3.34 



ARTHROPODA 



ladder-like nervous system consists, as was pointed out (p. 113), of a 

 dorsal brain (supraoesophageal ganglia) and a ventral chain of ganglia, 

 all connected by longitudinal 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 varying extent in different 

 species, the extreme being reached in the spiders and crabs (fig. 402) } 



B 



C 



D 



FIG. 370. 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). D, Thelyphonid (after _ Blanchard). a, abdomen; g s ,_ g 3 , 

 ganglia of ventral cord; gi, infracesophageal ganglion; gs, supracesophageal ganglion; 

 o, eye; p'-p", walking feet; ir, lung books; i, chelicerai; 2, pedipalpus. 



where the whole ventral chain forms a single ganglionic mass. In all 

 cases, however, the brain remains distinct from the rest, its position dorsal 

 to the oesophagus precluding its fusion with the ventral chain. 



Two types of eyes are recognized, the simple (ocellus, stemma) and 

 the compound (faceted). The ocelli are small. In their highest develop- 

 ment, as in spiders (fig. 371), they are composed of lens, vitreous body, 

 and retina. The lens is formed by the cuticula, the rest from the epider- 

 mis. The lens differs from the rest of the cuticle in being transparent, 

 and is usually thickened to a biconcave body (i) which converges the 

 light upon the retina. Only exceptionally (larvae of Ephemerida) is the 



