CHAPTER XXXI 



ANIMAL PARASITES AND DEGEN- 

 ERATION 



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 because 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 para- 

 sitic habit is shown by an animal called Sacculina (fig. 205), 

 which belongs to the crab and crayfish class. 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 feelers; it is now simply a pulsating tumor- 



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