HYALONEMA (PRIONEMA) FIMBRIATUM. 297 



rounded. The apical (proximal) ray is 0.6-2.1 mm. long; the lateral rays are 

 0.18-0.7 mm. The latter are inclined inward and enclose with the axis of the 

 proximal (apical) ray an angle of 75°-89°, usually about 80°. 



The hexadine megascleres (Plate 63, figs. 1-5) measured were 0.5-2.2 mm. 

 in diameter and had rays 10-55 ^ thick at the base. Some fragments of such 

 spicules observed, which were up to 62 ^ thick, indicate that considerably larger 

 hexactines also occur. The rays of the hexactine megascleres are straight or 

 slightly curved, and usually conic and pointed. One (Plate 63, figs. 4, 5) or 

 two (Plate 63, fig. 1) of the rays may be reduced in length and terminally rounded. 

 The rays of the same spicule are either about equal in size (Plate 63, fig. 3) or 

 unequal. The inequality is usually due to two rays lying opposite being longer 

 than the other four (Plate 63, figs. 1,2). 



Most of the rhabds of the body proper are centrotyle amphioxes, but diactines 

 with one ray reduced, and rounded and thickened at the end, also occur. These 

 spicules resemble tylostyles. 



The centrotyle amphioxes (Plate 63, figs. 10, 11) are nearly straight or slightly 

 curved, 1.2-1.8 mm. and more long, and 9-21 m thick near the centre. The 

 central tyle measures 12-26 n in transverse diameter. The relation between 

 its thickness and the thickness of the adjacent parts of the spicule is 106-163, 

 usually about 120 : 100. 



The tylostyle-Uke rhabds with one ray reduced and terminally thickened 

 (Plate 63, figs. 12-14) are 1-1.5 mm. and more long and usually 12-24 n thick. 

 Their terminal tyle is 18-36 ix in diameter. 



The slender-rayed, long-spined acanthophores of the protuberance, from which 

 in life the stalk arose (Plate 62, figs. 20-26), have four to six rays, and are con- 

 nected by transitional forms with the pinules. They measure 130-220 ju in 

 diameter, and have straight or shghtly cm-ved, pointed rays 3-4.5 m thick and 

 beset AAath numerous spines. The spines on the middle-parts of the rays are 

 usually the largest, and are 3-21 /^ long. Proximally and distally they decrease 

 in size. The spines are either all directed obliquely outward, or only the distal 

 ones are thus inclined, the proximal ones arising vertically. The rays of the 

 same spicule are usually unequal in respect to their spinulation and for a certain 

 extent also in respect to their size. Sometimes (Plate 62, figs. 20, 21, 23, 25) 

 this inequahty is inconsiderable, sometimes (Plate 62, figs. 22, 24, 26) it is very 

 marked. In the latter case one of the rays is usually longer and provided with 

 longer spines than the others. These spicules, which often resemble pinules 

 quite closely, connect the more regular slender-rayed long-spined basal spicules 



