Prof. W. Thomson on the “Vitreous” Sponges. 121 
kinds of these spicules are more or less completely fused to- 
gether, forming a continuous anastomosing network. 
‘T'wo forms of free spicules are extremely abundant through- 
out the group. The first are simply fusiform, frequently 
slightly curved, and often enlarged and tuberculated or other- 
wise armed or ornamented at one or both ends. These spicules 
vary greatly in length—from °05 to ‘5 of an inch in the skele- 
tons of Habrodictyon and of the sponge-mass of Dictyonema, 
where they are grouped in fascicles and make up the greater 
part of the flexible network, to 3 or 4 inches in the silky fringe 
at the base of Huplectella, and to the enormous length of from 
18 to 20 inches in the wonderful vertical wisp which is popu- 
larly known as the “‘ glass rope” of Hyalonema. 
hese spicules have all essentially the same structure ; they 
consist of extremely thin concentric layers of silica separated 
by films of sarcode, and are traversed throughout their entire 
length by a delicate canal, occupied in the fresh state by a 
sarcode axis. 
The second form is called by Dr. Bowerbank “ cylindro- 
rectangulated hexradiate”’ (British Sponges, vol. i. figs. 185, 
186). It consists of a central shaft with the ends often spined 
or tuberculated as in the fusiform spicules. Near the middle 
of the shaft four secondary branches, at right angles to one 
another, form a cross, the radii at right angles to the axis of 
the shaft. The central canal is very distinct in the main axis, 
and sends branches into the four diverging radii. Rarely only 
_ two secondary branches are produced, but this is evidently by 
suppression. ‘l'hese spicules are large, sometimes -05 inch in 
length. They are scattered irregularly among the fusiform 
spicules in Habrodictyon, Euplectella, and Hyalonema, and are 
sometimes aggregated in groups. 
The fusiform and the hexradiate spicules are modifications 
of one type. About the middle of one of the fusiform spicules, 
whether it be taken from the coil of Hyalonema, from the fringe 
of Huplectella, or from the general skeleton of any vitreous 
Sponge, if we use sufficient care and a sufficient magnifying- 
_ power, we can always detect one or two fine cross canals cut- 
ting the axial canal at right angles. When the cross canals 
have an appreciable length, two or four slight bulgings on the 
outer surface of the needle indicate their position (PI. IV. 
fig.1¢). It is remarkable that this hexradiate type of spicule, 
which is so abundant in the vitreous Sponges, is unknown in 
any other order. 
In Hyalonema the hexradiate spicules of the second form are 
usually if not always perfect and symmetrical. In Habrodictyon 
they are very frequently distorted, the rays twisted, and in 
