254 DIVISION III. ARTICULATED ANIMALS. — CLASS II. CRUSTACEA. 
as to prevent them from collapsing after a long penury of water, and thus 
completely stopping the circulation. ; 
While in fishes the water that serves for respiration flows from the front 
backward, so as not to impede their motions, the stream of water traversing 
ill of the Crustaceans is made to flow from behind forward, and thus 
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harmonizes perfectly with their retrograde movements. So wonderfully has 
the anatomical structure of these animals, like that of all other living things, 
been suited to their peculiar mode of life. 
All Crustaceans, however different their external aspect may be, are 
formed according to the same plan or fundamental idea. Among the lower 
orders the body consists of a number of almost equal-sized rings, each 
furnished with a pair of crawling or swimming legs. But as we ascend in 
the scale, we find the rings coalescing more or less to larger pieces, particu- 
larly in the crabs, whose broad, chalky carapace indicates its compound 
nature only by the number of pairs of legs which rise from its lower 
surface. 
ORDER I. DECAPODA (‘Ten-footed). 
The animals of this order have a shell or covering, which envelops the 
body, limbs, and head, the latter of which is fixed compactly to the thorax. 
They are slow of growth, and of long life. Some of the species attain the 
leneth of a foot. “Their claws, as is well known, are extremely powerful. 
They ordinarily reside in the water, but are not immediately killed by being 
removed into the air: indeed, some species pass a considerable part of their 
existence out of the water, which they only seek in order to deposit their 
eggs in it. They are, nevertheless, compelled to reside in damp situations 
and burrows. They are naturally voracious and carnivorous : some species, 
indeed, are said to frequent the cemeteries in order to feed upon dead bodies. 
Their limbs are renewed, when injured, with great quickness ; but it is neces- 
sary that the fracture should have been made at the junction of the joints : 
they, however, have the instinct to effect this if the wound has been of a 
different nature. When desirous to change their skins, they seek for some 
retired spot, where they may be at rest and secure from their enemies. The 
moulting then takes place, the body being at first soft and of a delicate 
flavor, as in the ease of the Black Crab of the West Indies, which is kept 
in cages expressly for the table. The chemical analysis of the old shell 
proves that it is formed of carbonate of lime and phosphate of lime in differ- 
ent proportions. Dy the action of the heat the epidermis assumes a bright- 
red color, the coloring principle being decomposed by the action of boiling 
water.” 
