Chapter IV: Appendix: On Trematoda and Nematoda 753 (1)

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

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| 391 " 281 |
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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.