Mr. W. Clark on the Branchial Currents in the Bivalves. 803 



XXIX. — On the Branchial Currents in the Bivalves. 

 By William Clark, Esq. 



To the Editors of the Annals of Natural History, 



Gentlemen, Exmouth, September 1853. 



I REQUEST permission to reply to Messrs. Alder and Hancock's 

 comments in the 'Annals of Natural History/ vol. viii. p. 370. 

 PI. XV. N. S., on my branchial theory, which appeared in a 

 paper on the Phohdidce in vol. vi. p. 313 of that publication. 

 I was so engaged last year in the examination of a splendid 

 harvest of rare animals, that I had no time to consider their re- 

 marks, but having now an unlimited supply of Pholades, I will 

 endeavour to acquit myself of my engagements with these gen- 

 tlemen. 



They commence by quoting parts of my theory, and say that 

 I announce " that y^ths, if not all, the branchial water is admitted 

 by the pedal gape." On this point they observe — 



" Let us for a moment consider what would be the conse- 

 quence of Mr. Clark^s supposition, that these animals obtain 

 water only by the pedal gape. Nearly all of them pass their 

 lives buried in sand or mud, or immured in solid stone, with only 

 a small aperture externally, the pedal opening being beneath, 

 and the siphonal tubes in communication with the sea. Yet 

 Mr. Clark would have these animals receive only the small quan- 

 tity of water charged with sand and mud that finds its way to 

 the bottom of the cavity, rather than draw their supply from the 

 pure element on the surface, by means of their long siphons." 



To this quotation T reply, that a fresh and very extended exa- 

 mination of four species of the Pholades fully supports me in 

 maintaining all my positions, and I think I shall demonstrate 

 that Messrs. Alder and Hancock's system of branchial currents 

 is erroneous- The only correction I have to make is, that I have 

 clearly ascertained that the branchial, like the anal siphon, is 

 both inhalant and exhalant. 



With respect to their observations on the habits of the Pho- 

 lades, it is only necessary to admit, that these animals often in- 

 habit sand and shingle, mixed with clay, and are not always im- 

 bedded in hard rock. But we contend that the cavities in which 

 they dwell afford sufficient passage for the sea water, and the 

 areas of their habitats are saturated thei'ewith, as when the tide 

 withdraws, much of its waters is retained by the various strata, 

 which by filtration reaches the burrows in a pure state, and not 

 " charged with sand and mud." 



Continual watchinffs for months of multitudes of these ani- 



