17 



of a sponge, let U3 return to the kind we are more immediately 

 concerned with, the order of 



Siliceous Sponges. 



Let us take the beautiful Euplectella as an example : — "In 

 life the glassy framework masked by a soft brown earthy coating 

 of sarcode. It would be difl&cult to imagine that the thick some- 

 what clumsy brown tube, perforated with irregular openings 

 contained any arrangement of support, so delicate and sym- 

 metrical." — Sir Wyville Thomson, Good Words, July, 1873. 



Now besides this siliceous or flinty skeleton the whole mass 

 of sponge-flesh, or sarcode, which surrounds it, was crowded with 

 needles of silica — spicula as they are termed. These vary much 

 in form in different genera of sponges. Generally speaking 

 they are exceedingly minute but sometimes of a size distinguish- 

 able by the unaided eye. Whence is the siliceous matter of these 

 spicula and of the skeleton derived ? Undoubtedly from the 

 waters of the ocean in which the sponge organism lives ; and 

 this although the quantity of silica in the waters of the sea is 

 so exceedingly small that the most refined analysis can only 

 detect " a trace " of it. 



These siliceous sponges still cover the bottom of the sea in 

 some localities, where the conditions of existence are favourable, 

 in enormous numbers. Sir Wyville Thompson, in his " Depths 

 of the Sea," speaks of a certain species of siliceous sponge which 

 forms a kind of bush or shrub and appears to clothe the bottom 

 in some places like heather on a moor. 



There must, therefore, in thf? course of years, be an enormous 

 accumulation of silica on the floor of the ocean, especially, as 

 Sollas informs us, that many of the siliceous sponges shed their 

 spicules. 



There are, no doubt, several here who can remember what a 

 sensation was caused in the scientific world when the first deep 

 dredging of the Atlantic revealed the fact that foramimfera 

 were building up the chalk floor of the bottom of the Atlantic 

 precisely as in times past they built up the chalk mud of the 

 ancient ocean on which we are now living. That was more than 

 thirty years ago, and since then we have accumulated much 

 additional evidence as to the life of the sea and the conditions 

 under which it exists. 



'' This calcareous mud," says Sir W. Thompson, " is the 

 home of multitudes of exquisitely formed glassy and other 

 siliceous sponges. . . . The mud was entirely filled with the 

 delicate siliceous mot fibres of the sponges, binding it together like 



