HYALONEMA (HYALONEMA) TENUIFUSUM. 
225 
in length, rounded at the end, and thickened so as often to attain a transverse 
diameter nearly equal to that of the “central” tyle, which in these spicules is of 
course very eccentric. 
The spicules forming the stalk are, at the point where they arise from the 
sponge-body, sometimes 0.5 mm. thick. Fragments of rhabds 20-40 n thick 
found in spicule-preparations of the interior are probably parts of young stalk- 
spicules. All the stalk-spicules observed were smooth. 
The stout acanthophores (Plate 67 , figs. 6, 7) have from one to six, most 
frequently from two to four rays. The rays usually taper distally, more rarely 
they are cylindrical. The end-part is densely covered with spines, generally 
somewhat thickened, and terminally rounded, or more rarely, pointed. The 
diactine forms are centrotyle. The three- to six-rayed forms are 335-580 n 
in diameter, and have rays 10-20 n thick. In the three-rayed forms all the rays 
lie in the same plane; two generally in a straight line, and the third at right 
angles to these. In the four-rayed forms the rays also lie in one plane, and the 
adjacent ones enclose angles of 90°. These spicules therefore appear as crosses 
(stauractines) . In the rare pentactine forms four rays extend in a plane, enclose 
angles of 90° with each other and appear as lateral rays, whilst the fifth is verti- 
cal to the plane of the four others and appears as an apical ray. The rare six- 
rayed forms are regular hexactines. 
In the diactine acanthophores the rays lie in a straight or slightly curved 
line. These spicules are 675 /x— 1.1 mm. long and 10-12 n thick. Their central 
tyle is 17-30 /x in transverse diameter. They are connected by transitional forms 
with the ordinary centrotyle amphiox megascleres. These transitional forms 
are about as thick as the true diactine acanthophores, but longer, reaching 2.6 
mm. in length, and have smaller central tyles. The monactine forms are tylo- 
styles 0.8-1 mm. long and 10-13 /x thick. The terminal tyle is about 22 n in 
diameter. 
The regular microhexactines (Plate 67 , figs. 10, 11) have six regularly dis- 
tributed, conical and sharp- pointed, straight or slightly curved rays. The 
curvature is, when present, usually greater in the proximal than in the distal 
part of the ray. The rays are nearly smooth, or slightly roughened by the 
presence of exceedingly minute spines, which appear to be directed backwards 
towards the centre of the spicule. In specimen (a) the regular microhexactines 
are 110-155 u in total diameter, and have rays 1.5-2 ^ thick at the base. In 
specimen ( b ) these spicules are somewhat smaller, 80-105 n in diameter, and 
have rays 1-2 n thick at the base. 
