Mr. W. Clark on the Branchial Currents in the Bivalves. 311 



counter-statement ; if these gentlemen had read a little more at- 

 tentively, they would have seen, in the paper on which they have 

 passed their strictui-es, that Mr. Clark states, " the application of 

 the mercuiy to that tube gradually filled the entire range of the 

 branchial vessels, which exhibited a very elegant appearance, but 

 no fluid escaped from them into the branchial sac." 



It is proper to state, that the Pholas crispata is the species 

 that has furnished my controversialists with their remarks on my 

 branchial theory, which is illustrated chiefly by the P. dactylus. 

 I am not aware that this circumstance is of much moment, as 

 we may safely conclude that the gills of all the Pholades have in 

 essentials the same character. But I ought to mention, that the 

 framework of the respiratory apparatus in some tribes of the 

 Bivalves presents a very difl"erent arrangement. For example, there 

 are several British families whose species I have seen alive, and 

 which fortunately can be obtained, that have a peculiar branchial 

 construction, which appears as to general configuration closely 

 analogous to that lately described in the ' Annals ' to exist in 

 the Chamostrea albida and Myochama anomioides of authors, but 

 the 2}articular parts of the mechanism in my species do not ac- 

 cord; I think the narrow reticulated ribands on the external 

 surface are not permeable, and do not lie on apertures that com- 

 municate with the interbranchial tubes. I refrain, at present, 

 from extending these remarks, but in a fitting time I shall be 

 prepared with some comparative notes on certain species that 

 have only a single complete gill-lamina and a rudimentary one 

 on each side the body, which seem to me to difi'er essentially 

 in structure from the descriptions that have been promulgated 

 on the composition of the branchial mechanism of the species 

 that have been alluded to. 



I now enter on the counter-statement to the last quotation, 

 and beg to observe, that Messrs. Alder and Hancock, in the ex- 

 planatory sketch of their Pholas cri&pata, PI. XV. vol. viii. N.S., 

 give a very intelligible outline of their theory. Though entirely 

 dissenting from it, I cannot but admire the ingenious dehneation, 

 particularly fig. 3. of the gill-laminse, showing the aspect of the 

 meshes ; it has however one fault — it exhibits them all with sym- 

 metrical longitudinal fissures called " orifices," which I think are 

 ruptures of the membrane of each mesh, not one of which exists 

 naturally in the three species I have examined. 



Since May 1853 I have often performed " the simple experi- 

 ment " detailed by Messrs. Alder and Hancock in the third pa- 

 ragraph of their paper, p. 374 ; it is by far the most important 

 of the series, as the problem of communication, with them, be- 

 tween the anal and branchial siphons, depends on it : — by the in- 

 jections of more than 200 Pholades with mercury and coloured 



