THE ORGANISM AND ITS ENVIRONMENT 365 



is attached to the intestinal wall. Posterior to the scolex the animal 

 consists o£ a chain of portions, each very much Hke the next, called 

 PROGLOTTiDS. The number of proglottids in a mature worm may be 

 several hundred. A mature proglottid consists essentially of repro- 

 ductive organs and developing embryos. Both ovaries and testes are 

 found in the same proglottid and the tgg is fertilized by sperms 

 from the testes of the same section. Proglottids distended with fer- 

 tilized eggs break off and are passed with the fasces. If the food 

 supply of swine is in some way contaminated by faecal material 

 containing tapeworm proglottids, the larval worms infest the mus- 

 cles of the animal and there form cysts. Within a cyst the larva 

 becomes a sac-shaped structure in which is the inverted head of the 

 new individual. If the meat of an infected swine is eaten without 

 having been heated sufficiently to kill these bladder-worms, or 

 CYSTicERcus as they are called, the larva develops into a mature 

 worm in the human intestine. The life cycle of this parasite requires 

 one principal and one intermediate host, the human body and the 

 swine. Other parasitic worms have more complicated life histories, 

 in some cases involving more than one intermediate host. 



The body of the adult tapeworm shows the structural degenera- 

 tions that accompany its parasitic source of food and its protected 

 habitat, for digestive organs are completely lacking and the nervous 

 mechanism is degenerate. Still more extreme are the degenerative 

 characters of an arthropod, Xenocceloma, that parasitizes a marine 

 annelid, Polycirrus. After this small shrimp-like animal has estab- 

 lished itself in the body wall of the annelid, practically its entire 

 body is absorbed by the host, except for the ovaries and testes and 

 the genital ducts. These remain functional and constitute the whole 

 of the adult parasite. Even the muscles that control the exit of the 

 genital products from the ducts of the parasite are derived from 

 host tissue, not from the parasite. The ability of a parasite to con- 

 trol the metabolism and growth of its host to suit the purposes of 

 the parasite is also shown by the protective galls that are formed 



