196 PHOLADID.E. 



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 nitration, even if pores existed, through 

 the gill-laminae 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 aeration being unquestionably effected in the 

 branchial vault, we may inquire, what is the object of this 

 partial labyrinthine exit for the water instead of its being 

 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 being 

 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 interbranchial 

 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 



