I NERVOUS SYSTEM AND BODY WALL I 3 



fairly regular arrangement from the region included between the 

 first and the last pair of true feet. There are nine or ten of them 

 between each pair of feet. They pass along the ventral wall of 

 the body, perforating the ventral mass of .longitudinal muscles. 

 On their way they give off nerves which innervate the skin. 



Posteriorly the two nerve-cords nearly meet immediately in 

 front of the generative aperture, and then, bending upwards, fall 

 into each other dorsally to the rectum. They give off a series 

 of nerves from their outer borders, which present throughout 

 the trunk a fairly regular arrangement. From each ganglion 

 two large nerves {pn) are given off, which, diverging somewhat 

 from each other, pass into the feet. 



From the oesophageal commissures, close to their junction 

 with the supra-oesophageal ganglia, a nerve arises on each side 

 which passes to the jaws, and a little in front of this, apparently 

 from the supra-oesophageal ganglion itself, a second nerve to the 

 jaws also takes its origin. 



The supra-oesophageal ganglia (Fig. 9) are large, somewhat 

 oval masses, broader in front than behind, completely fused in 

 the middle, but free at their extremities. Each of them is pro- 

 longed anteriorly into an antennary nerve, and is continuous 

 behind with one of the oesophageal commissures. On the 

 ventral surface of each, rather behind the level of the eye, is 

 placed a hollow protuberance (Fig. 9, d), of which I shall say 

 more in dealing with the development. About one-third of the 

 way back the two large optic nerves take their origin, arising 

 laterally, but rather from the dorsal surface (Fig. 9). Each of them 

 joins a large ganglionic mass placed immediately behind the retina. 



The histology of the ventral cords and oesophageal commis- 

 sures is very simple and uniform. They consist of a cord almost 

 wholly formed of nerve-fibres placed dorsally, and of a ventral 

 layer of ganglion cells. 



The Body Wall. 



The skin is formed of three layers. 



(1) The cuticle. 



(2) The epidermis or hypodermis. 



(3) The dermis. 



The cuticle is a thin layer. The spines, jaws, and claws are 

 special developments of it. Its surface is not, however, smooth. 



