THE TREMATOD A. 57 



of themselves deeper into the mucous integument, and to 

 the getting rid of the tail, which is no longer of any use 

 to them as an organ of locomotion. 



For this pm-pose they contract the body into an oval 

 form, and lash out the tail forcibly on both sides, upon 

 AA'hich the body is again slightly moved, whilst the tail is 

 kept stul, but only that it may be again lashed about 

 with greater vehemence than before, when the body has 

 acquired 2l point d'ajjj^ui for its further contraction, and 

 for the more strenuous movements of the tail. These 

 repeated efforts end at last in the casting off from the 

 body, of the tail, which becomes a lifeless and irregularly 

 contracted mass, and at the same time the tailless animal 

 itself assumes so completely the appearance of a Distoma 

 or fluhe, that it could not fail of being recognized as be- 

 longing to that genus, in case it were met with in this 

 condition in the viscera of other animals. However it 

 undergoes a further remarkable transformation before it 

 becomes a true entozoon in the common acceptation of the 

 w^ord. 



In various Cercnrice a copious mucous secretion is ob- 

 servable on the surface of the body, even before the loss 

 of the tail, and this secretion apparently increases dming 

 the efforts of the animal to cast off its appendage, and 

 especially dming the strong contractions of the body. 



As soon as the tail has been got rid of, the Cercaria 

 begins, by extending and contracting itself, partly to 

 move about, but partly, and in fact chiefly to tm^n itself 

 as it were round and round in the same spot. By this 

 sort of movement it makes for itself a cncular cavity 

 within the mucus, which gradually hardens, and forms a 

 tough, nearly transparent case around it. This is the 

 noted pupa state of the Cercaria, observed first by 

 Nitzsch'^ and afterwards by Siebold. The tailless Cer- 

 carice lie concealed under their transparent case, which is 

 arched over them like a small, closely-shut watch-glass ' 



* Nitzsch. Beitra^^ zur Infusorienkunde, oder Naturbeschreibung der 

 Zercarien und Bacillanen. Halle, 1817. 



