344 HYALONEMA (OONEMA) DENSUM. 
ally the thickest part of the ray does not lie at its proximal end but farther out, 
some distance from the centre of the spicule. The largest hexactine observed 
was of this kind. In this spicule the two longer opposite rays measured 1.1 mm. 
and 2.4 mm. in length respectively. The longer of the two is 70 u thick at the 
base. Its point of maximum thickness is 0.4 mm. from the centre of the spicule, 
and here the ray measures 80 u in transverse diameter. At the rounded end 
it is 15 u thick. In the proximal part of the ray the axial thread is simple and 
quite thin (0.5 » in diameter); distally it gradually increases in thickness to 
5 » at the end of the ray. In its distal and middle-parts it is not simple but 
provided, at frequent intervals, with verticillate groups of strongly inclined 
branches with a maximum length of 15 uz. 
The superficial and ordinary choanosomal amphioxes are centrotyle, usually 
0.6—2.2 mm. long, and 7—27 uw thick near the middle. The central tyle, which not 
infrequently protrudes much more on one side of the spicule than on the other, 
is 13-37 uw in transverse diameter, that is 1-18 » more than the adjacent parts 
of the spicule. 
The large axial amphioxes and rod-shaped fragments found in the central 
part of the sponge are 25-1304 thick. The largest intact one observed is a 
fusiform amphiox, blunt at both ends, 5 mm. long, and 28 yu thick. 
The acanthophores (Plate 94, figs. 34-36) have from two to six, most fre- 
quently two or four rays. The dimensions of these spicules are tabulated 
below. 
ACANTHOPHORES. 
from higher up in the from the lower end of the 
sponge sponge 
Number of Nl 
rays more or| total length | total length 
less developed (maximum (maximum 
diameter) of | basal thick- diameter) of basal thick- 
spicule | ness of rays spicule ness of rays 
a | M Me Me 
2 620-1560 7-19 100-790 12-30 
3 390-490 12-15 170-300 28 
4 390-610 | 13-29 120-565 11-388 
5 615 21 217-540 20-26 
6 300-400 14-19 600 21 
