INTKODUCTION. XIX 



cature of the walls dipping into the thoracic cavity, and 

 filled by the muscles which move the limbs. 



The abdomen of the Brachyura is very moderately de- 

 veloped.* It folds entirely underneath the thorax, against 

 which it is ordinarily closely applied. It consists, essen- 

 tially, of seven segments, of which, however, in many 

 cases, a greater or less number are so united as to be 

 scarcely distinguishable. In the Macroura* they are far 

 more extended, and serve the purposes of locomotion, 

 being elongated, very moveable upon each other, and 

 furnished at the extremity with a fan-shaped fin, formed 

 of five pieces, of which the centre is the terminal ab- 

 dominal segment. 



In the lower forms, as the Edriophthalma, the rings of 

 the body are more similar to each other, and constitute a 

 nearly regular series of more or less perfect rings. Those 

 of the head, however, are ordinarily much condensed, and 

 soldered together; whilst the thorax consists of seven 

 very distinct nioveable segments, and the abdomen of 

 either the same number, or nearly so ; as in some cases 

 the seventh is wanting, and in others the two anterior 

 ones are united. 



Between these two extreme cases, there are numerous 

 intermediate modifications, which will be seen in the 

 various families and genera. 



The members or appendages to the different segments 

 or annuli above described, form a very interesting and 

 important part of the tegumentary system of these ani- 

 mals. Theoretically speaking, every segment has its pair 

 of appendages, and, vice versa, each pair of appendages, 



* See the figures of the various species. 



b 2 



