Mr. W. Clark on the Branchial Currents in the Bivalves. 309 



again, failure always occurred ; as the animal, after receiving the 

 coloured fluid, which was apphed without difficulty, in general 

 immediately discharged it by the pedal gape, or by the branchial 

 aperture overwhelming with coloured matter both tubes, the ori- 

 fices of which by theii* inflection by the animal were so retracted 

 and blended together as to be undiscoverable ; of course, any issue 

 of Hquid from a particular tube was undistinguishable. ^^^len, 

 in any example, the fluid, which was coloured by archil, re- 

 mained a Uttle time without expulsion, I opened the branchial 

 cavity lo see if the gill-laminae and interbranchial tubes showed 

 any increased inflation or colour from the filtration of the injec- 

 tion, but no unusual appearance presented itseK. I also opened 

 the anal vault and collected with a camel's-hair brush as much 

 of the moisture as possible ; this was applied to a veiy small 

 quantity of distilled water, but no trace of colour appeared ; we 

 may then presume that none of the injection had passed from 

 one siphon to the other. But when the coloured fluid was ad- 

 ministered anally, all the interbranchial tubes were at once filled 

 and remained inflated more than an hour, representing minute 

 well-filled hoses, which bore the pressure of a delicate wooden 

 stylet, and exhibited the fluctuations of the liquid, which, on its 

 removal, instantly reverted to the points of displacement without 

 any escape into the branchial chamber. We may therefore con- 

 clude, that the interbranchial tubes are imper\-ious 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 afl'ord 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 matura- 

 tion 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-laminse, or within the 

 interlaminar cavities, or in the ci-ypts of the anal vault ; still the 

 " genitabile fempus" may be later; nevertheless the ovaria were 

 well filled with germs of various sizes. Under all the circum- 

 stances of this experiment, I think, though it may not be im- 

 practicable, that it cannot be depended on even if the gill-laminae 

 are permeable ; 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-cun-ents in the Bivalves, produced by the action of cilia 

 through separate siphons. They say, — 



