The colouring matter produced by the sponge itself is of two kinds--pigment, which is probably a waste product, and the substances produced directly by the ingestion of food or in the process of its digestion. When pigment is produced it takes the form of minute granules lying in the cells of the parenchyma, the dermal membrane being as a rule colourless. Very little is known about the pigments of freshwater sponges, and even less about the direct products of metabolism. It is apparently the latter, however, that give many otherwise colourless sponges a slight pinkish or yellowish tinge directly due to the presence in cells of the parenchyma of minute liquid globules. In one form of _Spongilla carteri_ these globules turn of a dark brown colour if treated with alcohol. The brilliant colour of _Pectispongilla aurea_ is due not to solid granules but to a liquid or semi-liquid substance contained in the cells. 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. [Illustration: Fig. 5.--Part of a type-specimen of _Spongilla lacustris_ subsp. _reticulata_ (nat. size).]