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



opposites likewise. This is, however, better expressed in the figures of corresponding 

 spicules in other species, e.g., Hyalonema apertum (PI. XXXVIII. fig. 5). 



The dermal skeleton is composed of large, strong, liypodermal pentacts, of autodermal 

 pentact pinuli, and of autodermal amphidiscs of various kinds. The rays of the hypo- 

 dermal pentacts are quite smooth and are directed at right angles to one another. They 

 either run gradually to a sharp point or terminate conically. There is no hint whatever 

 of the missing distal ray. The proximal ray is usually larger than the four equal-sized 

 tangentials, but is sometimes smaller, and sometimes of equal length. The autodermal 

 pinuli are pentacts. The proximal ray is no longer recognisable, but the distal is dra-wTi 

 out into a slim fir-tree-like form, about 0'3 mm. long, and beset with obliquely inserted 

 and somewhat bent and pointed branchlets (PI. XXVII. fig. 21). The basal end of the 

 distal ray usually remains perfectly smooth. The four tangential rays, which lie at 

 right angles to one another and to the distal ray, are straight and of equal size. Only near 

 the somewhat blunt distal extremity do they exhibit some distally directed protuber- 

 ances, while the large proximal portion of each ray is quite smooth (PI. XXVII. fig. 21). 



The very large kind of amphidisc which occurred in Hyalonema sieboldii is not 

 present in this species. The largest form of amphidisc has a length of 0"14 mm. and a 

 breadth of 0"06 mm. The axial rod is covered with protuberances, the highest of which 

 are in the middle, and arranged in a circle. The eight terminal umbel rays on each 

 side, which are disposed in a bell-like form, have a leaf- or spade-like shape, and 

 terminate in a somewhat rounded lancet-like point. The basal plate is comparatively 

 short. The individual terminal rays are sometimes laterally separated at their base by 

 grooves, but sometimes, on the other hand, they are laterally united. The terminal 

 arching of the beU-shaped umbel is not very broad, though strongly marked as the result 

 of the divergence of the umbel rays. The extremities of the ojiposite umbel-rays 

 approach one another to within a fourth of the whole length of the amphidisc. These 

 larger amphidiscs always have their median portion inserted in the dermal membrane, 

 so that the one half lies within the body of the sponge, while the other projects freely 

 above the surface of the skin; but, besides these, several other smaller forms occur as 

 represented in PI. XXVII. figs. 15, 16, 17. These all exhibit a relatively broad arching 

 of the much shorter terminal umbels, some of w^hich consist of more than eight umbel 

 rays. I have observed as many as thirteen umbel-rays in some of the medium-sized 

 amphidiscs (PL XXVII. figs. 16, 17). The axial rod either exhibits a few small, 

 irregularly distributed tubercles, or in addition to these four coarser, cruciately arranged, 

 median prominences, which may also occur alone. The umbel-rays have narrow bases, 

 they end in lancet-like fashion, and approach one another to the extent of not more 

 than one-third of the total length (0-03 mm.) of the amphidisc. I never found these 

 median and smaller amphidiscs except l>dng flat, that is, parallal to the surface of the 

 dermal membrane. 



