112 



THE INVERTEBRATA 



new oscula appear at the ends of the branches (Fig. 98). A higher 

 grade is reached when, as in the calcareous sponge Sycon (Fig. 99), 

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

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

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

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



inh.c. 



Fig. 99. 



Fig. 98. A branched calcareous sponge of the first (Ascon) type. From 



Sedgwick, after Haeckel. 



Fig. 99. A semidiagrammatic view of a simple Sycon, opened longitudinally, 



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



chamber. 



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

 pores, now known as prosopyles, into the excurrent canals, leaves 

 these through the openings, known as apopyles, by which they 



