CRUSTACEANS 245 
across the arms, and is said to reach even greater proportions. 
Crustaceans live on land and sed, 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 a 
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 
