THE PARASITES OF MAN. THE PARASITES OF MAN, AND THE DISEASES WHICH PROCEED FROM THEM. A TEXT-BOOK FOR STUDENTS AND PRACTITIONERS. BY RUDOLF LEUCKART, FESSOR OF ZOOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LEIPZIG. TRANSLATED FROM THE GERMAN, JVITH THE CO-OPERATION OF THE AUTHOR, BY WILLIAM E. HOYLE, M.A. (Oxon.), M.R.C.S., F.R.S.E. NATURAL HISTORY OF PARASITES IN GENERAL. SYSTEMATIC ACCOUNT OF THE PARASITES INFESTING MAN. Protozoa— Cestoda. EDINBURGH : YOUNG J. PENTLAND. 1886. EDINBURGH : PRINTED FOR YOUNG J. PENTLAND BV SCOTT AND FERGUSON AND BURNESS AND COMPANY, PRINTERS TO HER MAJESTY. 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(Entozoa and Parasites of Man), writings which cover the same ground as my own ; but Kiichenmeister's work is entirely out of date, while Cobbold aims at giving a general sketch rather than a complete delineation of the group.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 V1 author's prepack. 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 author's preface. VII 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 modem 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-Eot, and Trichinosis. In spite of the importance attributed to the medicinal aspects of V1U AUTIIOII'S PREFACE. tho. 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, " Traite 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, m 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 has 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 LEUCKAET. LEirziG, September 18S6. 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 are in all cases indicated by my own initials. X TRANSLATOR'S PBEFACE! Thanks to the writings of Cobbold and others, our language already possesses equivalents for most of the technical terms in this work, hut it has always appeared to me that it would he 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 passu 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. CONTENTS. SECTION I. NATURAL HISTORY OF PARASITES IN GENERAL. CHAPTER I. NATURE AND ORGANIZATION OP 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 OP PARASITES. Abundance— Distribution— Respiration and Respiratory Organs— Ectoparasites and Entoparasites— Nutrition and Mouth -Organs— Encystation, . CHAPTER III. THEORY OF THE ORIGIN OF PARASITES REGARDED HISTORICALLY. Theory of Spontaneous Generation— Heterogeny— Linne 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 Kuehenmeister— Its further development, . . 22-41 CHAPTER IV. LIFE -HISTORY OF PARASITES. Sexual Maturity— Eggs and Embryos— Developmental Stage of Eggs when laid — Migration of the Eggs — Worm-Nests — Continuous development and Reproduction (Rhabditis) — Hsmatozoa — Development of the Eggs externally — Influence of Moisture — Constitution of the Egg-Shell — Influence of Temperature — Duration of development, . . . 42-57 Migration of the Young Brood — Eggs with contained Embryos — Escape of the Embryos after digestion of the Shell — Escape from the- Host — Free Embryos Xll CONTENTS. 57-6(3 66-70 or Larvae-Entrance of Free-living Larvae (Active Migration)-PaRsivo Migration (with Food)— Viability of the Germs, Development of the Germs after Migration-Direct development- Wandering within the Body of the Host-Development of the Larval or intermediate stage ("Helminths of the Second Developmental Stage ") — Sexually mature Larva, Change of Host-Development and Migration of the Distomes-Wandering of Strongylus-Oi Bladder- Worms-Action of the Digestive Juices-Migration of Pmtaatomum— 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 ' • CHAPTER V. - • THE ORIGIN OF PARASITES, AND THE GRADUAL DEVELOPMENT OP PARASITIC LIFE. Various kinds . of Parasitism— Relations to Free-living Animals— Free-living Nematodes — Mabdoncma nigrovenosum — Parasitic Nematodes with Rhabditiform Larvte — 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 atage 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 SECTION II. SYSTEMATIC ACCOUNT OF THE PARASITES INFESTING MAN. INTRODUCTION. Number of Human Parasites — Larval and Adult Parasites — Entozoa and Epi- zoa — Zoological position, ....... 173-174 CONTENTS. Sub-Kingdom I. — PROTOZOA. Characters and Classification-Unicellular Onanisms - Protophyta-Parasites resembling normal Cells, . • • • Class I.— RHIZOPODA. Organization— Modes of Reproduction— Foraminifera— Radiolaria— History of Parasitic Forms, Amoeba, Ehrenberg, Amoeba coli, Lbsch, • . Organization and Vital Phenomena— Mode of Infection— Pathological results — Experimental investigation, . Class II.— SPORQZOA. Organization and Occurrence — Gregarines— Pseudonavicellse — Psorosperms — Coccidia — Miescher's Tubes, . . . •-" . • •" Coccidirim, Leuckart, Coccidium oviforme, Leuckart, . . . . • • Organization — Development — Coccidia and Psorosperms — Pathological sig- nificance, ........ Class 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, Donno, .... Trichomonas vaginalis, Donne, Trichomonas intestinalis, Leuckart, . Order II. — Ciliata. Definition — Organization, Xlll VAGK 175-182 182-185 185 186 186-191 191-202 202 203 203-228 228-237 237-240 240 • 242 242-246 246 248 250 252-254 Family Buusaiue/E. Balantidium, Claparbde and Lachmann, 251 xiv CONTENTS. PAGE Balantidium coli (Malmsten), Stein, • .... 254 Definition— History— Occurrence— Structure and Mode of Life— Repro- duction— Infection, ...... 254-264 Sob-Kingdom IL-VERMBS. Definition — History— Subdivisions, ...... 265-268 Class I.— PLATODES. Definition and general characters, ...... 269-270 Order I. — Cestoda. Definition — Polyzootic nature — Head and Segments, .... 270-279 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, . 279-330 The Development of Cestodes — Historical — Migrations of the Embryos — Structure and Development of the Bladder- Worms — Cysticercoid Larvte of the Tseniadoe — Of the remaining Cestodes — Development of the Dibothria — Modification into the definitive state — General survey, . . . 330-387 SYSTEMATIC ACCOUNT OF THE CESTODES. Classification — Synopsis of Human Tape- Worms, .... 388-390 Family I. — Teniae. Definitions — General Structure — Fixing Apparatus— Malformations — Sub- divisions, ......... 391-400 Division I.— Cystic Tape-Worms (Cystici). Definition— General Characteristics— Rostellum— Specific distinctness of the various forms, 400-403 Subgenus Cystotffinia, Leuckart, ...... 404 Characters— Number and Distribution of the Species, . . • 404-406 Taenia saginata, Gbze, 406 Definition— Tape-Worms known to the Ancients— Rectification of Nomen- clature, 406-422 Growth and Structure of the Tape -Worm— Formation of the Head- Reproductive Organs— Development of Reproductive Organs— Unripe and Mature Uterine Eggs— Malformations— Defective and Super- numerary Joints— Prismatic Tape- Worms— Perforated Worms, . 423-458 Development and Structure of the Bladder- Worm— Experimental Rearing— Acute Cestode Tuberculosis, . . • • • ^ Distribution and Frequency— Modes of Infection— Duration of Life- . . .„ . . 476-488 Medicinal significance, . CONTENTS. -xv 488 488-490 490-498 Taenia solium, Rudolphi, . Definition— General Characters, Origin and Development from the Bladder-Worm of the Pig-Breeding Experiments-Breeding of the Bladder-Worms-Of the Tape-Wonn- Occurrence of Cyslicercus ccllulosw, • Development and growth of Tcenia .rfiam-Development 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, . 498-518 Organization of Tcenia solium— Ripe Proglottides— Head and circlet of Hooks— Development of the Generative Organs— Pipe Eggs, . 519-528 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 — Cyslicercus racemosus, Cyslicercus turhinatus — Oldest record of Bladder- Worms in Man— Symptomatology of the Disease, 521-561 Taenia acanthotrias, Weinland, ...... 561 Definition and History, ....... 561-563 Taenia marginata, Batsch, ...... 563 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, .... 563-578 Subgenus Echinococcifer, Weinland, ..... 578 Peculiarity of the Cysticercoid Stage — Specific distinctness — Metamor- phosis of the Hooks— Historical Account of the Echinococcus — Acephalocysts, ....... 578-586 Taenia echinococcus, von Siebold, ..... 586 Definition — Description of the Adult Worm — Generative Organs — Duration of development — Supposed occurrence in Man, . . . 586-594 Development of the Echinococcus-THladder — Experimental Breeding — Structure of the Cuticle — Absence of Vascular System, . . 594-603 Structure and Development of the Ech in ococcus -Heads — Brood-Capsules — Budding of the Heads, ...... 603-611 The Formation of Daughter-Bladders — Echinococcus simplex or granosus — Interlamellar Budding — Echinococcus hydatidosus— Metamorphosis of Heads into Bladders — Metamorphosis of Brood-Capsules into Bladders —Direct formation of Daughter-Bladders — Echinococcus mulli- loculuris— Chemical Constitution of the Bladder- Wall and Fluid, . 611-631 xvi CONTENTS. Occurrence and Medical Significance— Multiplo Echinococci— Etiology- Distribution and Frequency of the Disease— Echinococcus 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, .... Division II.— Ordinaky Tape- Worms (Cystoidei). General Characters — Larval Stages — Number of Species, Subgenus Hymenolepis, Weinland, Taenia nana, von Siebold, . Definition — Development — Eggs, . Taenia flavo-punctata, Weinland, Definition and Characters, Taenia madagascariensis, Davaine, Definition and History, Subgenus Dipylidium, Leuckart, . Taenia cucumerina, Rudolphi, Definition — Historical Account — Development — Egg- Masses, .... Melnikoff's Discovery- 632-652 652-656 657 657 657-661 881 661-663 663 663-665 665 665 665-673 Family II. — Botheiocephalim:. Definition — General Characters — Head, Nerves, and Excretory Organs — Generative Organs — Number of Species, . . .' . . . 674-682 Bothriocephalus, Rudolphi, 682 683 Bothriocephalus latas, Bremser, ..... Definition — Historical Sketch — Anatomy — Muscles — Vessels — Generative Organs — Male Organs — Female Organs — Their Development — Abnor- malities, . 683-714 Occurrence — Historical Sketch — Braun's Discovery— Early Stages of Development — Ciliated Embryos — Metamorphosis, . . . 714-729 Distribution and Medical Significance — Modes of Infection — Specific and Individual Differences, ...... 729-735 Bothriocephalus cristatua, Davaine, . Bothriocephalus cordatus, Leuckart, . Definition— Occurrence in Man— Description and Specific Distinctness- Peculiarities of Young Forms, 735 736 736-745 745 Bothriocephalus liguloides, Leuckart, Definition — Historical Account — Occurrence in Man — Anatomy- Disposition of the Organs — Structure of the Head, . . . 745-751 LIST OF ILLUSTRATIONS. Fin. PAGE Aeanthobothrium coronatum, larval state of, after van Beneden, Oil Amozba coli ■ in intestinal mucus, with blood-corpuscles, Schizomycetes, 186 and similar bodies (after Liisch), .... 94 Anthomyia canicular is — 143 Larva of, . 88 Larva of, from the intestine of man, .... 7 15 Archigctes Sicboldi (x 60), 47 69 73 116 >> ... 220 376 >> ji ..... 353 676 f3 SI ... 356 681 A scaris lumbricoidcs, Eggs from, ... 32 53 Aspidogaster conchicola (after Aubert), .... 48 69 ,, !>••••* 74 115 Balantidium coli — In conjugation (after Wising), ..... 129 260 In various stages of division, ..... 130 261 With widely opened peristome (dorsal view), 127 255 Bladder-worm — From the brain, with a spirally coiled body ( x 12), 301 556 From the pig, after the digestion of the bladder ( x 20), 287 513 Head after digestion of the caudal bladder, 54 75 Head of, from the pike, ...... 390 727 In the anterior chamber of the eye, after de Wecker ( x 3), 299 553 Longitudinal section through the head process ( x 40), . 286 506 Of the pig, with evaginated head ( x 2), 23 36 M jj ... 285 506 With invaginated head ( x J) . 23 36 jj >> .... 284 506 Sagittal longitudinal section through the protruded head of, from the pike, ....... 392 128 The head and receptacle of, from a muscle about 6 mm. in size ( x 25), 281 503 Transformation of, into a tape-worm (Twnia scrrata), 26 37 With extruded head, ...... 53 75 Young, from the pike, with invaginated head, . 389 727 >> ))•-.. 390 727 Bladder-worm of the rabbit — . Cephalic end of a young ( x 45), ..... 189 347 Longitudinal section through the head of ( x 60), 193 350 Metamorphosis of, into the young tape-worm ( x 4), 224 382 Transverse section of the anterior end of, at the level of the suckers ( x 40), ...... 192 503 Undo saltans (after Stein), 118 237 b xviii LIST OF ILLUSTRATIONS. Bolhrioccphalus — p,° Diagrammatic representation of the course and connections of the vagina, as seen in longitudinal section, Egg of, with imperfectly developed embryo, being expelled by compression, .... Eggs of, with operculum, Encapsuled larva of a, from the smelt, . Head of a, reared in the cat from bladder-worms from the pike (after Braun), . Larva of, from the skink ( x 20), Larva of, from the smelt, Segment of, with yolk chambers and "yellow ducts" (after Eschricht), 370 Transverse section through the body of a larva of, Young, from the alimentary canal of the dog, Young, from the intestine of the cat, after feeding with bladder- worms from the pike, Bothriocephalus cordatus — A number of mature joints of, From man, ..... >> Head of, from the side and from above ( x 8), . Four young specimens of, ..... Head and anterior portion of ( x 5), Transverse sections of the head of ( x 20), Uterus of, Bothriocephalus latus — ....... >> jj • ,, ,, (cephalic end) ( x 8), Ciliated embryo of ( x 500), ..... Club-shaped head of, ..... Development of the reproductive organs in, Diagrammatic representation of the course and connections of the vagina, as seen in longitudinal sections, Egg of, with embryo, ...... Egg, showing yolk-cells and shell ( x 300), Eggs of ( x 300), ....... Embryo of, escaping from its ciliated envelope, . Embryo of, in the egg, ...... Female generative organs of, from the ventral surface ( x 20), . * j j j .... Female sexual organs of, showing the uterus, ovary, shell-gland, and yolk -gland ( x 12), Free ciliated embryos of ( x 500), .... Free embryo of , . Free-swimming embrvo of, .... (x 500) (x 600), . . Free-swimming embryo of, with the protoplasmic threads, &c. Generative organs of (ventral aspect), .... Head of ( x 8), . Larva of ( x 55), . Larva of, with protruded head, Larvaj of, from the pike, 371 709 384 722 38 59 375 715 380 719 352 674 221 377 370 708 391 728 376 716 379 719 395 737 350 674 397 739 398 739 401 743 140 278 394 737 399 739 400 742 396 737 137 275 357 684 226 389 177 327 393 734 372 712 367 701 36 57 171 321 359 685 388 725 385 723 366 700 369 705 157 305 386 723 40 60 70 110 351 674 374 715 387 725 141 278 358 684 354 676 378 718 360 686 377 717 LIST OF ILLUSTRATIONS. Longitudinal diagrammatic representation of the three generative ducts, . Male and female sexual organs of ( x 20), Male generative organs of, seen from the dorsal surface Mature joint of ( x 8), Ovum of, with yolk-cells and shell, Ovum of, after Schauinsland, with four embryonic cells and en- veloping cells on the granular yolk ( x 600), Another ovum, with covering cells apposed to the body ( x 600), . Ripe joint of, with the uterine rosette ( x 6), Series of joints with double genital apertures, Sexual organs of, from the ventral side ( x 20), . Transverse section through the body of, at the level cirrhus-pouch ( x 10), Transverse section through the head of a young ( x 55), Transverse sections through the body of ( x 10) . Bothriocephalus liguloides — . . . • Head of ( x 5), Transverse section through the larval body of, . Bothriocephalus proboscideus, Excretory apparatus of, after Steudener (x 32), Bothriocephalus salmonis, Embryonic development of, after ( x 300), ...... Brain of a lamb with tracks of Cmnurus, x 20), embryonic of the Kolliker 363 170 365 362 381 382 282 368 373 159 364 355 361 402 404 403 155 178 81 206 Brood-capsule — Closed and ruptured, showing their connection with the paren- XIX PAGE 694 318 698 693 721 722 722 702 713 306 695 678 689 746 751 749 301 328 135 359 chymal layer ( x 40), .... 325 606 Development of, and of the appended heads ( x 90), 328 610 Metamorphosis of the, into bladders, after Naunyn ( x 90), 332 621 With heads of Echinococcus in the interior ( x 40), 324 605 Cercaria — A free, ....... 50 72 A free and an encapsuled, the latter without tail, 21 and 22 34 An encysted, without tail, .... 51 73 Oercomonas from the liver (after Lambl), 122 244 Gercomonas intestinalis (after Davaine), 121 242 Cercomonas muscce at different stages (after Stein), 117 ■ 237 Coccidia — Development of psorosperms in, . 111 213 Enclosing psorosperms, ..... 112 214 From the human liver, ..... 114 223 From the intestine of the domestic mouse, 100 197 jj n • 113 219 From the kidneys of the garden snail, 101 198 From the liver, ...... 110 210 Coccidium-nodvle, Cross section of a, slightly enlarged, 108 208 C'occidium oviforme — From the liver of the rabbit, .... 102 198 » >, (x 550), . 106 204 Gmnurus — Head and body of, m situ ( x 100), 197 352 Heads of ( x 25), 203 356 Passages of, in the brain of a lamb, 181 340 XX LIST OF ILLUSTRATIONS. Cucullanus, Embryo of, Cysticercus — ' Head rudiment of an adult ( x 12), ' Subretinal, in the eye (after de We'cker), With evaginated head ( x 3), Cysticercus acanthotrias— Head and circlet of hooks seen from above, after Weinland ( x 60) Hooks of, after Weinland ( x 280), . Cysticercus arionis — With head retracted and protruded ( x 50), Cysticercus celluloses — Completion of the head formation in ( x 15), Head of, with rudiment of the receptacle ( x 25), Metamorphosis of the head-process into the head proper '( x 20)' The begmning of the bending of the head inside its receptacle (x 25), . Various stages in the formation of the head of (x 45), With the formation of the head just beginning ( x 10), With the head in the receptacle ( x 2), Cysticercus fasciolaris, Cysticercus glomcridis — (After Villot) ( x 50), . (x 200), . (x 50), . Cysticercus pisiformis — A piece of liver from the rabbit, showing passages made by, Before the development of the head, with granular sheath and cyst ( x 60), ..... Head and body of, in completely evaginated state ( x 19), Head of, with vascular system ( x 45), . Head of, just mature ( x 40), Metamorphosis of the head of ( x 45), Piece of rabbit's liver with passages of ( x 10), With head half evaginated ( x 6), Young, .... Cysticercus racemosus — (After Marchand), (After von Siebold), Cysticercus Tcenice saginatce — Embedded in the muscle, Evaginated head of ( x 30), Head of, with frontal sucker and vascular ring ( x 30), Head-rudiment of, before and after the development of the suckers ( x 25), , Longitudinal section through the head in situ ( x 30), . Cysticercus tenebrionis, Development of, after Stein ( x about 100), Cysticercus tenuicollis — (After Bremser), ..... Anterior end of, with retracted neck and ribbon-like appendage Exit of a young, from the liver, .... Longitudinal section through the head of an adult ( x 20), Fro. 65 268 298 269 302 303 209 336 283 279 282 280 188 278 235 202 236 210 214 337 46 183 199 190 191 194-196 82 207 198 12 300 59 267 270 248 265-66 271 208 305 313 83 312 PAGE 102 465 552 465 562 562 362 652 505 502 505 503 346 501 404 355 405 363 368 652 69 341 352 349 349 351 136 360 352 19 554 84 465 466 437 463 466 360 564 577 136 576 LIST OF ILLUSTBATIONS. XXI PIO. I'AUK 310 573 311 575 310 573 309 572 the body-cavity 213 366 29 45 68 104 69 108 35 57 39 60 The head process ( x 15), Three months old, Young, . Young, hi situ, . Development of an EchinococcusAike cysticcrcoid from of the earth-worm, after Mecznikoff ( x 25), Distomum haematobium, male and female, Distomum hepaticum — (Natural size), . Ciliated embryo of, with an eye speck, . Egg of, with embryo, Eree embryo of, . Distomum luteum (young), with suckers and viscera (after de la Valette), 1 6 Dochmius duodenalis, Cephalic extremity of ; profile and front view, . 10 18 Dochmius trigonocephalus — A, Free living young form ; B, Young parasite, 63 99 Rhabclitis-\\k% condition of young stage of, . . 43 62 Echeneibothrium minimum — (After van Beneden) ( x 8), . . . • . . 226 389 Chain of joints of, . . . • • .134 273 ,, isolated living head and tape-worm, . 24 and 25 37 Isolated living head of, from the intestine of Trygon pas- ■ tinaca, ..... • 133 273 Strobila and proglottis of (after van Beneden), . . 135 and 136 274 Echinococcus, . . . . • ■ • .13 19 Before the beginning of the segmentation ( x 15), . . 319 591 Bladder, eight weeks old ( x 20), . . . .321 597 Brood-capsule of, with adherent heads in various stages of de- velopment ( x 36), ...... 204 357 Brood-capsule of, with retracted head and with two appended buds at different stages ( x 100), . . . .314 579 Diagrammatic representation of a proliferating, . . . 205 357 Head, metamorphosis of an, into a bladder in the interior of the brood-capsule, after Naunyn ( x 60), . . . 331 620 Head, with the anterior part of the head invaginated ( x 90), . 323 604 Heads, development of the, from those hanging freely into the internal cavity of the brood-capsule, after Wagener ( x 90), 327 608 Hooks (x 600), ....... 315 581 In its natural size and position, ..... 329 613 Proliferation of the membrane of an, .... 330 614 Young, four weeks old, escaping from the capsule ( x 50), . 320 594 Echinococcus multilocularis ( x 30), ..... 335 629 Section through an, ...... 333 624 Echinococcus raccmosus, . . . . . .334 628 Echinococcus vclcrinorum — . .... 317 587 Brood-capsule of, with adult and hollow rudimentary heads (x 40), 326 608 Head of ( x 90), ....... 322 603 Echinorhynchus angustatus, male, . . . . . 11 18 Embryos of ; (A) the profile; (S) ventral view,. . . 72 112 l'i li 'inorhynchus gigas, Egg of, with embryo, .... 37 57 Krldnorhynchus spirula, natural size (after Westrumb), . . 71 110 Eggs of worms found in the alimentary canal of man, . . .90 146 Entozoa in the second stage of development, .... 45 68 xxii LIST OF ILLUSTRATIONS. Ftiaria sanguinis hominis (after Lewis), -Flea, Larva of the, Flesh of pig with bladder-worms (natural size), Flesh of pig with Trichina ( x 45), Gregarines, encapsuled ; (A) after conjugation \ (B) after formation of pseudonavicellae, (A) Monocystis agtiis ; {B) Gi-egarina cuneata ; (C) Stylorhynchus ohgacanthus, Hcxamita intestinalis, in the young and adult states (after Stein) Hn-udo medicinalis, Cephalic end of, with the three mandibles at the base of the oral cup, Infusorians with undulating longitudinal membrane from the'intestine of the hen (after Eberth), . Lacerta vivipara, Unarmed cystic worm from the body-cavity of ( x 30), Liver of a rabbit with Coccidiwm-nodules, Measly pork, " '> • Miescher's tubes, Extremity of one of, with its contents, Monostomum cqpitellatum — )> j, Ciliated embryo of, . Monostomum mutabile, Infusorian-like embryos of, with the " necessary parasite," Musca vomitoria, Larvse of, (Natural size and enlarged), Muscle- Trichina;, seven weeks old, in the distended sarcolemma-sheaths Nasua socialis, Kidney of, with Eustrongylus in the distended pelvis Oxyuris ambigua (young), Oxyuris vermicularis, Eggs of, . Paramwcium coli (after Malmsten), Pediculus (Phthirius) pubis, Pentastomida, Lung of rabbit infected with, Pcntastomum, Lung of a rabbit infected with, Pentastomum constrictum (after Aitken), Pcntastomum dcnticulatum, From the liver of man, . Piece of a rabbit's liver with bladder-worm passages ( x 10), Piestocystis variabilis, Longitudinal section of an unarmed cystic worm from the lung of a crow ( x 30), Pseudonavicellae, with germinal rods in their interior, . 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 Lieberkuhn), Psorospermise from the connective tissue of the human kidney (after Lindemann), ....... Psorosperms, (A), from the urinary bladder of Gadus lota; (B), from the gills of the bream, ...... Pulex penetrans, male and female, ..... Rainey's bodies, one of, within an isolated muscular fibre, enlarged 100 diameters, ........ Rainey's kibes enlarged about 40 diameters, .... Redia, Bojanus' "kingsyellow worms," from the pond-snail, . With brood of Distomes in the interior, (A ) with germs ; (B) with Cercariie in the interior ; (C) free Cercarise, Fin. PAGE 31 50 41 60 91 155 92 155 96 192 95 192 120 240 9 17 124 248 185 343 107 204 57 83 277 494 105 201 16 30 69 108 17 31 8 15 89 144 77 129 76 129 64 101 34 56 128 255 2 7 55 78 84 137 85 137 27 41 56 78 5 14 182 341 184 343 97 194 116 227 109 209 98 196 115 226 99 197 28 44 104 200 103 200: 18 31 75 118 19 31 excreta. LIST OF ILLUSTRATIONS. Rhabditis terricola, Adult female and young, Embryo of, ■ • 1 ' Rhabdonema (Ascaris) nigrovenosum, Rhabditoid form of, Rkabdonema nigrovenosu.ru, Mature embryo of, . Sarcoptes scabiei, Scabies norv'cgica, Crust of, with mites, their borings, eggs, and Sclerostomum tetracanthum, encysted, Scoleces ( x about 30), . • • • Spiroptera murina, Young form of, from the meal-worm, Sporocyst and Redia, with Cercarte in the interior, Strong ylus filar ia, Embryo of, . Tmnia, Double joint of, with three sexual openings, Embryo of ( x 100), Tmnia cwnurus ( x 10-15), Cephalic end of, with hexamerous symmetry ( x 25), Connection between the different parts of the female generative apparatus in, . . • Eorm of uterus in, Joint of, with excretory vessels and generative organs ( x 10), Larger and smaller hooks of ( x 280), Sexual organs of ( x 10), Siebold stages of If Tmnia crateriformis, Embryonal hooklet of, from a bird, after (x 700), . Taenia cucumerina, Cysticercoid of ( x 60), Cysticercus of, from the dog-louse, Head of, with rostellum and hooks in different contraction ( x 140), Proglottides of, in a sexually mature state. ( x 20), Rostellum of ( x 140), .... Taenia echinococcus ( x 10), .... Adult specimen of ( x 12), Generative organs of ( x 80), Sexual organs of ( x 100), Tmnia elliplica, Recently formed egg of ( x 600), Sexually mature proglottis of, Tcenia flavo-punctata, after Weinland — . Ripe proglottides of, one barren ( x 40), Tmnia marginata, Embryonic development of ( x 550), Eorm of uterus in ( x 6), Hooks of ( x 280), Recently formed egg of ( x 600), Tmnia mcdiocanellata (natural size), Tcenia nana ( x 18), Head of, with retracted rostellum ( x 100) ; (A) an isolated hook (x 600), .... . . XXU1 FIG. PAGE 30 48 60 95 42 62 61 97 62 97 6 14 86 140 87 140 14 20 217 372 45 68 49 71 65 102 258 451 80 132 179 308 568 232 396 163 314 308 568 154 299 OAT Oil/ 00/ 158 306 165 314 174 322 347 666 211 364 ooo DOo 348 669 45 68 346 665 349 672 229 393 226 389 138 277 316 587 318 590 164 314 172 321 143 279 344 661 345 662 176 326 30S 568 304 564 1 79 ^91 67 104 340 657 341 658 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), Tcenia nymphcea, Embryo of ( x 400) Tcenia perfoliata, Male and female organs of, after Kahane (x Mature joint of, with uterus ( x 10), Nervous system of ( x 20), _ Sexual organs of, from the horse, after Kahane f x 15) lcenia saginata, . " Cephalic end of, in retracted and extended state '( x 8) Cross section of a joint of ( x 38), Development of the efferent generative organs in, Development of the germ-producing organs ( x 5), Eggs of, after E. van Beneden ( x 550), Eormation of the first lateral branches of the uterus ( x Four last joints of, about to be liberated, Generative organs of ( x 10), Half-ripe joint of ( x 2), . 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, . 5), Joint of, . Joint of, with unusually simple structure of the uterus J oints of, with two and three genital openings, Longitudinal section of (young chain of joints) ( x 25), Longitudinal section through (a young chain) ( x 25), Lower end of the vagina, showing its connection with the uterus (* 30), Mehlis' body in connection with the various parts of the female productive organs ( x 30), . Prismatic proglottides, With double porus genitalis, In transverse section through the porus genitalis ( x 8) Proglottides of, in various conditions of contraction, Proglottides of, in motion, Ripe segment of ( x 2), Series of joints with perforated proglottides, Supernumerary joint of ( x \), Supernumerary joints of, . Tape-worm form of, Var. abietina, Eipe segment of, after Weinland ( x 2), Young bladder- worms of ( x 30), Young bladder-worms of, with rudimentary head ( x 30), Tcenia 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), 15), 2), PIG, I* AGE 175 322 342 659 167 316 343 660 173 321 166 315 168 316 152 296 i tin 313 237 407 246 433 148 292 252 446 253 447 256 449 255 448 150 294 249-50 439-43 296 528 244 431 247 435 238 408 132 271 240 423 169 316 243 427 257 450 147 291 149 293 245 431 254 447 251 444 260 453 262 456 261 455 44 65 241 425 239 408 263 457 233 399 259 452 131 271 272 479 186 345 264 460 145 282 146 287 176 326 308 568 153 298 306 566 LIST OF ILLUSTRATIONS. Longitudinal section of a young, consisting almost entirely of head and neck ( x 60), . ii " Rudimentary heads of, at the beginning and at the end of the protrusion of the head (x 20), . . ■ 200 and Twenty hours old, with incipient segmentation ( x 10), . Young bladder-worms of, with rudiment of head ( x 12), Tcenia seligera, Two proglottides of, from the goose, after Eeuereisen, Ttenia solium — Apex and hooks of, Apical surface and circle of hooks in ( x 80), ii » Cephalic end of, . Cross section of, showing middle and cortical layers under low power, ...» Cysticercus of, from the pig, Egg of, with shell and yolk-membrane ( x 400), Eggs of, with and without primitive vitelline membrane ( x 450), Embryo containing egg (without yolk-membrane) ( x 400), Generative organs of, Half -ripe and ripe joint, . Half -ripe joint of ( x 2), . Head of ( x 35), .... Head of, from the intestine of a rabbit ( x 25), Head of, from the intestine of a rabbit, in different stages of motion ( x 25), ..... Larger (anterior) and smaller (posterior) hooks of ( x 280), Proglottides of, with slightly branched uterus ( x 2), Reproductive organs of ( x 10), .... Two joints of, with branched uterus ( x 2), Two proglottides of, with uterus ( x 2), . Two segments of, with branched uterus ( x 3|), . Tcenia torulosa — Young form of, in Cyclops serrulatus, after G-riiber ( x 25). ii ii ii Tcenia uncinata, Generative organs of, after Stieda ( x 25), Tcmia undulata, Rostellum of, after Nitsche ( x 100), . Tape-worm — Egg of, from a bird, Tcenia nymphcea, Eggs of, with six-hooked embryo, Piece of a mummified, ..... Tenebrio molitor, Encapsuled tape -worm embryo, and the resulting cystii worm from, after Stein ( x 100), .... Tctrarhynchus — Cysticercoid, from a Mediterranean percoid ( x 20), Longitudinal section of a still imperfectly developed ( x 25), Longitudinal section of an isolated head of ( x 10), Tetrarhynchus sepice ( x 12), (Isolated head) ( x 12), ..... Thetis, Blood-corpuscles of, partly with enclosed granules of pigment, after Hteckel, ..... C 151 228 234 201 225 187 160 139 231 292 4 144 45 175 297 173 142 275 295 227 274 291 223 288 293 290 294 156 276 289 212 339 161 230 33 20 273 180 215 219 222 216 226 93 XXV PAOB 295 392 401 354 383 345 311 278 395 521 8 281 68 322 528 321 278 489 528 391 489 521 381 513 523 520 526 305 489 520 365 653 312 394 55 32 485 331 370 375 380 371 389 178 xx vi LIST OF ILLUSTRATIONS. M/^-caps.de, with connective-tissue covering (in situ), in B, calcified, Trzchma spvralv>-A, Embryo ; B, Intermediate form ; C, Sexual form (ummpregnated female), Trichinosed pork ( x 45), Trichocephalus dispar, >, in situ, Trichomonas batrachorum (after Stein), " >> Trichomonas intestinalis, after Zunker, Trichomonas vaginalis, after Kolliker, Ureter, with excrescences due to the presence of Distomum, Uterus of a free proglottis ( x 2), Worm aneurism of the horse, P^GB 15 21 66 103 58 83 32 53 3 7 119 239 123 247 126 250 125 249 78 130 242 426 52 74 79 131 SECTION I. NATURAL HISTORY OF PARASITES IN GENERAL. CHAPTEE L 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 simp]y 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 purpose of this work, and confine it entirely to animals living as parasites upon other animals. A 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 wxder 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., Rlmbditis) 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.2 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.3 It follows, therefore, from a case like this, that certain animals, such as the larvae 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. 2 The case mentioned in the text is not the only one known. Recent investigations have shown me that Anguillula stercoralis, occurring ih cases of " Cochin-China diarrhoea," is the Rhabditiform generation of A. intestinalis (Leuckart, Bericht math. phys. CI. k. Sachs. Gesellsch. d. Wiss., p. 85, 1882). There lives also in the peritoneal cavity of Hylobius pini a strange parasite, Allantonema 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. 3 Chapter VIII. 4 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 debris 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 microscope. R KLATION OF PARASITIC TO FREE LIFE. & and in my opinion, also for frogs, snakes, and spiders,' which have been stated by many authors to have existed for years m 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.3 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 usually some error has been introduced through a variety of circumstances. If, for instance, everything that is found mixed with the feces 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 lebende Amphibien im lebenden Korper," Midler's Archivf. Anat. u. Physiol., p. 430, 1849. 4 NATURE AND ORGANIZATION OF PARASITES. The parasites of man and the higher vertebrates belong exclusively to them. J 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 larva? (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 the means of approaching 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 (eg the bed-bug), sometimes even wings (midges and some other flies1), or' swimming appendages (fish-louse). When present, these organs aUow 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 (e.g., 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 Lernseadee, 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, which, in this respect, are somewhat 1 Hippobosca, Ornithomyla, &c. NATURE AND ORGANIZATION OF PARASITES. analogous to stationary parasites. I only need to mention certain caterpillars and other insect larva?, many of which lead a stationary lite like the intestinal worms, and, furthermore, resemble them in that, in many cases (e.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 Triclwce'plialm (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 in their structure and arrangement, which correspond always to the needs and cir- cumstances of the individual parasite. Some- times, as in the flukes (Fig. 1), muscular suckers are present, which work by atmospheric, or, more correctly speaking.by hydraulic pressure ; FlG. 1. — Distomum In- teum (young), with suckers and viscera (after de la Valette). 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 Tcenia solium (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 witli 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 beina displaced in this or that direction, according to their disposition. By the possession of sefee of this kind, the male Distomum (Bilharzia) haematobium 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 Tcenia solium, which has Fig. 2. — Pccliculus (Phthirius) pubis. Fig. 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 differences in their arrangement as in their structure. 8 NAT [IRE AN D ORGANIZATION OF PARASITES. iMT-m-^-*7 1 h°Pe' b6en made Clear that the sfcationary paiasite 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 setse 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 from the eggs laid by the fly. 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 Gordiacese, 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 oertain remarkable cases of parasitism, to Fig. 4.— Cephalic end of Tarda solium. COMMENSALISM. 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 m organization ; never- theless they are not true parasites, inasmuch as they do not feed upon ^"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 m man and the domestic auimals, which form the subject of the present treatise, it being supposed, of course, that the conception of the term is uot 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,1 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. Sci. Nat., ser. 3, t. xv., p. 302, 1851. CHAPTER II. OCCURRENCE OF PARASITES. J ust 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 larvae (Pteromalinse.) In the case of the Nematode, Tricliosomum crassicaiida, which infests the rat, we find three or four males living parasitically within the uterus of the female. 1 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 abundance2 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 {BoneUia), the male of which in a Bimilar fashion lives within the genital duct of its own female. See Kowalewsky, " Du male planariforme de la BoneUia," Revue des Sci. Nat., pL iv., 1875 (translated from the Eussian) ; Vejdovsky, Zeitschr. f. wise. Zool., Bd. xxx., p. 487, 1878 ; Spengel, Mitthcil. zool. Stat. Ncapel, Bd. i., p. 357 et scq. 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 Erzeugungder Eingeweidewlirmer," Berlin, 1782, p 12) found in a male bustard at least a thousand specimens of Tccnia viUosa, some of which were no less than 4 feet in length. Gbze also (" Versuch einer Naturgeschichte der EingeweidewUrmer," 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, Goze was astonished ABUNDANCE OF PARASITES. 11 ever opinion may be held-whether we regard the presence of parasites invertebrates7 as a necessary preliminary to their sojourn m 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 Ct number of species found in the same host.* Thus, for examp e, 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.g., Trichina spiralis, when encysted, is found only in striated muscle, the sexually mature Cestodes and Ecliinorhynchus 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. Gysticercus cellulosce, 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 Ligulce 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 (A rchivf. Naturgcsch., 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 Disloma hians from the oesophagus, 5 Distoma (D. hians ?) from between the coats of the stomach, and 1 Distoma cckinatum 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 mcgaloccphala, 190 Oxyuris curvula, several millions of Strongylus tclracanthus, 214 Sclerostomum armatum, 69 Taenia 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. 12 OCCURRENCE OF PARASITES. connective tissue, the brain and eye, and many other localities; Uchinococcus is found in the liver, spleen, kidney, lung, hones, 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 Oxyuris vermicularis ; also Taenia 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 Ganis, 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 hepaticum has also a very wide distribution among warm-blooded animals, being found in most Eumi- nantia, Perissodactyla, Pachydermata, and Eodentia, 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. The same holds good RESPIRATION OF PARASITES. 13 in the case of Strongylus gigas, which is far more abundant m 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 oiven 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 Pedimlus 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 far more abundant than 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 he 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, thou gh 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 Insecta differ from their immediate allies by the absence of respiratory organs. On the contrary, the majority possess the normal tube-like AIR-BREATHING ENTOZOA. 15 lunas (the so-called « trachea "), 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 occasional y m 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 larvae 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 {Luciha hominivora and Sarcophaga WohlfarM, both belonging to the Musoi- dae) ; the larvae of flies (Musca vomitoria, Anthomyia canicidans, Tigs. 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 larvae of Qastrus equi are almost constantly found in the horse in this situation. Other air-breathing parasites live below the skin of mammals (as the larvae 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 larvae within the body-cavities of insects, the hinder Fig. 7. — Larva of Anthomyia canicularis from the intestine of man. Fig. 8. — Larvae 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 tracheae of the latter. The occasional presence of dipterous larvae in wounds, abscesses, even in the vagina, and under the praeputium and eyelids, is well known, and 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-larvas 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. I have frequently introduced the larva? of Musca vomiioria 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 than 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 already remarked, depends upon its host, and shares with it the same condi- tions of life. It usually possesses special organs of respiration, especially when living upon terrestrial animals, and being, therefore, in 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 shin. The Entozoa, besides having no special respiratory organs, are also with- out pigment, tbe 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 Vertebrate 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), or a boring apparatus, as in the common lice, bugs, fleas, and mosquitoes, 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 rrc. 9.— Cephalic end muscular apparatus which allows of an alternate of Eh-udo medicmalia, ... . . p , -i ,n . , with the three mandibles 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 Bochmius 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 is in this respect, therefore, analogous to an ectoparasite. Since most entoparasites are entirely nourished by 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 MM mwMm Fig. 10. — Cephalic extremity of Dochmms duodenalis ; profile and front view. not, however, absolutely necessary. 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 Ecliinorhynchus 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.1 In all probability, this way of taking in nutriment by endosmosis is not confined to the anenterous forms, but exists generally among Entozoa, though it undergoes various modifications in correspondence with the various differences of structure in the outer covering of the body. From this point of view, the Entozoa may be i In the Rhizocephalida {Sacculina, &c.) we have recently discovered a group of ectoparasitic Crustacea that have no alimentary canal. They obtain then- food like plants, by a number of branched prolongations, which pass through the body of then- host and ramify in its intestine. They are found generally on the ventral surface of the abdomen of crabs. With respect to these interesting parasites see especially kossman, « Suctoria and Lepadidte : " Heidelberger Habilitationssclmft, 18/3. Fig. 11. — A male Echi- norhynchus angustatus. (The internal organs con- sist of the sheath of the proboscis, with retractor muscle, lemniscus, and sexual organs. An in- testine is wanting.) 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 m 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,1 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 Fig. 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 Polystomum, mtegerrimum. This Trematode usually inhabits for a short time the gill cavity of tadpoles, and then wanders into the bladder, where it becomes sexually mature in about four years ; if it 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 hlood-vessels which traverse the capsule, and occasionally form a definite system with afferent and efferent vessels, supply fluid nutriment, which is ahsorbed 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 (Echino- 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 to a 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 be recognised as a continuous (inde- 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- Trichina (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, Zntsrhr. f. wiss. Zool., Bd. xxvii., p. 238 et seq., 1876. i Waldenburg is of opinion that this chitinous capsule is in some cases formed by the host. See Archivf. pathol. Anat. u. Physiol., Bd. xxiv., p. 157, 1862. Fig. 14. -Sclerostomum tetracanthum, encysted. CALCAREOUS CYSTS. 21 else than a calcareous excretory product of the worm itself, on the outside of which there lies the connective tissue capsule. Fig. 15.— TVicAima-capsule, with connective tissue covering (in situ), in B calcified. CHAPTER III. THE THEOEY OF THE ORIGIN OF PAEASITES REGARDED HISTORICALLY. Weee 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 of 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 "generality cequivoca" or " srjontaneous generation," as it is usually termed, was a pheno- menon commonly met with among the lower animals, the^origin ORIGIN OF INTESTINAL WORMS. 23 of intestinal worms could be readily explained They were a L^ Sanle of spontaneous generation, the existence of winch ™s already contended for in by far the greater number of the lower anhnlls At most, the discussion related to the particular tlZ^l 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 a 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 Eedi had the most profound influence, and entirely contradicted the earlier theories, that sexual aeneration 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. Eedi clearly proved by his researches and experiments that the maggots, which had been formerly considered as independent organ- ismT(Helcophagi), were in reality the larvse 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 ; 2 he was also well aware (according to the communications of the painter 0. Marsilius) that the parasitic larvse in caterpillars were the offspring of insects that were in the habit of laying their eggs beneath the skin of these same caterpillars.3 With respect to the intestinal worms, neither of these observers brought any direct evidence against the generally received opinions. Certainly not Eedi, who put forward a view as to their origin, which differed only by a somewhat metaphysical tinge from the widely spread theory of generatio ccquivoca. Swammerdam expressly guards against any application of his experiments concerning the development of insects to the Entozoa. It appears, indeed, as if he 1 Eedi, " Esperienze intorno agl' insetti," t. i. p. 23 : Venezia, 1712. 2 "Bibel der Natur" (aus dem Hollandisohen Ubersetzt), p. 37, 1752. " Ibid., p. 281. 24 THEORY OF THE ORIGIN OF i PARASITES. W^Lltd^ ^ ^ f^b6ing th°Ught that intestinal -ms were derived from insects and other free-living animals ; nevertheless he does not deny the theory that they originate from the ^ species " that have already existed in the intestines of other animis" but m 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 he 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 1 and Hoffmann 2 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. 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. 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 LinnV and subsequently by several other naturalists m different places We now know that this tape-worm {Bothnoceplvalus j ScMsto- cephalus soliclus) 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. Linne however, did not know these facts, and regarded the worm without hesitation as a young and incomplete specimen of the large human tape-worm (Bothriocephahis Mm), 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 ; Linne believed that he had also dis- covered the liver-fluke of the sheep and the Oxyuris 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 Anguillulidse for the latter.3 However small the evidence was, it appeared sufficient to esta- blish this idea, which was believed in by many naturalists after Linne, 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 Linnaean school, to attract the attention of zoologists. Men of high ability and wide knowledge, like Pallas, 0. 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 Linne" 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 " Amcenit. Acad.," t. ii., Erlangse, 1787, p. 93. 2 Steenstrup, Overs. K. dansk. Videnshdb. Selsk. Forhandl., p. 166, 1857 : Zeitschr. d. t/esammt. 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, " Abhandl. von der Erzeugung der Eingeweidewiirmer," p. 2, 1782. 3 " Systema Naturse," ed. x. , t. i., p. 648. Fasciola hepatica, "habitat in aquis dul- cibus ad radices lapidum, inque hepate pecorum." Ascaris vcrmieularis, "habitat in paludibus, in radicibus plantamm putrescentibus, in intestinis puerorum et equi." 26 THEORY OF THE ORIGIN OF PARASITES. and yet no locality was left unsearched • and graduaUy arose the conviction that the statements of the free existence of intestinal worms were, m the majority of cases, based upon a confusion of these worms with others closely resembling them, and that in those instances (e.g., the tape-worm found by Linne} where an intestinal worm appeared to have been found living free, the discovery could not be interpreted in the sense Lmne 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 fceces 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 1 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 m 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 2 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 0. F. Miiller,3 Bloch,4 and Goze.6 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. Bcitrdge, Bd. i., p. 43, Bd. ii., p. 80. 2 " Opere fisico med.," t. i., 1733. 3 Naturforschcr, Bd. xiv., 195, 1780. Ncu.es Hamburger Magazin, Bd. xx., 1784. 4 " Abhandlung von der Erzeugung der Eingeweidewlirmer : " Berlin, p. 37, 1782. 5 " Versuch einer Naturgeschichte der Eingeweidewurmer : " Blankenburg, p. 4, &o. 1782. THEORY OF INHERITANCE. 2^ migration was disproved. The eggs which are extruded with the feces are as far as the intestinal worm is concerned, lost,-though may serve as food for other animals (Goze). It was _ certainly astonishing that hy far the greater number of eggs should ^ this fate, but even this fact was brought into accordance with the theory It was asserted that the intestinal worms, which could not like othei 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 in many of its supporters, must not be considered as due to the theory itself;1 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 Ccenurus that almost invariably kills its host, and generally before it amvra 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,2 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. Entozoa 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. ext., p. 38 ; also Davaine, " Traite des Entozoaires," 2me ed., p. 11 : Paris, 1877. 3 "Abhandlung von den Wurmern in den Gedarmen des menschlichen Korpers," p. 106 : Leipzig, 1776. 28 THEORY OF THE ORIGIN OF PARASITES. know well, from transmitted germs, and 'only in conseguenec 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 ouo-ht 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 PaUas, Bloch, and Goze began a long list of helmintholo- gists, of whom the most eminent were Budolphi 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,1 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, or a 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 1 See specially the excellent work of Bremser, " Lebende Wiirmer im lebenden Menschen," pp. 1-16 : Wien, 1819. 9Q TEACHING OF RUDOLPHI. p „i a fhflnrv The microscope had been for some time ft J" not t^Wworthy agent, and a simple lens, or the nLrfye Ll l sTfficient to w'atoh the process of spontaneous tltion^ When this had once taken place, it was f the Entozoa increased by sexual generation, or else to wha ^po e had they been provided with generative organs ? The significance of th s sexual generation had been kept in the background ^ h prevailing opinion of spontaneous generation. The majority o .he loos were extruded without being further developed for if not, the extraordinary fertility of these creatures would entirely fill their ho t 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 2 who, in spite of all other qualifications, was ignorant oi the necessary details of the subject. As long as Eudolphi'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 Eudolphi'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. (At,, p. 65. Kudolphi, " Entozoomm Hist. Nat.," vol. i., p. 811, 1808. a " Medicinisch-praktische Vorlesungen liber Eingeweidewurmer,' p. 47 et. seq., 1803. 30 THEORY OF THE ORIGIN OP PARASITES. in eggs of certain Distomidee contained an embryo (Fig 16) whioh shape .and ciliation resembled an Infusorianf it was oLsionaUy provided with an eye-speck, and after being hatched swam about iTke a Infusorial 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 ofL'lTo— 1g£ h™ever the eggs had been laid, and the em-' turn. Diyos, 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 Kordmann 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." 2 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 Eeuropterous 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 3 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 " (Eedia), 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 Redise 4 gave rise, by a develop- 1 Oken's Ms, p. 190, 1831. 2 " Mikrographische Beitrage," Bd. ii., p. 140, Note, 1832. 3 Archivf. Naturgcsch., Jahrg. i., Bd. i., p. 69, 1835. Burdach, " Physiologie," Bd. ii., p. 208 : Leipzig, 1838. * Nova Act. Acad. Ca>s. Leop., t. xiii., p. 627, 1826. PROOF OK METAMORPHOSIS IN TKEMATODES. 31 Monostomum mutabile with the ^eui*/ ^ ' ' 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). c. Fig. 19. — Redite. (A) with germs ; (B) with Cercariae in the interior ; (0) free Cercariae. 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 ee« 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 (Fi* 20) arranged in pairs at the anterior pole of the body, and capable of bein* moved like levers. 1 The subsequent changes undergone by this embryo were for some time uncertain, though there was no doubt that thev could only pass into the fully formed animal "by a kind of meta- morphosis." Fig. 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 follow them up to their legitimate consequences. This was done some years later by Eschricht,2 who fully discussed the question of the origin of Entozoa for the first time since the days of JBremser, and who had also, in his masterly researches upon Bothriocephalus lahis,3 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,4 and also known to Bremser and Eudolphi, 1 Burdach, " Physiologie, " lor. cit. Previously to von Siebold, Goze 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. Jowrnal, 1841. 3 Nova Acta Acad. Gas. Leop., t. xix., suppl. 2, 1841. 4 Naturhistorisk Selsk. Skrifier, Bd. i., p. 53, 1790, ; see the remarks made on p. 24. ESCHRICHT UPON THE ORIGIN OF INTESTINAL WORMS. no 00 that Bothriocephalic (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 m all proba- bility many other Helminths wander in a similar way from one or-an 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 larva; of ichneumon-flies and hot-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's1 observations lent to them, that the majority of helminthologists continued to uphold the old theory of the spontaneous generation of intestinal worms.2 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,3 and quite simply, for Steenstrup con- nected their life-history with the above-mentioned Cercaria3. 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. a "Ueber den Generationswechsel : " Copenhagen, p. 50, 1842. Translation bv Ray Society, London, 1845. 34 THEORY OF THE ORIGIN OF PARASITES. Fig. 22. parasites. Steenstrup was not content with solving the enigma merely by hypothesis; he also endeavoured by direct observation to place his FlG- 21- opinions beyond any doubt. He discovered that these Cercariae (Figs. 21 and 22) fre- quently made their way into the body of 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 Cercaria;, erroneously formed the opinion Figs. 21 and 22. -A free that tlais process, instead of being the pre- and an encapsuled Cercaria, cursor of a further development, led merely the latter without tail. , ,-i n J 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 (Schistocephahis) 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 Cercaria? ; but Steenstrup went further than his predecessors in showing the identity of the " kingsyellow worm " and the "living matrix of the Cercarias" 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 i " Bericht liber die Leistungen, &c," Archivf. Naturgaclu, Jahrg. xiv., Bd. ii., p. 321, 1848. STEENSTRUP'S THEORY OF ALTERNATION OF GENERATIONS. material was to hand, but there was no one capable of using it. In spite of the close similarity between the Cercaria and the Distoimom, 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, Steenstrup1 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."2 This opinion was strikingly confirmed by a new discovery. Dujardin3 frequently discovered on the ground, and especially after a sudden rainfall, masses of a Filaria-like Nematode (Mermis), resembling very closely Gordius aqitaticics, 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 Mermithidae in the bodies of insects and insect larvae, and observing them in the act of wandering away ; but also by the discovery that Gordius was also sometimes parasitic.4 At the time of their migration the Gordiaceae 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 later5 in tracing the development of the embryo within the egg during the 1 Loc. cit, p. 111. 2 £oc. (At., p. 116, Note. 8 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. ) 4 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. Exptr., t. iii, p. 182 et seq., 1874. 6 Ibid., 1848, p. 290 ; 1850, p. 239,— Jahresb. d. schlcsischen Gescllsch. fur vaterl Cuttur, p, 56 : P.reslau, 1851. 00 THEORY OF THE ORIGIN OK PARASITES. winter, and proved that the young lame 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 Fig. 23. — The common bladder-worm of the pig, with invaginated head (A), and with extruded head (B). as to the tsenioid 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 blaclder-worm of the mouse and that of Taenia crassicollis of the cat.2 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 " Handwb'rterbuch der Physiologie," Bd. ii., p. 640 : Brunswick, 1843. » " Hist. Nat. des Helminthes," pp. 544 and 632, 1845. DISCOVERIES OK VOX SIEBOBD, DtJJARBIlf, 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 veiy wide application, and to explain the existence of many o her sexless worms (even Trichina), 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 then- host was swallowed by some other carnivorous fish in whose ali- Fig. 24. Figs. 24 aud 25. — Echinohothrium minimum (after van Beneden), isolated living head and tape-worm. Fig. 26. — Transformation of the bladder-worm into a tape-worm (Tcenia 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 1 Zeittchr. f. wies. Zorf., Bd. ii., p. 198, 1850. THEORY OF THE ORIGIN OF PARASITES. Beneden had investigated the Entozoa .of various predatorv fi«Tr specially the shark and ray/ and made observations" afa He frequently found in the stomach of the shark the dieted ^ remaTns of various osseous fish with Tetrarkynckus heads, sSneTS were still encysted, some free or nearly so, while others had already buried themselves in the intestine\f their new ho and budded out a longer or shorter chain of segments. Van Beneden's researches were so extensive, and dealt with&so many differen forms hat they fully justified the generalisation that the transference rf 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 ^ Sclustocephalus. 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 letrarhynchus. 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,2 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 3 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 1 " Les vers Cestoides " : Bruxelles, 1850. (Preliminary account in the Comptcs Jiendus Acad. Belg., 1849). 2 " Ueber die Metamorphose der Finnen in Band wiirmer," Prager Viertefy'ahrsachrift, 1852. 8 "Ueber die Contagiosity der Eingeweidewurmer : " Jena, 1844. KUCHENMBISTBR INTRODUCES HELMINTHOLOGICAL EXPERIMENT. 39 known to the older workers in this department. It has already been mentioned that Ahildgaard in this way proved beyond doubt the mioration 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 announced1 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.2 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 3 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,5 de la Valette,6 and Pagenstecher 7 proved, by means 1 Giinsburg's Zeitschr. f. hlin. Med., p. 448, 1853. 2 I 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. 3 GurWs Magazinfiir ges. Thicr-HeUkundc, 1854 and 1855. 4 " Die Blasenbandwiirmer und ihre Entwickelung : " Giessen, 1856, p. 38 ct seq. 6 " Mem. pour servir a l'hist. gene"t. des Trematodes : " Turin, t. i.-iii. 0 " Symbolse ad Trematodum evolut. Hist. :" Berolini, 1855. r " Trematodenlarven und Trematoden:" Heidelberg, 1857. " Ueber Erziehung von Distomum echinatum duroh Putterung," Archivf. Naturgesch., Bd. i., p. 246, 1857. 40 THEORY OF THE ORIGIN OK PARASITES. tteh-SUt' f* enTted DiSt°meS greW matui'e di^tly after and S V h°St t0 an°ther' aS VOn Siebold had bdieved, and that in this way a migration to another host, or another or*an thTSmT' Alth0Uf\lnthert0 the Vari0us st*§'es of development of fWnT SpeCie'\had n0t been experimentally proved, > as in the Oestodes, we must regard our knowledge as having been completed by the experimental verification of this fact, and especially by Zeller's es2 1! P??heS int° life-hist017 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 lon27.— Pentastomum refuse to renew the old disputes. The theory they denticidatum. fought for is an error that has been dissipated. 1 " Bau imd Entwickelungsgeschiclite der Pentastomen : " Leipzig, 1860. Preliminary account in Zeitschr, f. rationelle Medicin, Bd. ii., p. 48, 1857 ; Bd. iv., p. 78, 1858. CHAPTER IV. LIFE-HISTOEY OF PARASITES. trs^rj TeT„:tt°rfs o o- i v ■ ow that Parasitism only repre- sents a single phase m the life of an animal, which, in spite of L if ^ 1 TJ ^ ™oseVano h stage In fact, if we only know concerning a certain animal that it s a parasite, we know but little; thoroughly to understand it history We must follow out all the separate stages and colons fparast:^ ^er which it becoLs witMnTS Tiedrd nUmrUS th6Se ^ b6'the^ are c-ta-ed within fixed boundaries. There are certain standards, or rather certain types of parasitic life, under which the individual cases ar more or less definitely grouped. The knowledge of these condLn 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 then- 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 larvae of parasitic insects and the Gordiaceaa and Mermithidae, 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, SEXUAL MATURITY. 43 in locomotion and capture of its food, is small, generally less than m free-living animals, and the income, therefore, is large ; there are m fact, without going into any further detail, numerous causes which must be considered as having a most important effect m furthering sexual maturity. The large balance on the side of income explains the great fertility, upon which stress has already been laid, as ot extreme importance in the life-history of these animals.1 _ This however is merely en passant. Most important is the tact 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 Lerncm, where coition takes place while the ani- mals are swimming freely in the water,2 and differ but little from the free-living Copepoda, and also in the chigoe (Tulex 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.3 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 Tcenia 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 etpial 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 Lernaeocera, Peniculus, und Lernaea," Schriftcn der Oesellsch. zur Beforderuwj d. ges. Naturw. zu Marburg, Suppl. -Heft ii., p. 21, 1868. 3 See Vol. II. 44 LIFE-HISTORY OF PARASITES. also questionable whether in this latter group parasitism i. fined to the fpninlo ai™„ i i &^U1J paiasitism is ever con- the female alone, as has been very generally observed to be the case in the Lemcece and their allies. The simple fact that these are animals of which only the females 1 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 iu the following sentence : — In the majority of parasitic animals the 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 Tccnice, where the eggs remain m the proglottides and are extruded from the body of their host. Fig. 28. — Pulex penetrans. b. Male. a. Female. EGGS AND EMBEYOS. 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 so on. In some cases, however, at the time of opposition, parasites undertake special migrations like free-living animals. There is a human parasitic worm that does so— Distomum haematobium (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 » In the same manner the sucking of blood by the Culicidsa 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. DEVELOPMENT WITHIN THE ECO. 45 contained in a groove on the lower surface of the male, into the veins of the pelvis, where the eggs are deposited m 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 egg 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, e.g., 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 eggs; in ,v m„ • rnv n lnnrr -norinri matdbium, male and female, some cases they remain loi a long period— ^ ^ ^ ^ canalig almost until the young are hatched — in the gynEecophorus 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 eggs to hairs ; Dactylogyrus, Diplozoon, &c, attach them to the branchire 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 case, that we are not acquainted with a Fig. 29. — Distomum ha?- 46 LIFE-HISTORY OF PARASITES. of couJ *■ *~ va/es he same thing takes place in animals living iu ot^f0"Zlt; are tins shed from the body. In the same way the e<™s of thp bronen.a! parasite of the sheep, Strongylusfilaria, are^- moved Uh raeheal mucus, and the eggs of iWo^'i^i^^^ on 0af tLThy 0 the dog; leave the body ^ ^ * secretion of the Schneidenan membrane; the eggs of Str^aylvs aiaas and the embryos of *m are ^ ^ aJf ^h tL L r V neCeSSary that the Parasites should *™ iu organs ^erP^ 6 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 weU known, for instance, that the young lice arow 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 (Oxyuris 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. 1 Neither has the statement of Norman been con- firmed, according to which all the developmental stages Fig. 30.— Rhab- of Anguillula (Rhabditis) stercoralis should be abun- ditis tenicola? dantly met with in the viscera of persons suffer- ing from "Cochin-China diarrhoea." 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 A scar is lumbricoidcs, 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 Trichosomam described by von Siebold in the spleen of a shrew-mouse (Archiv f. Naturgesch., Jahrg. xiv., Bd. ii., p. 358, 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 pliysilc. med. Societat zu Erlangcn, Hft. 2, p. 20, 1872). H.'EMATOZOA. 49 mentioned, no true parasite, but the mature state of a heteromorphous species, the so-called Angitillula intestinalis. The young are born m the intestine of the host, and attain maturity (like Fhabtkhs) only after abandoning the latter; they live in the same way as Rhabditis temicola (Fig. 30), and then give rise to a new generation.1 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-Mke Nematodes, which he considered to be the embryos of Filaria attenu- ata,a 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 Filarial, more than an inch long, containing numerous embryos ; the latter he also observed circulating in the blood. Lewis has also shown that numerous Hsematozoa are found in dogs afflicted by Filaria sanguinolenta, and the same thing was observed by Gruby and Delafond; 5 and later by Leidy and Walch,8 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 sfcercoralis, u. deren Bezieliung zu d. sog. A. intestinalis," Bericht math. phys. CI. fc. Sachs. Gesellsch. d. Wiss., p. 85, 1882. 2 I use 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 Hcematozoa, 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 yVth of the whole 360 gr. net weight, would contain about 18,000,000. 4 Arrhivf. Anat. und Physiol., p. 189, 1842. 6 Comptcs Rendus, t. xlvL, p. 1217, 1858. 0 Monthly Mic.r. Jonrn., p. 157, 1873. D 50 LIFE-HISTOliY OF PARASITES. atten^Tttt ,Ha3matOZOa ha™ ^ eensiderable attention by then- discovery m man (Fig. 31), under circumstances Fig. 31. — FUaria 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 HcBinatozoon was Lewis of Calcutta,2 and he regarded it at first as an adult parasite (FUaria sanguinis) ; but subsequently considered it to be the young form of a Filaria-like worm,3 which, in the sexual state (as F. Bancrofti, 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 dfs- 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.4 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.6 This proves satisfactorily that the periodical appearance of 1 Since Magalhaes (0 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 Hsematozoon inhabiting Human Blood," Calcutta, 1872. Ed. 2, 1874. 3 Centralblatt f. d. medicm. Wiss., No. 43, 1877 ; more in detail— Lancet, Sept. 1877, p. 453. See also Cobbold, ibid., p. 495, and Vol. II. of this work. 4 Manson, Journ. Queckett Micr. Ghib, vol. iv., p. 239, 1881. r- Mackenzie, Lancet, August 27, 1881. FATE OF FLFMATOZOA. 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 ot the lymph due to these.1— R. L.J If these Hrematozoa arrived at complete maturity m 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 Hfematozoa 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 Haematozoa in the blood. These facts point to the conclusion that the intermediate stage between the Htematozoon and the fully developed parasite is passed outside the body of the host. The analogy of Trichina also lends support to tins 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,3 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 Trichina 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- Trichince, Fig. 15) until they are swallowed by a new host, when they recommence their wanderings. In Trichina, therefore, and in these Haematozoa, a change from one host to another is necessary before sexual maturity can be reached. From the observations of Ecker, that the Haeniatozoa 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 Trichina, 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 Scheube, "Die Filarien-Krankheit," in Volkmann's " Sammlung Klinischer Vortrage," No. 232, Leipzig, 1883. 4 Leuckart, " Untersuchungen liber Trichina spiralis," Leipzig, 186n. 2d ed. 1865. 52 LIFJS-HISTOBY OK PARASITES, with certain Nematode lame which nrP nrpa^t *i way or other and continue their life-history under oL^ST This supposition is strongly supported by what has been observed in human Haematozoa According to Lewis, these wormsboreijS^ capillaries of the kidney and make their way into the renal tubules that, after a day has elapsed, m 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 develop* it o the worm is s fall unknown.-P, L.] Up to the present, this observa- tion is certainly unique, and nothing similar has been observed in the Hamiatozoa 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 m man, whose urine contains in abundance not only the Hffimatozoa, but also a quantity of blood and albumen milled with them, which renders their presence obvious— there always remains the possibility that the Haematozoa 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 Hasmatozoa in any animals where the latter are present,2 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- ment in various places and under various conditions, as chance directs. 1 Borrell (Archiv f. patkol. Anat, Bd. lxv., p. 399, 1876) shows reason to believe that the Hajmatozoa of the crow leave the body by the bile-duct, but the above-quoted investigations of Kahane prove that no Filarial are present here, or in the cloaca, ureters, or bronchi, except, of course, there has been some mixture of blood. 2 Gruby and Delafond only found once, in twenty four dogs infested with Haematozoa, the Filarial from which these originated. According to Ercolani, Filar ia mil is 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 Filarial 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 posit ion. 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 oealities, 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.1 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 twmh'icoides, and B, THchoc^halua dispar ; a, fresh froiri the feces ; 6, 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 AnguilMalriticL, Vlnatit, p. 330, 1855,- or (more in detail) M6m. Hoc. Biolog., t. iii., p. 201, 1856. 54 UFJMHBTOBY 01? PARASITES. of a .simple piece of apparatus consisting' of a rin« of kW those with a thick shell (.Ws lumlHcoides, A. LahcMal W and many free-living Ehabditidee), are not merdy clmb le of enduring a complete desiccation lasting for weeks and eve/montiis bu also alternations between the moist and dry conditions. Development t2Z She hT™e " a damP ---ment tt it i thTt fh h ? ?i 'J61'617 m°ist ; indeed' ifc has aPPea^ to me that this is actually more favourable than wetting the egJs themselves T^lfd1'- /d d^iP 6artl1 deVel°Pmenfc advan- em tli be dried development is at once checked, without, however, de- stroying the vitality of the germs. 1 The same holds good for the embryos; by desiccation they are lendered 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 (e.g., Filaria Medinensis and Bhabditis). But all the experiments are not opposed to the general law that a moist environ- ment ts necessary for the further development of the eggs of Fntozoa 0 f course this is not the only necessary condition. The decree of this moisture, the nature of the environment in other respects its chemical composition and temperature, are factors which are of varied importance m 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 embryo2 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 faeces and urine, the eggs of Ascaris hcmbricoides are gradually destroyed ; they are likewise ofteu destroyed through the foulness of the water which surrounds them. 1 The statement of Davaine (Mem. Soc. Biolog., t. iv., p. 2/2, 1862), that the eggs of A scaridcs 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 germa of Greguri- ii< iid parasites (Cocoidiuni, Lt.). 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 inter 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 ecrcrs of these parasites are unusually thick, than to any peculiarity m 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 tmnioides, which inhabits the nasal cavity of clogs, have a folded outer layer which enables them to adhere to various bodies when they are ejected from the nose of their host. In a 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 (Fig. 32 a) on the egg- <. shell proper, serve to secure the attachment of the egg to any body with which it comes in contact. The eggs of Tcenia frequently leave the body of their host enclosed in a Pig. 33.— Egg of a tape-worm , . . . . -I,,- n • i from a bird, Tcenia nymphcea. 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 (e.g., Acantho- cephala, Tccnice, many Distomidee, &c.) the embryo is already formed ■ in others, again, the egg contains merely the original cell. The presence of an embryo, 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 forming the embryo, and the young is hatched. 56 UM-HISTOBY OF PARASITES. small aquaria brio ., !?' , Ug '° S°hubart and Center, in -1 4p eartl „ th'eve, J^TT^ ^ ^ ata* proved in the case of hf e If of 1 ™s has aim) and Trematodes ™™ious tope-worms (BMriocepk- those of ^Sl^^fSL' JfT^ °f 2°' °- as much as 40° r Th * , w?™^am differ from those of (%wns in being entirely undeveloped at the time that 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 the young are hatched. TricJwcephalus rarely completes its development with- 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 Bocknnus duodenahs (especially in warmer climates) are hatched in a few days Similar variations are found in Trematodes and Cestodes the eo°s being sometimes hatched in a few days (Friamqphorus), at other times requiring weeks (Lignla) or even months (Bothriocephalus latns, 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 mystax, 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 eggs 1 In sunshine Vix saw an active embryo develop in a quarter of an hour in the egga of Oxywis.—Zcitschn f, Pmjckiatrk, Bd. xvii., p. 65, 1860. Fig. 31.— Eggs of Oxyuris ver- micularis ; a, b, freshly laid ; c, with developed embryo. mk;i;ation of embryos. 57 have never been fertilised, and this view is supported by the fact Sat thTeggs 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 {e.g 2Mta«» duodenalls), the early stages are usually passed ^t^uSj e«m 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 m a living host may thus be a necessary preliminary to embryonic ^ThouXour knowledge with regard to the germinal activity of the egos of Entozoa rests at present upon a comparatively small number ofDexperiments and observations,1 these are so entirely m 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, Fig. 36.— Egg of Both- Fig. 37. —Egg of Fig. 35. — Egg of Distomum rioccphalus latus with Echinorhynchus gigas with hepaticum with embryo. embryo. embryo. 1 See the observations of von Willemoes-Suhm, Zcilsrhr. f. iviss. Zool., Bd. xxiii., 1873, p. 343, (Bothrioccphalus), and p. 337 (Trematoda). 2 This holds good also for the generative buds of Gregarines — the so - called PseudonavicellsB— which, earlier or later (in the body of their host or outside it), develop into embryos. 58 LIFJ3-HIST0KY OF I'A RAJ3ITES, tn the majority of cases in L p f f V"" " mmat any pomt of similarity between the Z , ere 18 llardJy worm._(Kg8. 35, 36 and 37) J S a"d tlle full>' fOT""' -for the reason ftatT^LlX" T***?-* greatest importance in their life-hiTtorte, and serves only as a means to further their distribution and migration. Instead of blind chance, 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 stase RHABDITIS-LIKE EMBRYOS. 61 was sufficient* to — Nematoda 1° thought it probable that under which W ^ be dlscovered SSt^^^L has been justified. My researches into L HfeSries of Nematodes/ have proved that there are numerous pec es specially among the Strongylidae (of human parasites Su LfaiK.), which in their young stage resemble in ^ u ture and habits the free-living Khabditidas (Figs 42, 43) and ite th m go on feeding and growing for a considerable time. They hen Xnoe their skin, lose the pharyngeal armature so very characteristic of BkaMUis, and enter upon a stage when they cease to take in nourishment and to increase in size, and need to become mrasitic I need hardly recall the life-history of Ascaris mgrovenosa, shortly described above (p. 2 ), which belongs to this type ; but is peculiar in that the ShaMMs-ltee form, which elsewhere is merely a young stage, is here developed into a special generation, which, as soon it is completed, enters again on a parasitic life Anion" other Helminths (Cestodes, Acanthocephala, Distomidse) 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 larvfe, as Newport and Fabre have shown in the Meloida? : the larva? 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.2 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. II. 2 The life-history of these young Meloidse 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 Kanunculacese, dandelions, and other plants rich in honey, that are much visited by bees. As soon as the larvae are hatched, they crawl up the stems of these plants and hide themselves in the corolla. When bees visit the flowers the larvae attach themselves to them by their powerful limbs, and are carried to the nest ; here they lose their appendages and change into inactive grubs. 62 LIFE-HISTORY OF PARASITES free iife ; ; i s 0/ ri^r"™4 itself on to the outer skin of its host, or in some organ easily accessible from he exterior. In this way we know that the W f Trematodes attach themselves to the skin or within the respirlrv cavities of water-snails. Others bore their way at once into he mtestines 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 gradnlly forces its way in. Considering the small size o the body, and the fact that many of these embryos are provided with special boring apparatus-as, for instance, the larva, of y^ocepludns Gordius, and several species of JDistomum-it is evident toat the difficulties to be overcome are not very great, provided that they at ack the right host. It is of course only animals with a delicate outer skin, such as larval Insecta, Crustacea, Molluscs 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 OBSERVATIONS OX THE MIGRATION OF FREE EMBRYOS. 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 SiebokV m the account of his researches into the Merinithidae, and their wandering into the bodies of minute caterpillars, makes the following remarks: "Thirteen larvae 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 larvae 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 a piece. I also made use of young caterpillars not more than three lines in length of Pontea cratcegi, Liparis chrysorhcea, 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 larvae, 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 larvae of Ephemera, whicli Meissner made use of for his experiments,3 only took place at night, and always through the appendage which served as a point of attach- ment for the young larvae. " All the Ephemerid larvae which were left for the night in a vessel with the GWZras-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 clone 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 Enlomol. Zcituwj, p. 239, 1860. ■ Zeitschr. f. wiss. Zool, Bel. vii., p. 132, 1856. 8 Villot considers that the larvie of Chironomus, and not Ephemera, are the proper hosts of the young Gordius. Archives d. Zool. cxper., t. iii., p. 186, 1874. 64 • LIPE-HISTOEY OF PABASITBS. mg away I found The number of parasites in one larva was sometiml I? ' w many as forty) that I am inclined to attribute to tW8 h 1 « ^ the sudden mortality which took place ^^jS^^ For a considerable time it was believed that the parasitism of these free embryos was always brought about by their own active nr^atfon nuto the body of a host ; ofcourse.it was possible that it mi^ht b effected in other ways, but there was no proof of this. At pfesen 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 trigonocephaly,* (A* 43 a b) to the alimentary canal of a clog, and saw them grow°' into the parasite after the lapse of a few days.* Man is infected in a simi- lar way by Dochmms duodcncdis, and the horse by Sclerostomum ezmnum It is probably only the free young stages of Nematodes which select the natural passages in order to become Entozoa • at anv rate they are the only forms that can, by the thickness of their skm, withstand the action of the digestive juice. Meissner, however and others have shown that this is not a complete protection • the' former observed numerous Gordius-embryos destroyed by the digestive fluids of Ephemerid larvae, and I have observed the same in Monosto mum. In a 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. 2 From the observations of Manson (Trans. Linn. Soc. land., 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, m 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 the embryos evacuated with the \vrine (which probably no more represent a useless production than the eggs of intestinal worms which pass out with the foeces) 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. 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 egg-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. 1 have myself observed specimens of Gammarus and Asellus aquaticus feeding upon eggs of Echinorhynchus 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 {Taenia serrata, T. marginata, T. ccenurus, and T. echinococcus), which live in the intestine of dogs. The " pro- glottides " of these worms crawl out of the faeces and deposit their eggs upon grass stalks. I may also mention here Taenia saginata (mediocanellata) of man, the eggs of which are transferred in the same Pig. 44. — Proglottides of Tcenia saginata in various conditions of contraction. way to the stomach of the ox (Fig. 44) ; the pig generally becomes in- fected with Tcenia solium by feeding directly upon human ordure, and the meal-worm (Tencbrio 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 Echinorhynchus gigas with the fasces 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 the mouth with food ; and higher E fifi LIFE-HISTOIIY OF PARASITES. animals, c.,j. fishes, may be occasionally deceived and dev„„, t worms nnder the delnsion that they are Stim,a i„T t^" moreover, evident that the further deL„ menf "is when hey have reached the body of some animal, is only posS' when . W ' emmT iatTab'te- TLen ^^oeeWes contain a living embryo. It is not easy to say how long the embrvo will retain its vitality; accidental and even constant ndZnf br^ about the greatest variations in this respect. In the eggs J TZ common thread-worm {Ascaris) I have seen active embryos^ven aft w n st, on he contrary, the eggs of tape-worms usually lose thei vitality within a few weeks, even when kept damp Ihe eggs first of all, we may suppose, reach, in a living condition ten 2 The 1 T T' 7h7> ^ dlg6StiVe **- be of sufficient strength he 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 TricJwcephalus by feeding it with the eggs containing embryos.3 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. Ktichen- meister and Davaine attempted to breed Ascaris lumbricoicles from eggs 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 {e.g., Bochmms trigonocephalies, 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 had elapsed he was able, by heating them, to induce signs of vitality. (Mem. Sec. Biol., t. iv., p. 261, 1862.) He also states that he was able to preserve alive for years eggs and embryos of Tcenia solium and Tcmia serrata. (Ibid., t. iv., p. 273, 1862.) 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. 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 Tccnia, Echinorhynchus, and Fentastomum, whose develop- ment I have traced, and numerous thread-worms— Spiroptera munna, Ascaris incisa, Sclerostomum eqninnm, &c. If we recall and com- pare with these facts the additional fact that the larvfe of Distomum, Bothriocephalic, &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 found their way into the body of some host do not at once become quiescent, but continue their wanderings, and traverse in various directions the t issues and organs of its body. 1 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, e.g., the embryos of certain Filarial (p. 49). In a few cases the presence of embryos of Tccnia in the blood has been actually observed (Leuckart, Eaum) ; 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 Trichina 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 (Mutter's Archiv f. Anat. u. Physiol., p. 227, 1851) observed in the blood of Mustelus Icevis and its foetus the same Filarice. However, this does not seem to occur always, since in the Mammalia the transference of Nematode Hsematozoa to the foetus has not been demonstrated (Chaussat). The wandering embryos of Trichina avoid the body of the fcetus. 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. Eathke, I find, anticipated me in this obser- vation (Archiv f. Nalunjesch., Jahrg. iii., 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, lac. cit., p. 11 ). 68 LUTE-HISTORY OF PARASITES. form passages of communication from one nart nf «,* k i of which the embryos avail themselves P b0dy t0 an°ther' issues of the organs themselves, and whether they commence at one metamorphosis, further development V 9 or JnhoTho7ULable T^' PerhapS' in ™ defi^ the IZr t Perhaps, or a snail, in the brain or in the hver. Here only 1S a further development possible. If as is frequent chance has brought it about that the young parasite find its way into some other animal or some other organ, ?t Portly d s but m many cases it leaves behind traces of its" presence' For in-' stance, m lambs that have been fed with embryos of Tama catnurus 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. r 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) B. c_ A . Fig. 45. — Entozoa in the second stage of development. A. Cysticercus of Tcenia solium from the pig ; B. Cysticercus of Taenia cucumerina from the dog-louse ; 0. 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 SECONDARY WANDERINGS. 69 merely to assist their wanderings are cast off, and replaced by new structures, which subserve their altered conditions of life. As a 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 seen, is formed by growth of Fig. 46. — A piece of liver from the rabbit, showing passages made by Cysticercus pisi- formis. the connective tissue, or secretion by the growing body of the parasite, and feed on the substances immediately surrounding them (Fig. 45). 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- worms1 (Tcenia ccenurus, T. serrata, T. marginatd) who'se 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,2 an unsegmented tape-worm of the family Caryophylteidae (Fig. 47), which is a parasite in the body- cavity of many Naidaj. This worm becomes sexual while yet a bladder-worm, which, in other Cestodes, is only an intermediate sta^e. 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 Vertebrate 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 emlryonic stage does not conclude the life-history of the parasite, which needs rather a radical change in its environment -in other words, a second migration. i See Leuckart, "Blasenbandwtlrmer," p. 124. .„-.., „ 7„v„,7^ t ■ Leuckart, " Archigetes Sieboldi, eine geschlechterexfe Cestodenamme, Zettschr. f. ^^^^^^^ u. s. w, d. Aspidogaster conohicoV Zeittchr. f. win. Zool, Bd. vi., p. 349, 1855. MIGRATION TO THE DEFINITIVE HOST. 73 means of the tail, approached the msect larva, and crept all ov 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 m 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 fig. 51.— An en- skin into the perivisceral cavity. The tail of the opted ^ Cem Cercaria always remained outside, and was no doubt 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 larva?, I could still observe the Cercaria? inside their body. They invariably remained quiescent, and assumed a spherical shape (Fig. 51), surrounding themselves with a cyst. The frontal spine Avas 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 Cercaria? 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-larva?, Copepoda, &c, devour the tissues of their host, and become encysted in its empty shell (Mcebius). In the quiescent stage the Cercaria? 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.1 Tlie 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 myeelf in Ephemerid larvae. Similar cases have been observed by other naturalists, e.g. , 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 I'Is&re, t. ii., p. 9, 1868), which, however, I have not seen. I 74 LIFE-HISTORY OF PARASITES. MLT Z!ated f eTliVi^ W * * trans- i r fete ofT ] VG haVG Spedally t0 notice wheu the nnai late of an asexual internal parasite conies to be treated of This hosts 6UCe ^ ^ n° meaUS alWajS the r6Sult 0f a chailSe of 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 Schrostomum eguinum (Stronyylus equinus), which originate from the above- mentioned (p. 61) Ehabditis-Wie 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.1 Eipe 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 Fig. 52. — Worm aneurism of the horse. 1 For a detailed account see Vol. II. 2 Occasionally the reverse is the case, as in certain tape-worms (Ligula, Schisto- cepkalus), which are often taken up by water-fowls directly from tho water (see p. 25). ACTION OF THE DIGESTIVE JUICES. 75 1Mj 1inrfUv adduce special cases to prove. The parasites are in tins w*> u» rnld-blooded to a warm-blooded •r^Ti'^A. «• a r * some To powerful foe, and is devoured by it: neither herbivorous nor carnivorous animals are secure from the invasion of parasites. The Poss bil y of the transference of parasites increases of course with 2 ^ number of animals that are devoured, and especially smce the bearers of encysted parasites are usually small invertebrates. The krJei an Lais which need more nourishment, thus take m a gradu- al increasing number of parasites; and it is easy, therefore, to SaSSTS; it is that of all animals the Vertebrata are most affected by these creatures (see p. 11). Onlv when the parasite has been transferred to the body of its ri«*t host, and other circumstances are favourable does it arrive at sexual maturity; otherwise it rapidly dies. In the same way, the eooa 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 ahout more freely, and also, possibly, bore into the walls of the stomach. A longer contact with the digestive juices 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 LeucocMoridium, and its brood of Distonies (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," M6m. Acad. BeUjique, t. xl., p. 23, 1873). [In the same way, the Entozoa of cold-blooded animals, when they have not arrived at maturity, stop their metamorphosis dming the winter, and produce no eggs, or only very few ; and also, under similar circumstances, Rediaa, instead of producing Cer- carise, give rise to new Itedise. — R. L.] Fig. 53. — Bladder- worm with extruded head. Eig. 54. — Bladder- worm head after di- gestion of the caudal bladder. 76 LIFE-HISTORY OK 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 Zt 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 ecro- 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 strono- 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. 3 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 larva? (p. 72),— the presence of a capsule is necessary to further development (de la Valette), but not in Tcenia, 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- Trichince arrive at maturity in the intestine of their proper host, and that the Echinorhynclms-embvjos of our common Gammarus and Asellus become adult in fish (Uchino- rhynchus proteus) and water-birds (Echinorliynclms 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 echinahtm 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. 2 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. 77 PERIODIC PARASITES. are developed ^^^^^ St :: e«in% of a Oy^s, which is Iw^owed Inl the water in which it lives. The young worms f ; ^ intestine, where they remain but a short time and rway oi, Th. ^« ^ lix^ " r culties of feth 'Eternal wandering increase with the growth of the worm. It no doubt a fact that large, full-grown thread-worms and 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 oi the worms is no doubt assisted by pathological conditions set up m 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 Pentastomum tcenioides, which has a life-history like that of a typical Entozoon. The young form (for- merly described as a distinct species, Pentastomum denticulatum _Eia 56) inhabits cysts in the liver and lung (Eig. 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 i See Vol. II. 78 LIFE-HISTORY OF PARASITES. Gordiaceae and MermithidaB are instances of this kind of ™™ v among the Ento.oa, and the nation from ^^^Z Fig. 55. — Lung of rabbit infected with Pentastomida. Fig. 56.— Pentastomum denticulalu proglottides and other sexual Helminths {e.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 (e.g., by being licked up) reach their next destination. The Ichneumonidas 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 Gordiaceae and Mermithiihv, 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 INTERMEDIATE AND DEFINITE HOSTS. 79 , . i. • f tfcn infected animal (sometimes even in the intestine, h end of this period it instinctively begins to travel, and leaves its Pi ce either by the natural passages (the gad-fly of the horse, or Stance, through the anus, that of the sheep MJ"£ 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.1 It is evident that, in spite of apparent differ- ences the parasitism of Gordius 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 a rough sketch of the life-history of para- sites, and maybe 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. 80 LIFE-HISTORY OF PARASITES. t — ^stage passing directly, and without migration, into the sexual All this, however, is quite exceptional, and the rule for the life history of parasites may be stated as follows -.-The life-history of pal sties t „ **** vato two stages-^) the larval, and (2) L se^MyZZe these two hosts may be merely two individuals of the same m the case of Tricks; but generally they are quite dffflTand may belong even to separate orders or classes. Tcmia crassicoUis inhabits the liver of the mouse while in the young cond^ion and t 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 Tcema solium of man is found in the youn^ condition Z SWme\ lU aisimilar wa^ the life of Ligula is divided between fish (Cypnnidae) and water-birds ; of Echinobothrium typus, between rays and Gammarma ; of Bistomum echinatum, between ducks and Paludince- of Amphwtomum subclavatum, between the frog and Planorbis ; of Pen- tastomum tcenioides, 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 docx the fish and the sea-gull. And this fact is not difficult to understanding 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 Taenia crassicollis 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;1 but in these cases, the latter inhabit the same 1 The statement of Von Siebold (" Handworterbuch 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 LARGE NUMBERS OF EMBRYOS. 81 localities, and have been probably swallowed accidentally along with ^ W conditions also are of great importance in the distribution of parasites, as has been shown by Melnikoff and myself,1 in the case of the dog-louse (Trichodectes), which harbours the young of Tcema eUip- 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.3 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, therefore, 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." 3 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 beobtained — 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 :T — "During the breeding Thomas and myself render it very probable that ruminants and other herbivorous mammals devour Distmmtin hepaticum along with plants, to which Cercariae are attached in the encysted state. — R. L.] 1 Archivf. Naturejesch. , Jahrg. xxxv., Bd. i., p. 62, 1869. See also Vol. II. 2 A tape-worm has an average life of two years. It produces in this time some 1 500 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 ajbout 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. 3 " Handworterbuch. d. Physiol.," Bd. ii., p. 650. F 09 LIFE-HISTORY OF PARASITES. season of the coral polyps, myriads of microscopic embryos swarm in t he neighbourhood of the parent stock ; millions of theYe a^Zh d out to sea and on to dry land, and perish, or fix themselves iTIZi tions where they cannot grow; but if only a single one find a spot suitable to xts growth, Nature has accomplished her purpose and this one have reached a spot, perhaps hundreds of miles Lather no after the lapse of time, rises as an island out of the sea These £&atto? thrfT to auy firm ^ but is - ^ leadmg them to select a favourable place ; Nature, therefore, produces tiiem m such countless numbers, that, on the theory of probability some are certain to obtain a suitable locality." i Who would deny that tms is precisely analogous to Helminths "losin^ 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 j but who would undertake to decide whether or no a particular host were " appointed ? " Von Siebold, however, goes stiU 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."2 Von Siebold maintained this opinion,3 even after Kuchenmeister had endeavoured experimentally to contradict it;4 indeed his words at Konigsberg 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 ? "6 1 Jahrcsh. d. Vcr. vaterl. Naturhunde Wurttcmberg, Bd. xvi., p. 31, 1860. 2 Loc. cit. 3 "Ueber Band- und Blasenwlirmer, u. s. w.," p. 65. : Leipzig, 1854. * Prager med. Viertety'ahrsckrift, Jahrg. ix., p. 106, 1852 ; " Ueber die Cestoden im Allgemeinen, u. a. w.," p. 12 : Zittau, 1853. 6 " Kiinigsberg Naturf. Versamml.," 1860. 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 Trichina} (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 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 Trichina} 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 Trichina, 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 von Siebold's theory, which has now hardly any supporters, in spite of the great reputation of its originator. 84 LIFE-HISTORY OF PAHASITES. application of the degeneration theory, and not to an equal extent 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 ^«*»■* »d ,„«,, ^ty years, whilst o~ZTr w "'ey '"'VVal f0™' ^ ends its life-history under various circumstances1 (a.) ihe larva migrates and becomes, after complete metamorphosis a free-hving animal (Oestrid* among other flies, Ichneu momdsB, Mermithidse, Gordiaceaj ) (i.) The larva arrives at sexual maturity in its intermediate host without further metamorphosis, as in the above-mentioned Arc/ugetes and Asjpidogaster (p. 69) (a) The larva remains in the intermediate host until it migrates into its definitive host, mostly in a passive way (with food) Ihe 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, Distomidaa and Pentastomida3). 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 (Distomidaj). (fi.) 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 larvae {Trichina). (3.) 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 Tricliocephalus and Oxyuris. 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 1 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 Ktlchenmeister (" 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 ! 91 FROM FREE-LIVING FORMS. 72 > other words, F* to« * — *** " ££Z SS?5Sb we thus assert for these creatures is in princ P e precisely the same as that which we also assert, in consonance the doctrine of descent, for the individual free-hvrng 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 dif- 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 decn-ee 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.1 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 J ohahn Muller2 have made us acquainted with a Mollusc (JEntoconcha miraUlis) 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 Entomolorjie, Bd. iii., p. 261, 1818), and for Entozoa by my uncle Er. S. Leuckart (" Versuch einer naturgemassen Eintheiking der Helminthen :" Heidelberg, 1827). The latter says {lor. 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 then- origin in the entirely different mode of life of the parasitic worms, and in their circum- scribed and completely isolated abode." 5 J. Midler, " Ueber Synapta digitata mid die Enstehung von Schnecken in Holo- thurien :" Berlin, 1852. GO 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 MoUuscan 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 lrom 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 Asellidse, or lead a predatory life like the free-living Copepoda (represented in our waters by the genus Cyclops),2 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 1 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. 2 See v. Nordmann, Mih-oyraphisckc Beitrdye, Bd. ii. : Berlin, 1832. DIFFERENT DEGREES OF PARASITISM. yo according to the degree to which their parasitism becomes stationary. Likewise" also, the sensory perceptions, with their degenerate. The body loses its segmentation and finally b comes changed into a cylindrical mass, which not only swells considerab y under the pressure of the rapidly growing 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- meada^ among the Isopoda by the Entoniscidaa,2 winch 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.3 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 C. Clans, " Beobachtungen iiber Lemaeoceva, &c. :" Marburg, 1866. a Fr. Mttller, ArcMv fur Naturgesch., Jahrg. xxviii., Bd. i., p. 10, 1862 ; Jcnaische Zeitschr., Bd. vi., p. 53, 1867 ; and Buchkolz, Zeitschr. f. wiss. Zoo!,., Bd. xvi., p. 103, 1866. :' See Claus, " Beitriige zur Kenntniss der Schmarotzerkrebse," Zeitschr. f. wits. Zool., Bd. xvi., p. 365, 1864. 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 ^ fonly/ecently become the subject of a close investig- ation. 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 mi 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 seq.) 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 ffliabclitis, above mentioned (Lcptodcra ; Pclodcra, 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 oesophageal tube, which encloses in its posterior 1 Especially by Bastian, Ebert, Schneider, Biitschli, Marion, and de Maan. FREE-LIVING NEMATODES— RHABDnma!. 95 dobularly expanded portion (the so-called "Bulbus") an armature usually formed of three valvular teeth (Fig. 60). Sexual maturity i aTaMed ^through abundant nutrition, mostly only m places .here 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 larva; 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 appendiadatum^ though he has more Te^niiy^^^^ correctly recognised it as a BhaMihs [Lepto- deray The researches of Schneider, and more especially of Claus,a 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 larvas have the opportunity of migrating into the black slug (Avion 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 oesophageal 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 larva; in this position.4 The parasites, however, attain 1 Zeitschr. f. wiss. Zool., Bd. x., p. 176, 1860. 2 " Monographic der Nematode!!," p. 159 : Berlin, 1866. 3 " Beobachtungen liber die Organization and Fortpflanzung von Leptodera appen- diculata :" Marburg u. Leipzig, 1868. * See Vol. IT. 96 THE ORIGIN OF PARASITES. sexual maturity only after abandoning this host, when they cast their skm, and lose their riband-shaped candal appendages, while the apertures of the alimentary and sexual organs break outwards through the cuticle. In the sexually mature state also the size and iormation of the tail characterise these animals as a peculiar form J^ven 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 Bhabditis 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. 1 Such is the case with the above-mentioned Bhabdonema (Ascaris) nigrovenosnm (p. 2), whose Bhabditis-fovm, living in the excrement of frogs, differs very little from the animals related to it. Like other species of Bhabditis 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 Bhabditida?., remain there until they have completely destroyed and devoured the internal organs. Also, at the com- 1 I have for some time been accustomed to call such an alternate succession of dimorphous sexual generations by the name " Heterogeny." RIIABDITOID PARASITES. 97 mencement, the young have the characteristics of the nus ^ chUs 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 Bhabclitis appendicu- Fig. 61.— Rhabditoid form of Jthabdoncma (Ascaris) niyro- venosum. A. Male ; B. Female, with embryos in various stages of development. Ftg. 62. — Mature em- bryo of Rhabdonema nigrovenosum. lata. When they reach the lungs of their host, the young parasites grow to a length of almost an inch, and possess scarcely the slightest G THE ORIGIN OF PARASITES. trace of similarity to their predecessors; they live for several months during which lime they produce a countless numher of eggs, 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 Rhabditidse (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 Rhabdonema 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 Ehabditidse, 1 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— e.g., von Siebold— so that the sibility of a parthenogenetic development is not yet entirely excluded. [It was to be expected a priori that Rhabdonema nigrovenosum could not be the only Nematode possessing so peculiar a life-history ; but the statement of Ercolani2 as to the descent of the A. inflcoxc and A. vesicularis^ of hens from certain free-living Rhabditis-f orms, has no foundation in fact. On the contrary, my recent researches y lead to the conclusion that the so-called Angwillula stercoralis (an unmistakeable Rhabditis found in the excreta of patients suffering from diarrhoea 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- art4), which is parasitic in the body-cavity of Hylobms pini, and con- 1 See Schneider, " Monogr. d. Nematoden," p. 313; and Vernet, Arch. Set. Phi/s. Nat., t. xlv., p. 61, 1872. 2 Ercolani, " Sulla dimorphobiosi, &c," Mem. Accad. Bologna, t. iv., p. 237, 1874, and t. v., p. 391, 1875 ; Abstr. Joum. de Zool., t. iii., p. 67, t. iv., p. 254. Leuckart, " Ueber d. Lebensgesch. d. sog. Angnillula stercoralis, u. deren Bezieh. zu d. sog. A. intestinalis," Bericlit d. math. phys. CI. k. Sachs. Gcsc/hc/i. Wiss., pp. 75-107, 1882. 4 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. CI. Te. Sachs. OeselUch. d. Wiss. M1AHDIT0ID LARArAL FORMS. 99 tains in the uterus-like terminal portion of its generative organs an innumerable quantity of Ehabditoid embryos, which become free by boring to the exterior, and grow into mature males and females without essential change of form. — E. L.1] But even the single example of Rhabdoncma 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 Bhctbditis 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 Rhabdoncma, for, according to the above-mentioned (p. 61) researches, there is a whole series of parasitic Nematodes (especially in the family StrongylidaB), among which the Rhahditis-iorm, instead of representing an indepen- Fig. 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 Rhabditidse, 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 Rhdbditis 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. -W I?6!?'0™ PaSSage b6en BUb8titUted by the author for nne in the Gel™-™ edition. 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 RhabditidaB. 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 is a 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 in 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 Bhabditis-Wko, 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, Triclwccphalus, Spiroptera, and others, with embryos, which are not hatched in a free state, but remain in the egg 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 Oxyuris. 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 ABSENCE OF A 1UIABDITOID STAGE. 101 animal-body is plastic, and capable of adapting itself to the con- Sns of I speciRc mode of life. Hence we must leave rt doubt- ful whether the unmistakeable similarity which Oxywis (Fig. 64) presents in many respects (especially in the form of the body, structure of the alimentary canal, and sexual apparatus) to Rhabclitis, 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 FlG_ QL_Oxyuris wmUgua 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 Rhabditis4orm 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 Ehabditidee, 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 Oucullanus or Dracunculus on the one hand, and Stroncjylus 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 Nematodes in correct and natural sequence, THE ORIGIN OF PABASITES. ha. 65.— Embryos, A. of OucuUcmus, and B. of Stroncjylus Jilaria. 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.1 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 larvce which 1 mentioned last as being allied to the llhabditis-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 Rhabditis-iovvas. But in fact this is somewhat deceptive, especially when one considers larval forms of certain species of Strongylidse, which, both by their organization and the systematic position of their parents, remind us strongly of the Bhabditis-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 Blitschli has attempted in a similar way to prove the relations that exist betw een the free-living and parasitic Nematodes. — Bcricht d. Scnkcnb. nalurf. Gcselhch., p. 56, 1872. COMPLETE PARASITISM OK TiilCHINA. 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 as a rule, completes its entire life-history m the body of its host Th embryos, which are horn alive, soon bore through the wall A. w FlG. 66. — Trichina spiralis. A. Embryo ; B. Intermediate form ; C. Sexual form ; (nnimpregriated female). of the intestine which shelters their parents, and thus reach the muscles, where they develop into a larval form, which, after trans- ference into another suitable host, directly completes its growth into the sexual form (Fig. 66). A lengthened existence in the free state is thus entirely excluded ; even embryonic development and migration occur during the period of parasitic life. It is exceptional, and only 104 THE ORIGIN OF PARASITES. wlu^lI;"w^deed',friSh th6 o£ a papism wmcn has lost every relation to the outer world. The Trema todee and Cestodes, as well as the Aeanthocephala are without" exoepfon, governed by the law that in their' you^ eonlta Fig. 68. — Distomum hepaticum (natural size). Fig. 67. — Tccnia mcdiocancllata (natural size). they reach the external world either as freely-moving embryos, or at least as eggs, and from thence they return into their hosts, by means either of an active or passive migration. We know of no case, AFFINITY OF CESTOUES TO TREMATODES. 105 however, in which, among these Helminths, the free life of the larva attains to greater biological independence than I have proved in various ways to be the case among the Nematodes. Where we do meet with a free larval form among them, its function is limited to the search for and invasion of a suitable host (p. 61). Everywhere, during this period of free life, nutrition and growth are m abeyance. It is evident, and has indeed been mentioned above, that on account of this fact the proof of the relations to free-living animal forms is made considerably more difficult. On account of an extensive adap- tation to the conditions of parasitic life, the systematic characters of the animals in question are considerably modified, and often rendered wholly unrecognisable. Among the groups here mentioned there are two, the Cestodes and Trematodes, which are very nearly related to each other, so nearly indeed, that it is difficult to draw a clear distinction between them. This announcement may seem startling, when merely the external form of a Tcenia (Fig. 67) and of a Distomum (Fig. 68) is taken into consideration, for at first sight there are scarcely two other Helminths which differ so widely from each other in their external appearance. In one case, we find a ribbon-like body, perhaps some metres in length, with head and segments ; in the other, a body short, simple, and flat ; in the one, suckers on the circumference of the head, in the other, in the middle line of the anterior portion of the body; in the former, an absence of mouth and of intestine, in the latter, a well developed alimentary apparatus. Who, at first sight, would expect to find resemblances among such oppos- ing characters ? But the question assumes another aspect, when we recognise that what we call a tape-worm is not a single animal like a caterpillar or millipede, but a whole colony, which furnishes segments in regular succession, immediately behind the so-called " head," which also represents a specialised individual — the " Scolex " (p. 37). Not the whole worm, but the single segment (Proglottis) must be compared with the fluke ; and then we shall find, especially in the structure of the sexual apparatus, which constitutes by far the greatest portion of the whole internal organs, that there are so many and such surprising similarities, that the close relation- ship can no longer remain doubtful. Of course there are certain differences between the two forms, especially in respect of the in- testine and of the organs of attachment, but even these lose their importance as soon as we extend our comparison over a large number of species. In the first place, it has been shown that among the entopani- sitic Trematodes there are a number of species which, like the 101:5 TSE ORIGIN OF PARASITES. Cestodes, have no alimentary canals In the case of a W free- ing animal such a want would, of course, be a very remarkable rrcumstance, since the possession of mouth and intestine is, according 0 our present knowledge, a most necessary requisite of such animals" -But the relations of parasitic life, which permit of nutriment bein« taken up through the skin, render the possession of these organs un° necessary or at least not indispensable (p. 18). Even in Nematodes we see the intestinal canal become atrophied hi a few cases This proves no more than that the parasites in question are so completely adapted to the conditions of their existence, that they have no further need for an intestine, and hence we can only interpret the absence ot this organ m the Cestodes as meaning that they are much further removed from the conditions of free life than the Trematodes. But the absence of hooks in the proglottides, like the absence of an intestine, results from the relations given above. They do not stand in such need of them as the solitary living Trematodes, since they belong to a community which is sufficiently firmly fastened by means of a hook -apparatus, with which the so-called head is provided (Fig. 4) ; the individual segments of the chain have thus a certain share in the hook apparatus situated on the head. If further proof of this assertion were required, it might be found in the existence of certain unsegmented Cestodes, which, like Cary- ophyUaius, AmpHjptyches, &c, represent in their simple body both head and proglottis, — that is, unite in themselves both a hook-apparatus and sexual organs like the Trematodes. That which in the common tape-worm was spread over two generations (head and sexual animal) has in these animals again become united in a single individual : and this has been pointed out above to be a frequent occurrence among these groups which present alternation of generations — for it is an alternation of generations which manifests itself in the mode of de- velopment of the tape-worms. The above-mentioned facts leave no room for doubt that the Ces- todes are very closely related to the Trematodes, that they represent in a certain sense Trematodes without an intestine, in which the organism has, according to the law of alternation of generations, separated itself into two genetically combined individual forms. That this affords certain advantages of great importance, especially to animals exposed to so many vicissitudes, as is the case with the intes- tinal worms, is apparent, especially when we remember that the young tape-worm (Scolex) is rendered capable, through the alternation of 1 Such is the case, according to a letter which I have received from Prof. Claus, in a Trematode allied to Distomum from the intestine of Delphinua delplm, as also, acoording to van Beneden, in Distomum fiUicolle. Dr. Taschenberg will shortly prove that these examples by no means complete the list of anenteric Trematodes. i 07 AFFINITY OF TREMATODKS TO ETRUDINEA. of multiplying tne nuiuuc moves that the snnmo- Lough a further adaptation to the conditions of parasitic SSoJta Trematodes or Trematode-like -cesto^en^es- tion reading the origin of these two groups resolves itself into one tliat^to say? the inquiry concerns itself only with the relations which the Trematodes bear to free-living worms. In the discussions of this question only two groups o known animals can be considered here ; these are the leeches and the Plan- arians, both of which show in their external appearance and internal structure a manifold resemblance to the Trematodes. The leeches, by their mode of life, show an analogy with the Trematodes, for it is well known that the greater number of them live as parasites, although they are to some extent predatory (Aulastomum vorax, for instance, feeds chiefly on earth-worms and snails). The smaller and weaker forms of leeches are almost as persistent in their parasitism as the ectoparasitic Trematodes, some of which they also resemble m size and appearance (e.g., Astacobdella, which is parasitic upon the cray-fish, and Uclonella parasitic upon Galigus). One might indeed he easily tempted to imagine a direct connection between these two groups. But upon closer comparison there are considerable difficulties opposed to this hypothesis. Not only do the leeches possess a dis- tinctly segmented body— the segmentation being evident also in their internal structure, especially in the formation of their nervous system and excretory organs— but also the mode of their development and the organization of their embryos manifest many and vital differences from the Trematodes, which at present forbid any attempt to connect them. What similarity there is between the two forms is either more ap- parent than real (structure of the intestine and sexual organs), or is only found in points of inferior importance (possession of suctorial discs, absence of body-cavity). It is evidently more in accordance with our present knowledge of the morphological relations of the Hirudinea to regard them as parasitic forms allied to the earth-worms, than to connect them with the Trematodes. But if the Hirudinea do not furnish a link to the Trematodes, there remain only the Planarians which can be regarded as their ancestors. These prove in reality to be very closely related to the Trematodes in their general structure, and the formation of their individual organs. In both cases, the short unsegmented parenchyma- 108 THIS ORIGIN OF PARASITES. Fig. 69. — Ciliated Embryos oiA,Bisto- mumhepaticum, and B, of Monostomum ca- •pitellatum; the former with an eye-speck. tons body contains a many-branched alimentary canal without anus sexual apparatus. The same agreement obtains in the structure and arrangement of the excretory vessels, the nervous system, and the muscles.' Even in respect of the histology there are many similar agreements. Finally, since the embryonic condi- tions also manifest great similarity to each other, there remains a differ- ence between the two groups, only inasmuch as the one consists of free- living animals, the other contains only parasites. The specific peculi- arities, however, of the Plauarians, as well as of the Trematodes may be ascribed to this difference; since the possession of a ciliated epithe- lium and special organs of sense, as we find them in the Planarians, cor- respond with the requirements of a free life, exactly in the same way as the presence of a hook-apparatus does to the conditions of parasitism The free swimming young forms of the Trematodes— even their entozootic species— are mostly provided with the ciliated epithelium of the Planarians, and often also with the eye-specks of their free- living relatives (Fig. 69). There are forms, even in the fully developed condition, which serve as connecting links between the two groups. As there are numerous species of Trematodes which, instead of inhabiting the internal organs, live upon the external surface of their host, and approach free-living animals in their pigmentation and pos- session of eyes, so also we are acquainted with Planarians, the posterior extremity of whose body presents a discoid organ of attachment (Monocclis caudatus, Oulian.), or even bears a true sucker (Monocelis protractilis, Greeff), by the help of which they attach themselves to foreign bodies. Leidy erects the Planarida?, with a suctorial disc at the posterior extremity of the body, into a distinct genus (Bdellura), and describes in it a species (Bdellura parasitica) which lives on the gills of Polyphemus occidentalis, and presents an instance of a true parasite. 1 Apart from the ciliated epithelium, it 1 Here may also be mentioned Malacobdclla, which was for a long time classed among the Trematodes, and, like the Entozoa, is parasitic in shell-fish, but notwithstanding belongs AFFINITY OF TltEMATODES TO PLANARIANS. 109 „M hP difficult to distinguish such forms from ectoparasitic Trema- Z£ bX mated coating is lost as soon as the pa- becomes stationery or permanent, and the change of the host takes nlopp onlv during the larval period. P lf3l ese observations, the relationship of the Trematodes to the freely Planarians may he taken as established, so that I may omit a ompariso°n of the young forms of these two groups I will only men- tion that the above described (p. 30) peculiar developmental relation of the embryos of Monostomum mutaUle occur also m certain worms closely related to the Planarians, perhaps even in the Planarians themselves. Likewise the fact that the embryos of the entozootic Trematodes often leave the egg without a differentiated intestine, and sometimes (namely, when they develop into the so-called " sporocysts, Ficx 49 p 71) never possess such an organ, will hardly seem peculiar in creatures resembling the Planarians. It has been proved that there are forms among the free-living Planarians which are devoid of a proper intestine (Acoela, Oulian.), its place being occupied by a readily move- able mass of protoplasm, which absorbs the nutriment that passes in through the mouth, as is well known to be the case in the Infusoria. The absence of an intestine in the internal parasites is thus not in all cases the result of a retrograde development, but, under certain circumstances, also the sign of an imperfect differentiation ; and this is the case not only in the embryos of the above-mentioned Distonridte, but also in those of the tape-worms, in which it is impossible to find even the rudiment of an intestine.2 This is a further proof that these latter animals are far more completely adapted to a parasitic life than the other related parasites. This is much more strikingly shown in the Tamiadre, however, than the Bothriocephalidse, by the fact that the former do not even possess the embryonic ciliated coating which is seen in the young forms of the latter (Fig. 70), as in the Trematodes,3 and which, as in these, subserves the function of free locomotion. The (as had been supposed to be the case by me in 1848) to the Nemertines, a gronp closely related to the Planaridfe. 1 In this connection, see the observations concerning the so-called "Desor's Larva," Max Schultze, Zeitschr. f. iciss. Zool, Bd. iv., p. 179, 1853; and Krohn, Matter's Archiv f. Anat. u. Plvysiol., p. 293, 1858, and especially Barrois, " Mem. sur l'embryologie des Ne"mertes," Ann. Sci. nat, ser. 6, t. vi., p. 1, 1877. 2 Huxley considers this circumstance so important, that it causes him to doubt the origin of the Helminths without intestine from animals with intestine ; and he throws out the suggestion that they may be independent of free forms, and be directly and continu- ously developed forms, that were from the commencement parasites without intestine. See " Anatomy of Invertebrated Animals," pp. 213, 652, 675 : London, 1877. 3 In many cases also among the Trematodes, and even Distomidae, the embryos are without a ciliated coat. Von Willemoes Suhm classes among the 28 known embryos of Trematodes 10 non-ciliated forms (Zeitschr. f. wiss. Zool., Bd. xxiii., p. 339, 1873). 110 THE ORIGIN OF PARASITES. Tn w T T lattel' int° °"e ^tematic Ir , apparatus (Fig. 71) of the Tieniada,, with their cylindrical rostellum, and of the TetmrhyncM axe brought into comparison. But all these siml fife pt^ scaroeIy rre thr a certain a=°reement » «» ^ of life. They represent merely adaptive relationships, and since the Fig. 7l.~JEc7unorhyncJms spirula, natural size (after Westrumb). morphological structure in the two groups manifests the greatest differences, they by no means permit the conclusion of a genetic relationship to be drawn. The presence of a muscular body-wall separated from the internal organs— not to speak of other peculiarities — prohibits their association with the fiat-worms. It is indeed useless to seek in other directions for forms with which, the Acanthocephali naturally agree. For a time it was supposed that they were allied to the Sipunculids, and might be regarded as parasitic forms of this group. But in this case also it was only a superficial similarity which gave rise to this view, the more so RELATIONS OF THE ACANTHOCEPHALI. 111 a olmnst exclusively to the external formation of the ^rSS2££52S of the SipuncuUda shows scarcely any' close elation to the Acanthocephali, unless the presence of an m egm nted dermal muscular tube be regarded in this sens. Also the fact that the gulf between the two groups is not bridged o^el t any ^intermediate forms, further lessens the probaluhty of a re- MioZip between them. We know, however,-thanks to recent esearchS-of a parasitic animal closely related to the Srpuncuhda, nanSy the male of Bonellia, which (p. 10) lives as a parasite m the seTal passages of the female; but nothing in the annual betrays approximation to the Acanthocephali. The structure reminds one rather of the condition in the Planarians, or the ciliated embryonic condition of other worms. Also the similarity to the peculiar genus Echwod&res* is limited to certain external characters (the presence of hooks upon a conical head), and does not justify the opinion of a genetic connection. . But thouoh it must be confessed that no group oi animals can be adduced to which the Acanthocephali could be directly traced, this fact does not, of course, in any way involve the conclusion that they have no relation to any other forms. This only may be learned from it, that these relationships, instead of being manifest as in other cases, are of a more hidden nature; in other words, that the Acantho- cephali are related to forms of animals which have succumbed to a deep-seated modification before the typical structure of the parasites in question was developed. The dropping out of the intermediate members, of course, causes the position of these worms to appear very isolated. If, from this point of view, we search for forms which might be considered as the starting-point of the Acanthocephali, then our attention will soon be drawn to the Nematodes, which like them are parasitic. I will base nothing on the fact that there are thread-worms which, being provided with a proboscidiform and armed cephalic ex- tremity, have occasionally been considered as Ecliinorhynchi. An erroneous interpretation cannot have the force of proof. But this would have been almost impossible, had not so many other similarities ob- tained between the two forms. In fact, both possess an elongated cylindrical body, the walls of which are formed of a strongly developed dermal muscular tube, surrounded by a firm integument. This tube is traversed by longitudinal vessels, and encloses a distinct body- 1 Schneider also attempts to support the relationship with the Sipunculidaa by means of the structure of the muscular apparatus, which in its arrangement differs from the conditions found in the Nematodes, and agrees more with those of the Sipunculida- (Miillcv's Archir f. Anat. u. Physiol., p. 592, 1864). 2 See especially Greeff, Archiv f. Naturgesch,, Jahg. xxxv., Bd. i., p. 72, 1869 ; and Pagenstecher, Zeitichr.f. wiss. Zool., Bd. xxv., Suppl., p. 117, 1875. 112 THE ORIGIN OF PARASITES. cavity, which in both cases contains a well developed malP emale sexua apparatuSj whose m ml^ZLTeZ on a superficial view, are scarcely more marked L , V , ^wuctT °Vhe saffie — "cXodTo^ Turbellana According to the above remarks, the absence of intest ne n the Acanthocephali can scarcely be regarded as an i^LZt ^s motion But the proboscidiform apparatus also, although of comp ! cated and peculiar structure, cannot form an objection to&the exTsfence o a relations np with the Nematodes, since we are acquainted amon" II'b^^T WS "* ^ ^ aprobosci^ In conclusion we may remember that the Acanthocephali mani- fest also m respect of their histology many resemblances to the con- ditions which obtain among the Nematodes. Among other tiling both agree m the structure of the muscular fibres and the oanolia In the cuticular character of the connective tissue, in the often colossal size of their cells, and in the complete absence of ciliated epithelium On consideration of these facts, it becomes evident that the Acantho- cephali must be regarded as peculiarly modified Nematodes. The relations of these two groups may be rightly compared to those which obtain between the tape - worms and Trematodes ; that is to say the Acanthocephali may be regarded as forms of Nematodes which have adapted themselves to the parasitic conditions of existence, to a higher and more complete degree than the others. The character of the young forms agrees with this conception, and we are led to believe that they are more closely related to the original B conditions, because they are (according to my observations) provided with the rudiments of an intestine,1 in which one can discern, notwith- standing its incomplete differentiation, a pharynx and an intestine. An oral aperture is wanting ; its place is occupied by a grove in the form of a slit, surrounded by a varying number of setaa, em- bedded in the retractile cephalic extremity — (Fig. 72). On comparing this young form with the common embryonic forms of the Nematodes, it would seem as though the above asserted similarity of SLLV^/~S Were only a sl^U one> but this °Pinion chanSes f/ustatus;A. the profile; when we consider the embryos of the genus Gor- B. ventral view. ^ yfaiGii we meet with relations (see specially the illustrations published by Villot) which in fact differ only very little from those of the embryos of Echinorliynchus. Gordius is a 1 See Vol. II. 1 1 3 GOEDIUS AND ECHINORHYNCHVK. thread-worm which differs from the real and typical Nematodes m nanv rZe^s among which may be mentioned the atrophy of the Sr'and the terminal position of the male and female sexual azures characters which approximate it to the AcanthocephaMa, Thi s however, only an additional reason for laying greater stress Ipon ii, since we have every reason to consider the Acanthocephalid* as vet more modified forms. The changes which lead the embryos of Gordius to their ultimate structure are unfortunately yet unknown to us. This fact is the more to be regretted, as they may acquaint us with relations winch would brin» the strange and in many ways remarkable metamorphosis of the EcUnorhyncH- nearer to the usual process of development than has hitherto been the case. In the meantime, in considering their re- lationship, we can lay only slight stress upon these peculiarities, for we are well aware that the developmental history often pursues various courses even in closely related animals ; in one case it may be direct, and hasten rapidly to its goal, in another, it may reach its conclusion by a circuitous route, passing through metamorphosis and alterna- tion of generations. The course of development of the Eclivnorhynchus is merely a metamorphosis— a metamorphosis, too, than which nothing more thorough and complete could be imagined, since in its course almost everything that the fully developed worm possesses is formed anew out of the older structures. After the foregoing account, the reader may decide for himself whether, and how far, I have succeeded in discovering the relation- ships of the Helminths, and in proving that they have originated from free-living worms by adaptation to a parasitic mode of exist- ence. But even suppose the matters just discussed were proved facts, and not mere possibilities, even then much in the life-history of these animals would remain problematical. We could only conclude from this that a worm is capable of exchanging a free life for a parasitic one, and of adapting itself in structure and mode of life to such altered conditions. Instead of a free creature, the worm be- comes a parasite, which departs, more or less, from its original form according to circumstances. It now attains sexual maturity in the interior of its host, instead of, as formerly, in the free state. It pro- pagates, and generally, in consequence of the favourable circumstances of nutrition, has usually a very numerous progeny, which pass to the exterior, and perhaps for a time live freely, but finally develop into sexually mature parasites. This is so in many instances, not only in stationary parasites, but also in many Entozoa, though very seldom ; for, as a rule, the first host ' Sec Vol. II. H 114 ' THE ORIGIN OF PARASITES. does- not bring the intestinal worm to complete development bnt to a cer am more or less advanced stage, after which the parasite att ns maturity only after transference into its definitive host intestinal worms undergo, for the most part, as has been shown at length above, a change of hosts, and in consequence their life-history and development are spread over two or more hosts Of this change of hosts we have hitherto taken no account in our discussion, and yet it is clear that it is a process which not only com- plicates, m an unexpected manner, the phenomena of parasitism, but also requires an interpretation from a genetic standpoint before we can obtain a complete insight into the nature of parasitic life At the outset only an ambiguous answer can be given to the question of the significance and mode of origin of the so-called « inter mediate hosts," provided that we do not wish to forsake the point of view we have hitherto occupied. The intermediate hosts have either been interpolated subsequently into the life-history of the parasites or they were originally true definitive carriers, which formerly brought their intestinal worms to sexual maturity, but have since become merely intermediate, because the history of development of the in- mates has extended itself over a greater number of stages by means of further formation and differentiation. That we have in both cases to do with a far-reaching adaptation needs scarcely to be expressly mentioned. If I express myself unconditionally in favour of the second of these possibilities, it is chiefly in consideration of the fact that the fully formed and sexually mature stages of the Entozoa are found, with few exceptions, in the vertebrates — that is, in creatures which have relatively only recently originated. The Invertebrate, of course, are not free from Helminths, but all the hundreds and thousands of species which they shelter are, with few exceptions, young forms, which require transference into a vertebrate in order to complete the cycle of their development. If this do not imply that the intestinal worms have arisen along with the Vertebrate, or that they became extinct in their oldest representatives, with the exception of a few remnants — and both seem unlikely upon unprejudiced consideration — then the only possible conclusion is that the Helminths of the Invertebrate' have in course of time changed their character, and have, during their further development in the Vertebrate, become mere larval forms instead of sexually mature animals. In view of these facts, we cannot doubt that the vertebrates afford a much more favourable soil for the development of the Helminths than the invertebrates. We must even admit that numerous forms have originated after the Verte- 1 See 1 a and b, 2 b, and 3, in the short review at the commencement of this chapter. OHIO IN OF INTEHMEDIATE HOSTS. 115 brata became separated as a distinct phylum ; some, even m relatively recent times, such as the Trichina and others, whose life-cycle is limited to mammals, most recently of all creatures. In many cases the ori-in of new Helminths may have gone hand in hand with the transformation, by means of which the hosts have gradually become new species. That the change of a sexually mature animal into a mere pre- paratory stage (a larva)-the process which we have adopted to elucidate the change of hosts— is biologically possible, cannot be doubted in view of the analogy of the so-called abbreviated develop- ment, frequently mentioned above, and whose counterpart it forms. If a series of different developmental phases may contract into a single continuous process, then, conversely, this latter can also spread itself out into a number of such phases. This is a process to which we must attribute a very important rSle in the formation of species ; for the present larval forms are to be considered, agreeably with the doctrine of descent, as the original sexually mature ancestors of those species which to-day represent their ultimate condition. The sum of the characters by which these latter differ from the larvae represents the gain which the original animal has gradually acquired under the changed relations of life, changes which become, as it were, added on to earlier ones, so that the development is protracted, and sexual maturity, which coincides with the conclusion of development, is delayed. The nature of those Entozoa, which are parasitic in invertebrates in a mature condition, lends a yet more definite support to our supposi- tion. They are, of course, few in number, if we except the entozootic Isopoda and a few others, and confine ourselves to the true Hel- minths; these belong mostly to the thread- worms. But we may mention also a Trematode living in the fresh-water mussel (Aspido- gaster conchicula), and a Cestode {Archigetes Sieboldi), described re- cently by me, and found in the body-cavity of Scenuris. All these forms develop, so far as we know their life-history (p. 70), without an intermediate host, and attain their sexual maturity im- mediately in the first host, as would naturally be the case provided our supposition were correct. In addition, the development and metamorphosis of these forms are very simple, so that the respective animals are but little removed from their hypothetical original form, and become sexually mature in a condition which in many respects stands on a par with the young and larval forms of their further advanced relatives. Thus the Nema- todes, sexually mature, found in invertebrates (mostly omnivorous insects and millipedes), follow closely the Rhabditidse (Oxyuris) in 116 LIFE- HISTORY OF PARASITES. their development ; that is, they resemble forms to which the para- sitic Nematodes bear relations, which have led us above to regard them as then- forerunners, and which are often seen represented in their young forms. An exception must be made in the case of a single very peculiar species {Splmrularia), which lives in the body- Fig. 74. — Aspidogaster conchicola. (A.) Embryo, (B. ) Young animal, not yet sexually mature, (after Aubert). Fig. 73. — ArcMgetes Sieboldi. cavity of the hibernating humble-bee, and shows relations of organi- zation which are as yet only incompletely understood.1 Likewise 1 See especially Sir John Lubbock, Natur. Hist. Rev., vol. i., p. 44, 1861, and Schneider, " Monogr. d. Nematoden," p. 322, whose opinions regarding the life-history and morphology of this strange worm differ widely from each other. [Recent investigations of Schneider (Zool. Beitrage, Bd. i. , p. 1, 1884) have made us acquainted with the interesting fact that the young Splia-rularia grows outside the body of its host into a sexually mature animal, resembling Anguillula, without essential change in its organization. I have con- vinced myself of the correctness of this observation, and believe I have obtained proof that these worms copulate while in the free condition, and that only the females find their way into the humble-bees, where they develop into the paradoxical Sp/ia ru/aria. If such be the case, Spharalaria can no longer be considered an exception to the rule above stated. — R. L.] SEXUAL PARASITES OF INVERTEBRATES. 117 Archives (Fig. 73) is, morphologically speaking, nothing else than a Cy Scoid-'a tape-worm-which concludes its metamorphose at a stfoe of development which, in the case of the common Cestode , represents merely a transitional form inhabiting an intermediate host Aspidogastcr also (Fig. 74) is wrongly classed among the otherwise ectoparasitic Polystomidse, on account of an absence of metamoi- phosis whilst its structure stamps it decidedly as a Trematode allied to Di'stovmm. Aspidogastcr resembles a Eeclia in its mode ot de- velopment and the formation of its intestinal apparatus in so remarkable a manner, that I see no objection to placing it, notwith- standing its sexual maturity, beside the true Distomidae, and so classity- ino it along with them, j ust as Archigetes is placed with the tape-worms. The presence of a ventral sucker can as little be opposed to this conception as the high development of the excretory system of vessels, since both structures must be regarded merely as the result of an adaptation to the animal's mode of life, which cannot be taken into consideration in determining morphological relationships. The Eediaj and the Sporocysts (Fig. 49), which have sprung from them by a retrograde formation of the intestine,1 are, m accordance with the above discussion, to be regarded as the oldest Distomidfe, in the same way as the Cysticercoids are the original tape- worms. This agrees with the fact that the Kedias are more closely related to the ectoparasitic Treraatodes (specially by the structure of the intestine) than are the fully formed Distomidte, and hence may be more easily and readily supposed to be derived from the former. It is, moreover, sufficiently known that the Bedise do not change directly into the mature Distomidne, but develop them in their body-cavity out of so-called " germ " cells, which are of the nature of eggs, and separate themselves from the body-wall (Fig. 75). The metamorphosis is divided over two generations, which spring from each other ; it thus becomes an alternation of generations, a common phenomenon, as has been shown above. The production of the new brood may perhaps in this case be directly connected with the former * [This supposition has found an unexpected confirmation in the discovery of the Orthonectida (see Giard, Journ. rite I'Anat. ct Phys., t. xv., p. 449, 1879, and Metschnikoff , Zeitschr. f. wiss. Zool., Bd. xxxv., p. 282, 1881) ; or rather through the establishment of the fact that these simple animals, parasitic on Ophiuroids and Turbellarians, are to be regarded morphologically as sexually mature Trematode-embryos, devoid of an alimentary canal (Leuckart, Arehiv f. Naturgcsch., Jahrg. xlviii., p. 96, 1879). Hence the Ortho- nectida stand at the lowest stage of that series of developmental stages represented by the Trematoda. Aspidogaster, therefore, which we have regarded as a sexually mature Redia, stands higher in the series than the Orthonectida. What influence these facts have upon our views of the gradual progress of parasitic life — how beautifully and naturally they come into accord with the views expressed in text— hardly needs any further com- ment.— R. L.] 118 LIFE- HISTORY OF RARASITES. oty one paras.te there will now originate a number-many d„™ ZIns7Zen m0re-a11 « »*« favoerable eoi aitions, of commencing new parasitic life * The newly formed DistomidaB, however, do not grow into sexually mature annuals within or beside their parent, but, as a rule It least Fig. 75.— Ftedise, with brood of Distomes in the interior. (A.) From Paludina impura (young and old) ; (B.) From Lymnceus (young and old). forsake the host as a Cercaria, and swim about in the free state for a time by means of an appendage which is not unlike the caudal bladder of Archigetes, and then migrate into a new host, generally once more an invertebrate animal (p. 72). The Cercaria thus undergoes a change of host, which does not immediately transfer it to a vertebrate, as is usually the case, but at first to an invertebrate again, such as a snail or a water insect. In the present Distomida, also, these two hosts are both 1 This conception receives a new confirmation from the life-history of AUantonema, alluded to above (p. 98). 2 Such a proliferation in the intermediate host we find in a few Cestodes, and especi- ally in Eclvmococcw; but in this case the young brood originates through budding, aud remains connected with its mother-animal in the interior of the body for life. PROGRESS OK PARASITISM IN THE D1STOMID.E. 119 intermediate hosts, but we may take it for granted that such was not Te from its commencement. On the contrary, these , second .into, mediate hosts brought their Trematodes to sexual maturity m the same way as was formerly the case, according to our supposition, with the Kedi Since the caudal appendage, by means o which the Cercarne swim about, is lost when they force their way into a ™w host « > the developmental condition of these sexual animals must m the mam have been like the present one. • i • i The entozootic Trematodes are accordingly Helminths, m which the change of hosts had already come about at a time when the vertebrates, which are now almost exclusively concerned m it, had not yet come into existence. . The supposition that the Cercariee originally attained sexual maturity in their hosts, and only later developed retrogressive^ into more intermediate forms, finds some support in the fact that these animals even now, under certain circumstances, become sexually mature, and produce ova in their intermediate hosts. On a former occasion (p. 73, note) some cases of this kind were cited, and others are continually forthcoming. These sexually mature Helminths are not separate species, possessing no other sexual condition; they are rather nothing more than certain specially privileged individuals belonging to species which, under other conditions, are accustomed to attain their maturity only after transference into a vertebrate^ It is, moreover, a common phenomenon that the Distomidee not only commence the formation of their sexual organs in the interme- diate hosts, but bring them to a state of complete functional capacity. This phenomenon we meet also in other intestinal worms, although individual species present great variations in this respect, so that many are undifferentiated sexually even when passing into their definitive host. "With respect to the latter, I may mention Cucullanus and Spiroptem, whilst others, like Hedruris and all the Echinorhynchi, assume all their external and internal peculiarities in their interme- diate hosts, which is certainly a case of persistence of an earlier state. Of course such differences are not without influence upon the length of time occupied by the development ; instead, perhaps, of weeks and months being necessary, as usual, the worm of the latter kind requires only a few days, after leaving its temporary host, in order to attain full maturity, and to acquire the ability to propagate its species by sexual means. CHAPTER VI. THE EFFECTS OF PARASITES ON THEIK HOSTS. Parasitic Diseases. ^•Tlhat^ 7 b6en Said °f the ^MBtaT of Parasites, and especially of the Entozoa, it is evident that they influence in a most important way the health and even the life of their hosts. But the existence and amount of this influence was firmly established only by the discoveries of recent decades. From this time a rational theory of parasitic diseases, and a true insight into the deep significance of this important branch of medical science, must date. Not that the idea of parasitic diseases was something absolutely new on the contrary, from the earliest times men knew and feared the injurious effects of these unbidden guests, and feared them perhaps even more than they knew them. In order to form a correct estimate of the pathological significance of parasites, it is necessary to cast a glance at the literature upon the question of the seventeenth and eighteenth centuries. 1 There was then no grievous and dangerous malady which parasites, and especially intestinal worms, were not thought capable of exciting! Dysentery, scurvy, hydrophobia, and even the dangerous epidemics of the Middle Ages, such as plague and small-pox, were all described as parasitic diseases. With each disease they associated a particular parasite, just as we now sometimes speak of the cholera-Bacillus, and other similar creatures, as the transmitters of certain specific diseases. They supposed, further, that these originators of disease lived either in the alimentary canal, or under the skin, or in the blood, and thence, according to their nature, infected the whole organism in diverse ways. Nor was this opinion held by individuals only, but by many, and partially even by the most famous representatives of the pathology of the time (Leeuwenhoek, Hartsoeker, Andry, and others). The possibility of such extravagant opinions is now the subject of incredulous wonder. To understand them it is necessary to realise the condition of medical science at that time. On the one side there was inaccuracy of diagnosis, and almost entire ignorance of pathological 1 I specially recommend Andry, " Traite sur la generation des vers dans le corps de 1 homme," Paris, 1700, of which a new edition and German translation have since appeared. EARLY VIEWS ON PARASITIC DISEASES. 121 anatomy on the other, the natural desire to reduce the different diels to definite etiological entities. Men then hit upon para. es/ as they did later upon magnetism and electricity, m part only because they knew so little about them. It occurred to them the more naturally to refer these diseases to parasites when they observed the exit of intestinal worms and conse- quent recovery; and also because, since the time of the Arabian physicians, the parasitism of a mite had been recognised as the cause of the widely distributed itch (see Fig. 6). In the eyes of many patho- logists the last-named fact served as direct proof of the correctness of a theory from which they anticipated the weightiest conclusions as to the nature of diseases. But these hopes were vain. Although helminthological knowledge was gradually more and more extended and consolidated, the idea of the °Morbi animati " found no new support. Men attempted in vain to place beyond doubt the existence of a Contagium vivicm in the above-mentioned diseases. They only formed the conviction that the earlier physicians, with their guesses at the existence of certain para- sites, had been much too generous. The so-called "heart-worms" were recognised as blood-clots, the "umbilical worms" as mere fancies. The existence of the itch-mite even was doubtful, since a number of experienced physicians and naturalists had sought after it in vain. Observations concerning the presence of intestinal worms in animals also increased, in which, in spite of this parasitism, no signs of illness were noticed. Under such circumstances, the earlier opinions became in the latter half of the last century more and more discredited. The Entozoa were still, it is true, considered on the whole as in- jurious guests, which might seriously affect the health, and sometimes even endanger the life of their host. But their specific relations to certain diseases gradually ceased to be traced ; and there were many who even denied that intestinal worms had any hurtful effects on their host whatever; some even considered their effects to be advantageous. Men like Goze and Abildgaard maintained among other views that intestinal worms aided digestion, by absorbing the mucus and ex- citing peristaltic contractions. Jordens even called them the good angels and unfailing helpers of children.2 It was also supposed (e.g., by Gaultier) that their movements and the resulting conditions ex- erted a favourable influence on the development of the lungs and viscera. 1 These speculations went so far, that this question, for example, was discussed (and answered mostly in the affirmative)— " An mors naturalis sit substantia verminosa?" 1 " Entomologie und Helminthologie des inensohliohen Korpers :" Hof, 1801. 122 THE EFFECTS OF PAIiASITES tON THEIR HOSTS. The belief in the absolute injuriousness of parasites, already shaken by these coupons and doubts, received a still ruder l^ lc a their wide distribution and frequent occurrennP in nl ' T became know, Men began notLy to°2£X Sffl^ he worm-diseases but to think themselves justified in maintafnin, ht th Tt T T °f ^ t0 CaUSG ^ distu^re of health. It is true, however, that such opinions were held for the most part by naturalists and helmiuthologists. The physicians for the most part still held to the old opinions. any doubt as to the nature and origin of a disease, worms were blamed • and worm-imtation," « worm-fever," and other worm-diseases were' very common terms both in theory and practice. And if by chance or in consequence of medical treatment, a worm left the patient, they con- sidered the diagnosis verified, and the cause of the disease established beyond a doubt. The professional helmiuthologists, headed by Eudolphi and Bremser, although, as we have said, decidedly opposing these views could not deny that certain pathological conditions, especially those of the digestive apparatus, were generaUy connected with the presence of worms. They were, however, disinclined to believe that these condi- tions were directly due to worms, but sought, in accordance with their theory of the spontaneous generation of Helminths, to show that there were certain conditions productive of worms. They spoke of a " pre- disposition to worm production" referable to definite pathological pro- cesses, of a " Diathesis verminosa" which they sometimes even called " verminatio," a worm disease without worms ! Thus Bremser,1 the famous Viennese helminthologist, says, "By a worm disease I'mean any disturbance or interruption in the functions of the primary or secondary digestive and nutritive organs, whereby substances are formed and collected in the abmentary canal, which under favourable circumstances may, but do not necessarily, produce worms : I mean, in short, the material factors of worm production. So that worms in the alimentary canal are not an original disease, and indeed can only rarely be regarded as a disease at all, but are much more frequently the sign of the diseased state of the organs in question, or of some in- terruption in the co-operation of these organs, from which state many results may arise without the presence of worms." After what has been already said concerning the life-history and origin of the Entozoa, it is unnecessary to criticise these opinions minutely. We may now regard it as completely established that parasites do not originate from a diseased condition, but from germs intruding or introduced, and they especially originate where these 1 '• Lcbwide Wurmer im lebenden Kdrper," p. 119. PARASITES A REAL CAUSE OF DISEASE. 123 ceruis find the conditions of their development fulfilled. Just in pro- Son to the number of germs introduced, and to the adaptation of Cnvkoament to their wants, will the number of parasites increase suppose that the developmental conditions of parasites involved a certain pathological state, and might also assume ffpIsL could not be developed except in »=i but in the impossibility of all proof this would only be blind adher- ence to a dogma. It is true that the same has been asserted m regard to the spores of fungi, and even m regard to bark-beetles (Bostrichm) and vine-insects {Phylloxera), but here also the assumption of a previously existing pathological state seems unwarrantable, having neither proof nor probability. _ What leads me most decidedly to this conclusion is the ease with which even the healthiest individuals may be experimentally in- fected with Entozoa. Of course, the experiment does not succeed with every kind of parasite, but only, as was before explained, with those which find suitable environment. Even then there may be a few cases in which the expected result fails. But our former observa- tions have prepared us for such experiences. For the development of a parasite requires the presence not only of certain specific factors, but also of many individual ones. It might even be granted that the health, and especially the nature of the organism to be infected, are not without effect on the imported brood (in the case above mentioned (p. 85), in which, after three weeks, the heads of Tcenia cosnurus showed hardly any traces of further change, the animal under investigation had been used some time before for an experiment with Trichina), but we have never found the slightest ground for believing that the development of the invading Helminth is promoted or even conditioned by any un- healthiness of the animal experimented upon.1 Meanwhile, it is safe to assume that wherever there is a real con- nection between the unhealthiness of a host and the indwelling para- sites, it is the latter who are the efficient causes. It is, however, not only on a priori grounds that we are warranted in maintaining that parasites may cause even very dangerous diseases. Experimental helminthology has securely established this position. 1 1 Statistics, which alone can decide in this case, show, on the contrary, that certain ill- ne8geH — eJj,t chronic, and especially intestinal catarrhs— tend to remove parasites from the diseased organ, or even to prevent their occurrence. Thus Gribbohm (" Zur Statistik menschl. Entozoen," Kieler Inauguraldissert., p. 8, 1877), in chronic intestinal catarrh, mostly in consequence of phthisical processes, found in 05 bodies only 16 (24-6 per cent.), and in chronic catarrh of the large intestine in 18 bodies only 3 (16'7 per cent.) cases of the stomachic Nematodes (Ascaris, Oxyuris, Trkhoceplialu.s), which are other- wise so very frequently present (on an average in 49"8 per cent, of the cases examined). 124 THE EFFECTS OF PARASITES ON THEIR HOSTS. refer especially to the experiments made with Tcenia ecerwrus* and Tnclnna sprrahs, which may well dispel all doubt on he Sett If avourably situated, a brood of these parasites acts like poTso'n and large dose is sure to kill the animal. P ' d Through these experiments, not only has a foundation of fact been laid for the study of parasitic diseases, but an exact method trea men has been attained While little progress has as yet be n made m this direction we have nevertheless established n any n" practically important facts. y a In speaking of parasitic diseases, we mean the various disturb- ances of health occasioned by these creatures, in contradistinction to the views of those who speak as if there were only one «ZZ disease —a specific helminthiasis. Parasites act in very different ways,' according to their size and mode of life, as also according to the nature of the inhabited organism Intestinal parasites produce different symptoms from brain parasites' and the effects of these are different, according as they are situated in the cortical layers of the hemispheres, or in the crura cerebri In the same way, MhmtUB duodenalis acts differently from Oxyuris or TricUna. The effects of many parasites never fail-as, for example m the case of Cysticercus in the eye and Strongylus in the kidney- and m the case of others, accidental causes determine whether or not' and in what degree, these effects shall show themselves Besides the' situation of the parasite and the individuality of the host, the number of imported germs in this connection is specially important, and to this, indeed, the effects produced and the dangers incurred are ever proportionate. Thus, it can be easily proved that the frightful symptoms of trichinosis occur only when the parasites live in masses in the alimentary canal, and thence invade the muscles3 in still 1 Especially instructive on this subject is the result of a feeding experiment which was made simultaneously in May 1854 by van Beneden in Louvain, Eschricht in Copen- hagen Gurlt m Berlin, and by myself in Giessen, with specimens of Tcenia c«™ sent us by Kuchenmeister (then in Bautzen). The animals became ill in all the places at exactly the same time, and exhibited exactly the same symptoms. Compare Haubner in Gurlt S Magazine, loc. oit, Leuckart, " Blasenbandwlirmer," p. 47, or van Beneden Comptes raidus, t.xxxix., p. 46, 1854. 2 We may take this opportunity of referring to the classical work of Davaine, rait6 des ent°z°a]res et des maladies vermineuses de l'homme et des animaux domes- tiques "-Paris, I860, 2d ed. 1877- which contains an almost complete collection of ex- periments concerning worm-diseases up to date, and is full of interesting particulars. The part of tins work treating of natural history is not so good even in the second edition, and contains many errors and anachronisms. 3 The number of muscle Trichina in an individual case has been estimated at from 60,000,000 to 100,000,000-a number which, on the supposition that half of the Trichina- arc females, and produce on an average 150U embryos, would correspond to from 100 000 to 120,000 sexually mature animals. LOSS OF NUTRIMENT RY THE HOST. 125 i fW if oniv a moderate number be introduced, larger masses, and that it oniy d mau i 1 to be attacked will be best realised when we state that Teases of the so-called "Cochin-China diarrhoea" the number of B^ stLoralis which have been known to be vidual cases, within twenty-four hours, amounts to seveial hundred thousand or even to a million.1 , Three points have to be noticed m regard to the way in wJncn narasites affect their host. In the first place, they grow and breed at Ce pe-lf their host, from whom they thus ab^ nntnUve material. Secondly, they produce alterations in apace a upon the surrounding tissues or obstruct the <*an^^ they live. Lastly, their movements, according to circumstances may give rise to pain, to inflammation varying m degree and n termination, or even to perforation and dissolution, - all which symptoms are, however, sometimes merely the result of continuous ^ The first of these three kinds of influence, although perhaps the one which occurs most frequently, is seldom of much P^ojpcal importance. There must be unusual influences at work, it the loss caused by the abstraction of nutritive material by the parasite for its metabolism, growth, and propagation is at all appreciable to the host, provided that he belong to the larger animals, and much exceed his parasites in size and nutritive requirements. A Bothriocephalus lotus, seven metres in length, weighs about 27"5 arms According to Eschricht, it throws off yearly a number of pieces, which measure altogether about 15 to 20 metres, and may represent a weight of about 140 grins. Even on the supposition that the animal, which of course undergoes continuous metabolism, abstracts three or four times that quantity from its host,2 and that the nutritive material consumed by all the parasites amounts to several pounds yearly, it is easy to see that so moderate a quantity is of hardly any account compared with the yearly consumption of the host. It is » Normand, " Mem. sur la diarrhea dite de Cochin-Chine," Paris, 1877, and Davaine, loccit., 2ded.,p. 968. i It does not require much proof to show that Heller's method of determining the host's loss of nutritive material simply by the bodily weight of the parasites is erroneous. (Art. " Darmschmarotzer " in "v. Ziemssen's Handb. d. sp. Path. u. Ther.," Bd. vii., Th. 2, p. 567 ; Eng. transl., "Cyclop. Pract. Med.," vol. vii., p. 678, London, 1877.) Moreover, there are some tape- worms whose growth is so rapid that Heller's method also would give extraordinary results. The tape-worm of the sheep, for example, which grows to the length of a hundred metres, has been found fifty-one metres long in lambs four weeks old.— Goze, " Versuoh einer Naturgesch., u.s.w.," p. 371. 126 THE KmoiH nf pARAsm,s ^ iheie Hosw about 550 grms. of organic martyr Tf t , he year ss^r— \ r°f — ~— rrzz thread-worm, for example, produces, as we saw before 42 «mnf nf™ - stance yearly ; and taking into account what iT^TiS b ism must deprive its host during this time of at lea 100 ™ Thus if, as sometimes occurs, there were 100 of them fthere havP W cases m which 1000 have been found at the same time in th T estine), they would cause a loss of 833 grins. 1^1^ ^ some circumstances, and especially in childhood, mus have a very rr^rtn fmilany; the ioo'oo° °f ™ rtacwofo (1 mm. long, 04 mm. in diameter) which are often evacuated daily by patients suffering from Cochin-China dLhtt represent by their mass alone-without > taking into account maS used in metabolism-a weight of about 200 grms. And if itTe con sidered further that the number of the evacuated worms m^ i rel" tenfold, it becomes easy to understand how a severe marasmus may ensue, even after a short illness. y ,i .V0 ^ !10wever' cite these instances in support of the view that the disturbances of the nutritive functions, and their manifold external consequences, which the physicians were so willing to in- terpret as symptoms of helminthiasis, are always to be regarded as the direct consequences of the amount of nutritive material ab- stracted by parasites. Even if it be certain that the disease is con- nected with parasites, it is quite possible that the relation between disease and parasite is of an indirect nature, and brought about by the state of the infected organ.2 Further, the continuance of any disease, however slight and partial at first, will affect the general con- ditions of the nutritive organs. Nor is the nature of the materials which serve as nourishment to the parasites an unimportant factor in considering the effects of the association. Thus a blood-sucking parasite, cceteris paribus, causes a greater loss than one which feeds on epithelial cells. To this is due 1 [As above mentioned (p. 46), these Rhabdoids are the offspring of AnguilMa intestumhs, which had been met with in the intestine by earlier observers, but whose genetic relation had remained unrecognised. Grassi and Perona have recently reported that they have found Anguillula intestinalis in patients suffering from catarrhal gastro- enteritis in Milan (Archivio scienze medic, t. iii., No. 4, 1879). R. L.] 2 A very good example is afforded by the Trichina. Tn their wandering into the muscular tissue they not only destroy fibres, but, by paralyzing the muscles used in masti- cation and swallowing, they affect the introduction of nutriment to such a degree that within a very short time the patient experiences considerable atrophy. MECHANICAL DISTURBANCE BY PARASITES. 127 the ereat clinical importance of Doohmim dnodenalis (Anchylosto- ThS 10) which occurs in masses in man in many tropical and in the north of Italy, and is generally o full o ^ bTo d that at first sight the intestine of the patent appear, to be covered with leeches. As a rule, the presence of this worm soon Idu s an anaemic condition.* This disease, with many others re su from it, is so common in Egypt, that nearly a quarter of the nS'populat on suffer from it (hence « GMorosu JSgygaca ), and Zny Perish for want of proper treatment, which ought of course, n the first place, to be directed towards the removal of these danger- ous ^ests In this case we have, of course, to consider not only the loss of blood which the parasites cause in filling their digestive apparatus, but that which results from the bleeding of wounds caused by their bites, which, according to Griesingei- is often very con- siderable. It is in this way that even leeches may become dangerous, or even fatal, to man. . At this point it may also be mentioned that a very considerable loss of blood may be caused by the hematuria produced by the wandering of embryos, as in the case of Filaria Bancrofti (F. sanguinis hominis) and of Distomum hcematohium. ' But, on the whole, the disturbances resulting from the loss of strength, and interruption of the nutritive supplies, are considerably less than those which are occasioned by the growth and multiplication of the Helminths. As soon as the worms exceed a certain size, they begin, like other foreign bodies, to exert a pressure on their surroundings, which in- creases with their size and with the inability of the surrounding struc- tures to resist it, The influence of this pressure is, of course, most frequently felt in the case of the larger parasites, and especially in the case of those which have taken up their abode in narrow channels or in parenchymatous organs. The parts which are most immediately exposed to the pressure begin to lose their character and normal appearance at the points of contact with the foreign bodies. The canals must widen when the size of the inmates exceeds their normal diameter. They form sinuses, and, if the circumstances permit, even change into closed sacs, which become thicker and thicker through hypertrophy of the connective tissue, until they can 1 In individual cases years may elapse before the disease breaks out in full vigour. The case of a workman in Vienna, who six years before had been a soldier in garrison in the north of Italy, led Henschl to suppose that Dochmius duodenalis, like the D. trlgono- ct phalus of the dog, lives at first on epithelial cells, and only after they are exhausted betakes itself to the vascular connective tissue of the intestinal villi [Mittlieil.