XVI. On the anatomy and physiology of the spongiadæ

Hitherto the Spongiadæ have been classified either by their external form or in accordance with their chemical constituents. In the second edition of Lamarck’s 'Anim. s. Vert.,’ 138 species are included in the genus Spongia, without the slightest reference to their internal structure; and they are divided into seven groups by external form only, the same characters serving also, in a great degree, to discriminate the species. Fleming, Grant, Johnston, and other modern naturalists, have made their principal divisions depend on their chemical constituents, and have therefore constructed three great divisions as genera:— Spongia, composed of keratose fibres unmixed, as it was supposed, with earthy matter; Halichondria, formed principally of siliceous spicula; and Grantia, having the skeleton composed of calcareous spicula. Included in the second of these divisions are the genera Tethea, Geodia, Pachymatisma, Spongilla, Dysidea, and Halisarca, and these nine genera are all that are contained in Dr. Johnston’s 'History of British Sponges.’