56 \VYMAN REED GREEX. 



h -pecies of Cladocera found capable of pure parthenogenesis 



its own specific requirements as to which environmental 



factors may vary and to what extent without it becoming zygo- 



genic. 



III. MATERIAL. 



( )n taking up this work much difficulty was encountered in 

 rearing algae ES foccl for the Daphnians. Numerous formulae 

 were tried with varying success, but for some reason they did 

 not thrive- on alga? artificially produced. A simple and satis- 

 factory solution of the difficulty was discovered in the following 

 method: A number of three- or four-gallon aquaria nearly full 

 of water are placed where they will not receive the direct rays 

 of the sun and stocked with several kinds of unicellular algae. 

 One to three frogs, depending upon the size, which have been in 

 captivity until little or nothing remains in the alimentary canal, 

 are placed in each of the jars. Of course the aquaria must be 

 covered, in part to retain the frogs, but the more important 

 reason for this is to insure the maintenance of a high percentage of 

 carbonic acid gas and the depletion of oxygen. Under such con- 

 ditions the algae grow rapidly until a condition of equilibrium is 

 reached. When a sufficient quantity of algae has developed a 

 single large Daphnian with brood pouch full of eggs or embryos 

 may be introduced and the frogs removed, or one may be left 

 in with good results. When the Daphnians have overstocked 

 the culture they may be strained out with a cloth and trans- 

 ferred to a new jar. The brown sediment of tin- old culture, 

 which is excreta and dead algae, should be caref jlly removed so as 

 not to loosen tin- alga' on the sides of the jar, new water added 

 and more algae grown in the manner indicated above. 



The above method has served me very satisfactorily for 

 Simocephalus vetulus, though it is not so well adapted to some 

 other forms. Daphnia pulex thrives in water which is more or 

 less filled with putrescent matter. In a pond frequented daily 

 by cattle and thus kept very roily and malodorous, I found this 

 species in such numbers that by a single dip of the net I secured 

 hundreds of them. Kor best results one needs to work out special 

 methods for each species. In all of my work I have used Simo- 



'Inilus vet ill-its unless otherwise stated. 



