108 



ZOOLOGY 



SECT. 



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 

 directly on the outer 

 surface by means of 

 inhalant pores, 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 repre- 

 senting the next stage 

 of development (Fig. 

 77, 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- 

 rent canals, lined by 

 ectoderm and com- 

 municating with the 

 exterior on the one 

 hand, either by a 

 wide opening or by 

 pores perforating a 

 pore-membrane, and 

 on the other by means 

 of small openings, the 

 prosopyles, with the 

 radial canals. In 

 some Sponges of this 

 grade, as in those of 

 the last described, the 

 whole endoderm may 

 consist of flagellate 



FIG. 77. 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 pa^pf the wall of Leucilla convexa ; D, vertical 

 section through Oxr<i,-<.//n ; ., spaces of the incurrent canal 

 system ; l>, spaces of the excurrent canal system ; os. oscu- 

 lum. (After Korschelt and Heider.) 



