Mp, W. Clark on the Branchial Currents in the Bivalves. 313 



regular currents can be formed by such a chaos of agency ; ra- 

 pidity and diversity is the natural character of the action of the 

 cilia, and it is only by the exhaustion of moisture, which can 

 never occur in natural sites, that a subdued and more delibei'ate 

 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 yellowish pale brown ; the other thicker, of a 

 more mucous quality and whiter colour ; this is seen by exami- 

 ning the edges of a section. Between these membranes which 

 form the substance of the gill-plates the network of the blood- 

 vessels is spread, as without such support it would fall to pieces : 

 perhaps 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 

 cannot be the agents of a communication from the branchial 

 chamber to the anal siphon. 



It is necessary to state that occasional lesions, and now and 

 then a perforation, are seen on the surface of the gill-laminse, 

 the evident effect of a casual imperfection ; with these exceptions, 

 entirety is the ruling aspect ; all my fellow-observers concurred 

 in this opinion ; and two pieces of gill-lamina containing several 

 interbi'anchial tubes were submitted to a distinguished metro- 

 politan microscopist, who thus reported on them : " I can find 

 no pores in them, unless a piece of leather may be called porous." 

 Since this opinion a great number of the gill-membranes of the 

 Pholas dadylus have been examined by transmitted light by one 

 of Mr. Ross's microscopes, with the i and \ of an iiich object- 

 glasses, a power more than sufficient to detect the presence of 

 natural symmetrical apertures or pores through which effective 

 permeation could be obtained ; indeed that power would be equal 

 to show pores through which no water could pass freely, and 

 scarcely by exudation. 



The gill-plates of the Pholas parva are more delicate than in 

 the ' dadylus.' IS o appearance of symmetrical apertures exists, 

 but only an excessively minute wiry tracery, studded in the inter- 

 stices with points, which, under a power of 300 linear, only pre- 

 sented a surface little larger than the point of the finest needle, 

 and had the aspect of prominent dots rather than pores. 



In the Pholas papyracea the gills are of the finest texture, but 

 Ann. 6,- Maff. N. Hist. Ser. 3. Vol. xii. 22 



