ECOLOGY OF TADPOLES 55 



water, and were not accompanied by any frog tadpoles. A short 

 distance away, frog tadpoles fed on the bottom of the pond, un- 

 accompanied by any toad tadpoles, and made journeys to the surface 

 for air. These visits were hurried affairs — when watcliing the surface, 

 a tadpole could be seen swimming up very rapidly. It would break 

 surface, and without pause would swim down again as rapidly as it 

 appeared. The impression was irresistible — there was no time to waste, 

 but air had to be taken. The lungs are only one of the organs of 

 respiration, but they may make all the difference between one kind of 

 Ufe and another, as I shall show in the next paragraph. 



In small ponds, fluctuations occur in both the food supply and in 

 the oxygen. The oxygen variations may be due to two distinct causes. 

 Plants respire continuously, but carry out photosynthesis only by day. 

 If the plant population is large, respiration by night may be so intense 

 that the oxygen, plentiful by day, may then be reduced to dangerous 

 levels. In the second method, shrinking pools in the summer may be- 

 come the site of so much bacterial action that oxygen is depleted in 

 this way, even by day. 



The lungs of a tadpole vary greatly in different species. B. bufo as 

 mentioned above, is lungless (Fig. i6 c). The Papuan, C98 species 

 (Fig. 16 a) has the largest I have seen. In this case, they are probably 

 used partly as floats, balancing the large gut, loaded with mud, but 

 the glottis is well developed, and the lungs may be of special importance 

 in tropical mud. Ventilation of these enormous sacs was probably 

 effected by pressure from the powerful tail muscles, for the lungs, 

 almost rectangular in section, were strongly adherent to them and 

 were packed between them and the notochord so that, when the 

 muscles contracted and became thicker, the flat surfaces of the lungs 

 must have been pressed together. It was doubdess in this way that the 

 gases in the lungs became interchanged with those in the pharynx. 

 R. temporaria (Fig. 16 h) has smaller lungs than C98, but is much 

 nearer to it in this respect than it is to B. bufo. It is obvious from the 

 figures that animals with such extreme differences in respiratory 

 anatomy as these can live very different lives. C98 was collected from 

 water in a wheel-rut, a very restricted environment. R. temporaria 

 can live and thrive in similar places, if it can escape drought. B. bufo 

 must have ponds with a rehable oxygen supply. Frog and toad tad- 

 poles are often found in the same ponds, but this is because frog tadpoles 

 can Hve in toad ponds, not because the toad is equally adaptable. 



