Mineral Deposit in the Rhizopods and Sponges. 75 
non-elastic*, no further deposit can possibly be effected from 
either surface unless by displacement of an equivalent volume of 
sarcode, whether of the cavity or stolon. The next layer may 
therefore be formed around the stolon as in layer 4, fig. 4, or 
by the retrocession of the boundary of the cavity, as in layer 3, 
fig.3. Most frequently the layers are deposited one within the 
other—this being the reason why the diameter of the first two 
layers (which generally represent the diameter of the spicule) is 
“nearly equal to the size of the external diameter,” as stated 
by Dr. Bowerbank. But nevertheless successive layers of silex 
may continue to be added externally as well as internally. If 
internally, the stolon gradually diminishes in diameter, and may 
ultimately be altogether obliterated. Therefore, as a general rule, 
the greater the diameter of the tubular cavity y of a sponge- spicule 
and the thinner its wall, the younger is the spicule, and vice 
versa F. 
It has been stated that the first two layers secreted are inva- 
riably in contact. The proof of this is afforded by the fact that 
spicules never occur in which there are two distinct hollow 
cylinders enclosed one within the other, and presenting a free 
*. It is scarcely necessary, I presume, to state that the siliceous substance 
of a spicule is resilient, but not expansible. 
+ When the stolon is continuous with the cavity at each extremity, the 
layers of silex do not constitute a sealed tube, as shown at the upper por- 
tion of the horse-shoe figures, but remain open, as seen at the lower. 
