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- 

 laminae, 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 in- 

 fluence of the water and isolate the former ; it resulted, that 

 whilst the water flowed into the branchial cavity, none, in an 



