THE CRABS. 275 



its corneal surface alone being visible, as it projects into 

 the orbit. 



Again, the ischiopodites of the external maxillipedes 

 are expanded into broad quadrate plates, which meet in 

 the middle line, and close over the other manducatory 

 organs, Hke two folding-doors set in a square doorwaj^ 

 Behind these there are great chelate forceps, as in the 

 crayfish; but the succeeding four pairs of ambulatory 

 limbs are terminated by simple claws. 



When the abdomen is forcibly turned back, its sternal 

 surface is seen to be soft and membranous. There are no 

 swimmerets ; but, in the female, the four anterior pairs 

 of abdominal limbs are represented by singular appen- 

 dages, which give attachment to the eggs ; while in the 

 males there are two pairs of stjdiform organs attached 

 to the first and second somites of the abdomen, which 

 correspond with those of the male crayfishes. 



The ventral portions of the branchiostegites are 

 sharply bent inwards, and their edges are so closely 

 appKed throughout the greater part of their length to 

 the bases of the ambulatory Hmbs, that no branchial 

 cleft is left. In front of the bases of the forceps, how- 

 ever, there is an elongated aperture, which can be shut 

 or opened by a sort of valve, connected with the external 

 maxillij)ede, which serves for the entrance of water into 

 the branchial cavity. The water employed in respiration, 

 and kept in constant motion by the action of the sca- 

 phognathite, is baled out through two apertures, which 



