178 Proceedings of Boy at Society of Edinhurgh. [march 5, 
mens rotating left-handed (as in the latter case) the hulk of the 
body-axis was towards the right side (diag. 1 6), consequently left 
mantle-lobe and gills were most outspread, so that the balance of 
power was on the left side, and drove the animal to the right or 
relatively weaker side, and conversely. It may be presumed that if 
the animals could be laid out flat they would move straight forward. 
Duration . — The movement lasted for nearly 21 hours, and I was 
fortunate in observing it at the very last. When moving very 
slowly, it is difficult to fix the precise time when it ceases to move 
or completes a round ; but, by recording the times for slight move- 
ments, it is possible to fix a time after which it did move a little. 
Taking the last recorded time after which it moved slightly, the 
duration of movement was 20 hours 51 minutes, or say 21 hours. 
The palps and foot were still sensitive although not moving. 
Direction and Rate . — Another specimen of the same size as the 
first was laid out, measuring when removed from the shell If long 
and 1 J inch broad. It was first placed at the bottom of a bottle 
of sea-water, where it rotated left-handed, the body-axis being 
towards the right side. The rate of motion was very slow, only 
half a round being performed in 44 minutes. Throughout this and 
all the succeeding movements the foot never appeared, only the 
byssus projecting slightly. 
It was next transferred to a plate, where it was laid out as before ; 
it rotated left-handed, doing the first quarter round in 12 minutes 
and the second in 1 hour. The same was again laid out with the 
body-axis towards the left, when it rotated right-handed, the first 
quarter round being performed in 14 minutes and the second in 23. 
It was finally placed with the body-axis towards the right side ; it 
now rotated as at first, left-handed. The first quarter round was done 
in 18 minutes, the second in 28 minutes, and the next two quarters 
took 3 hours 5 minutes, thus completing the round in 3 hours 
51 minutes. It was now 1 inch higher up than, and exactly above, 
its original position, i.e.y it had moved forward 1 inch in the course 
of one rotation, without ultimately shifting its position otherwise. 
The next quarter round took 2 hours 2 minutes, and one might 
think that its powers were beginning to fail, but, as will be seen 
immediately, the falling olf was probably due to its getting some- 
what out of gear. 
