244 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 pomt, 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 eapelled indiscriminately im 
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 ove aperture, 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 n- 
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 
* «The respiratory currents 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. 
