3o6 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 weU-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 adaptabihty of the organism, and 

 partly because the very aggressive parasites have probably 

 ehminated themselves from time to time by kilUng 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 hfe after they have kiUed 

 their hosts. The disastrous efiects 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 pecuhar crustacean 

 parasite called SaccuUna 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 



