COMPLEX SPONGE BODIES 



85 



into the excurrent canals, leaves these through the openings, 

 known as apopyles, by which they communicate with the para- 

 gaster, and flows outwards through the osculum. A third grade 

 is found in sponges such as the calcareous sponge Leucilla, where 



a 



y- 



ch^ 



Fig. 52. — Part of a longitudinal section of the wall of an Olynthus, including a 



portion of the rim of the osculum. 



a., Amoeboid cell ; ch., choanocyte ; e, flat covering cells (pinacocytes) of dermal layer ; e'., similar cells lining 

 the rim of the osculum ; j, jelly ; p, pore ; pc, young porocyte ; pc'., fully developed porocyte ; sp., 

 spicule ; spc, spicule cell. 



the wall of the paragaster is folded a second time, so that the 

 flagellated chambers, instead of opening direct into the paragaster, 

 communicate with it by exhalant canals lined with pinacocytes. 



The three grades of sponge 

 structure, in which successively 

 the choanocytes line the whole 

 paragaster, are restricted to flag- 

 ellated chambers, or are still 

 further removed by the presence 

 of exhalant canals, are known 

 as the ' Ascon ', ' Sycon ', and 

 ' Leucon ' grades. In many of the 

 sponges whose canal systems are 

 of the third grade, the flagellated 

 chambers are no longer thimble- 

 shaped, but small and round. As 

 the canal system has grown more 

 intricate, complication has taken 

 place also in the skeletogenous 

 layer. It has grown thicker, form- 

 ing outside the flagellated chambers a layer known as the cortex, 

 in which the inhalant canals ramify ; and there appear in it 

 branched connective tissue cells which can change their shape. 

 The effect of these advances in complexity is (a) to increase 

 the rate and precision of the currents of water through the 



Fig. 53. — A branched calcareous 

 sponge of the first grade. — From 

 Sedgwick. 



