I 



NERVOUS SYSTEM. 163 



E. The Endophragmal Skeleton (fig. 37, p. 156). 



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. 



F. 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 antenna). 



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



nerve-cords, 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-cesophageal ganglionic mass is an ovoid 



mass, lying a short way behind the mouth. 

 From it arise the nerves supplying the mand- 

 ibles, both pairs of maxillae, and the first and 

 second maxillipedes. 



d. The thoracic nerve-chain consists of six ganglionic 



masses, united by paired connectives, and 

 supplying the appendages from the third 

 maxillipedes to the last thoracic legs. The first 

 one is situated immediately behind the post- 

 cesophageal ganglionic mass, with which it is 

 closely connected. The connectives between 



