NEEVOUS SYSTEM 145 



E. The Endophiagmal Skeleton. 



In the thorax a series of calcified plates project inwards 

 from the cuticle of the ventral surface, forming an internal 

 skeleton, which gives origin to the muscles of the legs and to 

 the powerful flexor muscles of the abdomen. 



P. The Nervous System. 

 1. The central nervous system consists of a series of 

 ganglia arranged in pairs, and united by paired 

 bundles of nerve-fibres, or connectives, to form a 

 chain, , which runs along the whole length of the 

 body close to the mid-ventral line. The two ganglia 

 of each pair are usually fused indistinguishably. 



a. The pre-oesophageal ganglionic mass, or 'brain,' is 



a white mass of considerable size, situated just 

 behind and slightly above the bases of the anten- 

 nules. It gives off nerves supplying the eyes, 

 the antennules, and the antennae. 



b. The para-oesophageal connectives are a pair of long 



nerve-cords, one on each side of the oesophagus, 

 connecting the ' brain ' with the hinder part of 

 the nervous system. They are connected with 

 each other by a transverse commissure, imme- 

 diately behind the oesophagus. 



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 maxiUse, and the first and second 

 maxillipedes. 



d. The thoracic nerve-chain consists of six ganglionic 



masses, united by paired connectives, and supply- 

 ing the appendages from the third maxillipede to 

 the last thoracic leg. 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 slightly, and the sternal 

 artery (p. 150) passes between them. 



