STUDIES [N SPICULE FORMATION. Lo 
(B in text-fig. 7; also in figs. 9, 11, 22) for the crystalline 
form in question, and that this last does not occur in the 
development of any calcareous spicule. 
Tue ''RIRADIATE SPICULES. 
To continue the description of spicule formation in HE. 
esculentus. As already stated, the skeleton of one side of 
the larva not infrequently (in say 20 per cent. of larve) 
arises in several centres, i.e. as several spherical granules 
situated in more than one cell, and seeing that the longitu- 
dinal strand usually consists of from half a dozen more or less 
closely-apposed cells, this is only what we should expect, but 
normally a single spherical granule is produced in the interior 
of one of the centrally-situated cells.! This spherical granule 
thus situated becomes after a short time three-cornered, and 
these three corners later assuming the form of three rays, 
there is thus produced a young triradiate spicule ? (figs. 6, 7, 
13, 16, etc.). 
When we consider the conditions under which calcareous 
spicules are in general developed—when we study their 
development in distinct groups of animals (12, 16, 24, 28, 
etc.)—it will seem probable that the triradiate form is corre- 
lated in the pluteus larva, as in other instances, with the 
disposition of the secreting cells. That this correlation 
roughly exists in the present instance is obvious from the 
most superficial inspection of the figures provided. In the 
region of a lateral longitudinal strand it is evident that, given 
1 As to why lime should be first deposited in this particular region, if may 
be remarked that this deposition always occurs where the scleroblasts are most 
thickly clustered (see my paper on the Sycon spicules—24), these scleroblasts 
“during their lively activity supplying themselves with calcareous salts [1. e. 
with water containing these salts] in such a degree that it becomes impossible 
to keep them in a state of solution” (Théel), the mutual apposition of the 
cells doubtless facilitating the deposition. 
* In Arbacia punctulata “the first spicules to appear are four-rayed ” 
(Garman and Colton, 6). 
