Mr. W. Clai'k on the Branchial Currents in the Bivalves. 307 



as long as moisture remains. 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 from 

 the consideration that the cilia must act antagonistically from 

 opposite points ; one set to work the water in branchially, and 

 another to expel it through the anal duct after percolating the 

 gill-laminae and interbranchial tubes. I shall recur to the cilia, 

 and expect to prove that the new scheme of communication be- 

 tween the two siphons is very problematical, I may say imprac- 

 ticable. 



Messrs. Alder and Hancock go on to say, that any one may 

 convince himself of the existence of a branchial in-current and an 

 anal ex-current, by placing a Pholas " in a glass of sea water, 

 and then by gently adding a little fresh, slightly charged with 

 floating particles,'' he will perceive the two actions. I admit, by 

 this process, that currents will ensue, as the invigorating fresh 

 element causes the animal to expel that which has become effete 

 and take in a new supply ; but as I have, under every condition of 

 experiment, examined multitudes of these creatm'cs, I am bound 

 to declare that the currents have no continuous regularity : and I 

 think the mode of testing their presence by means of the water 

 being chai-ged with buoyant particles is fallacious ; these only 

 float on the surface, and are subject to many pertui'bations and 

 deceptions arising from depth of water, currents of air, the po- 

 sition of the animal, whether on the ventral or dorsal surface, 

 by its will and humour, state of exhaustion, and by an unnatural 

 confinement. My repeated examinations show that the par- 

 ticles are whirled in all directions ; sometimes they pass into the 

 branchial chamber, at others none will enter : capricious gyra- 

 tions, whether arising from the action of the animal or other 

 natural causes, are their ruling character. With respect to the 

 anal siphon, the floating particles are certainly repelled from its 

 orifice in a somewhat regular and continuous manner, being only 

 interrupted by the periodic reception of fluid to supply the ex- 

 haustions. The frequent repulsions of particles from the anal 

 orifice have been construed by Messrs. Alder and Hancock to 

 arise from the percolation of water from the branchial vault 

 through the gill-laminae and interbi'anchial tubes to an issue at 

 the anal siphon, produced by the agency of cilia. I think it will 

 appear that this complicated operation will meet with insur- 

 mountable difficulties, and though I admit the anal outflow, I 

 protest against its being considered of branchial oi'igin and regu- 



