202 



GEODIA MEDIA. 



quently six rays. The rays are straight, and in the many-rayed forms concentric 

 and fairly regularly distributed. In the few-rayed forms the rays are frequently 

 not uniformly distributed and sometimes not quite concentric; many of these 

 spicules have a somewhat metastrose character accordingly. The rays are conic 

 and fairly pointed, or cylindroconical and blunt, or cylindrical and truncate. 

 The asters with rays of the latter kind which appear as true strongylasters, are 

 however rare. Quite at the base the rays are, for a short distance, smooth. 

 Apart from this they are wholly covered with spines standing rather close 

 together. In size the spines are subject to considerable variation, but they are 

 never very minute. In some asters with particularly large spines I noticed 

 that the spines are directed backwards, towards the centre of the spicule. The 

 blunt, cylindroconic rays usually bear a terminal spine. In the digitate form 

 these asters are 21-41 /<, in the massive form 21-45 /.«, in total diameter. The 

 basal thickness of the rays is in the former 1-2.5 in the latter 1-3.5 [i. An 

 inverse proportion between the size and ray-number is clearly expressed in 

 regard to the thickness of the rays but not so well marked in regard to the 

 total diameter of the spicule. In Bowerbank's type, these asters have from 

 four to seven rays, 1.2-2.3 // thick, and measure 20-37 /i in total diameter. 

 Sollas gives their diameter as 26 fx. 



The subcortical oxysphaeixisters have a spherical centrum 3-4.5 /tin diameter, 

 from which from fifteen to twenty rays arise radially. These are regularly dis- 

 tributed, 4-9 /I long, straight, conic, sharp pointed, and 0.8-1.7 fi thick at the 

 base. A verticil of large vertically arising spines is observed some distance 

 below the end of each ray. Besides these large verticil-spines smaller ones, 

 distributed irregularly over the distal part of the ray, are often present. The 

 total diameter of these oxysphaerasters is 9-18 /Jt. I have found similar spicules 

 in Bowerbank's type, and he appears to have observed them, but they are not 

 mentioned by Sollas. In the centrifugal spicule-preparations I have met with a 

 few spineless oxysphaerasters with only six rays, about the size of the largest 

 spined oxysphaerasters. It seems to me doubtful whether these spineless oxy- 

 sphaerasters, which I have not observed in situ in the sections are proper to the 

 sponge. 



The small dermal syhaerasters (Plate 17, figs. 1-3, 4-6a, 13, 14, 16, 17, 18a, 

 19a) have a spherical centrum 1.7-5 /i in diameter, from which from seven to 

 eighteen rays arise radially. These rays are quite regularly distributed 1.3- 

 3.5 /( long and 0.8-1.6 ft thick at the base. The total diameter of these asters 

 is 4.6-8, usually 5.5-6.5 /(. The shape of the rays and the relative size of the 



