COMM3NSALISM AND PARASITISM 339 



especially abundant on domestic fowls ? live upon the out- 

 side of the bodies of their hosts, feeding upon the feathers 

 and dermal scales. They are examples of external 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 alimentary 

 canal, like the tape-worm, 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 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 Sac- 

 culina, 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 root- 

 like processes, which penetrate deeply into the crab's body, 

 and take up nourishment from within. The Sacculina is 

 itself a crustacean or crab-like 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 devel- 

 opment, 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 as caprice dictates ; 

 or, as in other cases, the parasite may pass some definite 

 part of its life as a free, non-parasitic 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 or internal parasites, is simply 

 a matter of convenience, and does not indicate at all 



