44 THE VOYAGE OF H.M.S. CHALLENGER. 



curved, tapering from the base to a sharp point, reaching 015 mm. in length with a 

 diameter of 0-01 (PL V. fig. Id). 

 In a a- quadriradiate spicules. — Basal ray straight, tapering from base to sharp point, usually rather 

 thinner than lateral rays, forming with each of them an angle of about 100°, length incon- 

 stant, varying from 01S mm. to 0"5 mm.; lateral rays curved inwards, tapering from the base 

 to sharp points, reaching 0-3 mm. in length, 0-0125 mm. in diameter; most of them are truly 

 quadriradiate, their apical ray being occasionally longer (0-2 mm.) than that of the outer 

 quadriradiate spicules ; its length is, however, variable, and there are amongst the inner 

 quadriradiate spicules others with a merely embryonal apical ray, and even quite deprived 

 of it (PL V. figs, le, If). 



Minute acerate spicules. — Straight or slightly curved, spindle-shaped, tapering from the centre to a 

 sharp point at either extremity, usually 01 mm. long, - 002 mm. in diameter. 



Skeleton of the radial tubes. — The tubar skeleton consists of subgastric triradiate spicules, reaching 

 with their centrifugally directed basal ray the zone of the cortical acerate spicules, of tubar acerate 

 spicules lying parallel to the baeal ray just mentioned, and of tubar quadriradiate spicules scattered 

 here and there at the bottom of the radial tubes. 



Subgastric triradiate spicules. — All rays of the same thickness (0 - 013 mm.) ; basal ray straight, 

 tapering from the base to a sharp point, its average length - 3 mm. ; lateral rays slightly 

 curved inwards, forming with basal ray an angle varying from 100° to 110°, rarely exceed- 

 ing 015 mm. in length (PL V. 6g. 1 a). 



Tubar quadriradiate spicules. — All rays in different planes, lateral rays forming one curve, 

 basal and apical rays another; basal ray bluntly pointed, cylindrical, 0'0025 mm. thick, 

 rarely longer than - 003 mm. ; lateral rays straight or slightly curved, tapering from the 

 base to sharp points, each forming with basal ray an angle of about 110°, reaching 0'05 

 mm. in length, with a diameter of 0'002 ; apical ray slightly curved, sharp-pointed, of the 

 same diameter as lateral rays, but usually three times shorter (PL V. fig. Iff). 



Tubar acerate spieides. — Straight or slightly curved, tapering from the centre to sharp points, 

 rarely longer than 0'3 mm., with a diameter of 0-005. 



Skeleton of the cortex. — The skeleton of the cortex consists of large spindle-shaped acerate, of 

 minute acerate, and of sagittal triradiate spicules, with the basal ray directed towards the closed end 

 of the Sponge. 



Large acerate spieides straight or slightly curved, tapering from the centre to a sharp point at 

 either end ; length varying from 1 to 3 mm., diameter from - 05 to 012 mm. 



Minute acerate spicules.— Like those of the gastric surface spindle-shaped, straight, or slightly 

 curved, tapering from the centre to the sharp points; rarely exceeding 015 mm. in 

 length, and 0-0028 mm. in diameter. 



Sagittal triradiate spieides.— Basal ray smooth, either of cylindrical form or tapering from the 

 base to a sharp point, reaching 075 mm. in length, with a diameter of 0005 ; lateral 

 rays sharp-pointed, forming with basal ray an angle of 112°, either straight or more 



