Mr. W. Clark on the Branchial Currents in the Bivalves. 305 



and 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-lamiusej 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 refer to the Rev. James Bul- 

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

 Journal/ vol. ii. p. 258. Messrs. 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 regular 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 

 find 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 structure of these ani- 

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

 declaring the gill-laminae and their inter branchial tubes per- 

 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 formed an erroneous conclusion : 

 I cannot accord with the monstrous position, that the impure 

 bi-anchial water, deprived of its oxygen by the cilia, and of the 

 alimentary matters by the palpi of the animal, is sent by filtra- 

 tion, even if pores existed, through the gill-laminae 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 

 pedal aperture and branchial siphon, at which it entered, agree- 

 ably to the simple laws of nature ? 



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



