803 THE ANIMAL KINGDOM. 



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 com- 

 municate 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. 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 endoderm 1 . 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 Chalimila 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 Chalimda, 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 pro- 

 truded 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 . 



The development of the freshwater Meyenia (Spongilld) 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 



1 The ectoderm cells in question of Sycandra (Sycori] may be differentiated in part before fixa- 

 tion (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. 



" In 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. 



