216 



CKSTODA. 



youngest of them is that nearest to the head, and the oldest 

 that furthest removed from it. Each segment appears in fact 

 to be a sexual individual, and is 

 capable of becoming detached and 

 leading for some time an indepen- 

 dent existence. In some cases, 

 e.g. Cysticcrcus fasciolaris, the seg- 

 mentation of the trunk may take 

 place while the larva is still in its 

 intermediate host. 



The stages in the evolution of 

 the Cestoda are shortly as follows : 



1. Stage with embryonic epi- 

 dermis either ciliated (Bothrioce- 

 phalus, etc.) or still enclosed in the 

 egg-shell. This stage corresponds 

 to the ciliated larval stage of the 

 Trematoda. 



2. Six-hooked embryonic stage 

 after the embryonic epidermis has 

 been thrown off. During this stage 

 the embryo is transported into the 

 alimentary tract of its intermediate 

 host, and boring its way into the 

 tissues, becomes encapsuled. 



3. It develops during the en- 

 capsuled state into a cystic worm, 

 equivalent to the sporocyst of 

 Trematoda. 



4. The cystic worm while still encapsuled develops a head 

 with suckers and hooks, becoming a Cysticercus. In some forms 

 (Ccenurus, Echinococcus) reproduction by budding takes place at 

 this stage. The head and trunk are known as the scolex. 



5. The Cysticercus is transported into the second and 

 permanent host by the infested tissue being eaten. The bladder- 

 like remains of the cystic worm are then digested, and by a 

 process of successive budding a chain of sexual proglottides are 

 formed from the head, which remains asexual. 



The above development " is to be regarded as a case of 



FIG. 99. ECHINOCOCCUS VETE- 

 RINORUM. (From Huxley.) 



A. Tsenia head or scolex. a. 

 hooks, b. suckers, c. cilia in 

 water vessel, d. refracting parti- 

 cles in body wall. 



B. single hooks. 



C. portion of cyst. a. cuticle. 

 b. membranous wall of primary 

 cyst. c. and e. scolex heads, d. 

 secondary cyst. 



