_Distomum hepaticum_ Abild. " _lanceolatum_ Mehlis. " _conjunctum_ Cobbold. " _spathulatum_ Leuckart (= _D. sinense_ Cobb., _D. japonicum_ R. Blanch.). " _rathouisi_ Poir. (probably = _D. crassum_ Busk, _D. buskii_ Lank.). " _heterophyes_ v. Sieb. " _pulmonale_ Bälz (= _D. ringeri_ Cobb., _D. westermanni_ Kerb.). " _oculi humani_ Ammon (= _D. ophthalmobium_ Dies.). _Monostomum lentis_ v. Nord. _Amphistomum hominis_ Lewis and M‘Connell. _Bilharzia haematobia_ Cobb. LIFE-HISTORIES OF THE DIGENEA.—The classification of Trematodes according to their life-histories, expressed in the divisions Monogenea and Digenea, though a very useful one, breaks down entirely in the case of certain forms. Thus the life-history of _Gyrodactylus_ is probably digenetic rather than monogenetic. _Aspidogaster conchicola_,[77] which lives in the pericardial cavity of the fresh-water mussel (possibly the only case of a Trematode becoming normally mature in an Invertebrate host, since other species of _Aspidogaster_ live in Chelonia), produces larvae which enter another _Anodonta_ and develop directly into the sexual form. In other words, _Aspidogaster_, though structurally a digenetic form, possesses a life-history which is direct and simple, _i.e._ monogenetic. The Holostomatidae, which live in birds of prey and aquatic birds, give rise to eggs from which a minute larva escapes. The fate of this aquatic larva is not directly known, {64}but in all probability after entering a host (Fish, Amphibian, Mollusc), it undergoes a gradual change into what has long been known as a _Tetracotyle_, from the frequent presence of four (sometimes only three) adhering organs. Fig. 31 exhibits a species which is abundant in the lens and vitreous humour of the eye of the Perch. Its further history is not known, but presumably the Perch is presently devoured by the final host in which the _Diplostomum_ attains maturity. Thus the Holostomatidae are "metastatic" (Leuckart), their (probably) direct development requiring the presence of two hosts.[78] The other Digenea, the life-histories of which are known, belong to the Distomatidae and Amphistomatidae, and we may distinguish the steps by which the complex life-history of the liver-fluke (_Distomum hepaticum_) has been brought about, by a consideration of that of _Distomum macrostomum_. [Illustration: FIG. 31.—_Diplostomum (Tetracotyle) volvens._ (After v. Nordmann.) × 130. _cv_, Contractile excretory vesicle; _d_, intestine; _e_, calcareous bodies in excretory tubules; _ex.o_, excretory aperture; _gl_, glandular adhesive body; _ms_, oral sucker; _ph_, pharynx; _vs_, ventral sucker.]