The chief enemy of the sponges at Port Canning is, however, not an animal but a plant, viz., a green filamentous alga which grows inside the sponge, penetrating its substance, blocking up its canals and so causing it to die. Similar algæ have been described as being beneficial to the sponges in which they grow[AA], but my experience is that they are deadly enemies, for the growth of such algæ is one of the difficulties which must be fought in keeping sponges alive in an aquarium. The alga that grows in _S. alba_ often gives it a dark green colour, which is, however, quite different from the bright green caused by the presence of green corpuscles. The colour of healthy specimens of the variety _bengalensis_ is a rather dark grey, which appears to be due to minute inorganic particles taken into the cells of the parenchyma from the exceedingly muddy water in which this sponge usually grows. If the sponge is found in clean water, to whichever variety of the species it belongs, it is nearly white with a slight yellowish tinge. Even when the typical form is growing in close proximity to _S. proliferens_, as is often the case, no trace of green corpuscles is found in its cells. [Footnote AA: See M. and A. Weber in M. Weber's Zool. Ergeb. Niederl. Ost-Ind. vol. i, p. 48, pl. v (1890).] 4. Spongilla cinerea*, _Carter_. _Spongilla cinerea_, Carter, J. Bombay Soc. iii, p. 30, pl. i, fig. 5, & Ann. Nat. Hist. (2) iv, p. 82, pl. iii, fig. 5 (1849). _Spongilla cinerea_, Bowerbank, P. Zool. Soc. London, 1863, p. 468, pl. xxxviii, fig. 19. _Spongilla cinerea_, Carter, Ann. Nat. Hist. (5) vii, p. 263 (1881). _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.