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



appressed. They spring from a projection and cover over a groove-shaped depression. 

 Finally, we have to note the occurrence (here and there very abundant) of small thin 

 oxydiacts, which exhibit on their surface fine tubercles or spines, or only irregular 

 rouo-hnesses. It seems to me improbable that the amphidiscs found in the parenchyma 

 really belong to it. I believe rather that they are present only by secondary disloca- 

 tion, having originated in the dermal, gastral or canalicular skin. 



The dermal skeleton includes strongly developed hypodermal oxypentacts of variable 

 size. "While the long proximal ray has usually a perfectly radial disposition, the four 

 tangentials extending below the dermal membrane do not always form right angles with 

 one another, and frequently exhibit near their origin a marked curvature to the side or 

 inwards. The rays may exceed 1 mm. in diameter, and that strength is indeed common 

 enough, though not at the point of origin, but at some distance up the ray. Between 

 pentacts of this sort, tetracts occasionally occur, arising by reduction of the proximal 

 radial ray. Not unfrequently, in some or in all, the ends are more or less markedly 

 rounded. The autodermal pentact pinuli, present in great number and in strong develop- 

 ment exhibit great variations in the size and form of the cjrpress-like or fir-tree-like free 

 distal ray. The four cruciately disposed moderately long and thin basal rays lie in the 

 same plane. Near their origin they are smooth, but bear on the larger outer end small 

 teeth and spines. They usually end in a point, but in the larger specimens the ends are 

 sometimes more or less bluntly rounded off (PL XLV. figs. 2, 3). The distal ray of the 

 larger dermal pinnies attains a length of 0'3 mm. and more. In form it resembles a 

 slender cypress, so closely lie the uniformly long, oblique, lateral spines (PI. XLV. fig. 2). 

 Between these long pinnies numerous smaller forms occur, which are usually much more 

 slender, and run out into a single long terminal point (PI. XLV. fig. 3). Less frequently 

 compressed, broad and bushy forms occur (PI. XLV. fig. 5). The tall cypress-Uke 

 pinuli are usually associated with the strong tangential rays of the large hypodermal 

 pentacts, while the slender and smaller forms occur on the sieve-like perforated dermal 

 meshes, which are enclosed by large hypodermal pentacts. 



The larger amphidiscs, which do not occur in what could be called abundance, attain 

 a length of 0"18 mm., exhibit a moderately thin, tubercled, axial rod, and short cam- 

 panulate, almost hemispherical terminal umbels, with eight smooth, terminally slightly 

 pointed rays (PI. XLV. fig. 8). Less frequently, somewhat broader ovoid amphidiscs 

 occur, in which the umbel rays almost, or actually meet the opposites (PL XLV. fig. 7). 

 Finally, a large number of small amphidiscs are found, with longish form, and almost 

 hemispherical umbels composed of eight uniformly slender rays (PL XL VI. fig. 5). 



The gastral skeleton exhibits the same strong and somewhat slimmer oxypentacts 

 which occur under the external skin. The numerous pentact pinuli otherwise resemble 

 the bushy forms of the outer skin, but have a much shorter bushy radial ray. On the 

 sieve-network in the gastral skin, which covers the large grouped apertures, there are 



