Echinoderm Hybridization. 145 



THE EFFECT OF A DECREASE IN THE ALKALINITY OF THE 



SEA-WATER. 



With a decreased alkalinity brought about by the addition of -^^ 

 acetic or ^"^ hydrochloric acid, the plutei obtained showed the effect 

 of an influence tending to swing them toward the Toxopneustes type. 

 The plutei whose skeletons are shown in the figures were taken at ran- 

 dom, no attempt being made to select plutei which would support any 

 theory. (See Toxopneustes $ X HipponoecJ', figs. 44-85, andHipponoe $ X 

 Toxopneustes c?, figs. 86-93.) 



Table IV gives a summary of results obtained with the acetic acid. 



Table IV. — Summary of results of cross-fertilization in sea-water of decreased 

 alkalinity, acetic acid. 



[Number of plutei studied in each fertilization, so. Temperature, 29° C] 



Further data : 



1909. Eggs fertilized in 400 c.c. sea-water+ 20 drops ^J^ acetic acid, 

 (i) 1910. Eggs fertilized in 500 c.c. sea-water+ 15 drops -^-^ acetic acid. 3 days. 



(2) 1910. Eggs fertilized in 500 c.c. sea-water4- is drops ij, acetic acid. 4 days. 



(3) 1 9 10. Eggs fertilized in soo c.c. sea-water 4- 15 drops i?, acetic acid, s days. 



(4) 1910. Eggs fertilized in 500 c.c. sea-water+ 2 c.c. ij, acetic acid. 2 days, 

 (s) 1 9 10. Eggs fertilized in 500 c.c. sea-water + 2 c.c. f^ acetic acid. 2 days. 



The table shows clearly that the effect of the addition of the acetic 

 acid had been to swing the dominance toward Toxopneustes side. That 

 this result is not due to the specific action of the acetic acid is indicated 

 by the results of fertihzation in sea -water whose alkalinity has been 

 decreased by the addition of hydrochloric acid. 



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