VARIATION IN ECHINOID PLUTEI 
663 
progeny of nine females and nine males. In the series I-V observa- 
tions were made on the second, third, fourth and fifth day of 
development. For each lot and for each day 100 plutei were 
measured. 
In Series VI observations were made on the third, fourth and 
fifth day, the plutei being fed after the third day. Here again 
100 plutei were measured for each day. 
In Series VII measurements on 61 plutei were made on the 
fourth day. 
In Series VIII measurements were made on 100 plutei on the 
sixth day. 
In Series IX measurements were made on 100 plutei on the 
sixth day. 
We find that the variations fall into three classes: 
1. Fluctuating variations. 
2. Defects. 
3. Multiplicities. 
The fluctuating variations are those of length and correlation 
and are grouped more or less symmetrically about a mean. Figs. 
1-7 show normal individuals from the end of the first to the end 
of the seventh da}^ of development. These figures were drawn 
from living plutei which were selected as individuals that would 
•develop through metamorphosis under favorable conditions, 
experience having shown that such individuals could be identi- 
fied at the beginning of the second day. Inspection of these 
figures will show that the skeleton in each arm consists of a straight 
rod, slightly roughened by prickles or thorn-like processes. The 
oral rods are connected with the anal cross bars by a short rod 
as shown in fig. 4. (See page 666.) 
The defects consist in the absence of a skeletal rod or of an arm, 
or of a deformation of part of the body. Figs. 8, 9, 10, 18, 19, 20. 
The multiplicities consist in the presence of more than the 
normal number of skeletal rods or in more than the usual num- 
ber of arms. Figs. 11, 12, 13, 14, 16, 17. 
An examination of the Tables of Constants, of the Graphic 
Representation of Variation and of the Correlation Tables, will 
give any one who is interested in the subject from the standpoint 
