148 THE CEAYFISH 



c. The post-oesophageal ganglionic mass is an ovoid 



mass, lying a short way behind the mouth. 

 From it arise the nerves supplying the mandibles, 

 both pairs of maxilla, and the first and second 

 maxillipedes. 



d. The thoracic nerve-chain consists of six ganglionic 



masses, united by paired connectives, and supply- 

 ■ rag the appendages from the third maxiUipedes 

 to the last thoracic legs. The first one is situated 

 immediately behind the post-oesophageal gan- 

 glionic mass, with which it is closely connected. 

 The connectives between the fourth and fifth 

 ganglionic masses diverge shghtly, and the sternal 

 artery (p. 152) passes between them. 



The thoracic nerve-chain lies beneath the endcyphragmal 

 skeleton, which must be removed, to expose it. 



e. The abdominal nerve-chain consists of six ganglionic 



masses, joined by connectives, and supplying the 

 abdominal segments and their appendages. The 

 hindmost ganglionic mass is larger than the 

 others, and supplies both the nineteenth segment 

 and the telson. 



2. The visceral nervous system. 



This can only be dissected in a specimen in which the 

 alimenta/ry canal has been left undisturbed. 



a. The anterior visceral nerve is a median nerve 



formed, just in front of the /oesophagus, - by the 

 union of two median nerves from the ' brain ' with 

 two pairs of nerves arising from ganglionic swell- 

 ings on the para-oesophageal connectives. Prom 

 the union of these, the nerve runs up in front of 

 the stomach to branch out on its dorsal wall. 



b. The posterior visceral nerve arises from the hind- 



most ganglionic mass of the abdominal nerve- 

 chain, and runs forwards along the ventral surface 

 of the intestine. 



