MODE OF EXAMINING THEM. 455 



in Fig. 219, the rays, though very numerous, are extremely 

 short ; in otlier instances the rays are much longer, and scarcely 

 any central nucleus can be said to exist. The varieties in the 

 form of Sponge-spicules are, in fact, almost endless; and a 

 single sponge often presents two or more (as shown in Fig. 219), 

 the stellate spicules usually occurring either in the interspaces 

 between the elongated kinds, or in the external crust. In one 

 curious Sponge described by Mr. Bowerbank (the Dusideia fra- 

 gilis), the spicules are for the most part replaced by particles of 

 sand, of very uniform size, which are found imbedded in the 

 horny fibre. The spicules of Sponges cannot be considered, 

 like the "raphides" of Plants (§ 230), simply as deposits of 

 mineral matter in a crystalline state. For the forms of many of 

 them are such as no mere crystallization can produce ; many of 

 them possess internal cavities, which contain organic matter; 

 and the calcareous spicules, whose mineral matter can be readily 

 dissolved away by an acid, are found to have a distinct animal 

 basis. Hence it seems probable, that each spicule was origi- 

 nally a cell or segment of sarcode, which has undergone calcifica- 

 tion, and by the self-shaping power of which, the form of the 

 spicule is mainly determined. 



297. Of the Reproductive process in Sponges, much has 

 yet to be learned. The following is perhaps the most probable 

 account of it : — Multiplication by gemmation is efiiected by the 

 detachment of minute globular particles of sarcode from the 

 interior of the canals, where they sprout forth as little protu- 

 berances, whose footstalks gradually become narrower and nar- 

 rower until they give way altogether ; these gemmules, like the 

 zoospores of Algse, possess cilia, and issuing forth from the 

 vents, transport themselves to distant localities, where they may 

 lay the foundation of new fabrics. But according to the recent 

 observation? of Mr. Huxley,' a true sexual generation also takes 

 place, as might be anticipated ; both ova and sperm-cells being 

 found imbedded in the substance of the Sponge. The bodies 

 distinguished as capsules, which are larger than the gemmules, 

 and which usually have their investment strengthened with 

 siliceous spicules very regularly disposed, are probably the pro- 

 ducts of this operation. They contain numerous globular par- 

 ticles of sarcode, every one of which, when set free by the rup- 

 ture of its envelope, becomes an independent Amoeba-like body, 

 and may develope itself into a complete- Sponge. 



298. With the exception of those that belong to the genus 

 S-pongilla, all known Sponges are marine ; but they difi:er veiy 

 much in habit of growth. For whilst some can only be obtained 

 by dredging at considerable depths, others live near the surface, 

 whilst others attach themselves to the surfaces of rocks, shells, 

 &c., between the tide-marks. The various species of Grantia, 

 in which alone of all ihe marine Sponges has ciliary movement 



' "Ann. of Nat. Hist." Ser. 2, vol. vii. 



