9 8 



SPONGES 



FlO. 68. 



Heteropegmanodusgordii, Pol., part of a transverse section. The external surface is upper- 

 most ; the gastral surface towards the lower side ; the spicules are represented by straight 

 continuous lines ; the flat epithelium by dotted lines ; the collar cells by numerous small circles 

 rendering the branching radial tubes dark. (After Polejaeff, Challenger Reports.) x50. 



The leuconoid type of canal system nas probably been evolved 

 from the syconoid type in more ways than one. There are at least 

 two modes of evolution which can be indicated with tolerable 



Fio. 69. 



Uie argentea, Pol., part of a transverse section. The concentric circles indicate transverse 

 sections of spicules, lying within the cortex. For other points see description of last figure. 

 (After Polejaeff, Challenger Reports.) xlOO. 



