSTRATIONS. XXV FIC. PAGE Longitudinal section of a young, consisting almost entirely of head and neck (x 60), F F F : ‘ 151 295 ” ss 33 228 392 ” ” ” 234 401 Rudimentary heads of, at the beginning and at the end of the protrusion of the head (x 20), : A . 200 and 201 354 Twenty hours old, with incipient segmentation (x 10), . 7 225 383 Young bladder-worms of, with rudiment of head (x 12), F 187 345 Tenia setigera, Two proglottides of, from the goose, after Feuereisen, 160 311 Tenia solium— Apex and hooks of, . : : ‘ 139 278 Apical surface and circle of hadlis § in (x 80), 3 . : 231 395 ss F ‘ F 292 521 Cephalic endl of, . ‘i 4 8 Cross section of, ee ates and partied — imide ley power, : r ‘ : : 144 281 Cysticercus of, from the pig, : 5 : 45 68 Egg of, with shell and yolk-membrane fs x 400), . 175- 322 Eggs of, with and without primitive vitelline membrane (x 450), 297 528 Embryo containing egg (without oa membrane) ( x 400), : 173 321 Generative organs of, : : F ‘ 142 278 Half-ripe and ripe joint, . 2 : : ‘ 275 489 Half-ripe joint of (x 2), . A . r ‘ ‘3 295 528 Head of (x 35), . F F ‘ F : ‘ 227 391 i r : i : ‘ , . 274 489 9 : ‘ : 291 521 Head of, from the: intnatine of a sabibit bs x 25), = : 223 381 Head of, from the intestine of a rabbit, in different heed at motion (x 25), F F 288 513 Larger (anterior) and smaller posterior} node ak (x 280), ‘ 293 523 Proglottides of, with slightly branched uterus (x 2), . % 290 520 Reproductive organs of (x 10), . . ‘ : 294 526 Two joints of, with branched uterus ( x 2), y ‘ : 156 305 Two proglottides of, with uterus (x 2), . : 2 : 276 489 Two segments of, with branched uterus (x 34), . ‘ 2 289 520 Tenia torulosa— Young form of, in aia serrulatus, after Griiber (x 25), 3 212 365 a ; 339 653 Teenia uncinata, Generative aegis of, after Gtieda f x 25), - : 161 312 Tenia undulata, Rostellum of, after Nitsche (x 100), . : é 230 394 Tape-worm— Ege of, from a bird, Tenia nymphea, . a 7 ¢ 33 55 Eggs of, with six-hooked embryo, : 3 F : 20 32 Piece of a mummified, . : 273 485 Tenebrio molitor, Encapsuled tape-worm sabeea, and the ee aati worm from, after Stein (x 100), . : 2 F Z 180 331 Tetrarhynchus— Cysticercoid, from a Mediterranean percoid (x 20), 3 é 215 370 Longitudinal section of a still imperfectly developed (x 25), . 219 375 Longitudinal section of an isolated head of (x 10), . 222 380 Tetrarhynchus sepic (x 12), 5 ; é . : : 216 371 (Isolated head) (x 12), . 226 389 Thetis, Blood-corpuscles of, partly sith, saclueadl omuaaed of gaeaeiy etter eke, - Digitized by Microsoft® - ja ee Ne ( XXvl1 LIST OF ILLUSTRATIONS. Trichina-capsule, with connective-tissue covering (in situ), in B, calcified, Trichina spiralis—A, Embryo ; B, Intermediate form ; C’, Sexual form (unimpregnated female), Trichinosed pork (x 45), Trichocephalus dispar, in situ, on batrachorum (after Stein), Trichomonas jlestinelis, after ‘Finiker, é Trichomonas vaginalis, after Kolliker, : Ureter, with excrescences due to the presence of ison, Uterus of a free proglottis (x 2), Worm aneurism of the horse, a ” Digitized by Microsoft® FIG. PaGE SECTION I. NATURAL. HISTORY . OF PARASITES IN GENERAL. Digitized by Microsoft® Digitized by Microsoft® CHAPTER I. NATURE AND ORGANIZATION OF PARASITES. THE term “ Parasite,” in its widest sense, includes all those creatures which inhabit a living organism, and obtain nourishment from its body. This definition includes not only vegetable and animal parasites (phytoparasites and zooparasites), but also parasites on plants and on animals. The larva that inhabits the wood of a tree or the pulp of a fruit is to be regarded as a parasite in no less degree than the thread-worm of the human intestine; and the beetle that defoliates our forests is quite as much a parasite as the louse upon the feathers of the swallow. Parasitic life, then, as thus understood, is an ex- ceedingly widespread phenomenon. So long as the term “parasite” was confined to certain special forms, as was the case formerly, it followed as a necessary con- sequence that parasitism was an isolated phenomenon, and bore no relation to any other mode of existence. Now, however, this view is known to be false, a matter of great importance when we come to study the subject from a historic point of view. It is not merely the intestinal worms and allied forms that are to be included among parasites, but also numerous creatures that sometimes resemble so completely certain free-living animals, except in the nature of their food, that an independent mode of existence has been actually as- cribed to them. Does it correspond with the common view of the peculiar nature of parasitism, that a creature which, according to the definition just given, ought to be regarded as a parasite, should be sharply distinguished from another free-living animal simply because it feeds upon a living branch instead of dead wood, or on green foliage instead of withered leaves? Do not the value and meaning of these differences appear less than those which obtain between car- nivorous animals on the one hand and herbivorous on the other ? The question raised here remains the same, when we limit more narrowly the conception of parasitism, which on practical grounds is advisable for the purpesezefy thisyywenko@e@d confine it entirely to animals living as parasites wpon other animals. A bo NATURE AND ORGANIZATION OF PARASITES. Under this limitation, the group of parasites appears at first sight to be considerably more restricted than it did from the former wider point of view, and in earlier days, when it was thought that parasites always existed as parasites, for the simple reason that they were unable to lead a free existence, even more restricted than now. Modern investigations have taught us that there are frequently stages in the existence even of the most thorough-going parasites, such as the intestinal worms, when they lead a free life in water or damp earth; and also that among the thread-worms there are many species (e.g., Rhabditis) that are occasionally parasitic, and capable of arriving at their full development in milk, meat, and other organic substances as rapidly as, if not more quickly than, in the interior of a living organism, In another thread-worm (Ascaris nigrovenosa, Auct.), we have an in- stance of an animal whose life-history consists of two alternate gene- rations,1 both sexually mature, which differ so much from each other in structure and mode of life, that, before their genetic connection had been discovered, they were referred to two distinct families.? This case, which has such an important bearing upon the meaning and right understanding of the phenomenon of parasitism, will be described more fully in a subsequent chapter.? It follows, therefore, from a case like this, that certain animals, such as the larve of many flies (Musca vomitoria, Sarcophaga carnaria, Anthomyia canicularis, &c.), which occasionally feed upon living animals, although usually found in dead putrifying organic matter, are by no means to be ex- cluded from the category of parasites. If this kind of parasitism is to be distinguished in any way, it may conveniently be termed “ occasional,” in contrast to the “constant” parasitism exhibited by other animals. The term “ pseudoparasite,” which has been fre- quently, even in recent times, applied to cases of this kind, ought to be confined to various objects, such as hairs, vegetable tissue, &c., which have been mistaken for parasites, and even described as such ;4 1 For a fuller account see Vol. II. * The case mentioned in the text is not the only one known. Recent investigations have shown me that Anguillula stercoralis, occurring in cases of “ Cochin-China diarrhea,” is the Rhabditiform generation of A. intestinalis (Leuckart, Bericht math. phys. Cl. k. Stchs. Gesellsch. d. Wiss., p. 85, 1882). There lives also in the peritoneal cavity of Hylobius pint a strange parasite, Al/antonema mirabile, Lkt., whose offspring leads a free existence, and gives rise to many generations of Rhabditis, like sexually mature worms (Leuckart, Tagebl. d. Magdeburg. Naturf. Versamml., p. 320, 1880.—R. L. ° Chapter VIII. * A list of this kind of pseudoparasites, including only the commonest, would be too long for insertion here. It may be remarked that all kinds of objects, not only the débris of food (orange-pips, raisin-stones, sinews, small bones, and so forth), but also pieces of thread, hairs, &c., have been mistaken for parasites, It is generally not difficult to dis- tinguish these by the help of the micros Digitize ‘By Microsoft® RELATION OF PARASITIC TO FREE LIFE. 3 and, in my opinion, also for frogs, snakes, and spiders, which have been stated by many authors to have existed for years in the human alimentary canal, although it is perfectly certain that animals of this kind cannot endure the damp heat of the body of a mammal for more than six hours.? This occasional parasitism sufficiently points out what has just been maintained from another point of view, that no broad line of demarcation can be drawn between parasites and free-living animals. It is not, however, in such instances alone that the transition between the free and parasitic modes of existence is found. Many animals (such as the leech) are only parasites so long as they obtain their nourishment at the expense of larger and more powerful creatures, becoming simply carnivorous when they prey upon other animals of their own size or smaller. A parasite is, in all cases, smaller and weaker than the animal on which it feeds. Being in- capable, therefore, of overpowering it, the parasite contents itself with plundering its host and drawing nourishment from its juices and flesh. Thus the parasitic and free modes of existence are related to each other in two distinct ways, both of which are connected with peculiarities of parasitism itself, one of these links being the nature of the food, the other the relation of the parasite to the animal which supplies the nourishment. Reflecting upon the significance, already pointed out, which the size and equipment of the parasites have with regard to their mode of life, it is not surprising that the various groups of the animal kingdom do not all furnish equal contingents to the army of parasites. Among the Vertebrata, for instance—the majority of which are strong and of large size—there are very few parasitic forms; while, on the other hand, among the comparatively small and feeble Arthropoda and worms there are entire families, all, or nearly all, the members of which lead a parasitic life. In fact, it may safely be asserted that these two groups contribute more parasitic forms than all the other divisions of the animal kingdom taken together. 1 In these cases, also, the microscope serves to dispel the illusion ; for the contents of the intestines of these pseudoparasites will contain substances that could not possibly have been obtained in the body of their host. In estimating the origin of various objects asserted to have been evacuated by a patient it is impossible to be too careful. In such cases there is frequently an attempt to deceive the medical man, but more usualy some error has been introduced through a variety of circumstances. If, for instance, everything ‘that is found mixed with the faces be, without further investigation, set down as having come from the body of the patient, then the famous helminthologist, Dr. Bremser, must have evacuated, as he humorously relates, a pair of snuffers ; for they were certainly found in the bed-pan at a time when he was slightly indisposed, without any one having placed them there. ; 2 Berthold, “Ueber lebendéWirphibitn inyiiehendenorper, ” Miiller’s Archiv f. Anat. u. Physiol., p. 430, 1849. : 4 NATURE AND ORGANIZATION OF PARASITES. The parasites of man and the higher vertebrates belong exclusively to them. Comparing together the various forms of animal life included here, in the group of parasites, we find numerous and striking differences, not only in structure,—which corresponds of course to their zoological position,—but also in their biological aspects—in the nature and degree of the parasitism exhibited. On the one hand, there are para- sites which only occasionally seek out their host, and only remain long enough to take in a sufficient supply of food, departing as soon as this is done, and subsequently, perhaps, seeking out a fresh host. On the other hand, there are parasites that pass a considerable time, perhaps a whole stage of their existence, in the body of their host, which thus serves as a dwelling-place as well as a source of nutriment. This difference may perhaps be best expressed by the terms “tem- porary ” and “ stationary ;” but it must be pointed out that between these two kinds of parasitism no absolute line of demarcation can be drawn, any more than between the parasitic and free modes of exis- tence; the terms, however, may be retained, as they express two degrees of parasitism, which are, generally speaking, sufficiently distinct and are sometimes widely separated from each other. Even among the older zoologists this distinction was recognised, but “temporary” parasitism was usually so called in contradistinction to life-long, instead of to merely “stationary” parasitism. At that time, however, the fact that even the most thorough-going parasites—the intestinal worms—are free during part of their life, was not known, and, accordingly, the contrast implied between the types of parasites was different altogether from that put forward here. Besides those parasites which exist as such throughout their whole life-cycle, there are others which lead a free life for a longer or shorter period, either in the adult condition (ichneumon-flies and gad-flies), or as larve (certain thread-worms). “ Stationary ” parasitism, therefore, manifests itself in two ways ; it may be (1) “permanent,” lasting for life; or (2) “periodic,” embracing only a stage in the life-cycle, and therefore involving at some time or other a change from parasitic to free life. The various kinds of parasitism just enumerated possess an interest and importance that depend not merely upon their relations to each other and to other modes of existence; they are interesting also from the fact of the influence which they have in modifying the structure of the body, so that an examination of any form of parasite enables one to foretell with moderate certainty the particular kind of parasitism which it exhibits. Thus, temporary parasites must evi- dently be provided with th ; ; nue “Bigihzed by MrosoReenins and abandoning EFFECTS OF PARASITISM. 5 their host; they must have organs of motion and of sense. This is in- variably found to be the case; temporary parasites possess powerful limbs (¢.g., the bed-bug), sometimes even wings (midges and some other flies!), or swimming appendages (fish-louse). When present, these organs allow of a more complex development of the vital activities, and that, perhaps, to such a degree, that temporary parasites, when away from their host, display hardly any recognisable peculiarities. Only the nature of their food, and the way in which they obtain it, compel us to regard them as parasites; it is not the refuse of organic life, but living organisms, that supply them with nutriment. As the power of movement becomes less, it becomes more and more difficult for the parasite to leave its host. In this way a tem- porary gradually changes into a stationary parasitism ; the host which was formerly only visited at intervals, and for a short time, now serves as a shelter to the parasite, and is seldom abandoned by it or changed for another. Among stationary parasites there are many (eg., the flea) which retain the power of movement, and sometimes abandon one host for another in search of a safer dwelling-place or more abundant nutriment. These forms present many analogies to the temporary parasites, not merely as regards their mode of life, but in their structure, especially in regard to the development of locomotor organs. In the majority of cases, however, the power of movement is reduced in stationary parasites, sometimes entirely lost, so that the animal remains for months, or even years, attached to the same host. Instances of this may be found in the bladder-worms and the female Lerneade, which live with their heads imbedded in the muscles of fish. Moreover, it is not only the organs of locomotion which become abortive in these cases. The sense organs, and especially the eyes, whose development is almost co-extensive with the variety and energy of the muscular activity, in like manner frequently degenerate. The graceful outline of the body and its segmentation commonly dis- appear, in adaptation to the present slight need of locomotion. In fact, a glance at the group of the so-called intestinal worms, which are all stationary parasites, shows clearly that the more sedentary the life of a parasite becomes, the simpler and more undifferentiated is the form of the body. Moreover, the simplification of the external structure of the body is no more a special peculiarity of stationary parasites than is the possession of wings and swimming feet a peculiarity of free-living animals. Among the latter we find numerous examples of a similar form of body, and especially among those creatures with limited capabilities of locomotion .gvhichcip this respect, are somewhat 1 Hippobosca, Ornithomyia, &e. 6 NATURE AND ORGANIZATION OF PARASITES. analogous to stationary parasites. I only need to mention certain caterpillars and other insect larve, many of which lead a stationary life like the intestinal worms, and, furthermore, resemble them in that, in many cases (¢.g., ichneumon-flies, &c.), they are occasionally or even constantly parasitic. Besides these negative characters, stationary parasites can in many cases be recognised by positive characters, such as the possession of hooks and suckers, which serve to fix them on to the body of their host. Structures of this kind are by no means con- fined to stationary parasites, but are also commonly found in temporary parasites, and occasionally even in free-living animals, where, however, they are never so conspicuous or so constantly de- veloped. The more the power of locomotion in a parasite diminishes, the more difficult it is for it to migrate to another animal, and it must therefore be provided with organs which will enable it to retain its position under the most adverse circumstances. These organs of attachment vary in character, in correspondence with the structure of that part of the body of the host upon which the parasite dwells— being generally stronger and larger in those forms which are parasitic upon the outer skin than in those which live in the interior of the body of their host. Among internal parasites, again, the organs of attach- ment are generally more developed in those species which live in the alimentary canal, since they have to withstand the pressure of its contents. Many intestinal worms, however, do not possess any hooks or other organs of attachment; but in these cases there is gene- rally some compensation. Among the thread- worms, for example, which we shall presently consider, the form and length of the body seem quite as fit to break the pressure of the intestinal contents as to strengthen its hold upon the intestinal wall. In Trichocephalus (Fig. 3) the whiplash-like anterior part of the body is actually imbedded in the mucous membrane. - In this case the form of the body in a certain way makes up for the absence of proper organs of attachment. When these : are present we find the greatest differences Fic. 1. —Distomum lu- ; : : teum (young), with suckers 12 their structure and arrangement, which and viscera (after de la correspond always to the needs and cir- Valette). eens % cumstances of the individual parasite. Some- times, as in the flukes (Fig. 1), muscular suckers are present, which k by ati ri ing i : work by atmospheric, OF are comect ly. speakins, by hydraulic pressure; ORGANS OF ATTACHMENT. 7 sometimes the organs of attachment consist of hooks and claws, which serve to penetrate the underlying tissue or to lay hold of various prominences. In Tenia soliwm (Fig. 4) and other tape-worms these hooks have their basal end sunk within the tissues of the parasite, or else, as in lice (Fig. 2) and the majority of the parasitic Arthropoda, they are situated upon the extremities of the limbs. The various bristles and other prolongations of the outer skin, so commonly met with, may be safely included in the category of organs of attachment. These, by contact with neighbouring parts of the body, not only in- crease the power of resistance of the parasite, but also prevent it from being displaced in this or that direction, according to their disposition. By the possession of setee of this kind, the male Distomum (Bilharzia) hematobium is able not only to retain its position in the vena cava of man, but also occasionally to advance against the blood stream into the venous plexuses of the urinary bladder and rectum, so as to enable the female, which it drags along with it, to lay its eggs in a convenient place. Frequently several kinds of organs of attachment are found upon the same parasite; an instance of this is Twnia soliwm, which has Fic. 2.—Pediculus (Phthirius) pubis. Fic. 3.—Trichocephalus dispar, in situ. just been mentioned. Besides the hooks which are arranged in a circle upon the summit of the head (Fig. 4), there are found a number of suckers, which, together with the hooks, enable it to attach itself so firmly that it is very difficult to remove it from its place. Comparing the four suckers and their position upon the head with the single terminal sucker of the leech and the two suckers of Distomum (Fig. 1), we see that the organs of attachment in the parasites offer quite as great differeiegtize dlisivVarrangéifent as in their structure. 8 NATURE AND ORGANIZATION OF PARASITES. It has by this time, I hope, been made clear that the stationary parasite differs much more from ordinary free-living animals, both in the outward form of the body and in its armature, than the tem- porary parasite. How great the difference really is between these two forms of life, is most distinctly seen in those para- sites which are free at one period of their existence, and parasitic at another; the free stage may perhaps be entirely unlike the parasitic, especially in those cases where the conditions of life enjoyed by the animal during its parasitic and free stages differ markedly from each other. The larva of Gastrus, which inhabits the stomach of the horse, has all the characters of a stationary parasite ; a simple cylindrical body without eyes and sense organs, and, instead of organs of locomotion, organs of attachment in the shape of powerful hooks at both sides of the mouth, and numerous variously sized sete upon the surface of the body- How different is the form of the adult free-living animal, with its segmented body, eyes, ten- tacles, legs, and wings! Who would believe that these two creatures were merely stages in the development of the same animal, had not actual observation demonstrated the fact, and shown that the worm-like larva was produced Fic. 4—Cephalicend from the eggs laid by the fly. ic ice But this striking difference, we cannot doubt, corresponds less to the needs of parasitism as such, than to the differ- ences which usually obtain between a stationary mode of life and a free existence. In this way we can understand the fact, already men- tioned, that metamorphoses quite similar to that of Gastrus are com- monly met with in other insects, where the young are not parasitic, but only live a stationary life like parasites. Conversely, there are periodic parasites, whose structure, during both stages of their life-history, is quite the same. This is the case with the Gordiaceze, which pass their young stage in the body-cavity of snails and insects, and afterwards live in water or damp earth without any further ingestion of nutriment. In this instance, how- ever, there is no great difference in the manifestations of life between the free and parasitic stages; in both, the animal leads a stationary existence, and it is only the medium in which it lives that changes. It has been already pointed out in this chapter that the characters of parasites cannot be said to have the value of specific peculiarities, and this is well shown in certain remarkable cases of parasiti t Digitized by Microson® Pinan rae COMMENSALISM. 9 which van Beneden first applied the term “commensalism.” Here we have creatures that live within the bodies of larger animals, like para- sites, to which they are generally very similar in organization ; never- theless, they are not true parasites, inasmuch as they do not feed upon the juices and tissues of their host, but share its food or live upon the refuse of its body. Although there are several instances of commen- salism among the lower aquatic animals, we do not find any in man and the domestic animals, which form the subject of the present treatise, it being supposed, of course, that the conception of the term is not extended to such parasites as live upon the internal excretory products, instead of the living tissues of their host. If it could be definitely proved that certain intestinal worms (such as Oxyuris curvula of the horse) really feed upon the undigested food of their host,! this statement would need some limitation; but it would at the same time tend to show that commensalism is connected with true parasitism by numerous transitional stages, as we have already seen that the free and parasitic modes of existence are connected. 1 Dujardin, Ann. Sct. Nat., sér. 3, t. xv., p. 802, 1851. Digitized by Microsoft® CHAPTER II. OCCURRENCE OF PARASITES. Just as there is hardly a single creature which is not preyed upon by some carnivorous foe, so it seems probable that every animal gives shelter at one time or another to some parasite. We are even acquainted with cases where the parasite itself is subject to the attacks of other parasites. Many of the parasitic Crustacea, for example, give shelter to mites or thread-worms ; the parasitic larva of the ichneumon-fly again is inhabited by other minute parasitic larve (Pteromaline.) In the case of the Nematode, Trichosomum crassicauda, which infests the rat, we find three or four males living parasitically within the uterus of the female. Neither small dimen- sions nor a concealed mode of life offer the slightest protection against these enemies. Nevertheless, every animal is by no means equally subject to the attacks of parasites; on the contrary, we find the greatest differences in this respect. In certain animals, the presence of parasites appears to be the rule, since they may be found in great abundance? in every individual examined; in certain others hundreds of specimens may be searched without finding a single parasite. On the whole, it may be safely stated that the Vertebrata are far more generally infested by parasites than the Invertebrata, and indeed it was thought for a long time that their occurrence in the latter was a mere accident. Which- 1 See Vol. II. The recent researches of Biitschli and von Linstow have fully de- monstrated this fact. Moreover, there is another free-living worm (Bonellia), the male of which in a similar fashion lives within the genital duct of its own female. See Kowalewsky, “ Du male planariforme de la Bonellia,” Revue des Sci. Nat., pl iv., 1875 (translated from the Russian); Vejdovsky, Zeitschr. f. wiss. Zool., Bd. xxx., p. 487, 1878 ; Spengel, Mittheil. zool. Stat. Neapel, Bd. i, p: 857 et seq. 2 The snipe, the goose—so long as it lives in meadows—the turbot, have their intestines almost always filled with numerous Helminths, generally Cestodes. How vast is occasionally the number of parasites is shown by certain cases of trichinosis and the Cochin-China diarrhoea. Even the larger intestinal worms are sometimes found in great numbers, Bloch (‘‘ Abhandlung von der Erzeugung der Eingeweidewiirmer,” Berlin, 1782, p. 12) found in a male bustard at least a thousand specimens of Tenia villosa, some of which were no less than 4 feet in length. Gdze also (“ Versuch einer Naturgeschichte der Eingeweidewiirmer,” 1782, p. 32, note) found the alimentary canal of a parrot so full of Cestodes, 20 ells in length and about the thickness of a straw, ‘‘ that it (intestine) was almost ready to burst.” When the whole mass was placed in water, Géze was astonished Digitized by Microsoft® ABUNDANCE OF PARASITES. 11 ever opinion may be held—whether we regard the presence of parasites in invertebrates as a necessary preliminary to their sojourn in the body of some vertebrate or not—the fact remains the same. The abundance of parasites within the Vertebrata may be more or less accounted for by the fact that there are normally several, if not a great number of species found in the same host.1. Thus, for example, _man has more than fifty distinct species of parasites, the dog and the ox some two dozen each, the frog perhaps twenty. These are of course not all found in the same place and under similar condi- tions, but are scattered throughout the various organs and systems. One takes up its abode in the skin, another in the intestines, a third in the connective tissue between the muscles, while others again inhabit the brain or even the eye. No organ or tissue, how- ever remote or concealed, is entirely free from parasites, and it is well known that even the embryo within the body of the mother is occasionally infested by them. What has been already said about the various species of animals, applies equally well to the different organs ; some are more liable than others to be inhabited by parasites. The outer skin of the body and the alimentary canal seem on the whole to contain the greatest number, and this because they are more easy of access: in man more than three-fourths of the total number of parasites are found in these two localities. Frequently the distribution of a given parasite is not confined to a single organ. There are numerous examples, however, of this—e,y., Trichina spiralis, when encysted, is found only in striated muscle, the sexually mature Cestodes and Echinorhynchus are confined to the intestines, and Phthirius pubis only inhabits those parts of the body that are thickly covered with hair; but the converse is almost more general, Cysticercus cellulose, for instance, infests the intermuscular at the enormous number, for there were several thousands. This same helminthologist found on another occasion no less than 82 Ligule in the intestines of a diver, some of which were 6 to 8 ells long and 8 lines broad. Frequently the intestinal worms of an animal belong to several different species. Nathusius (Archiv f. Naturgesch., Jahrg. iii., Bd.i., p. 53, 1837), took from a single black stork 24 specimens of Filaria labiata from the lungs, 16 Syngamus (Strongylus) trachealis from the trachea, more than 100 Spiroptera alata from the coats of the stomach, several hundred Holostomum cxcavatum from the small intestine, and about a hundred Distoma ferox from the intestine, 22 specimens of Distoma hians from the cesophagus, 5 Distoma (D. hians ?) from between the coats of the stomach, and 1 Distoma echinatum from the small intestine. This forms quite a helminthological museum ; but Krause of Belgrade found even a greater number in a horse two years old—over 500 Ascaris megalocephala, 190 Oxyuris curvula, several millions of Strongylus tetracanthus, 214 Sclerostomum armatum, 69 Tenia perfoliata, 287 Filaria papillosa, and 6 Cysticerci/ (See van Beneden, ‘‘ Animal Parasites,” p. 91.) 1 Von Linstow has recently published a useful compilation of the distribution of Helminths, which is the most complete in existence : ‘‘ Compendium der Helminthologie,”’ Hanover, 1878. Digitized by Microsoft® 12 OCCURRENCE OF PARASITES. connective tissue, the brain and eye, and many other localities; Echinococcus is found in the liver, spleen, kidney, lung, bones, nervous centres, beneath the skin, and, in short, almost everywhere in the human body. Similarly Filaria papillosa of the horse is not only found in the peritoneal membrane, but also in the peripheral connective tissue of various parts of the body, and occasionally in the body-cavity, inside the skull and vertebral column, sometimes even in the eye, either in the outer layers, the anterior chamber, or the vitreous body. The same principle holds good in regard to the relations of a parasite to its host. Some species are limited to a single host; others, again, are parasitic upon several animals, not merely at different periods of their existence—passing their youth in the body of one animal, and attaining their maturity in that of another—but also during the same phase of development. In the first group may be reckoned among human parasites, Pediculus capitis, Bothriocephalus latus, and Oxywris vermicularis; also Tenia crassicollis of the cat, and Echinorhynchus gigas of the pig. To the second group belong by far the greater number of parasites, such as Strongylus gigas, which is found in many Carnivora—in the genera Canis, Mustela, Nasua, &c., in the horse, the ox, and in man; Trichina spiralis, which not only infests man, the pig, and the rat, but also the hedgehog, fox, martin, dog, cat, rabbit, ox, and horse, and may be trans- planted even to birds. Distomum hepaticwm has also a very wide distribution among warm-blooded animals, being found in most Rumi- nantia, Perissodactyla, Pachydermata, and Rodentia, and also in the kangaroo, in man, &c. Although it is a general rule that a para- site infests several distinct animals, it is equally true that the distribution of parasites is governed by certain laws. The examples just cited show this clearly. The various animals which are infested by one and the same parasite are always more or less closely related to each other. It is most usual to find that the related species of a given genus, or the genera of a given family, harbour the same para- sites ; there are, indeed, only very few exceptions to this rule, such as Trichina spiralis, But even in these rare cases a certain relation can be observed between the different hosts ; and a parasite which in the same stage of its existence infests sometimes a mammal or a fish, sometimes a mollusc, is quite unknown. This fact becomes more evident when we examine not merely the number of hosts in which a given parasite is met with, but also the statistics of the distribution of parasites, and discover the number of times which it is found in each host ; for instance, in other words, Distomum hepaticum is only rarely found in man, the kangaroo, and rodents, while it is commonly met with in ruminants, especially in the sheep. Th holds ¢ Digitized by Microson® ne 20's good RESPIRATION OF PARASITES. 13 in the case of Strongylus gigas, which is far more abundant in car- nivorous than in herbivorous animals, while it has only been met with a few times in man and other hosts. By the help of the statistical method it is easy to find out what are the animals most frequented by a given parasite, and the results obtained show that the several hosts are always more or less allied to the one in which the parasite is most commonly found. The causes of this are no doubt various, and partly of a kind which will be discussed later on, when we come to examine into the life-histories, origin, and migrations of parasites ; but for the present it may be remarked that these causes are to be looked for partly in the hosts themselves (in their distribution, habits, manner of locomotion, and their food), partly also in the nature, con- dition, and needs of the parasites. The factors which we are considering now are nearly the same as those which govern the relations between carnivorous and herbivorous animals, inasmuch as they concern not merely the actual carnivorous instinct, but also the choice of the prey. We need not be surprised, therefore, that a carnivorous mode of life, as has been already pointed out, is unmistakeably related to a parasitic mode of life. A very cursory examination of the conditions of life proves that we are right in regarding the distribution of parasites as greatly de- pendent upon the nature of the host as well as of the parasite itself. It is clear, for instance, that a parasite with lungs and other organs that need a direct contact with the air, can only exist in those creatures whose structure and mode of life render this possible, and only in those parts of the body to which the air has free access. Thus, all parasitic air-breathing insects (including Arachnida) are, without exception, confined to terrestrial or amphibious animals, and generally to their external skin. The walrus, for example, harbours a Pediculus of considerable size. On the other hand, the external parasites of aquatic animals generally belong to the Crustacea or some group which breathes by means of gills, and therefore needs direct contact with water. Worms breathe by means of the skin, and hence the parasitic members of this group—the so-called Helminths—are some- times found as ectoparasites upon aquatic animals, while they are usually met with only in the interior of terrestrial animals, in organs where they are bathed in the oxygenating fluids of their host. Para- sitic worms are also found in similar situations within the bodies of aquatic animals, but this is quite intelligible, inasmuch as they are of all parasites the most widely diffused ; in fact, internal parasites, or “ Entozoa,” as they are usually termed, are mainly worms. With this wide distribution may be coupled the fact that parasitic worms are numerically fav Mére Wb tHaantvhen parasitic Arthropoda ; 14 OCCURRENCE OF PARASITES. the latter, moreover, live in comparatively similar circumstances, while the conditions of entoparasitism are most varied. It must not be forgotten, however, that there are a few entoparasi- tic Arthropoda belonging to the Insecta and Arachnida. The most strik- ing example is furnished by the Pentastomida (Fig. 5), which, during the early stages of their existence, inhabit the internal organs of both terrestrial and aquatic animals, and on this account were included by the earlier helminthologists among the Helminths proper. A closer in- vestigation has shown that this classification, though hardly to be wondered at, is erroneous ; the Pentastomida are undoubtedly to be re- ferred to the Arachnida, but they differ from them by the entire absence of lungs, and in this respect approach the intestinal worms. Many mites also (Fig. 6) possess no respiratory organs, and agree with the Pen- tastomida in breathing by means of the skin ; this is facilitated by their small size, which implies a relatively large surface, and by the fact that they are usually to be found in damp situations, sometimes imbedded in the epidermis (Sarcoptes), almost entoparasitic, sometimes upon the hairy portions of the skin (Dermatodectes, &c.). But these instances must not be considered as proving that all entoparasitic Arachnida and H it is ! ata ABI Hail dia ste Fic. 5.— Pentastomum denticulatum from Fic. 6.—Sarcoptes scabiei. the liver of man, Insecta differ from their immediate allies by the absence of respiratory organs. On one oe the PAV TE, possess the normal tube-like AIR-BREATHING ENTOZOA. 15 lungs (the so-called “ trachez ”), and need therefore a direct contact with the air. To understand this properly, we must remember that contact with the air is by no means confined to the outer surface of the body ; many of the internal organs are either continuously or occasionally in communication with the outer air; and all these organs, in spite of their position in the interior of the body, are occasionally inhabited by air-breathing parasites. We often find the larve of flies within the nose and frontal sinuses of mammals, especially the sheep (Oestrus ovis) ; sometimes, as has been recently reported from Guiana, in man himself (Lueilia hominivora and Sarcophaga Wohlfarti, both belonging to the Musci- dee) ; the larve of flies (Musca vomitoria, Anthomyia canicularis, Figs. 7 and 8) are also sometimes found in the intestine, especially in its interior portion, where air frequently enters along with the food; in- deed the larvee of Gastrus equi are almost constantly found in the horse in this situation. Other air-breathing parasites live below the skin of mammals (as the larvee of Oestrus and the chigoe), and dwell not in enclosed spaces, but in passages open to the air; in these cases the apertures of the respiratory organs of the parasite are generally turned towards the exterior, to permit of a free exchange of air. Simi- larly, in parasitic larvee within the body-cavities of insects, the hinder Fie. 7.—Larva of Anthomyia canicularis from the intestine of man. Fie, 8.—Larve of Musca vomitoria. portion of the body with its tracheal opening is usually (as in the chigoe) protruded through the outer skin of its host, or is in com- munication with the trachee of the latter. The occasional presence of dipterous larvee in wounds, abscesses, even in the vagina, and under the preeputium and eyelite/ {gave WAGRRS QHD is easy to understand, 16 OCCURRENCE OF PARASITES. after what has just been said, since these parts of the body, being on the outside, are precisely the situations most convenient to parasites of this kind. Where respiration is impossible, there can evidently be no air-breathing parasites, and all notices of fly-larve discovered in such situations, as for example, within the internal urinary passages, are to be regarded as mere fables. The absolute need of access of air, which parasites of this description have, can be easily proved by experiment. Ihave frequently introduced the larve of Musca vomitoria at all stages, even as eggs, into the body-cavity of dogs and rabbits through apertures in the abdomen, but never in a single instance observed any further development take place; in most cases they died very shortly. From the foregoing remarks, it follows that parasites may be divided into two groups—ectoparasites (Epizoa, external parasites) and entoparasites (Entozoa, internal parasites). I am well aware that in certain cases this distinction is not more easy to make than that be- __ tween internal and external organs, and that the two groups by no means include all the peculiar forms of parasitic life; but it is on the whole convenient to retain it, to express the general conditions of parasitism with which we are for the present concerned. The ectoparasite inhabits the most readily accessible organs of the body of its host, which it frequently abandons at pleasure. The group which we have already alluded to as temporary parasites are, with a few exceptions, ectoparasitic. In the same way, the semi-stationary parasites are usually found upon the outer skin, where the least hindrance is offered to their movements, while the entirely stationary parasites are more commonly met with in the internal organs. It follows, therefore, that ectoparasites can generally be recognised as such by their outward form, especially by the structure. of their organs of locomotion. In certain ectoparasites, which have but a slight locomotive capa- city, there are usually found, either upon the organs of locomotion (Fig. 2), or (as in ectoparasitic worms) in their stead, powerful organs of attachment, which are generally more strongly developed Ga, in the Entozoa. These structures enable them to cling very firmly, and prevent them from being detached by the movements of the animal upon which they live. The great differences that exist between these organs in different parasites are greatly dependent upon the mode of life of their host and the structure of its outer skin. With regard to respiration, the ectoparasite, as has been alread remarked, depends upon its host, and shares with it the same cen tions of life. It usually possesses special organs of respiration, especially when living upon terrestrial animals, and being, therefore. in di Digitized by Microsoft® "71 direct ORAL APPENDAGES—ENTOZOA, 17 contact with the air. The possession of these organs is an almost exclusive attribute of ectoparasites ; for the Entozoa belong, with a few exceptions, to the group of worms which breathe by means of the skin. The Entozoa, besides having no special respiratory organs, are also with- out pigment, the skin being whitish and transparent: in this they agree with many other creatures, which, like themselves, are removed from the influence of light. The ectoparasites, on the other hand, especially the temporary parasites, agree in these respects with free-living animals. The modifications undergone by parasites, to adapt them to the various conditions, are also to be noticed in the structure of the mouth organs. Parasites upon the outer skin, of the higher Vertebrata at any rate, can obtain no other nutriment than a more or less firm horny sub- stance belonging partly to the epidermis and partly to the struc- tures that originate from it; it is needful, therefore, that they should possess some apparatus strong enough to gnaw through these hard tissues, and this we find, in the form of powerful jaws, in many lice, and especially in the Mallophaga. In the same way, parasites that feed upon the blood of their host must be able to bore through its epi- dermis, in order to reach their food and then suck it up. In these cases we find either mandibles, surrounded by a circular lip that plays the part of a sucker, as in the leech (Fig. 9), ora boring apparatus, as in the common lice, bugs, fleas, and mosquitoes, Ags which has the advantage of working rapidly, and is therefore specially adapted to these parasites which only visit their host for a short time. The necessity of a special mouth apparatus can only be dispensed with in those ectoparasites that live upon an animal which has a soft skin, as is generally the case in aquatic animals. The para- site, then, is provided with some contrivance that enables it to suck; generally a pharynx, or some Fic. 9.—Cephalic end muscular apparatus which allows of an alternate heirs Sines a widening and narrowing of the mouth cavity, or at the base of the oral which under other circumstances may cause merely cup. a peristaltic action. The Entozoa generally possess some apparatus of this kind in contradistinction to the ectoparasites, and are but rarely provided with jaws like the latter, except in a few cases, such as Dochmius duo- denalis (Fig. 10), which, although parasitic in the intestine, lives upon the blood of its host, and not upon the epithelial lining or contents of the intestine; and ispiajphigy BesHe cto therefore, analogous to an ectoparasite. Since most entoparasites are entirely nourished by B 18 OCCURRENCE OF PARASITES. the fluid or semi-fluid substances which surround them, the presence of the above-mentioned sucking organs is quite intelligible; they are Fie. 10.—Cephalic extremity of Dochmius duodenalis ; profile and front view. not, however, absolutely mecessary. Many Entozoa have no muscular pharynx, and are some- times even entirely destitute of an alimentary canal, and must absorb their food through the surface of the body, after the fashion of a plant, without the action of any further process. The Cestodes and Lchinorhynchus belong to this class, and their outer skin possesses the requisite permeability to a high degree, as may be easily proved by placing the animals in water, when they swell up rapidly. Of course, it is only substances dissolved in fluids that can find their way into the interior of the bodies of these parasites; but they usually live in situ- ations where they are surrounded by nutritive fluids to such an extent that they may be re- garded as almost swimming in them.? In all probability, this way of taking in nutriment by endosmosis is not confined to the anenterous norhynchus angustatus, forms, but exists generally among Entozoa, Leese ie “RCs though it undergoes various modifications in proboscis, with retractor Correspondence with the various differences of muscle, lemniscus, and structure in the outer covering of the body. ual organs. An in- . ‘ i testine is wanting.) From this point of view, the Entozoa may be + In the Rhizocephalida (Sacculina, &c.) we have recently discovered a group of ectoparasitic Crustacea that have no alimentary canal. They obtain their food like plants, by a number of branched prolongations, which pass through the body of their host and ramify in its intestine. They are found generally on the ventral surface of the abdomen of crabs. With resprgittectl ipvergxbing fP@asites see especially Kossman, “Suctoria and Lepadide :” Heidelberger Habilitationsschrift, 1873. ENCYSTATION OF ENTOZOA. 19 regarded as really an integral part of their host; they are quite com- parable, in respect of the way in which they are nourished (and breathe), with a cell, or an embryo. They manufacture their food in a precisely similar manner out of the juices surrounding them, which, by chemical change, minister to the conditions of their life and growth and remove their waste products. The presence of a mouth and intestine is not, however, rendered superfluous by the universality of this method of taking in food by endosmosis ; they not only enable their possessor to feed upon other semi-fluid or solid matters,+ but also serve the purpose of increasing the absorbent surface in cases where solid food-matter is not utilised. All that has been said hitherto refers to Entozoa that live in absolute contact with the tissues of their host, which is generally, but by no means always, the case. In the parenchymatous organs, a membranous cyst usually surrounds and isolates the parasite, with which it has no direct connection. It is a part of the infected organ, a hypertrophy of the surrounding connective tissue, which gradually encloses the parasite completely; a similar cyst is, indeed, formed round other foreign bodies introduced into the organ, and becomes very like a serous membrane, owing to the development on its free surface of a more or less thick epithelial layer (endothelium).— (Figs. 12-14.) Fig, 12.—Cysticercus pisiformis Fic. 13.—Echinococcus. (young). This capsule is regarded, and no doubt rightly, as an organ for the protection of the infected part; but, at the same time, it must not be 1 How great an influence the quality and abundance of the food has upon the parasite is strikingly shown in the case of Polyst integerri . This Trematode usually inhabits for a short time the gill cavity of tadpoles, and then wanders into the bladder, where it becomes sexually mdtieufiz abbut/lolbf/ gears 0 fs remain longer in the branchial cavity, it only takes twenty-seven days to reach sexual maturity. It is not merely the 20 OCCURRENCE OF PARASITES. forgotten that it is of no less importance for the nourishment of the contained parasite. The blood-vessels which traverse the capsule, and occasionally form a definite system with afferent and efferent vessels, supply fluid nutriment, which is absorbed by the parasite through its mouth or skin, and which varies in quality according to the structure of the capsule. On the whole, it appears that worms encysted in parenchymatous organs do not receive a great deal of nutriment, since they often remain unchanged for years, and even longer periods, while the same worms, under other circumstances— by migration to the intestine, for instance—rapidly grow and undergo further development. This capsule is most conspicuous in the so- called bladder-worms, especially in those which grow to a large size, and inhabit organs rich in connective tissue. In these cases (Zehino- coccus) the cyst becomes occasionally several millimetres in thickness, and so firm that it can be easily removed without injury from the surrounding parenchyma.—(Fig. 13.) Traces of a cyst are, however, found in all worms which remain for any length of time in parenchymatous organs, even when they only attain toa small size. In these cases, how- ever, the hypertrophy of the con- nective tissue, caused apparently by the irritation set up through the presence of the parasite, can hardly Fre. 14.—Selerostomum tetracanthum, be recognised as a continuous (inde- encyated. pendent) cyst. Many worms which inhabit parenchymatous organs secrete on the inner surface of their connective tissue capsule a cuticular cyst, which is, of course, sharply marked off from the former by its histological structure. It appears in the form of a homogeneous membrane, consisting of concentric layers; it resists the action of alkalies, and belongs, ap- parently, to the chitinous formation so generally met with in the lower animals.1. This chitinous cyst is most usually found in the Trematodes, but is not wholly absent in the other groups, being found, for example, in Tetrarhynchus, which lives in fishes, and even in the muscle-7richina (Fig. 15), the cysts of which are nothing rapidity of development which characterises these individuals ; they differ also from the common form in a number of anatomical peculiarities, notably by the absence of copulatory organs. See the interesting observations of Zeller, Zeitschr. f. wiss. Zool., Bd. xxvii., p. 238 et seg., 1876. 1 Waldenburg is of opinion that this chitinous capsule is in some cases formed by the host. See Archiv f. patheh figts a Dy Witbr sores” p. 157, 1862. SP is St CALCAREOUS CYSTS. 21 else than a calcareous excretory product of the worm itself, on the outside of which there lies the connective tissue capsule. Fic. 15.—Trichina-capsule, with connective tissue covering (in situ), in B calcified. Digitized by Microsoft® CHAPTER III. THE THEORY OF THE ORIGIN OF PARASITES REGARDED HISTORICALLY. WERE the parasites infesting animals confined to those which are temporary and external, their origin and descent would present no difficulties to the observer. But numerous forms are found deep in the interior of living bodies, in the brain, kidneys, and other apparently inaccessible organs. It is very surprising, when we expect to meet with only the blood, nerves, and other constituent tissues of the body, to find independent living animals, frequently of large size, which have left no trace to show how they reached their dwelling-place, and, indeed, are often incapable cf moving about. Under these circumstances, we can easily understand that the presence of parasites has an unusual interest, and that their origin was one of the subjects most eagerly investigated by biologists. The importance of parasites from a medical as well as from a zoological point of view, caused both physicians and naturalists to examine more closely into these facts, which appeared as mysterious and incomprehensible as the origin of life itself. In its most general aspect, the question of the origin of internal parasites can be answered only in two ways; either they originate in the tissues in which they are found, or else reach them from the external world. In the former case, they must be spontaneously generated ; in the latter they may, after the ordinary method, be developed from fertilised ova. Indeed, all the conjectures and hypotheses as to the origin of Entozoa brought forward in former centuries can be reduced to these two theories, though the greatest diversities, depending partly upon the views current at the time, and partly upon individual opinions, are to be seen in the way in which these theories were stated. Where facts are silent, there imagination is the more eloquent ; and it is only in our time that a definite solution has been put forward as to the origin of parasites, which rests at the same time time on a firm basis of fact. As long as it was believed that a “generatio aequivoca,” or “spontaneous generation,” as it is usually termed, was a pheno- menon commonly met with among the lower animals, the origin Digitized by Microsoft® ORIGIN OF INTESTINAL WORMS. 23 of intestinal worms could be readily explained. They were a striking instance of spontaneous generation, the existence of which was already contended for in by far the greater number of the lower animals. At most, the discussion related to the particular formative material out of which the newly created organism was. made, and whether it was first an egg or appeared at once as an adult. Sometimes it was the blood and juices of the body, at another time the excretions of the alimentary canal or the digested food, that was supposed to be the formative substratum of the spontaneous generation ; and it was disputed as to whether fermenta- tion or putrefaction, or a special organizing principle, gave the first impulse to its creation. These were the opinions held by the Ancients, and throughout the Middle Ages, so fruitless in scientific research. It was not until the seventeenth century that the theory of the generation of animals was reformed, and at the same time an entire revolution in the opinions as to the origin of Entozoa inaugurated. The researches of Swammerdam and Redi had the most profound influence, and entirely contradicted the earlier theories, that sexual generation was confined to the higher animals. They showed that sexual generation, precisely similar to that of birds, mammals, &c., was found in many of the lower animals; such as the insect, whose development and metamorphosis were for the first time worked out by these two naturalists ; not even the parasitic insects being neglected. Redi clearly proved by his researches and experiments that the maggots, which had been formerly considered as independent organ- isms (Helcophagi), were in reality the larve of flies, and that they were only developed when the fully formed insects were allowed access to deposit their eggs.1 Swammerdam, in the same way, showed that lice were developed from eggs ;? he was also well aware (according to the communications of the painter O. Marsilius) that the parasitic larvee in caterpillars were the offspring of insects that were in the habit of laying their eggs beneath the skin of these same caterpillars. * With respect to the intestinal worms, neither of these observers brought any direct evidence against the generally received opinions. Certainly not Redi, who put forward a view as to their origin, which differed only by a somewhat metaphysical tinge from the widely spread theory of generatio wquivoca. Swammerdam expressly guards against any application of his experiments concerning the development of insects to the Entozoa. It appears, indeed, as if he 1 Redi, “ Esperienze intorno agl’ insetti,” 4. i. p. 28: Venezia, 1712. 2 “ Bibel der Natur” (aus dem Hollandischen iibersetzt), p. 37, 1752, 8 Ieid., p. 281, Digitized by Microsoft® 24 THEORY OF THE ORIGIN OF PARASITES. chiefly wished to prevent it from being thought that intestinal worms were derived from insects and other free-living animals ; nevertheless, he does not deny the theory that they originate from the eggs of species “that have already existed in the intestines of other animals.” But in spite of the anathemas which Swammerdam hurled against the theory of the heterogeneous development of the Entozoa, this theory shortly after was very generally accepted. While, on the one hand, the existence of sexual generation in animals was being shown to be more and more universal, and became more definite, the microscope, newly applied to scientific researches, revealed a whole host of minute creatures, which, in spite of their wide distribution, had hitherto escaped attention on account of their small size. Animalcules were found in drinking water and in food, in the earth, and were supposed to exist even in the air: was it not natural that under the influence of these discoveries the theory of the heterogeny of Entozoa fell upon a fruitful soil? The introduction of these creatures into the human body appeared almost inevitably to lead to the conclusion that, when acted upon by the warmth and abundant nutriment in the body, they increased in size and became veritable Entozoa. It is not surprising, therefore, that men like Boerhaave ! and Hoffmann * traced back the Cestodes and Nematodes to animals which, when existing in the free state, were totally” different in appearance. The creatures that were supposed to be the progenitors of the Entozoa were by no means the Infusoria alone, but sometimes other larger creatures, such as free-living worms, and specially such worms as possessed a superficial resemblance to the Entozoa. Although a theory of this kind appears to us now entirely unscientific, we must not forget that at that time discoveries in the metamorphosis of animals were too recent and incomplete to allow of a just appreciation of the law of stability of species and their cyclical development. The actual nature of parasitic worms did not long remain unknown. Not only did naturalists gradually come to see that the occasional change of free-living animals into Entozoa was in entire contradic- tion to the common phenomena of generation and development, but they learnt to recognise the Entozoa as sexual animals, whose organic structure marked them out as representatives of special classes of animals. ice At the same time, however, it appeared that these creatures did not exist exclusively as Entozoa, but that they were also capable of a free existence. By a careful and systematic examination of our rivers and streams, a number of animal forms were discovered that appeared 1 Aphorism.,” 1360. 2 Opera,” t. iii, p. 490, Digitized by Microsoft® ORIGIN FROM FREE-LIVING ANIMALS. 25 strikingly like the Entozoa, and sometimes were even actual Entozoa. Of special import was the discovery of a tape-worm in fresh water by Linné,t and subsequently by several other naturalists in different places. We now know that this tape-worm (Bothriocephalus v. Schisto- cephalus solidus) inhabits originally the body-cavity of the stickleback, which it abandons at a certain stage of its development, and passes some time in the water, being finally swallowed by a water-fowl.? Linné, however, did not know these facts, and regarded the worm without hesitation as a young and incomplete specimen of the large human tape-worm (Bothriocephalus latus), and believed, therefore, that this worm was conveyed into the body from the exterior, where it already existed fully formed, in water. Moreover, this assertion was not confined to the tape-worm ; Linné believed that he had also dis- covered the liver-fluke of the sheep and the Oryuris of man leading a free existence; but there is no doubt that he mistook a Planarian for the former and one of the free-living Anguillulide for the latter.® However small the evidence was, it appeared sufficient to esta- blish this idea, which was believed in by many naturalists after Linné, chiefly because the facts known at that time about the Entozoa, as well as other parasites which we shall have to consider, were extremely fragmentary. To illustrate the small degree in which helminthology was known at that time, it may be mentioned that in spite of the vast numbers of existing Entozoa, not more than a dozen —and these almost entirely human parasites—had been described. Soon after this commenced a new era in helminthology. The knowledge of intestinal worms, which was till then chiefly of medical interest and cultivated by medical men, gradually began, under the influence of the Linnean school, to attract the attention of zoologists. Men of high ability and wide knowledge, like Pallas, O. F. Miiller, and others, bestowed upon this science their special attention, and increased our knowledge of parasites in all directions. But every new parasite and new host rendered less probable the idea of Linné that these animals lived sometimes freely and sometimes as parasites. The number of known Helminths soon became very considerable, but all attempts to find them living in a free state were in vain, 1 Ameenit, Acad.,” t. ii., Erlange, 1787, p. 93. 2 Steenstrup, Overs. A. dansk. Videnskab. Selsk. Forhandl., p. 166, 1857: Zeitschr. d. gesammt, Naturwiss., Bd. xiv., p. 475, 1859. In a similar manner Ligula frequently leaves the body of the fish in which it is parasitic at a certain stage of its development, and leads a free life, see Bloch, ‘‘ Abhand]. von der Erzeugung der Eingeweidewiirmer,” p. 2, 1782. 3 “Systema Nature,” ed. x., t. 1, p. 648. Fasciola hepatica, ‘‘ habitat in aquis dul- cibus ad radices lapidum, inque hepate pecorum.” Ascaris vermicularis, ‘habitat in paludibus, in radicibus plantarum putrescentibus, in intestinis puerorum et equi.” Digitized by Microsoft® 26 THEORY OF THE ORIGIN OF PARASITES. and yet no locality was left unsearched; and gradually arose the conviction that the statements of the free existence of intestinal worms were, in the majority of cases, based upon a confusion of these worms with others closely resembling them, and that in those instances (¢,g., the tape-worm found by Linné), where an intestinal worm appeared to — have been found living free, the discovery could not be interpreted in the sense Linné supposed. A new theory took the place of the old one. Basing his opinion upon the facts that the eggs of intestinal worms are expelled with the feeces of the animal in which they live, sometimes enclosed in a por- tion of the body of their parent, as in the Cestodes, and that they remain unaltered for a long time in water, Pallas} put forward the view that Entozoa agree with other animals in originating from eggs which can be carried from one animal to another. “It cannot be doubted,” he says, “that the eggs of the Entozoa are scattered abroad and undergo various changes without loss of vitality, and that im- mediately they reach the body of a suitable animal, through the medium of its food or drink, they grow into worms.” Of course the eggs in this way could only reach the alimentary canal; but since the Entozoa were found not only here, but also in other organs—liver, muscles, brain—the only possible explanation was that the eggs entered the blood-vessels from the intestine, and’ were carried “by the blood stream ” to those various and apparently inaccessible organs. By the help of the blood-vessels, Pallas believed that the eggs occasionally reached the body of the embryo before it was born ; in this way intestinal worms could also be inherited by one host from another. This was not, however, the first time that the theory of the “inheritance of Entozoa” had been propounded. Even in the days of Leeuwenhoek, Vallisnieri had endeavoured to explain the presence of Entozoa by supposing them to be transmitted from parents to children; and this hypothesis had many supporters, including certain of his illustrious contemporaries (Hartsoeker, Andry, &c.) and numerous later helminthologists, as O. F. Miiller,® Bloch, and Goze. On this hypothesis, the intestinal worms must have originated in the way just indicated; they must have been innate, or at least have been received by direct transference (for instance by kissing or being suckled). Otherwise, a subsequent 1 Neue nord. Beitrige, Bd. i., p. 48, Bd. ii, p. 80. 2 ‘© Opere fisico med.,” t. i, 1733. 3 Naturforscher, Bd, xiv., 195, 1780. Neucs Hamburger Magazin, Bd. xx., 1784. + “ Abhandlung von der Erzeugung der Eingeweidewiirmer: ” Berlin, p. 37, 1782. 5 ‘ Versuch einer Naturgeschichte der Eingeweidewtirmer:” Blankenburg, p. 4, &c. 1782. Digitized by Microsoft® THEORY OF INHERITANCE. 27 migration was disproved. The eggs which are extruded with the feeces are, as far as the intestinal worm is concerned, lost,—though they may serve as food for other animals (Goze). It was certainly astonishing that by far the greater number of eggs should incur this fate, but even this fact was brought into accordance with the theory. It was asserted that the intestinal worms, which could not, like other animals, deposit their eggs in a chosen place, must leave it to chance whether they passed into the blood-vessels or not ; and furthermore, that the probability of such a haphazard transference was far less than that they should be extruded from the body before it could take place (Bloch). That this view, under the influence of that theory of evolution which was then dominant, degenerated into wonderful subtleties and refinements iu many of its supporters, must not be considered as due to the theory itself;+ but in other respects it shows so many weak points, that it seems hardly necessary to refute it by calling to mind the various worm-epidemics (sheep-cough, liver rot, &c.), or the Cenurus that almost invariably kills its host, and generally before it arrives at sexual maturity. The influences, however, which led to this opinion are not difficult to understand. On the one hand was the undeniable fact of the sexuality of the Entozoa and their striking fertility ; on the other, the difficulty, apparently even impossibility, of tracing the origin of these animals to the eggs extruded with the feces. The idea of hereditary transmission seemed to afford a way out of this dilemma, and appeared all the more feasible, seeing that many observers stated that they had found Entozoa not only in the young of animals, but even in the embryo within the body of the mother. “Whether the cases here alleged were reliable or not,? is a matter of indifference to us, but it is surprising, and hardly agrees with this theory of inheritance, that these cases were extremely few in number. It was, accordingly, hardly unjustifiable in Pallas to use not only the directly transmitted eggs, but also those evacuated from the body, to explain entoparasitism. He did not succeed, however, in proving his opinions by direct experi- ment, any more than his illustrious contemporary, van Doeveren,* who also endeavoured to explain the distribution of Entozoa by the theory of the transference of similar germs, but we must not forget to pay our acknowledgment to the clear and accurate perceptions of this great naturalist. Hntozoa do actually originate, as we now 1 According to Eberhard’s “ Neue Apologie des Socrates” (Th. ii., p. 333), the parasites were present as eggs during the age of innocence, but were hatched after the Fall. 2 For a list of these cases see Bloch, loc. cit., p. 38; also Davaine, ‘‘ Traité des Entozoaires,” 2me ed., p. 11: Paris, 1877. 8“ Abhandlung von den Wiirmern in den Gedirmen des menschlichen Kérpers,” p. 106: Leipzig, 1776. Digitized by Microsoft® 28 THEORY OF THE ORIGIN OF PARASITES. know well, from transmitted germs, and only in consequence of a process of generation, similar to that which exists in the rest of the animal kingdom. In spite of the accordance between our present knowledge and the theories of van Doeveren and Pallas, the one is not the direct outcome of the other. The path of science strays now on one side, now on another side of the direct line of truth, and we ought not, therefore, to be surprised that the theory just quoted was pushed out of the way by other theories before it had time to take root. With Pallas, Bloch, and Goze began a long list of helmintholo- gists, of whom the most eminent were Rudolphi and Bremser. Thousands of animals were examined for their parasites, and with such success, that the number of the known Entozoa was soon esti- mated at many hundreds. As the material got larger, the science of helminthology became gradually more and more separated from zoology, and treated as a distinct specialty. This distinction had its evil effects. It caused helminthology to become a mere descriptive enumeration, hardly at all concerned with the life-histories and de- velopment of the animals so carefully registered. This one-sided way of looking at parasites was hardly suitable for solving the ques- tions concerning their origin by careful and unprejudiced experiment. That all previous attempts to explain the presence of these remark- able creatures by the theory of their introduction into the body of their host from without were more or less conspicuously faulty was never at any time doubtful, and perhaps least of all at the present moment. Instead of increasing the number of known facts by the empirical method, and getting, where possible, fresh support on which to base some theory, which might, although not completely proved, furnish numerous and weighty arguments for induction, helmin- thologists were content to point out the insufficiency of earlier investi- gations, and return again to the almost forgotten theory of spontaneous generation,* which was at any rate a convenient and simple method of cutting the knot. Those were the times when the all-powerful “ vital force,” governed the organism. And it seemed an easy thing for this “ vitality ” to organize a mass of mucus, a villus of the intestine, ora fragment of con- nective tissue, perhaps by an intensified abnormal process of develop- ment, into a bladder-worm instead of a simple hydatid. The structure of the Entozoa was regarded as comparatively simple, and it appeared, therefore, that from this point of view no great difficulties stood in +See specially the excellent work of Bremser, ‘‘Lebende Wiirmer im lebenden Menschen,” pp. 1-16: Wien, 1819. Digitized by Microsoft® TEACHING OF RUDOLPHI. 29 the way of such a theory. The microscope had been for some time laid aside as a not very trustworthy agent, and a simple lens, or the naked eye alone, was sufficient to watch the process of spontaneous generation.t When this had once taken place, it was supposed that the Entozoa increased by sexual generation, or else to what purpose had they been provided with generative organs? The significance of this sexual generation had been kept in the background by the prevailing opinion of spontaneous generation, The majority of the eggs were extruded without being further developed, for, if not, the extraordinary fertility of these creatures would entirely fill their host with their progeny. The supporters of this view, from being well- known authorities in their subject, had such weight, that they readily crushed the evidence advanced against their theory by other observers. This misfortune was partly the fault of their opponents, generally men like Brera,? who, in spite of all other qualifications, was ignorant of the necessary details of the subject. As long as Rudolphi’s teaching was followed, and the theory of vitality was generally accepted, this view just stated of the origin of the Entozoa was the only one that obtained credence; and it appeared to be strengthened by the discovery of a continually increasing number of bladder-worms and encysted Helminths which were entirely destitute of organs of generation, and were unable, therefore, to propagate themselves by the sexual method. Except by a theory of spontaneous generation, the existence of these worms appeared inexplicable. And yet this appearance was decep- tive—so much so, that it is by the help of these very sexless worms that we are now able to show the error of Rudolphi’s theory. The general acceptance of this erroneous theory was not, however, over- thrown at a single blow. It was necessary to bring forward numerous facts in order to shatter the belief in spontaneous generation, and set a more credible theory in its place; but these facts would not have been recognised for a long period, had not a change in the direction and method of biological study given a fresh impulse to helmintho- logy. The majority of these fundamental facts were discovered by means of the microscope, which v. Baer, Purkinje, Ehrenberg, and others had again used for scientific investigation, whereby the most brilliant results had already been obtained in other regions of zoology. The first discoveries made by the microscope in helminthology had a most important bearing on the origin of the intestinal worms. In the year 1831 Mehlis made the remarkable discovery that the 1 Bremser, loc. cit., p. 65. Rudolphi, ‘‘ Entozoorum Hist. Nat.,” vol. i, p. 811, 1808. 2 ‘ Medicinisch-praktische Vorlesungen tiber Eingeweidewiirmer,’ p. 47 et. seq., 1803. Digitized by Microsoft® 30 THEORY OF THE ORIGIN OF PARASITES. eggs of certain Distomid contained an embryo (Fig. 16) which in shape and ciliation resembled an Infusorian; it was occasionally provided with an eye-speck, and after being hatched swam about like . in a Infusorian.1 What a blow was this simple discovery to the earlier theories as to the fate of the eggs of Entozoa. It had certainly been known from the time of Goze that there were a few viviparous Entozoa, but these were in every case thread- worms, whose young so closely resembled the parent form, that they might easily be supposed to attain to their full development without any migration or further change. In the case discovered by Mehlis, Fic. 16.—Ciliatedembryo however, the eggs had been laid, and the em- se asnee iigl capitella- twos, entirely unlike their parent, seemed, from their eye-specks and coating of cilia, fitted for a free existence. We recall at once the opinions expressed by Leeuwen- hoek and Pallas, and it is quite intelligible that von Nordmann, who first confirmed the observation of Mehlis, remarked that these parasites, instead of originating by spontaneous generation, “ always sojourn during their first life-period in water, and subsequently enter the body of some animal, where they lose their eye-specks and become sexually mature.”* Von Nordmann certainly acknowledges that this sounds “fabulous” in comparison with the generally accepted theory, but, after further reflection, he insisted upon it, since he found in the gut of a Neuropterous larva, three-quarters of a line long, a species of Nematode with a conspicuous red eye, which was found also living independently in water. Soon afterwards von Siebold* added to these observations the remarkable fact that the ciliated embryo of Monostomum mutabile (Fig. 17), a parasite of water-birds, sheltered within its body another creature (a “necessary parasite,” as it is termed), which so strikingly recalled the “kingsyellow worm” (Redia), found by Bojanus in pond- snails (Fig. 18), that one might almost believe “that this creature continued to live after the death of its jailor and perhaps grew into a similar form.” Unfortunately this idea could not be proved, although its demonstration would have been of the greatest importance. Von Baer had previously shown that these Redie* gave rise, by a develop- 1 Oken’s Isis, p. 190, 1831. 2 “ Mikrographische Beitrage,” Bd. ii., p. 140, Note, 1832. % Archiv f. Naturgesch., Jahrg. i., Bd. i., p. 69, 1835. Burdach, ‘‘ Physiologie,” Bd. ii., p. 208: Leipzig, 1838. + Nova Act. Acad. Ces. Leop., t. xiii., p. 627, 1826. Digitized by Microsoft® PROOF OF METAMORPHOSIS IN TREMATODES. 31 ment of germ-granules in their interior, to a brood of animals which resemble the tailed Trematodes, but, unlike them, swim about freely Fic. 17.—Infusorian-like embryos of Fic. 18.— Bojanus’ “kingsyellow worms” Monostomum mutabile with the (Rediz) from the pond-snail. “necessary parasite.” . in water (Fig. 20), and had for this reason been included by the older naturalists among the Infusoria (under the name of Cercaria). Fic. 19.—Rediew. (A) with germs ; (B) with Cercariz in the interior ; (C) free Cercariz. Digitized by Microsoft® 32 THEORY OF THE ORIGIN OF PARASITES. The investigations of von Siebold were not confined to the eggs of Trematodes, but were extended to the eggs of other intestinal worms, and led to the important discovery that in the tape-worms also the egg contained an embyro before it was laid. Here also the embryo was totally different from the parent, a simple spherical mass, dis- tinguished only by the possession of six stylet-shaped hooks (Fig. 20) arranged in pairs at the anterior pole of the body, and capable of being moved like levers. The subsequent changes undergone by this embryo were for some time uncertain, though there was no doubt that they could only pass into the fully formed animal “by a kind of meta- morphosis.” Fic. 20.—Eggs of the tape-worm with six-hooked embryo. Whether von Siebold perceived at that time the important bearings of his observations, must be left undecided. In any case, he neglected to foliow them up to their legitimate consequences. This was done some years later by Eschricht,? who fully discussed the question of the origin of Entozoa for the first time since the days of Bremser, and who had also, in his masterly researches upon Bothriocephalus latus,® decidedly opposed the idea of spontaneous generation. In this work Eschricht collected all the facts that had been lately discovered about the metamorphosis of intestinal worms, and endeavoured to support the view that these phenomena were commonly found among the Helminths. He adduced the great development of the generative organs and the fertility of the Entozoa (the number of eggs produced annually by a single Bothriocephalus latus must be reckoned at at least a million, and of a female thread-worm at 64,000,000!) as evidence that did away with the enormous difficulties besetting the theory of a transmission to “suitable localities.” Finally, he recalled the fact, first discovered by Abildgaard,* and also known to Bremser and Rudolphi, * Burdach, “ Physiologie,” Joc. cit. Previously to von Siebold, Géze had seen these embryos, but his description and figures (‘‘ Versuch, &c.” tab. xxii., figs. 20-22,) are so insufficient, and for the most part so incorrect, that no conclusions can be drawn from them. 2 Edin. New Phil. Journal, 1841. : 3 Nova Acta Acad, Ces. Leop., t. xix., suppl. 2, 1841. 4 Naturhistorisk Selsk. Skrifter, Bd. i., p. 58, 1790, ; see the remarks made on p. 24. Digitized by Microsoft® ESCHRICHT UPON THE ORIGIN OF INTESTINAL WORMS. 33 that Bothrtocephalus (Schistocephalus) solidus and Ligula only at- tained to full development when they passed from the body-cavity of a fish to the intestine of a water-fowl, and stated that in all proba- bility many other Helminths wander in a similar way from one organ of their host to another. By these and other facts, Eschricht arrived at the conclusion that the life-history of Entozoa must be considered as analogous on the whole to that of the parasitic larvee of ichneumon-flies and bot-flies, but that each instance demands a special explanation, on account of the complexities possibly introduced. In the meantime, however, no details could be given, but in all pro- bability the various asexual parasites so frequently met with encysted in the muscles and connective tissue, such as bladder- worms, Filaria (including Trichina spiralis), and Echinorhynchus—the latter being occasionally during the summer found in thousands in the flesh of fishes—must be regarded as immature forms, retaining their primitive larval situation. We shall find later on that Eschricht had hit upon the truth in pointing out that change of place and of form were the most im- portant facts in the life-history of parasites. But there were none of the necessary details forthcoming to prove his explanation, and it appears, therefore, in spite of his statements and the support _ which Valentin’s! observations lent to them, that the majority of helminthologists continued to uphold the old theory of the spontaneous generation of intestinal worms. ? But gradually more and more light was thrown upon the obscurity which enveloped the whole subject of parasitic worms. Shortly after the publication of Eschricht’s researches appeared Steenstrup’s famous work upon the alternation of generations, which rendered intelligible so many facts in the developmental history of the lower animals that had been previously but incompletely appreciated. The discoveries and arguments brought forward by Steenstrup proved conclusively that there are animals whose descendants in the second or third gene- ration return to the original form of the sexual animal, and that numerous intestinal worms belong to this class. The proof of alternation of generations was most completely obtained from the Trematodes,* and quite simply, for Steenstrup con- nected their life-history with the above-mentioned Cercariw. By discovering that these latter, in spite of their independent origin, were really larval Trematodes, he determined the fate of a large group of 1 Repertorium f. Anat. u. Physiol., Bd. vi., p. 50, 1841. 2 Creplin, Art. ‘‘ Enthelminthologie” in Ersch u. Gruber’s ‘‘ Allgem. Encyclop. d. Wiss.,” Leipzig, 1818-1846, Bd. xxxv. 3 “Ueber den Generationswechsel :” Copenhagen, p. 50, 1842. Translation by Ray Society, London, 1845. Digitized by Microsoft® Cc 34 THEORY OF THE ORIGIN OF PARASITES. parasites. Steenstrup was not content with solving the enigma merely by hypothesis; he also endeavoured by direct observation to place his Fie. 21. opinions beyond any doubt. He discovered that these Cercarie (Figs. 21 and 22) fre- quently made their way into the body of Fic. 22. water-snails by boring through the muscular wall, and that, after losing their tails, they became encysted, resembling closely small and as yet asexual Trematodes. These facts were certainly not absolutely new, but those few naturalists who had anticipated Steenstrup in the discovery of the encysted condition of Cercariz, erroneously formed the opinion Fras, 21 and 22.—A free that this process, instead of being the pre- and an encapsuled Cercaria, cursor of a further development, led merely whe everett, to the death of the parasite. Moreover, Steen- strup himself fell into an error when he supposed that the tailless Cer- caria arrived at complete maturity within the body of the original host; von Siebold,1 who shortly after adopted the opinions of the illustrious Dane, rightly compared the development of the Cercaria to that of Bothriocephalus (Schistocephalus) solidus and Ligula, and believed that its further growth would not take place until the original host was devoured by some other animal. The older investigators (von Baer, see p. 30) had already demon- strated the origin of the Cercariz; but Steenstrup went further than his predecessors in showing the identity of the “kingsyellow worm” and the “living matrix of the Cercarize” with the “necessary parasite” within the body of the embryonic Monostomum, though the resemblance had been previously pointed out by von Siebold. According to Steenstrup, the egg of the Trematode, expelled from the body of- its host, gave rise to a free larva, which after a period of independent existence changed again into a parasite (the “generative sac”) after casting its skin. This parasite, how- ever, did not at once become a Distomum, but still remained a larval form (the asexual generation or so-called “nurse”), and in it was subsequently developed, asexually from germ-granules, another active larval form, the Cercaria from which the sexual adult then took its rise. If it had been known before that the life-history of an animal could be divided into several cycles, this process of development would have been thoroughly understood some years earlier. The 1“ Bericht wher die Leistungen, &c.,” Archiv f. Naturgesch., Jahrg. xiv., Bd. ii, p. 321, 1848. Digitized by Microsoft® STEENSTRUP’S THEORY OF ALTERNATION OF GENERATIONS. 35 material was to hand, but there was no one capable of using it. In spite of the close similarity between the Cercaria and the Distomum, no one ventured to state that one was the young of the other, since they had been found in the bodies of quite different animals. The phenomenon of alternation of generations threw a fresh light upon the well-known “sexless” Entozoa. According to earlier opinions, these were either independent, spontaneously generated organisms, or, as Eschricht thought, immature forms. The theory of alternation of generations rendered it possible that they played the part of an inter- mediate generation, the “nurse.” In fact, Steenstrup! had no hesi- tation in speaking of many of these forms, especially the bladder- worms, as “ nurses.” What stress was laid upon the migrations of the embryos by Steenstrup is sufficiently shown by his statement, based upon firm conviction, that the Entozoa are generally only parasitic for a longer or shorter period ; and that at other times, perhaps in different stages or generations, they lead an independent existence, or, as it is termed, “have a geographical distribution in nature (e¢g., in water) outside the body of a host.”? This opinion was strikingly confirmed by a new discovery. Dujardin® frequently discovered on the ground, and especially after a sudden rainfall, masses of a Filaria-like Nematode (Mermis), resembling very closely Gordius aquaticus, which had been known for some time as an inhabitant of water. This appearance (the so-called “worm-rain”) could only be explained by supposing that these creatures had left the bodies of insects or snails, upon which they are parasitic, for the purpose of laying their eggs in the damp earth. Von Siebold proved that this explanation was the right one, by finding not merely these Mermithide in the bodies of insects and insect larve, and observing them in the act of wandering away; but also by the discovery that Gordius was also sometimes parasitic. At the time of their migration the Gordiacee are mature; but copulation and ovi- position take place subsequently, in water in the case of Gordius, and in damp earth in the case of Mermis. Von Siebold succeeded later® in tracing the development of the embryo within the egg during the 1 Loe. cit., p. 111. 2 Loc. cit., p. 116, Note. 3 Ann. Sci. Nat., t. xviii, p. 129, 1842. (Similar observations have been fre- quently made since the publication of this paper by numerous observers, and among others by myself.) + Entomolog. Zeitung, p. 77, 1843. Villot has recently stated that the presence of Gordius in insects is an accidental wandering, while he believes that minnows and loaches are its normal hosts. See Archives de Zool. Expér., t. iii., p. 182 et seg., 1874. 5 Ibid., 1848, p. 290; 1850, p. 289,—Jahresb. d. schlesischen Gesellsch, fiir vaterl. Cultur, p, 56: Breslau, 1851 Digitized by Microsoft® 36 THEORY OF THE ORIGIN OF PARASITES. winter, and proved that the young larve hatched in the spring make their way into the interior of young caterpillars, just out of the egg— an important addition to our knowledge of the life-history of Entozoa. But before these observations had been brought to a close, von Siebold had already attempted to fashion the theory of the origin of the intestinal worms according to the newer views, which, as we have already seen, were receiving more and more support from the progress of discovery; and, with this end in view, he published a complete account of all the known facts relating to the development and gene- ration of these animals.1 This work, as might have been expected from the wide knowledge of the author and the well-deserved reputa- tion he enjoyed as a naturalist, made a great impression, and spread abroad the conviction that the secrets of the phenomena of ento- parasitism were to be sought for in the migrations and transference of parasites, and were not explicable by any hypothesis of spon- taneous generation. This work did not contain much that was abso- lutely new in the department of helminthology ; for even the opinion Fic, 23,—The common bladder-worm of the pig, with invaginated head (A), and with extruded head (B). as to the tenioid nature of bladder-worms (Fig. 23) had been some time previously advanced by Dujardin, although it was treated of in detail for the first time in the article by von Siebold, and based upon the striking resemblance (already pointed out by Pallas and Goze) between the head of the bladder-worm of the mouse and that of Tema crassicollis of the cat.? Concerning the development of the bladder-worms, von Siebold had, however, peculiar views. He did not agree with Dujardin in regarding them as larval stages or “nurses,” but considered them to be pathological formations caused by certain external circumstances, 1 Art. “Parasiten” in Wagner's ‘“ Handworterbuch der Physiologie,” Bd. ii., p. 640 : Brunswick, 1843. 2 « Hist, Nat. des Helminthes,” pp. 544 and 632, 1845. Digitized’ % y Microsotte DISCOVERIES OF VON SIEBOLD, DUJARDIN, AND VAN BENEDEN. 37 such as that the germ of the tape-worm had “lost its way ”—that is, arrived at some place that was not suitable to its requirements. We shall have to examine this theory of “straying” later on, but for the present it may be remarked that von Siebold believed it to be of very wide application, and to explain the existence of many other sexless worms (even Zrichina), which had not come to a full development on account of having strayed into unsuitable localities. Later von Siebold made the developmental history of tape-worms the subject of a special memoir,! in which he sought to prove, with special reference to Tetrarhynchus, that the tape-worm heads found so abundantly encysted in predatory fish, originated from embryos that had wandered there, and that these developed into the sexual adult by the formation of segments (Figs. 24 and 25), when their host was swallowed by some other carnivorous fish in whose ali- Fic, 24. i f A Ff ft Oe Py?) i Fics. 24 and 25.--Echinobothrium minimum Fic. 26.—Transformation of the (after van Beneden), isolated living head and bladder-worm into a tape-worm tape-worm. (Tenia serrata). mentary canal these chains of tape-worm segments were formed. Von Siebold based these arguments upon induction, but their cor- rectness was subsequently certified by a direct proof of the metamor- phosis and migration of these tape-worm heads. Contemporaneously with von Siebold, or even earlier, van 7 ZeitseitZE DY MiG sofro!*8, 1850, 38 THEORY OF THE ORIGIN OF PARASITES. Beneden had investigated the Entozoa of various predatory fish, especially the shark and ray,! and made observations at all stages. He frequently found in the stomach of the shark the digested remains of various osseous fish with Tetrarhynchus heads, some of which were still encysted, some free or nearly so, while others had already buried themselves in the intestine of their new host, and budded out a longer or shorter chain of segments. Van Beneden’s researches were so extensive, and dealt with so many different forms, that they fully justified the generalisation that the transference of asexual Entozoa takes place by means of the food of their host, which had been, up till the present time, only proved in the case of Ligula and Schistocephalus. It is not, however, our purpose here to enter particularly into van Beneden’s statements as to the development of Cestodes ; we shall recur to it in a future chapter, and content our- selves for the present with mentioning that a bladder-worm, according to this celebrated zoologist, is by no means a pathological condition, but is closely allied in structure and development to the head of a Tetrarhynchus. The correctness of this opinion was soon verified by a new ex- periment, which showed that bladder-worms, as von Siebold had previously stated was the case in certain forms, after losing the bladder, become developed into true tape-worms in the intestine of a suitable animal (Fig. 26). The history of helminthology does not, perhaps, contain a single other fact that created such a marked sen- sation. It was, however, not merely the proof that bladder-worms, which had for so long a time formed an impregnable fortress for the theory of spontaneous generation, were really the immature stage of tape-worms, that excited so wide an interest, but it was also the cir- cumstance that Kiichenmeister,? the discoverer of this fact, did not discover it merely by chance, but by~direct experiment, by the method of feeding, which is so easy to control and repeat, and has furnished the same results in other hands. The idea of using this method of proving the nature of bladder- worms was suggested by previous discoveries, but it had, notwith- standing, been made use of by no observer. I say no observer, for the attempts of Klenke in this direction * have really not the slightest claim to be mentioned. The method has only proved of value in modern times. The meaning of helminthological experiment was * “Tes vers Cestoides”: Bruxelles, 1850. (Preliminary account in the Comptes Rendus Acad, Belg., 1849). * “Ueber die Metamorphose der Finnen in Bandwiirmer,” Prager Vierteljahrsschrift, 1852. * “Ueber die Contayiositiit der Kingeweidewirmer :” Jena, 1844, Digitized by Microsoft® KUCHENMEISTER INTRODUCES HELMINTHOLOGICAL EXPERIMENT. 39 known to the older workers in this department. It has already been mentioned that Abildgaard in this way proved beyond doubt the migration of Schistocephalus solidus from the body-cavity of the fish to the intestine of the water-fowl. Also Pallas, Bloch, and Goze made the attempt to decide certain questions by the introduction of Hel- minths, or their germs, into various animals, without, however, getting any results of great importance. Besides the widespread belief in spontaneous generation, which arrested so powerfully the progress of helminthology, the manifest unfruitfulness of the experimental method gradually caused it to drop into oblivion. It was reserved for Kiichenmeister to reintroduce this method, and to show its importance for all time. A new and active vitality was thus breathed into helminthological science, so that observations and discoveries came thick and fast. Hardly a year had elapsed after the first trial of his method before Kiichenmeister announced? that he had succeeded in obtaining bladder-worms from the bodies of animals fed with the ripe proglottides, and thus com- pleted the whole cycle of the life-history of Cestodes.? The earliest experiment was made upon a sheep which died before the complete maturity of the bladder-worms, obviously on account of the experiment. Without a thorough knowledge of the development of the bladder-worms, which was the condition of naturalists at that time, the result of the experiment might have been doubted, had not Haubner ? and Leuckart* completely demonstrated that fact, by rearing almost all known bladder-worms, on an extensive scale, in suitable animals. But this experimental method was not confined to bladder-worms and tape-worms ; it was also applied to other Entozoa, and in these cases also the same facts were strikingly shown. De Filippi,® de la Valette,* and Pagenstecher? proved, by means 1 Giinsburg’s Zeitschr. f. klin. Med., p. 448, 1853. 2 T am unable to understand how Kiichenmeister can complain “ that German science hardly thanked him for the services that he had rendered ”—-(This passage is reproduced from the first into the second edition of his ‘‘ Parasiten des Menschen,” 1878, Preface)— nor yet why he reproaches me with neglecting no opportunity of attacking him in an unfair manner. On the contrary, I feel satisfied that I have always plainly stated what science does owe to him in the way both of discovery and suggestion (see Preface to the first edition of this work, p. iv.). I have also corrected his unfortunately numerous errors, but only in those cases where it could not be avoided. Had I really wished to attack him, there was plenty of material at my disposal, at any rate more than Kiichenmeister in his most recent work has endeavoured to bring up against me. > Gurlt’s Magazin fiir ges. Thier-Heilkunde, 1854 and 1855. + “Die Blasenbandwiirmer und ihre Entwickelung :” Giessen, 1856, p. 38 et seq. 5 « Mém. pour servir & l’hist. génét. des Trematodes:” Turin, t. i.-iti. 8 « Symbole ad Trematodum evolut. Hist. :” Berolini, 1855. 7 « Trematodenlarven und Trematoden :” Heidelberg, 1857. ‘Ueber Erziehung von Distomum echinatum durgh Bitexengyy AethinkoNeurgesch., Bd. i., p. 246, 1857. 40 THEORY OF THE ORIGIN OF PARASITES. of experiment, that encysted Distomes grew mature directly after their migration from one host to another, as von Siebold had believed, _ and that in this way a migration to another host, or another organ, was necessary in the Trematodes, before they could attain to sexual maturity. Although hitherto the various stages of development of the same species had not been experimentally proved,! as in the Cestodes, we must regard our knowledge as having been completed by the experimental verification of this fact, and especially by Zeller’s beautiful researches into the life-history of the ectoparasitic forms, especially Polystomum integerrimum of the frog,? which have com- pleted our knowledge in another direction. The parasitic Nematodes resisted investigation for a very long time. In the year 1863, when the first volume of my work on para- sites appeared, I was only able to mention a single Nematode besides the Gordiacee just referred to (p. 35), whose developmental history was thoroughly known. This was Z'richina spiralis, which Virchow’s*® and my own‘ researches showed to be developed in the intestines of rabbits, swine, and other Mammalia into a sexual form, which had previously escaped attention. The young of this worm are produced viviparously, and wander away from the intestine, becoming finally encysted in the well-known way in the muscles. Since this time, however, our knowledge has been considerably advanced by the observations recorded in the second volume of my manual. We are now acquainted not only with the life-history and migrations of the Acanthocephala, but also of numerous thread-worms belonging to different groups, and can show experimentally that the germs are extruded from the body of the host, and give rise to young, which, at a particular period of their existence, return to the body of their definitive host. Many of my further observations have greatly widened our conceptions of the parasitic mode of life, and have especially placed beyond a doubt the fact that a change of host is by no means a necessity in the life-history of all Entozoa. In many Nematodes, as we shall learn in the next chapter, the young stages are passed in an entirely free existence, and often (especially in certain Strongylide) under conditions similar to those enjoyed by free-living animals. The life-history of the Pentastomida shows, however, that a migration from one host to another is not confined * Even the experiments of Wagner upon Distomum cygnoides leave a considerable gap in respect of its migration into the body of the frog, ‘‘ Beitrag zur Entwickelungsgeschichte der Eingeweidewiirmer,” p, 29 ct seg. : Haarlem, 1857. ® Zeitschr, f. wiss, Zool., Bd. xxii., p. 1, 1872, and Bd. xxvii., p. 288, 1876. 5 Archiv f. pathol, Anat., vol. xviii., p. 830, 1860, * Zeitschr. f. vrationelle Medicin, Bd, viii., pp. 259 and 335, 1860. Digitized by Microsoft® FURTHER INVESTIGATIONS BY THIS METHOD, AND THEIR RESULTS. 41 to the Helminths. The development of these animals, which cer- tainly closely resemble the Helminths in their parasitic mode of life, was worked out by myself some years ago;} I succeeded in rearing the so-called Pentastomum denticulatum in the intestine of the rabbit from the eggs of Pentastomum tenioides, and traced the development of the young Pentastomum denticulatum into the adult Pentastomum tenioides by placing the embryo in the nasal cavity of the dog. Although these results of experimental helmin- thology appear so important and numerous, there yet remains much to discover. There are always forms, even among the most common of the Helminths, whose origin is unknown. We must admit that there is much in the natural history of parasites that seems problematical and hardly explicable, in accordance with our present experience, but who would dare, in the face of all the results that have been already achieved, to fill the gaps in our know- ledge with the remnants of some antiquated theory ? If Rudolphi and Bremser, and all the other cham- pions of former helminthological theories, were I present to-day, they would lower their colours, and py¢,27,—Pentastomum refuse to renew the old disputes. The theory they @enticulatum. fought for is an error that has been dissipated. Ha aR 11 ERS ya full gS 1 “Bau und Entwickelungsgeschichte der Pentastomen:” Leipzig, 1860. Preliminary account in Zeitschr. f. rationelle Mecdicin, Bd. ii., p. 48, 1857 ; Bd. iv., p. 78, 1858. Digitized by Microsoft® CHAPTER IV. LIFE-HISTORY OF PARASITES. Att that has been hitherto said about the origin, metamorphosis, and migration of parasites demonstrates plainly that the older observers, who denied any remarkable changes in their life-history, were entirely wrong. We now know that parasitism only repre- sents a single phase in the life of an animal, which, in spite of its importance and extent in many cases, always presupposes another stage. In fact, if we only know concerning a certain animal that it is a parasite, we know but little; thoroughly to understand its history, we must follow out all the separate stages and conditions of its existence, and especially the circumstances under which it becomes a parasite. However varied and numerous these may be, they are contained within fixed boundaries. There are certain standards, or rather certain types, of parasitic life, under which the individual cases are more or less definitely grouped. The knowledge of these conditions not only renders the individual cases intelligible, but it also enables us to cast a comprehensive glance over the whole field of parasitism, and therefore we may be thoroughly justified in prefacing the detailed study of individual types by a general sketch of their life-history. We commence with the period of sexual maturity, since this leads to the beginning of a new life-cycle. Between different parasites there is a striking difference with respect to the sexual maturity ; for, in agreement with what has already been stated concerning parasitism —that it is sometimes perpetual, and sometimes only temporary—we find some parasites whose period of sexual maturity coincides with the parasitic period, and others that do not attain to sexual maturity until they have commenced to lead a free existence. On the whole, however, the last-mentioned class is but small, and contains only the larve of parasitic insects and the Gordiacee and Mermithide, so that it may be confidently asserted as a law, that parasites, and especially the Helminths, attain sexual maturity while in the parasitic stage, and therefore reproduce themselves in the body of their host. A closer examination shows that this fact is entirely in harmony with the conditions of parasitic life. The position of a parasite is—economically considered—most fortunate ; its expenditure, Digitized by Microsoft® SEXUAL MATURITY. 43 in locomotion and capture of its food, is small, generally less than in free-living animals, and the income, therefore, is large ; there are in fact, without going into any further detail, numerous causes which must be considered as having a most important effect in furthering sexual maturity. The large balance on the side of income explains the great fertility, upon which stress has already been laid, as of extreme importance in the life-history of these animals. This, however, is merely en passant. Most important is the fact that sexual maturity and generation take place in most parasites during the time of their parasitic life. Copulation is often accom- plished in the lower animals before the female is fully developed, and occasionally before the stage of parasitism commences. This is the case, at least, in the Lernaw, where coition takes place while the ani- mals are swimming freely in the water,? and differ but little from the free-living Copepoda, and also in the chigoe (Pulex or Rhyn- choprion penetrans, Fig. 28)—it being supposed, at least, that only stationary parasitism is to be taken into consideration. It is, moreover, as is well known, only the female that is a stationary parasite. While the male retains the ordinary form and habits of a flea, the female bores her way into the skin of the foot in man, dogs, and other mammals, and becomes, by the enormous development of the ovary, a simple, motionless bladder. It is improbable, however, that there is anything analogous to this in the Helminths. It was thought at one time (Carter), but wrongly, that the Guinea-worm was fertil- ised before it became parasitic; but, as a matter of fact, this Nema- tode is only found leading an independent existence in its earliest stages, when the sexual organs are totally undeveloped? It is 1 We may give this instance of remarkable fruitfulness, in addition to that of the Nematodes, to which allusion has already been made (p. 32). In Tenia solium, the con- tents of the uterus of each of the segments is about 6 cubic millimetres, and it holds some 53,000 eggs, each egg having a diameter of 0°06 mm. ; seeing that a tape-worm produces yearly at least 800 segments, the total number of eggs will be thus some 42,000,000, a number that under favourable circumstances—(instances are known of tape-worms budding off five or six fresh segments daily)—is even occasionally exceeded. The extent of this fertility may be estimated by the following calculations :—The 64,000,000 eggs, which, according to Eschricht, a tape-worm brings forth in the course of a year, represent (each egg being ‘05 mm. in diameter, and having a specific gravity equal to that of water), a mass of 41,856 mgrm. (1 egg = 0000654 mgrm.). The adult worm itself weighs about 2°4 grm. or 3°4 grm., including the ovarian tube, and produces therefore yearly 174 gr. per cent. of eggs, about thirteen times as much as the queen bee, whose fertility is about 13 gr. per cent. A woman in giving birth to a child is deprived of about 7 per cent. of her weight, so that a thread-worm is as fertile as a woman would be if she brought forth seventy children every day ! 2 Claus, ‘‘ Beobachtungen iiber Lerneocera, Peniculus, und Lernea,” Schriften der Gesellsch. zur Beforderung d. ges. Naturw, zu Marburg, Suppl.-Heft ii., p. 21, 1868. > See Vol. II. Digitized by Microsoft® 44 LIFE-HISTORY OF PARASITES. also questionable whether in this latter group parasitism is ever con- fined to the female alone, as has been very generally observed to be the case in the Lernwe and their allies. The simple fact that these are animals of which only the females} are parasitic is of great interest ; this one-sided parasitism has never yet been observed in the male, except in the already quoted case (p. 10, note) of Bonellia. We must take into considera- tion here that only in a few cases is there a smaller expenditure in proportion to the produc- tion of sexual tissue, while for the female, on the contrary, the economic advantages of parasitism are of great importance. All that has been said concerning the coinci- dence of sexual maturity with the parasitic stage may be summed up in the following sentence :—Zn the Fic. 28.—Pulex penetrans. a. Female. majority of parasite animals the ye Male, eggs are produced, fertilised, and de- posited while they are in the parasitic stage. Although it is usually the case that the eggs are deposited in the host in which the parasite dwells, there are a few exceptions, such as many Tonia, where the eggs remain in the proglottides and are extruded from the body of their host, EGGS AND EMBRYOS. In general the eggs of parasites are deposited in those places where the parent lives; thus the Epizoa lay their eggs upon the outer skin; the intestinal parasites deposit them in the intestine of their host, and soon. In some cases, however, at the time of oviposition, parasites undertake special migrations like free-living animals. There is a human parasitic worm that does so—Distomwm hematobium (Fig. 29); this worm usually lives in the portal vein, but when sexually mature, as we learn from Bilharz, migrates in pairs, the female being 1 In the same manner the sucking of blood by the Culicide is confined to the females. The males possess a suctorial apparatus with which they can take up fluid nourishment, but it is not so strongly developed as to enable them to pierce the'skin. See Dimmock on “* The Anatomy of the Mouth-Parts of some Diptera,” p. 20: Boston, 1881.—R. L. Digitized by Microsoft® DEVELOPMENT WITHIN THE EGG. 45 contained in a groove on the lower surface of the male, into the veins of the pelvis, where the eggs are deposited in masses. With respect to the stage of development which the eggs have attained when they are laid, the differences in various species are considerable ; every stage, from the egg just fertilised to that which contains a fully developed embryo, is represented. According to the length of time which the fertilised ege passes in the ovarian duct, it is either unchanged, or has commenced to segment, or may even contain a fully developed embryo ; it happens sometimes, eg., in Trichina spiralis, that the embryos are hatched while in the body of their mother, which thus becomes viviparous instead of oviparous. It is not uncommon to find all these different ways in animals very closely allied, and it follows therefore that the mode of giving birth to its _ young affords no clue to the systematic // position of a parasite. Quite as varied also is the subsequent history of their eges; in ye, 29,—Distomum he- some cases they remain for a long period— ™4tobwm, male and female, : : the latter in the canalis almost until the young are hatched—in the gynexcophorus of the former, identical spot where they were deposited; while in other cases they are immediately extruded from the body of their host, and undergo their further development at large. The latter is the most usual, and may be taken for granted where circum- stances favour the dispersion of the eggs. There are numerous exceptions in individual instances, especially among the Epizoa, which often deposit their eggs in a more or less elaborate manner upon various processes of the body (lice, for instance, attach their eges to hairs; Dactylogyrus, Diplozoon, &c., attach them to the branchie of their host). When in such cases the ordinary means of attachment are not sufficient, the egg-shell is provided, as in the species just men- tioned, with some special apparatus of attachment in the shape of suckers or tendril-like processes. These structures are as important to the eggs of parasites as the various similar structures already alluded to (p. 6) are for the parasite itself. It very commonly happens among intestinal parasites that the eggs are early extruded from the body of the host, since they are continually being pressed onwards by the semi-fluid contents of the intestine ; this is so often the rage tbr Weare not acquainted with a 46 LIFE-HISTORY OF PARASITES. single parasite? that undergoes all its life stages without a change of locality. The number of eggs evacuated with the feces varies of course with the fertility and the number of the parasites, and is sometimes so considerable, that a very superficial microscopic examination is sufficient to show their presence. Moreover, the intestinal parasites are not the only ones whose eggs are evacuated ; the same thing takes place in animals living in other organs—the eggs of Distomum hepaticwm reach the intestine through the bile duct, and are thus shed from the body. In the same way the eggs of the bronchial parasite of the sheep, Strongylus filaria, are removed with the tracheal mucus, and the eges of Pentastomum tenioides, which lives in the nasal cavity of the dog, leave the body along with the secretion of the Schneiderian membrane ; the eggs of Strongylus gigas and the embryos of Filaria Bancrofti are passed out along with the urine.2 Nor is it necessary that the parasites should live in organs that are in direct communication with the exterior; there are instances where such communications are made by some subsequent abnormal process. The eggs and embryos of Distomum hematobium break through the wall of the urinary and rectal blood-vessels in which they are originally laid, into the surrounding spaces, where they form abscesses. The same thing is seen in Stephanurus (the “ kidney-worm ” of the Americans), which lives near the kidneys in swine, and bores its way into the pelvis of the kidney. The embryos of Dracunculus (Filaria Medinensis), which, as is well known, live between the muscles, reach the exterior through an abscess, which is formed as soon as the head of the worm comes into contact with any part of the cutis. If we bear these and other similar cases in mind, and also keep in view the fact that by far the greater number of sexual Helminths live in the alimentary canal, it is evident that we are right in considering the widespread occurrence of these migrations to be important in the life-history of parasites. But those other cases, where the eggs remain upon the spot where they were de- posited until the escape of the young, become, on this account, all the more striking and interesting. We have already mentioned that, to attain this latter end, the eggs of the Epizoa are fastened in various ways to the outer skin. There is no need of anything of this kind in the internal organs, where the inaccessibility of the position 1 In the German text Anguillula (Rhabditis) stercoralis was here mentioned as an exception, but, as above mentioned (p. 21, footnote), this form has proved to be merely a developmental stage of a parasite already known under the name of Anguillula intestinalis, —R. L. 2 A proper microscopical examination of the feces and excreta under such circum- stances generally furnishes a reliable diagnosis. Digitized by Microsoft® WORM-NESTS. 47 is sufficient of itself to ensure the safety of the eggs. In these cases the eggs are usually laid in the tissue in masses, which are often so large that they form conspicuous tubercle-like bodies—the so-called “worm-nests ” or “ worm-knots.” Formations of this kind are often met with in the lungs of mammals, especially of sheep, oxen, and rabbits, sometimes in such profusion that inflammation sets in, and soon kills the animal.t_ Actual worm epidemics are sometimes caused in this way. The parasites which lay the eggs belong to the Strongylidee?— (in the sheep, S. jilaria ; in the ox, S. micrwrus and S. rufescens ; in the rabbit, S. commutatus = Filaria leporis pulmonalis Frohl.)—a group of thread-worms which are commonly found in the trachea and air passages of our domestic animals, and also occasionally of man.? Some species of Filaria, in like manner, form “worm-knots.” Ecker* records the discovery in a rook of a tumour as large as a pea, which contained a full-grown Filaria attenuata and amass of its eggs. Generally, this species is found free in the intestine of its host, or in the loose con- nective tissue, under conditions unfavourable to the accumulation of large masses of eggs. This phenomenon, exceptionally found in Filaria attenuata, is very general in other members of the same genus. Thus, Filaria sanguinolenta of the dog, which in hot countries is found in almost every third individual, occurs on the aorta and the cesophagus in the sexual condition, massed together in great quantities, with eges in every stage of development.*® Nothing of the kind has been hitherto observed in man, with the exception of the ege-masses of Distomum (Bilharzia) haematobium, in the veins of the urinary organs, which only continue for a short time. There are some descriptions of worm-knots in the human body, but the evidence is not quite satisfactory.° 1 Bugnion (“Sur la pneumonie vermineuse des anim. domest.,” Comp. Rend. Soc. Helvét. & Andermatt, 1875), places with these cases the cysts of Ollulanus, described by me (see Vol. II.) in the lung of the cat. He believes, in opposition to my views, that these are not formed by embryos which have lost their way and died, but considers them— as did also Henle, who was the first to describe a case of this kind (Allgem. Path., Bd. ii., pp. 789 and 798)—as eggs in various stages of development. Since Stirling (“On the Changes produced in the Lung by the Embryos of Ollulanus tricuspis,” Quart. Journ. Micr, Sci., N.S., vol. xvii, p. 145, 1877) has confirmed my opinion, I need say no more about Bugnion’s views. I may also mention the fact that occasionally Ollulanus causes worm epidemics, 2 Vol. IT. 3 Diesing described a certain Strongylus longevaginatus, from the lungs of a child that had died of pneumonia (see Vol. II.) which is probably identical with Strongylus paradoxus from the lungs of the pig. + Archiv f. Anat. u. Physiol., p. 501, 1845. 5 Lewis, “The Pathological Signification of Nematode Hematozoa :” Calcutta, 1874. ° In the case quoted by Gubler (Gaz. méd,. de Paris, p. 657, 1858, and, in detail, Mém. Soc. Biol., t. v., p. 61, 1859), the bodies thought to be the eggs of Helminths were Digitized by Microsoft® 48 LIFE-HISTORY OF PARASITES. It is, moreover, not without significance that all these cases of “worm-nests” belong to the Nematodes. Seeing that it is an un- doubted fact that there are parasites whose eggs remain, without further development, in the same place where they were deposited, and are not extruded from the body, as is the rule in other cases, the fate of the embryos that arise from such eggs remains to be examined. The most evident supposition is that these embryos grow to maturity in the same spot by the side of their parent, and this is quite true of certain parasites. It is well known, for instance, that the young lice grow to maturity on the spot where they were born, and the investigations of Wagner, Zeller, and myself have shown that this is also the case with the above-mentioned gill-parasites, at least Dactylogyrus, Diplozoon, &c. The life-history of such parasites thus becomes extraordinarily simple. One generation follows another without any change being necessary, either to another host or another organ. If there be any migration, it is due to a mere accident. So far as we know, it is only Epizoa which have a simple life-history of this kind, though it has been attempted to prove that certain entoparasites, especially thread-worms, reach maturity without a change of locality. This opinion has, however, been shown to be incorrect, even in the case of the maw-worm (Ozyuris vermicularis), which is generally found in vast quantities in the human alimentary canal, and on that account would seem most apt to support such a theory.? Neither has the statement of Norman been con- firmed, according to which all the developmental stages Fre. 30.—Rhab- Of Anguillula (Rhabditis) stercoralis should be abun- ditis terricola? dantly met with in the viscera of persons suffer- ing from “Cochin-China diarrhea.” This worm is, as above in reality Psorosperms (Coccidium, Lt.), which used frequently to be mistaken for eggs (see postea). Virchow described (Archiv f. path. Anat., Bd. xviii, p. 523) a genuine case from the liver; the eggs, however, proved not to be Pentastomum, as Virchow thought, but Ascaris lumbricoides, from an examination that I made of some specimens that were sent to me, which were previously forgotten. Thus there is one case of the presence of a thread-worm in the bile duct (see Vol. II.) So, too, with the “ worm-nests” of Trichosomum described by von Siebold in the spleen of a shrew-mouse (Archiv f. Naturgesch., Jahrg. xiv., Bd. ii, p. 858, 1858). The bodies of the worm were found twisted together in knots near the eggs. 1 In support of this statement, which is at variance with the opinions of Kiichen- meister (‘‘ Parasiten des Menschen,” first ed., p. 229) and Vix (“Ueber Entozoen bei Geisteskranken,” Zeitschr. f. Psychiatrie, Bd. xvii.), I may quote my own observations de- scribed in Vol. II. of this work, which have also been confirmed by Zenker (Abhandl. der physik. med, Societét zu Erlangen, Hft. 2, p. 20, 1872). Digitized by Microsoft® HAMATOZOA, 49 mentioned, no true parasite, but the mature state of a heteromorphous species, the so-called Anguwillula intestinalis, The young are born in the intestine of the host, and attain maturity (like Rhabditis) only after abandoning the latter; they live in the same way as Rhabditis terricola (Fig. 30), and then give rise to a new generation. It appears, therefore, that the following generalisation may be safely made :—There are no intestinal worms, at least among the typical and constant parasites, whose embryos come to maturity near the parent ; or, in other words, there are none which pass their whole life-cycle in one locality.? If we now turn to the embryos arising from these so-called worm- nests, it seems clear that they by no means reach further develop- ment in the body of their host, but after a longer or shorter period abandon it for a free external life. All the little that we know by direct experiment agrees with this. Ecker discovered in the body- cavity and blood-vessels of his rook numerous small Filaria-like Nematodes, which he considered to be the embryos of Filaria attenu- ata,® and he found them in a later stage as small worms measuring about a line, encysted in the mesentery and other places. Vogt has made similar observations; 4 he discovered in the body-cavity of a frog two large Filariw, more than an inch long, containing numerous embryos ; the latter he also observed circulating in the blood. Lewis has also shown that numerous Heematozoa are found in dogs afflicted by Filaria sanguinolenta, and the same thing was observed by Gruby and Delafond; * and later by Leidy and Walch,® in cases where Filaria immitis was present in the right heart of the same animal. In the case last mentioned the embryos have no difficulty in getting into the blood, since they inhabit from the first an organ which they could reach otherwise only by means of an active migration. 1 Leuckart, “ Lebensgeschichte der sog. Anguillula stercoralis, u. deren Beziehung zu d. sog. A. intestinalis,” Bericht math. phys. Cl. k. Stchs, Gesellsch. d. Wiss., p. 85, 1882. 2 Luse the term “intestinal worms” instead of “ Entozoa”’ advisedly, since among Gregarine parasites there are many which regularly reach maturity near their parents. In other cases, where the germs grow to embryos at large, there is a regular migration, as in intestinal worms, to and from the body of their host. 3 Hematozoa, arising from Filaria attenuata, are very commonly met with at Leipzig. Of 38 crows which Kahane examined for this parasite at my suggestion, 28—i.e. 80 per cent.—contained it, and sometimes in such abundance that the smallest drop of blood contained quantities of them. By examining a certain amount of blood, the weight of which had been previously ascertained, it was found that 1 mgrm. of blood con- tained 601 embryos, which means that the whole of the blood, reckoning it at th of the whole 360 gr. net weight, would contain about 18,000,000. + Archiv f. Anat. und Physiol., p. 189, 1842. 5 Comptes Rendus, t. xlvi., p. 1217, 1858. ® Monthly Micr. Journ., p. Bigitzea by Microsoft® 50 LIFE-HISTORY OF PARASITES. The Nematode Hematozoa have lately attracted considerable attention by their discovery in man (Fig. 31), under circumstances Fic, 31,—Filaria sanguinis hominis (after Lewis). where they must have a considerable pathological signification. The Nematode appears to be very widely distributed in the tropics of the new! as well as the old world. The first discoverer of this human Heematozoon was Lewis of Calcutta,? and he regarded it at first as an adult parasite (Milaria sanguinis); but SUD SCUES EN considered it to be the young form of a Filaria-like worm,® which, in the sexual state (as F. Banerofti, Cobb.), is found viviparous in the subcutaneous connective tissue, more especially of the scrotum. [The embryos of this worm probably reach the blood through the lymphatic system. According to Manson’s interesting dis- covery they were usually found in blood only at night, and ap- peared to be entirely wanting during the day. At midnight the number of these embryos in the blood attained its maximum.‘ Such at least is the case when the patient preserves the usual order of life, but the reverse happens if he sleep by day and wake by night. This proves satisfactorily that the periodical appearance of 1 Since Magalhaes (O progresso medico, Rio de Janeiro, p. 375, 1878,) has discovered in blood the urinary worm of Wucherer, I cannot doubt that the Brazilian form is identical with the Indian parasite. The worm has also been observed in Japan and Australia. 2 On a Hematozoon inhabiting Human Blood,” Calcutta, 1872. Ed. 2, 1874. 3 Centralblatt f. d. medicin. Wiss., No. 43, 1877 ; more in detail—Lancet, Sept. 1877, p. 453. See also Cobbold, ibid., p. 495, and Vol. IT. of this work. * Manson, Journ. Queckett Micr. Club, vol. iv., p. 239, 1881. 5 Mackenzie, Lancet, August 27, 1881. Digitized by Microsoft® FATE OF HASMATOZOA. 51 the worms is to be explained by the state of the host as regards digestion and muscular exertion, as well as on the motion of the lymph due to these.1—R. L.] If these Heematozoa arrived at complete maturity in their host, one would expect to find, not merely a vast and increasing number of adults, but also all the intermediate stages. But no one has hitherto observed anything of the kind ; the Hematozoa remain for months, and even years (Gruby and Delafond), in the same developmental stage, and without altering in size. Even in cases where the adult worms exhibit some variation in their stages of development, as Lewis observed in certain parasites of the dog, there is a considerable gap between the youngest of these and the Hmatozoa in the blood. These facts point to the conclusion that the intermediate stage between the Hematozoon and the fully developed parasite is passed outside the body of the host. The analogy of Zrichina also lends support to this opinion. The young of this Nematode are produced viviparously, and like the embryos of the above-mentioned Filaria, wander about in the body of their host,? the only difference—and that an important one—being that they abandon the blood-vessels and betake themselves to the intermuscular connective tissue. In both instances we have a wander- ing from one part of the body to another, though it differs in kind in the two forms. But in 7richina also the result of this wandering is by no means the direct degeneration into the parasitic condition of the adult; the embryos, on the contrary, remain within the muscles, and, after developing up to a certain point, become encysted, and remain in this condition (as muscle-Zrichine, Fig. 15) until they are swallowed by a new host, when they recommence their wanderings. In Trichina, therefore, and in these Hzematozoa, a change from one host to another is necessary before sexual maturity can be reached. From the observations of Ecker, that the Hematozoa of the rook encyst themselves in the mesentery. of their host, one would be inclined to believe that the life-history of Filaria attenuata is to be regarded exactly in the same light as that of Zrichina, and that the transference into a new host is brought about by the encysted form. I myself, however, believe that this is really not the case, and that the encysted worms have nothing to do with the developmental cycle of Filaria attenuata, not merely because in this event they ought to be far more abundant than they actually are, but because the contents of these cysts, in the instances that I personally examined, agreed entirely 1 §cheube, ‘‘ Die Filarien-Krankheit,” in Volkmann’s ‘Sammlung Klinischer Vortrage,” No. 232, Leipzig, Be. . 2 2 Leuckart, ‘‘ Dratennck ang slOltizesian MichaSOHBi mri, 1860. 2d ed., 1865, 52 LIFE-HISTORY OF PARASITES. with certain Nematode larvee, which are present in the same situation in other birds, entirely free from Filaria attenuata or any other Heematozoa. The Hzematozoa, then, after a longer or shorter sojourn in the blood-vessels, would appear to leave the body of their host in some way or other, and continue their life-history under other conditions. This supposition is strongly supported by what has been observed in human Heematozoa. According to Lewis, these wormsborethrough the capillaries of the kidney and make their way into the renal tubules, and thence to the exterior. [This emigration takes place so rapidly that, after a day has elapsed, in most cases very few worms, some- times not even one, can be found, unless a fresh introduction have taken place. The significance of this migration to the future development of the worm is still unknown.—_-R. L.] Up to the present, this observa- tion is certainly unique, and nothing similar has been observed in the Heematozoa of other animals, though investigations have been carried on. If future researches throw no fresh light upon the subject,—and it is always possible that the emigration is different and more difficult to observe than in man, whose urine contains in abundance not only the Hematozoa, but also a quantity of blood and albumen mingled with them, which renders their presence obvious—there always remains the possibility that the Hematozoa continued to live in the blood, without change, until set free by the death of their host, which enables them to undergo further metamorphosis; and this is rendered more possible by the fact that no one has succeeded in finding the worms that originate from these Hematozoa in any animals where the latter are present,” and it is evident that they must at some time or other have been there. We have hitherto been considering those embryos only which, after being hatched, remain for some time in the body of their host; but these are only a small number of examples. The general rule is, that the eggs, as soon as they are laid, are evacuated from the body of their host together with its excreta, and undergo their further develop- mentin various places and under various conditions, as chance directs. 1 Borrell (Archiv f. pathol. Anat., Bd. lxv., p. 399, 1876) shows reason to believe that the Hzmatozoa of the crow leave the body by the bile-duct, but the above-quoted investigations of Kahane prove that no Filariw are present here, or in the cloaca, ureters, or bronchi, except, of course, there has been some mixture of blood. * Gruby and Delafond only found once, in twenty-four dogs infested with Hamatozoa, the Filarie from which these originated. According to Ercolani, Filaria mitis is to be found not only in the heart, but also in the connective tissue under the skin, where it might be easily overlooked (Rivolta, ‘(Studi fatti nel gabinetto di Pisa,” 1879). Similarly, among the above-mentioned thirty-eight crows, there were only three in which the presence of Jilarice could be proved ; of course it is probable that the sexually mature worms may have escaped observation, here and there, on account of their concealed position. Digitized by Microsoft® EFFECT OF DESSICATION. 53 ~In many cases, however, the circumstances and environment are by no means favourable. It may be stated generally that some degree of moisture is necessary to ensure further growth. In dry localities, the eggs lose their power of development, not merely for the time, but permanently, while in damp localities and in water they retain this power for a considerable period. In this respect the eggs agree with the full-grown animals, as do also, even to a greater degree, the embryos, which are frequently hatched in the body of the host and then evacuated. We cannot, however, make a hard and fast rule, since there are a number of Helminths whose eggs and embryos can withstand com- plete desiccation with impunity: these are chiefly Nematodes, a group which will be considered later on. The Nematoda, in spite of the simplicity of their organization and development, or perhaps rather because of it, display a variation in the conditions under which they live greater than that of any other group of Helminths. Not only are there parasitic, semi-parasitic, and free-living species, but numerous others, that infest plants, in many of which (wheat, rye, teazle, and clover) they give rise to actual diseases. That these parasites are liable to undergo a process of desiccation at regular intervals is hardly surprising, considering the periodicity of the developmental cycles of the plants which serve as their hosts. As an instance may be cited the wheat-grains which are infested by the young of a Nematode. When the seed is sown, the young parasites are brought into condi- tions favourable for their migration and further development.* This capability of withstanding desiccation is not, however, confined to Nematodes parasitic upon plants, but is occasionally found in those species that infest animals. : Fig. 32.—A, Eggs from Ascaris lumbricoides, and B, Trichocephalus dispar } a, fresh front the feces ; b, after long exposure to the open air. To investigate the influence of desiccation upon the capability for development possessed by the eggs of Nematodes, I have made use 1 See the excellent researches of Davaine on Anguzllula tritici., [ Instit., p. 330, 1855; or (more in detail) Mém. Soc. Biingizediiny MityasssT® 54 LIFE-HISTORY OF PARASITES. of a simple piece of apparatus consisting of a ring of blotting-paper enclosed between two glass slides. By alternately damping and drying the paper, the eggs could be brought into a moister or drier atmo- sphere. The conclusion to which I have been led by the use of this apparatus is, that the eggs of numerous Nematodes (Fig. 32), especially those with a thick shell (Ascaris lumbricoides, A. megalocephala, A. mystaxz, and many free-living Rhabditidee), are not merely capable of enduring a complete desiccation lasting for weeks and even months, but also alternations between the moist and dry conditions. Development does not proceed, however, save in a damp environment, but it is sufficient that the air be merely moist ; indeed, it has appeared to me that this is actually more favourable than wetting the eggs themselves with water. In damp earth development advances rapidly, but if the earth be dried, development is at once checked, without, however, de- stroying the vitality of the germs. The same holds good for the embryos; by desiccation they are rendered quiescent, but resume their vital functions on being moistened, as has been known for some time with respect to those species with free-living young (¢g., Filaria Medinensis and Rhabditis). But all the experiments are not opposed to the general law that @ moist environ- ment is necessary for the further development of the eggs of Entozoa. Of course this is not the only necessary condition. The degree of this moisture, the nature of the environment in other respects, its chemical composition and temperature, are factors which are of varied importance in different cases. Unfortunately, our knowledge on these points is defective, but one fact may be stated with confidence, and that is, that the eggs of certain Nematodes, especially those having a thick shell like Ascaris, possess an extraordinary power of resistance, and can remain a long time without injury to the development of the embryo? even in spirit, turpentine, chromic acid, and various poisonous liquids, fatal to the fully grown worm (Bischoff, Leuckart, Munk). Sometimes the degree of concentration of the liquid has an effect. Vix found that the eggs of Ascaris were de- stroyed by a solution of soap of 0:5 per cent., while in a solution of 1 per cent. they continued to develop. Similarly, as I have experi- mentally demonstrated, by means of small holes, artificially dug in the earth and filled with decomposing feces and urine, the eges of Ascaris lumbricoides ave gradually destroyed ; they are likewise often destroyed through the foulness of the water which surrounds them. + The statement of Davaine (Mém. Soc. Biolog., t. iv., p. 272, 1862), that the eggs of Ascarides inhabiting terrestrial animals undergo development when dried up, rests upon an error. ° This is the case also with the so-called Psorosperms, which are the germs of Gregari- noid parasites (Coccidium, Lt.). Digitized by Microsoft® CONDITIONS OF DEVELOPMENT. 55 All that these experiments show is that there is a limit to the power of resistance possessed by the eggs of Nematodes. All the cases just cited, however, by no means lead us to infer that power of resistance is not shown by the eggs of other Helminths, though certainly they do not show it to so great an extent as the Nematoda; but, compared with other animals, unfavourable conditions of environment take a much longer time to destroy the eggs, and this is no doubt owing rather to the simple fact that the shells of the egos of these parasites are unusually thick, than to any peculiarity in their protoplasm. In this connection it is important to notice that the eggs of Helminths are not only usually provided with a thick firm shell, but frequently possess in addition a simple or more complex accessory covering of some kind, which occasionally gives them a remarkable and characteristic appearance. This additional protective covering, besides serving to increase their power of resistance, often has other functions ; for example, the eggs of Pentastomum tanioides, which inhabits the nasal cavity of dogs, have a folded outer layer which enables them to adhere to various bodies when they are ejected from the nose of their host. Ina similar way the various filamentous or tufted prolongations of the outer egg-shell (Fig. 33), or the coating of albumen which is sometimes to be found (Eig, 32 a) on the egg- shell proper, serve to secure the slachuens of the ege to any body with which it comes in contact. The eggs of Tenia frequently leave the body of their host enclosed in a Fic. 33.—Egg of a tape-worm from a bird, Tenia nymphea. living covering—the proglottis— which pos- sesses a certain capability of locomotion, and therefore aids consider- ably in the dispersion of the contained ova, which are thus rendered more independent of external agents. In spite of all these arrange- ments, thousands of the eggs of Helminths are destroyed by the unsuitableness of the environment; but this is of no importance, considering their immense fertility. Assuming that the eggs attain to favourable conditions, let us now trace out the further course of their development. In the first place it must be remembered that the eggs reach the exterior in very different stages of development; in many instances (¢g., Acantho- cephala, Teenie, many Distomide, &c.) the embryo is already formed ; in others, again, the egg contains merely the original cell. The presence of an sibs, however, is the preliminary condition of any further change. The eggs that, when extruded from the body of their host, are either not at all or only incompletely developed, at once undergo the process of frigitizethe ein aia the young is hatched, 56 LIFE-HISTORY OF PARASITES. This is known to be the case especially in the eggs of Nematodes, which were not only hatched, according to Schubart and Richter, in small aquaria, but also, as already mentioned, in a damp atmosphere and damp earth with even greater certainty. This has also been proved in the case of the eggs of numerous tape-worms (Bothrioceph- alus) and Trematodes. In many, almost in the majority of instances, embryonic develop- ment only progresses during the summer months, and in many species only under the influence of a considerable degree of warmth; thus the eggs of Ascaris lumbricoides require a temperature of 20° C., those of Zrichocephalus 22:5° C., and those of Oxyuris vermicularis as much as 40° C. The eges of the latter develop a complete embryo in a few hours, and when the temperature is increased, in a still shorter time, while the egos of Ascaris and T'richocephalus, which an e differ from those of Oxyuris in being fe \ EE entirely undeveloped at the time that ee they are laid, require several weeks; : and when the temperature varies, as & it generally does in this country in the & " summer, several months elapse before ic; Aine so Oxpuvle ee the young are hatched. Lrichocephalus micularis ; a, b, freshly laid ; c, with rarely completes its development with- developed embryo: in the year; Ascaris lumbricoides, in the natural course of events, requires three or four months, and Ascaris mystax some three weeks. On the other hand, the young of Dochmius duodenalis (especially in warmer climates) are hatched in a few days. Similar variations are found in Trematodes and Cestodes, the eggs being sometimes hatched in a few days (Zriwnophorus), at other times requiring weeks (Ligula) or even months (Bothriocephalus latus, Dis- tomum hepaticum, &c.) for their full development. This, however, only applies to the summer months; in winter, even in a heated chamber, development goes on slowly and irregularly; in Ascaris mystaa, for example, the first traces of cleavage appear only after several months. Besides temperature, other circumstances are of considerable importance. There are individual differences between eggs them- selves ; embryos rarely develop in them simultaneously ; one egg may have hardly commenced to divide, while another contains a fully formed embryo. Numerous eggs also, under conditions favourable in other respects, never develop, but undergo a process of degeneration in which the whole mass becomes granular and semi-transparent, and all the details of its structure vanish. It may be that these eges ' In sunshine Vix saw an active embryo develop in a quarter of an hour in the eggs of Oxyuris.—Zeitschr. f. Psychiatrie, Ba. xvii. p. 65, 1860. ; Digitized by Microsoft® MIGRATION OF EMBRYOS. 57 have never been fertilised, and this view is supported by the fact that the eggs of unfertilised females among the Nematoda de- generate in the same way without any apparent cause. In Entozoa that develop in a short space of time (¢y., Dochmius duodenalis), the early stages are usually passed through while the eggs are traversing the alimentary canal of their host. Occasionally the whole process takes place in the body of the host, especially when they remain there for a considerable period. A longer sojourn in a living host may thus be a necessary preliminary to embryonic development. Though our knowledge with regard to the germinal activity of the egos of Entozoa rests at present upon a comparatively small number of experiments and observations,? these are so entirely in harmony, that there is no doubt about the general facts. We can therefore state with confidence that the embryos of oviparous forms develop after the eggs are laid, while those of viviparous (or ovo-viviparous) forms are developed previously,—in other words the eggs of all parasites at some time or other, either sooner or later, develop an embryo,” provided that they meet with favourable conditions. MIGRATION OF THE YOUNG BROOD. The embryos of Entozoa by no means exactly resemble their parents. On the contrary, they never do so, even in the Nematodes, Fic. 36.—Egg of Both- Fic. 37. —Egg of Fic. 35.—Egg of Distomum riocephalus latus with LEchinorhynchus gigas with hepaticum with embryo. embryo. embryo. 1 See the observations of von Willemoes-Suhm, Zeitschr. f. wiss. Zool., Bd. xxiii., 1878, p. 343, (Bothriocephalus), and p. 337 (Trematoda). 2 This holds good also for the generative buds of Gregarines—the so - called Pseudonavicellee—which, earlier or later (in the body of their host or outside it), develop into embryos. Digitized by Microsoft® 58 LIFE-HISTORY OF PARASITES. which are commonly said to go through no metamorphosis, the resemblance of the young to the adult is more apparent than real. In the majority of cases (in the Cestodes, Distomide, Lchinorhynchus, and Pentastomum) the differences are so great, that there is hardly any point of similarity between the young and the fully formed worm.—(Figs, 35, 36, and 37.) It is not so much for zoological reasons, to complete our knowledge of the organization of parasites, that these facts are brought forward, as for the reason that the heteromorphism of the embryo is of the greatest importance in their life-histories. Seeing that the structure of an animal is by no means a matter of chance, but depends upon the capacity for certain actions and modes of life, it is not surprising to find that the embryos of Entozoa, which live under different conditions from the adults, are different from them in form; and these peculiarities are all the more important, because the fate of the embryo is intimately connected with the character of its life-history. Let us consider the actual results of observation. It appears that the history of the young parasites that have reached the exterior from the body of their host, whether as eggs or developed embryos, may follow one of two directions ; either the young leave the egg and live in a free state for a longer or shorter period, or they remain within the egg until it is taken into the body of a new host, where they are then set free. In the latter case, there is no free-living stage, for it is always the eggs and not the embryos that are found at large. But it may be objected that it is impossible to draw a sharp line between a living individual and a fully developed egg. This is no doubt true; but it must be remembered that the relations between the embryos and the outer world are quite different while it is still enclosed within the egg-shell, though an embryo just hatched can hardly be said to be at a higher stage of development than a fully formed embryo still within the egg. Whether the embryo of a parasite, when fully developed, be free or not, depends in a great measure on the character of the egg-shell. The latter, when thick and strong, imposes an increased resistance to the exit of the embryo, and sometimes renders it quite impossible for it to leave the egg by its own unaided efforts. This is effected very often by the action of the gastric juice of the new host, which dis- solves the shell, or makes it so weak that the embryo can force its way out without special difficulty. My experiments? with the eggs of tape-worms show clearly that the hatching of the embryo is some- times merely a question of the digestive activity of its host. In some ‘ Leuckart, ‘‘ Blasenwurmer,” p. 100. Digitized by Microsoft® INFLUENCES OF DIGESTIVE JUICES. 59 cases, corresponding to the chemical and physical qualities of the egg- shell, the solvent power of the digestive juices must vary, in order to set free the enclosed embryo. The differences in the digestive activities of various animals are but slightly understood, and, in fact, are merely known to exist; but we cannot doubt that they exercise a profound influence upon the presence and distribution of parasites, when we remember that the eggs of the common tape-worm are digested by mammals, but not by frogs. It would appear that, on the whole, the digestive activity of cold-blooded is less than that of warm-blooded animals, since the larvee of flies, wood-lice, millepedes, &c., and the shells of the eggs of tape-worms and of Ascaris lumbricoides are able to pass through the alimentary canal of the former without being digested. Moreover, since, as we have seen, it depends upon the charac- ter of the egg-shell whether the embryo of a parasite be hatched outside the body of its host or not, we are right in assigning to the first group eggs with thin, delicate shells ; and this is especially the case in the Nematodes (Dochmius, Sclerostomum, &c.) But these thin-shelled eggs do not afford so much protection to their contents as those with a stouter shell; they are not found, therefore, in all species with free embryos, and are always absent in those cases where the time of in- cubation is longer. In these cases the eggs are thick-shelled, but provided at one end with a kind of lid, which yields to pressure from within, and can be raised up by the embryo (Fig. 38). These are found in the Distomide, Bothriocephalus, and in many ectoparasites, ¢.g., the louse. But it must not be supposed that the absence of a lid of this kind hinders in every case the exit of the young ; it is quite possible that the embryos are enabled, by the possession of head spines Bre, 98)—Hegs of Both: or other similar structures, to bore their way riocephalus, with opercu- through the egg-shell, as do the young of many 1™ 5 the one is empty. other animals; and in the case of Gordius, among Entozoa, this has been actually proved. It is also possible that a damp environ- ment may help to soften the shell, and so facilitate the escape of the embryo, as has been observed in many thread-worms. Be that, however, as it may, the main fact of interest to us is that there are numerous parasites which lead a free existence whilst young. The majority pass this stage in water, in localities that the ego has reached, in a more or less direct way, before the escape of their embryos. Sometimes they swim about by the help of a covering of cilia (Bothriocephalus, Monostomum, and other Trematodes—Figs. 39 and 40) or special appendages (fish-lice); sometimes they remain at the bottom, and make DIbRB ear AY pate, theesmud. Other species, 60 LIFE-HISTORY OF PARASITES. especially Nematodes, live in damp earth instead of water; and there are other parasites, but only air-breathing insects, that inhabit drier Fic. 39.—Free embryo of Fie. 40.—Free embryo of Bothrio- Distomum hepaticum. cephalus latus. localities. As a well-known example of this, may be adduced the larva of the flea (Fig. 41), which is found in quantities in retired Fic. 41.—Darva of the flea. of blind chance, spots in the neighbourhood of mouldering organic matter, such as dusty corners of rooms, and in the straw of hen-houses, &c. The comparison of a flea- larva to the young of Helminths in this particular does not, however, imply that they agree in all respects. The life of a flea-larva is of long dura- tion, and so noteworthy as regards growth and metamorphosis, that it must be considered quite as important as that of the adult. With the Entozoa, however, it is quite different—at least with the greater number; not merely do the young (except in some cases) take no nourishment during the free stage of their existence, but the period itself is of short duration, and serves only as a means to further their distribution and migration. Instead which in other cases directs the fate of the germs of parasites, we have to do with a definite and fixed order of events. This free stage of existence, in spite of its short duration, is long enough, under favourable circumstances, for the parasite to make its way into the body of some host. In the first edition of this work I was obliged to leave it uncertain whether any parasites existed in which the free stage Digitized by Microsoft® RHABDITIS-LIKE EMBRYOS. 61 was sufficiently prolonged to allow of their taking in nutriment, and so increasing in size. Regarding the manifold conditions under which the Nematoda lived, I thought it probable that examples of this kind, if they existed at all, would be discovered in this group. This opinion has been justified. My researches into the life-histories of Nematodes,! have proved that there are numerous species, especially among the Strongylidae (of human parasites Dochmius duodenalis), which in their young stage resemble in structure and habits the free-living Rhabditide (Figs. 42, 43), and like them go on feeding and growing for a considerable time. They then change their skin, lose the pharyngeal armature so very characteristic of Rhabditis, and enter upon a stage when they cease to take in nourishment and to increase in size, and need to become parasitic. I need hardly recall the life-history of Ascaris nigrovenosa, shortly described above (p. 2), which belongs to this type; but is peculiar, in that the Rhabditis-like form, which elsewhere is merely a young stage, is here developed into a special generation, which, as soon as it is completed, enters again on a parasitic life. Among other Helminths (Cestodes, Acanthocephala, Distomide) there is nothing of the kind known, and it would indeed be impossible in the two first-mentioned examples, inasmuch as the young has no alimentary canal. Where there are free-living stages in these forms, they serve only for an independent migration. Moreover, the Entozoa are by no means the only animals which have a “swarm-period” like this. It has often been observed in many other animals, such as corals, ascidians, and so forth, when the adult is entirely stationary, or possesses but limited powers of locomotion. Among the insects also we know of wandering larvee, as Newport and Fabre have shown in the Meloide: the larvee of these beetles live in the nests of various species of bees, to which they can only gain access in the young condition, owing to limited powers of movement of the adults.* As soon as the young parasite meets with its proper host, it abandons its previous course of life, and loses those organs which serve only to establish relations with the outer world, such as cilia, locomotor appendages, and organs of sense, when these are present, 1 For a fuller statement see Vol. IT. 2 The life-history of these young Meloide is such an interesting example of pilfering, that I cannot help giving an account of it here, especially as it affords many parallels and points of relation to the study of parasitism. The females lay their eggs in early spring at the roots of the Ranunculacee, dandelions, and other plants rich in honey, that are much visited by bees. As soon as the larve are hatched, they crawl up the stems of these plants and hide themselves in the corolla. When bees visit the flowers the larve attach themselves to them by their powerful limbs, and are carried to the nest ; here they lose their appendages and change into dmsiey BY Wi crosoft® 62 LIFE-HISTORY OF PARASITES. and in this way undergoes a metamorphosis which leads sooner or later to its definitive form. At the same time the parasite fixes Fic. 43.—Rhabditis-like condition of young stage of Fic. 42.—Embryo of Rhabditis Dochmius trigonocephalus; a, at commencement of the terricola, free life ; b, at the end of the free life, itself on to the outer skin of its host, or in some organ easily accessible ‘from the exterior. In this way we know that the larve of Trematodes attach themselves to the skin or within the respiratory cavities of water-snails. Others bore their way at once into the intestines or body-cavity. To attain this the parasite seeks a soft, slightly resisting part of the body, against which it presses with its anterior extremity, and gradully forces its way in. Considering the small size of the body, and the fact that many of these embryos are provided with special boring apparatus—as, for instance, the larve of Bothriocephalus, Gordius, and several species of Distomam—it is evident that the difficulties to be overcome are not very great, provided that they attack the right host. It is of course only animals with a delicate outer skin, such as larval Insecta, Crustacea, Mollusca, and so forth, that are attacked in this way by parasites. In many cases the process just described has been actually observed, and in other cases it is inferred by placing together the Digitized by Microsoft® OBSERVATIONS ON THE MIGRATION OF FREE EMBRYOS. 63 parasites and their hosts, and by subsequently finding the parasites within the bodies of the latter, which of course had been previously ascertained to be free from parasites. Von Siebold,+ in the account of his researches into the Mermithide,and their wandering into the bodies of minute caterpillars, makes the following remarks: “Thirteen larve of the spindle-tree moth (Hypomeneuta cognatella), which I had previously found by microscopical examination to be free from thread-worms, were placed in a watch-glass, in which was a quantity of damp earth containing active embryos of Mermis. After eighteen hours, I was able to detect these embryos in five of the caterpillars. In a second experiment, I carefully examined thirty-three caterpillars, to see that there were no Nematode larve in them to start with, and placed them in similar conditions. After the lapse of twenty-four hours, fourteen of them contained embryos of Mermis, six of them contained two worms a piece, two others contained as many as three apiece. I also made use of young caterpillars not more than three lines in length of Pontea crategi, Liparis chrysorhaa, Gastropacha neustria, which I took out of the webs in which they had hibernated. They were in a similar fashion placed in a watch-glass with damp earth and embryos of Mermis. On the following day I found that ten out of the fourteen contained embryos; in five there were two larvee, and in one there were no fewer than three.” Meissner 2 has recorded similar observations upon the embryos of Gordius. The wandering into the bodies of larvee of Hphemera, which Meissner made use of for his experiments,® only took place at night, and always through the appendage which served as a point of attach- ment for the young larve. “All the Ephemerid larve which were left for the night in a vessel with the Gordius-embryos were attacked by them; all the intruders, however, were found in the legs, usually in the neighbourhood of the first joint, but some had penetrated as far as the muscles of the coxa; some were quiescent, with the head and proboscis retracted, but the majority were actually moving about, and I was able to see them in the act of making their way between the muscle-bundles. This was done in a very peculiar way. The head was thrust forward, and the hooks, being directed outwards, obtained a firm hold of the tissues ; the head and proboscis were then drawn back, to be again thrust forward in the same way. The pro- boscis thus penetrated some distance, and the hole was then enlarged by the head with its circle of hooks. The contractions of the muscles of 1 Entomol. Zeitung, p. 239, 1860. 2 Zeitschr. f, wiss. Zool., Bd. vii., p. 132, 1856. 8 Villot considers that the larve of Chironomus, and not Ephemera, are the proper hosts of th Gordius, rehives d.. Zool. expér,, t, ii, p. 186, 1874. osta of the young Gordius. -d ret by Microsoh®”” 64 LIFE-HISTORY OF PARASITES. the Lphemera hindered this process to some extent by pushing away the Gordius-larvee, and rendering their attempts ineffectual. I found one specimen in the fat body endeavouring, but without success, to make its way between two huge drops of fat: every time that the larva pushed them asunder, they flowed together again directly. The longer the Ephemerid larvee remained in the infected water, the greater was the number of Gordius-embryos, which penetrated into their bodies ; I found them in all the organs, the legs, palpi, fat body, and especially in the body-cavity, and even in the dorsal vessel, lying some- times close to one of the valves, and moved to and fro by its pulsations. The number of parasites in one larva was sometimes so great (as many as forty) that I am inclined to attribute to this helminthiasis the sudden mortality which took place among the Ephemera.” For a considerable time it was believed that the parasitism of these free embryos was always brought about by their own active migration into the body of a host; of course, it was possible that it might be effected in other ways, but there was no proof of this. At present we know that many larval parasites find their way into the body of a host by means of drinking water. I transferred a quantity of muddy water containing embryos of Dochmius trigonocephalus (Fig. 43, a, 6,) to the alimentary canal of a dog, and saw them grow into the parasite after the lapse of a few days. Man is infected in a simi- lar way by Dochmius duodenalis, and the horse by Sclerostomum equinum. It is probably only the free young stages of Nematodes which select the natural passages in order to become Entozoa ; at any rate they are the only forms that can, by the thickness of their skin, withstand the action of the digestive juice. Meissner, however, and others have shown that this is not a complete protection; the former observed numerous G'ordius-embryos destroyed by the digestive fluids of Ephemerid larvee, and I have observed the same in Monosto- mum. Ina similar fashion the often numerous specimens of Filaria sanguinis, which the mosquito sucks up with the blood of man, shortly perish almost without exception in its alimentary canal.2 1 See Vol. II. ® From the observations of Manson (Zrans. Linn. Soc. Lond., pp. 367-8, 1884) there can no longer be any doubt that the few embryos which can pass without danger to themselves through the intestine of the mosquito undergo further development in the body-cavity, in consequence of which they now differ in size and in the structure of the mouth parts from the embryo at an earlier stage. Manson is of opinion that embryos, having thus reached a certain stage in the body-cavity, get into water only on the death of the host, and that they are taken into the human body with the water. This statement still requires demonstration, but even were this proof forthcoming, there would yet remain a possibility that jhe embryos evacuated with the urine (which probably no more represent a useless production than the eggs of intestinal worms which pass out with the feeces) may be transported to certain small hosts, and by these means human beings may perhaps be infected more commonly than in the way pointed out by Manson.—R. L. Digitized by Microsoft® PASSIVE MIGRATION. 65 A passive migration which occurs only exceptionally in Entozoa with free-living embryos is the rule in those species which have no free young stage. In the latter group the embryos, still enclosed by the ege-shell, reach in some way or other the intestine of their host; the process of alimentation affords numerous oppor- tunities for this to happen, which may recur after intervals, varying according to the peculiarities of the mode of life. Many animals, especially smaller ones, actually use the eggs of Entozoa as food. I have myself observed specimens of Gammarus and Asellus aquaticus feeding upon eges of Lchinorhynchus which I had placed in their aquarium ; others again take in the eggs acciden- tally along with their food, in greater or less numbers, sometimes still protected by the covering of the body of their parent. In the latter way grass-feeding ruminants are infected with the eggs of several tape-worms (Zenia serrata, T. marginata, T. cenurus, and 7". echinococeus), which live in the intestine of dogs. The “pro- glottides” of these worms crawl out of the feces and deposit their eggs upon grass stalks. I may also mention here Tenia saginata (mediocanellata) of man, the eges of which are transferred in the same Fig. 44.—Proglottides of Tenia saginata in various conditions of contraction, way to the stomach of the ox (Fig. 44); the pig generally becomes in- fected with Tenia soliwm by feeding directly upon human ordure, and the meal-worm (Tenebrio molitor) devours, along with the excrement of mice, the contained eggs of Spiroptera murina, while the larva of the cockchafer takes in the eggs of Hchinorhynchus giyas with the feces of the pig. Man himself is frequently attacked by parasites in the same way; and dogs, when licking their master’s hand, deposit the eggs of Pentastomum, which are thus easily transferred to the alimentary canal. These few examples show how the germs of parasites are taken in with food. In aquatic animals this is even more easily accom- plished. In those that possess circlets of cilia or tentacles, the eggs may be readily swept into, Hyp amersBome food; and higher 66 LIFE-HISTORY OF PARASITES. animals, eg. fishes, may be occasionally deceived, and devour tape- worms under the delusion that they are nutritious food. It is, moreover, evident that the further development of these eggs, when they have reached the body of some animal, is only possible when the conditions are favourable, and when the eggs themselves contain a living embryo. It is not easy to say how long the embryo will retain its vitality; accidental and even constant conditions bring about the greatest variations in this respect. In the eggs of the common thread-worm (Ascaris) I have seen active embryos even after the lapse of two or three years,’ as also in the eggs of Echinorhynchus ; whilst, on the contrary, the eggs of tape-worms usually lose their vitality within a few weeks, even when kept damp. The eggs first of all, we may suppose, reach, in a living condition, the stomach of their host, where, if the digestive juices be of sufficient strength, the shell is dissolved ; variations in this respect have been already alluded to (p. 58). The embryo, which was hitherto sufficiently protected by its outer cuticle against dissolution, now becomes free, and acquires the possibility of growth and development. DEVELOPMENT OF THE GERMS AFTER MIGRATION. That the embryos of some Entozoa, directly they are hatched, leave the stomach of their host, and find their way into its intestine, where they arrive at sexual maturity, has been placed beyond doubt. I suc- ceeded in infecting a sheep with Trichocephalus by feeding it with the eggs containing embryos.? In a similar fashion, according to Ehlers, hens and other birds are infected with the tracheal parasite Syngamus, and man (according to Zenker and myself) with Oxyuris. Kiichen- meister and Davaine attempted to breed Ascaris lwmbricoides from egos by drinking water containing them, but numerous and careful experiments in this direction by Mosler and myself led invariably to a negative result. In some cases (¢g., Dochmius trigonocephalus, as above men- tioned) the free embryos also attain to maturity without change of locality. It is usual, however, for the development of the young parasites, whether hatched in the stomach or outside the body, 1 Davaine saw embryos alive after four years, and even after five years hail elapsed he was able, by heating them, to induce signs of vitality. (J/ém. Soc. Biol., t. iv., p..261, 1862.) He also states that he was able to preserve alive for years eggs and embryos of Tenia solium and Tenia serrata, (bid., t. iv., p. 273, 1862.) i 2 See Vol, II. The attempt here referred to is the first which has established the continuous development of an intestinal worm, Of course, Davaine and others had, before this, occasionally asserted such a development, but what they adduced was in no way convincing. : Digitized by Microsoft® WANDERINGS WITHIN THE HOST. 67 to follow a more complicated path. The young of Trichina, for example, perforate the intestinal wall, and bore their way into the surrounding organs or tissues. The same holds good for many species of Tenia, Echinorhynchus, and Pentastomum, whose develop- ment I have traced, and numerous thread-worms—Spiroptera murina, Ascaris incisa, Sclerostomum equinum, &c. If we recall and com- pare with these facts the additional fact that the larvee of Distomwm, Bothriocephalus, &c. bore their way from the exterior into the body of their host, and make their way into certain definite localities, we may state, in a general way, that the embryos of Entozoa which have Sound their way into the body of some host do not at once become quiescent, but continue their wanderings, and traverse in various directions the tissues and organs of its body.+ These wanderings are facilitated by the minute size and often elon- gated needle-shaped body of the parasite, or by the possession of a boring apparatus. It is, in fact, no harder for a Nematode to make its way through the tissues of an animal than for a bird to move through a thick covert, or a dog through a cornfield, and they leave as little trace of their progress, inasmuch as they rather push between than actually tear their way through the tissues. ’ The wanderings of parasites in the larger animals are also often assisted by their getting into the blood-vessels, and so being carried into the remotest parts of the body. Many of them even live for a time as Hematozoa, eg., the embryos of certain Milarie (p. 49). Ina few cases the presence of embryos of Tania in the blood has been actually observed (Leuckart, Raum) ; in other cases it has been sus- pected from the wide and uniform distribution of the parasites in the body of the host. This conclusion is, however, quite a necessary one, for my researches on Z'richina have proved that the connective tissues 1 If such a migration take place into a pregnant female, the young Entozoa may reach the body of the embryos. Leydig (Muller's Archiv f, Anat. u. Physiol., p. 227, 1851) observed in the blood of Mustelus levis and its foetus the same Filariw. However, this does not seem to occur always, since in the Mammalia the transference of Nematode Hematozoa to the foetus has not been demonstrated (Chaussat). The wandering embryos of Trichina avoid the body of the foetus. On the other hand, I found in a pregnant Lacerta agilis that nearly all the embryos—nine out of twelve—contained active sexless Nematodes in the pericardial cavity, in the cavities of the brain and spinal cord, and in the amniotic fluid. Most of the embryos harboured two or three parasites, or even four, and in different parts, without showing the least traces of how the worms made their way in. In the organs of the mother I could not find any of the parasites, nor even the sexual worms which had produced them. Rathke, I find, anticipated me in this obser- vation (Archiv f. Naturgesch., Jabrg. ili., Bd. i., p. 335, 1837), The presence of Entozoa in embryos under such circumstances need excite no wonder ; but the older assertions, according to which the embryos occasionally harboured sexually mature Helminths in the intestine and liver, seem most suspicious (Davaine, /or. cit., p. 11). igitized by Microsoft® 68 LIFE-HISTORY OF PARASITES. form passages of communication from one part of the body to another, of which the embryos avail themselves. Whether these wanderings take place through the blood-channels, through the connective tissue, or perhaps also directly through the tissues of the organs themselves, and whether they commence at one point or another, at the skin or the alimentary canal, one fact is certain, that they do not last long. Sooner or later the embryo loses its activity, and then, if the circumstances be favourable, undergoes, by growth and metamorphosis, further development. These favourable conditions occur, perhaps, in only one definite organ or host—in a mammal, perhaps, or a snail, in the brain or in the liver. Here only is a further development possible. If, as is frequent, chance has brought it about that the young parasite finds its way into some other animal or some other organ, it shortly dies; but in many cases it leaves behind traces of its presence. For in- stance, in lambs that have been fed with embryos of Tenia cenurus, which only attain to development in the brain, many other organs and tissues, such as the muscles, connective tissue, and liver, are found to be filled with minute cysts, which were no doubt at one time occupied by the worms. The nature of the further development, of course, varies with the species of parasite and the structure of the embryo, so that increase of size appears to be the only change which can be universally pre- dicated of parasites. Different species vary much in the dimensions which they attain; some stop short at a few millimetres in length, others only after exceeding three or four decimetres (Ligula). Fic, 45,—Entozoa in the second stage of development. A. Cysticercus of Tenia solium from the pig; B. Cysticercus of Tenia cucumerina from the dog-louse ; C. Young form of Spiroptera murina from the meal-worm. If the embryos differ from their parents in form, they undergo metamorphosis as well as increase of size. The organs that served Digitized by Microsoft® SECONDARY WANDERINGS. 69 merely to assist their wanderings are cast off, and replaced by new structures, which subserve their altered conditions of life. Asa general rule, Entozoa, in this second developmental stage, show a considerable likeness to the fully formed ani- mals, but differ in various direc- tions. The sexual organs, for instance, are incompletely de- veloped, or even absent, so that the organization is, on the whole, less differentiated, —in accor- dance, certainly, with the com- paratively simple and uniform conditions of life. The embryos remain quiescent, and imbedded in the tissue of organs, generally within a cyst, which, as we have Fic. 46.—A piece of liver from the rabbit, showing passages made by Cysticercus pisi- Sormis. seen, is formed by growth of the connective tissue, or secretion Fic. 48.—Aspidogaster conchicola. a. Embryo ; b. Young animal, not sexually mature (after Aubert). Fic. 47.—Archigetes Steboldi. by the growing body of the parasite, and feed on the substances immediately surrounding them (Hig.4)osoff® 70 LIFE-HISTORY OF PARASITES. Occasionally, however, this state of quiescence is not absolute ; the parasites move from place to place in a slow and gradual fashion, as might have been expected from the size of the parasites and the tissues that surround them. This is known to occur in certain tape- worms! (Tenia cenurus, TI. serrata, T. marginata) whose embryos develop in the brain or liver of mammals. The bladder-worms, which constitute the second developmental stage of the tape- worms, progress, so long as they remain of small size, in a definite direction by a peristaltic action, and form in this way tunnels and passages, which are subsequently invaded by a growth of connective tissue, and present a striking appearance. Sometimes these passages open into the neighbouring cavities of the body, into which the parasites then fall. This is most generally the case with the tape-worms found in the liver of rabbits and ruminants, which find their way into the body-cavity, where they again become encysted. The quiescent stage in the life-history of parasites never takes place in the intestine, but may do so in any other organ of the body, and most generally does so in the connective tissue be- tween the muscles and in the parenchyma of the alimentary canal; some sexually mature parasites are also found in these same organs, and hence the question arises, whether they may not be directly developed from the asexual forms, without any further migration. There are two species in which this certainly does occur; one is Archigetes,? an unsegmented tape-worm of the family Caryophylleide (Fig. 47), which is a parasite in the body- cavity of many Naide. This worm becomes sexual while yet a bladder-worm, which, in other Cestodes, is only an intermediate stage. Another instance is furnished by the genus Aspidogaster,® which inhabits the pericardial cavity of the fresh-water mussel (Fig. 48), and attains sexual maturity without any further change of habitation. All these creatures, however, are parasitic upon invertebrates, a fact of which the importance will appear later on. Among the internal parasites of the Vertebrata we do not know of a single analogous example. We may therefore lay down this general law, that the quiescent stage following upon the wandering embryonic stage does not conclude the life-history of the parasite, which needs rather u radical change in its environment,—in other words, a second migration. + See Leuckart, ‘‘Blasenbandwurmer,” p. 124. * Leuckart, “ Archigetes Sieboldi, eine geschlechtsreife Cestodenamme,” Zeitschr. f. wiss, Zool., Bd. xxx. (Suppl.), p. 593, 1878. : * Aubert, “Ueber das Wassergefisssystem, u. s. w., d. Aspidogaster conchicola,” Zeitshr, f, wiss. Zool., Ba. vi., p. 349, 1855. , Digitized by Microsoft® NECESSITY OF A SECOND MIGRATION. 71 CHANGE OF HOST—MIGRATION. _ Neglecting for the present parasites that develop directly (Tricho- cephalus, Oxyuris, Dochinius, &c.), and the other two instances just Fic. 49.—Sporocyst and Redia, with Cercarie in the interior. quoted, the second stage of development leads only to a certain point, which is more or less distant from the final stage of sexual maturity. But this stage lasts for a considerable time, even several years in many parasites ; indeed, until a favourable opportunity affords the ‘conditions suitable for further development. If this opportunity do not occur, they remain in the asexual state, and finally perish. The progress of recent research has made us acquainted with the fact that these intermediate forms sometimes, of their own accord, seek out a new host, in the body of which they arrive at sexual maturity. This has been proved in the case of some marine tape-worms .(Tetrarhynchus), and will perhaps be ultimately shown to be of more frequent occurrence. This migration is often accomplished by a brood produced asexually from the quiescent form of the second develop- mental stage,—it being, of course, supposed that this is active, and not, like the heads of bladder-worms, attached to the mother. This is what takes place in the Distomide and allied forms of Trematodes. The embryos (Fig. 49) are formed in the interior of saccular parasites, pro- vided or unprovided with apaimentary gual (Redize or Sporocysis), 72 LIFE-HISTORY OF PARASITES. which thus, in conformity with the law of alternation of generations, give rise asexually to a new generation. In these cases a number of generative cells develop, which become collected together in increas- ing numbers, and grow into parasites which are different from the foregoing generations,? and are, in fact, small sexless Distomes. In many cases this generation finds its way into the bodyfof a host while yet contained within the Sporocyst (or Redia). In Distomum macrostomum, for example, whose life-history has recently been worked out by Zeller,? the Sporocyst (the so-called Leucochloridium paradoxun), having the appearance and colour of a tailed fly-maggot, is swallowed, together with its living contents, by some insectivorous bird, after having bored its way through the tentacle of the Swccinia infested by it. About six days after, the young Distome, freed from the Sporocyst, has attained to sexual maturity, having cast off the earlier thick larval cuticle. Such a direct transference into the definitive host is, however, rare ; it is usual for the young parasite first to enter the body of another animal. : For this purpose, the young Distomum is generally provided with a tail, and often with a boring tooth at the anterior extremity. In this stage it was formerly regarded as a distinct animal, and named “ Cercaria.” These Cercarie (Fig. 50) abandon their host and live free for some time in water; they then seek out a new host,? which may belong to the Mollusca, Insecta, or Crustacea, and bore their way through its outer skin. Von Siebold* observed these parasites in the act of making their way into the body of their host, and he thus describes the process :—“I had = Ny obtained a quantity of Cercaria armata from the aaa common pond-snail (Lymneus stagnalis), and put Fie, 50.—A free Cercaria them into a watch-glass containing a number of larvee of Ephemeride and Perlide. I could observe, by the help of the microscope, that the Cercaria, swimming about in the water by * Moreover, we know of cases, especially during the winter time, in which the genera- ting cells of certain Redie give rise again to Rediw, It is more generally, however, the Sporocysts that, by division or budding, give rise to a ramified structure that pierces the tissues of its host in all directions. 2 Zeller, Zeitschr. f. wiss. Zool., Bd. xxiv., p. 564, 1874, * If there be no such change of hosts, the young Distomum has no tail. Some few possess instead a short process which looks like a sucker, and serves to assist them in creeping about, * “Ueber Band- und Blasenwiirmer,” p. 26, “ Handwirterbuch d. Physiol.,” Bd. ii. p- 669, 18438, Digitized by Microsoft® MIGRATION TO THE DEFINITIVE HOST. 73 means of the tail, approached the insect larve, and crept all over them in a restless fashion, evidently seeking something. I also noticed that every now and then they remained motionless, and pressed their frontal armature against the body of the larva. In no case, however, were these boring operations continued, until the Cer- caria happened to have lighted upon a soft portion of the integument between the segments of the insect; then they used their spine with- out ceasing until they had made an aperture in the skin, through which the flexible fore-part of the body could be introduced. This enlarged the opening, and rendered it possible for the whole body, much attenu- ated during the process, to pass through the outer pre 51 Anen- skin into the perivisceral cavity. The tail of the cysted Cercaria, Cercaria always remained outside, and was no doubt since detached by the sides of the aperture closing together after the body of the parasite had passed through. Having selected for these experi- ments young and delicate larvee, I could still observe the Cercariz inside their body. They invariably remained quiescent, and assumed a spherical shape (Fig. 51), surrounding themselves with a cyst. The frontal spine was detached during this process of encysting, and was generally visible, lying close to the Cercaria, and within the cyst. This spine, therefore, like the tail, is cast off as soon as its purpose is fulfilled.” The duration of the free life varies with the species. In our common Cercarie it is generally short, and many species (Distomum hepaticum) do not wait to make their way into the body of some host, but become encysted upon water-plants and other objects. The marine forms, on the other hand, remain longer in the free stage; some, after entering the bodies of worm-larvee, Copepoda, &c., devour the tissues of their host, and become encysted in its empty shell (Mcebius). In the quiescent stage the Cercariz are just like other Entozoa in the second developmental period. They await transference to a new host, where, if circumstances favour it, they attain maturity. The changes undergone in the intermediate host—in which they some- times remain for years—are no more than preparations for the final stage, and consist mainly in a slight increase in size, and the gradual formation of the generative organs.? The change to the last developmental stage is then (even in species 1 If this intermediate condition be prolonged to an unusual extent, the encysted Distomum often arrives at sexual maturity, as I have noticed myself in Ephemerid larve, Similar cases have been observed by other naturalists, eg., Linstow and Villot. The last mentioned has published a special paper on this circumstance (‘‘Observ. de Distomes adultes chez les Insectes,” Bullet. Soc. Statistque de lU’Isére, t. ii., p. 9, 1868), which, however, I have not_seen. Digitized by Microsoft® 74 , LIFE-HISTORY OF PARASITES. with an intercalated free-living stage) accomplished by a passive trans- jerence,—a process which we shall have specially to notice when the final fate of an asexual internal parasite comes to be treated of. This transference is, however, by no means always the result of a change of hosts. . . . In the mesenteric artery of the horse there is commonly to be found a more or less conspicuous aneurismal swelling. This is caused by parasitic Nematodes, belonging to the life-cycle of Sclerostomum equinum (Strongylus equinus), which originate from the above- mentioned (p. 61) Rhabditis-like embryos. The worms live in the fibrous lining of the aneurism (Fig. 52), and grow to an inch in length; they then, after casting their skin, change into the adult condition, which is characterised not merely by the development of the sexual organs, but by the possession of a conspicuous horny mouth-armature with a serrated margin! Ripe sexual products are indeed some- times absent, having been developed directly after the animal has aban- doned its first habitation for the intestinal canal. This wandering, then, as has been pointed out, takes place without the parasite having to leave the body of its host. Subsequently the worm becomes detached from the lining of the aneurism, and is carried by the blood stream into the branches of the arterial system of the intestine, until their decreasing size puts a stop to further progress, Here the parasite begins to bore through the wall of the intestine, which it accomplishes by the trephine- like action of its mouth-cavity, and reaches its ultimate destination. But such instances are particularly rare. Whenever we have had the opportunity of observing, under similar circumstances, the trans- ference of an Entozoon to its definitive condition, it is always ac- complished by the worm—and its host—being devoured by the definitive host.2 The importance of this for the distribution of Fic. 52,—Worm aneurism of the horse. For a detailed account see Vol. II. * Occasionally the reverse is the case, as in certain tape-worms (Ligula, Schisto- cephalus), which are often taken up by water-fowls directly from the water (see p. 25). Digitized by Microsoft® ACTION OF THE DIGESTIVE JUICES. 75 Entozoa we need hardly adduce special cases to prove. The parasites are in this way handed on from one animal to another,—from an aquatic to a terrestrial animal, from a cold-blooded to a warm-blooded creature.1 The bearer of the encysted parasite falls a prey to some more powerful foe, and is devoured by it: neither herbivorous nor carnivorous animals are secure from the invasion of parasites. The possibility of the transference of parasites increases of course with the number of animals that are devoured, and especially since the bearers of encysted parasites are usually small invertebrates. The larger animals, which need more nourishment, thus take in a gradu- ally increasing number of parasites; and it is easy, therefore, to understand how it is that of all animals the Vertebrata are most affected by these creatures (see p. 11). Only when the parasite has been transferred to the body of its right host, and other circumstances are favourable, does it arrive at sexual maturity ; otherwise it rapidly dies. In the same way, the eggs, unless they reach the body of their proper host, die and decay. The first change that takes place is the dissolution of the cyst, which, as in the case of the egg-shell, is accomplished by the action of the digestive juices of the stomach; the parasite then usually makes its way into the intestine. It re- mains for some time exposed to this action of the digestive juices, longer, perhaps, than the embryos hatched from the eggs, which, on account of their small size,can move about more freely, and also, possibly, bore into the = ye, 53,—Bladder- ‘Fic, 54.—-Bladder- walls of the stomach. A longer worm with extruded worm head after di- . é iS cela head. gestion of the caudal contact with the digestive Juices bladder. is but rarely dangerous, since they are protected by their large size, and relatively small super- ficies, as well as by the thickness of the cuticle. Sometimes, how- The same thing holds good for the so-called Leucochloridium, and its brood of Distomes (see p. 71). 1 ‘That temperature has an effect upon Entozoa is shown by the fact that the Dis- tomum of the bat undergoes no further development during the winter sleep of its host, (van Beneden, “ Les Parasites des chauves souris,” Mém. Acad. Belgique, t. xi., p. 23, 1873). [In the same way, the Entozoa of cold-blooded animals, when they have not arrived at maturity, stop their metamorphosis during the winter, and produce no eggs, or only very few ; and also, under similar circumstances, Rediz, instead of producing Cer- carie, give rise to new Rediz. .—R. = : Digitized by Microsoft® 76 LIFE-HISTORY OF PARASITES. ever, this is not the case, as in the so-called “caudal” bladder of bladder-worms (Figs. 53 and 54), which has a large surface and com- paratively thin walls. This bladder is frequently dissolved,* so that the only part which reaches the intestine of the host is the head, which is the most important part of the bladder-worm. There is also no doubt that the varying digestive power of the juices exercises a considerable influence on the fate of these parasites, just as we saw that it did upon the young individuals hatched from the egg in the alimentary tract of their host (p. 59). If the action of the diges- tive juice be not strong enough, as in the case of the frog, which is incapable of dissolving the cysts of Trichina, or if it be too strong, and therefore destroys the parasite as well as its cyst, there is evi- dently an end to the life of the intruder. In these cases the host is not the proper host, for it does not afford conditions suitable for further development.” Besides the action of the digestive juices, there are other im- portant factors to be taken into consideration. In the Trematodes, for instance,—at least, in those that perform their migration as free larve (p. 72),—the presence of a capsule is necessary to further development (de la Valette), but not in Tenia, perhaps because the former, in consequence of their small size and delicate covering, require some protection against the action of the digestive juices of the host. The nutrition required by the parasites themselves is variable in a still higher degree; but we will return to this point later. These processes that I have briefly noticed in the foregoing pages have been proved experimentally step by step. In this way we know that bladder-worms and muscle-7richinw arrive at maturity in the intestine of their proper host, and that the Hchinorhynchus-embryos of our common Gammarus and Asellus become adult in fish (Eehino- rhynchus proteus) and water-birds (Echinorhynchus polymorphus). Thus also the encysted Nematode of the meal-worm (Fig. 45, C.) has been shown to develop in the stomach of the mouse into Spiroptera murina (vel obtusa), and Distomum echinatum of the pond-snail (Paludina) to acquire sexual organs in the bodies of ducks. The life-history already quoted (p. 49) of Filaria sanguinolenta renders it probable that the sexually adult parenchyma-worms also 1 In another place I have experimentally shown that the same alteration takes place outside the body of an animal, ‘‘ Blasenbandwiirmer,” p. 156. ? The first changes often go on in the “wrong ”’ host, and in experiments by the aid of artificial digestion, as well as in the proper host. The Cysticercus of the pig, for instance, when introduced into the alimentary canal of the dog and rabbit, becomes on the follow- ing day a free tape-worm head, just as if it were in the human alimentary canal ; but it does not develop any further, and soon dies, Digitized by Microsoft® PERIODIC PARASITES. 77 are developed from younger stages that are also internal parasites. Filaria Medinensis has been shown by Fedschenko? to make its way into its host as a larva concealed in the body of a Cyclops, which is swallowed along with the water in which it lives. The young worms then reach the intestine, where they remain but a short time, and then bore their way out. The analogy of Filaria sangwinolenta, which is often met with in a larval condition in the so-called “worm- knots,” suggests this, and also the consideration that the difficulties of further internal wandering increase with the growth of the worm. It is no doubt a fact that large, full-erown thread-worms and Tenia do bore through the alimentary canal, and even the body-wall of their host; but this is rare, and when it does occur, the progress of the worms is no doubt assisted by pathological conditions set up in the tissues by their boring. These facts have no special importance in the life-history of parasites, and are rather to be looked upon as accidental, often indeed seriously affecting the life of the host. It does not at all follow that every Entozoon that lives outside the alimentary canal must necessarily pass through the latter to reach its desired locality ; Nature has many ways of achieving her ends. An instance of this is afforded by Pentastomwm tanioides, which has a life-history like that of a typical Entozoon. The young form (for- merly described as a distinct species, Pentastomum denticulatum —Fig. 56) inhabits cysts in the liver and lung (Fig. 55) of herbivorous mammals; presently the young animal breaks through its cyst, and makes its way into the body-cavity, after causing considerable injury to the tissues during its transit, and occasionally even causing the death of its host. Sometimes it wanders again into the viscera, most frequently the lymphatic glands. If the body of its host be devoured by a dog or some carnivorous animal, the young Penta- stomum, if not already encysted, finds its way directly through the nostrils (and perhaps also the posterior nares) into the olfactory cavity, where it attains sexual maturity. This habit of active migration accounts for the presence of special organs of locomotion, hooks and spines (Fig. 56), which are developed towards the close of the resting stage, and finally laid aside after they have served their purpose. If the young Pentastomum left of its own accord the body of its host, and sought out no fresh host, it would be an example of a periodic parasite attaining sexual maturity while leading a free life. That there are parasites with a life-history of this kind, was briefly stated at the commencement of this chapter ; they are mainly insects, especially flies and wasps. The e Vol. IT, Digitized D y Microsoft® 78 LIFE-HISTORY OF PARASITES. Gordiacezee and Mermithide are instances of this kind of parasitism among the Entozoa, and the migration from the body of their host of Fie. 55.—Lung of rabbit infected Fic. 56.—Pentastomum denticulatum. with Pentastomida, proglottides and other sexual Helminths (¢g., Oxyuris vermicularis) presents an approximation to the same phenomenon. The young of these periodic parasites, at least in the case of insects, show certain peculiarities induced by the fact that their migration into the body of a host is accomplished for them by their parents. The latter, possessing as they do the power of free locomo- tion, can evidently influence considerably the fate of their eggs, which is quite as evidently impossible to the Entozoa. Thus the gad-flies lay their eggs on the hair of certain mammals, in situations whence the young can easily in an active or passive manner (¢y., by being licked up) reach their next destination. The Ichneumonide make matters easier still for their descendants, by depositing their eggs directly in the perivisceral cavity of caterpillars, for which purpose they are provided with a suitably constructed boring ovipositor. The converse of this is illustrated in the Gordiacee and Mermithide, whose eggs are laid in water or damp earth, and the young when hatched find their own way by active migration into their proper host, as has already been said. Whether the embryo be conveyed passively or actively, it makes its way into the body of its host, and becomes in Digitized by Microsoft® INTERMEDIATE AND DEFINITE HOSTS. 79 the interior of the infected animal (sometimes even in the intestine, eg., Gastrus equt) a parasite which may be compared to the second developmental stage of a Helminth. Though there is but rarely an actual encystation (even in the Helminths this condition is sometimes absent), the parasite usually remains quiescent for a time, which it spends in growth and preparation for its future metamorphosis. At the end of this period, it instinctively begins to travel, and leaves its place either by the natural passages (the gad-fly of the horse, for instance, through the anus, that of the sheep through the nasal cavities), or if this be impossible, by boring through the tissues; the parasite thus arrives at sexual maturity at large, and often differs markedly in form from the preceding larval stage. _ This wandering often causes the death of the host when it is only a small animal, which is hardly surprising, considering the relative size of the parasite and the injuries it must cause by making its way out. In Gordius the life-history is more complicated, inasmuch as this parasite passes into a second host before commencing its meta- morphosis. There are some facts which show that this is not peculiar to Gordius, and that certain other Nematodes have in all probability a similar life-history. It is evident that, in spite of apparent differ- ences, the parasitism of Gordivs is fundamentally similar to the cases already mentioned, and may without any difficulty be classed with them. In both cases there are three life periods, generally distinguished by a difference in form—the embryo, the sexually mature adult, and an intermediate stage, which, in view of its outward characters, may be termed a“ pupal ” stage, if the use of this word will not bring us into a hazardous conflict with the customary terminology when we come to treat of the larvee of parasitic insects. Each of these three stages represents in its biological relations a special department of life. The embryo is destined to commence the parasitism ; it migrates, while the “pupa” resumes the prematurely broken development, and carries it on so far that, after passing to the third stage, sexual maturity appears. The migration, which is the cause of this transitional condition, is usually passive, requires no special advances in structure, and is not effected by any particular developmental conditions. This is, of course, merely arough sketch of the life-history of para- sites, and may he regarded as a generalised description, subject, there- fore, to manifold variations in the way of either greater complexity or greater simplification. Complications arise, for example, by the intro- duction of an intermediate generation with independent migrations. On the other hand, the life-history may be simplified by the inter- 1 For details see Vol. II. Digitized by Microsoft® 80 LIFE-HISTORY OF PARASITES. mediate stage passing directly, and without migration, into the sexual condition. All this, however, is quite exceptional, and the rule for the life- history of parasites may be stated as follows :—The life-history of para- sites is divided into two stages—(1) the larval, and (2) the sexually mature adult; and each of these is passed in the body of a separate host. Sometimes these two hosts may be merely two individuals of the same species, as in the case of Trichina; but generally they are quite different, and may belong even to separate orders or classes. Tenia crassicollis inhabits the liver of the mouse while in the young condition, and the intestine of the cat when adult; Tenia marginata, the connective tissue of sheep and oxen when young, and finally the intestine of wolves and dogs ; the adult Tenia sol¢wm of man is found in the young condition in swine. Ina similar way the life of Zigula is divided between fish (Cyprinide) and water-birds; of Hehinobothrium typus, between rays and Gammarina ; of Distomum echinatum, between ducks and Paludina; of Amphistomum subclavatum, between the frog and Planorbis ; of Pen- tastomum tenioides, between the dog and rabbit, and so forth. These examples do not merely prove the justice of the general principle just enunciated, but also bring out prominently the fact that the host of the young parasite is frequently an animal which serves as food for the definitive host ; thus the mouse yields to the cat not only its flesh, but its parasites, and the like happens with the rabbit and dog, the fish and the sea-gull. And this fact is not difficult to understand from a physiological as well as a teleological point of view. If one animal select as its food a certain other animal, it evidently follows that the latter is best suited to its nutritive requirements, hence the conditions of nutrition in both must be somewhat similar, and a parasite capable of living in one would probably also find the other in a great measure favourable to the conditions of its life. This idea, however, must not be pushed too far, since we find, for example, the young of Tenia _ erassicollis in many animals which are not preyed upon by cats; so also the human tape-worm is occasionally found in the asexual state in man himself,—a fact which, on the principles just enun- ciated, would seem to justify cannibalism from the stand-point of natural history. The presence of the young stages in Carnivora is certainly to be looked upon in the above light. The Herbivora also often contain parasites which live in the young stage in bodies of other animals; but in these cases, the latter inhabit the same + The statement of Von Siebold (“ Handwirterbuch d. Physiol.,” Bd. ii., p. 647), repeated recently by Ercolani, that the Herbivora become infected with their parasites through the medium of their food, because the parasitic Nematodes of many plants develop in their bodies, has no foundation. The Nematodes of plants are independent species, which are never parasitic upon animals. (On the other hand, the recent researches of Digitized by Microsoft® LARGE NUMBERS OF EMBRYOS. 81 localities, and have been probably swallowed accidentally along with the food. Local conditions also are of great importance in the distribution of parasites, as has been shown by Melnikoff and myself, in the case of the dog-louse (Zrichodectes), which harbours the young of Tenia ellip- tica (Fig. 45, B) and passes it into the dog. Although the life-histories of parasites largely depend, in the most varied manner, upon the mutual relations of the animals that are their hosts, it is also true that chance plays a very large part in their determination. It is quite by chance, for example, that the egg meets with its proper host, or that its host is subsequently devoured by some other suitable animal. The more complicated, in fact, does the life- history of the parasite become, the greater risk does it run of not being able to complete its life-cycle. Millions of germs perish for one that reaches maturity.2 We have, however, already spoken of this, and shown how it is compensated by the immense fertility of parasitic worms. “If the eggs and embryos of Helminths always attained to a suitable environment, the bodies of all men would be absolutely full of tape-worms, Nematodes, and other parasites.” And it need hardly be pointed out that the lives of the parasites, as well as of their hosts, would be greatly endangered by this. The compli- cated life-history of the parasites serves as a means of checking their too rapid increase, and their metamorphoses and migrations, theneters, are of the highest benefit to them. Von Siebold has considered that those Entozoa found in the bodies of the wrong hosts have “lost their way.” ® Nothing can be said against this simple statement, but the conclusions which: he has drawn from it are by no means correct. In the first place, it must be remembered that any animal which has wandered into a locality where its proper food cannot be obtained— a stranded whale, for instance—may be said to have “lost its way.” The expression ought not to be confined to parasites, although perhaps the occurrence is more general with them. Weinland speaks in the following way of the life-history of corals :1—“ During the breeding Thomas and myself render it very probable that ruminants and other herbivorous mammals devour Distomum hepaticum along with plants, to which Cercarie are attached in the encysted state. —R. L.] 1 Archiv f. Naturgesch., Jahrg. xxxv., Bd. i., p. 62, 1869. See also Vol. IT. 2 A tape-worm has an average life of two years. It produces in this time some 1500 segments (see p. 43, note), each containing 53,000 eggs, the total number of eggs being therefore about 85,000,000 ; since the number of tape-worms remains about the same, one only of these 85,000,000 of eggs reaches maturity. The probability, therefore, against each tape-worm arriving at maturity is as 85,000,000 to 1. 8 « Handworterbuch. d. Physiol.” Bd. ii., p. 650. Digitized by Microsoft® 82 LIFE-HISTORY OF PARASITES. season of the coral polyps, myriads of microscopic embryos swarm in the neighbourhood of the parent stock ; millions of these are washed out to sea and on to dry land, and perish, or fix themselves in posi- tions where they cannot grow; but if only a single one find a spot suitable to its growth, Nature has accomplished her purpose, and if this one have reached a spot, perhaps hundreds of miles away, where no corals previously existed, it founds a new colony, which possibly, after the lapse of time, rises as an island out of the sea. These embryos attach themselves to any firm point, but there is no instinct leading them to select a favourable place; Nature, therefore, produces them in such countless numbers, that, on the theory of probability, some are certain to obtain a suitable locality.” Who would deny that this is precisely analogous to Helminths “losing their way ?” Moreover, von Siebold does not say of those Helminths that they have wandered into the wrong host, but into the body of some animal “not appointed to be their host.” But this expression has really no de- finite meaning. If a parasite develop in any given locality, we may conclude that it finds there the necessary conditions of existence ; if it do not develop, we may likewise conclude that the conditions are unfavourable ; but who would undertake to decide whether or noa particular host were “appointed?” Von Siebold, however, goes still further ; he states that these parasites which have lost their way do not usually die, but continue to grow, “though, on account of the un- favourable environment, they do not thrive, and fail to attain sexual maturity,” and in fact “degenerate.” Von Siebold maintained this opinion,® even after Kiichenmeister had endeavoured experimentally to contradict it;# indeed his words at Kénigsberg in 1860 show that he was then still convinced of the accuracy of his opinion :—“I cannot understand why the possibility of degeneration in worms is not admitted, since the same thing has been shown to take place in © higher animals, as a result of unusual conditions of climate and changed food, and is regarded as a cause of the formation of new species. If in many races an extraordinary growth of hair take place, in some ruminants the horns become larger, the ears of certain domes- tic animals become larger and droop, and in others again a local deposition of fat takes place; why is it not possible that in many lower animals the influence of varying conditions of the body may give rise to the presence of a serous fluid in certain parts, a local dropsy ?”5 * Jahresh. d. Ver, vaterl, Naturkunde Wiirttemberg, Bd. xvi., p. 31, 1860. 2 Loe. cit. ® “Ueber Band- und Blasenwiirmer, u. s. w.,” p.65. : Leipzig, 1854. * Prager med. Vierteljahrschrift, Jahrg. ix., p. 106, 1852 ; “Ueber die Cestoden im Allgemeinen, u. 8, w.,” p. 12: Zittau, 1853. ° “ Konigsberg Naturf. Versamml.,,” 1860. Digitized by Microsoft® DEGENERATION OF ENCYSTED PARASITES. 83 These last words show that the author regarded the asexual bladder-worms as Helminths that had degenerated and become drop- sical, in consequence of having lost their way, and got into the body- cavity or muscles of their host instead of its intestine. Bladder-worms (Fig. 57) and encysted Trichine (at that time only known in the encysted condition—Fig. 58) were the only parasites regarded thus by von Siebold, and at that period neither the im- portance nor wide distribution of the encysted condition in the life- history of parasites was understood, the generally received opinion being Fie. 57. Measly port (natural size). Fic. 58. Trichinosed pork (enlarged 45 times). that the germs migrated immediately into the body of their definitive host. At this period, then, von Siebold’s hypothesis was an attempt to explain certain striking and unintelligible facts, but has now be- come out of date. It is just these bladder-worms and Trichine that have become by a remarkable concurrence of circumstances the very subject of experimental investigation, and we are now thoroughly acquainted with their natural history. There is not the slightest doubt that what von Siebold considered to be abnormal conditions are in reality the ordinary stages of development ; that 7richina, before arriving at sexual maturity, always passes through a stage in which it is encysted in muscle, and that in the same way tape-worms are invariably derived from bladder-worms. We can, therefore, lay aside ars: ; ; ‘ m von Siebold’s theory, BiuleedtBy now. hardly, any supporters, in spite of the great reputation of its originator. 84 LIFE-HISTORY OF PARASITES. Our remarks, moreover, are only directed against the practical application of the degeneration theory, and not to an equal extent against its theoretical truth. Degeneration per se is quite as possible among Helminths as any malformation, which is the result of an unusual or insufficient combination of the causes of development. In this way are to be regarded certain varieties in the form of rh ©) Helminths, as, for example, the so-called 7 Echinococcus multilocularis or Cysticercus ra- cemosust (Fig. 59), considered even quite Fre. 59.—Cysticercus racemosus Tecently to be “degenerated” (that is to (after von Siebold). say, pathologically altered) forms. We are, therefore, in principle brought back to von Siebold’s standpoint, or even to that of Pallas (1767) and Hartmann (1685), who were the forerunners of von Siebold in this theory,* but, nevertheless, we cannot agree with the applications of that theory, and the conclusions that were drawn from it. This matter, which has been briefly touched upon, leads us naturally to an inquiry into the conditions of development among Helminths, or, in other words, the influence which the environment exercises upon their development. An Entozoon, as we know, only develops when its host can fulfil its needs. What happens, then, if it cannot? It has been already said that a parasite under these circumstances perishes; but the con- ditions which give rise to this decay, and the time that it takes, are subject to great variations. In the next place, we may mention that the conditions which render development possible are remarkably restricted, so that circumstances of the most varied kind come into play. It is, for example, a well-known fact that the famous Ccenurus only develops in lambs. It has been proved experimentally that the older the lamb becomes the more difficult it is to rear the worms in its body. After feeding the animal with the young parasites, it can be stated with almost mathematical accuracy when it will begin to show the effects of the infection ; while it can be stated with the same probability that an older sheep will continue in good health though infected with an equal number of parasites. These facts are not peculiar to Ccenurus, but common among bladder-worms, though 1 See for a description of this peculiar form Heller in Ziemssen’s ‘‘ Handb. d. sp. Path. u. Ther.,” Bd. iii., p. 334 (Eng. transl., ‘‘ Cyclop. Pract. Med.,” vol. iii., p. 598,- London, 1875), and Marchand, Virchow’s Archiv f. pathol. Anat., Bd. lxxv., p. 104, 1879. Numerous other observations have been made on the frequently irregular forms of the bladder-worm of the brain in man by von Siebold, Krabbe, and others. 2 See the historical introduction in my ‘‘ Blasenbandwiirmer,” pp. 11-13: Giessen, 1856. ; Digitized by Microsoft® NUTRITIONAL RELATIONS OF HELMINTHS. 85 the immunity from their attacks enjoyed by older animals is not perhaps so strikingly shown as in this case. Nothing is known of the reason of these phenomena. It is quite uncertain whether it be the nutritive conditions that influence the parasite, or perhaps the greater readiness with which the young tissues yield a path for its wandering. It is not, however, age alone which limits the conditions of this individual development, for we may ob- serve, although rarely, even among old sheep, fresh cases of staggers ; whereas, on the other hand, among lambs we may find single instances of complete immunity from these parasites. Thus, for instance, Baillet mentions the case of a lamb which was fed with mature seg- ments of Tenia cenwrus nineteen times in the course of about eight weeks, and still no Ccenurus was developed.1. The experimental helminthologist has often, and even under similar conditions, to register most unexpected results. Thus the author had fed a dog with a multiple-headed Ccenurus, and in about ten days the intestine was filled with more than a hundred completely developed tape-worms ; while in another similar experiment, after three weeks there were found in the intestine of the dog only heads of tape-worms, with bands of segments an inch long attached to them in a few cases only ; and in a third experiment a decidedly negative result was obtained. These three cases are, of course, exceptions, for we may accept it as a rule that the Coenurus-heads develop completely within three weeks into sexually mature tape-worms. Usually, however, even after this time one may find single immature tape-worms, sometimes even here and there an isolated head; but these are irregularities which depend rather upon the parasite than upon the host. Analogous cases are also observed in other helminthological experiments. Just as old sheep cannot be infected by means of the embryos of Tenia cenwrus, 90 in a similar manner muscle-7richine develop only rarely in dogs, even though the embryos are found to wander in masses from the intestines into the body-cavity. Similarly Pagen- stecher and I were unable to obtain muscle-Zrichine in birds, although thousands of pregnant animals lived in the intestine. In pigeons the introduced worms never reached sexual maturity. They grew and became similar in appearance to mature animals, but the sexual organs remained without germinal substance (Leuckart). From these instances, it is apparent that the conditions of develop- ment of parasites are circumscribed—that is to say, besides the “right” 1 Ann. Sci. Nat., sér. 4, t. x. p. 190, 1858. In a similar manner Fiedler (“ Zur Trichinenlehre,” Deutsches Archiv fiir klinische Medicin, Bd. i., p. 68, 1865) reports the case of a man who ate a piece of raw meat, which was strongly infected with Trichine, without suffering from it. Picitized by Microsoft® 86 LIFE-HISTORY OF PARASITES. and “wrong” hosts, there are also hosts which only partially satisfy the needs of parasites. In such hosts the invading parasites do not perish immediately after their. introduction; on the contrary, they begin to develop as usual, and continue to do so up to a definite point, without, however, completing their development. Whether the worms continue for a longer time in this immature condition, or whether they perish earlier, will apparently depend upon the degree to which the conditions of development influence the life of the parasite. These may vary in individual cases. Moreover, the above-mentioned phenomenon may be confirmed by other: examples. If a rabbit be fed with the bladder-worms of the conmnon tape-worm of the dog, they not only undergo the usual changes during their sojourn in the stomach (as has been already observed above (p. 75) with regard to the bladder-worms of the pig originating from Tenia solium), but they also delay for some time in the small in- testine, fastening themselves to the intestinal wall just as in ordinary cases. Some of them even develop a short segmented chain, which differs from the normal beginning of the body of the tape-worm only in that its segmentation is less complete. But here the development stops, until after about ten to twelve days the young worms have all perished (von Siebold, Kiichenmeister). A similar result is obtained by feeding with Tania cenurus. Since the bladder-worm of this tape- worm usually develops only in the brain of lambs, one would expect that the embryo would migrate only thither. But this is not the case, as has been already mentioned (p. 67). The embryos scatter hither and thither from the alimentary apparatus into the diverse organs of the host ; but everywhere, with the exception of the brain, they perish very soon after their entrance. If the infected lamb be examined about three weeks after the feeding, one finds, besides a number of small rounded bladders situated in the brain, which are the first beginnings of the future Ccenurus, also numerous white knots, having the appearance of tubercles, which are situated in the liver and lungs, and more especially between the muscles, and upon closer exa- mination can be recognised as cysts, which have developed round the embryos. Sometimes these cysts may contain the young of the invad- ing tape-worm—small, more or less opaque, and shrunken bladders— more rarely they are unchanged and alive, like those in the brain, although for the most part less in size.! In rare cases one of these small bladders may develop in time into a complete Coenurus.? 1 See on this head, and on the development of Coenurus in general, Haubner, Gurlt's Magazin fir pathol. Anat., pp. 248 and 375, 1854. 2 Thus Eichler found » developed Coenurus under the skin of the sheep, Nathusius under the skin of an ox. Also in the rabbit the Coenurus has been observed in the peripheral organs, but never as yet in the brain. Digitized by Microsoft® DURATION OF LIFE AND ULTIMATE FATE. 87 The causes of these diversities are not yet known to us. We can- not even explain why the liver or the intermuscular connective-tissue has a different influence upon the development of the brood from that of the brain. The supposition that the cause lies in certain differ- ences in the supply and quality of the nutrition cannot of course satisfy us. Here and there, however, the space afforded by the different organs, and the peculiarities in the anatomical arrangement of the tissues, may influence the development of the parasites. In this way the bladder-worm in the brain of man grows in the subarachnoid space into sinuous strings, dilated here and there into vesicles, which may reach a length of 25 cm., but only rarely develop a head, so that the true nature of this formation (the above-mentioned Cysticercus race- mosus) has only recently been recognised. Also the so-called “ multi- locular” Echinococcus is perhaps only due to the position it occupies in the body; whilst, on the other hand, the sterile Behinococeus (Acepha- locyst) and the hydatid—two other forms which can scarcely be con- sidered as normal—point to conditions of development which have to be sought in another direction. There are only few animals in which the metamorphoses and their individual phases are dependent upon the influence of external factors to such a degree and in such an evident manner as is the case with the Entozoa. The young parasite, if the opportunity of migrating to its definitive host be wanting, remains stationary for life at a staye of its development, which its more fortunate associates, and perhaps even their descendants, have long ago left behind them. Moreover, it is not only the periods of migration and development which cause Helminths to be dependent on external circumstances to such an unusual degree. In their later period of existence also their in- dependence is only a limited one. All the different disturbing influences which attack the host, and endanger its health and existence, have more or less an indirect influence upon the indwelling parasite. Some may by certain steps be expelled from the intestine and other internal organs, whilst others may perish through the inflammatory condition of their dwelling. For this reason also it is difficult to determine with certainty the natural length of life of parasites. Concerning some, of course, we know that they continue to live in exposed situations, not only for several years, but even through a whole decade (Bothriocephalus latus, Tenia saginata); but others again scarcely live longer than a few weeks. On the whole, the period of existence of Entozoa may be assumed to be longer than that of free-living animals of a similar size. 1 Thread-worms often forsake the intestine on the occurrence of diarrhea. Digitized by Microsoft® 88 LIFE-HISTORY OF PARASITES. This is especially true with regard to immature encysted forms, which often live for several decades. With regard to Hchinococci and muscle- Trichine, it is well known that in some instances they have lived over twenty years, whilst their corresponding sexual forms perish in a few weeks, The changes undergone by those Entozoa that die in their host, and remain there, differ according to circumstances. Some pass through the process of mummification, others that of fatty degenera- tion, and others again calcify. Digitized by Microsoft® CHAPTER VY. THE ORIGIN OF PARASITES, AND THE GRADUAL DEVELOPMENT OF PARASITIC LIFE. Ir we endeavour to summarise our knowledge of parasitic life as we have delineated it above, and to express its principal modifications in a few words, we shall arrive at some such scheme as the following: I. Temporary Parasitism.—To this category belong almost ex- clusively ectoparasites, which differ from their free-living relations in respect only of the quality and source of their nutriment. II. Stationary Ectoparasitism.—The animals in this group show, on the whole, only slight peculiarities. They either pass through all their developmental stages (from the egg onwards) on the host, or at first lead an independent existence under a form more or less different from the adult. IIL. Lntoparasitism.—The entoparasite is always stationary, but, with the exception of Rhabditis, which is apparently only occasionally parasitic, never passes through all its developmental stages in one host. The young brood is expelled, either in the form of free embryos or of more or less developed (perhaps wholly undeveloped) eggs. In the latter case the embryonic development occurs in the free state. But from this stage onwards the fate of the embryos varies in different directions. (1.) The embryos of Entozoa lead a free life for some time under an altered form (Nematodes, with Rhabditis-like young stages). They are not only capable of free motion, but take in food just in the same way as other creatures. (a.) In the course of this free life the young form arrives at sexual maturity, and thus only the sexually produced descendants re- turn again to parasitism (Rhabdonema (Ascaris) nigrovenosa).* (v.) The young form itself becomes again a sexually mature parasite at a certain stage of its development. From the commence- ment it enters its definitive host, and its development ends there, although this happens occasionally, as in the case of 1 Since the name Ascaris is quite incorrect to apply to these animals, I shall adopt henceforth the new generic e.Rhabdonema,to designate them. semis BiG zeh DY MicrosBne 90 THE ORIGIN OF PARASITES Sclerostomum equinum, in a provisional situation. To the former class, among others, belong certain Strongylide. (2.) The embryos find, in pursuance of an active or passive migration —without ever having led a free life—an intermediate host, in the organs of which they develop into a larval form, which then ends its life-history under various circumstances. * The larva migrates, and becomes, after complete metamorphosis, a free-living animal (Oestride among other flies, Ichneu- monide, Mermithide, Gordiacez.) (v.) The larva arrives at sexual maturity in its intermediate host without further metamorphosis, as in the above-mentioned Archigetes and Aspidogaster (p. 69). (c.) The larva remains in the intermediate host until it migrates into its definitive host, mostly in a passive way (with food), The developmental history in such cases is extended over two different hosts. This is the form of life-history which we discover in the majority of intestinal worms, and may be considered as really typical among Entozoa (Cestodes, with the exception of Archigetes, Acanthocephala, Distomide, and Pentastomide). In individual cases there are here certain modifications, thus— (a.) The number of intermediate hosts increases, whilst the larva either migrates of itself, and seeks a new host (certain Cestodes), or produces asexually a new generation, which then enters upon a similar change of host (Distomide). (8.) The intermediate and definitive hosts become one and the same when the embryos do not forsake the latter, but simply wander into its peripheral organs, and there develop into larvee (Trichina). (8.) The embryos pass at once, whilst they are yet enclosed by the egg- shell, in a passive manner into the intestine of their definitive host, and here complete their further development. To this class belong numerous Nematodes, especially Trichocephalus and Oxyuris. (a. ~~ The order in which we have drawn up the various modifications of parasitic life affords us at the same time a picture of the gradual in- crease in development of which this life is capable. The first + The form of entoparasitic life which is here shortly characterised is that which was earliest known to us, and when the first edition of this work appeared, it was the only one known. The existence of the other forms (1 a. and b., and 3) has only been proved later through my researches, especially with regard to Nematodes. (See Vol. II.). To these Helminths, which develop according to the newly discovered laws, belong the most impor- tant human parasites ; and yet Kuchenmeister (‘‘ Parasiten,’”’ 2d Ed., Preface) maintains that to the knowledge of parasites “nothing new or of practical importance can be added” beyond what he had asserted ! Digitized by Microsoft® FROM FREE-LIVING FORMS. 91 beginnings are lost, as has been remarked above (p. 1), in the pheno- mena of ordinary life, which latter evidently forms the starting-point of parasitism, or in other words, parasites have, by acconvmodating themselves to the conditions of a parasitic life, in course of time sprung Jrom creatures originally free. The mode of origin which we thus assert for these creatures is in principle precisely the same as that which we also assert, in consonance with the doctrine of descent, for the individual free-living forms, when we maintain their development to have been brought about by means of various influences, either directly from one and another, or from a common original form. The manner of adaptation is of course dit- ferent, inasmuch as in the case of free-living animals there is usually ‘a development of faculties which bring about a more extended and complicated capacity, whereas parasites, on the contrary, have a cor- respondingly limited relationship to the outer world, according to the degree of their parasitism. It is only under the influence of ever- changing surroundings, and in the full enjoyment of unembarrassed activity, that an organism can develop itself in every respect and fully form its capacities. Limitation of function is succeeded by stunted growth, and this it is which gives to parasites—at least to stationary parasites—their peculiar features. The organs and arrange- ments which serve to act upon the outer world, and are excited by it, disappear under the influence of a confined existence ; and by thorough- going parasites this is the case often to such a degree, that the whole organism, which at other times is so artistically formed, degenerates into a simple tube, whose capabilities are almost entirely expended in nutrition and generation.? These influences of parasitic life are especially apparent in those forms, the near relations of which lead a life either completely, or at least to a great extent, free. The classical researches of Johann Miller? have made us acquainted with a Mollusc (Entoconcha mirabilis) which, in its young form, possesses the usual attributes of these animals, and does not differ from related young forms any more than the latter do from each other; it lives also, for a time, in the ordinary free state, 1 This view had already been advocated previously to the rise of the Darwinian theory. In the case of Epizoa by Nitzsch (Magazin der Entomologie, Bd. iii., p. 261, 1818), and for Entozoa by my uncle Fr. S. Leuckart (“Versuch einer naturgemissen Eintheilung der Helminthen:” Heidelberg, 1827). The latter says (Joc. cit. p. 10), “ The Helminths show a manifold relationship and likeness to other orders and classes, but at the same time present important differences from the related forms of animals, which, without doubt, have their origin in the entirely different mode of life of the parasitic worms, and in their circum- scribed and completely isolated abode.” 2 J. Miiller, ‘‘ Ueber Synapta digitata und die Enstehung von Schnecken in Holo- thurien :” Berlin, 1852. ate . Digitized by Microsoft® i 92 THE ORIGIN OF PARASITES. but ultimately becomes parasitic, at the same time losing not only its shell—which is also the case with certain other snails—but also its locomotor, sensory, and alimentary organs, and degenerates into a simple sac filled with sexual products. In the form of this “snail- sac” the parasite is found in the body-cavity of the vermiform Holo- thurian (Synapta digitata), having its thickened knob-like anterior ex- tremity inserted into the intestinal vessel of its host, so that it may easily be mistaken for a true organ of the latter. No one, without knowledge of the young form, could recognise its Molluscan nature. If we regard this retrograde development as a consequence of para- sitism, we do not thereby mean to imply that this exerts its influence from the commencement, and with full force in each individual animal, and that the same process is repeated de novo each time in a similar manner. The influence which the external relations of life exert upon the development of an organism in the present case, as everywhere else, can only have been a gradual one, which must have continued to work for many generations before it could produce such extreme effects. It is not a sudden transformation, but a slow and steady progressive adaptation to the conditions of a parasitic mode of life, of which we see the results in the above-cited organism. We must accept the conclusion that the Mollusc—to continue with our example —has not exhibited this particular form of parasitism from the com- mencement, but has only gradually adopted the above-described mode of life. When the number of parasites in any group of animals is in- creasing, we often see also the various stages of parasitism in exist- ing forms allied to each other. The sum of the degeneration and transformation is then seen to be of different extent in different species, for the transformation of the organism in no case goes further than the circumstances of the parasitic life require. Step by step we can see how, under such circumstances, animals that feed usually on organic detritus, like the Asellide, or lead a predatory life like the free-living Copepoda (represented in our waters by the genus Cyclops),? exchange their free life for a parasitic one. Often they are only tem- porary parasites, differing from the most nearly related forms perhaps only in the possession of more powerful hooks, whilst in other cases they continue for a longer period upon their host. They lose the power of locomotion they previously possessed, since their extremities atrophy in consequence of disuse, and become stunted in their growth *T have followed in the account given above the usually accepted view, but I may add that the transformation of the snail into the so-called snail-sack, has not as yet been directly observed. ‘ * See v. Nordmann, Mikrographische Beitrdge, Bd. ii, : Berlin, 1832. Digitized by Microsoft® DIFFERENT DEGREES OF PARASITISM, 93 according to the degree to which their parasitism becomes stationary. Likewise, also, the sensory perceptions, with their corresponding organs, degenerate. The body loses its segmentation, and finally becomes changed into a cylindrical mass, which not only swells considerably under the pressure of the rapidly erowing sexual organs, especially the ovaria, but often becomes deformed in a most irregular manner. Such extreme cases are exhibited among the Copepoda by the Ler- neade,? among the Isopoda by the Entoniscide,? which live an entirely entozootic life. But even in these extreme cases the parasitic Crustacea possess, in their young state, the same organization as do the allied free-living forms, and, with a similar form, they lead also at first a similar life. The transformation into the definitive condition is slow and gradual, and is brought about by a metamorphosis which runs parallel with every change in the relations of life? That the metamorphosis is retrogressive on the whole, and that it advances to different degrees according to circumstances, has been mentioned above; I will only add that—in correlation with a previously mentioned fact (p. 44)— it often reaches a higher degree in the female than in the male. In the same manner also, as in the case of the parasitic Crustacea, the natural relations of the Gregarines, of the itch-mites, and of the mosquitoes, may be determined to the free-living forms related severally to each of them. But among the parasitic insects there are forms in which the relations are less evident, and the intermediate connecting links are wanting. For instance, the lice and fleas stand, notwithstanding their large number of species, to a large extent isolated from their related forms. They possess characteristics so different that no connecting links have as yet been found, so that even the systematic position of these animals appears in no way deter- mined. The same is the case with the greater number of the so-called intestinal worms. The groups Cestodes, Trematodes, and Acantho- cephala consist entirely of parasites, although they differ from each other in the degree of their parasitism, especially the Trematodes. The tape-worms and Acanthocephala are capable only of a parasitic life, through the want of a mouth and alimentary canal; for a free life presupposes the capacity of taking up nutritive substances into the body directly by means of a permanent or temporary opening. Among the intestinal worms there is only a single group which 1 ©, Claus, ‘‘ Beobachtungen tiber Lerneocera, &c.:"” Marburg, 1866. 2 Fr, Miiller, Archiv fiir Naturgesch., Jahrg. xxviii, Bd. i, p. 10, 1862; Jenaische _ Zeitschr., Bd. vi., p. 53, 1867; and Buchholz, Zeitschr. f. wiss, Zool., Bd. xvi., p. 103, 1866. 3 See Claus, “ Beitrige zur Kenntniss der Schmarotzerkrebse,” Zeitschr. f. wiss, Zool., Ba, xvi, p. 865, 1864. Digitized by Microsoft® 94 THE ORIGIN OF PARASITES. has related forms living in the free state, and that in considerable numbers, namely, the round-worms, or Nematodes. But the free-living Nematodes have only recently become the subject of a close investi- gation.! Only a few decades ago, scarcely half a dozen of these forms were known, and these only imperfectly, so that naturalists, mistaking their natural relations, were inclined to class them with the Infusoria rather than with the Nematodes. Under such circumstances it seems easy to understand how the older helminthologists entertained the view that the internal parasites stood isolated, not only biologically but also systematically, from other animals. They united them into a single class (Entozoa), which, although nearly approaching the free- living worms, was understood to have no close relation to them. It will be obvious that such a connection helped greatly to displace the processes of entozootic life from their natural connections. Under its influence parasitism appeared in science as a phenomenon swt generis, which could not be judged according to the laws of ordinary animal life, but, on the contrary, was thought to be opposed to these in many of its relations. On a former occasion (p. 22 et seg.) it has been shown at length how for a long time special and peculiar laws were supposed to govern the existence and origin of the Entozoa, and howthese had been invented, for the most part by systematic helminthologists, until they ultimately learned to judge facts more correctly and more in accordance with nature; and thus the relations of the Entozoa to the free-living animals have found a more proper recognition. As has been mentioned, the relations are most evident among the Nematodes, which are a group of animals whose representatives, far from being exclusively Entozoa, have in the free state such a wide dis- tribution, and under such varying circumstances, that the number of parasitic forms, although also great, is far outbalanced by the former. It would, of course, be impossible here to attempt a full description of these free Nematodes. For our purpose, it will be sufficient to remark that they live in the sea, in fresh water, in mud, and in the earth ; and that sometimes they lead a predatory existence, at other times they live on decaying matters. To the latter belong the best known and most widely distributed forms, the species of Dujardin’s genus Rhabditis, above mentioned (Leptodera ; Pelodera, Schneider). They are animals of small size, which live everywhere in large num- bers where the earth is impregnated with decaying organic substances, and differ from their related forms, especially in the structure of their alimentary and sexual organs. Especially characteristic is the highly muscular cesophageal tube, which encloses in its posterior 1 Mspecially by Bastian, Ebert, Schneider, Biitschli, Marion, and de Maan. Digitized by Microsoft® FREE-LIVING NEMATODES—RHABDITID&. 95 globularly expanded portion (the so-called “Bulbus”) an armature usually formed of three valvular teeth (Fig. 60). Sexual maturity is attained only through abundant nutrition, mostly only in places where a mass of decaying matter has been formed. In such localities the generations follow upon one another often so closely, that the young worms may be found there in large numbers and in all stages of development. When this decay- ing matter ceases to exist, either through being exhausted or dried up, then the creatures scatter and continue in the larval state, until some favouring fortune grants them the possibility of further development. In this young state, pro- vided with a cystic larval membrane (with oc- cluded mouth and anus), they can withstand desiccation for a considerable time without perish- ing. Under certain circumstances these mouth- less larvee reach the interior of living animals, where they then, evidently in consequence of their parasitism, enter upon a course of develop- ment which differs considerably from their usual life-history. This is specially the case with a species which was first described by its dis- coverer, Schneider, under the name Alloionema appendiculatwm,+ though he has more recently Be opera rae adic correctly recognised it as a Rhabditis (Lepto- “°") °"** *° Youns- dera).? The researches of Schneider, and more especially of Claus,® show that the parasitism of this interesting form is a purely optional one, and that it can be abandoned without change of its specific characters. In the latter case the life-history follows the ordinary . course; but it is otherwise when the larve have the opportunity of migrating into the black slug (Arion ater). In this they de- velop into animals which reach double their size (over 4 mm.), not- withstanding the absence of a mouth; they also lose the chitinous cesophageal teeth and awl-shaped caudal point they formerly pos- sessed, but there develop instead two finely streaked long cuticular bands at the posterior extremity of the body, whose function is most probably that of organs of touch, seeing that they occur also in other Nematode larve in this position. The parasites, however, attain 1 Zeitschr. f. wiss. Zool., Bd. x., p. 176, 1860. : 2 Monographie der Nematoden,” p. 159: Berlin, 1866. * “ Beobachtungen iiber die Organization and Fortpflanzung von Leptodera appen- diculata :” Marburg u. Leipzig, 1868. + See Vol. IT. oh ee 5 Digitized by Microsoft® 96 THE ORIGIN OF PARASITES. sexual maturity only after abandoning this host, when they cast their,skin, and lose their riband-shaped caudal appendages, while the apertures of the alimentary and sexual organs break outwards through the cuticle. In the sexually mature state also the size and formation of the tail characterise these animals as a peculiar form. Even the internal organization shows many differences. The uterus contains at least 500 to 600 eggs, whilst in the female developed from the free larva it encloses two or three dozen eggs at the utmost. In both cases, however, the eggs develop within the body of the female into embryos, which are exactly alike in size, form, and organization; and may also attain to sexual maturity in the free state in the presence of nitrogenous food material, without the need of migration into slugs. Hence there is no doubt that the parasitism in this case is merely collateral with the free state, and is of importance in the maintenance of the species only so far as—in agreement with the relations pre- viously indicated—it affords the possibility of producing a more numerous progeny. At the same time it is evident that the devia- tions in the structure of the parasitic generation are in correspon- dence with the altered circumstances of its life, and are conditioned by them. The appearance of parasitic generations side by side with free- living ones, which in the case of the above-mentioned Rhabditis appendiculata was only possible under certain circumstances, is more conspicuous in other instances, and becomes ultimately a constant phenomenon. The parasitic generations intercalate them- selves between the free-living, in regularly alternating succession, just as do the so-called “nurses” between the sexual animals in the case of alternation of generations. But the intermediate generations are not asexual like the nurses, which, as is well known, produce their successors asexually, but they are complete sexual animals, equivalent morphologically to the free-living generations, and in some respects even occupying a position superior to them. Such is the case with the above-mentioned Rhabdonema (Ascaris) nigrovenosum (p. 2), whose Rhabditis-form, living in the excrement of frogs, differs very little from the animals related to it. Like other species of Rhabditis of small size (Fig. 61), it attains sexual maturity within a short time, and produces several embryos, which are hatched within the body of the female, and, as has also been observed in the case of other Rhabditide, remain there until they have completely destroyed and devoured the internal organs. Also, at the com- 1 T have for some time been accustomed to call such an alternate succession of dimorphous sexual generations by the name ‘‘ Heterogeny.” Digitized by Microsoft® RHABDITOID PARASITES. 97 mencement, the young have the characteristics of the genus Rhabditis, but lose them while yet in the maternal body; after they have attained a certain size, they cease to eat, and undergo further de- velopment only after having found an opportunity of becoming trans- ferred into the lung of a frog, and thus exchanging their former mode of life for a parasitic one. The adaptation to the circumstances of parasitic life is much more complete in these worms than is the case in Rhabditis appendicu- Fic. 61.—Rhabditoid form of Rhabdonema (Ascaris) nigro- Fic. 62.—Mature em- venosum. A. Male; B. Female, with embryos in bryo of Rhabdonema various stages of development. nigrovenosum. lata. When they reach the lungs of their host, the young parasites leneth of almost, an inch, and possess scarcely the slightest pee aise Digitized by Microsoft® = 98 THE ORIGIN OF PARASITES. trace of similarity to their predecessors; they live for several months, during which time they produce a countless number of egos, which are hatched while yet in the uterus, and afterwards pass into the intestine of their host. During their stay in the intestine the embryos escape from the shell; they again become small perfect Rhabditide (Fig. 62), and remain in this form in the cloaca, unaltered, until they are expelled with the excrement, when, if surrounded by putrescent matters, they complete their life-cycle in a few days. The remarkable circumstance that the parasitic fhabdonema nigrovenosum is always found only in the female form, at first led me to suppose that they propagate their species by parthe- nogenesis; but I have since found—as also Bischoff had previously done—that in several individuals there were seminal corpuscles in the posterior portion of the ovary among the eggs; so that I am now prepared, with Schneider and Claus, to regard this form as a herma- phrodite, which, as is also known to be the case in certain instances of free-living Rhabditidse,t produces seminal corpuscles in sexual organs of otherwise female structure for some time before the ova make their appearance. But I must add, that in many cases I have sought in vain for these seminal corpuscles; and other helminthologists have also experienced the same difficulty—eg., von Siebold—so that the sibility of a parthenogenetic development is not yet entirely excluded. [It was to be expected « priori that Rhabdonema nigrovenosum could not be the only Nematode possessing so peculiar a life-history ; but the statement of Ercolani? as to the descent of the A. injlewa and A. vesicwlaris of hens from certain free-living Rhabditis-forms, has no foundation in fact. On the contrary, my recent researches* lead to the conclusion that the so-called Anguillula stercoralis (an unmistakeable Zhabditis found in the excreta of patients suffering from diarrhea in warm countries, and especially Cochin-China) pro- duces sexually a new generation, which becomes transformed in the intestine into the so-called A. intestinalis, represented, like Rhabdo- nema nigrovenosum, only by female individuals. The same is true of a sausage-shaped anenteric Nematode (Allantonema mirabile, Leuck- art*), which is parasitic in the body-cavity of Hylobius mini, and con- 1 See Schneider, ‘‘ Monogr. d. Nematoden,” p. 313; and Vernet, Arch. Sci. Phys. Nat., t. xlv., p. 61, 1872. : . ® Ercolani, “ Sulla dimorphobiosi, &c.,” Mem. Accad. Bologna, t. iv., p. 237, 1874, and t. v., p. 391, 1875; Abstr. Journ. de Zool., t. iii., p. 67, t. iv., p. 254, * Leuckart, ‘‘ Ueber d. Lebensgesch. d. sog. Anguillula stercoralis, u. deren Bezieh. wa d, sog. A, intestinalis,” Bericht d. math. phys. Cl. k. Stichs. Gesellsch. Wiss., pp. 75-107, 1882. * Leuckart, ‘‘ Ueber einen neuen heterogenen Nematoden,” Bericht d. Versamml. deutsch. Naturf. Magdeburg, p. 320, 1884; a more detailed account will shortly appear in Bericht. d. math. phys. Cl, k. Stichs, Gesellsch. d. Wiss. Digitized by Microsoft® RHABDITOID LARVAL FORMS. 99 tains in the uterus-like terminal portion of its generative organs an innumerable quantity of Rhabditoid embryos, which become free by boring to the exterior, and grow into mature males and females without essential change of form.—R. L.1] But even the single example of Rhabdonema is sufficient not only to place beyond doubt the special relations between parasitic and free life, but to prove further that the former, instead of being collateral, or even subsidiary to the latter, as in the case of Rhabditis appendiculata, may, under certain circumstances, become more conspicuous ; the im- portance of the free life, of course, becoming less in the same proportion. This alteration in the relative importance of the two conditions of life has by no means reached its extreme point in RLhabdonema, for, according to the above-mentioned (p. 61) researches, there is a whole series of parasitic Nematodes (especially in the family Strongylide), among which the Rhabditis-form, instead of representing an indepen- Fic. 63.— Dochmius trigonocephalus. A. Free-living young form ; B. Young parasite. dent generation which precedes the parasitic, is limited to the young stage of this latter, and passes on at once into the parasitic condition. After the manner of the common Rhabditide, these worms live at first free in mud and damp earth, where they feed and grow until they have attained a definite size. With the shedding of their skin the characters of the genus Rhabditis are lost, and also the possi- bility of their former mode of sustaining life. The worms, however, continue to live for some time under the former conditions, but only so long as the reserve material gathered in their interior is sufficient to meet their necessities. In order to grow further, and to complete their metamorphosis, they must exchange their former free life for a parasitic one, and only in the interior of a living animal do they find the conditions for their complete development. ' The above passage has been substituted by the author for one in the German edition, a Digitized by Microsoft® 100 THE ORIGIN OF PARASITES. Notwithstanding all differences, the constitution of the young form points unquestionably in all these cases to the relations which obtain between it and the Rhabditide. The differences, moreover, are not so great as they might seem at first sight, for, on the whole, they are limited to the fact that the former condition of life, which was spread over two generations, is now drawn together into one; and this isa ‘phenomenon which we often meet with in animal life. I need only remind the reader, by way of example, that in nearly related forms the alternation of generations is often represented by a metamorphosis in which the former preliminary generation is represented only by the characters of the young form. But even these traces of a former independence may be more or less completely lost, for we know that besides the species with alternation of generations and metamorphosis, there are very often others in which the state which was passed through by the former as a free larva is relegated to the period spent i ovo; so that thus birth occurs at a stage of development which was previously attained only in the free state. In such cases, of course, all those properties remain latent which enabled the respective conditions to obtain external manifestation; and the form which in the previous case was living and mature, is now indicated only in such faint outline as is necessary for accomplishing the transit into a new stage of de- velopment. Such being the case, we have, then, no right to make the existence of a Rhabditis-like larva the exclusive criterion for the rela- tions which obtain between the parasitic and free-living Nematodes. By means of a continuous and ever-increasing adaptation to the con- ditions of parasitism, this larval form may disappear, or, more correctly, it may become unrecognisable in the processes of development in ovo, Through such abbreviations of the history of development there may then arise forms like Oxyuris, Trichocephalus, Spiroptera, and others, with embryos, which are not hatched in a free state, but remain in the ege until they have found a host (p. 66). The differences which exist between these species must of course be considered in exactly the same way as the specific differences between free-living creatures. In every case the characters of an animal are the factors which determine its mode of life; so that if two animals deviate from each other, their capacities also vary, and that in exact proportion to the degree in which they differ Trichocephalus and Spiroptera live under other conditions than Oxywris. Although they are all Entozoa, and even inhabit the same organs, yet they differ in manner of locomotion, nutrition, and propagation, as well as in other functions. It is these very differences which find expression in the peculiarities of the external and internal structure, since the Digitized by Microsoft® ABSENCE OF A RHABDITOID SAGE. 101 animal-body is plastic, and capable of adapting itself to the con- ditions of a specific mode of life. Hence we must leave it doubt- ful whether the unmistakeable similarity which Oxyuris (Fig. 64) presents in many respects (especially in the form of the body, structure of the alimentary canal, and sexual apparatus) to Rhabditis, is the consequence of such a secondary adaptation; or whether it may be interpreted as a mark of closer genetic rela- tion. But it is not only the developed animals which present such conditions of adaptation, but also the embryos. Whether these remain where they have become free, or forsake the place of their birth and migrate; whether in their migra- tion they break through tissues and organs of a particular character ; whether their locomotion be rapid and energetic or not ;—all this finds expres- sion in form and structure, and often expresses itself in forms which, notwithstanding a common »,, ¢ £ Dapntaanblyue type, frequently differ widely from each other. (young). In this way may also be explained the fact that there are Nema- todes whose embryos exist without a Rhadditis-form for a time in the free state, until they migrate into their host in some way or other. Such embryos do not lead a true free life, like the Rhabditide, for they neither feed nor grow, but resemble free-living animals, in so far as they have the power of independent locomotion. It is owing to this circumstance that they are able to escape many of those casualties which otherwise determine the distribution and transference of helminthic germs. There are, then, certain advantages connected with such a larval form, and it may be these which have brought about its existence. It is plain that the form and structure of the embryos change in manifold ways, according to the varying conditions (locality, mode of locomotion, character of the skin to be penetrated); and this fact is obvious on even a superficial examination of the embryonic forms, say of Cucullanus or Dracunculus on the one hand, and Strongylus filaria on the other (Fig. 65), and may be estab- lished even by a most superficial research. The impossibility of ob- taining nutriment naturally makes it necesssary in all cases that the duration of such larval stage must be short; and, generally, the shorter the more lively is the locomotion which the embryo exhibits. I must of course leave it undetermined whether I have suc- ceeded in the above attempt to develop the phenomena of the parasitic life among the Mematades; in .comest and natural sequence, 102 THE ORIGIN OF PARASITES. from their earliest manifestation. Owing to the impossibility of checking reasoning by experiment, all such attempts have a more Fic. 65.—Embryos, A. of Cucullanus, and B. of Strongylus filaria. or less subjective character. It was not my intention to draw up a phylogenetic tree for the parasitic Nematodes, since that could be done only in reference to their relations, and might prove illusory in a very short time. What I aimed at was not more than to prove the possibility of such a relationship be- tween the free-living and parasitic Nematodes as would clearly allow of a derivation of the latter from the former, on the basis of biological knowledge.t I will therefore also grant that the connections may with equal, and perhaps even greater, right be sought in other directions than that followed by me. Thus, for instance, one might perhaps interpret the freely moving larvee which I mentioned last as being allied to the Rhabditis-like condition of other Nematodes, instead of explaining them to be only a subsequent adaptation, as I endeavoured to do; and one might, by the hypothesis of one diminished function (merely of locomotion), derive them from other Nematodes, and thus regard them in a certain way as degene- rated Lhadditis-forms. But in fact this is somewhat deceptive, especially when one considers larval forms of certain species of Strongylide, which, both by their organization and the systematic position of their parents, remind us strongly of the Rhabditis-like embryos of Dochmius and other Nematodes. Still, as above men- tioned, these are only possibilities, and hence remain always arbitrary. But thus much is established, that the parasitism of the Nematodes 1 Biitschli has attempted in a similar way to prove the relations that exist between the free-living and parasitic Nematodes.—Bericht d. Senkenb. naturf. Gesellsch., p. 56, 1872. Digitized by Microsoft® COMPLETE PARASITISM OF TRICHINA. 103 exists In various degrees, and, as a rule, attains its complete develop- ment only at the expense of a free life. The most complete case of this parasitism has not, however, hitherto found a place in our exposition. I refer to Trichine, which, as a rule, completes its entire life-history in the body of its host. The embryos, whicli'are born alive, soon bore through the wall C is SSSaae re