The spicula and skeleton of most of the marine sponges are siliceous and singularly beautiful; the skeleton of the Dactylocalyx pumiceus of Barbadoes is transparent as spun glass; and a species from Madagascar has numerous simple transparent and articulated spicules implanted in the siliceous fibres of the skeleton. The Cristata, Papillaris, Ovulata, and many more have siliceous skeletons, some garnished with spicules of various forms, and the surface occasionally covered with a layer of siliceous granules.
The variety in the size, structure, and habits of the marine sponges is very great: temperate and tropical seas have their own peculiar genera and species; some inhabit deep water, others live near the surface, while many fix themselves to rocks, sea-weeds, and shells, between high and low water mark. There are very few dead oyster, whelk, scallop, and other shells that escape from the ravages of the Cliona, an extremely minute burrowing sponge of the simplest structure, which has a coat of siliceous spicules supposed to be the tools with which it tunnels a labyrinth through the mid-layer of the shell, in a pattern that varies with the species of the sponge. A communication is formed here and there with the exterior by little round holes, through which the sponge protrudes its yellow papillæ. From the force exhibited by this little sponge, it may perhaps be inferred to possess a rudimentary muscle and nerve.[14]
Sponges are propagated twice in the year by minute ciliated globules of sarcode, detached from the interior of the aquiferous canals, which swim like zoospores to a distance, come to rest, and lay the foundation of new sponges. The little yellow eggs of Halichondria panicea are lodged in the interstices between the interior canals; when mature, they are oval and covered with cilia, and are carried out by the currents; and after swimming about for some days fix on a solid object, become covered with bristles, spread out into a transparent film, charged with contractile vesicles of different sizes in all degrees of dilatation and contraction, as well as with sponge ovules. Spicules are developed at the same time, and these films ultimately become young sponges, and if two happen to meet they unite and are soldered together.[15] Besides eggs, larger bodies covered with radiating spicules are produced, containing granular particles of sarcode, each of which when set free by the rupture of the envelope, becomes an Amœba-like creature, and ultimately a sponge.