Introduction 617

I.--AMŒBIC DYSENTERY 618 II.--TRYPANOSOMIASES 620 African Sleeping Sickness 620 South American Trypanosomiasis 623 III.--FLAGELLATE DIARRHŒA AND DYSENTERY 623 IV.--LEISHMANIASES 626 A. Kala-azar 626 Indian 626 Infantile 627 B. Oriental Sore, due to _Leishmania tropica_ 627 Naso-oral (Espundia) 628 V.--SPIROCHÆTOSES 629 A. Relapsing Fevers 629 B. Yaws or Frambœsia tropica 632 C. Syphilis 632 D. Bronchial Spirochætosis 632 VI.--MALARIA 633 VII.--BALANTIDIAN DYSENTERY 637 *Plathelminthes* (Flat Worms) 638 FASCIOLIASIS 638 _Fasciola hepatica_ 638 _Fasciolopsis buski_ 638 PARAGONIMIASIS 639 _Paragonimus ringeri_ 639 _Clonorchis sinensis_ 640 BILHARZIASIS 641 _Schistosoma hæmatobium_ 641 CESTODES 644 General 644 _Dibothriocephalus latus_ 658 _Sparganum mansoni_ 659 _Dipylidium caninum_ (_Tænia cucumerina_) 659 _Hymenolepis nana_ 661 _Tænia solium_ 662 _Tænia saginata_ 667 NEMATODES 674 _Strongyloides stercoralis_ 674 _Dracunculus medinensis_ (Dracontiasis) 675 _Filaria bancrofti_ 676 _Loa loa_ 678 _Trichuris trichiura_ 678 _Trichinella spiralis_ 680 _Eustrongylus gigas_ 681 _Ancylostoma duodenale_ (Ancylostomiasis) 682 _Ascaris lumbricoides_ (Ascariasis) 687 _Oxyuris vermicularis_ (Oxyuriasis) 694 *Hirudinea* (Leeches) 699 *Arthropoda* 702 ARACHNOIDEA 702 _Leptus autumnalis_ (Grass, Harvest, or Gooseberry Mite) 702 _Kedani, Akaneesch_ (The Japanese River or Inundation Disease) 703 _Dermanyssus gallinæ_ (_avium_) 703 _Ixodes reduvius_ (_ricinus_ ) 704 _Sarcoptes scabiei_ (Scabies) 704 _Demodex folliculorum_ 708 _Demodex folliculorum canis_ 709 INSECTA 709 _Pediculus capitis_ (Head Louse) 709 _Pediculus vestimenti_ (Clothes Louse) 710 _Phthirius inguinalis_ (_Pediculus pubis_) (Crab Louse) 711 _Cimex_ (_Acanthia_) _lectularia_ (_Cimex lectularius_) (Bed Bug) 713 _Pulex irritans_ (Human Flea) 714 _Dermatophilus_ (_Sarcopsylla_) _penetrans_ (Sand Flea) 714 _Myiasis_ 715 _Myiasis externa_ 715 _Gastricolous Oestridæ_ (Creeping Disease) 729 APPENDIX ON PROTOZOOLOGY 733 I.--NOTES ON RECENT RESEARCHES 733 Differences between _Entamœba histolytica_ and _E. coli_ 733 Phagedænic Amœbæ 733 _Endamœba gingivalis_ 733 _Entamœba kartulisi_ 734 _Craigia_ and Craigiasis 734 Human Trichomoniasis 734 _Chilomastix_ (_Tetramitus_) _mesnili_ 735 _Giardia_ (_Lamblia_) _intestinalis_ 736 _Cercomonas hominis_ 736 Transmissive Phase of Trypanosomes in Vertebrates 737 _Trypanosoma lewisi_ 737 Blepharoplastless Trypanosomes 737 The Experimental Introduction of certain Insect Flagellates into various Vertebrates, and its bearing on the Evolution of Leishmaniasis 737 The Transmission of _Spirochæta duttoni_ 739 _Spirochæta bronchialis_ 739 The Spirochætes of the Human Mouth 740 Coccidia in Cattle 741 The Hæmosporidia 742 The Leucocytozoa of Birds 742 II.--FORMULÆ OF SOME CULTURE MEDIA 742 Culture Media for growing Amœbæ 742 Culture Media for the growth of Protozoa parasitic in the Blood 744 III.--BRIEF NOTES ON GENERAL PROTOZOOLOGICAL TECHNIQUE 745 Fresh Material 745 Stained Material 747 Fixatives 748 Stains 749 APPENDIX ON TREMATODA AND NEMATODA 753 TREMATODA 753 _Artyfechinostomum sufrartyfex_ 753 _Metagonimus_ (_Yokogawa_) _yokogawai_ 753 _Opisthorchis sp._ 753 _Schistosome cercariæ_ 753 _Distomata cercariæ_ 753 Group. _Ferrocercous cercariæ_ 753 Family. _Schistosomidæ_ 753 _Cercaria bilharzia_, Leiper, 1915 754 _Cercaria bilharziella_, Leiper, 1915 754 _Schistosoma mansoni_, Sambon, 1907 754 NEMATODA 754 Ancylostomiasis 754 Ground Itch 754 _Ascaris lumbricoides_ 754 Filariasis 755 _Onchocerca volvulus_ 755 _Strongyloides stercoralis_ 755 BIBLIOGRAPHY 756 INDEX 836 LIST OF ILLUSTRATIONS. FIG. PAGE 1 _Amœba coli._ (After Loesch) 29 2 Encysted intestinal amœbæ. (After Grassi) 31 3 _Entamœba coli_, life-cycle. (After Castellani and Chalmers) 32 4 _Entamœba coli_, so-called autogamy. (From Minchin) 34 5 _Entamœba histolytica_ (_tetragena_ form). (After Hartmann) 35 6 _Entamœba histolytica_, ingestion of red blood corpuscles. (After Hartmann) 35 7 _Entamœba histolytica_, section through infected intestinal ulcer. (After Harris) 36 8 _Entamœba histolytica_ (_tetragena_), trophozoite and nuclei. (After Hartmann) 38 9 _Entamœba histolytica_ (_tetragena_), cysts. (After Hartmann) 39 10 _Entamœba buccalis._ (After Leyden and Löwenthal) 43 11 _Entamœba kartulisi._ (After Kartulis) 44 12 _Amœba miurai._ (After Ijima) 46 13 _Chlamydophrys enchelys._ (After Cienkowski) 48 14 _Chlamydophrys enchelys_, encysted. (After Cienkowski) 49 15 _Leydenia gemmipara_, Schaudinn 50 16 _Trichomonas vaginalis._ (After Künstler) 53 17 _Trichomonas intestinalis._ (After Grassi) 54 18 _Trichomonas intestinalis._ (Original, Fantham) 55 19 _Lamblia intestinalis._ (After Wenyon, from Minchin) 58 20 _Lamblia intestinalis._ (After Grassi and Schewiakoff) 59 21 _Cercomonas hominis._ (After Davaine) 61 22 _Cercomonas hominis_, from an echinococcus cyst. (After Lambl) 61 23 _Monas pyophila._ (After Grimm) 62 24 _Prowazekia urinaria._ (After Sinton) 64 25 _Prowazekia urinaria_, excystation. (After Sinton) 65 26 _Trypanosoma brucei_ in division. (After Laveran and Mesnil) 70 27 _Trypanosoma lewisi_, rosettes. (After Laveran and Mesnil) 71 28 _Trypanosoma gambiense._ (After Dutton) 73 29 _Trypanosoma gambiense_, development in vertebrate host. (Original, Fantham) 73 30 _Trypanosoma gambiense_, development in _Glossina palpalis_. (After Robertson) 75 31 _Trypanosoma rhodesiense._ (After Stephens and Fantham) 77 32 Chart showing daily counts of number of Trypanosomes per cubic millimetre of peripheral blood from a case of Rhodesian sleeping sickness. (After Ross and Thomson) 79 33 _Trypanosoma cruzi_, schizogony. (After Chagas, from Castellani and Chalmers) 84 34 _Trypanosoma cruzi_ in muscle. (After Vianna, from Castellani and Chalmers) 85 35 _Trypanosoma cruzi_, development in _Triatoma megista_. (After Chagas, from Castellani and Chalmers) 86 36 _Trypanosoma cruzi_, forms found in salivary glands of _Triatoma_. (After Chagas, from Castellani and Chalmers) 87 37 _Trypanosoma lewisi_, from rat’s blood. (After Minchin) 89 38 _Trypanosoma lewisi_, from stomach of rat-flea. (After Minchin) 91 39 _Trypanosoma lewisi_, from rectum of rat-flea. (After Minchin) 92 40 _Trypanosoma brucei._ (After Laveran and Mesnil) 94 41 _Trypanosoma evansi._ (Original, Fantham) 96 42 _Trypanosoma equinum._ (After Laveran and Mesnil) 96 43 _Trypanosoma equiperdum._ (Original, Fantham) 97 44 _Trypanosoma theileri._ (After Laveran and Mesnil) 98 45 _Trypanosoma vivax._ (Original, Fantham) 100 46 _Trypanosoma congolense._ (Original, Fantham) 100 47 _Trypanosoma uniforme._ (Original, Fantham) 100 48 _Trypanosoma rotatorium._ (After Laveran and Mesnil) 101 49 _Herpetomonas_, _Crithidia_, _Trypanosoma_. (After Porter) 103 50 _Leishmania donovani._ (After Christophers, Patton, Leishman; from Castellani and Chalmers) 106 51 _Toxoplasma gondii._ (After Laveran and Marullaz, from _Trop. Dis. Bulletin_) 113 52 _Toxoplasma pyrogenes._ (After Castellani, from _Trop. Dis. Bulletin_) 113 53 _Spirochæta balbianii._ (After Fantham and Porter) 114 54 _Spirochæta duttoni._ (After Fantham) 117 55 _Spirochæta duttoni_ and its coccoid bodies in the tick. (After Fantham) 118 56 _Treponema pallidum._ (After Bell, from Castellani and Chalmers) 124 57 _Treponema pallidum_, apparatus for cultivation of. (After Noguchi) 125 58 _Treponema pertenue._ (After Castellani and Chalmers) 127 59 _Monocystis agilis._ (After Stein) 130 60 _Gregarina longa_, stages of growth of trophozoite 130 61 _Xyphorhynchus firmus._ (After Léger) 131 62 _Gregarina munieri._ (After Schewiakoff) 131 63 _Monocystis agilis_, spores. (After Bütschli) 132 64 Gregarines, conjugation and spore formation. (After Calkins and Siedlecki, modified) 133 65 _Stylorhynchus oblongatus_, cyst and gametes. (After Léger) 133 66 Gregarines, various spores. (After Léger) 134 67 _Eimeria_ (_Coccidium_) _schubergi_, life-cycle diagram of. (After Schaudinn) 139 68 _Eimeria avium_ in gut epithelium of grouse chick. (After Fantham) 143 69 _Eimeria avium_, life-cycle, diagram of. (After Fantham) 144 70 _Eimeria stiedæ_ in section of rabbit’s intestine 145 71 _Eimeria stiedæ_, oöcysts from rabbit’s liver. (After Leuckart) 146 72 _Eimeria stiedæ_, spores. (After Balbiani) 146 73 _Eimeria stiedæ_, schizogony. (After R. Pfeiffer) 146 74 _Eimeria stiedæ_, section through infected nodule of liver 147 75 _Isospora bigemina._ (After Stiles) 150 76 _Hæmoproteus_ (_Halteridium_) _columbæ_, life-cycle. (After Aragão, from Castellani and Chalmers) 152 77 _Leucocytozoön lovati._ (After Fantham) 153 78 Hæmogregarines from lizards. (After França) 154 79 _Leucocytogregarina canis_, life-cycle. (After Christophers, from Castellani and Chalmers) 155 80 _Plasmodium vivax_, life-cycle. (After Schaudinn and Grassi) 160 81 Malignant tertian malarial parasite in intestine of _Anopheles_. (After Grassi) 162 82 Oökinete of malignant tertian malaria in stomach of _Anopheles_. (After Grassi) 162 83 Section of stomach of _Anopheles_ with malarial oöcysts. (After Grassi) 163 84 Sporulation of malarial parasites in _Anopheles_. (After Grassi) 163 85 Tertian malarial parasite in human red blood corpuscles. (After Mannaberg) 165 86 Quartan malarial parasite in human red corpuscles. (After Manson) 166 87 Malignant malarial parasite in human red corpuscles. (After Manson) 168 88 Malarial crescents. (After Mannaberg) 168 89 Section through tubule of salivary gland of _Anopheles_ infected with malarial sporozoites. (After Grassi) 169 90 _Nuttallia equi_, life-cycle in red blood corpuscles. (After Nuttall and Strickland) 173 91 _Babesia_ (_Piroplasma_) _canis_, life-cycle in blood of dog. (After Nuttall and Graham-Smith) 175 92 _Theileria parva._ (After Nuttall and Fantham) 179 93 Myxosporidian spores and infected gill of fish. (After J. Müller) 181 94 _Myxobolus mülleri_, spore. (After Bütschli) 181 95 _Myxobolus_, schema of spore. (After Doflein) 182 96 _Chloromyxum leydigi._ (After Thélohan) 182 97 _Myxobolus pfeifferi_, spore formation. (After Keysselitz, from Minchin) 183 98 _Nosema apis._ (After Fantham and Porter) 185 99 _Nosema bombycis_ from silkworm. (After Balbiani) 186 100 _Nosema bombycis_, spores. (After Thélohan) 186 101 _Hexactinomyxon psammoryctis_, spore. (After Stolč) 187 102} 103} _Sarcocystis miescheriana_ in muscle of pig. (After Kühn) 188 104 _Sarcocystis miescheriana_, mature trophozoite 189 105 _Sarcocystis tenella_ in section, as seen in œsophagus of sheep 190 106 _Sarcocystis tenella_, young trophozoite. (After Bertram) 190 107 _Sarcocystis miescheriana_, end portion of trophozoite. (After Bertram) 190 108 _Sarcocystis blanchardi_ from ox. (From Wasielewski, after van Eecke) 190 109 _Sarcocystis tenella._ (After Laveran and Mesnil) 191 110 _Haplosporidium heterocirri._ (After Caullery and Mesnil) 195 111 Haplosporidian spores. (After Caullery and Mesnil) 195 112 _Rhinosporidium kinealyi_, portion of ripe cyst. (After Minchin and Fantham) 197 113 _Balantidium coli._ (After Leuckart) 200 114 _Balantidium coli_, free and encysted. (After Casagrandi and Barbagallo) 200 115 _Balantidium minutum._ (After Schaudinn) 204 116 _Nyctotherus faba._ (After Schaudinn) 205 117 _Nyctotherus giganteus._ (After Krause) 206 118 _Nyctotherus africanus._ (After Castellani) 206 119 Trachoma bodies in conjunctival cells. (Original, Fantham) 209 120 Half of a transverse section through _Fasciola hepatica_, L. 214 121 _Harmostomum leptostomum_, Olss. 215 122 Median section through the anterior part of _Fasciola hepatica_ 217 123 _Polystomum integerrimum._ (After Zeller) 218 124 _Allocreadium isoporum_, Looss. (After Looss) 218 125 Terminal flame cell of the excretory system. (Stephens) 219 126 Diagram of female genitalia. (Stephens) 220 127 Diagram of male and part of female genitalia. (Stephens) 220 128 Ovum of _Fasciola hepatica_, L. 223 129 Miracidium of _Fasciola hepatica_. (After Leuckart) 223 130 A group of cercariæ of _Echinostoma_ sp. 225 131 Development of _Fasciola hepatica_, L. (After Leuckart) 226 132 Young redia of _Fasciola hepatica_. (From Leuckart) 227 133 Older redia of _Distoma echinatum_ 227 134 Cercaria of _Fasciola hepatica_. (After Leuckart) 228 135 Encysted cercaria of _Fasciola hepatica_. (After Leuckart) 228 136 _Watsonius watsoni._ (After Shipley) 234 137 _Watsonius watsoni_: ventral projection composed from a series of transverse sections. (After Stiles and Goldberger) 235 138 _Gastrodiscus hominis._ (After Leuckart) 236 139 _Fasciola hepatica_, L. 238 140 _Fasciola hepatica_, showing the gut and its branches 239 141 _Fasciola hepatica_, L. (After Claus) 239 142 _Fasciola hepatica_: egg from liver of sheep. (After Thomas) 240 143 _Limnæus truncatulus_, Müll. (From Leuckart) 240 144 Young _Fasciola hepatica_. (From Leuckart) 242 145 _Fasciola gigantica._ (After Looss) 243 146 _Fasciolopsis buski_, Lank. (After Odhner) 245 147 _Fasciolopsis rathouisi_, Poir. (After Claus) 246 148 _Fasciolopsis fülleborni._ (After Fülleborn) 248 149 _Paragonimus ringeri_, Cobb. (After Katsurada) 250 150 _Paragonimus ringeri_, Cobb. (After Kubo) 250 150A _Paragonimus westermanii_, Kerb. (After Leuckart) 250 151 Egg of _Paragonimus ringeri_, Cobb. (After Katsurada) 251 152 Egg of _Opisthorchis felineus_ 253 153 _Opisthorchis felineus._ (After Stiles and Hassall) 253 154 _Opisthorchis pseudofelineus._ (After Stiles) 254 155 _Parapisthorchis caninus._ (After Stephens) 256 156 _Amphimerus noverca_, Braun. (After McConnell) 257 157 _Metorchis conjunctus._ (After Cobbold) 258 158 _Clonorchis sinensis._ (After Looss) 259 159 Ova of _Clonorchis sinensis_. (After Looss) 259 160 _Clonorchis endemicus._ (After Looss) 260 161 _Clonorchis endemicus_: eggs. (After Looss) 260 162 _Metorchis truncatus_ 262 163 _Heterophyes heterophyes._ (After Looss) 263 164 _Metagonimus yokogawai._ (After Leiper) 264 165 _Dicrocœlium dendriticum_ 265 166 Eggs of _Dicrocœlium dendriticum_ 266 167 Miracidia of _Dicrocœlium dendriticum_. (After Leuckart) 266 168 _Echinostoma ilocanum._ (After Brumpt) 268 169 _Echinostoma ilocanum._ (After Leiper) 268 170 _Echinostoma malayanum_, Leiper. (After Leiper) 269 171 _Schistosoma hæmatobium._ (After Looss) 270 172 Transverse section through a pair of _Schistosoma hæmatobium_ in copulâ. (After Leuckart) 271 173 Anterior end of the male _Schistosoma hæmatobium_. (After Looss) 271 174 _Schistosoma hæmatobium._ (After Leuckart) 276 175 _Schistosoma hæmatobium_, ovum of. (After Looss) 277 176 _Schistosoma japonicum._ (After Katsurada) 278 177 _Schistosoma japonicum._ (After Katsurada) 279 178 _Schistosoma japonicum._ (After Looss) 279 179} 180} _Schistosoma japonicum_ from dog. (After Katsurada) 280 181} 182 _Schistosoma japonicum._ (After Catto) 281 183 _Schistosoma japonicum._ (After Katsurada) 282 184 Schematic representation of a small part of a transverse section of _Ligula_ sp. (After Blochmann) 287 185 Half of a transverse section through a proglottis of _Tænia crassicollis_ 288 186 _Dipylidium caninum._ (After Benham) 289 187 Longitudinal section of the head and neck of _Tænia crassicollis_ 290 188 _Tænia cœnurus._ (After Niemisec) 291 189 Young _Acanthobothrium coronatum_. (After Pintner) 292 190 Scolex of a cysticercoid from _Arion_ sp. (After Pintner) 292 191 Proglottis of _Tænia saginata_, Goeze, showing genitalia 293 192 _Dibothriocephalus latus._ (After Benham and Sommer and Landois) 294 193 Diagram of genitalia of a Cestode. (Stephens) 295 194 Part of a transverse section through a proglottis of _Dibothriocephalus latus_ 296 195 Egg of _Diplogonoporus grandis_. (After Kurimoto) 298 196 Uterine egg of _Tænia saginata_. (After Leuckart) 298 197 Oncosphere of _Tænia africana_ (after v. Linstow) and oncosphere of _Dipylidium caninum_. (After Grassi and Rovelli) 299 198 Diagram of a cysticercoid. (Stephens) 301 199 Diagram of a cysticercus. (Stephens) 301 200 Diagram of development of a cysticercus. (Stephens) 303 201 Section through a piece of a _Cœnurus cerebralis_ 304 202 Median section through a cysticercus. (After Leuckart) 304 203 _Cysticercus pisiformis_ in an evaginated condition 304 204 Various chains of segments of _Dibothriocephalus latus_ 311 205 Transverse section of the head of _Dibothriocephalus latus_ 311 206 Fairly mature proglottis of _Dibothriocephalus latus_ 311 207 _Dibothriocephalus latus._ (After Benham and Schauinsland) 312 208 Plerocercoid of _Dibothriocephalus latus_ 313 209 A piece of the body wall of the Burbot, _Lota vulgaris_ 313 210 Cephalic end of _Dibothriocephalus cordatus_. (After Leuckart) 315 211 _Diplogonoporus grandis_, Lühe, 1899. (After Ijima and Kurimoto) 317 212 _Diplogonoporus grandis._ (After Ijima and Kurimoto) 317 213 Cephalic end of _Sparganum mansoni_, Cobb. (After Leuckart) 318 214 _Sparganum mansoni._ (After Ijima and Murata) 318 215 _Sparganum prolifer._ (After Ijima) 319 216 _Sparganum proliferum._ (After Stiles) 319 217 _Dipylidium caninum._ (After Diamare) 320 218 _Dipylidium caninum._ (After Benham and Moniez) 320 219 _Dipylidium caninum_: central portion of a proglottis. (After Neumann and Railliet) 321 220 _Dipylidium caninum_: development of embryo. (After Benham, Grassi, and Rovelli) 321 221 Larva (cysticercoid) of _Dipylidium caninum_. (After Grassi and Rovelli) 322 222 _Hymenolepis nana_, v. Sieb. (After Leuckart) 324 223 _Hymenolepis nana_: head. (After Mertens) 324 224 _Hymenolepis nana_: an egg. (After Grassi) 324 225 Longitudinal section through the intestinal villus of a rat. (After Grassi and Rovelli) 324 226 _Hymenolepis nana_ (_murina_): cross-section of proglottis from a rat. (After v. Linstow) 325 227 _Hymenolepis nana_: longitudinal section of an embryo. (After Grassi and Rovelli) 325 228 _Hymenolepis diminuta._ (After Zschokke) 326 229 _Hymenolepis diminuta._ (After Grassi) 326 230 _Hymenolepis diminuta._ (After Bizzozero) 326 231 _Hymenolepis diminuta._ (Stephens, after Nicoll and Minchin) 327 232 _Hymenolepis lanceolata._ (After Krabbe) 328 233 _Hymenolepis lanceolata._ (After Wolffhügel) 328 234 Scolex of _Davainea madagascariensis_. (After Blanchard) 330 235 Two fairly mature proglottids of _Tænia solium_ 332 236 Head of _Tænia solium_ 332 237 Large and small hooks of _Tænia solium_. (After Leuckart) 333 238 _Tænia solium._ (After Leuckart) 333 239 Two mature proglottids of _Tænia solium_ 333 240 Large and small hooklets of _Tænia marginata_. (After Leuckart) 338 241 Mature segment of _Tænia saginata_ 339 242 Cephalic end of _Tænia saginata_ 339 243 _Tænia saginata._ (After Leuckart) 339 244 A piece of the muscle of the ox, with three specimens of _Cysticercus bovis_. (After Ostertag) 340 245 Mature segment of _Tænia africana_. (After v. Linstow) 342 246 Proglottis of _Tænia africana_. (After v. Linstow) 343 247 Head of _Tænia africana_. (After v. Linstow) 343 248 _Tania confusa._ (After Guyer) 344 249 _Tania confusa._ (After Ward) 344 250 _Tania echinococcus_ 345 251 _Echinococcus veterinorum._ (After Leuckart) 347 252 } 252A} Diagrams of mode of formation of brood capsule and scolices (Stephens) 348 253 Section through an invaginated echinococcus scolex. (After Dévé) 350 254 A piece of the wall of an _Echinococcus veterinorum_ stretched out and seen from the internal surface 350 255 _Echinococcus hominis_ in the liver. (After Ostertag, from Thomas) 351 256 Section through an echinococcus scolex in process of vesicular metamorphosis. (After Dévé) 351 257 } 257A} Diagram of transformation of a scolex into a daughter cyst. (Stephens) 352 258 Hooklets of echinococcus. (After Leuckart) 355 259 _Echinococcus multilocularis_ in the liver of the ox. (After Ostertag) 357 260 Diagram of a transverse section of _Ascaris lumbricoides_. (After Brandes) 362 261 Anterior end of an _Ascaris megalocephala_. (After Nassonow) 362 262 Transverse section through _Ascaris lumbricoides_ at the level of the œsophagus behind the nerve ring. (After Goldschmidt) 364 263 Schematic representation of the nervous system of a male _Ascaris megalocephala_. (After Brandes) 365 264 Diagram of female genitalia 368 264A Diagram of male genitalia of a strongylid 368 265 Transverse section through the ovarian tube of _Belascaris cati_ of the cat 369 266 Male of the rhabditic form of _Angiostomum nigrovenosum_ 370 267 Transverse section through the posterior extremity of the body of _Ascaris lumbricoides_ (male) 370 268 Hind end of a male _Ascaris lumbricoides_ cut across at the level of the dilator cells of the gut. (After Goldschmidt) 371 269 A piece of the trunk muscle of the pig with encapsuled embryonic Trichinæ 373 270 _Strongyloides stercoralis_, female. (After Looss) 380 271 _Strongyloides stercoralis_, male. (After Looss) 380 272 _Strongyloides stercoralis_, female. (After Looss) 382 273 _Strongyloides stercoralis._ (After Looss) 382 274 _Strongyloides stercoralis._ (After Looss) 383 275 _Gnathostoma siamense._ (After Levinsen) 385 276 Guinea worm (_Dracunculus medinensis_). (After Leuckart) 387 277 Anterior extremity of Guinea worm. (After Leuckart) 387 278 _Dracunculus medinensis._ (After Claus) 387 279 Transverse section of female Guinea worm. (After Leuckart) 388 280 _Cyclops virescens_, female 389 281 _Filaria bancrofti._ (After Leiper) 391 282 _Mf. bancrofti_ in thick film, dried and stained with hæmatoxylin. (After Fülleborn) 397 283 Schematic drawings of the anatomy of _Ml. loa_ and _Mf. bancrofti_. (After Fülleborn) 399 284 _F. demarquayi._ (After Leiper) 403 285 _Mf. demarquayi_ in thick film, dried and stained with hæmatoxylin. (After Fülleborn) 404 286 _Filaria_ (?) _conjunctivæ_. (After Addario) 405 287 _Filaria_ (?) _conjunctivæ_. (After Grassi) 405 288 _Setaria equina._ (After Railliet) 408 289 _Setaria equina_: anterior end. (After Railliet) 408 290 _Loa loa_: the anterior end of the male. (After R. Blanchard) 410 291 _Loa loa_: anterior portion of the female. (After Looss) 410 292 _Loa loa_ in situ. (After Fülleborn and Rodenwaldt) 410 293 _Loa loa_: male and female. (After Looss) 410 294 _Loa loa_: the hind end of a male and of a female. (After Looss) 411 295 _Loa loa_: lateral view of tail of male showing papillæ. (After Lane and Leiper) 411 296 _Loa loa._ (After Leiper) 411 297 _Mf. loa_: in thick film, dried and stained with hæmatoxylin. (After Fülleborn) 413 298 _Acanthocheilonema perstans._ (After Leiper) 414 299 _Mf. perstans._ (After Fülleborn) 415 300 _Dirofilaria magalhãesi._ (After v. Linstow) 417 301 _Trichuris trichiura_ 420 302 _Trichinella spiralis._ (After Claus) 422 303 Isolated muscular fibre of a rat, invaded by Trichinella. (After Hertwig-Graham) 425 304 Calcified Trichinella in the muscular system of a pig. (After Ostertag) 426 305 Various phases of the calcification of Trichinella of the muscles 426 306 _Dioctophyme gigas._ (After Railliet) 432 307 Eggs of _Dioctophyme gigas_. (After Railliet) 432 308 _Metastrongylus apri._ (Stephens) 433 309} 310} _Trichostrongylus instabilis._ (After Looss) 434 311} 312} _Trichostrongylus probolurus._ (After Looss) 435 313} 314} _Trichostrongylus vitrinus._ (After Looss) 436 315} 316} _Hæmonchus contortus._ (After Ransom) 437 316} 317 _Mecistocirrus fordi._ (After Stephens) 439 318 _Ternidens deminutus._ (After Railliet and Henry) 440 319} 320} _Œsophagostomum stephanostomum_ var. _thomasi_. (After Thomas) 442 321} 322} _Œsophagostomum stephanostomum_ var. _thomasi_. (After Thomas) 444 323} 324} _Ancylostoma duodenale_, male and female. (After Looss) 446 325 _Ancylostoma duodenale_, showing ventral teeth. (After Looss) 447 326 _Ancylostoma duodenale_: diagrammatic representation of excretory system. (After a drawing by Looss) 448 327 _Ancylostoma duodenale._ (After Railliet) 449 328 _Ancylostoma duodenale_: bursa of male. (After Looss) 450 329 _Ancylostoma duodenale_: eggs in different stages of development. (After Looss) 451 330 _Ancylostoma duodenale_: larva. (After Leichtenstern) 452 331 _Ancylostoma duodenale._ (After Looss) 453 332 _Ancylostoma ceylanicum._ (After Looss) 456 333 _Ancylostoma braziliense._ (After Gomez de Faria) 456 334 _Necator americanus._ (After Looss) 457 335 _Necator americanus_: lateral view. (After Looss) 458 336 _Necator americanus_: bursa of male. (After Looss) 458 337 _Syngamus kingi_: anterior end of male. (After Leiper) 460 338 _Syngamus kingi_: anterior end of female. (After Leiper) 460 339 Bursa of _Syngamus trachealis_. (Stephens) 461 340 _Physaloptera mordens_, Leiper, 1907. (After Leiper) 462 341 _Ascaris lumbricoides._ (From Claus) 463 342 Ovum of _Ascaris lumbricoides_ 463 343 Ovum of _Toxascaris limbata_ 466 344 Transverse section through the head part of _Belascaris cati_ from the cat. (After Leuckart) 466 345} 346} Male female of _Oxyuris vermicularis_ 468 347 _Oxyuris vermicularis_: egg freshly deposited 468 348 _Oxyuris vermicularis_: egg twelve hours after deposition 468 348A The male of _Echinorhynchus augustatus_ 476 348B Anterior portion of the female apparatus of _Echinorhynchus acus_. (After Wagener) 476 348C Egg of _Echinorhynchus gigas_. (After Leuckart) 477 348D The internal organs of the leech. (After Kennel) 480 348E _Hirudo medicinalis._ (After Claus) 481 349 _Leptus autumnalis._ (After Gudden) 485 350 _Leptus autumnalis._ (After Trouessart) 485 351 The kedani mite. (After Tanaka) 487 352 _Tetranychus telarius_ var. _russeolus_, Koch. (After Artault) 488 353 _Pediculoides ventricosus._ (After Laboulbène and Mégnin) 489 354 _Nephrophages sanguinarius_: male, ventral surface. (After Miyake and Scriba) 490 355 _Nephrophages sanguinarius_: female, dorsal aspect. (After Miyake and Scriba) 490 356 _Tydeus molestus._ (After Moniez) 491 357 _Dermanyssus gallinæ._ (After Berlese) 492 358 _Dermanyssus hirundinis._ (After Delafond) 492 359 _Ixodes ricinus_, male. (After Pagenstecher) 498 360 Female of _Ixodes ricinus_. (After Pagenstecher) 498 361 _Argas reflexus._ (After Pagenstecher) 506 362 _Argas persicus._ (After Mégnin) 507 363 _Tyroglyphus farinæ_: male. (After Berlese) 512 364 _Tyroglyphus longior_, Gerv. (After Fum. and Robin) 512 365 _Rhizoglyphus parasiticus_: male and female. (After Dalgetty) 514 366 _Histiogaster_ (_entomophagus_ ?) _spermaticus_. (After E. Trouessart) 515 367 _Sarcoptes scabiei._ (After Fürstenberg) 518 368 _Sarcoptes scabiei_: male, ventral aspect. (After Fürstenberg) 519 369 _Sarcoptes minor_ var. _cati_. (After Railliet) 521 370 _Demodex folliculorum_ of the dog. (After Mégnin) 522 371 _Linguatula rhinaria_: female 524 372 Larva of _Linguatula rhinaria_ (_Pentastoma denticulatum_). (After Leuckart) 524 373 _Linguatula rhinaria._ (After M. Koch) 525 374 Mouth-parts of _Pediculus vestimenti_. (After Denny) 533 375 Ovum of the head louse 533 376 Head louse, male 533 377 _Pediculus vestimenti_, Burm.: adult female 533 378 _Phthirius inguinalis_, Leach 534 379 Head of the bed bug from the ventral surface 535 380 _Dermatophilus penetrans_: young female. (After Moniez) 544 381 _Dermatophilus penetrans_: older female. (After Moniez) 544 382 _Pulex irritans_ 546 383 Larva of flea. (After Railliet) 546 384 _Pulex serraticeps_ 546 385 Head of a male and of a female Anopheles. (After Giles) 549 386 Head of a male and of a female Culex. (After Giles) 549 387 Mouth-parts of _Anopheles claviger_. (After Grassi) 550 388 _Anopheles maculipennis._ (After Nuttall and Shipley) 550 389 Longitudinal section of an Anopheles, showing alimentary canal. (After Grassi) 551 390 _Anopheles maculipennis_, Meigen. (After Grassi) 552 391 Larva of _Anopheles maculipennis_, Fabr. (After Grassi) 553 392 Larva of Culex. (After Grassi) 553 393 Pupa of _Anopheles maculipennis_, Meig. (After Grassi) 554 394 Heads of Culex and Anopheles. (After Daniels) 556 395 Eggs of Culex, of Anopheles, of Stegomyia, of Tæniorhynchus, and of Psorophora 557 396 Diagram showing the structure of a typical mosquito. (Theobald) 558 397 Types of scales, head and scutellar ornamentation, forms of clypeus. (Theobald, etc., etc.) 559 398 Neuration of wing. Explanation of wing veins and cells. (Theobald) 560 399 Wing of _Anopheles maculipennis_, Meigen 566 400 Wing of a Culex 575 401 Wing of Simulium 579 402 Wing of Chironomus 579 403 A Ceratopogon, or midge 580 404 An owl midge, _Phlebotomus_ sp. (From Giles’s “Gnats or Mosquitoes”) 581 405 Larva of _Homalomyia canicularis_ 585 406 Larvæ of _Calliphora vomitoria_ 585 407 Larva of _Chrysomyia macellaria_. (After Conil) 585 408 The screw-worm fly (_Chrysomyia macellaria_) 587 409 Ochromyia larva on the skin of man, South Africa. (After Blanchard) 590 410 Head end of “larva of Natal.” (After Gedoelst) 591 411 Lund’s larva. (After Gedoelst) 593 412 _Dermatobia noxialis_, Goudot 597 413 Larva of _Dermatobia cyaniventris_. (After Blanchard) 597 414 Larva of _Dermatobia cyaniventris_. (After Blanchard) 597 415 The ox gad fly (_Tabanus bovinus_, Linn.) 601 416 The brimp (_Hæmatopota pluvialis_, Linn.) 602 417 Head of _Glossina longipalpis_. (After Grünberg) 604 418 Antenna of _Glossina pallidipes_, male. (After Austen) 604 419 _Glossina palpalis_ and puparium. (After Brumpt) 607 420 The tsetse-fly (_Glossina morsitans_, Westwood) 608 421 The stinging fly (_Stomoxys calcitrans_, Linn.) 609 422 _Trichomonas_ from cæcum and gut of rat. (Original, Fantham) 735 423 _Chilomastix_ (_Tetramitus_) _mesnili._ (Original, Fantham) 736 -------------------------------------------------------------------- |We regret to have taken without permission from the “Transactions of| |The Society of Tropical Medicine and Hygiene,” London, the following| |diagrams:-- | | | | Pages Figures | | 268 No. 169 | | 269 " 170 | | 391 " 281 | | 411 " 295 and 296 | | 414 " 298 | | 460 " 337 and 338 | | | |and tender our regret to the Society in question for having done so.| -------------------------------------------------------------------- ERRATA. P. 31, line 6 from bottom: _delete_ “human,” as Leidy really worked with _Endamœba blattæ_, parasitic in the gut of the cockroach. P. 43, line 12 from bottom: _for_ “John’s” _read_ “Johns.” P. 44, line 13 from bottom: _for_ “_Amœba buccalis_, Sternberg,” _read_ “_Amœba buccalis_, Steinberg.” P. 46, line 13 from top: _for_ “breath” _read_ “breadth.” P. 53, In footnote ^1, line 6 from bottom: _insert_ “see” before _Arch. f. Protistenk_. P. 75: To paragraph regarding development of the parasite in the fly’s salivary glands, _add_ that the crithidial phase takes two to five days. P. 111, line 8 from top: the date of Sangiorgi should be 1911. P. 142, line 7 from top: _insert_ “Genus.” before *Eimeria*. P. 252, _Insert_ heading “Family. *Opisthorchiidæ*, Braun, 1901,” _above_ “Sub-family. *Opisthorchiinæ*, Looss, 1899.” P. 351, description of fig. 255, line 3: _for_ “Thoma” _read_ “Thomas.” P. 471, line 15 from bottom: _for_ “alcohol 100 parts” _read_ “alcohol 100 c.c.” P. 472, line 11 from bottom: _for_ “Or (2) 10 _per cent. formalin_,” _read_ “Or (2) _fix in hot_ 10 _per cent. formalin_.” P. 493, line 21 from top: _for_ “Conoy” _read_ “Couvy.” P. 589, line 2 from top: _for_ “*carnosa*” _read_ “*carnaria*.” P. 620, line 15 from top: _for_ “fo” _read_ “of.” P. 622, line 12 from bottom: _delete_ comma after quantity. P. 626, line 6 from bottom: _delete_ comma after Mackie (1915). P. 638: _insert_ title “*TREMATODES*” above that of “Fascioliasis.” P. 709, line 9 from bottom: _omit_ second Pediculus capitis. P. 748, line 8 from top: _for_ “cytologica” _read_ “cytological.” P. 753, line 4 from bottom: _for_ “*Fercocercous*” _read_ “*Furcocercous*.” P. 755 line 7: _for_ “*Oncocerca*” _read_ “*Onchocerca*.” ON PARASITES IN GENERAL. By the term PARASITES is understood living organisms which, for the purpose of procuring food, take up their abode, temporarily or permanently, on or within other living organisms. There are both plants and animals (Phytoparasites and Zoöparasites) which lead a parasitic life in or upon other plants and other animals. Phytoparasites are not included in the following descriptions of the forms of parasitism, but a very large number of animal parasites (zoöparasites) are described. The number of the latter, as a rule, is very much underrated. How great a number of animal parasites exists may be gathered from the fact that all classes of animals are subject to them. Some of the larger groups, such as _Sporozoa_, _Cestoda_, _Trematoda_ and _Acanthocephala_, consist entirely of parasitic species, and parasitism even occurs among the vertebrates (_Myxine_). It therefore follows that the characteristics of parasites lie, not in their structure, but in the manner of their existence. Parasitism itself occurs in various ways and degrees. According to R. Leuckart, we should distinguish between OCCASIONAL (temporary) and PERMANENT (stationary) PARASITISM. Occasional parasites, such as the flea (_Pulex irritans_), the bed-bug (_Cimex lectularius_), the leech (_Hirudo medicinalis_), and others, only seek their “host” to obtain nourishment and find shelter while thus occupied. Without being bound to the host, they usually abandon the latter soon after the attainment of their object (_Cimex, Hirudo_), or they may remain on the body of their host throughout their entire development from the hatching of the egg (_Pediculus_). It follows from this mode of living that the occasional parasites become sometimes distinguishable from their free-living relatives, though only to a slight extent. It is, therefore, seldom difficult to determine the systematic position of temporary parasites from their structure. In consequence of their mode of life, all these temporary parasites live on the external surface of the body of their host, though more rarely they take up their abode in cavities easily accessible from the exterior, such as the mouth, nose and gills. They are therefore frequently called EPIZOA or ECTOPARASITES; but these designations do not cover only the temporary parasites, because numerous epizoa (as for instance the louse) are parasitic during their entire life. In contradistinction to these temporary parasites, the permanent parasites obtain shelter as well as food from their host for a long period, sometimes during the entire course of their life. They do not seek their host only when requiring nourishment, but always remain with it, thus acquiring substantial protection. The permanent parasites, as a rule, live within the internal organs, preferably in those which are easily accessible from the exterior, such as the intestine, with its appendages. Nevertheless, permanent parasites are also found in separate organs and systems, such as the muscular and vascular systems, hollow bones and brain, while some live on the outer skin. Here again, the terms ENTOZOA and ENDOPARASITES do not include all stationary parasites; to the latter, for instance, the lice belong, which pass all their life on the surface of the body of their host, where they find shelter and food and go through their entire development. The ectoparasitic trematodes, numerous insects, crustacea, and other animals live in the same manner. All “HELMINTHES,” however, belong to the group of permanent parasites. This term is now applied to designate certain lowly worms which lead a parasitic life (intestinal worms); but they are not all so termed. For instance, the few parasitic TURBELLARIA are never classed with the helminthes, although closely related to them. The turbellarians, in fact, belong to a group of animals of which only a few members are parasitic, whereas the helminthes comprise those groups of worms of which all species (_Cestoda_, _Trematoda_, _Acanthocephala_), or at least the majority of species (_Nematoda_), are parasitic. Formerly the Linguatulidæ (_Pentastoma_) were classed with the helminthes because their existence is also endoparasitic, and because the shape of their body exhibits a great similarity to that of the true helminthes. Since the study of the development of the Linguatulidæ (P. J. van Beneden, 1848, and R. Leuckart, 1858) has demonstrated that they are really degenerate arachnoids, they have been separated from the helminthes. It is hardly necessary to emphasize the fact that the helminthes or intestinal worms do not represent a systematic group of animals, but only a biological one, and that the helminthes can only be discussed in the same sense as land and water animals are mentioned, _i.e._, without conveying the idea of a classification in such a grouping. It is true that formerly this was universally done, but very soon the error of such a classification was recognized. Still, until the middle of last century, the helminthes were regarded as a systematic group, although C. E. v. Baer (1827) and F. S. Leuckart (1827) strenuously opposed this view. Under the active leadership of J. A. E. Goeze, J. G. H. Zeder, J. G. Bremser, K. A. Rudolphi and F. Dujardin, the knowledge of the helminthes (helminthology) developed into a special study, but unfortunately it lost all connection with zoology. It required the intervention of Carl Vogt to disestablish the helminthes as one class of animals, by uniting the various groups with those of the free-living animals most closely related to them (_Platyhelminthes, Nemathelminthes_). PERMANENT PARASITISM in the course of time has caused animals adopting this mode of life to undergo considerable, sometimes even striking, bodily changes, permanent ectoparasites having as yet undergone least alteration. The latter sometimes bear so unmistakably the likeness to the group to which they belong, that even a superficial knowledge of their structure and appearance often suffices for the recognition of their systematic position. For instance, though the louse, like many decidedly temporary parasites, has lost its wings--a characteristic of insects--in consequence of parasitism, yet nobody would deny its insect nature; such also occurs in other temporary parasites (_Cimex, Pulex_). On the other hand, the changes in a number of permanent ectoparasites (such as parasitic Crustacea) are far more considerable, and correspond with those that have occurred in permanent endoparasites. These alterations depend partly on retrogression and partly on the acquisition of new peculiarities. In the former case, the change consists in the loss of those organs which have become useless in a permanent parasitic condition of existence, such as wings in the louse, and the articulated extremities seen in the larval stage of parasitic Crustacea. The loss of these organs goes hand in hand with the cohesion of segments of the body that were originally separate, and alterations in the muscular and nervous systems. In the same manner another means of locomotion is lost--the ciliated coat--which is possessed by many permanent parasites during their larval period. To all appearances, this character is not secondary and recently acquired, but represents a primary character inherited from free-living progenitors, and still transmitted to the altered descendants, because of its use during the larval stage (_e.g._, the larvæ of a great many Trematodes, the oncospheres of some Cestodes). Amongst the retrogressions, the loss of the organs of sense may be mentioned, particularly the eyes, which are still present, not only in the nearest free-living forms but also in the free-living larvæ of true parasites. It is only quite exceptionally that the eyes are subsequently retained, as a rule they are lost. Lastly, in a great many cases the digestive system also disappears, as in parasitic Crustacea, in a few nematodes and trematodes, in all cestodes and Acanthocephala. There remain at most the rudiments of the muscles of the fore-gut, but these are adapted to entirely different uses. The new characters which permanent parasites may acquire are, first of all, the remarkably manifold CLASPING and CLINGING ORGANS, which are seldom (as in parasitic Crustacea) directly joined on to already existing structures. In those instances in which organs for the conveyance of food are retained, these likewise frequently undergo transformation, in consequence of the altered food and manner of feeding. Such alterations consist, for instance, in the transformation of a masticating mouth apparatus into the piercing and sucking organs of parasitic insects. HERMAPHRODITISM (as in Trematodes, Cestodes, and a few Nematodes) is a further peculiarity of many permanent parasites; moreover, the association in couples that occurs, especially in trematodes, may lead to complete cohesion and, exceptionally, also to re-separation of the sexes. In many cases the females only are parasitic, while the males live a free life, or there may be in addition the so-called complementary males. Occasionally the male alone is parasitic, and in that case lives within the female of the same species, which may live free, like certain Gephyrea (_Bonellia_); or the female also may be parasitic, as _Trichosoma crassicaudum_, which lives in the bladder of the sewer rat (_Mus decumanus_). We have numerous proofs that demonstrate how considerably the original features of many parasites have become changed. We need only draw attention to the aforementioned Linguatulidæ, also to many of the parasitic Crustacea belonging to various orders. In all of these a knowledge of the larval stages--in which there is no alteration, or at most only a slight degree of change--serves to determine their systematic position, _i.e._, the nearest conditions of relationship. The most remarkable changes are observed in those groups that contain only a few parasitic members, the majority leading a free life. A striking instance is afforded by a snail, the well-known _Entoconcha mirabilis_, Müller. This mollusc consists merely of an elongated sac living in a Holothurian (_Synapta digitata_). It possesses none of the characteristics of either the Gastropoda or any molluscs, and in its interior there is nothing to be observed but the organs of generation and the embryos. Nevertheless, the _Entoconcha_ is decidedly a parasitic snail, as is clearly proved by its larvæ, but it is a snail which, in consequence of parasitism, has lost all the characteristics of molluscs in its mature condition, but still exhibits them in the early stages of development. Certain nematodes show very clearly to what devious courses parasitism may lead. The _Atractonema gibbosum_, the life-history of which has been described by R. Leuckart, and which lives in the larvæ and pupæ of a dipterous insect (_Cecidomyia_), exhibits, in its early stage, the ordinary characteristics of other threadworms. A few weeks later--the males having died off immediately after copulation--the females are transformed into spindle-shaped bodies, the mouth and anus of which are closed. They carry with them an irregularly shaped appendage, in which the segmenting ova are situated, and in which the further conditions of life of the _Atractonema_ are accomplished. A minute examination has demonstrated that this appendage is the prolapsed and enlarged vagina of the animal which has become merely a supplementary attachment. The conditions present in the _Sphærularia_, the nematoid nature of which was long undiscovered, are still more remarkable. It was only when Siebold proved that typical nematodes were hatched from their eggs that their nature was recognized. The nematodes thus produced have not the slightest resemblance to the parent. The researches of Lubbock, A. Schneider, and more particularly of R. Leuckart, have shown that what we call _Sphærularia bombi_ is not an animal but merely an organ--the vagina--of a nematode worm. This vagina at first grows, sac-like, from the body of the tiny nematode; it gradually assumes enormous dimensions (2 cm. in length); it contains the sexual organs and parts of the intestine. The remaining portion of the actual animal then becomes small and shrivelled; it may be easily overlooked, being but an appendage to the vagina with its independent existence, and it finally disappears altogether. The GREAT FERTILITY of parasites is another of their peculiarities, though this may be also the case to a certain degree with some of the free-living animals, the progeny of which are likewise exposed to enormous destruction. More remarkable, however, is the fact that the young of the endoparasites only very exceptionally grow to maturity by the side of their parents. Sooner or later they leave the organ inhabited by the parents, frequently reach the open, and after a shorter or longer period of free existence seek new hosts. During their free period, moreover, a considerable growth may be attained, or metamorphosis may take place, or even multiplication. In the exceptional cases in which the young remain within the same host, they nevertheless usually quit the organ inhabited by the parents. They likewise rarely attain maturity within the host inhabited by the parents, but only, as in other cases, after having gained access to fresh hosts. These transmigrations play a very important _rôle_ in the natural history of the internal parasites, but they frequently conceal the cycle of development, for sometimes there are INTERMEDIATE GENERATIONS, which themselves invade intermediate hosts. Even when there are no intermediate generations, THE SYSTEM OF INTERMEDIATE HOSTS is frequently maintained by the endoparasites. According to the kind of food ingested by parasites, it has recently become usual to separate the true parasites from those animals that feed on the superfluity of the food of the host, or on products which are no longer necessary to him, and to call the latter MESSMATES or COMMENSALS. As examples, the Ricinidæ are thus designated, because, like actual lice, they dwell among the fur of mammals or the plumage of birds. They do not, however, suck blood, for which their mouth apparatus is unsuited, but subsist on useless epidermic scales. These epizoa, according to J. P. van Beneden, are, to a certain extent, useful to their hosts by removing deciduous materials which under certain circumstances might become harmful to them.[1] This investigator, who has contributed so greatly to our knowledge of parasites, assigns the Ricines to the MUTUALISTS, under which term he comprises animals of various species which live in common, and confer certain benefits on one another. The mutualists are usually intimately connected in a mutually advantageous association known as “symbiosis.”[2] [1] According to Sambon, the Ricinidæ are by no means advantageous to their hosts. These Hemipterous parasites give rise to an intolerable itching which may cause loss of rest, emaciation, and sometimes even death. Birds suffering from phthiriasis of the Ricines are usually in bad health. [2] For further information on these conditions, see “Die Schmarotzer des Thierreichs,” by P. J. van Beneden, Leipzig, 1876; and “Die Symbiose,” by O. Hertwig. _Incidental and Pseudo Parasites._--In many cases the parasites are confined to certain hosts, and may therefore be designated as _specific_ to such hosts. Thus, hitherto, _Tænia solium_ and _Tænia saginata_ in their adult condition have only been found in man; _Tænia crassicollis_ only in the cat; _Brandesia_ (_Distoma_) _turgida_ and _Halipegus_ (_Distoma_) _ovocaudatas_ only in _Rana esculenta_, and so forth. In many other cases, however, certain species of parasites are common to several, and sometimes many, species of hosts; _Dipylidium caninum_ is found in the domestic cat as well as in the dog; _Fasciola hepatica_ is found in a large number of herbivorous mammals (nineteen species), _Diplodiscus_ (_Amphistomum_) _subclavatus_ in numerous urodele and ecaudate amphibia, _Holostomum variabile_ in about twenty-four species of birds, and so on. In these cases the hosts are almost invariably closely related, belonging, as a rule, to the same family or order, or at any rate to the same class. _Trichinella spiralis_, which is found in man, and in the pig, bear, rat, mouse, cat, fox, badger, polecat and marten, and is capable of being artificially cultivated in the dog, rabbit, sheep, horse, in other mammals, and even in birds, is one of the most striking exceptions. Some parasites are so strictly confined to one species of host that, even when artificially introduced into animals very closely related to their normal host, they do not thrive, but sooner or later, often very quickly, die off, and very rarely establish themselves. For example, repeated attempts have been made to rear the adult _Tænia solium_ in the dog, or to rear _Cysticercus cellulosæ_ in the ox, or the _Cysticercus_ of _Tænia saginata_ in the pig, but they have always proved unsuccessful. Only exceptionally has it been possible to transfer _Cœnurus cerebralis_, the larval stage of a tapeworm (_Tænia cœnurus_) of the dog from the brain of the sheep to that of the domestic goat. On the other hand, in the case of the Trichinellæ transference to a different host is easily accomplished. Under natural conditions, it is not uncommon for certain kinds of specific parasites to occur occasionally in unusual hosts. Their relationship to the latter is that of INCIDENTAL PARASITES. Thus _Echinorhynchus gigas_, a specific parasite of the pig, is only an incidental parasite of man; _Fasciola hepatica_ and _Dicrocœlium lanceatum_ are specific to numerous kinds of mammals, but may be found incidentally in man. On the other hand, _Dibothriocephalus latus_, a specific parasite of man, may occasionally take up its abode in the dog, cat and fox. As a rule, all those parasites of man that are only rarely met with, notwithstanding that human beings are constantly being observed and examined by medical men, are termed INCIDENTAL PARASITES OF MAN. In many cases we are acquainted with the normal or specific host of these parasites. Thus we know the specific host of _Balantidium coli_, _Eimeria stiedæ_, _Fasciola hepatica_, _Dipylidium caninum_, etc.; in others the host is as yet unknown. In the latter case the question partly relates to such forms as have been so deficiently described that their recognition is impossible, partly to parasites of man in various regions of the earth, the Helminthes and parasites of which are totally unknown or only slightly known, or finally to early developmental stages that are difficult to identify. Animals that usually live free, and exceptionally become parasitic, may likewise be called incidental parasites. In this category are included a few _Anguillulidæ_ that have been observed in man; also _Leptodera appendiculata_, which usually lives free, but may occasionally become parasitic in black slugs (_Arion empiricorum_): when parasitic it attains a larger size, and produces far more eggs than when living a free life. In order to avoid errors, the term “incidental parasites” should be confined to true parasites which, besides living in their normal host, may also live in other hosts. Leuckart speaks of FACULTATIVE PARASITISM in such forms as _Leptodera_. L. Oerley[3] succeeded in artificially causing _Leptodera_ (_Rhabditis_) _pellio_ to assume facultative parasitism by introducing these worms into the vagina of mice, where the parasites remained alive and multiplied. _Leptodera pellio_ dies in the intestines of mammals and man; it remains alive in frogs, but always escapes into the open with the fæces. [3] Oerley, L., “Der Rhabditiden und ihre medizinische Bedeutung,” Berlin, 1886, p. 65. Recently the incidental parasites of man have also been called “PSEUDO-PARASITES” or “PSEUDO-HELMINTHES.” Formerly, however, these terms were applied not only to living organisms that do not and cannot live parasitically, and that only exceptionally and incidentally get into man, but also to any foreign bodies, portions of animals and plants, or even pathological formations that left the human system through the natural channels, and the true nature of which was misunderstood. Frequently these bodies were described as living or dead parasites and labelled with scientific names, as if they were true parasites. A study of these errors, which formerly occurred very frequently, would be as interesting as it would be instructive. It is better not to use the expression pseudo-parasites for incidental parasites, but to keep to the original meaning, for it is not at all certain that pseudo-parasites are not described, even nowadays. _The Influence of Parasites on the Host._--In a great many cases, we are not in a position to state anything regarding any marked influence exercised by the parasite on the organism, and on the conditions of life, of the host. Most animals and many persons exhibit few signs of such influence, an exception being infestation with helminthes and certain other parasites which produce eosinophilia in the blood. As a general rule, the parasite, which is always smaller and weaker than its host, does not attempt to endanger the life of the latter, as simultaneously its own existence would be threatened. The parasite, of course, robs its host, but usually in a scanty and sparing manner, and the injuries it inflicts can hardly be taken into account. There are, however, numerous cases[4] in which the situation of the parasites or the nature of their food, added to their number and movements, may cause more or less injury, and even threaten the life of the host. It stands to reason that a _Cysticercus cellulosæ_ situated in the skin is of but slight importance, whereas one that has penetrated the eye or the brain must give rise to serious disorders. A cuticular or intestinal parasite is, as a rule, less harmful than a blood parasite. A helminth, such as an _Ascaris lumbricoides_ or a tapeworm, that feeds on the residues of foodstuffs within the intestine, will hardly affect its host by depriving it of this material. The case is different when the parasites are very numerous, especially when the heavily infested host happens to be a young individual needing all it ingests for its own requirements, and therefore unable to sustain the drain of numerous intruders in the intestine. Disturbances also set in more rapidly when the intestinal helminthes are blood-suckers, the injury to the host resulting from the kind of food taken by the parasite. [4] Lühe, M., “Ueber d. Fix. d. Helm. a. d. Darmwand ihrer Wirthe u. die dadurch verursachten path-anat. Veränderungen d. Wirthsdarmes,” _Trans. of IVth Intern. Zool. Cong._, Berlin, 1901; Mingazzini, P., “Ric. sul var. modo di fiss. delle tenie alla par. int. e sul loro assorbimento,” _Ric. Lab. Anat. Roma e altri Lab. biol._, vol. x, 1904; Shipley, A. E., and E. G. Fearnsides, “The Effects of Metazoan Parasites on their Hosts,” _Journ. Econ. Biol._, 1906, i, 2. Generally, the disorders caused by loss of chyle are insignificant when compared with those induced by the GROWTH and agglomeration of the helminthes. The latter may cause chiefly obstructions of small vessels or symptoms of pressure in affected or contiguous organs, with all those complications which may arise secondarily, or they may even lead to the complete obliteration of the organ invaded. Of course the symptoms will vary according to the nature of the organ attacked. In consequence also of the MOVEMENTS of the parasites, disorders are set up that may tend to serious pathological changes of the affected organs. The collective migrations, undertaken chiefly by the embryos of certain parasites (as in trichinosis, acute cestode tuberculosis), are still more harmful, as are also the unusual migrations of other parasites, which, incidentally, may lead to the formation of so-called worm abscesses or to abnormal communications (fistulæ) between organs that are contiguous but possess no direct connection. Recently, several authors have called attention to the fact that the helminthes produce substances that are TOXIC to their host; and the effects of such poisons explain the pathology of helminthiasis far more satisfactorily than the theory of reflex action. In a number of cases these toxic materials (leucomaines) have been isolated and their effects on living organisms demonstrated by actual experiments. It also appears that the absorption of materials formed by the decomposition of dead helminthes may likewise cause toxic effects. However, our knowledge of these conditions is as yet in its initial stage.[5] [5] Moursson et Schlagdenhauffen, “Nouv. rech. clin. et phys. sur quelq. liquides organ.,” _C. R. Acad. Sci._, Paris, 1882, p. 791; Debove, “De l’intox. hydat.,” _Bull. et Mém. Soc. méd. des Hôpit._, 1888; Linstow, v., “Ueb. d. Giftgehalt d. helm.,”_Internat. Monatsschr. f. Anat. u. Phys._, xiii, 1896; Peiper, “Z. Symptomatol. der thier. Paras.,” _Deutsche med. Wochenschr._, 1897, No. 40; Mingazzini, P., “Ric. sul veleno d. elm. int.,” _Rass. intern. d. med. modern. Ann._, 1901, ii, No. 6; Vaullegeard, A., “Etud. exp. et crit. sur l’action d. helm.,” _Bull. Soc. Linn. de Normandie_, 1901, 5, Ser. T, vii, p. 84, and others. Nearly all the symptoms caused directly or indirectly by parasites are of such a nature that the presence of the parasites cannot be diagnosed with any certainty, or only very rarely. The most that can be done is to deduce the presence of parasites by the exclusion of other causes. Fortunately, however, there are sufficient means by which we may confirm the diagnosis in a great many cases. Such means consist not only in a minute examination of the patient by palpation, percussion and local inspection, but also in the microscopical examination of the natural secretions and excretions of the body, such as sputum, nasal mucus, urine and fæces. Though such examinations may entail loss of time, they are necessary in the interest of the patient. It appears, moreover, that quackery, which has gained considerable ground even in the treatment of the helminthic diseases of man, can thus be considerably limited. _Origin of Parasites._[6]--In former times, when the only correct views that existed related to the origin of the higher animals, the mode of multiplication of parasites as well as of other lowly animals was ascribed to SPONTANEOUS GENERATION (_generatio æquivoca_), and this opinion prevailed throughout the middle ages. The writers on natural science merely devoted their time to the interpretation of the views of the old authors, and perpetuated the opinions of the ancients on questions, which, even in those days, could have been correctly explained merely by observation. [6] Die Geschichte der “Klinisch wichtigen Parasiten,” behandelt H. Vierordt im “Handb. d. Gesch. d. Med. hrsg.” v. M. Neuburger u. J. Pagel, Bd. ii, 1903. It was only when observations were again recommenced, and the microscope was invented, that the idea of spontaneous generation became limited. Not only did the microscope reveal the organs of generation or their products (eggs) in numerous animals, but Redi succeeded in proving that the so-called _Helcophagi_ (flesh maggots) are only the progeny of flies, and never appear in the flesh of slaughtered animals when fully developed flies are prevented from approaching and depositing their eggs on it. Swammerdam likewise knew that the “worms” living in the caterpillars of butterflies were the larvæ of other insects (ichneumon flies) which had laid their eggs in their bodies; he also discovered the ova of lice. The two authors mentioned were, however, unwilling to see that the experience they had gained regarding insects applied to the helminthes. Leeuwenhoek also vehemently opposed the theory of a spontaneous generation, maintaining that, on a basis of common-sense, eggs, or at all events germs, must exist, even though they could not be seen. The use of the microscope also revealed a large number of very small organisms in the water and moist soil, some of which undoubtedly resembled helminthes. Considering the wide dissemination of these minute organisms, it was natural to conjecture that after their almost unavoidable introduction into the human system they should grow into helminthes (Boerhave, Hoffmann). Linnæus went even further, for he traced the descent of the liver-fluke of sheep from a free-living planaria (_Dendrocœlum lacteum_), the _Oxyuris vermicularis_ from free-living nematodes, and the _Tænia lata_ (_i.e._, _Dibothriocephalus latus_) from a tapeworm (_Schistocephalus solidus_) found free in the water. Linnæus’ statements met with general approval. However, we must bear in mind that at that time the number of helminthes known was very small, and many of the forms that we have long ago learned to differentiate as specific were then regarded as belonging to one species. Linnæus’ statements were partly supported by similar discoveries by other investigators, such as Unzer, and partly also by the discovery of eggs in many helminthes. It was believed that the eggs hatched in the outside world gave rise to free-living creatures, and that these, after their introduction into the intestine, were transformed into helminthes. By means of these eggs the old investigators tried to explain the HEREDITARY TRANSMISSION of the intestinal worms, which was universally believed until the commencement of the last century. Some authors went so far as to regard the intestinal worms as congenital or inherited; they maintained the possibility of direct transmission, as in suckling, and denied that the eggs reaching the external world had anything to do with the propagation of the parasites. The more minute comparison between the supposed free-living stages of the helminthes and their adult forms, and the impossibility of finding corresponding free forms for the ever-increasing number of parasitic species, revealed the improbability of Linnæus’ statements (O. Fr. Müller). It was the latter author also who recognized the origin of the tapeworms (_Schistocephalus, Ligula_) found free in the water. They originate from fishes which they quit spontaneously. However, in spite of the fact that van Doeveren and Pallas correctly recognized the significance of the eggs in the transmission of intestinal worms, these statements remained disregarded, as did Abildgaard’s observation, experimentally confirmed, that the (immature) cestodes from the abdominal cavity of sticklebacks became mature in the intestines of aquatic birds. Moreover, at the end of the eighteenth and the commencement of the nineteenth centuries, after helminthology had been raised to a special branch of study by the successful results of the investigations of numerous authors (Goeze, Bloch, Pallas, Müller, Batsch, Rudolphi, Bremser), many of whom experienced a “divine joy” in searching the intestines of animals for helminthes, some authors reverted to _generatio æquivoca_, without, however, entirely denying the existence of organs of generation and eggs. The fact that a few nematodes bore living progeny--a fact of which Goeze was already aware--had no influence on the erroneous opinion, as in such cases it was considered that the young continued to develop beside the old forms. There were also many helminthes known that never developed sexual organs and never produced eggs, and which therefore were referred to _generatio æquivoca_. People were convinced that the intestinal mucous membrane or an intestinal villus could transform itself into a worm, either in a general morbid condition of the body, or in pathological changes of a more local character. The appearance of helminthes was even regarded as useful and as a means for the expulsion of injurious matter. These views, firmly rooted and supported by such eminent authorities as Rudolphi and Bremser, could not easily be overthrown. First, a change took place in the knowledge of the trematodes. In 1773, O. Fr. Müller discovered _Cercariæ_ living free in water. He regarded them as independent creatures and gave them the name that is still used at the present time. Nitzsch, who also minutely studied these organisms and who recognized the resemblance of the anterior part of their bodies to a _Fasciola_, did not, however, arrive at a correct conclusion. He regarded the combination rather as that of a _Fasciola_ with a _Vibrio_, for which he mistook the characteristic tail of the cercaria. He also noticed the encystment (transformation into the “pupa”) on foreign bodies of many species of these animals, but was of opinion that this process signified only the termination of life. Considerable attention was attracted to the matter when Bojanus first published a paper entitled “A Short Note on Cercaria and their Place of Origin.” He pointed out that the cercariæ creep out of the “royal yellow worms,” which occur in freshwater snails (_Limnæa, Paludina_), and are probably generated in these worms. Oken, in whose journal, _Isis_ (1818, p. 729), Bojanus published his discovery, remarks in an annotation, “One might lay a wager that these Cercariæ are the embryos of Distomes.” Soon after (1827), C. E. v. Baer was able to confirm Bojanus’ hypothesis that the cercariæ as a “heterogeneous brood” originated from spores in parasitic tubes in snails (germinating tubes). Moreover, Mehlis (_Isis_, 1831, p. 190) not only discovered the opercula of the ova of _Distoma_, but likewise saw the infusorian-like embryo emerge from the eggs of _Typhlocœlum_ (_Monostomum_) _flavum_ and _Cathæmasia_ (_Distoma_) _hians_. A few years later (1835) v. Siebold observed the embryos (miracidia) of the _Cyclocœlum_ (_Monostomum_) _mutabile_, and discovered in their interior a cylindrical body that behaved like an independent being (“necessary parasite”), and was so similar in appearance to the “royal yellow worms” (Bojanus) that Siebold considered the origin of the latter from the embryos of trematodes as, at all events, possible. Meanwhile, v. Nordmann of Helsingfors had in 1832 seen the miracidia of flukes provided with eyes swimming in water; v. Siebold (1835) had observed the embryos, or oncospheres, of tapeworms furnished with six hooklets in the so-called eggs of the Tænia; while Creplin (1837) had discovered the “infusorial” young of the _Diphyllobothrium_ (_Bothriocephalus_) _ditremum_, and conjectured that similar embryos were to be found in other cestodes with operculated eggs. At all events, the fact was established that the progeny of the helminthes appeared in various forms and was partly free living. The researches of Eschricht (1841) were likewise of influence, as they elucidated the structure of the Bothriocephali, and proved that the encysted and sexless helminthes were merely immature stages. J. I. Steenstrup (1842) was, however, the first to furnish explanations for the numerous isolated and uncomprehended discoveries. Commencing with the remarkable development of the Cœlenterata, he established the fact that the Helminthes, especially the endoparasitic trematodes, multiply by means of alternating and differently formed generations. Just as the polyp originating from the egg of a medusa represents a generation of medusæ, so does the germinal tube (“royal yellow worm”) originating from the ciliated embryo of a Distoma, etc., represent the cercaria. These were consequently regarded as the progeny of trematodes, and Steenstrup, guided by his observations, conjectured that the cercaria, whose entrance into the snails he had observed accompanied by the simultaneous loss of the propelling tail, finally penetrated into other animals, in which they became flukes. Part of this hypothetical cycle of development was erroneous, and in other particulars positive observation was lacking, but the path pursued was in the right direction. Immediately after the appearance of Steenstrup’s celebrated work, v. Siebold expressed his opinion that the encapsuled flukes certainly had to travel, _i.e._, to be transmitted with their bearers into other hosts, before becoming mature. This view was experimentally confirmed by de Filippi, La Valette St. George (1855), as well as by Pagenstecher (1857), while the metamorphosis of the ciliated embryo of Distoma into a germinal tube was first seen by G. Wagener (1857) in _Gorgodera_ (_Distoma_) _cygnoides_ of frogs. All that we have subsequently learned from the works of numerous investigators about the development of endoparasitic trematodes has certainly increased our knowledge in various directions, and, apart from the deviating development of the _Holostomidæ_ has, as a whole, confirmed the briefly sketched cycle of development. Steenstrup’s work on the cestodes did not attract the same attention as his work on trematodes. Steenstrup always insisted on the “nurse” nature of the cysticerci and other bladder-worms. Abildgaard (1790), as well as Creplin (1829 and 1839), had already furnished the information that certain sexless cestodes (_Schistocephalus_ and _Ligula_) from the abdomen of fishes only become mature after their transference to the intestine of aquatic birds. These passive migrations were confirmed in an entire series of other cestodes, particularly by v. Siebold (1844, 1848, 1850) and E. J. van Beneden (1849), not by actual experiment, but by undoubted observation. It was correctly believed that the ova or oncospheres penetrate into certain intermediate hosts, in which they develop into unsegmented larvæ. Here they remain until, with their host, they are swallowed by some predacious animal. They then reach the intestine, being freed from the surrounding membranes through the process of digestion, and settle themselves there to form the adult chain of proglottides. Though some few scientists, such as P. J. van Beneden and Em. Blanchard, deduced from these observations that the bladder-worms (Cysticerci), which had hitherto been regarded as a separate class of helminthes, were only larval Tæniæ, this correct view was not at first universally accepted. The foundation was too slight, and van Beneden was of opinion that the Cysticerci were not necessary, but only appeared incidentally. v. Siebold was a strenuous opponent to this theory, notwithstanding his experiences on the change of hosts of the Tetrarhynchus. Together with Dujardin (1850) he conjectured that the Tæniæ underwent a deviating cycle of development. He was of opinion that the six-hooked oncospheres left the intestine, in which the older generation lived, and were scattered about with the fæces, and finally re-entered _per os_ (_i.e._, with water and food) a host similar to the one they had left, in the intestine of which they were directly transformed into tapeworms. A change of host such as occurred in other cestodes was not supposed to take place (the history of the cestodes was at this time not entirely established). As the oncospheres of the Tænia are enveloped in one calcareous or several softer coverings which they cannot leave actively, and as, in consequence of this condition, innumerable oncospheres cannot penetrate into an animal, and others cannot reach the proper animal, v. Siebold conceded, at least for the latter, the possibility of a further development. But this was only supposed to occur because they had either invaded wrong hosts, or, having reached the right hosts, had penetrated organs unsuitable to their development, and had thus gone astray in their travels, and had become hydropically degenerated tæniæ. This was v. Siebold’s explanation of bladder-worms. Naturally, v. Siebold himself conjectured that a recovery of the diseased tapeworm might occur, in a few exceptional cases, after transmission into the correct host, as, for instance, in the _Cysticercus fasciolaris_ of mice, the host of which is the domestic cat, and in which there is a seemingly normally developed piece of tapeworm situated between the caudal vesicle and the cysticercus head. Guided by correct views, F. Küchenmeister undertook in Zittau the task of confirming the metamorphosis of _Cysticercus pisiformis_ of hares and rabbits, into tapeworms in the intestine of the dog by means of feeding experiments. The first reports on the subject, published in 1851, were not likely to meet with universal approval, because Küchenmeister first diagnosed the actual tapeworm he had been rearing as _Tænia crassiceps_, afterwards as _Tænia serrata_, and finally as _Tænia pisiformis_ n. sp. However, in any case, Küchenmeister, by means of the reintroduction of experimental investigation, rendered a great service to helminthology. The publication of Küchenmeister’s works induced v. Siebold to undertake similar experiments (1852 and 1853), which were partly published by his pupil Lewald in 1852. But the positive results obtained hardly changed Siebold’s opinion, for although he no longer considered the bladder-worms as hydropically degenerated tapeworms, he still regarded them as tæniæ that had strayed. The change of opinion was partly due to an important work of the Prague zoologist, v. Stein (1853). He was able to examine the development of a small bladder-worm in the larvæ of the well-known meal-worm (_Tenebrio molitor_) and to demonstrate that, as Goeze had already proved in the case of _Cysticercus fasciolaris_ of mice, first the caudal vesicle is formed and then the scolex, whereas Siebold believed that in bladder-worms the posterior end of the scolex was formed first, and that this posterior end underwent a secondary hydropic degeneration. In opposition to v. Siebold, Küchenmeister successfully proved the necessity of the bladder-worm stage by rearing tapeworms in dogs from the _Cysticercus tenuicollis_ of domestic mammals and from the _Cœnurus cerebralis_ of sheep. He, and simultaneously several other investigators independently, succeeded, with material provided by Küchenmeister, in rearing the _Cœnurus cerebralis_ in sheep from the oncospheres of the _Tænia cœnurus_ of the dog (1854). R. Leuckart obtained similar results in mice by feeding them with the mature proglottides of the _Tænia crassicollis_ of cats (1854). Küchenmeister also repeatedly reared the _Tænia solium_ of man from the _Cysticercus cellulosæ_ of pigs (1855), and from the embryos of this parasite P. J. van Beneden succeeded in obtaining the same _Cysticercus_ in the pig (1854). As Küchenmeister distinguished the _Tænia mediocanellata_, known to Goeze as _Tænia saginata_, amongst the large tæniæ of man (1851), so it was not long before R. Leuckart (1862) succeeded in rearing the cysticercus of the hookless tapeworm in the ox. It is particularly to this last-named investigator that helminthology is indebted more than to any other author. He followed the gradual metamorphosis from oncospheres to cystic worms in all its details. In view of all the researches that were made, and which are too numerous to mention individually, the idea that bladder-worms are abnormal or only incidental forms had to be abandoned. Everything pointed to the fact that in all cestodes the development is divided between two kinds of animals; in one--the host, the adult tapeworm is found; while in the other, the intermediate host, we find some form or other of an intermediate stage (cysticercus in the broadest sense). The practical application of this knowledge is self-evident. If no infected pork or beef is ingested, no tapeworm can be acquired, and also the rearing of cysticerci in the human body is prevented by avoiding the