The green corpuscles of the Spongillidæ are not present in all species. There is every reason to think that they represent a stage in the life-history of an alga, and that they enter the sponge in an active condition (see p. 49).
A fourth cause for the coloration of freshwater sponges may be noted briefly. It is not a normal one, but occurs commonly in certain forms (e. g., Spongilla alba var. bengalensis). This cause is the growth in the canals and substance of the sponge of parasitic algaæ, which turn the whole organism of a dull green colour. They do not do so, however, until they have reduced it to a dying state. The commonest parasite of the kind is a filamentous species particularly common in brackish water in the Ganges delta.
External Form and Consistency.
Fig. 5.—Part of a type-specimen of Spongilla lacustris subsp. reticulata (nat. size).
The external form of sponges is very variable, but each species, subspecies, or variety of the Spongillidæ has normally a characteristic appearance. The European race of Spongilla lacustris, for example, consists in favourable circumstances of a flattened basal part from which long cylindrical branches grow out; while in the Indian race of the species these branches are flattened instead of being cylindrical, and anastomose freely. The structure of the branches is identical with that of the basal part. Many other species (for instance, Spongilla bombayensis and S. ultima) never produce branches but always consist of lichenoid or cushion-shaped masses. The appearance of Spongilla crateriformis, when it is growing on a flattened surface which allows it to develop its natural form, is very characteristic, for it consists of little flattened masses that seem to be running out towards one another, just as though the sponge had been dropped, spoonful by spoonful, in a viscous condition from a teaspoon. Some species, such as Trochospongilla phillottiana, cover large areas with a thin film of uniform thickness, while others (e. g., Spongilla alba and Ephydatia meyeni) consist of irregular masses, the surface of which bears numerous irregular ridges or conical, subquadrate, or digitate processes. In a few forms (e. g., Corvospongilla burmanica) the surface is covered with small turret-like projections of considerable regularity, and some (e. g., Spongilla crassissima) naturally assume a spherical or oval shape with an absolutely smooth surface.
The production of long branches is apparently rare in tropical freshwater sponges.
The form of the oscula is characteristic in many cases. No other Indian species has them so large, or with such well-defined margins as Spongilla carteri (Pl. II, fig. 1). In many species (Pl. II, fig. 3) they have a stellate appearance owing to the fact that grooves in the substance of the sponge radiate round them beneath the external membrane. In other species they are quite inconspicuous and very small.
Fig. 6.—Radial section through part of a dried sponge of Spongilla crassissima (from Calcutta), × 5.