THE LOBSTER AS A TYPE OF CRUSTACEA 15 



first small and incomplete^ formed, the second in 

 the form of a short cylinder, having the eye at 

 its end. There are, however, reasons for doubting 

 whether the eye-stalks are really appendages. 



The hard outer covering of the Lobster not only 

 protects and gives support to the internal organs, 

 but also affords points of attachment for the muscles 

 by means of which the animal moves. In other 

 words, it plays the part of a skeleton ; but since, 

 unlike the skeleton of vertebrate animals, it is outside 

 instead of inside the soft parts of the body, it is 

 known as an exoskeleton. Closer examination shows 

 that this outer covering is really continuous over the 

 whole of the body and limbs, but is thin and soft 

 at the joints, allowing the parts to move one upon 

 another. It is composed of a horn-like substance 

 known as chitin, which, except at the joints, is 

 hardened by the deposition in it of carbonate and 

 other salts of lime. 



As this external covering does not increase in size 

 after it has been formed, and as it cannot stretch to 

 any great extent, the Lobster requires to cast its 

 shell at intervals as it grows. In this process of 

 moulting the integument of the back splits between 

 the carapace and the first abdominal somite. The 

 body and limbs are gradually worked loose and 

 withdrawn through the opening, leaving the cast 

 shell with all its appendages almost entire. The 

 new covering, which had been formed underneath the 



