1128 



CRUSTACEA. 



Posterior feet. — The posterior pair of thoracic feet in the female is 

 usually quite small or nearly obsolete and naked, though occasionally 

 approaching the size of the first pair of natatories. The two are often 

 a little unequal, with slight differences at times in the terminal spine 

 or seta, but without anything very marked in form. 



In the males, the left is small, something like those of the female. 

 The right terminates in a large cheliform hand. The carpus is a 

 stout oblong joint. The hand is large, and of a very different shape 

 in different species. It is articulated with the inner apex of the 

 carpus, and when at rest, it folds back against the inner margin, so 

 as to lie between the carpus and the left leg. The basal extremity is 

 prolonged outward into a spine, or bears a spinous process, which 

 answers to a thumb or immoveable finger; it is short, or very long, 

 according to the species. At the opposite extremity there is an 

 oblong finger, sometimes very stout, sometimes like a long claw, and 

 sometimes having a spoon-shaped or spatulate extremity. It closes 

 against the thumb, and between the two when closed together there 

 is usually a space of considerable size. The inner margin of the 

 finger, and also of the hand, is at times furnished with one or two 

 setae or spines, or with short villi (Plate 82, fig. 6/). 



Abdomen. — The abdomen has from two to five segments. The 

 number is not constant in the same species. It is probable, that it 

 increases with the later metamorphoses, as we have observed only two 

 segments in smaller individuals of a species, in which those of full 

 size had four. The relative sizes also vary. In some species the 

 female has but two or three joints, when the male has four or five; and 

 the form of the abdomen in the female may be gibbous or ovoid, when 

 it is regularly terete or decreasing in the male. Five is a common 

 number for adult males, and is not frequent in females. 



It is often the case also that the abdomen is distorted. Sometimes 

 one of the joints is widened on one side by a process, and yet this 

 character is confined to a few individuals of a species. This takes 

 place usually on the right side, though not always so ; and it is observed 

 in both sexes. When there are five joints, it is commonly the third 

 joint that is modified. Sometimes the abdomen is gibbous above, and 

 occasionally so below. 



On account of these strange variations, it is difficult to draw satis- 

 factory specific distinctions from the abdomen. Yet, when we are 



