192 SUPPLEMENT | 
theory of this movement is in other respects precisely the same. 
as that of the reptation of ordinary gasteropods. 
The movements of the acephalous mollusca are often con- 
fined to the trifling opening of the valves of their shell, and to 
their complete occlusion. The first is the natural position, or 
the repose of the animal ; and in fact it is only then that it can 
receive the water which brings it nutriment, especially when, 
its mantle is not provided with external tubes., It is produced 
by the disposition of the ligament of the hinge, the perpendi- 
cular fibres of which, at each valve, are drawn or, compressed, 
according to their position without or within the point of sup- 
port, when an attempt is made to cause the two valves’ to 
touch. Their closure is, on the contrary, entirely active; that 
is to say, owing to the contraction of. the fibres of the adduc- 
tor muscles, which are the antagonists of the ligament. Willis, 
and more lately, Dr. Leach, have considered, that in oysters, 
a part of the central adductor muscle was formed of elastic 
substance, an antagonist of the other part which should be 
alone contractile. Some doubt seems to attach to this notion. 
The family of the palliobranchia contains many genera, in 
which, instead of a ligament, the two valves of the shell are 
united at their summit by a long elastic ‘tube, which is fixed 
to submarine bodies, and which might even perhaps be a little 
contractile. Nevertheless, the animal has no other movement 
than that of opening and closing the shell, like the other 
acephala. 
In the species fixed immediately by the shell, or by a tube, 
—such are the only motions permitted,—there is then not the 
slightest degree of transference of situation. In all the others 
there is one, though to very different degrees. Thus many 
species are almost in the situation of those of which we have 
just spoken ; that is to say, they are fixed, but it is with a cer- 
tain degree of mobility. Such are they whose attachment is 
