80 REPORT— 1876. 



In this genus the sternal plate, as an anatomical part of the animal, is 

 wanting, or represented only in a theoretical character by the median line 

 of fusion. 



The coxae are existent without fusion with each other for some extent, 

 visible on the ventral surface before their close contact reaches ossification so 

 perfect that their line of union is represented by marks of depression only 

 on the external surface, and corresponding crests or ridges on the internal 

 surface. Dorsally this appears to be similarly repeated, and the lines of con- 

 tact are imperfect in their fusion until the plates have thinned out into a 

 membrane. Laterally the walls of the coxoe of the several pairs of appen- 

 dages are so closely compressed that their lines of union are with difficulty 

 determined not to be fused together. That they exist for some distance as 

 thin plates in close contact is certain ; but they ultimately reach a point 

 where the distinction is lost in perfect ossification. The internal plates ap- 

 proach the corresponding ones on the opposite side in the first two somites 

 only, which form a bridge that supports the posterior extremity of the stomachic 

 region ; behind this the ventral surface rapidly widens, but the apodema or 

 internal plates abruptly terminate, leaving a large expansion for the internal 

 viscera to occupy. 



In the Brachyura the central fusion of the sternal plates is still more perfect, 

 and the ventral portion of the somites appears to be covered entirely ; this 

 exists in a vertical plate that appears to be formed by being compressed 

 between the coxee of the corresponding pairs of appendages, the external 

 surface of which may be traced to a sinus (PI. II. fig. Grt) that opens in the 

 median line between the third and fourth somites. The segments of the 

 pereion in this order of Crustacea, as may be seen in the genus Cancer, 

 are very closely compressed, and apparently overlap each other dorsally, 

 while ventrally the several appendages, from their proportionate dimensions, 

 preclude the possibility of too close a contact. The consequence is that the 

 general arrangement of the entire muscular system that moves the appen- 

 dages or the pereion, together with the osseous structure that supports them, 

 is arranged in a circular fonn, the superior or extensor muscles forming the 

 upper or dorsal arc, and the inferior or flexor muscles forming the lower or 

 ventral arc. The plate, therefore, that is produced internally in the median line 

 is in continuation with the anterior portion of the ventral floor of the pere- 

 ion, and is the homologue of the sternal plate. This tendency of the muscles 

 to form round a common osseoiis centre appears to give a similar relation of 

 the several somites to one another. Thus we find that the apodema narrows 

 the dorsal extremity corresponding to each somite to such a degree that a 

 deep notch or fold takes place over the fourth pair of appendages, at which 

 point the curvature is greatest (fig. 5). It is this circular portion of the 

 muscles that facilitates that peculiar arrangement by which the posterior 

 two pairs of legs in Dromia, Boripe, &c. appear to be attached to the dorsal 

 surface of the animal, which enables them to adhere to floating pieces of 

 wood or weed, or seci:rely attach themselves to univalve shells by means of 

 these appendages. 



The pleon, or that portion of the animal to which the appendages are 

 attached which, in their most perfect condition, are adapted for swimming, 

 undergoes a great variety of forms. It is perhaps most perfectly developed, 

 in accordance with the value and usefulness of its parts, in the Macrurous 

 division of Crustacea. 



In the Edriophthalmia it is perhaps more simple in character ; but it is in 

 the Anisopoda, or that intermediate stage that unites the Isopoda and the 



