30 



EECEEATIVE SCIENCE. 



but the sponges were, perhaps, the most 

 numerous. Consisting of sUica, with a very- 

 perishable organized tissue, these were ready 

 at any time to undergo petrifaction, if 

 circumstances were favourable. Bring two 

 globules of mercury near to each other, and 

 see how readily they run together into one 

 mass. There you have a mechanical example 

 of the way in which silica was precipitated 

 from the water, and aggregated into nodules 

 about the spiculsB of the sponges, and by de- 

 grees filled up the whole skeleton, preserv- 

 ing its form, but destroying its substance, and 

 thus changing sponges into nodules of flint. 

 Mr. Brande has imitated this very process in 

 experiment, and has seen the formation of 

 flinty nodules about a nucleus, when finely 

 powdered silica has been mixed with other 

 earths, and the whole diffused through water. 

 Every separate sponge offering a separate 

 nucleus, suffices to explain why flints should 

 commonly appear, as they do, with such de- 

 cided individuality of character ; they are 

 petrified sponges, formed in much the same 

 way as those petrified forests travellers tell 

 us of, where the trees are all fiint; the woody 

 fibre has disappeared, but the original struc- 

 ture is still traceable in the mass of silex in 

 which the perished organisms are now repre- 

 sented. That silica abounded in the seas of 

 the period in which the chalk beds were depo- 

 sited IS certain ; but we are far from having 

 arrived at a clear idea as to the chemistry of 

 the whole subject to which flints introduce us. 

 We can see the sponges in the flint, and the 

 flint in the sponges, and the more we ob- 

 serve, reason, and compare, the more are we 

 convinced of their geological and chemical 

 relations. 



Then comes the question, what is a 

 sponge? Down here, in this wet hollow, 

 we are sure to find some ; in dark places, 

 where the water is of some depth, almost 

 every fragment of sea-weed has attached to 

 it some living species of sponges, and they 

 Vary in size and structure, from mere specks 



to large and substantial masses. ]^ow and 

 then we may find them on the shells of 

 oysters and crabs; and once, in our aqua- 

 rium, a fine hermit came out of his shell to 

 die, and was foTind to have a sponge as large 

 as a hazel-nut attached to his soft body, just 

 below the insertion of the last pair of legs. 



Animal life may be said to begin or end 

 in the sponges, they are the very lowest in 

 the scale of animated nature ; but it is quite 

 certain they are not members of the veget- 

 able kingdom. Take a piece of sponge, such 

 as is commonly used on the toilet table, and 

 dip it into a thin solution of size, and you 

 have a fair resemblance of its condition 

 when living. The sponge proper is the skele- 

 ton, the gelatinous coating is organized and 

 animal; and the best proof of the fact is 

 afforded by the microscope, which reveals 

 ciliary motion, and there is an end of the 

 difficulty as to what place it should occupy. 

 The openings in the sponge are chambers, 

 interlaced with sHicated fibres, and, by the 

 play of the cilia on the gelatinous surface, 

 the water is made to circulate from chamber 

 to chamber, so that the sponges obtain their 

 food by the same process as a vorticella 

 or rotifer — namely, by creating currents 

 through the agency of cilia. The exterior 

 film is the life of the sponge, the skeleton is 

 a deposit. But the film must be under- 

 stood as pervading the inner as well as the 

 exterior chambers, so that the currents of 

 water pass through the entire mass, and 

 carry nourishment to all the mouths for 

 which the cilia work so incessantly. A very 

 dead sort of creature is a living sponge. It 

 has none of the organs of sense which dis- 

 tinguish terrestrial animals, and not even 

 the irritability which makes a sea-anemone 

 of so peevish or spasmodic a temper. But it 

 has its history, however brief, like others of 

 the great class of zoophytes. The sponges 

 increase by gemmation. Little buds appear 

 within the openings of the reticulated mass, 

 and these at last detach themselves, and 



