1888-89.] Mr D. M'Alpine on Bivalve Molluscs. 
735 
the same process was repeated as occasion arose. In the left inner, 
matter was also seen passing rapidly across from the inner to the 
outer margin, gradually getting loosened from the hody of the palp, 
and ultimately being thrown off from the tip backward. There 
was none of that literal shaking off of rubbish met with in the 
palp of Mytilus, but it separated almost insensibly. In the face 
of the above facts, it does sound strange to read in a lecture delivered 
by Professor Huxley at the Poyal Institution, on i( Oysters and the 
Oyster Question” (English Illustrated Magazine , vol. i. p. 52) — 
“The anterior ends of each pair of hemi-branchise are attached 
between the two palps of the side to which they belong. The 
applied surfaces of the palps, between which lies the commencement 
of the mouth-cleft, are ridged and richly ciliated, so that anything 
brought by the ciliary current of the gills is led directly into the 
oral cavity.” Instead of that, it is generally led directly away 
from it, and we have already seen that in the sea-mussel it is the 
same when attached, so that the onus of proof must rest with those 
who make such a statement as the above, in future. 
The larval oyster is unprovided with palps, and so it would 
seem to have its mouth unprotected at an age when such protection 
was most needed. But there is an apparent substitute for them on 
the relatively large oval ciliated disc or velum, which overlies the 
larval mouth. Whatever development may say as to the future 
of the velum, it probably functions partly in the larva, as do the 
palps in the adult, i.e., in addition to the locomotive, and pos- 
sibly respiratory, function, it has the function of guarding the 
mouth against unsuitable food as it swims about with its velum in 
front. And thus the suggestion of Loven, that the velum becomes 
the palps, may at least have a functional basis. 
It may be suggested that the palps act both as guards and 
guides to the mouth, seeing that they can vary their direction of 
rotation, and consequently the direction of the ciliary current, the 
latter when feeding, and the former at other times. Still the 
unguarded statement must not henceforth be made, that the cilia 
of the palps act constantly and mechanically in the direction of 
the mouth. 
Since the palps are all capable of reversing the direction Of 
their rotation, and since a palp (the right inner) has been actually 
