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 in 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-laminae, 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. 



p 2 



