212 PHOLADHLE. 



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 absolutely antagonistic to water, and impermeable ; there- 

 fore the gill-laminae of the Pholades and other bivalves are 

 impermeable. 



I apprehend, that ciliated mucous membranes are neither 

 absorbents nor emuuctories, 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 

 constitution of the branchial plates informs the anatomist and 

 physiologist, that there cannot through them be a communi- 

 cation from the branchial to the anal chamber. 



It would be lost labour to prolong this disquisition, in which 

 I fear my observations have been too often repeated, but the 

 importance 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 pre- 

 sented, 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 this 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 history many observers, besides the present controversialists, 



