_Sponge_ forming masses of considerable area, but never of more than moderate depth or thickness. Surface smooth and undulating or with irregular or conical projections; sponge hard but brittle; colour white or whitish; oscula of moderate or large size, never very conspicuous; radiating furrows absent or very short; external membrane adhering to the substance of the sponge. _Skeleton_ forming a moderately dense network of slender radiating and transverse fibres feebly held together; little spongin present; the meshes much smaller than in _S. lacustris_ or _S. proliferens_. _Spicules._ Skeleton-spicules smooth, sharply pointed, slender, feebly curved. Gemmule-spicules (fig. 8, p. 71) slender, cylindrical, blunt or abruptly pointed at the ends, feebly curved, bearing relatively long backwardly directed spines, which are usually more numerous at the ends than near the middle of the shaft. Flesh-spicules very numerous in the parenchyma and especially the external membrane, as a rule considerably more slender and more sharply pointed than the gemmule-spicules, covered with straight spines which are often longer at the middle of the shaft than at the ends. _Gemmules_ usually of large size, with a moderately thick granular layer; spicules never very numerous, often lying horizontally on the external surface of the gemmule as well as tangentially in the granular layer; no foraminal tubule; a foraminal cup sometimes present. 3_a_. Var. cerebellata, _Bowerbank_. _Spongilla cerebellata_, Bowerbank, P. Zool. Soc. London, 1863, p. 465, pl. xxxviii, fig. 16. _Spongilla alba_ var. _cerebellata_, Carter, Ann. Nat. Hist. (5) vii, p. 88 (1881). _Spongilla cerebellata_, Weltner, Arch. Naturg. lxi (i), p. 117 (1895). _Spongilla cerebellata_, Kirkpatrick, Ann. Nat. Hist. (7) xx, p. 523 (1907). This variety is distinguished from the typical form by the total absence of flesh-spicules. The gemmule-spicules are also more numerous and cross one another more regularly. 3_b_. Var. bengalensis*, _Annandale_. (Plate I, figs. 1-3.) _Spongilla lacustris_ var. _bengalensis_, Annandale, J. Asiat. Soc. Bengal, 1906, p. 56. _Spongilla alba_ var. _marina_, _id._, Rec. Ind. Mus. i, p. 389 (1907). The sponge is either devoid of branches or produces irregular, compressed, and often digitate processes, sometimes of considerable length and delicacy. Flesh-spicules are usually present throughout the sponge, but are sometimes absent from one part of a specimen and present in others. Some of the gemmules are often much smaller than the others. Perhaps this form should be regarded as a phase rather than a true variety (see p. 18). All forms of _S. alba_ can be distinguished from all forms of _S. lacustris_ by the much closer network of the skeleton and by the consequent hardness of the sponge; also by the complete absence of green corpuscles.