54 



ARTHROrODA 



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

 dorsal brain i^supraresophageal ganglia) and a ventral chain of ganglia, 

 all connected bv longitudinal ner\'e cords, the brain being connected witli 

 the rest by cords or commissures passing on either side ol the a-sophagus. 

 The ventral chain shouki coniaiir as many pairs of ganglia as there are 

 somites, but tliis is not the case except in the embryo. The tendency is 

 rather towards a fusion of ganglia, especial))- of those somites wlrich unite 

 or fuse. This fusion of ganglia occurs to a -wir^ing extent in dilTerent 

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



.-1 



D 



Fig. },-o. — Different degrees of concentration of the ventral cord of .\rthropod3 

 (from t'.cgenbaur). .i. Termite (.after LespesL B, water beetle (.after Hlanchard). 

 r, Ih- (after Blanchard). D, Ttielyphonid (after Blanchard'). a. abdomen; .!;-, .•;% 

 ganglia of ventral cord; gi. infraeesopliageal ganglion; .;,',(. supraa-sophageal ganglion; 

 0, eye; p'-p", walking feet; /r, lung books; i, chelicera-; 2, pedipalpus. 



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

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

 to the oesophagus precluding its fusion with the veittral chain. 



Two types of eyes are recognized, the simple [ocellus, stcwma) and 

 the compound (facetetl). The ocelli are small. In their Itighest develop- 

 ment, as in spiders (fig. 371), (hey 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, 

 ami is usually thickened to a bicoricave body (1) which converges the 

 light upon the retina. Oirly exceptionally (larva^ of Ephemerida) is the 



