Jackson, the Amphipod Hyalella Knickerbockers. 
51 
difficult to select specimens which are similar to each other in 
color and pattern unless a very large number are examined. The 
predominant shade of color seems to be light brown or some shade 
of green; others may be distinctly bluish, dark purple, dark brown, 
while a very few have a reddish tint. Older animals appear to 
have more uniformity of color, a large number being light brown, 
but even here there is considerable variation. This tendency 
towards a brown color in adults is probably due to the thickening 
of the cuticle which then obscures the underlying pigment. There 
is also a tendency for the animals to assume a light brown color 
preceding the molt, due to the loosening of the cuticle from the 
body wall, an effect quite similar to a thickened cuticle. I was at 
first inclined to believe that the green color of some individuals 
was due principally to chlorophyl—containing algae in the digest¬ 
ive tract. This is true to a certain extent; among several light 
brown individuals, which were isolated and fed upon green algae, 
were two which acquired a green shade of coloration. However, 
many green Hyalellce were isolated, some of which were starved, 
others were fed upon nonchlorophyl-containing matter; and all 
of these animals still retained their green color after several days 
of isolation. Careful examination of sections of these showed the 
alimentary tract to be free from chlorophyll, the green color being 
due to pigment in the hypodermis. 
SIZE. 
The largest specimen of this species which I have seen meas¬ 
ured 7.6 millimeters in length. The average length of adult speci¬ 
mens is considerably less than this, the majority having a length 
between 5.2 millimeters and 6.5 millimeters. The females average 
somewhat shorter than the males. The depth and width of the 
females are proportionately to the length greater than in the 
males. I measured a number of each sex and found the depth to 
vary from 15.8%’ to 23.3%, of the length. The males had a depth 
varying from 15.8% to 21.2%, of the length; the females a depth 
varying from 18.1% to 23.3% of the length. The average depth 
of the males was 18.4% of the length; the average depth of the 
females was 20.9% of the length. These measurements were taken 
through the deepest part of the thorax. 
