ANIMALS OF POND AND STREAM 35 



The smooth pearl lining to the shell is what gives it value for "pearl but- 

 tons." What service does the pearl lining render to the animal? Do 

 you find little rounded knobs on this pearly lining in some shells? Can 

 you explain them ? What are baroque pearls ? What is the value of the 

 pearls obtained in the United States from fresh-water clams ? of the buttons 

 made from their shells ? Where is the chief center of this button industry ? 



Frogs. The commonest frogs in our region are the bullfrog, green frog, 

 leopard frog, pickerel frog, cricket frog, and tree frog. Learn to recognize 

 these and also the common garden toad. When is the latter an animal 

 of the ponds and streams ? What advantage would it be to have toads in 

 the garden? How may you provide inducements for them there? On 

 what does the bullfrog feed ? How does the frog or toad breathe ? Watch 

 the frog swim in the pond or aquarium. What is the advantage of the 

 webbed foot? The tree frogs climb on plants to secure insects as food. 

 Keep one in a jar with a spray of plant and watch to see it climb the sides 

 of the jar or on the plant. Draw a foot to show the climbing device. 

 Cut a circular piece of leather two inches in diameter from an old shoe top 

 and with a needle run a string through its center. Tie a knot in the end of 

 the string so it cannot pull out. Soak the leather a while in water. Apply 

 it to a piece of glass, knot side down. Press the edges against the glass. 

 Pull on the string, trying to lift the leather from the glass. What does the 

 experiment show and what is its significance at this point ? 



The common tree frog (Hyla versicolor) is said to change its color readily 

 when on varicolored backgrounds. Put one in a glass-topped box lined 

 with green paper. Does it become green? Reline the box with brown 

 and put the frog in it; does it change its color? How long does it take to 

 change ? Try orange, red, and black. 



Frogs' eggs. About the middle of April obtain some frogs' eggs from 

 the pond early in the morning. Toads' eggs may be secured three or four 

 weeks later. Some of the eggs will be found in early stages with about 

 half the surface white, half black, and a single faint line crossing the black 

 portion. When the eggs are laid by the female, each is a single large cell, 

 a spherical mass of living protoplasm more or less filled with yolk granules. 

 At the same time the males discharge the sperm into the water and one of 

 them unites with an egg, so fertilizing it. (Recall fertilization of the flower.) 

 The egg now proceeds to divide and subdivide; watch this process in the 

 schoolroom, taking a few eggs for the purpose in a shallow glass dish with a 

 little water and placing them under the simple magnifying glass. Later 

 the spherical mass of small cells formed by the continued subdivisions 

 changes its shape to become the tadpole. Supplement your own observa- 

 tions by those of other members of the class if you cannot stay by the 



