CHAPTER XIX 

 ANIMAL PARASITES 



An animal parasite is an animal which lives and feeds 

 for all or part of its life on or in the body of another 

 which is called the host. Fleas, dogticks, and lice are 

 familiar parasites ; they are not very pleasant to think 

 about perhaps, but their mode of life is interesting be- 

 cause it presents one way of getting a living which has 

 been adopted by many different kinds of animals, and 

 which always results in a more or less marked change in 

 their structure. This change usually involves the loss 

 or imperfect development of some part of the body. 



Degeneration of parasites. — Fleas and lice are insects, 

 but, unlike most of their kind, they have no wings. 

 Being carried about by the host they do not need to fly. 

 One of the most striking examples of loss of parts due to 

 a parasitic habit is shown by an animal called Sacculina 

 (fig. 231), which belongs to the crab and crayfish group. 

 The young Sacculina, hatched from eggs laid in ocean 

 tide-pools, has legs and eyes and a mouth and feelers, 

 and can swim actively about. It looks much like a 

 young crab or prawn. But after a short period of free 

 active life it finds a full-grown crab and attaches itself 

 to its body. There grow out from the Sacculina and 

 penetrate the body of the crab slender root-like processes 

 by means of which the parasite sucks up the juices of its 

 host. Soon it moults and loses its legs, eyes, and feel- 

 ers; it is now simply a pulsating tumor-like sac fastened 



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