CESTODA. 



253 



neck, which presents even at this stage traces of segments (Fig. 

 205, e). The head and neck together constitute the scolex. In some 

 cases (Echinococcus) the irregularly shaped maternal vesicle produces 

 from its internal walls one or two generations * of secondary vesicles 

 which project into it ; and the Cestode heads originate' in special 

 small brood-capsules on these secondary vesicles (Fig. 206, a). In 

 such cases the number of tape-worms which arise from one embryo 



FIG. 206. a, brood-capsule of Echinococcus with developing heads (after R. Leuckart). fo, brood- 

 capsule of Echinococcus (after G. Wagener). c, heads of Echinococcusstill connected with the 

 wall of the brood-capsule one is evaginated ; Vc excretory canals. 



is naturally enormous, and the parent vesicle may reach a very 

 considerable size, being sometimes as large as a man's head. In 

 consequence of this enormous growth the vesicles frequently obtain 

 an irregular shape ; while on the other hand, the tape-worms which 

 are developed from them remain very small, and carry, as a rule, 

 only one ripe proglottis (Fig. 207). The cyst in which the bladder- 

 worm lies, and which is caused by it, is called an hydatid cyst. 



* In Cysticerci (C. longicollis, tenuicollis) also, sterile daughter vesicles are 

 sometimes budded off. 



