196 METHODS IN TEACHING 



Explain why to the pupils. Note the interruption of the 

 blood current by the pulse. Note also the currents of water 

 flowing past the gills. These currents are kept up by rap- 

 idly moving cilia, difficult to see even with the high powers, 

 owing to their small size and rapid motions. 



To see all these things requires time and much patience, 

 as the little creature frequently moves out of the field of 

 view and must be brought back again by moving the watch 

 glass so as to bring the animal again under the objective. 

 It is worth all the trouble, however, for there is probably 

 no better object lesson on the circulation of the blood. 



A little later in the season the frogs and toads appear in 

 great numbers in the ponds, adding their choruses to the 



other subtle sounds and influences of spring. 

 Frogs 



The eggs are similar to those of the sala- 

 mander, but they are loosely coherent in masses of fifty 

 or more, attached to plants or floating free according to 

 the species by which they are deposited. The development 

 of the tadpole is similar to that of the larval salamander. 

 It has no external gills except when very young, and it 

 feeds almost exclusively upon vegetable matter. The legs 

 develop in the reverse order to those of the salamander, and 

 in the final change the tail is completely absorbed. At the 

 same time the digestive system undergoes pronounced 

 changes to fit it for the exclusive insect diet of the adult 

 stage. Respiration in frogs and salamanders takes place 

 not only through the gills, but through the skin every- 

 where, and through the widely expanded tail-fin, which is 

 plentifully supplied with capillary vessels. In the adults 

 of both forms lungs take the place of gills. 



