PHOLAS. 211 
contemporaneous succession of fluid to compensate any pos- 
sible exhaustion. One would rather suppose that a tendency 
to a vacuum, instead of existing in the anal chamber, the 
point of issue, would be formed in the branchial vault, the 
source of supply, from a possible deficiency of fluid: a river 
shows no appearance of vacuity at its debouchure or elsewhere, 
whilst its sources maintain their integrity. 
I can conceive in a running stream that the pressure of one 
portion of water on another produces an impulsion, not a 
vacuum; but how is this impulsion from mere declivity of 
gradient to operate in the Bivalves, in which the natural 
position of the siphons is almost invariably at an angle of 90° 
in reference to the horizon? How is the flow out of water to 
be effected in them? Are we called on to believe that the 
cilia, besides eliminating the oxygen for the blood, perform 
the function of a pumping apparatus? Surely I need not 
further entertain such an absurdity; we may therefore con- 
clude that the water is expelled at intervals of two to five 
minutes from both chambers, by the powerful adductor 
muscles i combination with the siphonal retractors of the 
animal operating on the valves; these agents act as a force- 
pump ; there is no other adequate exhausting mechanism. 
I do not think the idea of ciliary currents, independent of 
those for the extraction of the oxygen, can be sustained. I 
also cannot admit, with my views of the impermeability of 
the gill-laminz, that the concluding hypothesis of these gen- 
tlemen throws “some light on the sustentation of the Lamel- 
libranchiate mollusks;” I believe the gills are strictly a 
respiratory machine, with the exception that they may be 
subservient in some or all the Bivalves to reproduction. I 
consider that the palpi are the purveyors and locomotive 
agents of the alimentary matters. 
As a last argument I submit a syllogism, which perhaps 
some of your readers will say, from its decisive character, had 
better have been placed at the head, instead of the end of this 
paper, and thus they and myself would have escaped the 
trouble of wading through long accounts of optical and other 
experimental tests. 
P2 
