TADPOLES AND FROGS. 187 
poles cling in clusters to plants, or other objects; life 
being sustained by the food-yolk within the body. 
After a few days, a mouth with horny jaws develops 
and the animals begin to feed. 
The tadpole now eats plants, being especially fond 
of the green conferve so common on the sides of 
aquaria. 
Growth is now rapid, and in a few months the legs 
appear and the tail is absorbed. 
E 
Fic, 143.—Growth of Frog’s Lung from Primitive Food-tube. 
Movements. The most important organs of motion 
are the muscles. A muscle causes movement by 
shortening itself, and thus bringing nearer together its 
two ends with whatever is attached to them. In gen- 
eral, muscles are attached to bones. How muscle and 
bone together produce movement may be well under- 
stood by studying Fig. 148 in connection with the 
movements of the human arm. 
