VARIATION IN ECHINOID PLUTEI 
661 
The measurements were made with an ocular micrometer scale 
in a Zeiss compensating ocular no. 4 with objective AA. The 
plutei were placed under a large cover and the slides gone over 
with the aid of a mechanical stage. 
All of the arithmetical calculations in this paper are in these 
micrometer scale units. With the lenses used each eye piece unit 
is equivalent to .019 mm. 
Four measurements were made on each pluteus, namely, from 
the posterior end of the body to the end of the right anal arm; 
from the posterior end of the body to the end of the left anal arm ; 
from the posterior end of the body to the end of the right oral 
arm, and from the posterior end of the body to the end of the left 
oral arm. The measurements in each case being that of the com- 
bined body and arm length. (Fig. A.) 
I have selected the skeletal rods as the basis of measurement. 
In experimental w^ork on echinoid hybrids various investigators 
have made use of different characters. These characters have 
been: 
1. Form of larvae. 
2. Skeleton. 
3. Number of chromatophores. 
4. Pigment content of chromatophores. 
5. Arrangement of chromatophores. 
6. Number of primary mesenchyme cells. 
7. Size of larvae. 
Fig. A 
