PHOLAS. 205 
exists; as when there is really an imperfection in the mem- 
brane it cannot thus be resolved, but under every phase of the 
instrument the hiatus of a solution of continuity is seen. The 
shallow depressions are the uncovered patches of the mem- 
branous base of the scales or epithelium, incident to all the 
Mollusca ; from them the numerous vibratile cilia spring, which 
present the most discordant and particular motions that operate 
from every point ; sometimes they appear as if each entire pit 
was whirled on a vertical axis, at others a compact mass of 
strands dilates and contracts like the heart, then a fasciculus 
of cilia is seen beating the water with irregularity; some- 
times only a single cirrhus is raised in quick succession, 
hike "a hammer in a mechanic’s hand; but it is impossible 
to describe all the varieties of motion. In a fresh animal, 
the action and strokes exhibit the greatest rapidity ; it seems 
utterly impracticable that regular currents can be formed by 
such a chaos of agency; rapidity and diversity is the natural 
character of the action of the cilia, and it is only by the ex- 
haustion of moisture, which can neyer occur in natural sites, 
that a subdued and more deliberate motion is attained, and 
even then their direction is as variable as ever; I can only 
consider them as the eliminatmg mechanism of the oxygen. 
The epithelium is pretty regularly deposited on the upper area 
of a compound membrane, one lamina being thin, horny, and 
of a pale yellowish brown ; the other thicker, of a more mucous 
quality and whiter colour: this is seen by examining the edges 
of a section. Between those membranes which form the sub- 
stance of the gill-plates the network of the blood-vessels is 
spread, as without such support it would fall to pieces: per- 
haps the roots of the cilia pass through the epithelium and its 
supporting membrane, and impinging or centring on the coats 
of the blood-vessels, by a capillary or porous action supply 
them with the air they extract from the water. It is scarcely 
possible to view a more interesting object than the structure of 
the branchial mechanism and operation of the cilia, by trans- 
mitted light, under a power of 300 or 400 diameters. I think 
these data will almost convince naturalists that these organs 
