256 THE POPULAR SCIENCE MONTHLY 



epithelial layer, whereas the central nervous organs of the earthworm 

 are solid masses of nerve-cells, fibers, and neuropile entirely distinct 

 from any epithelium. But this condition is apparently a recent acqui- 

 sition on the part of the earthworm, for in another annelid, Sagalion, 

 the ventral cord (Fig. 3) and the brain are still a part of the superficial 



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V 



Fig. 4. Transverse Section of the Ventral Nervods Cord of an Earthworm, 

 showing the ganglion-cells on the ventral side (v) and the nerve-fibers and neuropile 

 on the dorsal side (d). 



ectoderm and differ from the condition in the ccelenterates only in that 

 they represent a concentration of nervous elements in certain regions 

 instead of a diffuse 'condition as in the sea-anemones, etc. In Nereis 

 the brain is epithelial, but the cord by a process of delamination has 

 broken away from the integument, and in the earthworm the whole 

 central nervous system, brain as well as cord, has delaminated. It is 

 chiefly this concentration and separation of the nervous organs from the 

 skin that justifies, in my opinion, the statement that an earthworm has 

 central nervous organs and a sea-anemone has not. 



The fact, however, that the central nervous system of the earthworm 

 has developed on the lines of the ccelenterate, has left its mark in the 

 distribution of nervous materials in the ventral cord of this animal. In 

 the ectoderm of the ccelenterate the cell-bodies of the nervous mech- 

 anism are nearer the exterior of the animal than are their processes, the 

 fibrillar mass, and the same is true in the ventral ganglia of the earth- 

 worm (Fig. 4) ; here the cell-bodies are on the ventral side of the gang- 

 lion, i. e., next the integumentary epithelium, and the neuropile and 

 nerve-fibers are on the opposite or dorsal side of the ganglion. This 

 peculiarity in the distribution of nervous materials is apparently true 

 for most higher metazoans. 



Another point of comparison between the nervous mechanism in 

 ccelenterates and in the earthworm is the presence of nerves in the 

 latter and their absence in the former. As already pointed out, the 

 nerves in the earthworm are bundles of independent fibers which course 

 more or less together between their end-organs and the central apparatus. 



