with descriptions of new Species. . Sat 
below, which is almost entirely made up of tubes. The form of 
these bodies, Pls. XII. & XIII. fig. 10 a. & fig. 2 6, is now ob- 
served to be somewhat like that of the mulberry, and on closer 
examination they are found to be composed of a stout central 
axis, near each extremity of which is placed a whorl of six or 
eight large, irregularly quadrate nodules ; the extremities of the 
axis being each formed of a nodule similar to those of the whorls. 
These bodies measure ;+-th of an inch long, are colourless, refract 
light powerfully, and are as brilliant as the spicula, and in like 
manner are unaflected by strong nitric acid, how long soever sub- 
jected to its action. 
It is to the above-described peculiar siliceous bodies on the 
surface of the excavating sponges that I attribute the power they 
possess of burying themselves in calcareous substances. The 
spicula may perhaps assist in Cliona; but they seem ill-adapted 
for the purpose in Thoosa, and indeed are not always present. In 
the former they undoubtedly penetrate the surface, and originally 
I was inclined to look upon them as the chief agents employed. 
The discovery of the mulberry-like bodies on the surface of Thoosa 
led me to examine more closely that of Cliona, and after finding 
- there those beautiful gem-like crystals, so well adapted for cut- 
ting, their homology cannot be doubted; and I am compelled to 
adopt the view just expressed. 
The surface then of these animals will very much resemble 
what I have elsewhere described the cutting surface to be in the 
boring mollusks; in the former as in the latter every portion of 
it will cut with the keenness of glass-paper ; and as Clionais ad- 
mitted on all hands to be highly contractile, there can be no dif- 
fieulty respecting the capabilities of the excavating apparatus as 
just described. All that is necessary is, that each siliceous gra- 
nule, or cluster of granules, should be put in motion. Action,— 
very limited,—not more extensive than that of vibratile cilia, 
would be sufficient; and it would seem not at all improbable 
that it may be of the same nature. From Ehrenberg’s inves- 
tigations it would appear that the motion of these minute organs 
is produced by a contractile tissue on which they are based, and 
that in some of the animalcules they have a rotatory motion. 
Now if we suppose these siliceous bodies of Chona and Thoosa to 
be in connexion with a similar contractile tissue, the whole sur- 
face of the sponge would be composed of thousands of minute 
drills quite able to cut into calcareous substances of the hardest 
nature. 
Were the action of this character, the walls of the chambers 
would be drilled full of little holes, and would present just the 
appearance we haye already seen they possess. And as the calea- 
reous particles were remoyed they would be carried away by the 
