8 SELECTION IN CLADOCERA ON THE BASIS OF 



between 60 and 100 collections of S. exspinosus material from this 

 pond were made at intervals of several days and no males or ephippial 

 females were found. Males might readily be overlooked in col- 

 lections not carefully examined microscopically, but ephippial 

 females are unlikely to be overlooked by one who is accustomed to 

 handling Cladocera material. However, sexual reproduction in 

 Cladocera is spasmodic and frequently of short duration, so that the 

 results of the examination of the collections do not entirely preclude 

 the possibility (though making it seem improbable) that sexual 

 reproduction may actually have occurred during the period of fre- 

 quent examinations of material from this pond. 



The same general reactiveness appeared to exist in all D. pulex 

 lines (see figs. 2c, 3c, 4, 5, 6, 7c, 8c, 9, and 10D), although they came 

 from two ponds (Pond I and Pond II), were collected at different 

 times, and presumably may have belonged to more than one clone. 

 Sexual reproduction in this species was several times noted in a nearby 

 pond (Pond III) from which the culture-water was obtained. 



Table 1 shows in condensed form certain data concerning the 

 lines of Cladocera reared in the laboratory and used for the selection 

 experiments. 



CULTURE METHODS. 



The culture-water in which these animals were reared was at 

 first obtained from the spring-fed pond (Pond II). Beginning a year 

 and a half later, it was gotten from Pond III, the barnyard pond. 

 This pond receives all its water from the surface drainage of a small 

 pasture lot and from a barn and barnyard in which live-stock is kept. 

 The amount of organic matter carried into the pond by the surface 

 drainage is relatively large and the water is rich in organic solution 

 constituents. The water is usually heavily colored a reddish or 

 yellowish brown. 



In being collected, the culture-water is dipped up in a manner 

 to obtain as much as possible of the loose, fluffy sediment from the 

 bottom of the pond. After being brought into the laboratory it is 

 strained through fine "India linen. " The lighter portion of the 

 sediment is gently rubbed through the straining-cloth and the coarser 

 residue is discarded. The strained water is then allowed to stand 

 for two days, when it is again (sediment included) strained through 

 India linen. After a second interval of from one to several days the 

 water is thoroughly stirred and a third straining is made through a 

 standard silk bolting-cloth having 130 meshes to the inch. The 

 water is then ready for use as culture-water. The first straining 

 removes any Cladocera or Copepoda which may be in the pond- 

 water and removes most (at least) of their eggs. The second strain- 

 ing removes any young which as eggs may have passed through the 



