318 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



the gallert, so that the main cause of the formation of a Sub- 

 epithelial layer in other Coelenterata does not influence the 

 Sponges. And as there also do not seem to be any nervous 

 elements in tbe Epithelia of the Sponge, no reason exists why a 

 Sub-epithelial layer should be formed. (The origin of the Sub- 

 epithelial layer in other Coelenterata ; compare O. and R. 

 Hertwig, Das Nervensystem und die Sinnesorgane der Medusen.) 



The ground substance, together with the cells imbedded therein, 

 is to be considered as Mesoderm, whilst the Epithelia are partly 

 Ectodermal and partly Entodermal. We shall commence with 

 the description of the Ectoderm. 



1. Ectoderm. 



The whole outer free surface of the Sponge is covered with a 

 low Epithelium, consisting of flat covering cells, each of which 

 may possess one swinging cilia. The tubes which connect the outer 

 surface with the inner canal system direct, or with the subdermal 

 cavities are covered by a continuation of the Ectodermal Epi- 

 thelium of the outer surface. The subdermal cavities and the 

 canals leading from them to the ciliated chambers are also covered 

 with the same Epithelium. Here the Ectoderm ends and all other 

 surfaces of the Sponge are covered with Entoderm. (F. E. 

 Schulze. Plakinidas.) 



According to the interesting observations of Marshall, it does 

 not appear improbable, that the Epithelium on the surface also of 

 those canals through which the water flows towards the ciliated 

 chambers is to be considered as Entoderm. 



In Reniera filigrana the parts of these canals adjacent to the 

 ciliated chambers certainly are covered with Entoderm. But 

 Marshall shows also in an extremely ingenious manner, that in 

 those cases where these canals are originally rarely formed by the 

 Ectoderm, they may be considered as Entodermal structures, 

 because they are formed by an invagination, which takes place very 

 early, and may therefore be considered as a sort of Gastrular 

 invagination. 



