398 r. W. GAMBLE AND FREDERICK KEEBLE. 



indicates the iiiimecliate negative pliototropio movement 

 induced bj^ suddenly raising the liglit-intensity. 



Unlike this transitory " negative " effect — of the nature of 

 shock, — the "positive" movement induces a remarkably 

 permanent disposition. The animals remain the whole day 

 long massed up in the brighter side of the vessel, but only 

 so long as the light conditions remain fairly constant. A 

 shadow thrown over them suffices to disturb and to cause them 

 to mass themselves evenly throughout the vessel. 



From the above it follows that though phototropism only 

 occurs when the light-intensity is high, yet exceedingly slight 

 differences of intensity serve as stimuli. 



As is the case with green plants, external conditions, 

 especially light conditions, exert normally and continually 

 a certain general influence on the condition and state of 

 irritability of Convoluta (tonus or paratonus). In both 

 plants and in Convoluta the nervous states and metabolic 

 activity is in part the outcome of a constant rain of internal 

 and external stimuli. When we speak of tone, we refer to the 

 modified tone compounded of the autonomous and of the 

 induced tone. This light-induced tone (phototonus) is one of 

 the most striking features of Convoluta. When the tonic 

 influence of light is withdrawn — as, for example, by main- 

 taining the animal in darkness for several days — Convoluta 

 passes into a sluggish condition. In this state it lies motion- 

 less, its muscles contracted so that it becomes shorter and of 

 a darker green colour by reason of the massing together of 

 its green cells. On re-exposure to light this state rapidly 

 passes away. From what we have said previously with respect 

 to the marked negative gootropism of dark-kept Convoluta, 

 and from the above observations, it follows that dark-rigor 

 only subvenes gradually, and that phases of dark-rigor are 

 succeeded by progressively shorter periods marked by high 

 irritability to the stimulus of gravity. 



We infer from the phenomena presented in dark-kept 

 Convoluta that light exerts a constant (phototonic) influ- 

 ence on Convoluta. But when the light intensity is 



