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. [Illustration: Fig. 6.--Radial section through part of a dried sponge of _Spongilla crassissima_ (from Calcutta), × 5.]