The Annual Distribution of the Crustacea. 301 



THE ANNUAL DISTRIBUTION OF THE CRUSTACEA. 



/. 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 DeG-. 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. 



8 The specific identification is not certain. 



3 Formerly classed as a variety of D. Kahlbergiensis or D. cucullata. 



