406 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



crab, all the ffan^lia behind the cerebral become fused 



/ O O 



into one large mass,- which still retains evidence of its 

 composite character by giving off a large number of 

 separate nerve fibres (Fig. 175). A similar series illus- 

 trating the phenomenon of the fusion of nerve centres 

 may be observed in Araclinida and Insecta. 



We note, then, that the 

 loss of that plexiform arrange- 

 ment, of which we have 

 spoken so often, is succeeded 

 by an aggregation of ganglionic 

 cells, which form distinct- 

 masses in every segment oi 

 the body ; at first each mass 

 is composed of two distinct 

 halves. The anterior regior 

 becomes more and more pre- 

 dominant, and the " arcliicere- 

 brum," or simple anteric 



Fig. 175. Nervous System 

 of a Crab. 



c, Cerebral ganslion ; o, optic; 

 a, antennary nerve ; c ; rpso- 

 phageal commissure, T, fused 

 ventral ganglion. 



enlargement, becomes a " syii- 

 CgrpbrTLtr^" or compound one, 

 As the segments of the body, 

 which in the earthworm, for 

 example, are all alike and have 

 nearly all just the same func- 

 tions, become arranged in 

 groups which, as in the cray- 

 fish, take on different duties, or exhibit division of 

 labour, the nervous centres likewise become affected, 

 so that while Apus has a separate ganglionic mass 

 for each of its sixty segments, the crayfish has the 

 first six of its ventral ganglia fused together, and 

 the short-tailed crab has all the ventral ganglia in a 

 single mass (Fig. 175) ; so, again, the Myriopod has 

 ganglia in every one of its segments, the scorpion 

 has the first nine ventral ganglia united, and in the 

 short-bodied spider there is only one ventral ganglion. 



