^Ir. W. Clark on the Branchial Currents in the Bivalves. 305 



aud expulsions of water ? Will not every reasonable person 

 acknowledge that they can only serve for branchial purposes — 

 the receptions, to administer water to the gill-laminte, and 

 the expulsions to remove it when effete by the contraction of 

 the adductors of the valves and siphonal retractors ? In corro- 

 boration of the above, I particularly refej* to the Rev. James Bul- 

 wer^s account of the Isocardia cor, published in the ' Zoological 

 Joui"nal/ vol. ii. p. 258. Messi-s. Alder and Hancock cannot 

 controvert these facts, and therefore in relation to my theory 

 say, " This is, however, a special case having nothing to do with 

 the rcffular branchial currents, as has before been pointed out to 

 Mr. Clark.^^ 



My opponents may find their special case a general law, and 

 their system of regular branchial currents an illusion. 



They, having discovered that no ascertained communication — ■ 

 [this is a condition of my theory] — existed between the branchial 

 and anal chambers, thus express the fact : — " We certainly 

 lind no opening between the foot and the gills, nor between the 

 gills and the mantle ;" and in consequence of their favourite 

 doctrine being in jeopardy, they " found it necessary to make a 

 careful examination of the anatomical stnicture of these ani- 

 mals," and have infonned us of the discovery of a channel, by 

 declaring the gill-laminse and their interbranchial tubes ])er- 

 meable, on which — to them a most important fact, if true — they 

 emphatically observe, "Thus in an instant the secret was ex- 

 plained ; the currents communicate through minute openings in 

 the laminae of the gill-plates." 



I think these gentlemen have foi*med an erroneous conclusion : 

 I cannot accord with the monstrous position, that the impure 

 branchial water, deprived of its oxygen by the cilia, and of the 

 alimcutai'y matters by the palpi of the animal, is sent by filtra- 

 tion, even if pores existed, through the gill-laminse and inter- 

 branchial tubes, which are the supports of the delicate blood-ves- 

 sels 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 ad- 

 mit 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 labyrin- 

 thine exit for the water instead of its being wholly ejected by the 

 ])eclal aperture and branchial sii)hon, at which it entered, agree- 

 ably to the simple laws of nature ? 



In connexion with these views, I state a fact that may have 



