$28 Mr. A. Hancock on the Excavating Powers of Sponges, 
the branches bifureate ;—in short the young Cliona has now 
assumed the character of the mature sponge (fig. 2)... Thus we 
can trace Cliona from its earliest stage of growth,—not. larger 
than the gemmule of Halichondria and resembling it in form,— 
up to its perfect development, step by step, excavating its compli- 
cated habitation in sound shell, within which it hes closely im- 
bedded, but unobscured by its pearly envelope which is perfectly 
free from decay and is untouched by worms: to neither of which 
by any constrained imagination can the chambers in this mstance 
be attributed ;—Cliona makes them for itself. And now having, 
I trust, established this fact, we shall endeavour to ascertain the 
nature of the apparatus by which these sponges work out their 
abode,—a subject of much difficulty. 
The mollusks being furnished with a shell, the shenetianians 
into the nature of their excavating instrument is much compli- 
eated. The burrowing sponges, however, having no such hard 
covering, we have in them only the animal to look to for an ex- 
planation. The excavations of Cliona and Thoosa can only be 
effected by the surface of the sponge, aided either by some 
minute mechanical instruments in connexion with it, or by a 
solvent: unless, indeed, the water-currents of the papille, as 
hinted by Professor Grant, be thought. equal to perform the 
task. But were these currents of sufficient magnitude to 
penetrate rapidly into shell or hard limestone, it is difficult to 
see how they could be brought into effective operation. The 
papillz are closely adherent to the sides of the orifices through 
which they protrude; and here the water could have no effect; and 
yet these orifices are at first small, and are afterwards consider- 
ably increased in size. » And at those points where the water is 
drawn into the sponge, the currents, of course, cannot be sup- 
posed to act in the way proposed. To show, however, how in- 
adequate these minute currents are to work out the chambers of 
Clona, which we have seen are formed very rapidly, we have 
only to reflect on the comparatively slow action of the enormous 
currents of the sea,—of the tidal currents, and of those resulting 
from the lashing of the waves. The puncturing of the sides of 
the chambers also seems unfavourable to such an hypothesis. We 
shall not therefore stop to discuss this branch of the subject 
further, but at once inquire how far a solvent is likely to be the 
agent. 
"The extreme simplicity of the organic structure of these beings 
forbids a belief in the existence of a special secreting apparatus. 
If therefore a solvent fluid be the agent, it must be supposed to 
exude from the entire surface of this humble animal. The 
character of the excavations would -also. lead to the same con- 
clusion ; for it is evident that the form of the sponge is influen- 
