Mr. 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 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, 

 homy, 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-laminae, 

 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 

 interbranchial tubes were submitted to a distinguished metro- 

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

 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 dactylus have been examined by ti-ansmitted light by one 

 of Mr. Ross's microscopes, with the ^ and ^ of an inch 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 'dactylus.' jNo appearance of symmetrical apertures exists, 

 but only an excessively minute wiry tracerj-, 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 i-ather than pores. 



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

 Ann. ^- May. N. Hist. Ser. 2. Vol. xii. 22 



