50 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



that Polyplacophora can only digest those algal cells that happen to be 

 pierced by the teeth on the radula, an organ that has some resemblances 

 to the cuticular teeth of the tadpole. It is a wasteful method of feeding, 

 but in both cases it works. In tadpoles, we know that sometimes it 

 would seem to be much better if more efficient methods were in use, 

 for then there would be enough to go round even in the lean years. 

 But this is how the tadpole has evolved, and it is the total effect that 

 counts. If the rate of reproduction is suitably geared to allow the 

 occasional catastrophes to occur without extinction, that is the end of 

 the matter, for the species is not concerned with human ideas of 

 efficiency. 



If we now follow the food in its journey through the tadpole, we 

 come soon to another problem. At one time, the tadpole may be 

 feeding on such large objects as Daphnia, or even larger animals (a 

 spider was once found), and at the next, it may be collecting thousands 

 of small unicellular algae, or even bacteria. One could hardly expect 

 that the same mechanism would serve both ends and, in fact, there are 

 two. 



Pharyngeal Anatomy and Physiology 



When the pharynx of a tadpole is opened from the dorsal side, a 

 conspicuous organ, the velum, can be seen. The velum is actually a 

 single organ in the form of a tilted annulus with a gap in it at the centre 

 of the dorsal portion, but the structure and function of the dorsal and 

 ventral parts are so different that it seems better to treat these parts as 

 if they were separate organs. 



Fig. 17 shows a sectional view of an actual tadpole, and Fig. 18 a 

 diagrammatic view. The ventral velum is a substantial, somewhat 

 muscular organ, and to judge from the different positions of its edge in 

 different specimens it seems probable that it directs the entering stream 

 of water differently at different times. When the edge of the velum is 

 raised, the water must strike the roof of the pharynx at a point anterior 

 to the dorsal velum. This is a much thinner fold of tissue than the 

 ventral velum, and can be opened like a sail if a stream of water is 

 directed at it. Consideration of the hydrodynamic relations of the 

 parts makes it certain that behind the dorsal velum there would be an 

 eddy. Particles collect in eddies and this is where, on dissection, one 

 finds food particles. Bles (1905) described cihated grooves on each side 

 of the pharynx of Xenopus laevis and said that water green with 



