STRUCTURE AND FUNOTION OF THE SPHARIDIA, 47 
results were obtained by the use of chromic-osmic-acetic solu- 
tion according to Flemming’s formula (Fol, loc. cit.). The 
spheridia are supplied from the tentacular nerve-trunk, 
branches of which pass into the globule at the joint. In order 
to obtain a general view of the nerve-cells of the canal system 
it is necessary to remove the calcareous matter, and this is 
satisfactorily accomplished, without injuring the soft tissues, by 
the use of the acid solution already referred to. It is then easy 
to trace the nerve-cells from the epithelium to the interior of 
the globule where they form a network of filaments with here 
and there irregular knots into which two or more filaments pass. 
The latter are the nucleated portions of the cells. As repre- 
sented in figs. 23 and 25, the cells fill partially or completely 
the canals. 
The nervous system of the canals consists of numerous long 
branching cells provided here and there with ganglionic swel- 
lings usually containing a nucleus. These enlargements 
correspond to lacunz in the canal system as do the fine fila- 
mentous branches to the finer canals (figs. 23, 25.) Such 
slender filaments are frequently knotted with small granular 
enlargements exactly such as are found among the nerve-cells 
of the Medusz. The branches of the cells anastomose with 
each other and form thus ring-like meshes in the cell (figs. 25, ¢, 
IV, V, VI.) These cells reach from the central canals of the 
globule to the exterior and end in the epithelial layer between 
the epithelial cells. They pass out of the globule as fine fila- 
ments but enlarge at once after passing the orifices of the 
canal system. Considerable interest attaches to this manner of 
ending since it reminds one strongly of the end organs of the 
nerves of higher animals; but is of course not to be compared 
with them since here we have to do from first to last with a 
simple cell. The ends are club-shaped or pyramidal with 
the larger part directed outwards. Whether these nerve-ends 
are ciliate or not I cannot say, I have never observed anything 
that would indicate the presence of cilia. 
In Spatangus there is usually a quantity of black pigment 
in the soft tissues of the neck near the base of the head, and 
