180 Mr. J. Alder on the Branchial Currents 



and never, under any circumstance, through the ventral or pedal 

 opening." On the contrary, I admit the flowing in of water 

 through all the apertures (not voluntarily closed) as a natural 

 consequence of the opening of the valves*, which action the ani- 

 mal has of course frequent occasion to perform. I have stated, 

 too, in recording my observations on Turtonia minuta, that " at 

 first the water was observed to pass into the widely-open mantle 

 of this little mollusk at all parts of the base of the shell f ;" and 

 that "in Montacuta bidentata the principal ingress current is 

 decidedly anterior, though the water is admitted occasionally 

 through the whole length of the open mantle J." The occasional 

 expulsion through all the orifices is treated as an acknowledged 

 fact throughout ; the only reservation I make is that it must be 

 considered an occasional action, unconnected with the regular 

 branchial currents. Mr. Hancock has also distinctly stated in 

 his paper on Chamostrea that, when the valves are suddenly 

 closed and the siphons withdrawn, the contained water will 

 escape by the pedal opening and the minute fourth opening, 

 which is found in this and some other bivalves with closed 

 mantles §. In our joint paper "On the Branchial Currents in 

 Pholas and Mya," a description is given of the action of the 

 currents, observed in an individual of Mya arenaria, in situ. 

 This instance is adduced in illustration of the general law of the 

 distinct and simultaneous action of separate currents through 

 the siphons. The question of occasional action under different 

 circumstances or in other families is not gone into, so that no 

 such extreme opinion on this matter as Dr. Williams attributes 

 to Mr. Hancock, can, by any fair construction of words, be de- 

 duced from it. Indeed the holding of such opinions we both 

 distinctly deny. Concerning this joint paper I take the oppor- 

 tunity of stating that, Mr. Clark having brought forward the 

 non-connexion of the branchial and anal chambers in Pholas as 

 a proof that the currents could not pass in at one siphon and out 

 at the other, I felt myself unable, from the want of sufficient 

 skill in delicate anatomical investigations, to give a correct solu- 

 tion of the difficulty, though aware of the fact of communication 

 from repeated observation of the currents. I therefore had re- 

 course to the assistance of my friend Mr. Hancock. The result 

 was the discovery of the communication existing through the 

 minute network of the gills, which Dr. Williams rightly attri- 

 butes to Mr. Hancock. He considers it very extraordinary, 

 however, that Mr. Hancock should not have been aware of the 

 discovery of the same fact by Dr. Sharpey, published ten years 



* Ann. Nat. Hist. 2nd Ser. vol. iv. p. 49. f Vol. iv. p. 243. 



X Vol. v. p. 211. § Vol. xi. p. 106. 



