Fresh-Water Sponges

Abstract

Nearly all sponges are sea animals, but one family, the Spongillidae, live in fresh water. There are five British species, two common ones being the River Sponge (Ephydatia [or Spongilla] fluviatilis) and the Pond Sponge (Euspongilla [or Spongilla] lacustris). Both these species form greenish or yellowish encrusting growths over the surfaces of stones and water-plants. The colour depends on whether the sponge is growing in a good or poor light, because the greenness is due to tiny one-celled plants similar to those which live inside Hydras (p. 17). These plants only develop their chlorophyll (the green colouring matter in plant cells) when they are exposed to light. The river sponge is common in many places, it forms flat encrustations on the underside of stones, when its colour is yellowish or dirty white; green specimens are found on plant stems or pieces of timber which are exposed to the light. Rivers and canals are its chief habitats. The pond sponge is found in still water only. It has a similar encrusting habit, and is characterized by a large number of finger-shaped projections on its surface (Fig. 1). It is generally a brighter green than the river sponge and it has a coarser texture.