CRUSTACEANS 245 



across the arms, and is said to reach even greater proportions. 

 Crustaceans live on land and sea, and in both fresh and salt water ; 

 they may be parasitic, sedentary, or free and active. There are 

 said to be over ten thousand species, which include crabs, lobsters, 

 shrimps, beach-fleas, wood-lice, barnacles, and water-fleas. Nine 

 tenths of the species are marine; of these some are pelagic, 

 and their transparent forms constitute a part of the plankton 

 Others live on the bottom in deep water and attain a large size. 

 A vast number live in the littoral zone and form a considerable 

 part of that crowded community. They are scavengers and free- 

 booters, being great fighters, as well as predatory in their habits. 

 As a rule, they feed on dead organic matter, which they consume 

 in surprising quantities ; some have also a vegetarian diet. They 

 have, like starfishes, the singular power of throwing off their 

 limbs and growing new ones. 



The Crustacea derive their name from the nature of their crust 

 or covering. This is earthy and brittle, not stony, like the shells 

 of mollusks. This crust, or exoskeleton, has various degrees of 

 firmness, from that of a delicate polished cuticle, seen in small 

 forms, to the heavy armor of some crabs and the shell-like char- 

 acter of the covering of barnacles. 



The lobster and crab are perhaps the most comprehensive 

 examples as well as the largest in size of the class, the vast 

 majority of the species being minute forms. The ocean swarms 

 with varieties too small to be noticeable to the naked eye. 



It is the province of Crustacea to consume decaying organic 

 matter, both animal and vegetable, and in doing this they per- 

 form a great service in purifying the waters of the pool, the shore, 

 and even the sea. Not only in this are they serviceable, but to ?., 

 great extent they serve as food for other animals, their flesh being 

 palatable and preferred to that of other animals whose diet would 

 seem to make them more wholesome. The small forms exist in 

 such myriads, and increase so rapidly, that, although extremely 

 small, they furnish an inexhaustible supply of food. Even some 

 whales subsist upon these minute animals, and for man the larger 

 species are articles of diet that are counted among the luxuries. 

 The crustaceans are remarkable for their varied forms and for their 



