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 antennae 



