CRUSTACEANS. 153 



stock of shelly matter having been for some time accumulating 

 in its stomach, in the form of two hard balls, commonly called 

 crafts-eyes. This substance is supposed to be taken up and distri- 

 buted to the surface, so that when the new crust has again acquired 

 consistence, these concretions are no longer found. The whole pro- 

 cess occupies from one to three days. The supposition that the 

 moulting in these aifimals takes place every year must probably 

 be restricted to the period of their growth, beyond which the 

 change of shell would seem to be unnecessary. A specimen of 

 the common crab has been taken, the shell of which was covered 





. 



FIG. 157. CRAB COVERED WITH OYSTERS. 



with oysters of six years' growth, besides Actinice and other para- 

 sites. The crab was full grown and in perfect health, and it is 

 clear that it could not have cast its shell for six years previously. 



The external forms and structure of the limbs of Crustaceans 

 are infinitely diversified ; but as they are all constructed in accor- 

 dance with the same plan, we will select the lobster for special 

 description, as illustrative of the entire group. 



On examining a lobster with a little attention, it will be seen 

 that its head is furnished with four antennae, a circumstance 

 which is distinctive of the class. Its eyes are compound, like 

 those of an insect, and are supported upon a pair of moveable 

 pedicles.- There are five pairs of jointed limbs placed on each 



