J. L. BROOKS 



B 



91 



Fig. 2. Phenotypic variation in Daphnia. These three specimens of 

 Daphnia sch0dleri Sars exemplify the variation in body form that charac- 

 terizes most species of Daphnia. The principal differences are in the length 

 of the shell spine and the extension of the exoskeleton along the head mar- 

 gins. In lateral view the extent of the crest, as this lamellar extension is 

 called, can be judged by the distance between the head margin and such 

 structures as the optic vesicle and the origins of the antennal muscles. 

 Dorsal views of specimens A and C indicate lamellar nature of the crest. 



