The Annual Distribution of the Crustacea. 
801 
THE ANNUAL DISTRIBUTION OF THE CRUSTACEA. 
I. General Relations of the Plankton Crustacea. 
Figs. 6-11. 
Lake Mendota has eleven species of limnetic Crustacea, which 
may be grouped as follows: 
A. Perennial species — 
a. Appearing in great numbers — 
Copepoda. 
Diaptomus Oregonensis Lillj. 
Cyclops brevispinosus Herrick. 
Cyclops Leuckartii Sars. 
Cladocera. 
Daphnia hyalina Leyd. 
Chydorus sphaericus O. F. M. var. minor Lillj. 1 
b. Usually appearing as isolated individuals — 
Copepoda. 
Epischura lacustris Forbes. 
Ergasilus depressus Sars. 2 
B. Periodic species — 
a. Appearing in great numbers — 
Cladocera. 
Daphnia pulex DeGr. var. pulicaria Forbes. 
Daphnia retrocurva Forbes. 3 
Diaphanosoma brachyurum Sars. 
b. Appearing as isolated individuals — 
Cladocera. 
Leptodora hyalina Lillj. 
To these might be added Bosmina of which a very few indi¬ 
viduals appear, chiefly in winter, but of which there are never 
enough to make a fair determination of their number a possi- 
’Sometimes absent but not properly periodic. 
2 The specific identification is not certain. 
3 Formerly classed as a variety of D. Kahlbergiensis or D. cucullata. 
