442 'Journal of Comparative Neurology and Psychology. 



more slowly and the tips of the circle of branchiae remained pro- 

 jecting a trifle from the mouth of the tube. The time during 

 which the animals remained contracted was noted in each case and 

 was found to vary in a way which will be described later. 



A considerable number of tests were made to determine the sensi- 

 tiveness to shadow alone. In most of these the shadows were 

 given at unequal intervals; for instance, if the animal contracted 

 in response to the first shadow, the second was given as soon as the 

 worm was again fully expanded. The time thus varied between 

 ten seconds and a minute or, in a few cases, two or three minutes. 

 If the animal did not respond, the shadows were repeated at short 

 intervals, from five to ten seconds. Ten such trials were given in 

 succession. In the greater number of these tests (i6 out of 27) 

 with different specimens the animals responded only the first time, 

 or possibly from one to three times, and then gave no further 

 response throughout the ten trials. This fact seems to be com- 

 parable to the results obtained by Jennings* (p. 172) with attached 

 infusoria which reacted the first time to a jet of water striking 

 against the disk or to other faint stimuli, or a few times to a 

 stronger stimulus, but did not respond to later repetitions of the 

 stimulus. This behavior Jennings interprets as due, not to 

 fatigue, since the number of reactions is few, but to a change in the 

 organism itself (p. 173), a change which has distinctly a regulatory 

 character in the behavior of the animal. Hargitt, on the con- 

 trary (p. 301), who observed that when shadows fell rhythmically 

 on Hydroides and "the experiments were repeated with any con- 

 siderable frequency, specimens sooner or later became somewhat 

 irresponsive," is inclined to regard this as the result of fatigue. 

 Evidence, however, from a later series of the present experiments, 

 tends to show that fatigue is not the cause, since the animals are 

 capable under stronger stimulation of contracting a large number 

 of times in rapid succession without apparent fatigue. 



In ten cases of the twenty-seven mentioned above, the worms did 

 not respond the first time nor later during the ten trials. Whether 

 this is due to individual variation in sensitiveness or whether 

 the animals had responded previous to the experiments to the 

 shadows which fell upon them by chance while still in the aqua- 



'Jennings, H. S. Behavior of the Lower Organisms. New Tork, Macmillan. Pp. 366, figs. 144. 

 1906. 



