PORIFERA 



203 



Sponges of this simple ascon type are few. As a rule sponges are more 

 massive and have a more complicated canal system (iigs. 161, 162). The 

 first step towards complication is seen in the Sycon type, in which the gas- 

 tral cavity consists of numerous radial chambers or ampulla: which alone 





FIG. 162. Section of cortex of Chondrillanucula, the skeleton omitted (after Schuize). 

 c l , afferent canals; c 2 , efferent canals; g, ampullae; m, cloaca; o, osculum. 



contain the collared cells, while the central cavity, now called cloaca, is 

 lined with pavement epithelium. By increase of mesoderm and corre- 

 sponding thickening of the body wall the ampulla become separated from 

 external and cloacal surfaces (Leucon type). They nevertheless retain 

 their connection with both surfaces by means of cavities which may 



sSSfr 



A 



rf 



FIG. 163. FIG. 164. FIG. 165. 



FIG. 163. Surface view of dermal pores of Aplysina aerophoba (after Schuize). 

 FIG. 164. Ascyssa acufera (after Haeckel). 

 FIG. 165. Leucetta sagittata (after Haeckel). 



be lacunar (fig. 161) or consist of a system of canals. The canal system 

 is double; one part is incurrent and leads from the dermal pores to the 

 ampulke; the other or excurrent, from the ampulla- to the cloaca, the two 

 being connected by the ampulke alone (fig. 162), the canals from the pores 



