TYPE PLATYHELMINTEES. 157 



pendently to the exterior a little behind the genital pore. 

 The vagina serves as a duct for the spermatozoa during 

 copulation, and corresponds with the canals opening at the 

 sides of the anterior end of the body in the Polystomeae (see 

 p. 146), while the uterus serves for the retention of fertilized 

 and mature ova. In Bothriocephalus the opening of the genital 

 pore is in the middle line of one of the surfaces of the pro- 

 glottid ; in Taenia, however, it occurs on the lateral margin of 

 the proglottid, and in some cases each proglottid may have an 

 opening in each of the lateral margins, there being a duplica- 

 tion in such cases of the genital ducts. Furthermore, in the 

 Tsenias the vitellarium is much less voluminous than in 

 Bothrioce'phalus, and produces an albumen-like secretion in- 

 stead of yolk-cells, and in addition the uterus has no special 

 opening to the exterior and is relatively small, though it may 

 become fairly voluminous by its walls being pushed out into 

 pouch-like sacculations by the contained ova. 



Developmerd of the Cestoda. — Accompanying the differences 

 in the arrangement of the reproductive apparatus differences 

 in the development are found in the two groups. In Bothrio- 

 cephalus (taking this form again as the example of the one 

 group) the egg is richly provided with yolk-cells, among 

 which the germ-cell lies imbedded. The embryo leaves the 

 egg in the form of a spherical ciliated body, provided with 

 six chitinous hooks, arranged more or less distinctly in pairs. 

 After swimming about for a time the cilia and their cells are 

 thrown off, and the six-hooked embryo makes its way into the 

 body of the first host, where it becomes enclosed in a thin 

 cyst, within which it develops directly to a scolex. If this 

 be swallowed by the second host, the worm fastens itself to 

 the walls of the digestive tract, and soon develops to the 

 sexually mature strobila. 



In the Tsenias, however, the ova are much smaller, the 

 yolk-cells being replaced by an albumen-like substance, 

 relatively small in amount, and the embryos when they hatch 

 out are destitute of cilia, resembling the six-hooked embryo 

 of Bothriocephalus after it has lost its ciliated covering (Fig. 

 83, A). In this condition it makes its way into the primary 

 host, in whose tissues it becomes encysted, and develops in 



