204 CRUSTACEA. 



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 covkred AvirH ov^teij5. 



with oysters of six years' growth, besides Actinice and 

 other parasites. 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 accordance with the same plan, we 

 will select the lobster for special description, as illus- 

 trative 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 

 side of the mouth, which are evidently adapted to 

 assist in seizing and conveying into the mouth the 

 substances used as food. 



