S i: M AKOSTOM KJE AURKLLI A . 



1121 



respond momentarily by contractions to all sons of stimuli, electrical, chemical, thermal, or 

 mechanical. Small parts of the disk with sense-organs attached pulsate somewhat more 

 slowly than large ones. Romanes succeeded in maintaining the rhythmical movement ol the 

 medusa in parts of the disk without marginal sense-organs by stimulating weakly with a con- 

 stant or alternating current of electricity. 



Romanes also found that a stimulus too weak to cause a response would if repeated 

 eventually give rise to a contraction. This phenomenon is known as the summation of stimuli, 

 and it is interesting to see that Lee and Morse, lyio (Proc. Soc. Experimental Biology and 

 Medicine, New York, vol. 7, p. 38), find that this effect may be due to a rise in irritability, 

 brought about by the action on the living substance of small quantities ot certain products 

 of metabolism, especially carbon dioxide and lactic acid, the same substances which in gn .iti i 

 concentration are important factors in fatigue. 



Synopsis of the Races or Species of Aurellia Continued. 



