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Show 1867.] DR. J. E. GRAY ON SPONGES. 493 posed to divide sponges into those with calcareous and those with siliceous spicules, and those without any spicules and having only a horny skeleton, and to call them respectively Spongia calcarea, S. silicea, and S. keratosa. Dr. Bowerbank aud several zoologists have adopted this arrangement, changing the names of one or more of the divisions. The division between the calcareous and the siliceous sponges is very distinct and natural; the separation between the siliceous and keratose sponges, on the other hand, is very indistinct and separates nearly allied genera. Some siliceous sponges are entirely formed of siliceous spicules, covered with a very small quantity of sarcode; in others the spicules are enclosed in a very thin coating of horny or keratose matter; in others the horny matter increases in thickness, and the spicules diminish in quantity until they almost entirely disappear ; and sometimes the place of the spicules in the horny fibres is supplied by sand or other siliceous matter, which the fibres take up as they are formed; and the skeleton of some sponges is entirely formed of horny matter; and the passage of these forms into each other is so gradual that I believe it is better to unite the siliceous and keratose sponges of these zoologists into one group or order. The spicula that form the main part of the skeleton of these siliceous sponges are of three shapes :- Fusiform (acerate, Bowerb.), more or less cylindrical, and pointed at each end. Needle-shaped (acuate, Bowerb.), cylindrical, blunt at one and sharp at the other end. Pin-shaped (spinulate, Bowerb.), cylindrical, with a more or less spherical head and a tapering point. There are spicula of many other shapes which have been represented by Bowerbank, Quekett, Carpenter, Oscar Schmidt, and other authors; but they are for special purposes, are found in certain parts of the sponge, and are peculiar to certain forms of sponges. Many spicula offer great variation in form in the same species of sponge, and also in the different parts of the same specimen and according to their state of development. These spicules may he divided into three series, thus :- (1) The spicula that Dr. Bowerbank calls retentive, and designates as anchorate, (bi- or quadri-) hamate, umbonate, and bi- or tri-rotulate, all belong to the same series, and each presents several modifications, sometimes in the same species of sponge, and they all gradually pass into each other. In the same way (2) the stellate spicules and (3) the three-pronged spicules each belong to a separate series offering many modifications. The stellate are usually scattered in the sarcode; and the three-pronged are what Dr. Bowerbank calls tension and defensive spicules, supporting the outer surface of the sponge, and sometimes for this purpose even extending beyond their surface. The forms of the spicules are characteristic of the different families, if they are not always absolutely peculiar to them, thus : - The many-rayed stellate, with rays on all sides, and the three- I |
