110 E. A. Andrewfi — Anatomy of the Spider Crab. 



ciiiials {I>c^ titf. 1) which run ui)\vartl just l)enoath the floor of the 

 l)iaiicliial chamber and communicate witli the pericanlium. Tlie 

 canals from the posterior two branchia' unite to form one, and those 

 from llie anterior branchiie also unite. The two large canals thus 

 formed have a common opening into the pericardium, nearly opposite 

 to the lateral ^■alve of the heart, and Iteneath the apex of the bran- 

 chial chamber. 



Nervous System. — The nervous system (fig. 20) comprises the two 

 large ganglionic masses connected by commissures ; the nerves ema- 

 nating from them ; and the stomato-gastric system connected with 

 the commissures. 



The thoracic ganglion {tg) is a large, flattened, oval mass, lying 

 horizontally immediately above the sternal artery in the central part 

 of the thorax. From its anterior end two stout commissural cords 

 (c) run upward and forward to the cei)halic ganglion {eg), passing on 

 either side of the cesophagus {pe) just behind which they are connected 

 by a transverse cord {Jc). 



The cephalic ganglion is large, but much smaller than the thoracic 

 ganglion, flattened, transversely elongated, and lies against the up- 

 turned antennulary sternum. From each outer anterior angle of this 

 cephalic, or cerebral, ganglion, a large optic nerve (i) passes outward 

 and forward into the ophthalmic peduncle ; it is closely accompanied 

 along its posterior side by a smaller motor nerve [mo) supplying the 

 muscles of the pedunc'.e. From the lower face of the ganglion, 

 somewhat behind the origin of the optic nerves, a pair of smaller 

 nerves pass downw^ard and forward into the antennules. From 

 each lateral edge a large nei-ve runs directly outward and forks near 

 the edge of the carapace {chi), the anterior and posterior branches 

 supplying the dermis. From the posterior edge the two cireum- 

 oesophageal commissures {c) pass back to the thoracic ganglion. 

 Between the lateral dermal nerves and the commissures, but from 

 a loAver level, there arises on each side a nerve which passes down- 

 ward and forward into the antenna (iii). Thus the cerebral ganglion 

 gives off" five pairs of nerves. 



From the posterior end of the tlioracic ganglion two nervous 

 cords, unequal in size {an), run horizontally backward and enter the 

 abdomen, passing on either side of the descending sternal artery (/•"). 

 In the abdomen the filaments of these cords become separated, some 

 l)assing back toward the anus below jmd by the sides of the intes- 

 tine, while each appendage receives one nerve which j)asses outward 

 from near the median line. 



