[Illustration: FIG. 69.—Larva of _Gellius varius_ shortly after fixation. The pigmented pole, originally posterior, is turned towards the reader. _R_, Marginal membrane with pseudopodia; _x_, hinder pole. (After Maas.)] EPHYDATIA FLUVIATILIS. In the fresh water of our rivers, ponds, and lakes, sponges are represented very commonly by _Ephydatia (Spongilla) fluviatilis_, a cosmopolitan species. The search for specimens is most likely {175}to be successful if perpendicular timbers such as lock-gates are examined, or the underside of floating logs or barges, or overhanging branches of trees which dip beneath the surface of the water. The sponge is sessile and massive, seldom forming branches, and is often to be found in great luxuriance of growth, masses of many pounds weight having been taken off barges in the Thames. The colour ranges from flesh-tint to green, according to the exposure to light. This fact is dealt with in a most interesting paper by Professor Lankester,[202] who has shown not only that the green colour is due to the presence of chlorophyll, but that the colouring matter is contained in corpuscles similar to the chlorophyll corpuscles of green plants, and, further, that the flesh-coloured specimens contain colourless corpuscles, which, though differing in shape from those which contain the green pigment, are in all probability converted into these latter under the influence of sufficient light. The corpuscles, both green and colourless, are contained in amoeboid cells of the dermal layer;[203] and in the same cells but not in the corpuscles are to be found amyloid substances.