EYES. SKIN. 559 



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

 jaws also takes its origin. 



The supra-oesophageal ganglia (Fig. 339) are large, somewhat 

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

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

 prolonged 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. 339, d), of which we 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. 339). 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 

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

 of ganglion cells. 



The eyes (Fig. 340) consist of hollow vesicles, the cavity of 

 which is occupied by a cuticular lens (I). The front wall consists 

 of a layer of somewhat cubical epithelial cells lying immediately 

 beneath the cornea (cor), and the hinder wall is much thickened, 

 consisting of retina (Re) and of pigment (pi.r). The optic 

 ganglion is closely applied against the pigment. The cuticle 

 over the eye is thin and transparent and lies immediately over 

 the epidermis which here constitutes the corneal epithelium. 

 The eye is developed as an invagination of the embryonic brain 

 while it is still part of the ectoderm, and its connexion with the 

 brain by means of the optic nerve is a persistent remains of this 

 primitive connexion. 



The skin is formed of thiee 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, 

 but is everywhere, with the exception of the perioral region, 

 raised into minute secondary papillae, which in most instances 

 bear at their free extremity a somewhat prominent spine. The 

 whole surface of each of the secondary papillae just described is 

 in its turn covered by numerous minute spinous tubercles. 



The epidermis, placed immediately within the cuticle, is 



