1888.] Mr D. Alpine on Bivalve Molluscs. 195 
the mouth; in the outer, the currents running towards the inner 
margin, so that particles thrust off at the anterior end of the outer 
gill are driven towards the inner margin of the palp, and so con- 
veyed aioay from the mouth. The highly vascular nature of the 
palps suggests or indicates that they have a respiratory function. 
3. Mantle- Lohes . — There can be little doubt that the inner sur- 
face serves a respiratory purpose, by reason of the ciliary currents 
towards the margin. The muscular margin, too, has its own proper 
work to do, fitted as it is by its superior strength and eminent con- 
tractility, to expel intruders and rejected products from the body. 
In Mytilus the direction of the ciliary current is outward on the 
body of the mantle and posterior along the inner edge of the 
muscular margin. 
4. In the foot in the byssal groove the ciliary current is towards 
the tip, and since this groove may be converted into a closed 
canal, it would serve remarkably well to convey the generative 
products out of the body and beyond reach of its own currents, 
that is, in front of the animal (see Leuwenhoeck, Select Works^ 
1800, vol. i. p. 84). Or it may be that the ventral byssal groove, 
with its tipward ciliary current, is used for directing the byssal 
filaments to their appropriate spot before fixing. It may like- 
wise keep off other sedentary animals from settling down upon the 
shell, at least round about the anterior end. But it also removes 
unnecessary material from the body, as will be shown more fully 
afterwards, and thus the tongue-shaped foot, which might serve as 
accessory to nutrition, and behave like a tongue to the headless 
mollusc, co-operates with the palps in removing matters from the 
neighbourhood of the mouth. From experiments made on the 
direction of the currents, it is found that small pieces of coloured 
sea-weed travel along the free margin of the inner gill towards the 
mouth at an average rate of 2 inches per minute (Bronn’s Thier- 
reichs, vol. hi. p. 415), but that both pairs of palps arrest these 
pieces at the mouth. 
Particles on the margin of inner gill are taken up by the inner palp. 
It can sweep along the margin with its tip, but it is towards its 
attached end that particles are picked up, then passed for a short 
distance backward along its outer margin, and finally thrown off. 
The same applies to the outer gill, only here it is the outer palp 
