Cadlina Sp.? Dark brown, flattened.
Aeolidia sp. White to pink, appendages brown.
W. A. H.
Central Nervous System of the Sand
Dollar Dendraster Excentricus Esh
WILLIAM A. HILTON
There seems to be little or no literature on the central nervous system of this form of echinoderm. As might be expected, the general arrangement of radial and circumoral bands are much as in sea-urchins, such as shown especially by Delage and Herouard 1903. There are however some interesting features which make the study of this type of special value.
In this paper only the chief mass of the central nervous system is considered. The more evident parts of the central nervous system are arranged in general as in other forms. The circumoral nerves issue from under the lantern and run along the oral, cross over at the edge of the shell and then run along the aboral side. The five radial nerves converge at the five ocular areas near the center of the aboral region. The circumoral nerve ring is looped over and under parts of the lantern. Fig. [1] shows a part of the lantern and parts of three loops of the circumoral nerve trunk. In the center of the figure one fifth of the lantern is drawn in and from under it a radial nerve is shown in the lower part of the figure. To the left and to the right of the central bony part of the lantern the union of a radial with a circumoral nerve is shown. At the junction of each radial nerve with the circumoral, is a little thickening which seems to be a special cellular mass such as I have not found in other forms. Fig. [7] is a section through a part of a circumoral strand, much enlarged. There are only a few nerve cells, from one to two layers.
As the radial nerves leave the lantern they are quite evident in dissected specimens as they are close to the bony skeleton with very little connective or other tissues to obscure them. The use of aqueous methylene blue aids in following the smaller branches. Near the lantern the branches are small as shown in fig. [2]. When the region is reached where the upper and lower surfaces of the shell begin to fuse, the branches become larger and more irregularly arranged, as shown in the lower part of fig. [1] and fig. [2]. After the nerve turns to run on the aboral side there is no change in arrangement until the region of the tube feet is reached. In the region of the tube feet the nerves become more numerous, smaller and more regular. The general distribution of the nerves and the arrangement of the tube feet nerves are shown in fig. [4] which is from part of the upper end of the aboral nerve. The holes in the skeleton for the tube feet are shown as circles on each side of the diagram.