LARVAE OF ARBACIA PUXCTULATA 431 



Decreased size is correlated vriih reduced ^4gor. 



Individuals subnormal as to size, when subjected to an unfa- 

 vorable envdronment, undergo more extreme modifications than 

 do normal specimens when subjected to similar conditions. 



7. The larvae of Arbacia punctulata show greater plasticity' in 

 respect to decrease in size than in respect to increase. In 

 such modifications of the medium as occurred in the foregoing 

 experiments, the range of variation, extended, at the time of 

 maximum size, from a 5.1 per cent increase above the size of the 

 control, to a 63.3 per cent decrease. 



This range of variation probably corresponds approximately 

 to that of the larvae of .Irbacia in their natural environment. 



The subject of this paper was suggested by Dr. D. H. Tennent 

 and the work has been carried out imder his direction. The 

 experimental part was done at the ^Marine Biological Labora- 

 tory at Woods Hole, ^Massachusetts, during the summers of 1914 

 and 1915. I am very glad to express my gratitude to Dr. 

 Tennent for his constant help and guidance in this research 

 during the two years of its progress. 



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