180 ANIMAL LIFE 



94. Kinds of parasitism. The bird-lice (Mallophaga), 

 which infest the bodies of all kinds of birds and are found 

 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 ani- 

 mals. 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 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 root-like processes 

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

 nourishment from within. The Sacculina is itself a crus- 

 tacean or crab-like creature. The classification of para- 

 sites 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, 



