Sponge forming large, flat sheets, never more than a few millimetres in thickness, without a trace of branches, compact but very friable, of a dark greyish colour; oscula small and inconspicuous or moderately large, never prominent; membrane adhering closely to the sponge.
Skeleton with well-defined but slender radiating fibres, which contain very little spongin; transverse fibres close together but consisting for the most part of one or two spicules only.
Spicules. Skeleton-spicules short, slender, sharply pointed, minutely serrated or irregular in outline, almost straight. Gemmule-spicules very small, rather stout, cylindrical, pointed, covered with relatively long and stout spines which are either straight or directed towards the ends of the spicule. Flesh-spicules fairly numerous in the external membrane but by no means abundant in the parenchyma, very slender, gradually pointed, covered uniformly with minute but distinct spines.
Gemmules very small, only visible to the naked eye as minute specks, as a rule numerous, free in the substance of the sponge, each provided with a slender foraminal tubule and covered with a thick granular coat in which the gemmule-spicules are arranged almost horizontally; a horizontal layer of spicules also present on the external surface of the gemmule; gemmule-spicules very numerous.
Fig. 10.—Gemmules and fragment of the skeleton of Spongilla cinerea (from type specimen), × 35.
This sponge is easily distinguished from its Indian allies by the form of its skeleton-spicules, which are, as Bowerbank expresses it, "subspined"; that it to say, under a high power of the microscope their outline appears to be very minutely serrated, although under a low power they seem to be quite smooth. The spicules also are smaller than those of S. alba, the only species with which S. cinerea is likely to be confused, and the gemmule has a well-developed foraminal tubule; the skeleton is much closer than in S. proliferens.
Type in the British Museum; a piece in the Indian Museum.
Geographical Distribution.—S. cinerea is only known from the Bombay Presidency. Carter obtained the original specimens at Bombay and the only ones I have found were collected at Nasik, which is situated on the eastern slopes of the Western Ghats, about 90 miles to the north-east.
Biology.—Carter's specimens were growing on gravel, rocks and stones at the edge of "tanks," and were seldom covered for more than six months in the year. Mine were on the sides of a stone conduit built to facilitate bathing by conveying a part of the water of the Godaveri River under a bridge. They were accompanied by Spongilla indica and Corvospongilla lapidosa (the only other sponges I have found in running water in India) and in the month of November appeared to be in active growth.