196 The Scyphophori, Haplomi, and Xenomi 
There are properly then two pupils, an upper and lower, sepa- 
rated by a band consisting of the two flaps, which may probably, 
by moving upward and downward, increase or diminish the 
size of either pupil; an upward motion of the flaps increasing 
the lower pupil at the expense of the other, and vice versa.” 
This division of the pupil into two parts permits the fish, 
when swimming at the surface of the water, as is its usual cus- 
tom, to see in the air with the upper portion and in the water 
with the lower. It is thus able to see not only such insects as 
are upon the surface of the water or flying in the air above, 
but also any that may be swimming beneath the surface. 
According to Mr. E. W. Nelson, ‘the individuals of this species 
swim always at the surface and in little schools arranged in 
platoons or abreast. They always swim headed upstream 
against the current, and feed upon floating matter which the 
current brings them. A platoon may be seen in regular for- 
Fic. 153.—Round Minnow, Cyprinodon variegatus Lacépéde. St. George Island, 
Maryland. 
mation breasting the current, either making slight headway 
upstream or merely maintaining their station, and on the qui 
vive for any suitable food the current may bring. Now and 
then one may be seen to dart forward, seize a floating food 
particle, and then resume its place in the platoon. And thus 
