ON PARASITES IN GENERAL. 



BY the term PARASITES is understood living organisms which, 

 for the purpose of procuring food, take up their abode, temporarily 

 or permanently, on or within other living organisms. There are 

 both plants and animals (Phytoparasites and Zooparasites) which 

 lead a parasitic life in or upon other plants and other animals. 



Phytoparasites are not included in the following descriptions of 

 the forms of parasitism, but a very large number of animal parasites 

 (zooparasites) are described. The number of the latter, as a rule, 

 is very much underrated. How great a number of animal parasites 

 exists may be gathered from the fact that all classes of animals are 

 subject to them. Some of the larger groups, such as Sporozoa, Cestoda, 

 Trematoda and Acaiithocephala, consist entirely of parasitic species, 

 and parasitism even occurs among the vertebrates (Myxine). It 

 therefore follows that the characteristics of parasites lie, not in. their 

 structure, but in the manner of their existence. 



Parasitism itself occurs in various ways and degrees. According 

 to R. Leuckart, we should distinguish between OCCASIONAL (temporary) 

 and PERMANENT (stationary) PARASITISM. Occasional parasites, such 

 as the flea (Pnlex irritaiis), the bed-bug (Cimex lectularius) , the leech 

 (Hirndo uiediciiialis), and others, only seek their " host " to obtain 

 nourishment and find shelter while thus occupied. Without being 

 bound to the host, they usually abandon the latter soon after the 

 attainment of their object (Cimex, Hirudo), or they may remain on 

 the body of their host throughout their entire development from the 

 hatching of the egg (Pedicuhis). It follows from this mode of living 

 that the occasional parasites become sometimes distinguishable from 

 their free-living relatives, though only to a slight extent. It is, there- 

 fore, seldom difficult to determine the systematic position of temporary 

 parasites from their structure. 



In consequence of their mode of life, all these temporary parasites 

 live on the external surface of the body of their host, though more 

 rarely they take up their abode in cavities easily accessible from the 

 exterior, such as the mouth, nose and gills. They are therefore 

 frequently called EPIZOA or ECTOPARASITES ; but these designations 



