Spongillidæ differ greatly in consistency. Spongilla crassissima and Corvospongilla lapidosa are almost stony, although the former is extremely light, more like pumice than true stone. Other species (e. g., Trochospongilla latouchiana) are hard but brittle, while others again are soft and easily compressed, as Spongilla lacustris, the variety mollis of S. carteri, and S. crateriformis. The consistency of a sponge depends on two factors—the number of spicules present, and the amount of spongin. In Corvospongilla lapidosa the number of spicules is very large indeed. They are not arranged so as to form a reticulate skeleton but interlock in all directions, and there is hardly any spongin associated with them. In Spongilla crassissima, on the other hand, the number of spicules although large is not unusually so; but they form a very definitely reticulate skeleton, and are bound together by an unusually profuse secretion of spongin. In S. carteri var. mollis both spicules and spongin are reduced to a minimum, and the parenchyma is relatively more bulky than usual.
Variation.
Sponges are very variable organisms, and even a slight change in the environment of the freshwater species often produces a considerable change in form and structure. Some species vary in accordance with the season, and others without apparent cause. Not only have many given rise to subspecies and "varieties" that possess a certain stability, but most if not all are liable to smaller changes that apparently affect both the individual and the breed, at any rate for a period.
(a) Seasonal Variation.
Weltner has shown in a recent paper (Arch. Natg. Berlin, lxxiii (i), p. 276, 1907) that in Europe those individuals of Ephydatia which are found (exceptionally) in an active condition in winter differ considerably both as regards the number of their cells and their anatomy from those found in summer. In Calcutta the majority of the individuals of Spongilla carteri that are found in summer have their external surface unusually smooth and rounded, and contain in their parenchyma numerous cells the protoplasm of which is gorged with liquid. These cells give the whole sponge a faint pinkish tinge during life; but if it is plunged in spirit, both the liquid in the cells and the spirit turn rapidly of a dark brown colour. Specimens of Spongilla crateriformis taken in a certain tank in Calcutta during the cold weather had the majority of the skeleton-spicules blunt, while the extremities of the gemmule-spicules were distinctly differentiated. Specimens of the same species taken from the same tank in July had the skeleton-spicules pointed, while the extremities of the gemmule-spicules were much less clearly differentiated. I have been unable to confirm this by observations made on sponges from other tanks, but it would certainly suggest that at any rate the breed of sponges in the tank first investigated was liable to seasonal variation.
(b) Variation due directly to Environment.
The characteristic external form of freshwater sponges is liable in most cases to be altered as a direct result of changes in the environment. The following are two characteristic instances of this phenomenon.
Certain shrubs with slender stems grow in the water at the edge of Igatpuri Lake. The stems of these shrubs support many large examples of Spongilla carteri, which are kept in almost constant motion owing to the action of the wind on those parts of the shrubs that are not under water. The surface of the sponges is so affected by the currents of water thus set up against it that it is covered with deep grooves and high irregular ridges like cockscombs. Less than a hundred yards from the lake there is a small pond in which Spongilla carteri is also abundant. Here it grows on stones at the bottom and has the characteristic and almost smooth form of the species.
My second instance also refers in part to Igatpuri Lake. Corvospongilla lapidosa is common in the lake on the lower surface of stones, and also occurs at Nasik, about thirty miles away, on the walls of a conduit of dirty water. In the latter situation it has the form of large sheets of a blackish colour, with the surface corrugated and the oscula inconspicuous, while in the clear waters of the lake it is of a pale yellowish colour, occurs in small lichenoid patches, and has its oscula rendered conspicuous, in spite of their minute size, by being raised on little conical eminences in such a way that they resemble the craters of volcanoes in miniature.
Both the European and the Indian races of Spongilla lacustris fail to develop branches if growing in unfavourable conditions. In specimens obtained from the River Spree near Berlin these structures are sometimes many inches in length; while in mature specimens taken under stones in Loch Baa in the Island of Mull the whole organism consisted of a minute cushion-shaped mass less than an inch in diameter, and was also deficient in spicules. Both these breeds belong to the same species, and probably differ as a direct result of differences in environment.