PHOLAS. 209 



Hitherto the Pholades have been more particularly the 

 object of consideration it may now be not amiss to turn our 

 attention to a group of Bivalves which, though essentially the 

 same, differ materially in the configuration and arrangement of 

 many of their organs ; they may perhaps assist us in searching 

 out the truth, by the discordancy of their attributes with those 

 of their precursors. 



What view am I to take of the Anomia and Ostrea, that have 

 open mantles and no tubes ; in which the water must enter at 

 every 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 ven- 

 tral cavity. Am I to idealize, and suppose that in the same 

 branchial vault there is a distinct current of ingress and an- 

 other of egress ? I may observe, 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 Anomm and Ostrea ; 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 prove that there is no per- 

 meation in either case ? 



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

 without having recourse to an " clla podrida," or hash of 

 currents, to conclude, that when the animal opens the shell 

 for the admission 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 



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