Ill 



PHYLUM AND CLASS PORIFERA 



117 



A somewhat more complex type of structure than that of Ascetta 

 is exhibited by those 

 sponges in which the 

 wall 



becomes thick- 

 ened and perforated 

 by radially-arranged 

 canals, which open di- 

 rectly on the outer sur- 

 face by means of inhal- 

 ant pores or cstia, and 

 lead directly into the 

 paragastric cavity by 

 means of ftpcpyle* 

 the whole inner sur- 

 face as well as the 

 radial canals being 

 lined with flagellate 

 endoderm cells. In 

 forms which may be 

 regarded as represent- 

 ing the next stage 

 of development (Fig. 

 86, B: see also the 

 figures of Sycon gela- 

 tinosum), there are 

 formed by infolding 

 of the surface, in the 

 intervals between the 

 radial canals, canal- 

 like spaces, the incur- 

 rtnt canals, lined by 

 ectoderm and com- 

 municating with the 

 exterior on the one 

 hand, either by a 

 wide opening or by 

 pores (ostia) perfor- 

 ating a pore-mem- 

 brane, and on the 

 other by means of 

 small openings, the 

 prosopyles (the equi- 

 valents of the inhalant 

 pores of the OlynlJnis), 

 with the radial canals. 

 Sponges similar to 

 Sycon gelatinosum, 



3\ If a If 



FIG. 86. Diagram of the canal system of various sponges, the 

 ectoderm denoted by a continuous narrow line ; the flat- 

 tened endoderm by an interrupted line ; the flagellate 

 endoderm by short parallel strokes. A, cross-section 

 through a part of the wall of an Ascon ; B, cross-section 

 through a part of the wall of a Sycon ; C, cross-section 

 through a part of the wall of LewciUa conrexa ; D, vertical 

 section through Oscarella ; a, spaces' of the incurrent canal 

 system ; b, spaces of the excurrent canal system ; os. osm- 

 ium. (After Korschelt and Heider.) 



