188 HYALONEMA (HYALONEMA) AGASSIZI. 
The diactine uncinates (Plate 44, figs. 6-14, 17a) are generally straight or 
slightly curved, simple amphioxes; considerably curved and centrotyle ones, 
however, also occur. The ordinary amphiox uncinates in form A are 330-800 u 
long, 5.5-12 » thick in the middle, and beset with spines. As far as I could make 
out these spines are 0.7-1.5 » long, and about 1 » thick at the base. Sometimes 
it appeared as if they were continued in a fine terminal filament which was, 
however, too thin to be distinctly projected even with the 280 » light. At one 
end of the spicule these spines are numerous, rather close together, and strongly 
inclined toward the opposite end. Toward the other end they become much 
scarcer and less inclined. Some of the spines nearest the latter end are vertical 
or even inclined in the opposite direction. In the centrotyle uncinates the cen- 
tral tyle is 15% to 45% thicker than the adjacent parts of the spicule. 
The monactine uncinates appear as tylostyles. In form A they are 260— 
293 » long and 9-12 » thick just below the rounded end. The rounded end itself 
is thickened to a more or less spherical tyle 14-16 y» in transverse diameter. 
The large stalk-spicules of form A (Plate 41, fig. 2; Plate 43, figs. 1-7) have 
a maximum length of 42 em. and all are broken off at the lower distal end. Where 
they arise from the sponge they are 0.05-0.95 mm. thick; 30 cm. lower, where 
most of them are stoutest, they are 0.5-1.3 mm. thick. 
One (Plate 43, fig. 1), which I studied in detail, is 160 » thick at the upper 
end, and rapidly increases in thickness to 730 » at 7 cm. from the end; it then 
gradually thickens down to 28 em., where it attains its maximum thickness 
of 1050 ». Farther on it again becomes thinner, and at the lower end, 42 em. 
from the tip, is 760 u thick. Its axial thread is for the most part thin. It is 
thickened, however, here and there in an irregular manner. The silica is very 
clearly stratified. The surface of the upper, proximal part of the spicule is quite 
smooth. Where the spicule attains its maximum thickness fine transverse lines 
(Plate 43, fig. 7) make their appearance on its surface, and 1 cm. above the distal 
end its surface, for a short distance, has quite a peculiar structure (Plate 43, 
fig. 4). Here a silica-layer is exposed which consists of lamellae overlapping 
like tyles, and composed of parallel rods about 10 » thick and lying close to- 
gether. These rods extend nearly but not quite paratangentially and longitudi- 
nally. They deviate slightly both radially and laterally from the direction of the 
axis of the spicule. The radial deviation is due to their forming the overlapping 
lamellae, and like the lamellae themselves they slightly diverge from the axis 
below. The lateral deviation is due to their lying somewhat obliquely in elon- 
gated spirals. 
