HYALONEMA (SKIANEMA) UMBRACULUM. 385 



28 M in transverse diameter, that is 1-3 n more than the adjacent parts of the 

 spicule. 



The diactine tylostyles are much more frequent in the smaller specimen 

 than in the larger. They are about 1 mm. long, and at the morphological cen- 

 trum, where a slight thickening is to be noticed, are 9-16 n in transverse diameter. 

 The rounded end is 20-28 /x, and the attenuated "neck," separating it from the 

 rest of the spicule, is 11-15 ix thick. 



The rhabds of the axial skeleton, wliich form the upper continuation of the 

 stalk within the body of the sponge, are 50-100 ^ thick. As nearly all these 

 spicules found in the preparations are broken, I could not determine their length. 



The few large spicules of the stalk proper are 540-630 ^ thick. 



The acanthophores of the basal part of the sponge (Plate 101, figs. 15-17) 

 ha\-e from two to four rays. The tri- and tetractines are 340-580 ^ in maximum 

 diameter, their rays being 14-40 ^ thick at the base. The extreme tips of the 

 rays are generally spineless, smooth, simply rounded, and dome-shaped. On 

 this smooth end-part follows a spiny belt, usually occupying from a quarter 

 to a half of the whole ray. Proximally the spines in these belts become smaller 

 and smaller until they disappear altogether, leaving from half to three quarters 

 of the ray entirely smooth. 



The microhexactines (Plate 101, figs. 4-7, 11-14) are 40-100 ^ in diameter, 

 on an average 64.3 m- The rays are either equal, or two opposite ones exceed 

 the other four in length. The basal part of the rays is quite straight, the distal 

 part, usually a little less than half of the ray, uniformly curved and so strongly 

 that the directions (tangents) of the basal half and the tip of the ray enclose an 

 angle often as small as 90° or even smaller. The rays are conical, 1.1-1.8 m 

 thick at the base, and end in fine points. They are fairly smooth or only slightly 

 roughened by barely visible spines. 



Besides these regular microhexactines a few diactine microhexactine-derivates 

 (Plate 101, fig. 14) have been observed. These spicules appear as centrotyle 

 amphioxes with fine, curved end-parts. Their surface is more rough (spiny) 

 than that of the regular microhexactines. 



The measurements of a typical spicule of this kind are: — length 91 n, 

 diameter of central tyle 3 fi, basal thickness of rays 1.5 /a. 



Morphologically two main kinds of amphidiscs can be distinguished: — 

 those with relatively large and broad terminal anchors and those with inter- 

 mediate or relatively small, not particularly broad anchors. The former, which 

 are 78-280 m long, I consider as macramphidiscs ; the latter, which are 16-99^ 

 long, as micramphidiscs. 



