22 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



nutritive function, because the mouth is still closed. However, 

 Krizenecky and Podhradsky (1924) found that tadpoles were capable of 

 absorbing dissolved substances through their skins, and in a review 

 Krogh (193 1) came to the conclusion that such absorption was rare in 

 vertebrates but did occur in tadpoles. Since the envelopes are dis- 

 integrating at this stage by bacterial action, which might break down 

 the mucoprotein to simpler and diffusible materials, it still seemed 

 possible that there would be some nutriment for the tadpoles in the 

 water. However, when a comparison was made between tadpoles 

 left on the envelopes and those removed from them, it was found that 

 there was no difference in the fmal concentration of carbohydrate in the 

 two sets (Fig. 2), and it was concluded that the disintegrating envelopes 

 have no nutritive function. They do, however, play a part in depleting 

 the oxygen in the water of the aggregations, for the water among a 

 quantity of disintegrating envelopes of an aggregation in a pond was 

 found to contain 210,000 bacteria per ml but the water of the same pond, 

 about two metres from the aggregation contained only about 4,800, 

 and other determinations have given similar results. Bacterial action 

 of this kind always makes a heavy demand on the dissolved oxygen. 



It now remains to consider what useful function is performed by 

 this habit of aggregation. Of course, some advantage in temperature 

 still remains, for not only is the carpet of tadpoles black and therefore 

 a good absorber of solar radiation, but Gay da (1921) has shown that 

 small quantities of water in which toads' eggs were developing were 

 as much as i*5°C higher than the controls. Perhaps there is some 

 protection from predators, for an environment deficient in oxygen 

 would repel animals dependent on aquatic respiration. 



In the aggregating habit, we may perhaps see another case in which 

 the bizarre habits of an exotic frog could have been derived from one 

 that had only those of our famihar R. temporaria. Some species, for 

 example Hyla rosenbergii, build basins of mud at the edges of ponds, and 

 inside these harbours, as Noble (1927) has described, the tadpoles live 

 in a special environment produced for them by the parent frogs. The 

 water is teeming with bacteria and deficient in oxygen, and the 

 tadpoles have enormous pinnate gills that adhere to the surface film 

 of the water. Our tadpoles do not quite achieve these extraordinary 

 modifications, but the parent frogs, by laying all their eggs in con- 

 tact with each other, help to create a special environment which the 

 tadpoles continue to maintain and extend, so that ultimately an 



