PHOLAS. 199 
will meet with insurmountable difficulties, and though I admit 
the anal outflow, I protest against its being considered of 
branchial origin and regular; the regularity is fallacious, 
though most naturalists appear to have adopted that idea, 
without perhaps. sufficient examination, and others have been 
careless in their observations. But the diligent observer of 
cause and effect will perceive that there is as much water 
inhaled as expelled by the anal siphon, and that its fluctuation 
in the branchial chamber, produced by the contraction and 
dilatation of the four gill-plates, which can often be seen by a 
lens through the orifice of a large P. dactylus, aided by the 
respiratory circulation, causes a pressure and an impulse on 
the interbranchial tubes; these, as before shown, are filled 
every two to four minutes by a reception of water anally, 
which after performing its function, of whatever nature it may 
be, is thus for a similar period made to reflow into the anal 
cavity, and from thence is discharged by an insensible con- 
traction of the siphonal muscles until the exhaustion of the 
fluid: this is very evident by the failure of the current, which 
only recovers its full action on the periodic renewal of the 
water. I have thus, perhaps, explained the mystery of the 
so-called branchial current. 
It is problematical what are the precise functions of the 
water that is received into the interbranchial tubes and anal 
vault ; I have hereafter alluded to some of them conjecturally, 
and for the present will only observe, that as this tube acts as 
a conduit to the contents of the rectum, one probable use of 
the water is to break down and remove the dejections ; and it 
would indeed be strange if it had no other entry, except from 
the branchial vault by the devious route of filtration through 
the interbranchial canals. 
In further support of the view that the anal ex-current is 
not the effect of a percolation of liquid through the gill- 
lamince, I will for a moment digress, and relate a short inci- 
dental experiment. As the anal siphon is somewhat longer 
than the branchial, it is easy to subject the latter to the im- 
fluence of the water and isolate the former; it resulted, that 
whilst the water flowed into the branchial cavity, none, in an 
