PHOLAS. 201 
impervious tissues, and are supplied through the orifices of the 
crypts from the water sucked in by the anal siphon; and one 
of their uses is probably, by being filled, to afford a sufficient 
tension to the network of the blood-vessels that they may the 
better receive the action of the cilia: they may also possibly 
be the receptacles for the maturation of the ova, agreeably to 
the opinions of some naturalists ; but in the multitudes I have 
examined I cannot corroborate this view, as during the months 
of May, June and July I failed to see any deposit of ova 
either on the gill-lamine, or within the interlaminar cavities, 
or in the crypts of the anal vault ; still the “ genitabile tempus” 
may be later; nevertheless the ovaria were well filled with 
germs of various sizes. Under all the circumstances of this 
experiment, I think, though it may not be impracticable, that 
it cannot be depended on even if the gill-lamine are per- 
meable ; but as I confidently believe no communication exists 
through them, I must conclude that these gentlemen were 
mistaken in supposing they had detected an issue of coloured 
fluid from the branchial vault through their exhalant siphon. 
I have now to consider the principal experiment, which 
Messrs. Alder and Hancock think will settle the disputed 
problem of in- and ex-currents in the Bivalves, produced by 
the action of cilia through separate siphons. They say, 
“ But a simple experiment will at once solve this difficulty. 
Having killed a specimen of Pholas crispata with the siphonal 
tubes contracted as little as possible, and having placed it in 
diluted spirit a few hours to render the tissues firm without 
hardening them too much, we had again recourse to the blow- 
pipe, charged as formerly with coloured fluid. The specimen 
was opened down the ventral margin, exposing to view the 
whole of the gills stretched along the roof of the branchial 
cavity. The nosle of the blowpipe was passed into the anal 
siphon, and on removing the finger from the top of the 
pipe, the contained fluid immediately filled the anal chamber 
behind the gills, and then passing at once down the tubes 
between the lamine of the gills, issued through ten thousand 
pores, and dyed the water in the branchial chamber. Thus 
in an instant the seeret was explained ;—the currents com- 
