BY R. VON LENDENFELD, PH.D. 325 



Now a small cavity makes it appearance in the centre of the 

 Ccenoblastem, which may have the shape of a ring (Plakina) or 

 a lens. This original Gastral cavity is perfectly homologous to 

 the Gastral cavity of all those Embryos of other Metazoa which 

 produce their Entoderm by delamination and not by direct 

 invagination. From this simple cavity soon sack-shaped excresences 

 grow forth. The ciliated chambers produced at their termini 

 form short excrescences which connect them with the surface. In 

 this manner tubes are formed, the commencement of the intro- 

 ductory canal system. In a similar manner the Oscular tube is 

 formed from the gastral cavity. 



As to the further development of the Sponge from this indis- 

 puted stage, there are two opposite opinions, and both from such 

 authorities, that it will be best to state both without commentary. 



Schulze believes that by a complicated process of continual 

 folding the sponge is finally produced in such a manner that the 

 introductory canals are nothing else than parts of the original free 

 surface of the Sponge, at the sides of which two portions of the 

 outer surface which have come in close contact by the formation of 

 a fold, have coalesced. 



Marshall on the other hand thinks that all these canals are 

 formed from the ciliated chambers by sending tubes centrifugally, 

 which when they reach the surface open there and so form the 

 introductory canal. The outer surface remaining from beginning 

 to end unfolded and unchanged. 



Further investigations will be necessary to show which Sponges 

 are developed in the former and which in the latter way. I refer 

 the reader to the statements made above under the heading 

 Ectoderm. 



IV. Physiology. 



The investigations on this subject are still fewer in number than 

 those on the former subject, and as yet hardly permit of generali- 

 sation. 



1. Skeleton. 

 a. Calcispongice. 



The skeleton of the Calcareous Sponges consists as the name 

 indicates partly of Calcium. 



