62 PLATYHELMINTHES TREMATODA 



Trematoda digenea (endoparasitica}. 



Occurrence and Habits of Digenea. Endoparasitic 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- 

 lt 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 

 PIG. SQ.Distmmtm luteum v. employed by the parasites against this 

 5rSS,tVE; dig^ive action, becomes still more 

 excretory vessels, x 50. marked when it is considered that if a 

 Trematode normally living in the stomach 



tile duct opens the finer o f one host be transferred to that of 



vessels end in flame -cells ; , . . ,., ,. , 



int, intestine ; m, mouth- another, it is usually speedily digested, as 

 sucker; ph pharynx; , i s shown (p. G5) in the case of IHstomum 



ventral sucker. (After la 



Vaiette.) 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, Eraun). 



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

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

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

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



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

 tomatidae of Frogs and Fishes may be found in Leuckant and Chun's Bibliotheca 

 Zoohxjica, Heft 16, 1894. 



