20 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



top of a shallow vessel, if they were stimulated by lack of oxygen. 

 This shortage of oxygen was sometimes produced by merely allowing 

 the tadpoles placed in the horizontal tube to exhaust that which was 

 present at first, and sometimes by removing oxygen from the water 

 artificially. When this was done, the Httle animals became restless, 

 and swam upwards. If these movements took them into a better- 

 oxygenated part of the vessel, they stopped swimming. This move- 

 ment was therefore a combination of a kinesis (random movement 



12 3 4 



CENTIMETRES 



Fig. 4. The Apparatus Used for the Experiments on the Reactions 

 of Young Tadpoles to Deficient Oxygen 



under the influence of a stimulus) and a negative geotaxis (a movement 

 upwards) when once movement was provoked by lack of oicygen. 



Transferring the results of the experiments to the field, it is now 

 possible to explain what happens. The tadpoles in the central parts of 

 the large aggregations live in water with very little oxygen. They 

 therefore swim upwards. Those at the edges of the aggregation are in 

 better-oxygenated water, and, not being stimulated, do not swim at 

 all. The colony therefore remains in the same place as a whole, but 

 the individual members of it are continually changing places. When 

 there are only a few clumps, there is enough oxygen for all, and no 

 swimming takes place. All that is seen in such cases are the gymnastic 

 exercises that tadpoles of this age execute wliile they hold on v^th the 

 aid of the adhesive gland with which they are provided. 



The Function of the External Gills 



The water of these places has another unusual feature. It is full of 

 the gelatinous remains of the envelopes, and during the determinations 



