PORIFERA. 803 



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 

 cells \ 



As to asexual reproduction, fission does not occur among Porifera, but 

 the washing Sponge (Euspongia) may be propagated artificially from frag- 

 ments. 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 

 question 2 . 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 con- 

 tinuous gemmation 3 . 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 



1 According to Ganin, the larval ectoderm persists ; the endodermal cavity becomes the internal 

 cavity, and outgrowths of it give rise to the ampullae, but the subdermal cavities originate indepen- 

 dently as spaces between the ectoderm and mesoglaeal cells. 



2 Whether or no buds are formed by the solid branching stolons of Esperia stolonifera from 

 the White Sea, described by Merejkowsky (Mem. Imp. Acad. St. Petersburg (7), xxvi. No. 7, p. 23), 

 seems uncertain. 



3 The ampullae probably multiply by division, or by the separation of a small part : see Keller 

 on Reniera semitubulosa, Z. W. Z. xxx. p. 579. Schulze states that isolated ampullae are met with 

 on the walls of the oscular tube where they are very thin in Oscaretta lobularis : see Z. W. Z. xxviii. 

 p. 23, PI. II. Fig. 9. The view held by some authorities that the ampulla represents an individual, 

 is negatived by the facts of embryology and the undoubted homology existing between ampullae and 

 radial cones. 



3 F 3 



