16 



THE BRACHYURA. 



I, The Beachyuea in General. 



1. Of the General Form of the Body in the Bmchyura. 



In the Crabs, as in Nephrops, the body is divided into two regions — a 

 cephalothorax consisting of 14 somites fused together, and an abdomen consist- 

 ing of 7 ; but the abdomen of the crab is very much smaller than the cephalo- 

 thorax, beneath which it is usually folded out of sight, being often, in fact, in 

 the male little more than a rudiment. 



As in Nephrops, the cephalothorax is covered by a large overlapping cara- 

 pace, which, however, is usually broader than long. 



Moreover, although the abdomen as a whole is moveable, yet it is very 

 common to find some of its somites fused together : the abdominal terga do not 

 overlap one another, hardly ever are any of the sterna calcified, and hardly ever 

 are there recognizable pleura. 



There are certain undoubted crabs (e.g., the Raninidm) in' which the cephalothorax is much 

 longer than broad and the abdomen, though reduced in size, is by no means folded out of sight. 



Moreover there are other Deoapoda besides crabs {e.g , the Lithodidse &nd Porcellanidse) in 

 which the carapace may be broader than long and the abdomen is folded beneath it. 



In the great majority of crabs the anterior end of the carapace is not pro- 

 longed to form a rostrum, but is, on the contrary, truucated and often deflexed. 

 As a result of this the eyestalks, instead of lying close together in the longi- 

 tudinal axis of the body, are forced asunder and pushed back, until they lie 

 almost in the transverse axis of the body. A similar but less marked change 

 of position is usually seen in the antennules and antennge. 



Moreover, both the eyes and the antennules become more or less enfolded 

 or embedded in pits. When these pits are shallow and the second joint of the 

 antennal peduncle remains free, as in the Dromides, we find a pair of common 

 orbito-antonuular fossee into which the eyes and antennules are, or are not, 

 completely retractile. But when, as in most crabs, the second joint of the 

 antennal peduncle becomes more or less fixed, we find distinct antennular fossar 

 into which the antennules can bo folded, and distinct orhits for the eyes, the 

 second joint of the antennal peduncle forming the partition between the two 

 cavities. 



There are numerous crabs (e.g , the Oxyrhyncha) in which tlie carapace is produced anteriorly 

 to form a rostrum, but in this case the rostrum always sends down from its under surface a 

 process that separates the antennules from one another. 



Again tliere are many crabs, besides the Uomolidea, in which the eyes are not concealed in 

 orbits, and there are some ci'abs in whieli the eyes, though broadly separated from one another, 

 . are not pushed out at right angles to the sagittal axis of the body. 



