384 Mr. J. Alder on the Animal of Kellia rubra. 



its motions cannot be observed^ and it is therefore necessary 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 egi'css-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 1 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 vibratory 

 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 tially withdrawn at the same time that the foot is contracted, — 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 branchiae 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 oeconomy 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 continuous 



