LARVAE OF ARBACIA PUNCTULATA 



387 



esses, the greater prominence of the ventral and dorsal branches, 

 the less highly fused condition of the arm-rods, and the reduced 

 size of the body-rods. Were this last named characteristic not 

 correlated with a proportionate increase of arm-length, we 

 might ascribe it merely to the dilution of some necessary con- 

 stituents of the sea-water, but since total size is not reduced, 

 we must conclude that it is a specific effect of the Na. 

 In these solutions, then we have four specific effects: 



a) A condition as to thickness of skeleton, diametrically op- 

 posed to that which occurs in the carbonate solutions, and 

 expressed in slender, perforated skeletons with conspicuous 

 processes. 



b) A growth exceeding that of the normal in the solutions of 

 slighter dilution, caused possibly by decrease in the propor- 

 tion of Mg and Ca. This excessive growth occurs during the 

 later periods. 



c) A tendency toward inhibition of growth during the first day 

 of the experiment, the time when decrease of alkalinity produces 

 its most marked effect. 



d) Decreased length of body-rods. 



Experiment 15 was made with solutions of 0.56M NaCl. 

 Three solutions were employed, a control, a; b, of 10 cc. 0.56M 

 NaCl to 90 cc. sea- water; and c, of 20 cc. 0.56M NaCl to 80 cc. 

 sea-water. 



The corresponding measurements were: 



Experiment 15. Plot 14- July 21, 1915 



