TREMATODA. 33 



perfect distomum ; at first, they form individuals to 

 which the name of cercarim has been given, and 

 which are unlike the adult parasites ; their bodies 

 are oval, very contractile, usually provided with tem- 

 porary organs, such as a tail of variable length, which 

 serves for locomotion, one or more hooks, which serve 

 to penetrate into tissues, and with permanent organs, 

 viz., suckers, a digestive tube, and an excretory appa- 

 ratus. When the cercarise have acquired a certam 

 degree of development within the sporocyst, the 

 latter is ruptured, and' places its progeny in a state 

 of freedom. The cercaria, having become free, swims 

 in search of a new animal in which to hve ; by the 

 aid of its buccal hooks it penetrates through the 

 integuments of some aquatic animal, usually the 

 larva of an insect, or some mollusc, loses its tail in 

 its passage, and becomes enveloped in a cyst ; in this 

 new situation, it assumes the form of a perfect dis- 

 tomum, but it is only when the animal in which it is 

 contained is swallowed accidentally by a vertebrated 

 animal that the yoiuig distomum, having arrived into 

 the organ and the animal which is suitable for its 

 existence, permanently acquires the characteristics of 

 the adult of its species. 



Thus, the different phases in the development of 

 a distomum are three in number : the embryo is a 

 grand-nurse, the sporocyst is a 7iurse, and the cer- 

 caria is a larva which ari^ives at the adult state hy 

 metamorphosis. 



The adult distoma never live in a free state ; 

 and when they are removed firom the organs which 

 they inhabit and are placed in water, they rapidly 

 become decomposed and perish. They are principally 



