306 THE WONDER OF LIFE 



Parasites affect their hosts in a great variety of ways. 



Their injurious influence may be trivial or serious, direct 



or indirect. Skin parasites which are unimportant may 



prepare the way for the entrance of very injurious microbes. 



Intestinal parasites may become so numerous that they 



interfere with the host's nutrition ; several hundreds of 



large threadworms have been taken out of a horse's stomach. 



They may cause very serious perforations of the wall of 



the intestine. In the well-known sturdie-worm of the 



sheep, the large bladderworm is found in the brain or in 



the spinal cord, and causes disastrous locomotor disorders, 



and often death. On the whole, however, the relation 



between parasite and host is remarkably unimportant, 



partly because of the adaptability of the organism, and 



partly because the very aggressive parasites have probably 



eliminated themselves from time to time by killing their 



hosts. Such a case as Ichneumon larvae and caterpillars, 



referred to elsewhere, is only possible because the insect 



larvae pass into a new phase of life after they have killed 



their hosts. The disastrous effects of parasites are usually 



the results of the infection of new hosts who have not become 



adapted to withstand the toxic and other deleterious 



influences of the intruders. 



Of great interest are those cases first rightly interpreted 

 by Professor Giard, where the parasite destroys the repro- 

 ductive organs of its host, effecting ' parasitic castration '. 

 Thus male crabs infected with the peculiar crustacean 

 parasite called Sacculina have their whole constitution 

 profoundly altered. The reproductive organ may be 

 destroyed and a small ovary producing ova may take 

 its place ; the shape of the abdomen approximates to 

 that of the female ; and the protruding parasite is actually 



