HYALONEMA (PRIONEMA) FIMRRIATUM. 
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 hexactine megascleres (Plate 63 , figs. 1-5) measured were 0.5-2. 2 mm. 
in diameter and had rays 10-55 m 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 rhabcls 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 ^ 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 tylo style-like 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 ^ thick. 
Their terminal tyle is 18-36 m 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 m in 
diameter, and have straight or slightly curved, pointed rays 3-4.5 m thick and 
beset with numerous spines. The spines on the middle-parts of the rays are 
usually the largest, and are 3-21 fx 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 inequality 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 
