CHARACTERISTICS 149 



the series within the Insecta, but we have already seen various 

 degrees of independent hfe in the nematodes, and have met 

 examples of this group and of Protozoa where there is no free- 

 living stage in the life-history at all. 



If we are to have a definition of a parasite which has any 

 practical value, we must extend the usual one by several clauses. 

 A parasite may be defined, with some approach to accuracy, 

 as an animal which, for an appreciable time, lives in or on the 

 body of another, considerably larger, animal, called the host, 

 to the detriment of the latter, but without causing its death 

 until reproduction of the parasite has occurred. This leaves the 

 zoologist free to put his own meaning on * appreciable time ' 

 and ' considerably larger '. Having digested this definition you 

 may remember that ornithologists agree in calHng the cuckoo 

 a parasite, although it lives neither in nor on the body of the 

 host, and so far from feeding on the host, is voluntarily fed by it. 



Fortunately, although few biological terms can be rigidly 

 defined, the general meaning is often clear. This is true of the 

 term parasite, and it is the typical parasites that we must now 

 consider. In the first place, it is convenient to divide them into 

 ectoparasites which Uve on their hosts, and endoparasites which 

 live in them, although, as we have seen, there are intermediate 

 forms. The modifications in both form and physiology which are 

 necessary in parasites differ in these two groups, as do their effects 

 on their hosts. 



EFFECTS ON HOSTS 



The effects of parasites on their hosts make up much of the 

 subject-matter of pathology, and it is impossible in a textbook 

 of zoology to go far in their study, but some generalisations are 

 possible. It is often said that the successful parasite does not kill 

 its host, and even that the really successful parasite does not 

 even harm it, but there does not seem to be any justification 

 for this view. Judged by their ubiquity, malarial parasites and 

 Trichinella are highly successful, but they often cause death ; 

 the ichneumon flies which live in the caterpillars of Lepidoptera 

 also appear to be highly successful, but they always completely 

 consume their host. It is true that there must be a limit to the 

 power of the parasites to kill, for if all hosts were killed there could 

 be no parasites, but in fact the reproduction of the hosts seems 

 to keep pace with the deaths, and a low density of population 



