SPONGE WEAVERS. 143 



abundance, will invariably be seen." And again : 

 " In the greensand large silicious nodules, known as 

 Polypothecea, are of frequent occurrence, and when 

 thin sections are examined their spongeous origin is 

 distinctly seen ; these nodules were, however, formed 

 under somewhat different conditions to the ordi- 

 nary chalk flint, the silica is distinctly crystalline 

 and doubly refractive, and polarises like quartz or 

 agate ; the sponges were also probably different from 

 those belonging to the chalk. A careful microscopic 

 examination of very many sections did not reveal 

 the presence of any form of spicule, they were most 

 likely allied to the recent keratode sponges ; in fact, 

 a thin slice of ordinary domestic sponge greatly 

 resembles a section of its silicified predecessor. The 

 reticulations are not solid, but tubular, and I have 

 been able, in many cases, to fill them with colouring 

 matter." 



No one ventures to doubt, or dispute, the evi- 

 dence, that sponges are amongst some of the 

 oldest inhabitants of this world of ours, and on that 

 account, if on no other, are worthy of our better 

 acquaintance. We have endeavoured to compress, 

 within a few pages, an epitome of a history which 

 would readily have expanded sufficiently to have 

 filled a volume, but even this fragmentary sketch 

 will show, that there is something worth learning in 

 the " home without hands," constructed in the sea 



