I20 



THE INVERTEBRATA 



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

 these through the openings, known as apopyles^ by which they com- 

 municate with the paragaster, and flows outwards through the 

 osculum. A third grade is found in sponges such as the calcareous 

 sponge Leucandra (Fig. loi), where 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 (or efferent) 

 canals lined with pinacocytes. 



The three grades of sponge structure (Fig. 102), in which suc- 

 cessively the choanocytes line the whole paragaster, are restricted to 

 flagellated chambers, or are still further removed by the presence of 

 exhalant canals, are known as the *' Ascon ", *' Sycon ", and *' Leucon " 



Fig. 1 01. Diagram of a section of the wall of the sponge Leucandra aspersa, 

 showing the direction of the currents. After Bidder. 



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, com- 

 plication has taken place also in the skeletogenous layer. It has grown 

 thicker, forming 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 sponges which we have so far considered have skeletons com- 

 posed solely of calcareous spicules, and their choanocytes are re- 

 latively large. They constitute a comparatively small group, the class 

 Calcarea. The majority of the phylum are without calcareous spicules 

 and have relatively small choanocytes. They have usually siliceous 



