SIL.ICIOUS AND HORNY SPONGES WILSON. 323 



1-3 mm. The latero-inferior surface bears no oscular elevations. It 

 is the pore surface and is marked by intercommunicating shallow 

 depressions, 2-3 mm. apart and about 1 mm. wide, which divide it up 

 into irregular and only very slightly prominent areas. 



The cortex is whitish, the interior darker, gray-brown and com- 

 pact. The cortex is about 1.8 mm. thick and is occupied almost 

 entirely by the sterrastral layer. The ectochrote over the bulk of the 

 sterrastral layer, except in fact in the sterraster-free areas, is rep- 

 resented only by a very thin dermal membrane. 



The pores lie in small and irregular sterraster-free areas abun- 

 dantly scattered over the latero-inferior surface of the sponge (pi. 47, 

 fig. 6, p. a.). The smaller areas measure about 200 [x in diameter ; the 

 larger ones are several times that size. The areas are vaguely defined 

 and by no means always sharply separated; the appearances are as 

 if the sterrasters had been shifted about a good deal after the closure 

 of the pores. Sometimes only one pore occupies an area ; larger areas 

 include several pores. The pores are mostly closed. There are some 

 open ones, however, and these are about 100 j/. in diameter. 



The cortex of the latero-inferior surface is traversed by radial 

 afferent chone-canals about 200 [i in diameter, each surrounded by 

 some collenchyma. Into the outer end of each there open a few 

 smaller oblique canals leading from the surface of the sponge. It is 

 into these doubtless that the pores open, and the actual connection 

 could be made out in a few cases. The afferent chone-canals are 2-3 

 mm. apart and end below, each, in a small endochone. The endo- 

 chones are contracted, appearing as conical masses of densely fibrous 

 tissue, with the included canal closed. They are much smaller than 

 the corresponding structures of the efferent system (pi. 47, fig. 7). 

 The apex of the chone projects into a subcortical canal. 



My data make it clear that the sponge is to be classed among 

 those with cribriporal afferents. 



The oscula are the uniporal apertures of radial efferent chone- 

 canals (pi. 47, fig. 7, ef. c. c.) The chone-canal from the surface 

 of the sponge to the lower limit of the sterrastral layer is open, al- 

 though somewhat constricted at several levels; the diameter varying 

 from 200 to 400 pt.. Round it is some finely fibrous collenchyma that 

 is evidently contractile. At the lower level of the sterrastral layer, 

 the chonal canal passes in the usual way into its very narrow inner- 

 most part, which occupies the axis of the endochone. This part of 

 the canal is closed but is marked out by a streak of asters. The 

 endochone has the usual character, being a conical densely fibrous 

 mass, the apex (ch. e.) of which projects into a subcortical canal. 



The megasclere skeleton has the arrangement that is usual in the 

 family — that is, the peripheral choanosome is occupied by numer- 



