DR. WYVILLE THOMSON ON HOLTENIA. 
705 
1 0 to 15 rap in length, occur in the sponge-substance, but these may, I think, be more pro- 
perly associated with the spicules of the next group. 
2. The spicules of the “ heard .” — It is extremely difficult to isolate the long simple spi- 
cules, of the beard. The sponges had been rolled about in the mud in the dredge, and the 
long spicules came up a close tangled mass. One spicule, however, which was not com- 
plete, I extricated to the length of 4 decimeters. These spicules are infinitely more deli- 
cate, and more flexible and elastic than those of the “glass-rope” of Hyalonema, more 
delicate even than the root-fibres of Euplectella. They arise by an extremely fine point 
in the columns of the sponge-substance (Plate LXX. fig. 1, a, h). They enlarge gradually, 
and after passing in bundles through the outer wall of the sponge into the chalk-mud, 
they maintain nearly the same diameter, from T to 0 mm T2, throughout their length. 
Their structure is not so compact as that of most other spicules. When sharply bent the 
layers of silica readily separate from one another, and they may be frequently seen with 
parts of the outer layers broken off (Plate LX VIII. fig. 6). I have never observed more 
than from ten to twelve layers in the portions which I have examined. The external 
surface of this spicule is perfectly smooth throughout the greater part of its length. 
I have not, however, seen a fully developed example complete at the distal extremity. 
Along with the bundles of these simple fibres there issue from the interior of the 
sponge numerous hamate, bihamate, and rarely trihamate spicules (Plate LXVIII. fig. 5, 
a-d , Plate LXX. fig. 1, <z, h). These spicules are from 5 to 10 mm in length, from -02 to 
•025 in diameter of the shaft, and the bihamate ones about ‘06 across the flukes. They 
are armed throughout their entire length with longer or shorter, sharper or blunter 
recurved spines. These spicules are found here and there in the sponge- substance ; 
but usually they run parallel to and are mixed with the sheaves of simple spicules of 
the beard. They resemble very closely in form the so-called “prehensile spicule” 
figured by Bowerbank and Claus from the base of Euplectella. 
Associated with these in the fascicles of spicules of the beard, but much more abun- 
dant in the bundles of shorter and more rigid spicules towards the upper portion of the 
sponge, we find the very beautiful and remarkable spicules, Plate LXVIII. fig. 4. 
These spicules are from about 5 to 8 mm in length and ‘015 in diameter. They are 
cylindrical, and come to a fine point at each end. All along the shaft minute elliptical or 
nearly linear depressions are arranged in a close double spiral. The distal end of each 
depression deepens slightly, so that the markings are somewhat like the scars left by 
the detachment of leaves. From the distal lip of each depression an extremely delicate 
spine, visible only with a rather high magnifying power, arises, and runs backwards over 
the depression, and nearly parallel with the axis of the spicule. 
Mixed with the rigid projecting spicules of the upper part of the sponge, tufts of 
extremely fine siliceous fibres pass out of the sponge-substance. I have found it impos- 
sible to isolate one of these fibres (Plate LXVIII. fig. 7) for its entire length, which 
seems to reach about 20 mm . These fibres are glossy, like the ultimate fibres of floss silk. 
Their diameter is about 0 mm, 002. 
