696 FRESH-WATER BIOLOGY 



32(31,33) Ocellus present; head helmeted. . Daphnia arcuata Forbes iSg 3, 



Very transparent. Vertex rounded, rostrum extending back- 

 ward and applied to margin of valves. Slender spine projecting 

 from middle of valves. Anal spines about 10-12; claws with 

 distal pecten of some 6 teeth. Length of 9 to 2 mm., besides 

 spine of about 0.5 ram. Wyoming, Wisconsin; in open waters of 

 lakes. 



This species forms a transition to the retrocurva forms. 



Fig. 1067. Daphnia arcuata. 

 0.1 MM. 



33(31,32) Ocellus absent; head helmeted. . Daphnia retrocurva FoihtSJ&&2. 



-■-7 



Body much compressed, pellucid. Eye small, with 

 numerous projecting lenses and little pigment; no ocellus. 

 Spine ordinarily above middle of valves, directed upward. 

 Crest very variable, often enormous. Claws with two 

 pectens, the distal of 7-9 teeth. Anal spines 7-12. Sum- 

 mer eggs ordinarily 2 ; sometimes as many as 6. Length, 

 $, to 2.0 mm., besides spine, which may reach 0.5 mm. 



Widely distributed 

 in limnetic region of 

 lakes. Shape of head 

 extremely variable; 

 all forms from var. 

 breviceps Birge, 

 where the crest is 

 hardly visible, to the 

 extreme of extension shown by retro- 

 curva proper. This species replaces 

 in the United States the European D. 

 cucullata, which is related to D. longi- 

 spina, much as this form is to O. ptUex. 

 D. retrocurva never has the extremely 

 acuminate form of head wbichcucullala 

 sometimes shows. 



Very probably study will show that all the pulex forms (31, 32, 33) must be imited into one 

 polymorphic species. 



34 (26) Claws without pecten 35 



35 (38) Ocellus present, though small. 



Daphnia longispina (O. F. Miiller) 1785 . . 36 

 Spine long; claws without pecten. Male without long papilla on posterior part of body. 

 This species is so variable that almost no characters can be given for it. It is less robust than 

 the pulex forms, ordinarily fairly transparent; often hyaline. Thispart of the species divides 

 at once into 2 sections or subspecies, each with numerous varieties which have never been 

 thoroughly studied in the United States. 



t).5 MM. 



Fig. 1068. Daphnia retrocurva. 



0.2 MM. ■- 



Fig. 1069. Daphnia lonsispitM. (See also Fig. 1050, p. 677.) 



