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, like two folding-doors set in a square doorway. 
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 styliform 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 
applied throughout the greater part of their length to 
the bases of the ambulatory limbs, 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 
maxillipede, 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 
T2 
