-and Function of the Spicules of Hyalonema, 293. 
coats like the rest of the body; in fact they seem to be in- 
creased in size by a layer of siliceous and animal matter being 
deposited on their whole surface, on the apices as well as the 
rest of the body. 
On the contrary, the spicules of Hyalonema are subcylin- 
drical, rather thicker above, and very slender at the base, 
formed of numerous concentric lamine round a central line ; 
but the outer layers do not reach the ends of the spicules. 
The ends are truncated, and there is a succession of lamine, 
each terminating shorter and shorter of the top of the spi- 
cules; so that the ends of the spicules are furnished with a 
succession of rings, each formed by the termination of a layer 
of siliceous and animal matter, as is well figured in the plates 
of Professors Max Schultze, Brandt, and Bocage. 
I believe that this difference in the structure arises from the 
very different functions which the spicules of the axis of Hya- 
lonema have to perform in the coral from those for which the 
spicules of the sponges are formed in the body of those beings. 
In Ayalonema the coral is gradually being extended in height, 
and the spicules are extended in length, part passw, to support: 
the coral as it grows. ‘To allow of this extension of length, the 
ends of the spicules are not finished off as they are in the sponges, 
where they are completed at once, and are only slightly 
thickened and lengthened to a certain limit as the sponge: 
grows. The spicules of sponges merely form a support for a. 
more or less massive sponge, and are of a definite size; while 
the spicules of the rope-like axis are continually increasing in. 
len hh and thickness to support a continually growing mass: 
of animals, which require a Macnee and stronger axis to sup- 
port them as the community assumes the adult form. The 
spicules of the rope-like mass are of different thickness; and 
_ they increase in number as the axis increases in diameter, 
those last formed being the most slender and consisting of the 
fewest concentric layers. 
It is to be observed that the short rugose cruciform spicules 
in the bark of the Hyalonema, which do not require to be 
lengthened as the coral increases 1n size, are formed like the spi- 
cules of sponges, and are complete in form when first deposited, 
and have complete ends. This difference in the structure of 
the spicules of the axis and of the bark, I think, goes far to 
confirm the theory I have propounded, that the spicules of the 
axis are of different structure, because they have a peculiar 
function to perform. 
_ This peculiarity in the structure and function of the spicules 
of the axis, I think, proves that they are no part of the sponge, 
but a secretion of the hard flesh of the polype that surrounds . 
