Ill 



PHYLUM AND CLASS PORIFERA 



117 





A somewhat more com 

 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 pares or ostia, and 

 lead directly into the 

 paragastric cavity by 

 means of ape/pyks — 

 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, U : 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- 

 rent centals, 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 

 prosopylcs (the equi- 

 valents of the inhalant 

 pores of the Oiynlhus), 

 with the"*radial canals. 

 Sponges similar to 

 Sycon gelatinosum, 



plex type of structure than that of Ascetta 



L 



Li| M 





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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 

 sndodena by short parallel strokes. A, cross-section 

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

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

 through a pirt of the wall of Livcilta ctmve&I / />, vertical 

 ii through OscarMa ; a, spaces of the incurrent canal 

 system ; b, spices of the excurrent canal system ; ot. oscu- 

 lum. (After Korseholt and Heider.) 



