chapter 3 



TPiE RELATION BETWEEN THE ECOLOGY OF 

 TADPOLES AND TPIEIR ANATOMY AND PHYSIOLOGY 



In the last chapter, the events in the ponds were described without 

 much attempt at explaining the underlying reasons. In this chapter, 

 I attempt to show that at least some of these events can be traced to 

 physiological and anatomical adaptations in the tadpoles. I shall, by 

 drawing also on what knowledge we possess of other species of tad- 

 poles, show how highly adapted to special environments even "normal" 

 pond tadpoles are. 



Food and its Digestion 



The gut of a tadpole is never empty. Normally it is packed from 

 one end to the other with some kind of material, and only in the 

 emaciated tadpoles from Lower Parkfield late in 1947 were there any 

 empty spaces. Yet the graphs clearly show that growth rates are often 

 widely different, and that sometimes growth ceases altogether. The 

 conclusion is inevitable — all that is in the gut cannot be food, and we 

 are unable to judge whether the contents are really food unless we 

 have simultaneous observations of the growth rates. Moreover, the 

 faeces often contained much undigested material. Leaves and roots of 

 duckweed were eaten in large quantities by the starving tadpoles in 

 Lower Parkfield in 1947, but seemed to pass through quite unchanged. 

 Hospital Pond in one year contained quantities of the spherical alga, 

 Volvox globator, and intact spheres passed into the faeces. Many of the 

 algal filaments and unicellular forms were found in the hinder parts of 

 the gut with their contents reacting to stains as if they had been collected 

 from the pond itself. 



There was obviously something peculiar about the digestive process 

 in tadpoles, and a test was arranged in which they were first fed a 

 coloured meal and then placed in Petri dishes for continuous obser- 

 vation. It was found that food passed through at a truly astonishing 

 rate. The average for four experiments on R. temporaria was only dj 

 hours, with one time of 4f, and a tadpole of JB. bufo passed coloured 

 faeces only 3I hours after the beginning of the experiment. 



44 



