348 EVOLUTION AND ANIMAL LIFE 



dition of an internal parasite than any other vertebrate. And 

 what about the flea? In its immature life it lives as a white 

 grub or larva in the dust of cracks and crevices, of floors and 

 cellars and heaps of debris; here it pupates, and finally changes 

 into the active leaping blood-sucking adult which finds its way 

 to the body of some mammal and clings there sucking blood. 

 But it can jump off and hunt other prey; it leaves the host body 

 entirely to lay its eggs, and yet it feeds as a parasite, at least 

 it conforms to the definition of parasite in the essential fact of 



FIG. 209. At the left, the red-tailed trichina fly, Winthemia, h-pastulata, the parasite 

 of the army worm, Leucania unipuncta ; at the right, the worms upon which the 

 fly has laid eggs. (After Slingerland.) 



being carried about on or in the host body, while feeding at the 

 host's expense. 



It is of course not particularly important that we distinguish 

 sharply between parasitic and predaceous animals, but as we 

 look on the degeneration of parasitic animals as the result of 

 their special habit of life, we must attempt a sort of classifica- 

 tion of the phases or degrees of parasitism, in order to asso- 

 ciate with them corresponding categories of degeneration. 



The bird lice (Mallophaga), which infest the bodies of all 

 kinds of birds and are found especially abundant on domestic 

 fowls, live upon the outside of the bodies of their hosts, feeding 

 upon the feathers and dermal scales. They are examples of ex- 

 ternal parasites. Other examples are fleas and ticks, and the 

 crustaceans called fish lice and whale lice, which are attached 

 to marine animals. On the other hand, almost all animals 

 are infested by certain parasitic worms which live in the ali- 

 mentary canal, like the tapeworm, or imbedded in the muscles, 

 like the trichina. These are examples of internal parasites. 

 Such parasites belong mostly to the class of worms, and some 



