194 PHOLADID.E. 



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 in 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 in 

 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 being domed by 

 the animal, a large ovally dilatable fissure is always left for 

 the water in the connecting membrane of the laminse of the 

 dome, in its centre, to correspond with the gape that has been 

 rendered ineffective. And in the linear 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 in 

 the membrane of the connecting valves. In the Mycs 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 being 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, in 

 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 



