284 University of California Publications in Zoology [Vol. 11 



greater than those of the normal mnsele. After one hundred 

 and seven contractions the phosphate muscle began to show signs 

 of depression. 



Leeches placed in a one-seventh gram molecular solution of 

 mono-potassium phosphate (KH^POj) showed signs of consid- 

 erable excitation extending over some length of time. Within 

 three hours the most of the leeches were in a condition near 

 complete depression. The muscles of the animal appeared flaccid, 

 showing no signs of paralysis. Recoverj' took place rather slowly 

 as compared with that following the use of carbon dioxide. 

 Within three to four hours after removal to clear water the 

 leeches were usually able to perform well co-ordinated movements, 

 though at a rather slow rate. By the next morning the leeches 

 had practically entirely recovered normal responsiveness. 



9. Lactic Acid 

 Leeches placed in a 0.08 per cent solution of lactic acid 

 (sp. gr. 1.21) required eight or more hours for depression to 

 occur as the result of the solution. With a 0.2 per cent solution 

 this result was secured in a much shorter period. Augmentation 

 of activity upon stimulation seemed to be marked almost to the 

 time of depression. In fact the action of lactic acid on leeches 

 was rather different from that of carbon dioxide and mono- 

 potassium phosphate. A much longer time was required for 

 depression to occur ; and when the animal reached this stage the 

 substance had so injured the animal as to prevent its recovery. 

 Whether this is due to the difference between lactic acid and 

 paralactic acid was not made the subject of investigation. Rich- 

 ter (1899), however, states that in all its transpositions para- 

 lactic acid behaves like ordinary lactic acid, and hence the same 

 chemical structure is accepted for it. The existence of the two 

 modifications is explained by him as due to the asymmetry of a 

 carbon atom in the acid. 



10. Discussion of Fatigue 

 Fatigue represents a distinct physiological state in an animal. 

 This state carries with it a more or less characteristic type of 

 response in the leech, and very materially influences its degree 



