118 



ZOOLOGY 



SECT. 



A somewhat more complex type of structure than that of Ascetta 



is exhibited by those 

 sponges in which the 

 wall becomes thick- 



becomes 

 ened and perforated 

 b y radially - arranged 

 canals, which open 

 directly on the outer 

 surface by means of in- 

 halant pores or ostia, 

 and lead directly into 

 the paragastric cavity 

 by means of apopyles 

 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. 87, 

 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 in- 

 current canals, lined 

 by ectoderm and com- 

 municating with the 

 exterior on the one 

 hand, either by a 

 wide opening or by 

 pores (ostia) perforat- 

 ing a pore-membrane, 

 and on the other by 

 means of small open- 

 ings, the prosopyles 

 (the equivalents 01 the 

 inhalant pores of the 

 Olynthus), with the 

 radial canals. Sponges 

 similar to Sycon gela- 

 tinosum, but with 





FIG. 87. Diagram of the canal system of various sponges, 

 the ectoderm denoted by a continuous narrow line ; the 

 flattened 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 Leucllla convexa ; D, vertical 

 section through Oscar ella ; a, spaces of the in current canal 

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

 osculum. (After Korschelt and Heider.) 



