368 



J. M. D. OLMSTED 



July 25. Specimens collected this morning 



In table 7 are given the limiting concentrations of the va- 

 rious classes of chemical agents. These were determined by 

 making at least two trials on each of five or more different in- 

 dividuals. If, with a certain concentration, definite responses 

 were obtained in half the number of trials, this concentration 

 was considered to be the limit. Upon trying double this con- 

 centration it was invariably found that a decided response to 

 every trial was given by every individual, and that to half this 

 concentration no reactions at all were shown. Table 8 has 

 been compiled for purposes of comparison. 



Synaptula hydriformis, in contrast to Synapta inhaerens 

 (Quatrefages, '42), is very sensitive to fresh water. Indeed 

 fairly slight changes of osmotic pressure call forth a very de- 

 cided reaction. 8 cc. ordinary sea-water + 2 cc. rain water 

 causes a vigorous reaction, while 9 cc. sea-water + 1 cc. rain 



