OCCURRENCE AND HABITS OF DIGENEA.—Endoparasitic Trematodes have been found in almost all the organs of Vertebrate hosts excepting in the nervous, skeletal, and reproductive systems. The alimentary canal, however, is the most usual habitat. From the buccal cavity to the large intestine, or even to the cloaca, its different regions are the resorts of various Trematodes. No Digenea have been found in the mouth, pharynx, or oesophagus of Mammals; but in Birds, Reptiles, Amphibia, and especially in Fishes, these parts are largely affected. It is a striking fact that Trematodes should occur in the stomach of (chiefly) large predaceous fishes, such as the Pike, Sharks, the Angler-fish, and others, considering the powerful digestive action of the gastric juice of these carnivores. The peculiar nature of the defence which must be employed by the parasites against this digestive action, becomes still more marked when it is considered that if a Trematode normally living in the stomach of one host be transferred to that of another, it is usually speedily digested, as is shown (p. 65) in the case of _Distomum macrostomum_. From these considerations the suggestion has been made that the cutaneous secretions of these Trematodes must act, not only as a protection against digestive or other ferments, but that the action in each case must be a specific one (Frenzel, Braun). [Illustration: FIG. 30.—_Distomum luteum_ v. Baer (immature), to show the arrangement of the excretory vessels. × 50. _ex.o_, Excretory aperture by which the terminal contractile duct opens—the finer vessels end in flame-cells; _int_, intestine; _m_, mouth-sucker; _ph_, pharynx; _vs_, ventral sucker. (After la Valette.)] It is, however, in the small intestine that most Trematodes occur, as the examination of the common Frog[75] will readily demonstrate. Both this and the edible Frog are attacked by a dozen Distomatidae, only a few of which, however, are common {63}to both hosts, and a number of Holostomatidae also pass a stage of their development within these Amphibia. Some idea of the extent to which animals, whose habits lead to infection, may be attacked by Trematodes (to say nothing of Cestodes and Nematodes, which often occur also) may be gathered from the fact that in dissecting a black stork, Nathusius found several hundred _Holostomum excavatum_ and about a hundred _Distomum ferox_ in the small intestine, twenty-two _D. hians_ in the oesophagus, five others in the stomach, and one _D. echinatum_ in the intestine. Snipe, Woodcock, Sandpipers, Dunlin, Gulls, Bittern, Geese, and Wild Ducks are, to mention a few cases, greatly infested by members of this group. The following Trematodes have occurred in man[76]:—