318 IMr. W. Clark on the Branchial Currents in the Bivalves. 



the Bivalves to reproduction. I consider that i}\Q palpi are.tb^ 

 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. 



In a gill-membrane in which cilia are planted, epithelium is 

 always present, and it and its supporting tissue cannot exist 

 without a membranous and mucous substratum ; these are abso- 

 lutely antagonistic to water and impermeable ; therefore the gill- 

 lamiuse of the Pholadcs and other bivalves are impermeable. 



I apprehend, that ciliated mucous membranes are neither 

 absorbents nor emunctories, though the vessels of such glands 

 may pass through them to the surface ; they are probably a 

 product by exudation from the blood-vessels, for the formation 

 of an upper membrane and the epithelium. Thus the very con- 

 stitution of the branchial plates informs the anatomist and, 

 physiologist, that there cannot through them be a communica- 

 tion from the bi'anchial to the anal chamber. 



Jt would be lost labour to pi-olongthis disquisition, in which I 

 fear my observations have been too often repeated, but the im- 

 portance of the problem is my apology. If I have failed to 

 convince, I have at least supplied matter for reflection, which 

 may perhaps lead malacologists to doubt whether the doctrine 

 of inhalant and exhalant currents by cilia and distinct apertures 

 can be maintained against the evidence I have presented, and to 

 admit that this long-entertained theory may prove a delusion. 



I conclude with a remark of Sir William Napier, who thus 

 offers an apology — all will exclaim, a needless one — for writing 

 the history of what he terms "a thrice-told tale," the Peninsular 

 War : the eloquent historian says, " that two men observing the 

 same object will describe it diversely, according to the point of 

 view from which either beholds it ; in the eyes of one it shall 

 be a fair prospect, to the other a barren waste, and neither may 

 see aright.'" 



Are the northern naturalists and myself in tliis category ? He 

 adds, " wherefore truth being the legitimate object of history, it 

 is better that she should be sought for by the many than by few, 

 lest for want of seekers, amongst false lights, she be lost 

 altogether." 



Let us then apply these views, and hope that in this branch of 

 historv many observers, besides the present controversialists, will 

 step in to announce the truth and dispel the mists of prejudice. 

 I am. Gentlemen, your niost obedient servant, 



Willi vm C'lark. 



