CHAPTER IV. 



Poeifera (Sponges). 

 Continued. 



Though the horny or keratose Sponges are distinguished 

 from those which have calcareous or siliceous spicula in 

 them, this distinction must not be understood to imply- 

 that the former are totally destitute of these 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. 



