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



Thomson on the " Porcupine " expedition, off the Strait of Gibraltar, and very 

 probably the same form as was figured in the Depths of the Sea, p. 419. To this 

 dried specimen the following description refers. 



In contrast to the cylindrical or barrel-like shape of most of the specimens of 

 Rossella antarctica, Carter, the form of Rossella velata, Wyville Thomson, is 

 perfectly ovoid, 6 '5 cm. long, by 4 "5 broad. The superior pole exhibits a circular, 

 sharp-edged aperture, 1"6 cm. in diameter, — the opening of the equally broad, cylin- 

 drical, gastral cavity (4 cm. in depth) into which the efferent canals open. The 

 roundish apertures of the latter are especially wide in the blind basal portion of the 

 cavity, and become gradually narrower towards the upper end. The external surface 

 of the body is not uniformly smooth, nor beset merely with minute, sharply-defined 

 papilla?, as in Rossella antarctica, but is rather to be described as hillocky, with 

 numerous gently convex protuberances, from 5 to 6 mm. in breadth, and not sharply 

 defined from the reticulate, connected, intervening depressions. They are distributed 

 over the whole surface, with some regularity, at intervals of about 10 mm. between the 

 summits. The network of beams, which extends radially in relation to these projecting 

 eminences, is doubtless covered during life by the fine rectangular lattice-work of the 

 smaller dermalia. The whole external surface of the sponge is covered by the greatly- 

 developed system of most beautiful prostalia, which project radially, in small groups, 

 from the apices of the hillocky elevations. These tufts of pleuralia consist of 

 simple pointed diacts and pentacts, in which the four tangential rays arise at 

 right angles to the radial ray, at a distance of about 10 mm. from the surface of 

 the body. They intersect, not at acute angles, but in a perfect cruciform fashion; 

 they are also directed mutually at right angles. At the upper end of the sponge, 

 near the oscular margin, only the radially disposed, long oxydiacts persist, forming a 

 marginal fringe, which attains the conspicuous length of 3 to 4 cm. At the lower pole 

 of the sponge-body, on a flat surface measuring 6 to 7 square cm., thick tufts of 

 spicules arise from the apices of hillocky elevations. The tufts bear twenty or more 

 basalia, 5 to 8 cm. in leno-th, and taken tosrether form a loose basal tuft. On most of 

 these long basalia one can recognise, even with the naked eye, at the outer extremity, 

 a small four-rayed anchor. 



The parenchyma contains, as in Rossella antarctica, medium-sized oxyhexacts and 

 oxydiacts, which are frequently roughened towards the pointed extremities. The 

 middle portions of the oxydiacts are either smooth, or provided with an annular swelling 

 or with four cruciate projections — traces of the undeveloped rays. Small oxyhexasters 

 with short principal rays, each bearing two long divergent terminals, are very 

 abundant. Between these there is a somewhat abundant, but locally variable occur- 

 rence of discohexasters similar to the above, but with toothed, somewhat incurved, 

 transverse discs at the ends of the terminal rays, and also of discohexasters in which the 



