62 



PLATYHELMINTHES TREMATODA 



inb 



Trematoda digenea (endoparasitica). 



Occurrence and Habits of Digenea. Endoparasitie Trema- 

 todes 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 Mam- 

 mals ; 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 

 action, becomes still more 

 marked when it is considered that if a 

 ^tmSllSS^l Trematode normally living in the stomach 

 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 considera- 

 tions 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). 



It is, however, in the small intestine that most Trematodes 

 occur, as the examination of the common Frog 1 will readily 

 demonstrate. Both this and the edible Frog are attacked by a 

 dozen Distomatidae, only a few of which, however, are common 



-ex.o 



Fig. 30. Distomum luteum v 



Baer (immature), to show j: " Qt ' vp 



the arrangement of the C 



excretory vessels. x 50 



ex.o. 



which the terminal contrac 



tile duct opens the finer of 



vessels end in flame -cells ; 



int, intestine ; m, mouth - 



sucker ; ph, pharynx ; vs, 



ventral sucker. (After la 



Valette.) 



1 An excellent and beautifully illustrated account, by Looss, of the Dis- 

 tomatidae of Frogs and Fishes may be found in Leuckart and Chun's Bibliotheea 

 Zoologica, Heft 16, 1894. 



