STUDIES ON THE COMPAEATIVE ANATOMY OE SPONGES. 9 



5. The skeleton of the base or stalk (where one is 

 present) . 



It will be evident from the following account that Grantia 

 labyrinthica conforms very closely in the arrangement of 

 the skeleton to the normal Sycon plan. 



The Dermal Skeleton. 



This forms a distinct cortex of somewhat varying thickness 

 over the outer surface of the sponge (figs. 23, 25). The main 

 mass of this cortex consists of large sagittal triradiates, arranged 

 so that the long unpaired ray points towards the base of the 

 sponge. Even within the limits of the dermal cortex the tri- 

 radiates exhibit considerable individual variation in shape and 

 size. In the inner portion of the cortex numbers of the 

 singularly beautiful sagittal triradiates represented in figs. 13 

 and 14 occur. 



The outermost portion of the dermal skeleton consists of 

 great numbers of small oxeote spicules of the form represented 

 in figs. 10 and 11, placed at right angles to the surface, with 

 their narrow ends embedded amongst the large triradiates, and 

 their broad ends projecting freely (fig. 22). In the pore-areas 

 the dermal skeleton is practically reduced to this outer layer 

 of small oxea, many of which appear to be quite flat in the 

 thin dermal membrane. 



The Gastral Skeleton. 



The gastral skeleton (figs. 22, 25) forms a protective cortex 

 over the inner surface of the cup, exactly as the dermal 

 skeleton does over the outer. The gastral and dermal cortex 

 do not differ very greatly in thickness, although in this respect 

 there seems to be a good deal of variation. In the specimen 

 before me as I write the gastral cortex is decidedly thinner 

 than the dermal. Like the latter it is made up princi- 

 pally of triradiate spicules interwoven to form a feltwork ; 

 as before, the spicules are usually sagittal, with the long arm 

 pointing towards the base of the sponge. The beautiful 



