178 



HOMOLOGY 



FIG. 73. The central nervous systems of the lob- 

 ster and the crab. The ventral chain of ganglia in 

 the crab are concentrated in one ventral mass, the 

 sternal artery passing through it. (From Gerstaecker 

 after Cuvier.) 



highly developed, the large ventral muscles 

 of the abdomen being remarkably power- 

 ful. These are inserted anteriorly on 

 the inner walls of the cephalothorax (Fig. 

 72), one on each side. In the abdomen 

 they twist around one another like a 

 huge muscular rope, and are intimately 

 connected with the ventral exo-skeletal 

 parts of each somite. Contraction of the 

 muscles results in the simultaneous ven- 

 tral turning of all the abdominal somites, 

 and the vigorous flop of the lobster 

 results. Similar, but smaller and straight 

 muscles lie on the dorsal surface of the 

 huge ventral muscles, and are similarly 

 connected with the dorsal exo-skeleton. 

 When these muscles contract, the abdomi- 



