. Comparative anatomy and physiology. 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
. Comparative anatomy and physiology. 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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