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



The strong, smooth, principal radial ray, which is always pointed towards the interior 

 of the sponge-body, is either simply pointed at the outer extremity, so that simple 

 oxydiact forms result, or gives origin to four exactly tangential rays (6 to 10 mm. in 

 length), which do not, however, intersect at right angles, but form with one another 

 three acute angles of 40' to 50°, so that the two outermost rays together form an angle 

 of 100° to 150°, rarely of 180° (PI. LV. figs. 9, 13). As from three to five of these 

 pen tacts are grouped together in a tuft, and so disposed that the points of the tangential 

 rays meet at an angle, a most beautiful veil is formed, which spreads over the sponge at 

 a distance of 5 to 12 mm. from the surface. Besides these peculiar pentacts, each tuft of 

 parietal prostalia includes several of the above-mentioned radial cliacts, which project 

 freely for 3 to 6 mm. beyond the surface of the pentact-veil. Near the oscular 

 margin the pentact prostalia disappear, and the oxydiacts become more prominent. The 

 latter are sometimes so numerous and closely grouped that they form a thick WTeath of 

 externally directed and pointed spicules (PI. LV. figs. 1, 7). The long, smooth, radial 

 ray of the pleural pentacts, is, as regards its surface, in marked contrast to the four 

 tangentials which spring from its outer extremity, and run out in a straight or slightly 

 curved course to a gradually narrowed point. As Carter has carefully described, the 

 whole surface of these tangentials is so thickly and uniformly beset with fine pointed 

 tubercles that it presents a thoroughly rough appearance. Between tliese small 

 tubercles, but at greater intervals, strongly developed spines project obliquely outwards 

 like the prickles of a rose. They occur with tolerable imiformity, but without 

 recognisable law, and gradually decrease in height towards the narrowed end (PI. LV. 

 figs. 9, 13). 



The spicules of Rossella antarctica undergo peculiar modification at the base, where 

 the sponge is either attached directly to some solid body, or fixed by means of processes 

 to various smaller objects. The parenchyma contains, in the first place, very abundant 

 small discohexasters, like those represented in PI. LV. fig. 6, but with more numerous . 

 and shorter terminals rising from comparatively broad basal discs. 



We have also to note that here all the longer spicules, and especially the long diacts, 

 exhibit terminally a club-shaped thickening with large tubercles, instead of the small 

 points or roughnesses which occur elsewhere. Finally, in the region where the sponge 

 comes into contact with the substratum, manifold modifications occur in the form of 

 outgrowths, fusions, and not unfrequently irregular reticulations, similar to those 

 represented in PI. LXIV. fig. 3, from Rhabdocalyptus mollis. Such modifications only 

 occur where some irritation is exerted on the sponge through contact with foreign 

 bodies. 



Whether the spicule figured in PL LV. fig. 12, which 1 found isolated in the basal 

 portion of a Rossella antarctica (Station 320), really belongs to the sponge or is a foreign 

 intrusion, I cannot determine. 



