4 NATURE AND ORGANIZATION OF PARASITES. 



The parasites of man and the higher vertebrates belong exclusively 

 to them. 



Comparing together the various forms of animal life included here 

 in the group of parasites, we find numerous and striking differences, 

 not only in structure, which corresponds of course to their zoological 

 position, but also in their biological aspects in the nature and 

 degree of the parasitism exhibited. On the one hand, there are para- 

 sites which only occasionally seek out their host, and only remain 

 long enough to take in a sufficient supply of food, departing as soon 

 as this is done, and subsequently, perhaps, seeking out a fresh host. 

 On the other hand, there are parasites that pass a considerable time, 

 perhaps a whole stage of their existence, in the body of their host, 

 which thus serves as a dwelling-place as well as a source of nutriment. 

 This difference may perhaps be best expressed by the terms " tem- 

 porary " and " stationary ;" but it must be pointed out that between 

 these two kinds of parasitism no absolute line of demarcation can be 

 drawn, any more than between the parasitic and free modes of exis- 

 tence; the terms, however, may be retained, as they express two 

 degrees of parasitism, which are, generally speaking, sufficiently 

 distinct and are sometimes widely separated from each other. 



Even among the older zoologists this distinction was recognised, but 

 " temporary " parasitism was usually so called in contradistinction to 

 life-long, instead of to merely " stationary" parasitism. At that time, 

 however, the fact that even the most thorough-going parasites the 

 intestinal worms are free during part of their life, was not known, 

 and, accordingly, the contrast implied between the types of parasites 

 was different altogether from that put forward here. Besides those 

 parasites which exist as such throughout their whole life-cycle, there 

 are others which lead a free life for a longer or shorter period, either 

 in the adult condition (ichneumon-flies and gad-flies), or as larva* 

 (certain thread-worms). 



" Stationary " parasitism, therefore, manifests itself in two ways ; 

 it may be (1) " permanent," lasting for life ; or (2) " periodic," 

 embracing only a stage in the life-cycle, and therefore involving at 

 some time or other a change from parasitic to free life. 



The various kinds of parasitism just enumerated possess an interest 

 and importance that depend not merely upon their relations to each 

 other and to other modes of existence ; they are interesting also 

 from the fact of the influence which they have in modifying the 

 structure of the body, so that an examination of any form of parasite 

 enables one to foretell with moderate certainty the particular kind of 

 parasitism which it exhibits. Thus, temporary parasites must evi- 

 dently be provided with the means of approaching and abandoning 



