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CLASS IV. CRUSTACEA. 



(CRABS.) 



NUMEROUS limbs formed of distinct joints are for the 

 first time met with in the animals now before us ; 

 they are arranged in pairs along the sides of the 

 body, but on the inferior or belly-surface. The body 

 is itself also divided into rings or segments, which 

 fit into one another, less numerous, but more dis- 

 tinctly marked, than those of the ANNELIDA. These 

 segments are, theoretically, twenty-one in number ; 

 but in every known species some or other of them 

 are so united, soldered as it were, to their fellow 

 segments, as to be undistinguishable ; and there- 

 fore it is only by an examination of a great variety 

 of forms, that the total number can be recognised. 

 The principal divisions are those of the head, 

 thorax, and abdomen, each of which contains seven 

 segments ; but those of the head are usually much 

 crowded and very minute ; those of the thorax are 

 more or less covered above by a great shield, called 

 the carapace, and are visible only beneath; but 

 those of the abdomen are generally well marked. 



Each segment is furnished with a pair of appen- 

 dages, more or less developed ; and the presence 

 of these is often sufficient to indicate a segment, 

 though no trace of division can be detected in the 

 body. The appendages take different forms in 

 different parts of the body; consisting of 1. the 



