Variations in the Crest of Daphnia hyalina

Abstract

THE characteristic tendency of Daphnia to form local varieties is expressed in D. zyalinza by a variability in the cephalic crest, such that every kind of crest displayed in the genus may be observed in the species, which at the same time remains relatively constant in other distinguishing characters of D. /iyaiina. The object of this paper is a comparison between the American and the European forms, with a more careful inquiry into the conditions for several lakes offering widely different varieties. My thanks are due to Professor E. A. Birge, of the University of Wisconsin, who furnished me with collections from northern Michigan and Wisconsin, and through whose kindness I had access to the literature on the subject; to Professor C. B. Davenport, of the University of Chicago, for many helpful suggestions in revising the results of my study; and to my brother, Karl E. Enteman, for assistance while collecting from southeastern Wisconsin. Historical. -D. 1/yalinza was first found by Leydiig in the depths of the Schlier See, and was given its name on account of its extremely pellucid character. Following are the chief points in its description as given in Leydiig's Aal\urgescihic/te der 9cph/uzideni, IS60: About as large as D. long-ispina, but somewhat narrowed, and at first sight distinguished from all known species by its extreme transparency; the head prolonged into a long, straight rostrum, without a ventral concavity, and so closely approximated to the thorax that the posterior outline is hidden between the valves of the carapace; macula nigra present; terminal claw smooth, showing under high power of microscope fine cross striations at base; the fornix much higher than in all known forms, the shell delicately 879