130 Prof. W\, Thomson on the “ Vitreous” Sponges. 
are so characteristic of Huplectella. The. long hexradiate 
clubbed spicules and the irregular hexradiate spicules make up 
the mass of the skeleton. 3 
The investing film of sarcode is thickly studded with ex- 
tremely minute spicules, which in the died sponge seem to be 
adhering to the larger spicules and irregularly massed among 
them. On shaking the sponge, quantities of these minute 
spicules of the sarcode fall out. They are of two marked 
types. The most abundant are small hexradiate stellate 
spicules (Pl. IV. fig. 16, x 250), with the rays nearly equal, 
formed exactly upon the same plan as the larger spicules of 
the skeleton, with a well-marked but extremely minute six- 
rayed tube occupying the centre of the rays. ‘The rays when 
broken show the same lamellar structure as the larger spicules. 
The second type is also very abundant; but it is rare to find 
these spicules at all perfect: it is the “‘ floricomo-hexradiate 
stellate”? of Bowerbank. The centre of the spicule is hex- 
radiate stellate, like the other spicules; but each ray, not more 
than four times its own width from the point of divergence, 
spreads out into several, probably six or eight, expanded plates: 
these plates curve outwards and form a cup; they then bend 
upwards and slightly inwards, and become so extremely deli- 
cate that they are little more than visible under a high magni- 
fying-power ; finally they sweep upwards and outwards, end- 
ing in a trifid expansion. ‘These trifid ends turn gracefully 
over, so that the processes from each of the six radii unite in 
producing a beautiful vase-like form. A perfect spicule with 
all its vases complete is an exquisite microscopic object. The 
star-like centres of these spicules are not very common; but 
the sarcode is full of the ultimate branches. (Pl. IV. fig. 1f) 
The vast number of these separate hooks may be explained 
by the extreme tenuity of their attachments to the central 
stars. The strain of the contraction of the sarcode in drying 
may probably be sufficient to break them off. These spicules 
appear to be most abundant near the edges of the openings in 
the network. . : 
The conical sponge is abruptly truncated above, the wall 
ending in a well-defined ridge or lip, somewhat more dense in 
structure than the rest of the wall. The tube is then closed 
by a very irregular, horizontal, netted lid, composed of fibres 
which are much more thread-like and closer in their texture 
than those of the wall. The ultimate elements of the fibres of 
the lid, however, are exactly the same as those of the wall- 
network ; only the large stellate spicules are less numerous, and 
the fusiform spicules are usually much shorter. ‘The spicules 
of the sarcode are equally numerous and of the same character 
throughout. 
