320 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



These ciliated chambers are the main characteristic of the 

 Sponges and have no homologue in other Coelenterata. The cells 

 which cover the inner surface of the chambers, always possess 

 a frill at their free end, and a cilia in the middle of the free 

 surface. Standing close together, and pressing each other laterally, 

 they attain the shape of hexagonal prisms. At the vaulted end 

 of the chamber a few of these cells here and there are wanting ; 

 here the pores are situated, through which the water enters the 

 chambers from the supplying Ectodermal canals. These apertures 

 are always very small, and it appears that the Sponge has the power 

 of closing some or all of them. 



Towards the other end of the chamber the cells get gradually 

 lower and pass into the Entodermal Epithel of the drainage 

 canals. There always is only one outlet, and this is of a large size, 

 generally about half as wide as the chamber itself. It cannot be 

 closed by the Sponge. 



The Entoderm covers the surfaces of all the drainage-canals, 

 and the oscular tube and ends at the margin of the latter. The 

 boundary between Ectoderm and Entoderm lies in the circum- 

 ference of the Osculum. 



3. Mesoderm. 



The same difference between the structure of the Mesoderm of 

 the Polypomedusa? on the one hand and the Ctenophorse on the 

 other, (Hertwig der Bau der Ctenophoi-en, Chun die Ctenophoren 

 des Golfes von Neapel), also appertains between the Mesoderm of 

 Polypomedusse and the Mesoderm of Sponges. 



In some respects the Sponge mesoderm shows affinities to the 

 Mesoderm of the Ctenophorse. Whilst the cells in the ground 

 substance of the Polypouiedusfe, that is the primary Mesoderm, 

 (von Lendenfeld : Encopella campanularia) have only a nourishing 

 function and are derived from the Entoderm ; the Mesoderm 

 cells of the Ctenophorfo and Sponges have other functions also, 

 and are in the Ctenophorte (Kawolevsky) and, perhaps, also in the 

 Sponges derived from the Ectoderm. 



