282 The Twenty -Ninth General Meeting. 



and often in the traces of internal structure, that the great majority 

 of them once were sponges, which always contain silicious spiculse, 

 and of which many deep sea species are largely composed of an en- 

 tanglement of fibrous spiculse, consisting of silica. The protozoic 

 animals {Porifera) that occupy the porous and tubular structure 

 of the sponge have their wants supplied by a circulation throughout 

 that structure, propagated by a ciliary movement of their own, of 

 the water of the ocean-bottom on which they live. 



The canals which this circulation traverses, as well as the whole 

 sponge itself, are lined with a gelatinous envelope, and the minute 

 zoids that build and inhabit the sponge take their nourishment at 

 orifices in this envelope. Now, supposing sea-water to contain dis- 

 solved silica, such a structure as a dead sponge is evidently well 

 calculated to form an apparatus for attracting the silica from the 

 water along the whole course of the canal-system, and in the interior 

 of the sponge so soon as the organism is no longer able to carry on 

 the necessary vital processes of ingestion and expulsion of the 

 materials brought to it by the sea-water for its nourishment. But 

 flints exhibit the structure of bodies other than sponges. Occasion- 

 ally the shell of a dead echinus or of some other denizen of the upper 

 water of the ocean is found silicified as flint ; and here it is evident 

 that silica has been deposited inside the shell, and not outside of it, 

 which is hard to be explained, except on the supposition that the 

 dialysis was in this case from within outwards, the silicic solutions 

 entering by the two orifices of the empty shell, in the case of the 

 dead echinus, and depositing their silica, while the more limpid 

 liquid found exit through the dead shell structure ; and this process 

 would continue until the whole was filled with silica, provided that 

 the entrance orifices were not closed to the sea-water. Where they 

 are so a hollow is left, and generally quartz crystals are then found 

 in it as in the case of the agates. Minute remains of diatomaceous 

 and desmidian plants in the form of silicious shields secreted by the 

 plant bear witness to a large development of this class of life at 

 and near the ocean-surface, the indestructible debris of these forms 

 of life falling gradually to the sea bottom, and adding to its silicious 

 material. 



