372 Messrs. Alder and Hancock on the Branchial 



tained. On replacing the animal in water, a powerful column was 

 observed to pass in through the pedal opening, while none was 

 seen to enter by the siphons, these latter being probably closed 

 by the animal in consequence of the unnatural iisage it had been 

 subjected to ; but whether this be the case or not, it was to be 

 expected that, by a known law of hydrostatics, the greater flow 

 of water would be through the larger aperture until the equili- 

 brium was restored. This, however, is a special case having 

 nothing to do with the regular branchial currents, as has been 

 before pointed out to Mr. Clark ; yet this experiment is consi- 

 dered to have finally disposed of the complicated scheme, as it is 

 called, of other authors, by which name is designated the simple 

 observed fact, that water for branchial purposes is received and 

 expelled by the action of cilia and by separate orifices. 



Let us for a moment consider what would be the consequence 

 of Mr. Clark's supposition that these animals obtain water only 

 by the pedal gape. Nearly all of them pass their lives buried in 

 sand or mud, or immured in solid stone, with only a small 

 aperture externally, the pedal opening being beneath, and the 

 siphonal tubes in communication with the open sea. Yet Mr. 

 Clark would have these animals receive only the small quantity 

 of water charged with sand and mud that finds its way to the 

 bottom of the cavity, rather than draw their supply from the 

 pure element on the surface, by means of their long siphons, 

 which, it is generally supposed, nature has furnished them with 

 for this very purpose. 



It has long been known to us from actual observation that the 

 Pholacles and Mijadm do di-aw in a current of water by the 

 branchial siphon, and that it is expelled by the anal one, as is 

 the case with the other bivalves. To convince himself of this 

 fact, it is only necessary for an observer to place a Pholas in a 

 glass of sea-water and allow it to remain undisturbed for some 

 time, in order that the animal may stretch out its tubes and re- 

 cover its natural quiescent state. Then by gently adding a little 

 fresh sea-water slightly charged with floating particles, a very 

 decided current will generally be seen to set into the branchial 

 siphon, and a counter-current to proceed from the anal one ; the 

 water round the pedal opening being at the same time undis- 

 turbed. 



The same arrangements are readily recognised in Mya. We 

 lately had an opportunity of observing Mya arenaria in its native 

 haunts, and watched the play of its siphonal currents under vei*y 

 favourable circumstances. This species, at the mouth of the 

 Tyne, buries itself to a depth of 6 or 8 inches in a stiffish clay, 

 mixed with shingle ; and, in shallow pools left by the tide, the 

 siphonal tubes may be seen just level with the surface of the 



