How infection occurs is a question to which at present no satisfactory answer can be made. The attempt to introduce embryos of Bilharzia into the common fresh-water animals of Alexandria has hitherto proved fruitless (Looss[[88]]), although there seems little doubt that the comparative immunity of Europeans from the disease is in some way owing to their drinking purer water than the natives. Possibly, as Leuckart suggests, the embryo becomes a sporocyst in man himself, somewhat as Taenia murina is known to develop in the rat without an intermediate host.[[89]] The immense numbers of the parasite in one host would then readily receive an explanation.
A Bilharzia, possibly B. haematobia, was found by Cobbold in the portal vein of Cercopithecus fuliginosus; and B. crassa infests the cattle of Egypt, Sicily, and certain parts of India, but does not produce haematuria.
Of the other Trematodes of man and domestic animals there is not room to speak fully. Distomum pulmonale, which occurs in the lungs of the cat, tiger, and dog, as well as in man, is especially common in Japan, China, Corea, and Formosa. D. sinense and D. rathouisi have been also found in inhabitants of these countries.
Bisexual Trematodes.—Zoologically, Bilharzia is interesting from its bisexual condition. It is not, however, the only bisexual Trematode. In cysts in the branchial chamber of Ray's bream, Brama raii, two worms are found, which are probably the slender male and the swollen female of the same species (Distomum okenii). The only doubt that can arise proceeds from the tendency in all Trematodes for the male organs to ripen before the female organs. Until we certainly know that the swollen egg-bearing form (♀) does not arise from a previously male form (♂), the case is open to suspicion. Since, however, Kölliker[[90]] never found intermediate hermaphrodite conditions, this Distomum may be almost certainly regarded as of distinct sexes. Didymozoon thynni (Monostomum bipartitum), from cysts on the gills of the Tunny (Thynnus), is another case. Two slender worms flattened posteriorly, come together, and the body of one becomes folded to receive that of the other. They fuse completely except for a small lateral opening through which the anterior parts of both worms may freely protrude. The enclosing individual contains a coiled uterus filled with eggs, and is the female, whereas the smaller individual never possesses eggs, and is probably the male.[[91]] Nematobothrium (Fig. 22, A), which occurs also in the Tunny, in the form of two immensely long individuals intricately wound about each other in a cyst, is, however, not bisexual.
Fig. 35.—Distomum okenii Köll. Showing male and female as they occur together in the branchial cavity of Bramaraii (Ray's bream). (From Bronn, after Kölliker.) Nat. size.
Table of Digenetic Trematodes and their Life-Histories.[[92]]
| Species. | Final host. | Host into which the larva enters, and in which Cercariae are eventually formed. | Host into which the Cercariae migrate and encyst; eaten by final host. |
| Diplodiscus (Amphistomum) subclavatus Göze | Rana, Bufo, Triton | Smaller species of Planorbis and Cyclas | Insect-larvae, Rana, Bufo, but frequently omitted |
| Distomum advena Duj. (D. migrans Duj.) | Sorex araneus | Not known | Limax |
| D. appendiculatum Rud. | Clupea alosa | Not known | Lucullus acuspes, Centropages hamatus (Copepoda) |
| D. ascidia v. Ben. | Species of Bats | Limnaea stagnalis Planorbis corneus | Ephemera, Perla, Chironomus plumosus |
| D. atriventre Weinl. | Frogs and Toads of N. America | Physa heterostropha | Not known |
| D. brachysomum Crepl. | The Dunlin (Tringa alpina) | Not known | Anthura gracilis |
| D. caudatum v. Linst. | Hedgehog (Erinaceus europaeus) | Helix hortensis | |
| D. clavigerum Rud. | Rana | Limnaea ovata Planorbis corneus | Not known |
| D. cygnoides Zed. | Rana | Pisidium, Cyclas | Limnaea sp. (Cercaria macrocerca Fil.) |
| D. cylindraceum Zed. | Rana | Limnaea ovata | Ilybius fuliginosus |
| D. dimorphum Dies. | Ardea, Ciconia (Brazil) | Not known | Different species of Fishes |
| D. echinatum Zed. | Cygnus, Anser, Anas | Species of Limnaea | Species of Limnaea, Paludina vivipara |
| D. endolobum Duj. | Rana | Limnaea stagnalis | L. stagnalis, Gammarus pulex, larvae of Limnophilus rhombicus |
| D. globiporum Rud. | Perca fluviatilis | Not known | Limnaea stagnalis, L. ovata, Succinea pfeifferi, S. putris, Physa fontinalis, Planorbis marginatus |
| D. hepaticum Abild. | Sheep, Oxen, Man, etc. | Limnaea truncatula | Omitted |
| D. hystrix Duj. | Lophius piscatorius | Not known | Marine Fishes |
| D. macrostomum Rud. | Warblers, Tits, Woodpeckers, etc. | Succinea putris | Omitted |
| D. militare v. Ben. | Common Snipe | Paludina vivipara | P. vivipara |
| D. nodulosum Zed. | Perca fluviatilis | Bithynia tentaculata | Cyprinus, Acerina cernua |
| D. ovocaudatum Vulp. | Rana esculenta | Species of Planorbis | Probably omitted. (Cercaria known as C. cystophora Wag.) |
| D. retusum Duj. | Rana | Limnaea stagnalis | L. stagnalis, larvae of Phryganeidae |
| D. squamula Dies. | Polecat | Unknown | Rana temporaria |
| D. signatum Duj. | Tropidonotus natrix | Unknown | Rana |
| D. trigonocephalum Rud. | Badger, Polecat | Paludina vivipara | Unknown |
| Gasterostomum sp. | Dogfish, Rays | Ostrea edulis, Cardium rusticum, C. edule | Belone vulgaris |
| G. fimbriatum v. Sieb. | Perca, Esox | Unio, Anodonta (Cercaria known as Bucephalus polymorphus) | Leuciscus erythrophthalmus |
| G. gracilescens Rud. | Lophius piscatorius | Unknown | Species of Gadus (e.g. G. aeglefinus), Molva, Lophius |
| Monostomum flavum Mehl. | Anas | Planorbis corneus | Omitted |
Classification of Trematodes.—We have seen (p. [63]) that it is hardly possible to carry out fully the division of Trematodes into Monogenea and Digenea. Nevertheless, pending further investigation on the doubtful points, this classification may still be used. Monticelli[[93]] has proposed the main divisions of a new classification, which has been also adopted by Braun, and is based on the nature of the suckers. These divisions are indicated below in brackets.
| A. Monogenea v. Ben. (Heterocotylea Mont.). | ||
| 1. Fam. | Temnocephalidae Hasw. | |
| Gen. | Temnocephala Hasw. | |
| 2. Fam. | Tristomatidae Tschbg. | |
| Sub-Fam. 1. | Tristomatinae Mont. | |
| Gen. | Tristomum, Nitzschia, Epibdella, Trochopus, Acanthocotyle, Phyllonella, Placunella, Encotylabe. | |
| Sub-Fam. 2. | Monocotylinae Tschbg. | |
| Gen. | Pseudocotyle, Calicotyle, Monocotyle. | |
| Sub-Fam. 3. | Udonellinae v. Ben.-Hesse. | |
| Gen. | Udonella, Echinella, Pteronella. | |
| 3. Fam. | Polystomatidae Tschbg. | |
| Sub-Fam. 4. | Octocotylinae v. Ben.-Hesse. | |
| Gen. | Octobothrium, Pleurocotyle, Diplozoon, Anthocotyle, Vallisnia, Phyllocotyle, Hexacotyle, Platycotyle, Plectanocotyle, Diclidophora. | |
| Sub-Fam. 5. | Polystomatinae v. Ben. | |
| Gen. | Polystomum, Onchocotyle, Erpocotyle, Diplobothrium, Sphyranura. | |
| Sub-Fam. 6. | Microcotylinae Tschbg. | |
| Gen. | Microcotyle, Gastrocotyle, Axine, Pseudaxine. | |
| 4. Fam. | Gyrodactylidae v. Ben. | |
| Sub-Fam. 7. | Gyrodactylinae Par. et Per. | |
| Gen. | Gyrodactylus, Dactylogyrus, Tetraonchus, Diplectanum. | |
| Sub-Fam. 8. | Calceostominae Par. et Per. | |
| Gen. | Calceostomum, Anoplodiscus. | |
| 5. Fam. | Aspidobothridae Burm. (= Aspidocotylea Mont.). | |
| Gen. | Aspidogaster, Platyaspis, Cotylogaster, Macraspis. | |
| B. Digenea v. Ben. (Malacocotylea Mont.). | ||
| 6. Fam. | Holostomatidae Brandes (= Metastatica Leuckart). | |
| Gen. | Diplostomum, Polycotyle, Hemistomum, Holostomum. | |
| 7. Fam. | Amphistomatidae Mont. | |
| Gen. | Amphistomum, Diplodiscus, Gastrodiscus, Homalogaster, Gastrothylax, Aspidocotyle. | |
| 8. Fam. | Distomatidae Mont. | |
| Gen. | Distomum (and sub-genera), Rhopalophorus, Koellikeria, Bilharzia. | |
| 9. Fam. | Gasterostomatidae Braun. | |
| Gen. | Gasterostomum. | |
| 10. Fam. | Didymozoontidae Mont. | |
| Gen. | Didymozoon, Nematobothrium. | |
| 11. Fam. | Monostomatidae Mont. | |
| Gen. | Monostomum, Notocotyle, Ogmogaster, Opisthotrema. | |