178 



CESTODA. 



its final host. This takes place by the part of one animal infested 



with cysticerci becoming eaten by the 

 host in question. In the alimentary 

 canal of the final host the connective- 

 tissue capsule is digested, and then the 

 vesicular caudal appendage undergoes 

 the same fate, while the head, with its 

 suckers and hooks, attaches itself to 

 the walls of the intestine. The head 

 and rudimentary trunk, which have 

 been up to this time hollow, now be- 

 come solid by the deposition of an 

 axial tissue; and the trunk very soon 

 becomes divided into segments, known 

 as proglottides (fig. 99 A). These seg- 

 ments are not formed in the same suc- 

 cession as those of Chaetopods; the 

 voungest of them is that nearest to the 



*/ O 



head, and the oldest that furthest re- 

 moved from it. Each segment appears 

 in fact to be a sexual individual, and 

 is capable of becoming detached and 

 leading for some time an independent 

 existence. In some cases, e. g. Cysti- 

 cercus fasciolaris, the segmentation 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 epidermis 

 either ciliated (Bothriocephalus, 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 encapsuled 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 ( Coenurus, 

 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. 



FIG. 99. ECHINOCOCCUS VETEKI- 

 NOKUM. (From Huxley.) 



A. Taenia 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. 

 I. membranous wall of primary 

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

 secondary cyst. 



