78 The Under-Water World 



visible to the naked eye being com- 

 paratively few and far between. 



Crustaceans are enclosed in a crust or 

 shell composed of a tough elastic sub- 

 stance allied to horn and often 

 strengthened by carbonate of lime. It 

 does not grow with the animal and has 

 consequently to be cast periodically at 

 frequent intervals in infancy and more 

 rarely as maturity approaches. When a 

 lobster moults, the fleshy parts of the 

 animal are withdrawn through a crack 

 in the middle of the carapace. The 

 operation is a lengthy and tiresome one, 

 since the large claws, the head including 

 the eyes, the body and even the lining of 

 the stomach must all be drawn out of 

 their cases. The new " shell/' which is 

 always in the process of forming beneath 

 the suit actually in use, hardens after 

 about a week's exposure to the water. 

 The power of regeneration of lost parts 

 is common to all crustaceans, damaged 



