All sponges, however, have not a horny framework, but some, which are thereby rendered useless in a commercial point of view, are supported by a skeleton composed of siliceous particles imbedded in a tough, fibrous material. These particles, or _spicula_, as they are termed, are so uniform in the species to which they severally belong, that, in the words of Professor Grant, if the soft portion be destroyed, and a "few of them brought from any pan of the world on the point of a needle, they would enable the zoologist to identify the species to which they originally belonged." Professor R. Jones, however, considers that this opinion should be received with considerable limitations. The last fact, trivial as it appears, assumes immense importance when we learn that to these spicula we must turn for an explanation of the isolated masses of flint which abound in various chalk formations. "The mere assertion," says Rhymer Jones, "that flints were sponges, would no doubt startle the reader who was unacquainted with the history of these fossil relics of a former ocean;" and yet a little reflection "will satisfy the most skeptical." For long ages the sponge is imbedded in the chalk, through which water is continually percolating. A well-known law of chemistry explains why similar matter should become aggregated; and thus the siliceous matter of the sponge forms a nucleus for the siliceous matter contained in the water, until at length the entire mass is converted into a solid flint. But we are not left, he adds, to mere conjecture or hypothesis on this point, "for nothing is more common in chalky districts than to find flints, which, on _being broken, still contain portions of the original sponge in an almost unaltered state_." There is every reason to believe that the sponge-fisheries of the AEgean are at present conducted precisely in the same manner as they were in the time of Aristotle. The sponge-divers are mostly inhabitants of the islands which lie off the Carian coast, and of those situated between Rhodes and Calymnos. These men--who form a distinct society, and are governed by peculiar laws, which prohibit their marriage until they shall have attained a prescribed proficiency in their art--go out in little fleets, composed of caiques, each of six or seven tons' burden, and manned by six or eight divers: each man is simply equipped with a netted bag in which to place the sponges, and a hoop by which to suspend it round his neck; and thus furnished, he descends to a depth of from five to twenty, or even occasionally thirty fathoms. The sponges which he collects are first saturated with fresh water, which destroys the vitality, and decomposing the gelatinous matter, turns it black; this matter is stamped out by the feet of the divers, and the sponges are then dried in the sun, and strung in circles, after which they are ready for sale and exportation.