The free larva has one of three forms, (I) It is an oval blastula with a large blastocoele. The cells are all similar, and the blastocoele is soon filled by the immigration of cells at all points as in Ascetta (Leucosolenia) primordialis, or at one pole as in A. (L.) blanca and A. (L.) clathrus among Calcarea Homocoela. The cells are collared and flagellate, and one pole is invaginated to form a gastrula in Oscarella lobularis. Or the posterior pole is a patch of rose-coloured non-ciliated cells as in Verongia (Aplysilla?) rosed1. (2) It is an oval amphiblastula, one half composed of large granular ectoderm cells, the other of columnar ciliated endoderm cells2, jointly inclosing a blastocoele. This larva occurs in all Calcarea save the three above-named and in the horny sponge Gumminea (Halisarca?) mimosa. In Sycandra (Sycon) raphanus the granular cells are derived by the growth of a circle of eight cells at one pole of the embryo after it has reached an advanced stage of segmentation (48 blastomeres): and the columnar cells are invaginated after a period of free larval existence, giving rise to a gastrula3. (3) It is solid, either oval or conical, with a posterior flattened or concave pole: its superficial cells are columnar, ciliated, disposed in a single layer, and form the ectoderm of the sponge.

They inclose (i) a mass of cells projecting posteriorly as in Chalinida fertilis, Espcria Lorenzii, Clione stationis (?); (ii) a mesoglaea with cells, all stellate as in Aplysilla sulfurea, stellate in the centre, hour-glass shaped at the periphery as in Spongelia pallescens; (iii) cells each contained in a clear capsule, the protoplasm reduced to a stellate figure by an accumulation of a clear liquid as in Euspongia; (iv) a granular material (coenoblastem of Marshall) with a few nuclei and subsequently many cells as in Plakina monolopha, or many nuclei as in Reniera jiligrana and apparently many other allied sponges. The origin of the contained nuclei or cells is not known except in Euspongia, where cells immigrate inwards during the progress of segmentation1. The posterior pole or the anterior (Reniera), sometimes both poles, are marked by a pigment patch. The larva is very-sensitive to light, and as a rule attracted by it, more seldom repelled (Remera). In Chalinula fertilis mesoglaea appears among the cells of the anterior and middle portions of the inclosed mass, and spicules, as is commonly the case, are formed in some of the cells near the periphery.

1 Halisarca Dujardini is said by Barrois to have a blastula; by Metschnikoff to have a solid larva with cells arranged in rosettes, and perhaps derived by immigration. H. pontica has solid larvae, and two or three of them may fuse to form a zygoplanula.

2 These ciliated cells are said to be collared: O. Schmidt, Z. W. Z. xxv. Suppl. pp. 129-30; cf. Haeckel, Kalkschwamme, i. p. 335.

3 The opposite, i. e. invagination of the granular non-flagellate cells, was said by Keller to happen in Ascandra (Leucosolenia) Lieberkilhnii and Leucandra (Leuconia) aspera, but the latter agrees with Sycandra (Sycori) raphanus, according to Metschnikoff. Barrois concluded that the granular cells gave rise to the endoderm in Ascandra (Leucosolenia) contorta, Sycandra (Grantia) compressa, S. coronata (Sycon coronatuni) and S. ciliata (Sycon ciliatum), but he observed neither invagination nor the act of fixation. Schulze states that abnormal or transitory invaginations of the granular cells, giving rise to a pseudo-gastrula, may occur in Sycandra (Sycon) raphanus: they are occasionally of great extent but have no significance in development. See Z. W. Z. xxxi. p. 266.

Fixation takes place by the gastrula mouth, which is subsequently closed in Sycandra (Sycon) raphanus and Oscarella lobular is 2; by the projecting cell-mass at first in Chalinula fertilis, afterwards by one of the two sides of the larva which has now become flattened; by the posterior pole in Plakina; by the central mass protruding at the posterior pole in Reniera filigrana. The ectoderm cells throw out short pseudopodial processes for the purpose of fixation, which are retracted as development progresses; they lose their columnar form and are gradually flattened. The stages subsequent to the larval have as yet been accurately observed only in a few instances. Sycandra (Sycon) raphanus becomes cylindrical with a flattened free pole, in the centre of which the osculum is formed. Before it appears, however, spicules are developed in mesoglaeal cells, and then protrude at the sides of the cylinder, which are pierced by pores. At the same time the endoderm cells, which lost their collars and flagella by absorption immediately after invagination, acquire them again. S. ciliata (Sycon ciliatum) may attain a height of I mm. before the radial cones begin to form as evaginations of the wall. The endoderm cells of the central tube flatten out and lose their flagella at a later period.

In Oscarella lobularis the gastric cavity or archenteron developes a peripheral circle of diverticula, some large, some small, parallel to the surface of the sponge, and directed aborally. These diverticula are in the course of growth thrown first into radial, i. e. zig-zag folds; the folds are then divided by constrictions into an outer and inner circle of chambers or ampullae, and the outer circle is 'doubled in number by the radial division of each ampulla. A cross section, however, shows that the ampullae form a single superficial layer, a condition which persists in the adult, and is seen also in some Plakinidae. The ampullae open into the central gastric cavity, the cells of which are now cubical and flagellate but no longer collared, by wide mouths subsequently narrowed, and they acquire an opening to the exterior either by the fusion of the superficial ectoderm with the endoderm of an ampulla, and the perforation of the fused spot, or by ectodermic invaginations which communicate with two adjoining ampullae. The osculum is formed by the growth of a solid aboral mesoglaeal cone, the invasion of this cone by a diverticulum of the gastric cavity, and the fusion and perforation of the ecto-and endo-derm at its apex.