SPONGE WEAVERS. 113 



which are soft, flexible, and of a substance some- 

 what resembling horn, but flexible, and chemically 

 different from horn ; then, secondly, there are the 

 flinty (or siliceous) sponges, to which Venus's Flower- 

 basket belongs, in which the skeleton is rigid, com- 

 posed of flinty fibres, not unlike " spun glass " ; and 

 lastly, the chalky (or calcareous) sponges, in which 

 the skeleton is mostly chalky, or carbonate of lime. 

 For general purposes this division is useful, although 

 some exception might be taken to it for scientific 

 purposes ; nevertheless, they are all sponges, and 

 the idea of a sponge should be so extended as to 

 embrace them all. Most of them live and flourish 

 in the sea, whilst only a very few occur in fresh 

 water. 



Accepting this general notion of a sponge, our 

 best authority on the subject says r 1 " Whatever 

 may be their form, or however they may differ from 

 each other in appearance, there are certain points in 

 their organisation in which they all agree. In the 

 first place, however variable in its form and mode of 

 structure, there is always a skeleton present, in which 

 the rest of the organic parts are based and main- 

 tained. Amidst this skeleton, and intimately incor- 

 porated with it, are the interstitial canals, usually of 



1 "A Monograph of the British Spongiadae," by J. S. Bower- 

 bank, LL.D. Ray Society. 1864, &c. 



I 



