536 Annals of the South African Museum. 



and lighted strongly from one side, the colonies can be watched 

 swimming towards the light ; in a very short time all the colonies 

 are collected on that side of the glass nearest the light, moving about 

 and jostling one another. This is specially marked if the material 

 has been kept in the dark or badly lighted for some time previously. 

 In half a minute practically all the colonies have passed to the side 

 nearest the light. Reverse the position of the light without moving 

 the watch-glass, and the colonies immediately start in the opposite 

 direction, again collecting on the side nearest the light. This gives 

 an average rate for the crowd of about 6 to 8 cm. per minute, but 

 some individuals obviously attain a much higher speed (cf. also 

 Klein, 1889, B, p. 47, for V. globator). 



From a few such simple experiments it becomes obvious that 

 Volvox is strongly phototactic, and that in this pursuit of light the 

 anterior pole is directed towards the light ; it would thus appear 

 that this pole is the light-sensitive region of the coenobium. 



Under natural conditions, of course, the upper surface of the 

 pool is the most strongly lighted, hence during its voluntary move- 

 ment Volvox tends to swim upwards and collect in the upper layers 

 of water. In the watch-glass experiment, however, if the one-sided 

 illumination continues to operate for a lengthened period, there 

 comes a time when the stimulus is no longer effective, the colonies 

 cease to respond, become quiescent, and gradually sink under their 

 own weight, collecting at the bottom of the glass. Similar behaviour 

 may be seen in their natural surroundings ; after remaining motile 

 near the surface for some time, if the light is strong, the coenobia 

 soon become quiescent, and slowly sink. Possibly this may be a 

 question of " fatigue," the cells ceasing to respond if the stimulus 

 is long continued. However that may be, the alternate swimming 

 towards and dropping away from the light is probably directly 

 connected with photosynthetic activity during exposure to light 

 carbohydrates are actively formed, accumulate in the cell until a 

 certain concentration is reached, when active movement ceases and 

 the plants slowly sink under their own weight. Further, it would 

 appear that the stimulus of light is necessary to initiate the move- 

 ment of the cilia, since at night the colonies collect at the bottom of 

 the vessel containing them. 



Interesting observations on the vertical distribution of Volvox in 

 deep waters at different times of the day were carried out on Lake 

 Monona, Wisconsin (Smith, 1917), but conditions there are, of course, 

 very different from those obtaining on the Cape Flats. Further 



