///. CE8T0DA. 



283 



being made to the description of the organs in the trematodes (p. 



The difference in the sexual apparatus has its influence on the 

 peculiarities of the egg (fig. 245). In Bothriocephalus it is large 

 (k), has a tough shell with a lid, and encloses a small &gg cell 

 with numerous yolk cells. The eggs of TcBnia (h, i) are small, 

 with a layer of albumen and a delicate shell which is lost early. It 

 is replaced by an embryonic shell, a radially striped envelope 

 secreted by the embijo in a somewhat advanced stage. It is in 

 this condition that one usually sees Taenia eggs. 



A further consequence is a difference in development. In most 

 Bothriocephalidse, as in the Trematoda, the egg must enter the 

 water for its further development. Here a ciliated oval larva 

 escapes which contains a six-hooked larva {oncosphcora, fig. 246). 



/fii'i^is 



Fig. 246.~Development of Bothriocephalus latiis. (From Leuckart.) A, ciliated 

 larva; B, same witll escaping six-hooked larva; C, young encysted Bothrio- 

 cephalus. 



The ciliated envelope is temporary and is cast off like the ciliated 

 coat of the trematode larva. The six-hooked larva in some 

 unknown way enters a fish, becomes encysted (pleurocercoid) in 

 muscles or viscera, and changes directly into the head of a Both- 

 riocA'plialus. This on being taken, in feeding, into the intestine 

 of the proper host develops into the adult. 



The longer and better known history of the Tcenias differs con- 

 siderably. The distinctions are early recognizable, since the six- 

 hooked larva lacks the ciliated envelope but is enclosed in its 

 homologue, the embryonic shell already alluded to. Since this 

 envelope cannot open of itself, the young must be freed from 

 it by its digestion in the stomach of the proper intermediate host. 

 Thus the eggs of Tcenia solmvi must pass into the stomach of the 

 pig (they are taken by admixture of the food with embryos con- 

 tained in faecal matter) and after being freed from their shell in 



