SEMAEOSTOMEiE — AURELLIA. 



621 



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

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

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

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

 stant or alternating current ot electricity. 



Romanes also found that a stmiulus 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, 1910 (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 ((uantitics of certain products 

 of metabolism, especially carbon dioxide and lactic acid, the same substances which in greater 

 concentration are important factors in fatigue. 



Synripsts of the Races or Species of Aurelliti — Continued. 



