46 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



In some cases, when the algae are deUcate, it seems likely that the 

 vigorous peristaltic movements of the intestines is sufficient to rupture 

 the cells and release the contents. Li and Lin (1936), studying the 

 tadpoles of Kaloula borealis, found some digestion of euglenoid 

 organisms such as Euglena and Phacus, and I have found that the cells 

 o£ Scenedesmus, a dehcate planktonic species, are often empty in the gut. 

 Li and Lin found that diatoms passed through undigested, which 

 contrasts with the observations of Fish (195 1) who found that unless 

 the fish Tilapia can eat diatoms, it cannot survive, for it, too, cannot 

 digest the cellulose of the other algae which may be abundant in its 

 lakes. Li and Lin considered that the algae did not form an important 

 source of food, but that their tadpoles hved chiefly on Protozoa, a 

 matter to which I shall return. I think that the evidence that the 

 tadpoles of R. temporaria, at any rate, can live on algae is conclusive. 

 Other food, however, is always taken. 



The two ponds. Dagger Lane and Hospital, were examples of ponds, 

 in which the tadpoles appeared to live mainly on algae, and in both 

 the tadpoles metamorphosed while still far short of the maximum size 

 that the species can reach. By contrast, some of the tadpoles in Lower 

 Parkfield, except in the abnormal year of starvation, and in Cat Hill, 

 grew to the largest size found in this investigation, and the main food 

 found in the gut consisted of Entomostracae, with much debris. The 

 tadpoles, in their ceaseless task of ingestion, get this nourishing food, 

 as it were, by accident. Tliis same habit probably serves to tide the 

 animals over a temporary shortage of food for, indigestible as the 

 Phanerogams may be, it is probably better to eat them than to eat 

 nothing. As for the mud which is often present in abundance, this 

 may well contain a sufficient quantity of food to be well worth eating, 

 when it is considered what enormous quantities pass through in one 

 day. The tadpole (C98) from Papua, of unkno^vn species, apparently 

 specialized in eating heavy mud, which was counterbalanced by the 

 extraordinary development of its fl.oat-like lungs (Fig. 16 a). 



Another result of this habit of somewhat indiscriminate engulfing is 

 that even the tadpoles that live largely on algae get animal food, for 

 the tests of the Protozoa Arcella and Difflugia, which climb about the 

 plants, are common in the gut. How many amoebae and other soft 

 animals were originally there can only be imagined, for they would 

 be rapidly digested. When it is considered that only a fraction of 

 the algae is digested, but probably all the soft animals, the relative 



