PHOLAS. 205 



exists ; as when there is really an imperfection in the mem- 

 brane it cannot thus be resolved, bnt 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, 

 like 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 never 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 eliminating 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 



