1913] Gee: Behavior of Leeches 259 



the animal is in a condition of quiet or "sleep" two or three 

 stimulations with a glass rod are usually necessary to produce 

 a reaction other than a mere local contraction. A stage of 

 increased tonus ensues in which the leech responds readily with 

 vigorous turns away from the stimulus, and rapid looping move- 

 ments. A still greater state of excitement follows upon continued 

 stimulation, and in this condition the swimming response seems 

 the dominant reaction. As is discussed in a succeeding; portion 

 of this paper, fatigue produces its characteristic depressed re- 

 sponsiveness. As for states of hunger and satiety, the first of 

 these seems to cause in general a condition of greater reflex 

 excitability; the latter, a lowered responsiveness to stimulation. 



All of these described physiological states in the planarians, 

 in the earthworm, and in the leech, seem to find their expression 

 through a modified reflex irritability of the particular organism. 

 They represent various degrees of excitement from that of com- 

 plete depression, as in the case of fatigue, to that of the most 

 intense excitement. If this be true, and this is the opinion of 

 the writer, we have a very suggestive viewpoint afl'orded in the 

 work of Von Uexkiill (1905). 



(e) Chain reflexes. — Von Uexkiill has shown that in "das 

 Gehen" of the blood-leech there is a certain sequence of reflexes 

 involved. He says : " Es ist schon dem Augenschein nach un- 

 zweifelhaft, dass hier eine Verkettung von Reflexen vorliegt, die 

 such zu einem Ringen aneinanderschliessen. " 



Such a condition seems to explain largely the different re- 

 sponses to the same stimulus of the anterior end in Dina micro- 

 stoma. The reflex ring necessary to produce such modifications 

 would have to be little, if any, more complex than that which 

 is used by Von Uexkiill in explaining the looping response. The 

 effectors c and d in the accompanying diagram (see fig. 6) repre- 

 sent a turn to the right and left respectively. Strong stimulation 

 of an intermediate region e produces first a turn to the right. 

 An overflow of stimulus set up by the turn to the right spreads 

 back to the left and a turn to the left is produced. Thus we 

 have the response of a turn to the right followed by a turn to 

 the left. Such a conception would also explain why a turn to 

 the right tends to be followed bv a turn in the same direction 



