SPECIALIZATIONS 15 



high degree, to adhere to or to make their way about in their 

 particular host, or the particular part of the host, in which they 

 find suitable conditions for existence. Examples of speciali- 

 zations of external parasites are the compressed bodies of fleas, 

 permitting them to glide readily between the hairs of their hosts; 

 the backward-projecting spines of fleas, which are of much assist- 

 ance in forcing a path through dense hair by preventing any 

 back-sliding; the clasping talons on the claws of lice; the barbed 

 probosces of ticks; and the tactile hairs of mites. In these same 

 parasites can be observed marked degenerations in the loss of 

 eyes and other sense organs, absence of wings, and, in some 

 cases, reduction of legs. Internal parasites are even more pe- 

 culiar combinations of degeneration and specialization. They 

 possess all sorts of hooks, barbs, suckers and boring apparatus, 

 yet they have practically no sense organs or special organs of 

 locomotion, a very simple nervous system, and sometimes, as 

 said before, a complete absence of the digestive tube. 



Still more remarkable are the specializations of parasites, in 

 their reproduction and life history, to insure, as far as possible, 

 a safe transfer to new hosts for the succeeding generations. 

 Every structure, every function, every instinct of many of these 

 parasites is modified, to a certain extent, for the sole purpose of 

 reproduction. A fluke does not eat to live, it eats only to re- 

 produce. The complexity to which the development of the re- 

 productive systems may go is almost incredible. In some adult 

 tapeworms not only does every segment bear complete male 

 and female reproductive systems, but it bears two sets of each. 

 The number of eggs produced by many parasitic worms may run 

 well into the hundreds of thousands. The complexity of the 

 life history is no less remarkable. Not only are free-living stages 

 interposed, and intermediate hosts made to serve as transmitting 

 agents, but often asexual multiplications, sometimes to the ex- 

 tent of several generations, are passed through during the course 

 of these remarkable experiences. 



Effects of Parasites on Hosts. The effects of parasites on 

 their hosts are almost as numerous and as varied as are the kinds 

 of parasites, and vary besides with the susceptibility of the in- 

 dividual concerned, his physical condition, and complication with 

 other infections. In general it may be said that a parasite damages 

 its host in one or more of three ways: (1) by robbing it of food 



