196 PHOLADID. 
I think these gentlemen have formed an erroneous con- 
clusion: I cannot accord with the monstrous position that 
the impure branchial water, deprived of its oxygen by the 
cilia, and of the alimentary matters by the palpi of the 
animal, is sent by filtration, even if pores existed, through 
the gill-lamine and interbranchial tubes, which are the sup- 
ports of the delicate blood-vessels for discharge at the anal 
siphon. 
As the capacity of the branchial chamber is at least three 
times greater than the anal, Messrs. Alder and Hancock must 
admit that two-thirds of its fluid is expelled agreeably to my 
theory ; it is therefore difficult to conceive a plausible reason 
why a part of the effete water, only one-third, should be got 
rid of by an issue, termed by them a branchial current. The 
sustentation and aération being unquestionably effected in the 
branchial vault, we may inquire, what is the object of this 
partial labyrinthine exit for the water imstead of its bemg 
wholly ejected by the pedal aperture and branchial siphon, at 
which it entered, agreeably to the simple laws of nature? 
In connection with these views, I state a fact that may 
have some weight even with the sceptical. The longitudinal 
retractors and transverse muscles of the siphons are of very 
great power; the office of the latter is to diminish the calibre 
of the tubes, that, in conjunction with the former, they may 
effect a more powerful expulsion of the impure fluid. As 
proof, if a dozen Pholas dactylus are placed in a large dish 
of sea-water, they will cause so great an ejection from the 
siphons, not from the effect of sudden disturbance or bemg 
startled, but of regular periodical emissions, as to cover the 
table several times during the twelve hours of the day and 
also throughout the quietude of the night: assuredly this 
circumstance serves to prove that the impure water is thus 
expelled, and that no part of it permeates the imterbranchial 
tubes. 
However, it still appears that Messrs. Alder and Hancock 
insist on a regular in-current by the branchial siphon, and an 
ex-current from the anal, effected by cilia, for the use of the 
respiratory apparatus; these are, as I think, strange and 
