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



through the fluid to show the floating particles ; for it is the size 

 of these, and not that of the aperture, which enables an observer 

 to distinguish the direction of a current. Mr. Clark could see 

 the excrements pass out of this small opening. What then should 

 prevent our seeing other bodies, if sufficiently visible under the 

 microscope, float in or out ? 



For argument, Mr. Clark would assume that the posterior slit, 

 as I state, shows no sign of an ingress-current. Yet no argu- 

 ment is founded upon it, for in the very next sentence the con- 

 trary fact is stated to he proved "by the contraction and dilatation 

 of the slit % (my dissent from this proof is already on record) ; 

 " especially," that gentleman adds, " as I have shown that the 

 analogous tubes " (the anal ones ?) " of the close-mantled mol- 

 lusca . . . must of necessity receive and discharge the fluid neces- 

 sary for the branchial oeconomy." Is this shown ? and where ? 



We have next an assumed case which is also called & proof, 

 put in these words : " Suppose Kellia rubra, instead of being an 

 open-mantled animal, is one of the closed mollusca, — where, in 

 this case, is the entrance to the branchial currents ? " All the 

 known closed mollusca have at least two if not three apertures. 

 A closed mollusk with a single aperture, if such did exist, would 

 be an anomaly, and its branchial arrangement might also be ex- 

 pected to be an exception to the general rule. But what argu- 

 ment can be founded upon this ? That where there are two or 

 more apertures, they cannot be set apart for different purposes ? 

 Certainly not ; — any more than we could argue that because some 

 animals exist where the alimentary and excretory functions are 

 performed through the same orifice, that in other animals where 

 two orifices are found they cannot perform different functions. 



" It may be asked," says Mr. Clark, " why has nature departed 

 from her usual scheme only in Kellia rubra and -ST. suborbicularisV 

 The only way in which the usual scheme is departed from in this 

 genus, is, not in giving the species a special inhalant siphon, but 

 in placing it before instead of behind : and perhaps for this some 

 reason might be found. Most bivalves live in sand, and they 

 require to have both tubes placed at that end of the shell which 

 usually communicates with the surface. The Kellia, on the con- 

 trary, never burrow in sand, but inhabit the sinuosities of rocks, 

 sea-weeds, and old shells : a simpler arrangement, by which the 

 water can be admitted direct to the mouth and anterior part of 

 the gills, is therefore not incompatible with its habits. But it 

 is added, " We will now inquire into the { cui bono ' of this fold 

 of the mantle, considered as a branchial appendage. It is well 

 known that nature never acts by way of surplusage ; and having 

 given Kellia rubra an open' mantle by which the currents can 

 enter, as in other analogous 'Open bivalves, we must conclude 



