SPONGES. 
39 
CHAPTER IY. 
Porifera {Sponges). 
Continued. 
Though the horny or keratose Sponges are distinguished 
from those which have calcareous or siliceous spicula in 
them, this distinction must not bo understood to imply 
that the former are totally destitute of those bodies, but 
only that they possess them in an excessively minute 
proportion. Mr Bowerbank, in his elaborate and valu- 
able investigations “ On the Keratose Sponges of Com- 
merce,” has found spicula of very minute dimensions 
imbedded in the substance of the horny fibres of various 
species.* Still the immense preponderance of the cor- 
neous structure fully warrants their isolation as a natural 
group. 
The horny fibres, as we have already said, form an 
irregularly netted mass, uniting to and separating from 
each other at various angles and distances, without the 
least order. They are not tubular, as has been sup- 
posed, but solid and of unequal thickness in different parts. 
Sometimes they are rigid and coarse, as we have seen in a 
large tubular Sponge on the shores of Jamaica, almost 
* Trans. Micr. Soc. i., p. 32. 
