Gemmule-spicules, which form a characteristic feature of the Spongillidæ, are very seldom absent when the gemmules are mature. They are of the greatest importance in distinguishing the genera. In their simplest form they closely resemble the free microscleres, but in several genera they bear, either at or near one end or at or near both ends, transverse disks which are either smooth or indented round the edge. In one genus (Pectispongilla) they are provided at both ends not with disks but with vertically parallel rows of spines resembling combs in appearance.
The simpler spicules of the Spongillidæ are formed in single cells (see fig. 2 E), but those of more complicated shape are produced by several cells acting in concert. Each spicule, although it is formed mainly of hydrated silica (opal), contains a slender organic filament running along its main axis inside the silica. This filament, or rather the tube in which it is contained, is often quite conspicuous, and in some species (e. g., Spongilla crassissima) its termination is marked at both ends of the megasclere by a minute conical protuberance in the silica.
Unless sponges are alchemists and can transmute one element into another, the material of which the spicules are made must ultimately come from the water in which the sponges live, or the rocks or other bodies to or near which they are attached. The amount of water that must pass through a large specimen of such a sponge as Spongilla carteri in order that it may obtain materials for its skeleton must be enormous, for silica is an insoluble substance. I have noticed, however, that this sponge is particularly abundant and grows with special luxuriance in ponds in which clothes are washed with soap, and my friend Mr. G. H. Tipper has suggested to me that possibly the alkali contained in the soap-suds may assist the sponge in dissolving out the silica contained in the mud at the bottom of the ponds. The question of how the mineral matter of the skeleton is obtained is, however, one about which we know nothing definite.
The spongin that binds the skeleton-spicules together takes the form of a colourless or yellowish transparent membrane, which is often practically invisible. When very abundant it sometimes extends across the nodes of the skeleton as a delicate veil. In some sponges it also forms a basal membrane in contact with the object to which the sponge is attached, and in some such cases the spongin of the radiating fibres is in direct continuity with that of the basal membrane.
Colour and Odour.
Most freshwater sponges have a bad odour, which is more marked in some species than in others. This odour is not peculiar to the Spongillidæ, for it is practically identical with that given out by the common marine sponge Halichondria panicea. Its function is probably protective, but how it is produced we do not know.
The coloration of freshwater sponges is usually dull and uniform, but Pectispongilla aurea is of the brilliant yellow indicated by its name, while many species are of the bright green shade characteristic of chlorophyll, the colouring matter of the leaves of plants. Many species are brown or grey, and some are almost white.
These colours are due to one of three causes, or to a combination of more than one of them, viz.:—(1) the inhalation of solid inorganic particles, which are engulfed by the cells; (2) the presence in the cells of coloured substances, solid or liquid, produced by the vital activities of the sponge; and (3) the presence in the cells of peculiar organized living bodies known as "green corpuscles."
Sponges living in muddy water are often nearly black. This is because the cells of their parenchyma are gorged with very minute solid particles of silt. If a sponge of the kind is kept in clean water for a few days, it often becomes almost white. An interesting experiment is easily performed to illustrate the absorption and final elimination of solid colouring matter by placing a living sponge (small specimens of Spongilla carteri are suitable) in a glass of clean water, and sprinkling finely powdered carmine in the water. In a few hours the sponge will be of a bright pink colour, but if only a little carmine is used at first and no more added, it will regain its normal greyish hue in a few days.
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.