GEODIA MEDIA. 



197 



covering the pits are the efferents. The fact that the pores in these sieves are 

 smaller than those on the smooth convex parts of the surface is on the other 

 hand in favour of the view that they are the afferents. 



The skeleton of the interior consists chiefly of amphioxes and large asters 

 (oxyasters and a few strongylasters). Besides these spicules also styles, various 

 angularly bent or branched amphiox- and style-derivates, and sterrasters occur 

 in small numbers. The amphioxes, styles, amphiox- and style-derivates are 

 arranged rather irregularly in the interior, but assume a regular radial arrange- 

 ment towards the surface. Here,, just below the cortex, plagiotriaenes, meso- 

 clades (mesomonaenes), and a few slender, amphiox-like mesoclade-derivates are 

 added to these megascleres. The clades of the plagiotriaenes lie at the limit 

 between the middle and inner layers of the cortex, the stcrraster-armour resting, 

 as it were, on the plagiotriaene-cladomes. Just below the cortex small oxysphaer- 

 asters, which are very numerous in some places, to a great extent replace the 

 large asters of the interior, and here also scattered sterrasters and small dermal 

 styles occur. The latter are situated radially, obliquely, or paratangentially (Plate 

 17, fig. 22d). The whole of the cortex, with the exception (jf the dermal layer, 

 contains sterrasters. These are scattered in small numbers in the thin, inner- 

 most, fibrous layer, and form a dense mass in the thick middle (sterraster-armour) 

 layer. Between the sterrasters a few small sphaerasters, dermal styles, and pro- 

 truding spicules occur. Small sphaerasters form a dense and continuous layer 

 on the outer surface of the dermal layer. Over the pits, where the latter is 

 greatly thickened (Plate 17, fig. 21), these sphaerasters are not confined to the 

 surface but are very numerous also for some distance below it. Tufts of small 

 dermal styles are met with on all parts of the surface. These styles are more 

 numerous in the thickened parts of the dermal membrane in the pits (Plate 17, 

 fig. 21b) than elsewhere. They are situated radially or obliquely; their rounded 

 ends lie proximally and are deeply imbedded in the sponge, their pointed ends 

 abut on the surface or pi'otrudc more or less beyond it (Plate 17, fig. 21). The 

 fact that these styles are abundant on the surface and rather plentiful also in 

 the subcortical layer, but comparatively rare in the sterraster-armour layer, 

 indicates that they move up from the distal parts of the choanosome, where 

 they are formed, to the surface, at first, till they reach the cortex, slowly, and 

 then, when they reach the sterraster-armour, rapidly, traversing the latter at 

 such a rate that at any time only a few are found in the act of passing 

 through it. 



Besides the spicules above described several other forms, not met with in 



