CESTOIDEA. 15 



the caudal vesicle ; when the echinococcus has reached 

 its fiiU development, this cord breaks or becomes 

 detached, and the echinococcus remains free in the 

 cavity of the hydatid. After a certain period the 

 germinal membrane is destroyed, and at a later 

 period the echinococci are themselves lost, so that 

 nothing then remains in the hydatid cavity excepting 

 the hooks of the echinococci. Those hydatids in 

 which no germinal membrane is developed have no 

 echinococci. 



The existence in the echinococcus of a rostellum, 

 of a double circlet of hooks, of four suckers, and of 

 calcareous corpuscles, indubitably place this parasite 

 in the class Cestoidea, and in the sub-class Tsenise. 

 Considered by itself, that is to say separately from 

 the hydatid, it represents a cystic worm, of wliich the 

 body has not been developed, and which is conse- 

 quently limited to a head and to a caudal vesicle. 



Primitive and Ultimate Phases of Development of 

 the Hydatid-Echinococcus. — The hydatid and the 

 echinococcus being two successive and transitional 

 phases in the development of a taenia, the six-hooked 

 embryo of the taenia ought to precede the hydatid ; 

 but whether the latter proceeds from the embryo by 

 metamorphosis, or by gemmation, is not yet known. 



The phase Scolex being always the last but one 

 in the life of a cestoid worm, the echinococcus, which 

 is a head or scolex, can produce nothing besides a 

 proglottis, which is the adult cestoid worm, and 

 forms the last link in the series. The various phases 

 of the development of the tape-worm, which may be 

 named the Tcenia Hydatigena, are consequently four, 

 at least, viz. : — 



