6 



Art. 3.- N. Yatsu: 



and comes in contact with any rhopaliuni (the only pigmented 

 region of the medusa), the tentacles and the pedalia are quickly 

 drawn in and the animal sinks to the bottom, thus increasing the 

 chance of escape from being preyed upon. Only in one individual 

 out of some hundred specimens I have examined, it was noticed 

 that one pedalium did not respond at all to the stroking of the 

 rhopalium, while the other three did. This may be due to some 

 congenital defect in the reflex mechanism. 



In order to see how soon fatigue comes to the pedalial muscles 

 three rhopalia were cut oft' and the fourth one w\as stroked 

 several times in succession as soon as the pedalia relaxed after 

 contraction. Tn one individual the bending l)ecame difiicult after 

 twelve or thirteen stimulations, and to the sixteenth stroking the 

 pedalia did not respond at all. In another medusa the contraction 

 became weak after seven or eight stimulations, and the fourteenth 

 did no longer cause bending at all. But it is hardly necessary to 

 add that a stronger stimulus, c g., the pinching of the basal part 

 of rhopalium or the sensory niche w^ith a pair of forceps, will call 

 forth response even in such fatigued 

 medusa3. 



In passing it may be mentioned 

 that by means of the rhopalium- 

 pedalium reflex, the course of the 

 nerve impulse from the rhopalium to 

 other parts of the body was determin- 

 ed. Three rhopalia were cut oft' and 

 the lower half of the bell w^as Insected 

 by two opposite incisions, as shown 

 in Fig. 3. Then the rhopalium was 

 stroked. This experiment was per- pig. 3. 



