Page:Popular Science Monthly Volume 75.djvu/260

256 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 acquisition 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

 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 cœlenterates 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 cœlenterate, has left its mark in the distribution of nervous materials in the ventral cord of this animal. In the ectoderm of the cœlenterate the cell-bodies of the nervous mechanism 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 earthworm (Fig. 4); here the cell-bodies are on the ventral side of the ganglion, 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 cœlenterates 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.