268 University of California Publications in Zoology [Vol. 11 



spending " desensibilisation " which is pronounced in the late 

 afternoon or evening. The results of the repeated excitation 

 are sho\vn differently throughout the hours of the daj'. To quote 

 him exactly : ' ' Quand une excitation se repete un certain nombre 

 de fois, on observe souvent que pendant un certain temps la 

 reaction devienent de plus und plus facile, c'est-a-dire se fait 

 avec un rapidite et une intensite de plus und plus grandes. 

 Enfin, vers le soir, au bout de quelques attouchements, le tentacle 

 devenait insensible." This condition he suggests is to be ex- 

 plained in terms of the chemical equilibrium within the cells 

 involved. It is suggested that under the influence of catalyzing 

 agents the vigor of a certain reaction is increased, due to the 

 liberation of the specific energies leading to that type of response. 

 The decline in response is to be attributed to the impoverishment 

 in the cells of these certain substances. After a period of repose 

 the organism replenishes its supply of the substances needed and 

 the normal response is again given. 



It had been observed in some unpublished experiments of 

 mine on planarians and earthworms that there seemed to be in 

 their response to several hundreds of repeated contact stimuli a 

 process very analogous to that found by Bohn in Cerianthus. 

 A difficulty presented itself in these forms due to the fact that 

 there was not even an approximately accurate method of quan- 

 titatively measuring this increase and decline of response. In 

 Dina microstoma, however, the dviration of the swimming re- 

 sponse seemed to be at least a fairly accurate gauge of the degree 

 of activity in the successive periods. Accordingly, three indi- 

 viduals were subjected one at a time to repeated contact stimu- 

 lation of the posterior end, and the duration of the responses 

 measured by means of a stop watch. 



The results of the three were found to be very much the 

 same. There was a phenomenon resembling in its general fea- 

 tures the fatigue of skeletal muscle as reported by the workers 

 cited above. There was at first a period of comparatively low 

 response from which the animal was aroused to an increased 

 degree of activity indicated both by the vigor of its response 

 and the duration of the swimming period. A more accurate 

 register of the degree of excitement of the leech would be its 



