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



some weight even with the sceptical. The longitudinal retract- 

 ors and transverse muscles of the siphons are of very great 

 power ; the office of the latter is to diminish the calibre of the 

 tubes, that, in conjunction with the former, they may effect a 

 more powerful expulsion of the impure fluid. As proof, if a dozen 

 Pholas dactylus are placed in a large dish of sea water, they will 

 cause so great an ejection from the siphons, not from the effect 

 of sudden disturbance or being startled, but of regular periodical 

 emissions, as to cover the table several times during the twelve 

 hours of the day and also throughout the quietude of the night : 

 assuredly this circumstance serves to prove that the impure water 

 is thus expelled, and that no part of it permeates the inter- 

 branchial tubes. 



However, it still appears that Messrs. Alder and Hancock 

 insist on a regular in-current by the branchial siphon, and an 

 ex-current from the anal, effected by cilia, for the use of the 

 respiratory apparatus ; these are, as I think, strange and impotent 

 motor agents. I have in a former paper expressed a belief that 

 the function of the cilia is to beat and subdivide the water, that 

 the oxygen may be the more easily extracted. I must now ob- 

 serve that all the testaceous Mollusca have many parts of their 

 bodies clothed with cilia, which show their action in a similar 

 manner to the Bivalves ; what then, in them, are the functions 

 of these appendages ? May we not reasonably conclude, the same 

 as in the Bivalves, to extract air from the water not only for their 

 branchiae, but perhaps to pass the vital fluid through the pores 

 of the body. One can hardly suppose that in either group their 

 duty is mechanically to create currents, when a more simple, 

 visible, and effective plan exists ; I therefore think the view is 

 untenable, that they effect the in- and out-flux of water in the 

 anal and branchial chambers. I believe a simple hydrostatic law 

 provides for this operation in all the Bivalves by a vacuum being 

 formed by the contraction of the valves in the expulsion of the 

 effete water, and that on opening them and relaxing the siphonal 

 orifices to take in a fresh supply the vacuum ceases. 



The action of the cilia is local. That they produce currents or 

 rather eddies on the gill- laminae and different parts of the body 

 of the Gasteropoda cannot be doubted ; these result from every 

 stroke of each that causes a displacement of fluid which instantly 

 reverts to its level, but they are not the locomotive agents of the 

 entrance or exit of the branchial water ; they are strictly parti- 

 cular, having no determinate line of operation, and act indiscri- 

 minately from every pole. As presumptive proof, examine an 

 oyster or a mussel from a provincial stall a few days after they 

 are received, when the cilia under the microscope will be found 

 in full action as if just taken from the sea, and will continue so 



