ADAPTATION 349 



very small and is provided with hooks and suckers by means of 

 which it attaches itself to the intestinal wall. The body behind 

 the head consists of a series of segments or proglottids. The 

 parasite has no mouth or alimentary canal of its own, the host's 

 food being absorbed directly through the body wall. The more 

 posterior proglottids become filled with eggs, which are probably 

 fertilized by spermatozoa from the same proglottid. As the 

 proglottids become laden with embryos, they break off and pass 

 out of the alimentary canal of the host. If the proglottids are 

 eaten by a pig, the embryos are released and bore through the 

 walls of the pig's alimentary canal, whence they make their 

 way to voluntary muscles where they encyst. Here the embryo 

 develops into an oval cysticercus, or bladder worm, and remains 

 as a cyst until the infected muscle is eaten (Fig. 233). When 

 the wall of the cyst is dissolved by the human gastric juice, the 

 scolex is protruded from the inverted position occupied in the 

 cyst, in the manner of a finger of a glove. The scolex then 

 attaches itself to the intestine, and with the development of 

 proglottids the bladder worm is transformed into the adult. 

 From this it is seen that in the usual life history of the pork tape- 

 worm two hosts are involved. Neither host is killed by the 

 presence of the parasite in the ordinary course of events. 



In other cases of parasitism the host is killed as a result of the 

 parasitic association. Thus, the chalcid flies deposit their eggs 

 in the bodies of caterpillars. The eggs hatch into larvae which 

 devour the caterpillar. Similarly the ichneumon flies lay their 

 eggs in caterpillars and other insects, which are destroyed by the 

 developing parasitic larvae. 



Regeneration. — Many animals have the power to replace or 

 regenerate new parts if old ones are lost. Ewplanaria is a genus 

 of the free-living flatworms occurring in fresh-water ponds. If 

 such an animal is cut transversely, the head end will regenerate a 

 new tail and the tail end a new head. If a crab loses a leg, a new 

 one is regenerated. The same is true of the cockroach before 

 the final molt. If one or more arms are torn from a starfish, 

 they are replaced by new ones (Figs. 198 and 199). Regeneration, 

 which enables an animal to survive the loss of considerable por- 

 tions of the body, is common among lower invertebrate animals. 

 Regeneration of lost parts is present in the larval stages of lower 

 vertebrates. Thus the limbs of amphibian tadpoles if cut off are 



