344 Mr. J. Alder on the Branchial Currents 



shell. This was perhaps owing to the gradual opening of the 

 valves, as afterwards the current appeared to be confined to the 

 posterior portion, and while it was flowing in at that point, I 

 could distinctly see an opposite current passing off at the poste- 

 rior siphon. This simultaneous action of currents in contrary- 

 directions, observed in all the instances mentioned, is surely suf- 

 ficient to prove the existence of some special motive power di- 

 stinct from the action of the valves*. 



We shall now turn to Mr. Clark's two additional ^ proofs,' by 

 which he " proposes to demonstrate '' that the water passes into 

 the branchial cavity by both the posterior siphons, in conjunction 

 with the pedal aperture, and that it is expelled indiscriminately in 

 various proportions by all. The argument is a little obscure, but 

 if admitted in its fullest extent could not demonstrate the whole 

 of this proposition. As far as I can understand it, it is this : — that 

 as '^ important prehensile organs " — cirrhi and cilia — clothe both 

 the anal and branchial siphons " to entangle and capture the 

 minute animalcules to be conveyed into the branchial cavity,'' 

 therefore a current of water must pass into each siphon to carry 

 them forward to their destination. But the premises are as- 

 sumptions that require in the first place to be proved. Accord- 

 ing to my observations, the cirrhi that surround the apertures 

 are not prehensile but only tentacular ; their use apparently being 

 to guard the orifices from the intrusion of anything hurtful. The 

 cilia that clothe the interior of the siphons (which I presume are 

 what Mr. Clark alludes to) are neither prehensile nor tentacular, 

 but perform the office usual to these minute organs in assisting 

 to create a current. But why should the food be seized and 

 detained by these organs at so great a distance from the mouth, 

 when it could (and does) flow freely into the branchial siphon by 

 means of the same current that brought it to the aperture ? The 

 hyaline valve of the anal siphon would obstruct the performance 

 of such a function by the cirrhi of that aperture. This argument, 

 therefore, instead of being ' irrefragable,' appears to me to prove 

 nothing. 



The next argument rests on the literal meaning of the word 

 ' aperture.' In those bivalves whose mantle is entirely open the 

 whole circumference forms only one apertui-e, consequently in 

 these species there cannot be two apertures (ingress and egress). 

 True. But there may be nevertheless an ingress- and an egress- 

 current at different points of the open mantle without their in- 

 terfering with each other : and such is the case in Anomia, where 

 a current may be seen to pass in at the anterior base of the shell 



9 



* " The respiratory eurrei)ts are occasioned by the action of cilia, and are 

 not dependent upon the opening and closing of the valves of the shell." — 

 Owen's Lect. Comp. Anat. vol. i. p. 283. 



