370 J. M. D. OLMSTED 



which has been evaporated to one-half its normal volume gives 

 a reaction, while 7 cc. ordinary sea-water + 3 cc. of the concen- 

 trated sea-water fails to call forth a reaction. The former con- 

 centration corresponds to 7/8 M. The range of concentrations 

 between which osmotic pressure does not serve as a stimulus 

 is therefore 1/2 M to 7/8 M. The significance of these limits 

 will appear in the discussion of the effect of certain agents such 

 as sugar and glycerine. 



Since changes in osmotic pressure caused such marked re- 

 actions, it was impossible to make up solutions in pure water. 

 5/8 M NaCl or KC1 in pure water produced violent reactions, from 

 which it required several minutes for the individual to recover. 

 Solutions were therefore made up in sea-water. Solid agents were 

 dissolved directly in sea-water, and the normality of the solution 

 was reckoned as if pure w^ater had been used. Liquids like HC1 

 were added in double the concentration desired to sea-water evap- 

 orated to one-half its normal volume. It is by no means claimed 

 that these limits are as accurate as the table suggests. The 

 first drops of the solution must necessarily be much diluted by 

 having to pass through several millimeters of sea-water before 

 reaching the Synaptula. The last part of the discharge must, 

 however, flow against the animal in approximately its original 

 concentration. If these limiting values err, they do so in being 

 too high; still they are significant for comparing the effects of 

 different agents on this one species, even if they may be less so 

 for comparing the degree of sensitivity of Synaptula with that 

 of other animals. 



Parker ( ; 12, p. 228) considers that aquatic vertebrates will 

 not respond to that class of chemical substances which in man 

 produces the sensation of sweetness. Crozier ('14 a, p. 289) 

 believes that some sugars may prove stimulating, since Holo- 

 thuria surinamensis responds to maltose and glycerine. The 

 1/2 M cane sugar and glycerine solutions to which Synaptula 

 responds (table 7) have a total concentration (salts in sea-water 

 plus sugar or glycerine) of 9/8 M, while the 1/4 M solution, to 

 which it does not respond, has a total concentration of 7/8 M. 

 It is evident therefore that we are dealing in the case of the 



