MpiWt^Clirk on the Branchial Currents in the Bivalves. 315 



fabric, — and the contractive qualities of alcohol are well-known, 

 — had, when the moisture was evaporated, the membrane of the 

 entire network of the branchial laminse broken by lesions and 

 contractions, and their tig. 3. in the plate has much the aspect 

 of such ruptures. " I cast this idea on the waters," as Southey 

 did " his little book," and it may have as much value as it deserves. 

 I had scarcely written these lines when I found that my conjecture 

 might be right. Having opened in a gill-plate an interbranchial 

 tube that retained the injected mercury, I cleared it of the 

 mineral, and being dry it was placed in water to recover pliability, 

 for fixing on a tablet, on which it was carefully spread without 

 stretching ; I found that in the central portion of the membrane 

 of the plate almost every parallelogram was ruptured, which 

 under the microscope showed no previous solutions of continuity, 

 and each fissure proved a facsimile of those delineated vol. viii. 

 N. S. PI. XV. fig. 3. 



The area of the portion of the plate examined contained about 

 2000 parallelograms in rows, and by its size caused the sphere 

 of contractibility to centre in the middle, whilst towards the 

 margins, a less resistance and greater elasticity prevailing, many 

 of the rows of network preserved their integrity. I then pre- 

 pared another portion of ten transverse and as many longitudinal 

 rows ; in this diminished area not a mesh was ruptured, and the 

 membrane of the blood-vessels remained perfect. It appears 

 then, that the moistening of the gill-plate with plain water — and 

 of course with alcohol a much greater efi\jct is produced — may 

 have caused all the fissures in Messrs. Alder and Hancock's 

 specimens, thus fully accounting for the singularly different 

 results of our respective injections of the anal siphon. 



If I am right in these points, the question of in- and ex-cur- 

 rents by cilia and separate siphons is disposed of. The data of 

 these gentlemen to show a communication between the anal and 

 branchial vaults through the membrane of the network of the 

 gill-laminae not being tenable, of course their theory falls to 

 the ground, on the principle of " sublata causa toUitur etfectus ; " 

 consequently mine, as published in the ' Annals,' 1850, has not 

 yet been proved incorrect. 



Hitherto the Pholades have been more particularly the object 

 of consideration ; it may now be not amiss to turn our attention 

 to a group of Bivalves which, though essentially the same, diifer 

 materially in the configuration and arrangement of many of their 

 organs ; they may perhaps assist us in searching out the truth, 

 by the discordancy of their attributes with those of their pre- 

 cursors. 



How am I to consider the Anomice and Ostrea, that have open 

 mantles and no tubes, in which the water must enter at every 



22* 



