CRUSTACEANS. 309 
of one ring encroaching on the others which are still 
there, although of very small size. We see here the 
same disposition to have a general plan that we see ey- 
ery where in the structures of nature. <A type is always 
adopted, and we see traces of this in the widest varia- 
tions from it. 
527. The covering of the Crustacea, which is their 
skeleton, is commonly quite hard; being made so by the 
carbonate of lime, of which it is in part composed. As 
this can not grow with the other parts, it must be shed 
from time to time, and a new and larger covering be 
formed. The manner in which the old shell is got rid of 
is very singular. At the proper time there is effected a 
separation between all parts of the animal and the shell. 
Then the shell gapes open at some part, and the animal 
works itself out. This opening, in the case of the Lob- 
ster, is down through the middle line of the back. The 
animal, on emerging, crawls into some by-place where it 
may be secure, and remains quiet for a day or two till a 
new shell is formed. The material is supplied from the 
blood, just as the material for our internal skeleton is 
supplied from our blood. 
528. The Crustacea are divided into fourteen orders. 
Of these I will notice only a few. 
529. The Decapoda, or Ten-footed Crustacea, include 
the Lobsters, Crabs, Crayfish, Prawns, Shrimps, etc. 
Nearly all the Crustacea that are used as food are con 
tained in this order. One marked peculiarity of this 
group is the situation of the eyes on the ends of foot- 
stalks. The habits of most of these animals are aquatic ; 
but the gills are inclosed in such a way that they do not 
soon become dry when the animals are in the air, and 
hence they live for some time after being taken out of 
the water. They are carnivorous and very voracious ; 
and the first pair of legs are made into powerful claws, 
by which they seize their food and convey it to the 
mouth. The mouth itself is quite a complicated appara- 
