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. 
Pig. 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 speci- 
mens 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 dis- 
tribution 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. 
The general structure of the chief central nerve trunks is quite similar as shown 
in sections. Figs. 6, 7 and 8. The nerve trunks have about one to two layers of 
cells, the main part of the nerves are composed of longitudinal fibers. There are not 
so many evident vertical fibers from cells as found in starfish and some other forms. 
This change in position of the fibers may be in part due to the general modification 
of structure. Whether this arrangement leads to other types of nerve association is 
a question. 
When the nerve trunks are removed, stained in methylene blue and examined 
with the microscope something of the arrangement of the cells may be seen. In the 
circumoral and oral radial nerves the nerve cells are thickly massed from side to side, 
but in the upper part of the aboral nerve there is an evident arrangement of nerve 
