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



lav ; the regularity is fallacious, though most naturalists appeal* 

 to have adopted that idea, without perhaps sufficient examina- 

 tion, 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. dacttjlus, 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 per- 

 forming 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 contraction 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, ex- 

 plained the mystery of the so-called bi-anchial 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-cuiTcnt is not 

 the efifect of a percolation of liquid through the gill-laminse, I 

 will for a moment digress, and relate a short incidental experi- 

 ment. As the anal siphon is somewhat longer than the branchial, 

 it is easy to subject the latter to the influence of the water and 

 isolate the former; it resulted, that whilst the water flowed into 

 the branchial cavity, none, in an hour's constant observation 

 under the lens, issued from the anal siphon, a sufficient proof of 

 the non-communication of the two; but as soon as the anal 

 siphon was allowed to reach the water and obtained a supply, the 

 current recommenced. 



I now come to another experiment from which IMessrs. Alder 

 and Hancock conclusively infer the connexion of the siphonal 

 currents. They state, "that the nosle of a blowpipe charged 

 with a coloured fluid was placed at the inhalant orifice of a 

 Pholas, and immediately a quantity was drawn into the animal. 

 Watching carefully the result, wc had soon the satisfaction of 

 beholding a blue-stained stream issue from the cxhalant orifice." 



To this I observe, that having tried the experiment again and 



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