PARASITISM AND DEGENERATION 349 



of them are very injurious, sucking the blood from the tissues 

 of the host, while others feed solely on the partly digested 

 food. There are also parasites that live partly within and 

 partly on the outside of the body, like the Sacculina, which 

 lives on various kinds of crabs. The body of the Sacculina 

 consists of a soft sac which lies on the outside of the crab's 

 body, and of a number of long, slender rootlike processes 

 which penetrate deeply into the crab's body, and take up 

 nourishment from within. The Sacculina is itself a crus- 

 tacean or crablike creature. The classification of parasites 

 as external and internal is purely arbitrary, but it is often a 

 matter of convenience. 



Some parasites live for their whole lifetime on or in the 

 body of the host, as is the case with the bird lice. Their eggs 

 are laid on the feathers of the bird host ; the young when hatched 

 remain on the bird during growth and development, and the 

 adults only rarely leave the body, usually never. These may 

 be called permanent parasites. On the other hand, fleas leap 

 off or on a dog apparently as caprice dictates; or, as in other 

 cases, the parasite may pass some definite part of its life as a 

 free nonparasitic organism, attaching itself, after development, 

 to some animal, and remaining there for the rest of its life. 

 These parasites may be called temporary parasites. But this 

 grouping or classification, like that of the external and internal 

 parasites, is simply a matter of convenience, and does not 

 indicate at all any blood relationship among the members of 

 any one group. 



Some parasites are so specialized in habit and structure that 

 they are wholly unable to go through their life history, or to 

 maintain themselves, except in a single fixed way. They are 

 dependent wholly on one particular kind of host, or on a par- 

 ticular series of hosts, part of their life being passed in one and 

 another part in one or more other so-called intermediate hosts. 

 These parasitic species are called obligate parasites, while 

 others with less definite, more flexible requirements in regard 

 to their mode of development and life are called facultative 

 parasites. These latter may indeed be able to go through life 

 as free-living, nonparasitic animals, although, with oppor- 

 tunity, they live parasitically. 



In nearly all cases the body of a parasite is simpler in 

 structure than the body of other animals which are closely 



