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



point of the periphery that is patent, contemporaneously with 

 the opening of the shell by the animal ? Here the water cannot 

 be passed off by what is called an anal tube, because none exists ; 

 it must therefore be discharged by the great ventral cavity. Or, 

 am I to idealize, and suppose that in the same branchial vault a 

 distinct in-current has its course and another out ? I may ob- 

 serve, that in the Gasteropoda there is a similar periodic entry 

 and expulsion of water from the branchial chamber as in the 

 Bivalves ; and after the cilia have extracted the oxygen, I have 

 witnessed a hundred times the forcible expulsion of the effete 

 fluid by a jet as decided as in them ; — am I here also to suppose 

 that there are two distinct opposite currents in the same undivided 

 cavity ? 



I have now to inquire how the gill-percolation, admitting for 

 argument that it exists, is disposed of in this tribe of Bivalves 

 without siphons. If the water permeates the gills of the Pholades, 

 it must do so in the Anomice and Ostrece ; in the former there is 

 a possible vent by the siphon, but none in the latter, therefore 

 it must revert to its source, the branchial cavity. Does not this 

 go far to pi'ove that there is no permeation in either case ? 



Then, may it not be permitted us, in this asiphonal group, with- 

 out having recourse to an " olla podrida," or hash of currents, 

 to conclude, that when the animal opens the shell for the ad- 

 mission of water to bathe the branchiae, and when that function 

 is accomplished, it ejects the effete fluid by the same channel it 

 entered, as no separate duct can be found? Will not the calm 

 consideration of this case make most men doubt the existence of 

 branchial currents either by distinct tubes, that is one inhalant 

 and branchial, and another exhalant and anal, or by what I term 

 supposititious ones ? The former position I think I have proved 

 in the Pholades by showing that there is no effective communica- 

 tion between the two chambers ; and in the Anomice and Ostrea, 

 the latter condition of the currents being imaginaiy, appears to 

 be the most correct view. It may therefore be considered that 

 in the Bivalves, whatever modification their siphonal mechanism 

 may present, all are subject to a general law of the water being 

 expelled from the same siphon or channel at which it entered, 

 aided by the pedal gape and pedal aperture where they exist ; 

 and in the Anomice and Ostreee, in which these organs are rudi- 

 mentary or entirely wanting, the water is simply received and 

 expelled through the ventral range, and not by an imaginative 

 inhalant and exhalant regular current, effected by cilia. 



The remainder of Messrs. Alder and Hancock^s paper requires 

 no further remark than a few words on their concluding experi- 

 ment, showing how the colouring matters collect in the neigh- 

 bourhood of the buccal aperture. I have observed these appear- 



