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



another species of the same genus, the walls are closed ; yet their 

 functions are surely analogous. A similar siphonal fold, though 

 less perfect, may be seen in some of the Modiolce : but the case 

 most in point is the siphon of the zoophagous gasteropods, which 

 is a prolongation and fold of the mantle similar to this, yet no 

 one that I am aware of has argued that it cannot be for the sup- 

 ply of water to the branchiae because it is continuously open with 

 the other parts of the cloak*. 



Mr. Clark thinks my views incorrect : " As in those bivalves 

 with open mantles the currents of water enter by the great pedal 

 orifice or rima magna of the mantle to aerate the branchice, and 

 the greater part of the impure fluid is expelled by the aperture 

 of ingress, a small portion, as before stated, passes out by the 

 posterior siphonal apparatus." Is this any more than a repetition 

 of the former statement, leaving out the opening and shutting of 

 the valves, and defining the purpose more distinctly to be, " to 

 aerate the branchiae" ? That it has reference to the same action 

 is evident from the words " as before stated." Mr. Clark must 

 therefore either think that the branchial currents are produced 

 by the opening and shutting of the valves, or he is confounding 

 two things that are distinct. If the pedal orifice is the principal 

 one by which the true branchial currents are received and ex- 

 pelled, of course my observations, and the views of almost every 

 author who has written on the subject must be wrong, but the 

 proof requires to be brought forward in some more definite form 

 than this. 



Again, Mr. Clark says, " In the mollusca with nearly closed 

 mantles, only a small portion of the fluid can enter by the re- 

 stricted pedal orifices ; the far greater portion must be inhaled 

 by the posterior siphons " (not necessarily by both), " and is often 

 expelled simultaneously at both orifices, as I have observed in 

 Pholadidea papyracea, the most closed of all the bivalves." This 

 fact of the occasional simultaneous expulsion of water at both 

 orifices seems to be the only one that Mr. Clark has satisfactorily 

 ascertained from observation in this species ; he might perhaps 

 have added that it was accompanied by a closing of the valves; — 

 at least such is the case with the allied Pholades as I have myself 

 witnessed. But this sudden ejection of water is only occasional, 

 and caused by other means than the regular ciliary currents. It 

 is probable that in the Pholades and some other bivalves with 



* I am sorry to have misunderstood Mr. Clark with respect to the sense 

 in which he took the words branchial and anal. I did not say, however, 

 that he used the words, but that he appeared to take them (as used by 

 others) in too restricted a sense. My reason for thinking so was, that he said 

 the posterior opening had " passed for the anus," and took some trouble to 

 show that the true anus is distinct from it. 



