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 Anomie and Ostree, 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 Anomie and Ostree; 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 astphonal group, 
without having recourse to an “cella podrida,’ or hash of 
currents, to conclude, that when the animal opens the shell 
for the admission of water to bathe the branchiz, 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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