Types. The types of the species and of the var. cerebellata are in the British Museum, with fragments of the former in the Indian Museum; that of var. bengalensis is in the Indian Museum, with a co-type in London.
Geographical Distribution.—India and Egypt. Localities:—Bombay Presidency, island of Bombay (Carter); Igatpuri, W. Ghats (Annandale): Bengal, Calcutta; Port Canning, Ganges delta (var. bengalensis) (Annandale); Garia, Salt Lakes, nr. Calcutta (var. bengalensis) (B. L. Chaudhuri); Chilka Lake, Orissa (var. bengalensis) (Gopal Chunder Chatterjee): Madras Presidency, Rambha, Ganjam district (Annandale): Nizam's Territory, Aurangabad (Bowerbank, var. cerebellata). The var. cerebellata has also been taken near Cairo.
Biology.—The typical form of the species is usually found growing on rocks or bricks at the edges of ponds, while the variety bengalensis abounds on grass-roots in pools and swamps of brackish water in the Ganges delta and has been found on mussel-shells (Modiola jenkinsi, Preston) in practically salt water in the Chilka Lake. Carter procured the typical form at Bombay on stones which were only covered for six months in the year, and "temporarily on floating objects." In Calcutta this form flourishes in the cold weather on artificial stonework in the "tanks" together with S. carteri, S. fragilis, Ephydatia meyeni, and Trochospongilla latouchiana.
The variety bengalensis is best known to me as it occurs in certain ponds of brackish water at Port Canning on the Mutlah River, which connects the Salt Lakes near Calcutta with the sea. It appears in these ponds in great luxuriance every year at the beginning of the cold weather and often coats the whole edge for a space of several hundred feet, growing in irregular masses which are more or less fused together on the roots and stems of a species of grass that flourishes in such situations. Apparently the tendency for the sponges to form branches is much more marked in some years than in others (see Pl. I, figs. 1-3). The gemmules germinate towards the end of the "rains," and large masses of sponge are not formed much before December. At this season, however, the level of the water in the ponds sinks considerably and many of the sponges become dry. If high winds occur, the dry sponges are broken up and often carried for considerable distances over the flat surrounding country. In January the gemmules floating on the surface of the ponds form a regular scum. S. alba var. bengalensis is the only sponge that occurs in these ponds at Port Canning, but S. lacustris, subsp. reticulata, is occasionally found with it on brickwork in the ditches that drain off the water from the neighbouring fields into the Mutlah estuary. The latter sponge, however, perishes as these ditches dry up, at an earlier period than that at which S. alba reaches its maximum development.
The larvæ of Sisyra indica are commonly found in the oscula of the typical form of S. alba as well as in those of S. lacustris subsp. reticulata, and S. carteri; but the compact structure of the sponge renders it a less suitable residence for other incolæ than S. carteri.
In the variety bengalensis, as it grows in the ponds at Port Canning, a large number of arthropods, molluscs and other small animals take shelter. Apart from protozoa and rotifers, which have as yet been little studied, the following are some of the more abundant inhabitants of the sponge:—The sea-anemone, Sagartia schilleriana subsp. exul (see p. 140), which frequently occurs in very large numbers in the broader canals; the free-living nematode, Oncholaimus indicus[[W]], which makes its way in and out of the oscula; molluscs belonging to several species of the genus Corbula, which conceal themselves in the canals but are sometimes engulfed in the growing sponge and so perish; young individuals of the crab Varuna litterata, which hide among the branches and ramifications of the larger sponges together with several small species of prawns and the schizopod Macropsis orientalis[[X]]; the peculiar amphipod Quadrivisio bengalensis[[Y]], only known from the ponds at Port Canning, which breeds in little communities inside the sponge; a small isopod[[Z]], allied to Sphæroma walkeri, Stebbing; the larva of a may-fly, and those of at least two midges (Chironomidæ).
The peculiarly mixed nature (marine and lacustrine) of the fauna associated with S. alba in the ponds at Port Canning is well illustrated by this list, and it only remains to be stated that little fish (Gobius alcockii, Barbus stigma, Haplochilus melanostigma, H. panchax, etc.) are very common and feed readily on injured sponges. They are apparently unable to attack a sponge so long as its external membrane is intact, but if this membrane is broken, they swarm round the sponge and devour the parenchyma greedily. In fresh water one of these fishes (Gobius alcockii, see p. 94) lays its eggs in sponges.
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.
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).