212 



CESTODA. 



The central mass of cells becomes developed, as in the other 

 type, into a six-hooked (rarely four-hooked) embryo (fig. 96 G), 

 but the superficial layer separates from the central, and either 

 disappears or becomes (Bothriocephalus proboscidcns) a cuticular 

 layer. Between the six-hooked embryo and the outer layer of 

 cells one or more thick membranes become deposited (E. van 

 Beneden). The eggs are carried out of the alimentary canal in 

 the proglottis and transported to various situations on land or 

 in water. They usually remain within the proglottis, invested 

 by their thick shell, till taken up into the alimentary canal of 

 a suitable host, or they may be swallowed after the death and 

 decay of the proglottis. They are subsequently hatched after 

 their shell has become softened by the action of the digestive 

 fluids. 



Before proceeding to describe their further history, the close 

 resemblance between the first developmental stages of Cestoda, 

 especially in the case of the ciliated larvae, and those of Trema- 

 toda, may be pointed out. 



In both there is a ciliated larva, and in both there is developed 



FIG. 96. DIAGRAMS OF VARIOUS STAGES IN THE DEVELOPMENT OF THE 

 CESTODA. (From Huxley.) 



A. Cysticercus. B. and C. Cysticerci in the everted (B) and inverted (C) con- 

 dition. D. Ccenurus. E. and F. Diagrams of Echinococcus. It is most probable that 

 Trenia heads are not developed directly from the wall of the cyst as represented in 

 the diagram. G. Six-hooked embryo. 



within the ciliated skin a second larva, which becomes freed by 

 the stripping off of the ciliated skin. 



The type of development has moreover many analogies with 

 that of the Nemertine larva of Desor, p. 163 (cf. Metschnikoff), 

 and is probably like that an abbreviated record of a long history. 



The suitable host for the six-hooked embryo to enter is 



