ARTIIROPODA. 



403 



as was pointed out (p. 124), of a dorsal brain (supraoesophageal 

 ganglia) and a ventral chain of ganglia, all connected by longi- 

 tudinal nerve cords, the brain being connected with the rest by 

 cords or commissures passing on either side of the oesophagus. 

 The ventral chain should contain as many pairs of ganglia as there 

 are somites, but this is not the case except in the embryo. The 

 tendency is rather towards a fusion of ganglia, especially of those 

 somites which unite or fuse. This fusion of ganglia occurs to a 

 ABC D 



FIG. 405. Different degrees of concentration of the ventral cord of Arthropods. 

 (From Gegenbaur.) A, Termite (after Lespes) ; B, water beetle (after Blanchard); 

 C, fly (after Blanchard); Z>, Thelyphonid (after Blanchard). o, abdomen ; </'*, <? 3 , 

 ganglia of ventral cord ; <;z, infracesophageal ganglion ; gs, supraoesophageal 

 ganglion ; o, eye ; p'-p'** walking feet ; tt\ lung books ; I, chelicerae ; 2, pedipalpus. 



varying extent in different species, the extreme being reached in 

 the spiders and crabs (fig. 441), where the whole ventral chain 

 forms a large ganglionic mass. In all cases, however, the brain 

 remains distinct from the rest, its position dorsal to the ossophagus 

 precluding its fusion with the ventral chain. 



Of the sense organs the best known are the eyes, of which two 

 types are recognized, the simple (ocellus, stemma) and the com- 

 pound (faceted). The ocelli are very small. In their highest 

 development, as in spiders (fig. 406), they are composed of lens, 

 vitreous body, and retina. The lens is formed by the cuticula, the 

 rest from the epidermis. The lens differs from the rest of the 

 cuticle in being transparent, and is usually thickened to a biconcave 

 body (1) which converges the light upon the retina. Behind the 

 lens comes a layer of transparent cells, the vitreous body (#), and 



