ARTHROPODS. CLASS CRUSTACEA 107 
hydroid colonies, with which their bodies harmonize in 
form and color. And, lastly, most bizarre creatures, known 
as “ whale-lice,” attach themselves to the skin of whales, of 
which each species acts as host for one or more kinds. 
106. Internal organization.—Most Crustacea are carnivo- 
rous, preying upon almost any of the smaller animals within 
convenient reach; a much smaller number live on vege- 
table food; and there are many, such as the crayfishes, lob- 
sters, and numerous crabs, which are also notorious scaven- 
gers. In these latter forms the food is held in one of the 
large pincers, torn into shreds by the other, and transferred 
to the mouth-parts, where, as in all Crustacea, it is soon 
reduced to a pulp by their rapid movements. In many 
species the food is now ready for the digestive process, 
but not so in the higher forms. If the stomach of any of 
these, for example, the crabs or crayfishes, be opened, three 
(Fig. 65, s) large teeth operated by powerful muscles will 
be noted, and beyond these a strainer consisting of many 
closely set hairs. In operation this “ gastric mill” takes 
the food passed on from the mouth-parts, and crushes and 
tears it until fine enough to pass through the strainer, 
whereupon it is dissolved by the juices from the liver and 
is absorbed as it passes down the intestine. 
The circulatory system is usually highly developed, and 
consists of a heart, in some species almost as long as the 
body, though usually shorter (Fig. 65), from which two or 
more arteries branch to all parts of the body. Here the 
blood, instead of emptying into definite veins, pours into a 
series of spaces or sinuses in among the muscles and other 
organs of the body, through which it makes its way back to 
the heart. During this return journey it is usually made 
to traverse definite respiratory organs, either situated upon 
the legs or, as feathery outgrowths, upon the sides of the 
body, and generally concealed under the carapace. A por- 
tion of the blood is also continually sent to the kidneys, 
which are located either at the base of the second antenne 
! 
