304 Mr. W. Clark on the Branchial Currents in the Bivalves 



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 re- 

 ceived 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 laminae of the dome, in its 

 centre, to correspond with the gape that has been rendered in- 

 effective. 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 Myce and other bivalves the water can 

 get access through the ventral and pedal apertures. Thus we 

 learn from these examples that when nature has denied the or- 

 dinary 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 puUastra, 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 induces a new disposition. 

 The entry and issue of the fluid, in conjunction with the pedal 

 gape and aperture, are thus performed : — the animal simulta- 

 neously closes the orifices of both siphons, which after a short 

 pause arc again opened; the effete water flows from both, and 

 fresh is received. But independent of these silent though very 

 visible operations, there is about every five minutes a powerful 

 and copious jet from both tubes, sometimes simultaneously, at 

 others at intervals, and that from the branchial tube in the 

 Pholades is almost always accompanied by a strong ejection from 

 the pedal gape, and also in the Venei-es from the pedal aperture, 

 though from the absence of a gape in their shells it is not so 

 visible. The periodic times of the in- and out-flux, of whichc\'er 

 character it may be, as the animal becomes exhausted, are more 

 and more prolonged ; they are only in vigour for twelve hours. 



What is the object of these copious and regular receptions 



