292 University of California P^iblications in Zoology [Vol. 11 



and the same is true of his individual muscles, which are incap- 

 able of performing as many contractions as the muscles of a 

 normal animal." 



As has been stated in the first part of this paper, leeches 

 starved for almost six months show considerable vigor of re- 

 sponse. This fact is indicated very clearly by the accompanying 

 curves (see fig. 13) of activity of normal and starved animals. 

 These curves are plotted from forty responses to contact stimu- 

 lation of the posterior end, and consequently represent only the 

 initial stages of a complete curve of activity. Proportionately 

 the starved specimens show a greater irritability than the normal 

 specimens. The relative nature of the curves seems to indicate 

 that the starved leeches would reach complete depression with 

 a smaller number of stimulations than would the normal speci- 

 mens. 



3. Responses op Undisturbed Leeches 

 There is in the behavior of the leech a condition corresponding 

 to that of "sleep" in the higher animals. In this state the 

 animal may occupy one of several positions. It may have the 

 anterior eighth of the body turned back over the rest of the body ; 

 sometimes it may be coiled slightly; and again, the body may 

 be held entirely straight, the animal hugging the bottom of the 

 dish. In this condition two or more stimuli are necessary to 

 arouse the animal to action. The first stimulus when the leech 

 is in this condition may produce little more than a local con- 

 traction ; the second, a turn to the left or right as the case may 

 be; while the third will produce a reversal of direction of the 

 anterior end together with swimming. This tjT)e of response is 

 just what would be expected in summation effects of stimulation, 

 and this is doubtless the true explanation for the behavior. 

 When the animal is awakened the responsiveness is sharp and 

 vigorous. A slight stimulus of the anterior end may produce 

 a reversal of direction of orientation and subsequent swimming. 

 This would indicate a heightened tonus for the body of the animal 

 as the result of its period of rest, the stimulus liberating a greater 

 supply of energy than normally because of the replenished stores 

 of energy producing material in the tissues of the bod.y. 



