PARASITISM 



could obviously assist ui host restriction (p. 156), but they are less 

 interesting than those which only develop in the presence of the 

 parasite and so confer an acquired immunity. It seems to be on 

 the whole rare for a host to acquire complete resistance or 

 immunity to an animal parasite, that is, to develop such a 

 chemical reaction that it cannot subsequently be infected by 

 the same species. Cattle may, however, acquire a resistance to 

 Trypanosoma hrucei and others, and man to Plasmodium vivax. 

 l^iVtial resistance to the cystercus stage of tapeworms is common, 

 but resistanre to the adults is probably never acquired. A pecuHar 

 form of resistance, called premunition, does, however, prevent 

 a second infection in a man who harbours a single beef or pork 

 tapeworm, and premunition also prevents subsequent infection 

 with Protozoa in a number of animals and diseases, including malaria 

 and trypanosomiasis, so long as any individuals derived from the 

 first infection are present ; these are themselves also kept in 

 check. Many bacteria confer a much stronger immunity, so that 

 all the original invaders are killed, and reinfection is impossible for 

 long afterwards. 



EFFECTS ON PARASITES 



In attempting to determine the effects which parasitism has 

 had on parasites one must compare them with what appear to 

 be their nearest free-living relatives. This is easy with insects like 

 the warble-fly or the ichneumons, where parasitic larvae grow into 

 normal adults, or the crustacean Sacculina, w^here the parasitic 

 adult is formed from a normal nauplius larva, but in the more 

 typically parasitic groups it is more difficult. Neither the Sporozoa 

 nor the Trematoda nor the Cestoda are at all closely related to 

 other members of their phyla, and though there are free-living 

 nematodes they seem to differ not at all from the parasites. 

 The ' adaptations to parasitism ' of elementary textbooks and 

 examination papers are, in fact, mostly imaginary ; there is no 

 evidence that any parasite possesses any physiological mechanism 

 or structural peculiarity that is not possessed or paralleled by free- 

 living animals. What is true is that parasites show an unusual 

 development of features which are known elsewhere, and that the 

 few parasitic members of the ' higher ' groups, chiefly the Crustacea 

 and Insecta, have lost almost all their characteristic features. 



Ectoparasites generally possess piercing and sucking mouth- 

 parts, for they typically feed on blood ; the Mallophaga or bird 



