STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 15 



which place the inhalant canals in direct communication with 

 the flagellated chambers. On looking down upon the wall of 

 a chamber the prosopyles are seen fairly regularly scattered 

 over it (figs. 23^ 25). In this way they are most readily 

 recognised, but in order to see the actual communication 

 between the flagellated chambers and the inhalant canals it is 

 necessary to examine very thin sections. We then see that 

 the amount of mesodermal tissue intervening between the 

 inhalant canals and the chambers which they supply is by no 

 means great. On approaching a prosopyle (fig. 26, pr.) it 

 thins away altogether^ and the flattened epithelial lining of the 

 inhalant canal meets the lining of collared cells of the chamber 

 around the margin of the circular aperture. The prosopyles 

 are scattered over the whole of the chamber, so that some are 

 found quite close to the exhalant aperture (fig. 26) ,• in such 

 cases the water can only just enter the chamber and leave it 

 again almost immediately. 



Mr. Carter gives a very diff'erent account of the prosopyles 

 in Grantia labyrinthica, which I have already quoted on 

 a previous page. They do not, as he suggests, " serve for the 

 purpose of intercommunication where the walls of the neigh- 

 bouring chambers are in direct contact with each other, or for 

 the purpose of allowing the ova developed in the intercameral 

 tissue to pass into the chamber and thus be expelled.^^ In 

 the first place it appears, as I shall show presently, that the 

 walls of neighbouring chambers never are in direct contact 

 with each other, nor have I found their cavities ever in direct 

 communication ; and, secondly, I have been able to prove (10) 

 that the embryos escape in a very different manner from that 

 suggested.^ In shorty the prosopyles of Grantia labyrin- 

 thica agree in function and position with those of other Sycons 

 as described and figured by Schulze (11), Polejaeff" (8), and 

 others. 



The Flagellated Chambers. 

 These have the usual Sycon character of more or less 

 ' Cf. next page. 



