Ti 



CHAPTER 6 



THE ORIGINS OF PARASITISM AND THE 

 EVOLUTION OF PARASITES 



How Mutability in them doth play 

 Her cruel sports . . . 



Edmund Spenser 



I HERE ARE no parasitic starfish or lampshells but in all the other 

 large groups we find animals which have abandoned a firee life in 

 favour of parasitism. Zoologists have Httle, if any doubt that all para- 

 sites have evolved from free-living animals. Many such organisms are, 

 of course, at different stages of evolution, some being much farther 

 removed structurally from their ancestral stock than others and 

 consequently resembling them less and less. Before the larval stages 

 of these animals were known their origins in many cases remained 

 obscure. Now that the life-cycles have been worked out their past has 

 been revealed. Thus, for example, the dog whelk has a free-swimming 

 larva known as a veliger and so has the worm-like, shell-less, footless, 

 colourless, toothless gastropod mollusc {Entoconcha mirabilis) which lives 

 as an internal parasite of sea cucumbers (Holothurians) . Fish-hce, too, 

 some of which as adults resemble Httle bags of blood fixed to the gills ol 

 fish, have an active swimming larva very similar to the larva of the 

 free-living copepods, which swarm in the sea. An endo-parasite 

 [Sacculina] which resembles a mass of roots ramifying through the inter- 

 nal organs of crabs, has a free-swimming larva which instantly reveals 

 its true nature and places it among the barnacles (Cirripedia). 



These are extreme cases. Most parasites are not modified beyond 

 recognition. As we have seen in the foregoing chapter certain struc- 

 tural alterations are associated with the parasitic mode of life, but we 

 can still find in most of them a well-marked resemblance to their free- 

 living relatives. There are, however, no free-living tapeworms or 



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