REPRODUCTION OF SPONGES. 531 



of it. Multiplication by Gemmation is effected by the detachment 

 of minute globular particles of sarcode from the interior of the 

 canals, where they sprout-forth as little protuberances, whose foot- 

 stalks gradually become narrower and narrower until they give way 

 altogether ; these gemmules, like the zoospores of Algfe, 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 observations of Mr. Huxley on the marine 

 genus Tethya,* a true sexual Generation also takes-place ; 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 products of this operation. They contain numerous 

 globular particles of sarcode, every one of which, when set free by 

 the rupture of its envelope, becomes an independent Amceba-like 

 body, and may develope itself into a complete Sponge. The pheno- 

 mena of Sexual reproduction and development have since been 

 more particularly studied in the Spongilla or Fresh-water Sponge, 

 especially by Carterf and Lieberkiihn. X 



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

 Spongilla, all known Sponges are Marine ; but they differ very 

 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 the marine Sponges has Ciliary movement yet been 

 seen, belong to this last category. They have a peculiarly simple 

 structure, each being a sort of bag whose wall is so thin that no 

 system of canals is required, the water absorbed by the outer sur- 

 face passing directly towards the inner, and being expelled by the 

 mouth of the bag. The cilia may be plainly distinguished with a 

 l-8th in. objective, on some of the cells of the gelatinous sub- 

 stance scraped from the interior of the bag ; or they may be seen 

 in situ, by making very thin transverse sections of the substance 

 of the Sponge. § It is by such sections alone that the internal 

 structure of Sponges, and the relation of their spicular and horny 



* * On the Anatomy of the genus Tethya,' in "Ann. of Nat. Hist.," 2nd 

 Ser., Vol. vii. (1851), p. 370. 



t See his Memoirs ' On Zoosperms in Spongilla ■' in "Ann. of Nat. Hist.," 

 2nd Ser., Vol. xiv. (1854), p. 334, and ' On the Ultimate Structure of 

 Spongilla,' in "Ann. of Nat. Hist.," 2nd Ser., Vol. xx. (1857), p. 21. 



X See the Memoirs of Lieberkuhn ' On the Development of the Spon- 

 gilla,' in "Miiller's Archiv." for 1856, and his 'New Researches on the 

 Anatomy of Sponges' in " Reichert's und Du Bois Reymond's Archiv." 

 for 1859. Abstracts of the former are contained in the "Annals of Nat. 

 Hist.," 2nd Ser., Vol. xvii. (1856), p. 403, and in the "Quart. Journ. of 

 Microsc. Science," Vol. v. (1857), p. 212. 



§ SeeDobie, loc. cit.; and Bowerbank, in "Trans, of Microsc. Soc," 1st 

 Ser., Vol. iii., p. 137. 



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