1888.] Mr D. M' Alpine on Bivalve Molluscs. 183 
and the palp turned irregularly from right to left, or left to right, 
without completing a round. All its varied movements were 
followed, which need not be given, but after about two hours it 
completed a regular round to the right in 3 minutes. After oscil- 
lating for a quarter of an hour, it completed a round to the left 
in 5 minutes, when it was accidentally stopped by a hair. 
This change of direction and return to the original is rather 
interesting to follow, since it shows that there are more than mere 
mechanical arrangements concerned in the movement, but as there 
is a very striking case of change of direction with the outer palps, 
the subject will be more fully referred to then. 
The left reversed performed twelve revolutions, at an average 
rate of 8 minutes. After the first round, which took 16 minutes, 
the rate was either 7 or 8 (diag. 3 a). 
In a second series of observations with another palp, twelve 
revolutions were performed at an average rate of 6J minutes. The 
first round took lOJ minutes, and afterwards they varied from 
to 5 minutes. 
Right . — For twenty-six recorded revolutions, the slowest was 
60 minutes, the quickest If minutes, and the average 8J minutes. 
It commenced with a revolution in 5 minutes, about the middle 
(14th) attained to the quickest in If minutes, and ended with 
the slowest in 60 minutes. A partial average for the more steady 
rounds, from the 6th to 19 th inclusive, gave minutes per 
round. The record was closed for the right after completing 
twenty-six rounds, when it became perfectly still, as if exhausted. 
It was still sensitive, however, as it quivered on being touched with 
a pin, and next morning it had shifted its position. 
The right reversed moved very slowly, although it rotated in the 
usual manner by making the base the pivot. The first round 
occupied an hour, but deducting time stuck, it only took 28 minutes, 
the second round 22, and the third 20 minutes (diag. 3 h). 
This was abnormally slow, but in a second specimen tried, 
twelve revolutions gave an average rate of 5^ minutes per round. 
The first round took 7J minutes, the last 8|- minutes, and the 
intermediate rounds from 4 to 5|- minutes. It was left rotating at 
the rate of 7 minutes per round, as shown by the 13th. 
In the outer palps the movement generally resembles that of 
