ECOLOGY OF TADPOLES 47 



importance of the types of food becomes difficult to assess. It is very 

 likely that even the "vegetarian" tadpoles obtained much of their 

 nourishment from animal food. 



Gudernatsch and Hoffman (1936) fed different amino acids to tad- 

 poles of R. syhatica. Some allowed growth without much differentia- 

 tion, others differentiation with little growth, and some allowed both 

 to occur. It is certain that the amino-acid composition of the very 

 varied diet of tadpoles must vary considerably, and tliis may be one 

 reason for the variations in size. 



One of the implications of the fact that in some ponds the tadpoles 

 Uve on animal food, and in others on vegetable food, is that there is 

 one more link in the food chain in the former. It is well known that 

 each link in the chain reduces the numbers of organisms of the same 

 size that can be supported in the same enviromnent. Macan and 

 Worthington say (195 1) that each link reduces the number to one 

 seventh. It is certainly true that the population in the ponds with the 

 large tadpoles feeding on animal food was far smaller than in the 

 others, and that disaster overtook those in such a pond when the 

 numbers were large. 



A point of interest, not yet solved, is how the tadpoles manage to 

 digest Crustacea in the short time available. The Crustacea are 

 invested with a remarkably complete cuticle of chitin, a material not 

 easily acted upon by enzymes, yet dead and swollen Crustacea are seen 

 in the gut. As pure conjecture, I suggest that with these quite small 

 animals, the peristaltic movements of the tadpole's gut, which are very 

 vigorous, may be sufficient to pump out the contents of the gut of the 

 Entomostracan, and to replace them with digestive juices, so that the 

 prey is digested from within. There is a small part of the gut of 

 Crustacea that is not lined with chitin and, after all, it is often only 

 a fraction of a milhmetre from either end. As another speculation, I 

 have wondered whether the gut contents of the prey might not be very 

 useful, consisting as it does of comminuted and pardy digested algae. 



The ingestion of Crustacea by tadpoles of American species is part 

 of the pattern of life, for the trematode Halipe^us eccentricus gains 

 access to tadpoles by first infesting Cyclops, which are then eaten by 

 the tadpoles (Thomas, 1939)- 



In the description of the behaviour of the tadpoles in Lower Park- 

 field, I wrote that on the date concerned the tadpoles were feeding on 

 the surface fdm. It may not be realized that the term "surface film" 



