Biology.—S. crateriformis flourishes in Calcutta throughout the year. Here it is usually found adhering to the roots of water-plants, especially Pistia and Limnanthemum. In the case of the former it occurs at the surface, in that of the latter at the bottom. When growing near the surface or even if attached to a stone at the bottom in clear water, it is invariably of a pale yellowish or greyish colour. When growing on the roots of Limnanthemum in the mud of the Gangetic alluvium, however, it is almost black, and when growing in the reddish muddy waters of the tanks round Bangalore of a reddish-brown colour. This appears to be due entirely to the absorption of minute particles of inorganic matter by the cells of the parenchyma. If black sponges of the species are kept alive in clean water, they turn pure white in less than a week, apparently because these particles are eliminated. When growing on stones the sponge, as found in India, often conforms exactly with Potts's description: "a filmy grey sponge, branching off here and there ... yet with a curious lack of continuity...."
The wide efferent canals of this sponge afford a convenient shelter to small crustacea, and the isopod Tachæa spongillicola, Stebbing (see p. 94), is found in them more abundantly than in those of any other sponge. This is especially the case when the sponge is growing at the bottom. On the surface of the sponge I have found a peculiar protozoon which resembles the European Trichodina spongillæ in general structure but belongs, I think, to a distinct species, if not to a distinct genus.
Subgenus B. EUNAPIUS, J. E. Gray.
Eunapius, J. E. Gray (partim), P. Zool. Soc. London, 1867, p. 552. Spongilla (s. str.), Vejdovsky, in Potts's "Fresh-Water Sponges," P. Ac. Philad. 1887, p. 172. Spongilla (s. str.), Weltner, in Zacharias's Tier- und Pflanzenwelt des Süsswassers, i, p. 214 (1891). Spongilla (s. str.), Annandale, Zool. Jahrb., Syst. xxvii, p. 559 (1909).
Type, Spongilla carteri, Carter.
Spongillæ in which the gemmules are covered with layers of distinct polygonal air-spaces with chitinous walls.
The gemmules are usually fastened together in groups, which may either be free in the sponge or adhere to its support as a "pavement layer"; sometimes, however, they are not arranged in this manner, but are quite independent of one another. The skeleton is usually delicate, sometimes very stout (e. g., in S. nitens, Carter).
The term Eunapius here used is not quite in the original sense, for Gray included under it Bowerbank's Spongilla paupercula which is now regarded as a form of S. lacustris. His description, nevertheless, fits the group of species here associated except in one particular, viz., the smoothness of the gemmule-spicules to which he refers, for this character, though a feature of S. carteri, is not found in certain closely allied forms. The use of "Spongilla" in a double sense may be avoided by the adoption of Gray's name.
The subgenus Eunapius is, like Euspongilla, cosmopolitan. It is not, however, nearly so prolific in species. Four can be recognized in India, two of which range, in slightly different forms, as far north as Europe, one of them also being found in North America, Northern Asia, and Australia.
8. Spongilla carteri* Carter (Bowerbank, in litt.). ([Plate II]. fig. 1.)