694 



FRESH-WATER BIOLOGY 



21 (2) Five or six pairs of feet. First and second pairs more or less prehen- 



22 (117) 



sile, others foliaceous. . . . Tribe II. Anomopoda . . 22 



Antennules attached to ventral side of head, not covered by for- 

 nices ....................... 23 



23 (76, 83) Antennules of female usually small, sometimes rudimentary; if 



large, never inserted at anterior end of ventral surface of 

 head. Dorsal ramus of antenna 4-jointed, ventral ramus 

 3-jointed. Intestine simple with 2 hepatic ceca. 



Family DAPHNIDAE Straus . . 24 



Five pairs of feet, the first two prehensile and without branchial lamella; the fifth with large 

 recurved seta, extending around branchial sac. Antennules in general small or rudimentary, 

 and when large not at the anterior extremity of the head. Antennae long, not strong, cylindrical, 



setae ~~ I ~ 3 . Post-abdomen distinctly set off from body, usually more or less compressed, 



1-1-3 



always with anal spines. Abdominal setae not borne on distinct projection or papilla. Claws 

 sometimes pectinate; always denticulate, unless worn by use; never with basal spine. Intes- 

 tine not convoluted, with 2 hepatic ceca. Eye large; ocellus usually small, sometimes want- 

 ing. Summer eggs ordinarily numerous; typical ephippium formed, containing i or 2 eggs. 

 $ usually with hook on first foot. 



24 (48) Rostrum present ..................... 25 



2 5 (39) No cervical sinus. Valves with polygonal, usually rhomboidal, mark- 



ing, and with a posterior spine. Crest on dorsal side of head. 

 Daphnia O. F. Miiller 1785 . . 26 



Form oval or elliptical, except as modified by crest of head (helmet) in some species. Body 

 always compressed, often greatly so. Valves reticulated; dorsal and ventral margins rounding 

 over toward each other and provided with spinules along posterior part. Rostrum well-marked 

 in 9 and pointed. Antennules small or rudimentary, not movable, placed behind rostrum. 

 Abdominal processes 3-4, all ordinarily developed; the anterior especially long, tongue-shaped 

 Ephippium with 2 eggs. Summer eggs often very numerous. 



and bent forward. 

 Head of $ with 

 seta or flagellum; first foot with hook and long flagellum. 



Head of $ without rostrum; antennules large, movable, ordinarily with long, stout, anterior 

 ell 



26(34) Claws with pecten ................ .. ., .. . 27 



27 (30) Heavy, thick-bodied forms. Fornix and secondary fornix (Fig. 1063) 



well developed. Distal pecten with more than 1 2 teeth. . 28 



28 (29) Antennules large for genus; post-abdomen with deeply sinuate 



margin .......... Daphnia magna Straus 1820. 



Form rounded or oval, body thick and heavy, not transparent. 

 Post-abdomen long, with deep indentation behind anus, breaking 

 through row of anal spines. These are about 1 2 in proximal and 

 8-1 o in distal set. Claws with two pectens of numerous teeth. 

 Ephippium characteristic; dorsal margin of valves separates with 

 it both behind and before; 2 eggs, placed obliquely. Summer 

 eggs numerous. Length, 9 , to 5.0 mm. ; $ , 2.0 mm. or more. The 

 largest of the family. Maine, Colorado, Nebraska, N. Dakota, 

 California. 



FIG. 1062. Daphnia magna, post-abdomen. 



