1888.] 
Mr D. Alpine on Bivalve Molluscs. 
197 
both of the outer and inner, do not drive particles towards the 
mouth but away from it, and the normal direction of rotation indi- 
cates the same. The only one I have found to question the asser- 
tion is Herr J. Thiele,* and it is a significant fact that he has 
examined the palps of no less than eighteen families of Lamelli- 
branchs. In dealing with the physiology of the palps, the first 
point he attempted to settle was the possibility of their having any 
function in relation to the ingestion of food. He cannot but 
recognise the outward direction of the currents, and at the same 
time he cannot shake his mind free from prejudice in favour of the 
palps, and so he attempts a compromise. He comes to the conclu- 
sion that the use of the marginal currents appear to be to drive 
away the water from which the food has been obtained, but he 
omits to notice the very point at issue, how the food is obtained — 
how the nutritive particles in the water can travel one way and the 
water another. According to one set of observers or describers, 
the gills strain off the solid particles from the water, but here it is 
the palps which remove the water from the solid particles of food. 
In this connection, we must not omit to notice the observations 
of Alder and Hancock,! entirely agreeing with our own, as far as 
they go. They observed the currents in a Pholas, laid out similarly 
to the Mytilus, by means of indigo particles. They found an out- 
ward surface current and a forward marginal current on the gills. 
The stream of indigo particles, constantly being reinforced, increased 
in volume towards the anterior extremity of the gill, and they were 
not loosely driven along but bound together in cords or threads by 
some tenacious fluid. This stream of particles continued to be 
formed and to move forward for hours. The rest will be given in 
their own words : — “Thus considerable quantities of indigo were 
accumulated in the vicinity of the mouth and oral tentacles. These 
accumulations were composed of ravelled threads, spun as it were 
by the branchial apparatus, from the scattered, nearly invisible 
particles of indigo, in the surrounding medium. This examina- 
tion of the gill in its living state throws some light upon the 
sustentation of the Laniellibranchiate molluscs; for it would 
appear evident that all the minute particles of matter sus- 
* Zeitschr.f. Wiss. Zool., xliv,, 1886. 
t A7in. Mag. Nat. Hist., vol. viii., 18.51. 
