Xll INTRODUCTION. 



as in the lobster itself, yet it would appear to bear a 

 near relation to their habits ; the presence of the cal- 

 careous substance hardening and solidifying the skeleton, 

 and thus rendering it an efficient protection against the 

 rocks and waves of the more exposed parts of the sea, be- 

 ing found in the greatest proportion in species exposed to 

 such agents; whilst the others are either small, active 

 creatures, swimming with great ease and constancy in 

 more open and shallow situations, or creeping safely 

 amongst fuci or under stones, and other protecting sub- 

 stances, or even attaching themselves to the surface of 

 different species of fish. 



The annulose character, typical of the great group to 

 which it gives its designation, has, in a great number of 

 the species composing this class, reached its maximum of 

 development. The segments which surround the body are 

 more complete, and more separately movable, whilst they 

 possess a greater degree of individual solidity than in any 

 others. They are also furnished with articulated appen- 

 dages ; each segment, whether remaining distinct or in- 

 timately united to others, bearing a single pair, in a more 

 or less developed, or in a merely rudimentary condition. 

 In numerous instances, from this intimate union or solder- 

 ing together of two or more segments, the only indi- 

 cation of their theoretical separate existence is the presence 

 of the normal number of these appendages ; but with 

 this aid it rarely happens, in the higher forms of Crus- 

 tacea, that they cannot be proved to exist. 



The true normal number of the segments, taking the 

 whole class, appears to be twenty-one, of which, accord- 

 ing to our present knowledge, seven must be considered as 

 belonging to the head, and an equal number respectively 

 to the thorax and the abdomen. Now, although it is 



