1888.] Mr p. M‘ Alpine on Bivalve Molluscs. 181 
and placed as shown in fig. 1, turn inwards, the left turning to the 
left, while the right turns to the right (diag. 2). Both, as already 
remarked, may move forward or backward or to the side, but 
through it all there is this revolving motion ; and when not roaming 
about, hut rotating steadily, the base is comparatively stationary. 
If, however, there are obstacles in the way, such as dirt-particles in 
the water, or solid bodies of any kind, then the sensitive tip, ever 
seemingly on the alert, soon backs out and clears away from it, even 
although it should involve a change of course. Thus I have seen a 
palp, when placed in a dirty liquid, turn reversely for a short 
distance, until it had shaken itself clear of adhering rubbish, 
and then go forward in its regular course as if nothing had 
happened. 
If either palp is reversed, then it might be anticipated that the 
direction of movement would also be reversed. The right reversed, 
just behaved^like the left already described (diag. 3 h), and the left 
reversed ought to have behaved like the right as already given, but 
it did not. The unexpected happened here, for the tip formed the 
pivot, and merely shifted a very little to the right — about \ inch — 
in twelve revolutions (diag. 3 a). The tip was curved inward upon 
the body of the palp, making the tip end truncated like the basal 
end. This mode of rotation was evidently exceptional, and so 
another specimen was tried. It rotated on its base, but, contrary 
to expectation, the rotation was right-handed (diag. 3 a'), and thus 
the very reverse of the right, inner side uppermost. Twelve 
revolutions were recorded for comparison, and, with the exception 
of momentary reversions, there was no change in the direction 
during this time. 
About two hours afterwards it was observed rotating in the 
same direction, completing its round in 9 minutes, but in an 
hour and a half afterwards it was observed rotating left-handed 
(diag. 3 a"). The rate was now more rapid, being \ round per 
minute. Almost every possible mode of rotation was here shown, 
on tip and base, right-handed and left-handed. This variability 
of the rotation of the palp, when detached, is a sign of its vita- 
lity — that it is not a mere rigid body blindly obeying impulsive 
forces. 
A right reversed was also tried again, and it too behaved 
