The King Crab. 113 



THE KING CRAB 



(Limulus polyphemus) . 



External Anatomy. Note, on examining the animal 

 from above, that the chitinous shell is divisible into a large 

 anteriorly lying cephalothorax and a smaller, polygonal ab- 

 domen, terminated posteriorly by a rigid caudal spine. 



The cephalothorax bears a pair of lateral, compound eyes, 

 and a pair of median ocelli. The latter occupy positions, 

 one on each side of the frontal spine. 



Is there anything on the dorsal surface of the abdomen 

 that would show that it might possibly be made up of sev- 

 eral fused segments or mctameres ? How many movable 

 spines are there along the margin? How many pairs of pits 

 each side of the median line ? 



Draw. 



Examining the lower, concave surface of the specimen, 

 observe the seven pairs of cephalothoracic limbs. The first 

 (chelicerce} , smaller than the remaining, are followed in 

 immature specimens and in females by four pairs of pincer- 

 bearing legs. Do the pincers open and close as in Homa- 

 rus ? The sixth pair of appendages are directed posteriorly. 

 Have they anything that might be of assistance in forcing 

 the animal through the mud? Note the exopodite-like 

 process at the base of each. 



The chilaria are supposed by some to represent a pair 



