1888-89.] Mr D. M c Alpine on Bivalve Molluscs. 
731 
constantly, but the rate varied considerably in the different speci- 
mens. One specimen with the outer surface uppermost began to 
move bodily to the right and to rotate right-handed; the first 
round was as usual the slowest, taking 19J minutes, the 30th 
taking only 3 minutes. Next morning it took 16 minutes. On 
the 3rd day, 38 minutes; on the 4th, 10 J and 13 J minutes; on 
the 6th day three rounds took 5, 9|, and 10 J minutes respectively; 
on the 8th day lateral movement was observed, but no definite 
rotation and no further movement was noted. Another specimen, 
with the inner surface uppermost, on the 2nd day was rotating 
left-handed at a rate of a round in 2J minutes ; on the 6th day 
in If to 2J minutes, now right-handed; on the 7th day the 
movement became much slower, and was again left-handed and more 
irregular, and it now began to move in a straight line. The move- 
ments ended on the 9th day. Reckoning from the last of the 
continuous rotation to the last of the recorded rounds, after which 
there was a little left-handed rotation, the time was exactly 1 day 
6 hours 52 minutes, so that the palp had been rotating more or 
less continuously for nearly 8 days of 24 hours each. 
On the 10th day, examining under the microscope, the cilia were 
seemingly as active as ever, but there was no movement of the 
palp; on the 11th day, however, left-handed rotation was again 
resumed after renewal of the sea W’ater, and half a round was 
completed, and the palp had moved § of an inch higher up. On 
the 12 th day another round was completed, one quarter during 
the night, the other during the day, that of the night taking about 
10 hours, and that of the day about 9 hours. During the night 
there was little change of position while rotating, but during the 
day, or last quarter round, there was a deal of progressive move-, 
ment combined with the rotation. In less than 2 hours the half 
of the last quarter round was about completed when the palp 
began to move in a curve in the direction of "the tip, with very 
little if any rotation for some time. The distance thus traversed 
varied for different periods of the course. The entire curve measured 
about 2-J inches, while for a given period of 1J hour the distance 
was of an inch, and for the last 2 hours 22 minutes it was | of 
an inch. At the close of the round the palp was almost exactly 
1J inch to the left of its first position, and the same lower down. 
