18G7.] DR. J. E. GRAY ON SPONGES. 501 



long cilia, supported by a skeleton consisting of calcareous or sili- 

 ceous spicules or horny network, The spicules have a distinct ani- 

 mal basis ; hence it seems probable that each spicule was originally 

 a segment of sarcode which has undergone calcitication or silicifica- 

 tiou, and by the self-shaping power of which the form of the spicule 

 is mainly determined. 



The mass of sarcode and spicules called the Sponge is permeated 

 by a series of canals having a distribution proper to each kind of 

 sponge. The ciliated cells seem to form the walls of the canals, 

 which may be said to commence in the small j^ores of the surface 

 and to terminate in the large vents or oscules ; and a current of water 

 is continually entering at the former and passing forth from the 

 latter during the life of the sponge, bringing in alimentary particles 

 and oxygen, and carrying out excrementitious matter (see Dr. Car- 

 penter's ' JNIicroscope,' p. 530). 



The sponges are reproduced or multiplied by gemmation, which 

 is effected by the detachment of minute globular particles of sarcode 

 (covered with a more or less distinct thin membranaceous skin) from 

 the interior of the canals, when they sprout forth as little protuber- 

 ances, whose foot-stalks gradually become narrower and narrower 

 until they give way altogether. These gemmules, like the zoospores 

 of Algae, possess cilia, and, issuing forth from the vent, transport 

 themselves to distant localities, where they lay the foundation of new 

 fabrics. 



According to the observations of Mr. Huxley on the marine 

 genus Tethya (Ann. & Mag. N. H. vii. 1851, p. 370), 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 (ovisacs), which are larger than the gemmules, and which 

 usually have their investments strengthened with siliceous spicules 

 very regularly disposed, are probably the product of this operation. 

 They contain numerous globular particles of sarcode, every one of 

 which when set free by the rujjture of the envelope (?) becomes an 

 independent fowa-ia-like body, and may develope itself into a com- 

 plete sponge. The phenomena of sexual reproduction and develop- 

 ment have since been more particularly studied in the Sponyillce. or 

 Freshwater Sponges, es|)ecially by Mr. Carter (Ann. & Mag. Nat. 

 Hist. xiv. 1854, p. 334, & xx. 1857, p. 21), and by Lieberkuhn in 

 IMiiller's 'Arch.' 1856, in ' Reichert und du Bois-Reymond's Arch.' 

 1859, abstracted in 'Ann. & Mag. Nat. Hist.' xviii. 185G, p. 403, 

 and the 'Quarterly Journ. of Microscopic Science,' v. 1857, p. 212. 



From the observations of iSIr. Carter (Ann. & Mag. Nat. Hist. iv. 

 1849, p. 81) the sponge appears to begin life a solitary amoeba ; and 

 it is only in the midst of an aggregation formed by the multiplica- 

 tion of these that the characteristic */)o??^(?-structure makes its ap- 

 pearance, the formation of the spicules being the first indication of 

 such organization. 



In this essay I have made free use of the very accurate and ad- 

 mirable figures of the spicules in the plates that accompany Professor 

 O. Schmidt's and, especially, Dr. Bowerbank's works and papers, 



