284 A. M. Banta 



2. AseUiis 



As in all other cases, Asellus and Caecidotea were experimented 

 with in succession and under the same conditions. The reactions 

 of Asellus will be described first. 



With vertical illumination of 6983 C.M.^ intensity the photokin- 

 etic effect upon Asellus was very marked. If the animals had only 

 recently been placed m the tank, the random movements brought 

 about by thigmotaxis or other influences due to transference to 

 the tank were likewise very marked, but because of photokinesis 

 the animals in the illuminated region were more active than those 

 in the dark region. When the influence of the transference to the 

 tank ceased, the difference in activity between the animals in the 

 illuminated region and those in the dark region was increased. 

 When given time to become acclimated to the tank before the 

 experiment was begun, many of the animals began to move about 

 within a half minute or less. With Asellus the characteristic inter- 

 mittent movements continued in most cases as long as the ani- 

 mals were in the light region, but the activity was always less 

 when they were in the dark region, so that there was a decided 

 photokinetic eff"ect. 



Concerning the usual movements of Asellus, it may be said that 

 normally the animal moves by short stretches or "runs," between 

 which it pauses for a time. When more active, its runs are longer 

 and its pauses shorter. It normally comes to a stop by a gradual 

 slowing of its movements, as it might appear to do if the stop were 

 due to a loss of momentum. When stimulated in any manner 

 just after the beginning of one of these runs, or near the end of it, 

 the animal often stops almost instantly. Sometimes, however, 

 the movement is immediately accelerated by the stimulus, for 

 the run is more rapid and longer than it would otherwise have 

 been. If the stimulus is applied during the middle of a run, it 

 seems less likely to be effective. When an Asellus has been stop- 



3 This intensity was produced by the use of a 772-c. p., 6-glower Nernst lamp at 30 cm. from the sur- 

 face of the water, which was 3 cm. deep. In calculating the intensity of the light at the bottom of the 

 dish, where the animals were, A per cent was deducted for each cm. of water through which the light 

 passed. 



