200 Proceedings of Royal Society of Edinhnrgk. [march 5 , 
out from among the gills and palps with sufficient force to send 
them to the muscular margin. 
Fourthly, the finer particles, which might he supposed to serve 
as food, are sifted out by the gills, and only the coarser particles 
are carried forward. 
Fifthly, the mouth-lips can discriminate suitable or unsuitable 
food-particles, and so the mouth-lobes or palps are not required for 
that purpose. The lips may be seen to pass to one side or another 
matters presented to them, and they can also pass them in as well 
as out. 
Sixthly, sensitive tips of palp readily reject, but do not retain 
solid particles. 
Seventhly, the palps may control to a certain extent the opening 
or closing of the lips, and may even allow matter to pass, but there 
is no evidence to show that they carry supplies to the mouth. 
It seems to me that in all this speculation about the mode of 
feeding one very obvious fact has been overlooked, viz., the counter - 
currents necessarily accompanying the original currents. 
A glance at fig. 1 will show what a network of currents exists in 
Mytilus, and they are only partly represented there. There are 
four inward currents towards the mouth-end, along the free margins 
of the gills, two on either side of the body, going at a rate of at 
least 1 inch per minute, and sometimes double that speed. And 
there are two outward currents, one on each mantle-lobe margin, 
streaming posteriorly at a rate of 2 inches per minute, and some- 
times even quicker. One effect of these currents will be to create 
counter-currents, and we have already noticed the counter-current 
of the right inner gill carrying particles posteriorly. 
The counter marginal current of each mantle-lobe will carry 
particles towards the anterior end. There the converging and 
diverging currents of the gills and mantle-lobes will create an eddy 
in the confined triangular space in front of the mouth, and food- 
particles will be brought within reach of the mouth. 
In this neighbourhood, too, the constantly renew^ed water will be 
as constantly bringing fresh supplies, and a portion of the micro- 
scopic food must inevitably enter such a wide opening as the mouth, 
which, moreover, has its own ciliary current. By the mere act of 
opening or closing its mouth, it can feed or abstain from feeding. 
