Mesoglaea appears between the ecto- and endo-derm after fixation in both sponges alike; its cells are derived in Sycandra (Sycon) from the ectoderm, in Oscarella from the endoderm1. The attached larvae of Chalinula fertilis, Plakina monolopha, and Reniera filigrana become flattened, but of the first-named subsequently raised into an eminence. The gastric cavity makes its appearance as an incomplete or complete ring, afterwards widening into a simple central space in Plakina, as a simple space in the other two named. The spaces are lined by a single layer of columnar cells delaminated from the common cell-mass. The ampullae originate in Chalinula from groups of deeply coloured cells, which acquire a cavity and open into the central space; in Reniera as outgrowths of the central space. In Plakina they are oval chambers lined by collared cells, opening as in Chalinula, but their mode of origin is not known. The cells of the central mass remaining after the differentiation of the endoderm become mesoglaeal cells, a gelatinous substance appearing between them. The osculum of Chalinula is central, of Plakina marginal, but in both formed seemingly as a simple perforation.
In Reniera the central space extends towards the surface and opens in the anterior protruded portion of the central mass of the larva. The pores and inhalent canals of Plakina are believed by Schulze to be of ectodermic origin. The outgrowths of the central space in Reniera not only give origin to ampullae and the exhalent canals, but outgrowths from the ampullae to other ampullae and to the inhalent canals. The latter open outwards through the ectoderm in protrusions of the mesoglaea (Marshall)2.
1The anterior pointed pole and the posterior concave pole of the conical larva are said to be sometimes non-ciliate, and the latter to be girdled by cilia of exceptional length. But Schulze found the concave pole ciliated in Spongelia and Euspongia, and Keller states that in Chalinula fertilis it is non-ciliate before the escape of the larva, that it then acquires cilia but loses them just before fixation. In Reniera filigrana, according to Marshall, the central mass forces its way out at each pole of the oval larva when it is mature, and the anterior bare patch is surrounded by very long cilia.
2Oscarella is attached in later stages by one or two foot-like processes ( = tentacles ?), and here and there by the base of the sponge, i. e. by the surface formed on the closure of the gastrula mouth. Heider states that the ectoderm covering the feet is columnar and secretes a clear cement.
The development of the freshwater Meyenia (Spongilla) fluviatilis is, if Goette's results are to be trusted, extremely peculiar in some respects. The primitive ovum divides; one of the cells thus produced grows into the definitive ovum, and of the remainder some perhaps take part in forming the follicle, whilst others are nutritive, and either atrophy or fuse with the ovum. Segmentation is total; the blastomeres are irregularly massed; a cap of cubical cells, the ectoderm, is differentiated and grows round the remaining cells, which inclose an 'endodermal' cavity excentrically placed near one - the apical - pole, but of no significance. The ectoderm becomes ciliated. The metamorphosis of the ovate larva may be accomplished after fixation, which takes place by the apical pole, during its free life or within the egg-follicle. Its two principal features are the loss of the ectoderm and the obliteration of the endodermal cavity. The cell-mass inclosed by the ectoderm forms the sponge. Its most superficial cells become epidermis. Spaces or gaps between the cells give rise to the pores and oscula. The ampullae are formed independently of one another from large cells which bud, a cavity-appearing in the cell-mass. Several such may fuse together.
The subdermal cavities, the in- and ex-halent canals, are developed from intercellular spaces, their epithelium from amoeboid cells1.
1The ectoderm cells in question of Sycandra (Sycon) may be differentiated in part before fixation (Metschnikoff). Heider thinks that the blastocoele of Oscarella is filled by a gelatinous fluid from which the first-formed mesoglaea is derived. The growth of the mesoglaea probably causes the formation of the diverticula and their resolution into ampullae, above described. It is possible that new ampullae may develope as evaginations of the endoderm at or near the base of the young Oscarella.
2In Halisarca Dujardini certain of the cells in the newly attached larva, which is solid, become grouped; within each group is formed a canal; the canals are at first independent, but afterwards fuse into a system with a central space. So, too, in Ascetta (Leucosolenia) primordialis and A. blanca, a portion of the cells lengthen and are radiately arranged round a common centre, where a space has been observed in the latter of the two sponges named. A delamination of the contained cell-mass into endoderm and mesoglaeal cells is probably characteristic of all solid larvae.
As to asexual reproduction, fission does not occur among Porifera, but the washing Sponge (Euspongia) may be propagated artificially from fragments. It is generally said that many sponges are colonial. Where individuals concresce, or where there is, as in Homoderma, a creeping tubular stolon connecting the individuals, there can be no doubt about the question2. And in those instances where a given sponge consists of columnar or cylindrical masses united by a common base or stem, and furnished each with its own osculum, it is natural to regard it as a colony. But oscula cannot be considered as the equivalents of mouths; they are often absent; and though a young sponge just developed from a larva possesses but one osculum, it is doubtful if an increase in the number of such apertures as the sponge grows in size necessarily indicates the occurrence of continuous gemmation3. Discontinuous gemmation, however, occurs, and falls under two types. (I) In the Calcarean Ascandra variabilis - Leucosolenia botryoides tubular outgrowths of the body-walls are developed and set free, the aperture formed by detachment becoming the osculum of the new sponge. (2) In Tetilla radiata, T. euplocamns (Ancorinidae), and Tethya maza (Tethyadae) mesoglaeal cells multiply and form a mass near the surface of the sponge.