CESTODA. 165 



the fine branches of longitudinal excretory tubes, which are 

 united in a ring in the head, are connected transversely at 

 each joint, and open terminally by one or more pores. All 

 tape-zvorms are hermaphrodite, and most, if not all, are 

 probably self-fertilising. The male reproductive organs in- 

 clude diffuse testes, a vas deferens, and a protrusible terminal 

 cirrus. The female organs include a pair of ovaries, yolk 

 glands, a shell gland, a vagina by which spermatozoa enter, a 

 receptacle for storing spermatozoa, and a ute?-us in which the 

 ova develop. The embryo develops within another host into a 

 proscolex or bladder-worm stage, which forms a "head" or 

 scolex. When the host of the bladder-worm is eaten by the 

 final host, the scolex develops into an adult sexual tape-worm. 

 With the conditions of endoparasitic life ?nay be associated the 

 occurrence of fixing organs, the absence of sense organs, the low 

 though somewhat complex character of the nervous system, the 

 entire absence of a food canal, and the prolific reproduction. 



Life history of Taenia solium. — This is one of the most 

 frequent of the tape-worms infesting man. In its adult state 

 it is often many feet in length, and is attached by its " head " 

 to the wall of the intestine. The head bears four suckers 

 and a crown of hooks, and buds off a long chain of joints, 

 which develop complex reproductive organs as they get 

 shunted further and further from the head. The last of the 

 joints or proglottides is liberated (singly or along with 

 others), and passes down the intestine of its host to the 

 exterior. It has some power of muscular contraction, and 

 is distended with little embryos within firm egg-shells. 

 When the proglottis ruptures, these egg-cases are set free. 



In certain circumstances, the embryos, within their firmly 

 resistent egg-shells, may be swallowed by the omnivorous 

 pig. Within the alimentary canal of this animal the egg- 

 shells are dissolved, and embryos bearing six anterior hooks 

 are liberated. They bore their way from the intestine into 

 the muscles or other structures, and there encyst. They 

 lose their hooks, increase in size, and become passive, vege- 

 tative, asexual " bladder- worms." A bud from the wall of 

 the bladder or proscolex grows into the cavity of the same, 

 and forms the future " head " or scolex. This is afterwards 

 everted, and then the bladder-worm consists of a small head 

 attached by a short neck to a relatively large bladder. But 



