56 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



Strawinski found, as stated above, that the gill filters have very little 

 respiratory function, and even that the gills themselves are not very 

 important. On the other hand, he found that the inner surface of the 

 operculum is highly respiratory. Perhaps we make too much of 

 anatomical distinctions, and should rather consider (a) lungs, {b) skin, 

 and (c) the whole respiratory channel as three units. The first two are 

 clearly distinct, but buccal cavity, gill filters, gills and operculum are 

 only subdivisions of one system {c), which perhaps may share the 

 functions in different proportions in different species. 



I have suggested (1952 and 1955) that the respiratory systems in 

 tadpoles are connected with the ecology in the following way. If a 

 tadpole hves in an environment rich in food, as many temporary or 

 poUuted ponds are, it does not need to pump much water to get its 

 food, and so does not need large gill filters. To use the oxygen under 

 these conditions, however, it must have large gills, and needs lungs to 

 tide it over emergencies. If, however, it lives in the oHgotropliic type 

 of pond, with plentiful and almost constant supplies of oxygen but 

 with a low concentration of food, it needs to pump much water, and 

 so must have large gill filters. With these, giUs might not be necessary, 

 because of the large surface of the filters (or rather, in view of Stra win- 

 ski's work, more probably, the associated large operculum). In 

 ordinary ponds, intermediate as habitats, the arrangements might be 

 expected to be intermediate also. 



There is a good deal of conjecture in all this, but the Microhyhds 

 seem to provide examples. Some have such large gill fdaments that 

 they trail in the opercular cavity in a way quite unlike those of Ranay 

 others have no gills but have enormous gill filters, for example 

 Glyphoglossus molossiis. Some, such as Hypopachus aguae, are inter- 

 mediate and live in ordinary ponds. R. temporaria also has a moderate 

 development of gills, filters and lungs, and lives in ordinary ponds. 



The respiratory arrangements in tadpoles underline what I beheve 

 to be points of fundamental importance in Anuran ecology — the great 

 degree of speciahzed adaptation to the environment, wliich enables 

 the different species to colonize habitats in great variety, from tree 

 holes to puddles, from lakes to cow-ponds, from mountain streams to 

 dry land. The common feature of most of these habitats is that they 

 confine the tadpoles within usually quite small hmits. If the conditions 

 in their pond turn out to be dangerous they cannot go in search of 

 better ones, as animals that hve on dry land or in the sea can do, but 



