PORIFERA 



203 



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

 massive and have a more comphcated canal system (figs. i6i, 162). The 

 tirst step tovi^ards complication is seen in the Sycon type, in which the gas- 

 tral cavity consists of numerous radial chambers or avijni-UcB which alone 





'■Mik 



Fig. 162. — Section of cortex oiChondrilla nucula, the skeleton omitted fatter Scliulze). 

 c', afferent canals; c-, effereni canals; g, ampuUas; m, cloaca; 0, 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 ampullie become separated from 

 external and cloacal surfaces (Leucon t}fpe). They nevertheless retain 

 their connection with both surfaces by means of cavities which may 



'f 









(sis 



Fig. 163. 

 Fig. 163. — Surface' 



ew of dermal pores of Aplysina aerofkoba (after Schulze). 



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 

 ampullce ; the other or excurrent, from the ampullae to the cloaca, the two 

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



