4i 6 ELEMENTARY ZOOLOGY 



but suffers corresponding disadvantage, often even the loss 

 of life itself. Parasitism is a phenomenon common in 

 all the large groups of animals, though the parasites 

 themselves are mostly invertebrates. There are parasitic 

 Protozoa, worms, crustaceans, insects, and molluscs, and 

 a few vertebrates. 



Some parasites, like the fleas and lice, live on the sur- 

 face of the body of the host. These are called external 

 parasites. Others, as the tapeworms, live exclusively 

 inside the body; such are called internal parasites. 

 Again, some, as the bird-lice, which are external parasites 

 feeding on the feathers of birds, spend their whole life- 

 time on the host; they are called permanent parasites. 

 Others, as a flea, which leaps on or off its host as caprice 

 directs, or like certain parasites which as young live free 

 and active lives, finally attaching themselves to some host 

 and remaining fixed there for the rest of their lives, are 

 called temporary parasites. Such a grouping is purely 

 arbitrary and exists simply for the sake of convenience. 

 It is not rigid, nor does it class parasites in their proper 

 natural groups. 



When various parasites are examined it will be noted 

 that practically in all cases the body of a parasite is 

 simpler in structure than the body of other animals closely 

 related to it; that is, species which live parasitically, 

 obtaining their food from and being carried about by a 

 host, have simpler bodies than related forms that live free 

 active lives, competing for food with other animals about 

 them. This simplicity is not primitive, but results from 

 the loss or atrophy of the structures which the special 

 mode of life of the parasite renders useless. Many para- 

 sites are attached firmly by hooks or suckers to their host, 

 and do not move about independently of it. They have 

 no need of the power of locomotion, and accordingly are 

 usually without wings, legs, or other locomotory organs. 



