^14 ERNEST WARREN. 



Measurements. — (1) The length of the spine was 

 measured along a line which passed from its apex to a point 

 on the ventral surface of its base (b c). Since Daphnia con- 

 tinue to grow throughout life, it was necessary to express 

 the length of the spine in terms of some other dimension. For 

 this purpose the total length of the body was chosen. 



(2) The body -length was measured along a line which 

 passed through the above point on the ventral surface of the 

 base of the spine, and cut the convex surface of the head 

 opposite the middle of the compound eye (a b). The measure- 

 ments were made under the microscope by means of Zeiss's 

 screw micrometer. 



Twelve adult Uaphnia were taken without selection from 

 the tank. 



Four of these (" A'^) were placed separately in four glass 

 tumblers containing 200 c.c. of water (that of the New River 

 Company) and a little lump of Conferva. At the bottom of the 

 tumblers there was a layer of mud taken from the Victoria 

 regia tank at Regent's Park. This mud contained many 

 Alg£e, &c., and constituted an adequate food-supply for the 

 Daphnia. The tumblers were covered with pieces of glass. 



Four others ("ft") were placed in tumblers which were in 

 everyway similar to those of "Series A" except that the 

 coverings of glass were not used, and rain water collected in 

 the country was added from time to time to supply the loss 

 due to evaporation. 



The remaining four Daphnia (" b ") were placed in tumblers 

 containing similar mud, but the water was changed at least 

 six times a week. 



A brood was obtained from each parthenogenetic female. 

 The mother was then measured, and the offspring were 

 allowed to grow up. When adult and about to produce 

 broods, all the children were measured except one. The one 

 which was left was any individual of the brood, for no selec- 

 tion was made. After this animal had produced a brood it 

 was measured, and the offspring were left in the tumbler. 



The results of the measui'ements on the spine are given in 



