BRACHYURA. 



segment of -the thorax is universal. But in a large portion of the 

 species, the verges proceed directly from the base of the posterior 

 legs; while in another large group, they pass from the sternum inside 

 of the base of the legs. This important distinction is well used in the 

 classification of Crustacea by Milne Edwards, and is neglected in that 

 by De Haan. In fact, the position of the verges is nearly the same 

 in all, as regards distance from the medial line; and the difference in 

 the external position arises from the greater or less width of the 

 sternum behind, which in the Grapsoidea throws the posterior legs 

 farther from the medial line. The female vulvae are situated in the 

 sternal plate. 



Passing from the branchial system, we next appeal to the organs of 

 ihe semes for distinctions. But here the differences among different 

 groups are mostly small. The eyes may be longer or shorter pedun- 

 culate ; but mere variations in length of peduncle is a character of a 

 low grade. They may arise from near the medial line of the body, or 

 remote from it, — sometimes a generic distinction and rarely of higher 

 value. They may have distinct orbits for retraction, or be without 

 such orbits, — a characteristic of considerable importance. They may 

 have the eye at the extremity of the peduncle, or the peduncle pro- 

 duced beyond the eye, — a striking instance of an anomaly which is 

 only trivial in value. 



The two pairs of antennce are organs of high rank, and afford impor- 

 tant distinctions, as would be gathered from the remarks on the 

 degradation of species, on a preceding page. The condition of these 

 organs is one of the most prominent marks of grade or rank. Yet it 

 should be noted, that in each of the several divisions of the Brachyura, 

 similar variations of grade exist, and the characteristics they afford are 

 not superior to those of general structure. 



The inner antennm may be longitudinal or transverse. When the 

 former, it is owing to the narrowness of the space between the eyes, and 

 the closer approximation of all the organs of sense, and as already ex- 

 plained, this position is often a mark of the higher grade, it charac- 

 terizing preeminently the Maia group. 



The outer antennce vary in position with reference to the eyes, and 

 also in the condition of the basal joint. They are sometimes so situated 

 that when the animal is in its natural position, the first basal joint 

 is directly beneath the eye and extends forward beyond it. Such 

 species — the Maioidea — have the front narrow, and the basal joint is 

 soldered firm to the shell outside. Combined with the form, it is an 



