CEUSTACEANS. 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 ev- 

 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 suoji 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- 



