PORIFERA 



119 



No sponge remains at this simple stage throughout its life. At the 

 least the body branches and thus complicates its shape, and then often 

 new oscula appear at the ends of the branches (Fig. 99). A higher 

 grade is reached when, as in the calcareous sponge Sycon (Fig. 100), 

 the greater part of the vase is covered with blind, thimble-shaped out- 

 growths, regularly arranged, and touching in places, but leaving 



inh.c. 



Fig. 100. 



Fig. 99. A branched calcareous sponge of the first (Ascon) type. From 

 Sedgwick, after Haeckel. 



Fig. 100. A semidiagrammatic view of a simple Sycon, opened longitudinally, 

 with a portion of the wall enlarged. i7ih.c. inhalent canal; fl.c. flagellated 

 chamber. 



between them channels, known as inhalant (or afferent) canals, whose 

 openings on the surface of the sponge are often narrowed and are 

 known as ostia. The thimble-shaped chambers are known as flagellated 

 chambers, and are lined by choanocytes, but these are now lacking 

 from the paragaster, where they are replaced by pinacocytes. Water 

 enters by the ostia, passes along the inhalant canals and through the 



