76 



PORIFERA. 



point (Sycetta). But in the more complex forms the walls of 

 adjacent chambers fuse more or less completely where they touch, 

 and the spaces between them are broken up and called the inlialent 

 canals or intercanals. These intercanals open by the prosopyles 

 into the chambers, and outwards on the surface of the sponge 

 (Sycoji). In still more complex forms the outer ends of the 

 chambers and the openings of the intercanals are covered by a 

 membrane called the dermal membrane or cortex. This cortex is 

 perforated by numerous pores the dermal pores (to be dis- 

 tinguished from the prosopyles) which lead into the intercanals 



FIG. CO. 1, Diagrammatic section of a PJiagon. H hypopliare ; osculum ; G gastral cavity ; 



c flagellated chambers ; pi prosopyle (from Perrier, after Sollas). 

 2, Diagram of a simple form of the Eurypyluus type of canal-system. The spcmgophare is 



folded. H hypopliare ; G gastral cavity ; o osculum ; i incurrent sinuses ; p dermal pores. 



(Grantia, Ute, Sycyssa, Heferopegma, Amphoriseus, etc.). In this 

 type the chambers may branch, and the dermal cortex may attain 

 a considerable thickness, as also may the wall of the central tube 

 (gastral cortex). A further complication is effected by the retreat 

 of the flagellated cells towards the distal (outer) ends of the 

 chambers, their place being taken by pavement epithelium. We 

 thus get exhalent canals coming off from the gastral cavity and lined 



