286 



PLASMODROMATA AND SARCODINA 



lives in the red cells of the blood. These latter may be subdivided 

 into ' periodical parasites,' which only spend a portion of their 

 life-history within a host, and ' permanent,' which are parasitic 

 throughout the whole life-cycle. As an example of the former 

 may be mentioned the larvae of certain flies which are parasitic, 

 while the fly itself is not, and of the latter Oxyuris vermicularis 

 (Linnaeus, 1767) is a good example. 



Animal parasites may be classified into ectoparasites or epizoa, 

 affecting the skin and exterior of the host; and endoparasites or 

 entozoa, affecting the internal organs or cavities of the host. 



Until recent years the greatest confusion existed as to the nature 

 of parasites. With our present knowledge it appears that parasites, 

 however altered in structure some of them may have become during 

 their parasitic existence, have been originally derived from free- 

 living forms. It will, however, be noted that there are great 

 differences in the structure and appearance between some of the 

 permanent parasites and their nearest free-living affinities. This 

 difference has been brought about mainly by environment. 



The permanent parasite, particularly if an entozoon, has food 

 prepared for it by its host in an easily assimilated form; hence 

 there is no necessity for a complicated digestive apparatus, which 

 in certain cases has entirely disappeared. 



Further, certain parasites, having reached an organ which supplies 

 them with suitable food, do not require organs of locomotion, for 

 they have only to remain where they are, and the tissues of the 

 host will subserve their purpose. Some of them have no necessity, 

 as a rule, for locomotion for sexual purposes, because hermaphro- 

 ditism is common, and even in cases where this is not found para- 

 sites often live in couples (male and female). Being enclosed in 

 the body of the host, there is no necessity for sense organs; therefore 

 these are much reduced, or are absent. 



On the other hand, the parasite requires something to enable it 

 to fix itself firmly to the tissues of the host; hence suckers and 

 hooks are often found, and also clasping and clinging organs. 



The chance of a parasite, locked up inside a host, successfully 

 reproducing its species is small; hence reproduction assumes im- 

 portant features associated with the production of large numbers of 

 new forms, which are often protected by shells or other coverings. 



Reproduction may be asexual, especially in the protozoa, or 

 sexual. Sexual reproduction in parasites is generally complicated, 

 ending in the production of large quantities of eggs or spores; and 

 very often the spores of the protozoa or the corresponding lar\al 

 forms of the metazoa may travel through one or more intermediary 

 hosts before they infect another definitive host — that is, one in 

 which the sexual life-history is gone through . 



With regard to the sexual process, it may be noted that often the 

 male is smaller than the female, and in fewer numbers, but there 

 are exceptions. 



Usually parasites keep strictly to certain hosts called ' normal 



