1883.] Zoölogy, 559 
cannot be made at any one season, as the Cladocera periodically 
disappear and occur at the bottom in the state of resting-eggs. 
As these pelagic creatures can never rest on a solid body, they 
have highly developed swimming organs and a light specific 
gravity, yet they are sluggish, and escape destruction rather by 
their perfect transparency than by their motions. Some feed on 
Algæ, but most on animal food. During the night they swira 
upon the surface, but in the day descend into the depths. 
have a strongly pigmented black, brown, or red eye. Professor 
Forel believes that the regular wind which at night blows from 
the land to the water has been the cause which has kept a certain 
portion of the entomostracous fauna of the lakes to the pelagic 
area until it has become modified to suit its surroundings. As 
the creatures sink during the day, they escape the breeze that 
then blows towards the land. 
Vertebrates— At the close of a valuable paper in the Proceed- 
ings of the Zodlogical Society of London, on the colors of feathers, 
r. H. Gadow sums up his conclusions as follows: We have 
to distinguish between several categories of colors in feathers. 
1, Objective chemical colors, directly produced by pigment. To these 
belong black, brown, red, orange and yellow. 2. Objective struc- 
tural colors. The feathers may contain no pigment at all, and the 
color be produced solely by special structural arrangement of the 
feather-substance, for instance, white, and frequently yellow; the 
latter, if the surface is composed of very fine and narrow longi- 
tudinal ridges. Or the feather contains a yellow to brownish-black 
Pigment, and the color actually observed, as green, d/ue and violet, 
1S produced by a specially produced and particularly constructed 
Sparent layer between the pigment and the surface of non- 
changing colors; blue and violet are always structurally objective. 
Green seems to be only in a few cases the result of yellow pig- 
ment combined with blue surface-structure. In most cases it 
depend on the position of the light and eye. They are produced by 
