Ill 



PHYLUM PORIFERA 



osculum. So far there is a considerable resemblance to 

 Sycon gelatinosum ; but the structure of the wall in Ascetta 

 is extremely simple. Regu- 

 larly arranged over the sur- 

 face are a number of small 

 rounded apertures, the in- 

 halant or incurrent pores ; 

 but, since the wall of the 

 sponge is very thin, these 

 apertures lead directly into 

 the central or paragastric 

 cavity, the long passages or 

 canals through which the 

 communication is effected in 

 Sycon being absent. The 

 wall consists of the same 

 three layers as in Sycon ; but 

 the middle one, though it 

 contains a small number of 

 spicules, is very thin ; the 

 endoderm, which lines the 

 paragastric cavity, consists 

 throughout of flagellate col- 

 lared cells similar to those of 

 the flagellate canals of Sycon. 

 The majority of Sponges, 

 however, are more compli- 

 cated in structure than Sycon, 

 one of the causes of their 

 complexity being that the 

 canals, instead of being simple 



and straight, become branched, forming a system, often 

 highly complicated, of ramifying channels. In these more 



FIG. 36. Ascetta primordialis. A 

 portion of the wall of the vase-like 

 sponge removed to show the para- 

 gastric cavity. (After Haeckel.) 



