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-laminae, 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-laminae 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 laminae of the gills, issued through ten thousand 

 pores, and dyed the water in the branchial chamber. Thus 

 in an instant the secret was explained; the currents com- 



