140 THE BIOLOGY OF THE SEA-SHORE 



has become an integral and unalterable habit, through other 

 forms where the inter-relationship is less close or stable, to 

 cases in which doubts may be entertained as to whether the 

 plant or animal in question is or is not a parasite. 



In particular, some confusion of thought would appear 

 to have arisen through the tendency to regard as parasites 

 creatures such as Ichneumon-fly grubs or Myxine, the 

 Hag-fish, which are really predatory animals eating from 

 within instead of from without. There is a sense, of course, 

 in which all predatory animals are parasitic, but the main 

 point of diff"erence here between a predatory and a truly 

 parasitic animal is that the former encompasses the death of 

 its victims, whilst the latter seeks merely to be tolerated. 



We may distinguish parasites as being temporary (or 

 occasional) when they use a host for a part of their life- 

 history or at feeding times only ; permanent (or stationary) 

 when the host is never left by the adult parasite ; faciiltative 

 in the case of animals resorting to parasitism only as 

 a second-string ; and incidental when they occur on unusual 

 hosts. Those which confine themselves to the exterior of 

 the host's body are known as ectoparasites, while those which 

 dwell within the host's body receive the term endoparasites . 

 We may summarise the most essential of these features as 

 follows : — 



Parasites : i.e. forms living i Ectoparasites : confining themselves to the 

 in or on other organisms exterior of the Iiost's body, 

 and obtaining food from ( Feeding on di- 



them without rendering j Endoparasites : dwelling ^^f ^^ ^"""^ °^ 

 benefit m return, and ^jtW the host's body. §",*•_ ,. . 

 yet without necessarily ' j r eedmg on living 



killing their hosts. \ V tissue. 



There is great diversity in the form, habit, and life-history 

 of parasites. In some instances we have species with 

 sedentary parasitic females and free-living males (or now 

 and again the reverse may occur), while in others both sexes 

 may be parasitic. An asexual reproductive phase may 

 alternate with the normal sexual manner of reproduction, 

 and so on. Nowhere are the many different possibilities 

 better illustrated than among Nematodes. There is in fact 



