32 ENTOZOA FOUND IN MAN. 



vibratile cilia, by means of which it swims in sur- 

 rounding fluids ; it does not usually undergo meta- 

 morphosis, but it perishes after having produced 

 one or more germs, which are developed in its 

 interior. These germs enjoy a separate existence, 

 and continue to develop themselves ; their organisa- 

 tion differs from that of the embryo wliich produced 

 them, and also from that of a perfect distomum. 

 They often become simple oval or cyhndrical sacs, 

 each provided with a rudimentary sucker, or with 

 ramifying tubes without any apparent internal 

 organs ; at other times they acquire a digestive tube, 

 and a buccal sucker. 



These organisms, which proceed from the embryo, 

 form a second phase of generation. The embryo, after 

 being hatched, lived in a free state, but the individual 

 which succeeds it always lives parasitically in the 

 interior of molluscous animals. The individuals com- 

 posing this second generation have been designated 

 sporocysts. They sometimes multiply themselves in 

 the same form, like the hydatids, either by division, 

 or by external or internal gemmation. The sporo- 

 cysts are not destined to become perfect distoma ; 

 they are endowed, in the early periods of their exist- 

 ence, with life and very active movements, and there 

 are subsequently formed in their interior germs which 

 increase rapidly; these germs by their accumulation 

 distend the body of the sporocyst, wliich, gradually 

 losing its vitality and its movements, is at length 

 reduced to the condition of a totally inert and mem- 

 branous sac. 



The germs developed in the sporocyst constitute 

 a thu"d phase of generation which will jiroduce the 



