266 MR. BRODERIP ON CLAVAGELLA. 
face. The inside of the loose valve of the latter specimen! is almost nacreous, and the 
distant and broad corrugations appear upon the internal surface. In both, the casts 
of the backs of the loose valves may be seen on the stony chamber as if they had 
been impressed on wax. A great portion of the fixed or ‘‘ soldered” valves, which are 
continued on to the tubes in both specimens, appears to have been surrounded by the 
naked chamber ; and the situation of the secretion of the tubules? (the area of which is 
very extensive in Mr. Cuming’s specimen) appears to have been varied according to 
the necessities of the case. In Mr. Miller’s specimen, a perforated shelly plate is situated 
close to the tube and to the umbones of the valves?, so that it comes against the upper 
part of the back of the loose valve, which almost hides it from view when in its place ; 
and tubules are visible at various points. 
We are left to conjecture the causes which operate to determine the animal in the 
choice of its abode, if indeed it can be called choice, for most probably Clavagella is the 
creature of circumstances ; and if, soon after its exclusion from the parent, (when I sup- 
pose it to be furnished with its two valves only, and to float free, with some voluntary 
impulse perhaps,) it arrives at the vacant hole of some small Petricola, Lithodomus, or 
other perforating Testacean which suits it, one valve soon becomes attached to the wall 
of the hole, and then the animal, being sedentary, proceeds to secrete the siphonic sheath 
or tube, to enlarge the chamber according to its necessities, and to form the shelly 
perforated tubular plate which is to give admission to the water at the practicable part 
of the chamber. 
How the excavation is carried on is also a matter of doubt. The chambers of the 
individuals of Clav. Australis were formed in a siliceous grit, those of Clav. elongata in 
the substance of an Astr@opora, that of Clav. lata in a calcareous grit, and those of Clav. 
Melitensis in an argillo-calcareous tufa. If the excavation be the work of a solvent se- 
cretion, it must be a solvent of extensive power. The situation of the glands detected 
by my friend Mr. Owen, leads me to think that they minister in some way to this 
operation ; and I do not see how the anterior or greater end of the chamber, at all 
events, can be operated on by mere mechanical attrition with such parts as must have 
been contiguous to it. 
It has been objected that any solvent which would act on a calcareous rock would 
equally act on the calcareous shell of the animal ; but there is, perhaps, more of point 
than of strength in this objection. Without laying too much stress on that law of na- 
ture by which chemical and vital forces are placed in a state of hostility’, and which 
may or may not be applicable to such a substance as shell, the gland for the secretion 
' Plate xxx. Fig. 7. 2 Ibid. Figg. 5, 6. 8 Ibid. Fig. 6. a. 
* John Hunter’s paper in the * Philosophical Transactions’ for the year 1772, ‘‘ On the Digestion of the 
Stomach after Death,” and Spallanzani’s experiments on that organ, will readily occur to the reader. 
