212 PHOLADID/. 
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-lamine of the Pholades 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 
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 
Penimsular 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 im 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 m this branch 
of history many observers, besides the present controversialists, 
