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 mav 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, 
