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 be 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 ceconomy.” Is this shown? and where ? 
We have next an assumed case which is also called a proof, 
put in these words: ‘ Suppose Kellia rubra, instead of bemg 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 K. suborbicularis?” 
The only way in which the usual scheme is departed from in this 
genus, 18, 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 Kelle, 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 Kelia rubra an open mantle by which the currents can 
enter, as in other analogous open bivalves, we must conclude 
