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The Life-Cycle of Cladocera, with Remarks on the Physiology 

 of Growth and Reproduction in Crustacea. 

 By Geoffrey Smith, M.A., Fellow of New College, Oxford. 



(Communicated by E. S. Goodrich, F.R.S. Received December 18, 1914.) 



1. Experiments on Daphnia pulex. 



In a paper on the life-cycle of Moina rcctirostris, published in 1913 (5), it 

 was shown by the late Mr. G. H. Grosvenor and myself that it was possible 

 to inhibit entirely the production of the sexual forms by isolating the 

 parthenogenetic parents soon after birth, and keeping them at a constant 

 high temperature of 25-30° C. It was proved that for a succession of 

 eight generations the isolated parents at this temperature gave no males or 

 ephippial females, while parents of the same generations kept crowded at a 

 temperature of about 14° C. or 5° C. gave about 50 per cent, males. We 

 were unable to determine how the effect of isolation and crowding of the 

 parthenogenetic parents influenced the production of the sexual forms, but 

 two alternative suggestions were made, either that in the crowded glasses the 

 animals were unable to obtain sufficient nutriment and were partially 

 starved, or else that some excretory matter accumulated in the crowded 

 glasses which influenced the production of males and sexual females. 



In order to confirm the above results and to throw some light on the 

 processes involved, breeding experiments have been carried on for some time 

 with another species of Cladocera, the common Daphnia pulex. Mr. Robert 

 Gurney very kindly gave me some dried mud from a pond which was known 

 to contain the resting eggs of these animals, and, on placing the mud in a 

 bowl of water, after some weeks some young Daphnia hatched out. One of 

 these was kept until it had produced young, and the offspring of these young 

 ones were used to start the first experimental generation. • 



D. pulex does not flourish on the food used for Moina, viz., manure infusion, 

 but I had previously found that they could be cultivated with great ease 

 if some green Alga, such as Protococcus, is added to the water in which they 

 are kept. In order to have a constant supply of the Alga, stock cultures 

 were made in a nutrient medium of inorganic salts to which a small amount 

 of organic material was added. The best medium for growing the Proto- 

 coccus was found to be a certain dilution of the mixture recommended by 

 Miquel for growing Diatoms, which Mr. H. G. Thornton and myself have 

 used for cultivating Euglena (8). By adding a pipette-full of the green 

 growth to each glass in which the experimental animals are kept, it is possible 



