5 8 CRUSTACEA. 



closely together than those of the other Crustacea 

 they have studied, li they dissect the crab, they find 

 a great concentration of the internal organs, especially 

 of the nervous system. Instead of many ganglia scat- 

 tered along a nerve-cord, all the ganglia behind the 

 mouth in the crab have coalesced to form a single 

 mass. Thus it is seen that not only are the two regions 

 of the body concentrated in the crab as in the lobster, 

 but the third or abdominal region is doubled up under 

 the cephalothorax, so that, when looked at from above, 

 the crab appears to be merely a perambulatory cephalic 

 shield. 



The crab moves sidewise, and when cornered faces 

 the enemy with its two arms thrown up, and ready for 

 action. The jaws are used, not only for capturing, 

 killing, and crushing the prey, but also perform the 

 additional labor of holding it up to the mandibles and 

 helping them to tear off pieces of suitable size for 

 chewing, thus taking the work done in the lobster bv 

 both the arms and the third pair of maxillipeds. They 

 are, therefore, not twisted so much as in the lobster, 

 but work vertically, and the arms are not too long, 

 being able to reach the mouth by a sharp bend. The 

 legs of one side are used to push with, and those 

 of the other to pull with, when the crab is in mo- 

 tion. Those of the same side do not, however, all 

 move together, but alternately, so that there is no 

 halting in their gait ; some of the legs are always in 

 the act of taking new steps, and by shoving and pulling 

 in unison a continuous motion is kept Up. This crawl- 

 ing by means of jointed appendages can be imitated 

 after having once seen a live crab. Cross the two 



