Currents in Pholas and Mya. 377 



This examination of the gill in its living state throws some 

 light upon the sustentation of the Lamellibranchiate Mollusks ; 

 for it would appear evident that all the minute particles of matter 

 suspended in the water are collected and carried to the mouth 

 without any apparent selection. The labial tentacles may pos- 

 sibly have the power of rejecting distasteful matters ; but it is 

 difficult to conceive how this can be, if the particles, as in the 

 present case, always form a continuous cord, which would have 

 to be severed before any part could be disengaged. And we may 

 mention in proof of the minuteness of the particles extracted 

 from the surrounding element by these animals, that on placing 

 five or six individuals of Pholas crispata in a small vessel of water 

 strongly coloured with indigo, they, in the course of twelve or 

 fourteen hours, deprived the water of nearly all its colouring 

 matter, leaving it scarcely tinged with blue, and the alimentary 

 tube of several that were opened was crammed full of indigo, and 

 that it was but little altered in appearance on passing in fsecal 

 masses from the anal siphon. 



The structure of the gills of Mya arenaria is similar to that of 

 Pholas ; those of Pullastra perforans are likewise formed much 

 after the same plan, and in this latter species the sedimentary 

 matters are collected on the surface of the gill in exactly the 

 same manner as in Pholas crispata. In Cardium edule the gills 

 are provided with interbranchial tubes, and they convey the food 

 to the mouth along their free margins. The anal and branchial 

 chambers of the oyster are as completely separated as in Pholas, 

 and the gills procure food and take it to the mouth in the same 

 manner as in that genus. In Mytilus edulis the branchiae are a 

 little modified. The four plates in this species are composed of 

 two laminae as usual, but only one of the laminae of each gill is 

 attached to the mantle behind ; the outer lamina of the external 

 gill, and the lamina next the foot of the internal gill being free : 

 the interbranchial tubes are iiTcgular and incomplete, extending 

 scarcely halfway up from the free margin of the gill. The 

 branchial plates of Mytilus are nevertheless efficient sieves, col- 

 lecting all sedimentary matters on their surfaces, and conveying 

 the same along their margins to the oral opening *. 



These are all the species we have yet examined, but perhaps 

 they may be considered sufficient to support a belief that the 

 sieve-like character of the branchije, and the mode of action here 

 described, predominate throughout the whole order of theZ«?«e/- 

 libranchiata, as well as in that of Tunicata, whose branchiae are 

 known to have a similar structure. 



* The gill-lamina; of Pecten opercularis are arranged in the same man- 

 ner as in Mytilus. The outer lamina is free and mnch narrower than the 

 inner, extending only about two-thirds up from the free margin. 



