194 PHOLADID A. 
Continual watchings for months of multitudes of these ani- 
mals prove beyond doubt that the water is not only copiously 
received at the pedal gape or aperture, but is often expelled 
with as much force, and with a similar formed jet, as from the 
branchial siphon; and my dredger, who during the last fifty 
years has excavated more Pholades than any man im existence, 
says, that he continually sees the water expelled from the 
pedal gape. This is an important fact in corroboration of my 
theory, as an in- and ex-current is established pedally im 
combination with the branchial siphon. 
I will now mention a decisive proof that nature, in all the 
bivalves, intends the water, under certain conditions, to be 
received and expelled by the pedal gape or aperture. When 
the gape of the Pholas papyracea is closed, by bemg domed by 
the animal, a large ovally dilatable fissure is always left for 
the water in the connecting membrane of the lamine of the 
dome, in its centre, to correspond with the gape that has been 
rendered ineffective. And in the lnear Solens, in which, from 
the quality of the foot and its singular position, the water 
cannot well enter pedally, a similar aperture is also left im 
the membrane of the connecting valves. In the Mye and 
other bivalves the water has access through the ventral and 
pedal apertures. Thus we learn from these examples that 
when nature has denied the ordinary pedal entry and exit for 
the fluid, she always supplies a compensation. 
The periodic entry and reflux of the branchial water present 
two distinct characters ; the one beg regular, the other more 
uncertain. With respect to the first, place a dozen Pholades 
and as many Pullastra pullastra, or P. decussata, in a dish of 
sea-water: it will be seen that each has a regular periodic 
action, the Veneres usually from one to two minutes, and the 
Pholades three to four, until a change of circumstances in- 
duces a new disposition. The entry and issue of the fluid, mn 
conjunction with the pedal gape and aperture, are thus per- 
formed :—the animal simultaneously closes the orifices of both 
siphons, which after a short pause are again opened ; the effete 
water flows from both, and fresh is received. But independent 
of these silent though very visible operations, there is about 
