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.