384: Mr. J. Alder on the Animal of Kellia rubra. 
its motions cannot be observed, and it is therefore neces to 
have some minute floating matter contained in it; but this is 
always the case in water obtained fresh from the sea, though it 
may appear pure to the naked eye. The next thing to ascertain 
was, whether a current of water passed in by any other channel. 
For this purpose I examined carefully the circumference of the 
mantle, but found no indication of an ingress-current at any 
other part. The floating particles in the water remained perfectly 
stationary, with the exception that an egress-current was occa- 
sionally seen to proceed from the posterior orifice: but this was 
more difficult to detect than the ingress-current, probably because 
the floating matter appeared to be all detained and appropriated, 
and partly because the flow was not continuous, occasioned by 
the alternate opening and closing of the aperture as is usual in 
the excretory siphon. In two or three cases I saw a current 
issuing from this orifice very distinctly, but never one entering 
it. In another instance where a delicate filament of extraneous 
matter was attached to the edge of the aperture, its vibrato: 
motion showed the presence of a current which I could not other- 
wise detect ; but this filament was always deflected outwards, and 
was never drawn towards the fissure, as would have been the case 
had an alternate current set in in that direction. 
We shall now turn to Mr. Clark’s observations. As Mr. Clark 
does not say that he has seen the currents of water in any in- 
stance, I infer that his conclusions are drawn from the appear- 
ance and motions of the parts only, which in all cases he states — 
very correctly. Mr. Clark considers that the only use of the an- 
terior tube-like fold is to assist the foot in progression. I do not 
exactly understand how this is to be accomplished, nor is its 
mode of operation distinctly explained. The tube is indeed par- 
tially withdrawn at the same time that the foot is contracted,— 
that is, while the body is drawn forward,—but this appears to be 
more easily and satisfactorily explained by supposing that the 
withdrawal of the tube at each step is for the purpose of regu- 
lating the admission of water while the body is advanced. These 
parts are known to be extremely sensitive, and contract on the 
slightest external motion. But Mr. Clark thinks he has disco- 
vered that the supply of water for the branchiz is received and 
expelled by the same aperture—the posterior one—in the man- 
ner of systole and diastole. To this I would reply, that such a 
supposition is contrary to the known ceconomy of the bivalves, in 
which the inhalant is always kept distinct from the exhalant 
current, and admitted by a separate aperture from that by which 
the latter is expelled. This seems to be necessary, as the currents, 
being caused by the motion of the branchial cilia, and not by the 
expansion and contraction of the walls of a cavity, are contimuous 
