From the Isis, growing on the wall of a lock, with Figure 12, illustrating the structure of Enspongia officinalis.

This specimen, like the preceding Hydroid (Prep. 50), is plant-like in appearance, consisting, as it does, of a base of attachment and two upright stems arising close together. Spongilla lacustris often attains a large size. Owing to its having been preserved in spirit, this specimen has the surface more fenestrated than in the living condition; its protrusible oscula or exhalent apertures are no longer visible; and its natural emerald-green colour is lost. The surface is rough with protruding bundles of the silici-cous spiculae which make up the skeleton of this sponge. The small inhalent orifices or 'pores,' characteristic of the class Porifera, are not distinguishable: indeed they are of microscopic size. It may be noted that the two stems have come into contact by their sides, and that at the point of contact they have fused or undergone 'concrescence.' Concrescence of this kind is an exceedingly common phenomenon in Porifera, and is one of the causes of the great variability in form of the species. But it does not occur between sponges of different species.

Yellow seed-like bodies, the gemmules (statoblasts), may be seen in the substance of the base of attachment: they are formed at the approach of cold weather in the European, at the approach of the dry season in the Indian, fresh-water sponges.

There appear to be two fresh-water Sponges in Great Britain - Spongilla lacustris and Meyenia (Spongilla) fluviatilis. The former is branched, the latter massive and lobate. The principal distinction between the two rests on the structure of the gemmule or statoblast. These bodies are more or less globular in shape, and possess at some one point a pore which is placed at the bottom of a funnel-shaped depression, through which the inclosed mass of sponge cells makes its exit at the proper season. A delicate membrane, finely reticulate, immediately invests this mass of cells, and protrudes slightly into the pore. Externally to it lie two coats - a yellowish chitinoid coat and a 'crust,' both of which are deficient at the pore. The crust varies in thickness. In the two British sponges it consists of a granular cell-like structure, which appears to contain silica. In Spongilla lacustris it lodges curved stout fusiform siliceous spicules, the surfaces of which are beset with stout recurved spines. They are arranged tangentially, and give the outer aspect of the crust an appearance like the lines of a so-called 'engine-turned' watch-case. In Meyenia fluviatilis the spicules of the crust are birotulate, and are known as amphidiscs.

They consist of a shaft terminated at each end by a disc deeply and irregularly denticulate at its margin. The amphidiscs are set parallel to one another, and vertically to the outer surface of the crust. Marshall states that the gemmule is formed thus. In autumn a number of amoeboid wandering cells collect in the inhalent canals or ampullae (infra), pass into the mesodermic tissue, and group themselves round one or two mesodermic cells. A pellicle appears on the surface of the mass. Mesodermic cells next form a capsule round it, and are transformed into the crust and its spicules. The parent sponge then dies away. The cells of the gemmule, at first distinct, gradually swell and form a syncytium, which emits a pseudopodium through the pore. In April or May the mass escapes, remains seated on the empty gemmule case, then floats for a couple of days, and finally comes to rest. A clear ectoderm is distinguishable from a more granular internal mass. An enteric cavity appears, before or after the osculum and inhalent pores, both of which may be absent. The young Spongilla becomes sexual. The sexes are separate. The males have no osculum or enteric cavity (?); the females have them.

The ovum is fertilised, and the ciliated embryo is set free.

The males appear to perish, but the females grow, and are stated to lose their oscula and enteric cavities (= ampullae) more or less completely. The ciliated embryo produces an individual which is never sexual, but gives origin in autumn to gemmules. There is therefore an alternation of generations (Marshall).

The green colour of Spongilla is developed only when the sponge grows freely exposed to light. If it is shaded, a pale flesh-colour takes the place of green. However, a piece of such a pale-coloured sponge turns green when dipped into sulphuric acid, as does the colourless saprophyte plant Neottia. A flesh-coloured sponge contains in its cells angular particles; these in pale-green specimens are found mixed with green concavo-convex discs, which appear to be derived from them, and which are present in great abundance in an ordinary full-green specimen. These green bodies have been considered by Brandt (op. cit. p. 245 ante) to be symbiotic algae. The green colouring matter itself appears to have a similar constitution to that of higher plants, but the proportions of the constituents are different. The cells of Spongilla, whether flesh-coloured or green, contain starch in solution.

The structure of Spongilla has not been properly worked out, and the anatomy of a sponge (Fig. 12, A.B.C.) is best illustrated by that of the sponge of commerce, Euspongia officinalis, which grows in quantity in the Mediterranean Sea. There are, according to Schulze (op. cit. infra), six varieties of it, differing in external shape, disposition of the oscula, and of the fibrous skeleton.

A. Section of Euspongia officinalis, after Schulze, op. cit. infra, P1. xxxvi. Fig. 2.

Fig. 12. A. Section of Euspongia officinalis, after Schulze, op. cit. infra, P1. xxxvi. Fig. 2.

B. Ampulla of same (Schulze, PL xxxvii. Fig. 11 in part).

C. Young secondary spongin fibre surrounded by spongoblasts (sp.) of same (Schulze, Pl. xxxvi. Fig. 6 in part).