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) both of 

 which lead a parasitic life in or upon other plants and other 

 animals. 



Phytoparasites are not included in the following descrip- 

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

 parasites (zooparasites) are described ; the number of the latter, as 

 a rule, are very much under-rated. How great a number of animal 

 parasites exist may be gathered from the fact that with the excep- 

 tion of the Echinodermata and the Tunicata, which, so far as we know 

 at present, do not harbour living parasites, all other classes of animals 

 are subject to them ; some of the larger classes, such as Sporozoa, 

 Cestodes, Trematodes and Acanthocephala, consist entirely of para- 

 sitic species, and parasitism even occurs amongst 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. Accord- 

 ing to R. Leuckart, we should distinguish between OCCASIONAL (tem- 

 porary) and PERMANENT (stationary) PARASITISM. Occasional para- 

 sites, such as the flea (Pulex irritans), the bed-bug (Cimex lectularia), 

 the leech (Hirudo medicinalis), and others only seek their " host " 

 to obtain nourishment and find shelter while thus occupied, but 

 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 develop- 

 ment from the hatching of the egg (Pulex). It follows from this mode 

 of living that the occasional parasites become sometimes distinguish- 

 able from^Blir free-living relatives, though only to a slight extent. 

 It is, therefore, never difficult to assign the systematic position of 

 temporary parasites from their structure. 



