THE BRACHYURA. 295 



inof nine on each side, and sometimes not more than seven. 

 The branchiostegite fits closely down upon the bases of the 

 four posterior pairs of thoracic limbs, and sometimes incloses 

 a space which is very large in proportion to the branchiae 

 This is particularly the case in the Land Crabs (Gecarcinns), 

 where the spacious branchial chamber is lined by a thick and 

 vascular membrane, which, in these almost wholly terrestrial 



7 7 J 



Crustacea, either takes on to some extent the respiratory 

 function, or serves to keep the air within the branchial cham- 

 ber saturated with moisture. 



The abdomen in the Brachyura is comparatively small ; 

 its sixth somite possesses no appendages ; and the others, if 

 they exist at all, subserve only a sexual purpose, the two an- 

 terior pairs commonly forming accessory copulatory organs 

 in the male ; while, in the female, so many of these append- 

 ages as remain give attachment to the ova, which are carried 

 about until hatched, between the thorax and the abdomen, 

 which is bent up against it. The female Brachyura also pos- 

 sess a spermatheca attached to each oviduct, which is absent 

 in the Macrura / and, in this sex, the abdomen is larger and 

 broader than in the males. In accordance with the rudimen- 

 tary condition of this part of the body, the abdominal gan- 

 glia are represented only by a cord, which proceeds from the 

 posterior part of the great thoracic mass. It is in the con- 

 struction of their skeleton, however, that the Brachyura 

 present the most interesting deviations from the Macrura. 

 Thus, if we select the common Shore-crab, Carcinus moenas 

 (Fig. 76), as a typical example of a Brachyuran, we find that 

 the carapace is a wide shield, broader than long, having a 

 somewhat pentagonal shape, and bent sharply inward at the 

 sides, instead of taking an even sweep down to the base of 

 the legs. It is in such close contact with the four posterior 

 pairs of thoracic limbs as to leave no passage or aperture 

 such as exists in Astacus, the only inlet for the water required 

 for respiration being placed above the basal joints of the che- 

 late anterior ambulatory limbs. The edges of the carapace pass 

 completely in front of the basis of the limbs, and then turn sud- 

 denly forward, parallel with one another and with the axis of 

 the body, as the pterygostomial plates of Milne-Edwards, to 

 join the antennary sternum, which is very wide, but short from 

 before backward. The space included between the edges of 

 the pterygostomial plates and the antennary sternum is the 

 "cadre buccal," or peristome; the antennary sternum itself 

 receives, as in the Astacus, the specific appellation of epi- 



