182 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
used further counteracted this error and also tended to equalize it in 
the different series. So on the whole, the number of days that the 
larvae remained in the glaucothoe phase was determinable with a 
fair degree of accuracy, and although some error must remain, it 
does not affect the comparability of the series. 
Series A. 
Normal. 
The glaucothoe were provided with dextrally spiral shells and given 
opportunity to take these whenever they pleased. The shells of the 
marine snail, Scycotypus, were used, being abundantly obtained from 
the egg cases. Eleven experiments were made, using 183 larvae, 
99 of which attained the moult to the sixth sta«;e. 
O 
6 glaucothoe 
6 percent 
remained in 
stage 
! 4 
day; 
3 
a 
3 
it 
a 
a 
it 
u 
it 
57 
a 
57 
it 
it 
a 
it 
5 
ii 
2 
it 
2 
u 
a 
u 
ii 
5 h 
a 
19 
a 
19 
a 
'u 
n 
it 
6 
a 
14 
u 
14 
u 
a 
u 
it 
Gh 
it 
4 
ti 
4 
a 
a 
tt 
ti 
7 
a 
2 
u 
2 
u 
it 
a 
a 
n 
u 
2 
ll 
2 
u 
it 
tt 
a 
8 
a 
The percentages for the duration of the stage may be arranged in 
a curve in which the ordinates represent days, the abcissae the per¬ 
centages (pi. 7, fig. 32, continuous line). This gives a very steep 
curve with the mode at the fifth day. 
Series B. 
Delayed Normal. 
In these experiments, a batch of larvae was separated into two 
parts. One half was provided at once with dextral shells to 
serve as a control ..experiment; the other did not receive shells 
immediately. The figures for the control records have already been 
