MANUAL OF ZOOLOGY 



SECT. 



Sycon belongs to a type of sponges intermediate between 

 the very simplest forms on the one hand, and the more 



complex on the other. The 

 simplest and most primitive 

 of known sponges is one 

 named Ascetta primordialis 

 (Fig. 37). It is vase-shaped, 

 contracted at the base to 

 form a sort of stalk, by the 

 expanded extremity of which 

 it is attached ; at the oppo- 

 site or free end is the circular 

 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 



FIG. 37. Ascetta primordialis. A 

 portion of the wall of the vase-like 

 sponge removed to show the para- 

 gastric cavity. (After Haeckel.) 



