330 henry b. ward: 



In dicyclic forms the first sexual period falls in the spring and 

 the second in the fall. 



Lundberg and Stingelin (97) discuss seasonal dimorphism 

 among Cladocera and shows that in some instances the suc- 

 cession of species is actually only a succession of broods. 

 Lauterborn (98a) shows great variations among Rotifera at dif- 

 ferent times of the year. 



The extreme change in environment in the case of those lakes 

 which are frozen in winter has attracted the attention of 

 numerous observers to the life of the water at that time. Ac- 

 cording to the observations of Lauterborn (94) the microfauna 

 under the ice is rich in species and often in individuals; even 

 the limnetic fauna endures through the winter, some species in 

 large numbers. Certain Rotatoria are found in summer only, 

 and some Protozoa in winter only. However, accordingly to 

 Zacharias (94e) in Lake Pliii), the Protozoa are the first to dis- 

 appear, then the Rotatoria, the Crustacea reaching a minimum 

 in February ami March. The periodicity of these forms is, he 

 believes, ruled not by temperature but by the surface and 

 depth of the water basin. in Finland also, Levander (94) 

 finds a rich limnetic fauna, consisting of various groups, 

 which persists under the ice of lakes and ponds. Of Cladocera, 

 according to Stingelin (95), most forms persist through the 

 winter though those with ephippia disappear, and many forms 

 manifest a marked seasonal dimorphism which as yet has been 

 worked out in only a few species. The investigations of Birge 

 (97a) show that 7 out of the 1 1 limnetic species of Crustacea in 

 Lake Mendota are perennial and present in considerable num- 

 bers in the winter plankton, and these numbers are singularly 

 uniform from January to March with a minimum near the first 

 of this period. The Rotifera and the phyto-plankton are also 

 regularly present in this period and become abundant before 

 the breaking up of the ice. Hartwig (Ubb, cf. 98a) gives 

 precise data for another lake concerning the occurrence and 

 abundance < >f numen »us winter species. Here also Lampert (96). 

 Sundvik believes that the fish may in some cases pass the 



