1. New data on the number of setae of limbs of different Chydoridae are summarized and compared with that of other families of Cladocera (see the table). These data may contribute to systematics of Cladocera. 2. The oligomerization of Cladocera takes place in appendages of different segments ind within the same segment. 3. In forms with numerous setae in homologous groups their number variates, new examples are described for Eurycerinae and Sayciinae. 4. In the size range within Cladocera the decrease in size of a structure, in itself, is not a case of depression of its appendages. 5. The general tendency of Cladocera to oligomerization is differently realized in different families which seems to be functionally determined and connected with their different mode of life.