Mr. W. Clark on the Branchial Currents in the Bivalves. 317 



ancesj but I am of opinion, that in an animal cut up from stem 

 to stem, with the so-called in-current, as they admit, annihilated, 

 little dependence can be placed on the action of the gill-laminae 

 floated in a shallow vessel, to account for the colouring matters 

 seen at its oral termination. And I cannot understand the 

 hydro-pneumatic statics of these gentlemen, nor the position 

 agreeably to their theory, that " a tendency to form a vacuum " 

 in the anal chamber and interbranchial tubes is effected by the 

 " flowing out " of the water from the ex-current siphon, com- 

 bined with ciliary agency, which actions, they add, are the 

 foundation of their " correct answer to this question ; How is 

 the matter, divided into such minute particles, collected on the 

 surface of the gills ? " 



But a fallacy with respect to a tendency to form a vacuum 

 seems to present itself, as in this case a flowing out involves the 

 idea of a flo^\•ing in, which militates against the vacuum, for the 

 fact is, that with the outflow there is in their theory a contem- 

 poraneous succession of fluid to compensate any possible ex- 

 haustion. 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 main- 

 tain 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 gra- 

 dient 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 elimi- 

 nating the oxygen for the blood, perform the function of a 

 pumping apparatus ? Surely I need not further entertain such 

 an absiu'dity ; we may therefore conclude that the water is ex- 

 pelled at intervals of two to five minutes from both chambers, by 

 the powerful adductor muscles in combination with the siphonal 

 i-etractors 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- 

 laminre, that the concluding hypothesis of these gentlemen 

 throws " some light on the sustentation of the Lamellibranchiate 

 mollusks; " I believe the gills arc strictly a respiratory machine, 

 with the exception that they may be subservient in some or all 



