PHOLAS. 197 



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 observe 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 reason- 

 ably 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 particular, having no determinate line of opera- 

 tion, and act indiscriminately from every pole. As presump- 

 tive proof, examine an oyster or a muscle 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 as long as moisture re- 

 mains. In this case these species, even if they had siphons, 

 could not produce in- and out-currents by separate ducts, 

 from non-access to water ; we are, therefore, bound to give 

 the preference to the idea that their functions are to eliminate 

 the oxygen. I may observe, that cilia are attached to the 

 different epithelia in all animals, from the monad to man. 

 The inconsistency of such a motive power will be apparent 



