208 PHOLADID. 
evaporated, the membrane of the entire network of the 
branchial lamine broken by lesions and contractions ; and 
their fig. 8. in the plate has much the aspect of such ruptures. 
“T 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 inter- 
branchial tube that retained the imjected 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 care- 
fully spread without stretching; I found that im the central 
portion of the membrane of the plate almost every parallelo- 
gram was ruptured, which under the microscope showed no 
previous solutions of continuity, and each fissure proved a 
fac-simile of those delineated in the ‘ Annals,’ vol. viii. N.S. 
ple lb sfiga3- 
The area of the portion of the gill-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 prepared 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 fresh water—and of course with alcohol a much 
greater effect 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 m- and ex- 
currents 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-lamine not being tenable, of course their 
theory falls to the ground, on the principle of “sublata causa, 
tollitur effectus ;” consequently mine, as published im_ the 
‘Annals,’ 1850, has not yet been proved incorrect. 
