PHOLAS. 193 



p. 370. pi. 15. N. S., on my branchial theory, which appeared 

 in a paper on the Pholadida in vol. vi. p. 313 of that publi- 

 cation. I was so engaged last year in the examination of a 

 splendid harvest of rare animals, that I had no time to con- 

 sider their remarks, but having now an unlimited supply of 

 Pholades, I will endeavour to acquit myself of my engage- 

 ments with these gentlemen. 



They commence by quoting parts of my theory, and say 

 that I announce " that T %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 be- 

 neath, and the siphonal tubes in communication with the sea. 

 Yet Mr. Clark would have these animals receive only the 

 small quantity 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 I reply, that a fresh and very extended 

 examination of four species of the Pholades fully supports me 

 in maintaining all my positions, and I think I shall demon- 

 strate 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. 



AVith respect to their observations on the habits of the 

 PholadeSj it is only necessary to admit, that these animals 

 often inhabit sand and shingle, mixed with clay, and are not 

 always imbedded 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 there- 

 with, 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." 



