BY E. P. KALLMANN. 395 



(ii.) of strongly developed, single fibres radiating therefrom out- 

 wards to the surface, at an average distance apart of about 200ii. 

 Also participating in the formation of the axial skeleton, but 

 occurring only interstitially, are from scarce to fairly numerous, 

 singly dispersed, long slender styli, directed longitudinally; 

 these spicules appear to be more abundant in the older portions 

 of the sponge. The spaces between the extra-axial fibres are 

 entirely free from spicules. The axial reticulation, as seen in 

 longitudinal transverse section, presents an irregularly sub- 

 renieroid pattern, not altogether unlike that characteristic of 

 the genus Petrosia; examined, however, in section parallel to 

 and in the mid-plane of the sponge (Plate xxi., figs. 3, 4) it is 

 seen to be formed {i.e., in its most central region) of somewhat 

 ill-defined, plurispicular strands (of the already mentioned small 

 styli) running longitudinally in close subparallelism, and inter- 

 connected by a paucispicular, plexus-like network of the same 

 spicules, as well as by occasional single spicules directed trans- 

 versely. The longitudinal spicule-strands are echinated by 

 scarce acanthostyli, which usually project almost perpendicularly; 

 and echinating acanthostyli also occur (sometimes rather abund- 

 antly) on the outward side of the outermost fibres of the axial 

 reticulation. In the youngest portions of the skeleton, spongin 

 is present only in the least possible quantity requisite to hold 

 the spicules together : even in the oldest parts of the sponge, it 

 seldom forms a distinct sheath to the spicule-strands, and (owing 

 to its colourlessness) is clearly discernible only when stained. 



The extra-axial fibres, — which vary in stoutness from 25 to 

 90/7-, — are composed, throughout the greater part of their length, 

 almost entirely of small smooth styli (similar to those of the 

 axial skeleton) closely packed together side by side; accompany- 

 ing these spicules there are also acanthostyli, mostly located at 

 the surface of the fibre, and in part projecting therefrom at a 

 small angle of inclination as echinating spicules. The latter 

 spicules (the acanthostyli) are at first relatively very few, but 

 towards the outer extremity of the fibre (though still at some 

 considerable distance below the dermal surface) they increase in 

 number at the expense of the former, sometimes almost entirely 



