ANIMAL PARASITISM 



739 



worms would long ago have become extinct except for the fact that 

 each tapeworm produces manj^ millions of eggs. The peculiar 

 habitat and mode of life of endoparasites also makes necessary 

 peculiar adaptations in the functions or physiology of the parasite, 

 A parasite in the intestine, for instance, must be able to carry on 

 respiration in almost complete absence of oxygen, must secrete 

 substajices to counteract the digestive juices of the host in order 

 to prevent its being digested, must be adapted to a high concen- 

 tration of salts, acids, and other substances in solution in the fluid 

 around it, and if in a warm-blooded animal must be able to live 

 at a relatively high constant temperature. The fact that no host 

 is immortal makes it necessary for a parasite to have some special 

 provision for its offspring to escape to another host, in order to 

 maintain the existence of the species ; this necessity is met by various 

 peculiar adaptations in the life cycle or development, such as the 

 complex succession of larval stages in the endoparasitic trematodes 

 and cestodes. 



Fig. 385. — Diagram of the tunnel of an itcli mite in human skin. The female 

 animal is depositing eggs. (Reprinted by permission from Introduction to Human 

 farasitologii by Chandler, published by John Wiley and Sons, Inc. Adapted from 

 Riley and Johannsen.) 



Some parasites are able to carry on parasitic activities without 

 injuring their hosts, Avhile others may weaken or destroy the host. 

 Parasites which injure their hosts are said to be pathogenic (disease- 

 producing), while those which cause no appreciable injury are said 

 to be nonpathogc7iic or commensal. Since most parasites cannot live 

 without their hosts, a parasite which shortens the life of its host 

 destroys its own home and means of livelihood ; nonpathogenic para- 

 sites are more likely to be successful in the long run, and are there- 



