82 



BOYS AND GIRLS IN BIOLOGY. 



and dash into the water in search of its food. It is one 

 of the funniest things in the world to see the hydra 

 catch its prey. I remember, in my old geography, a 

 picture of Indians catching wild-horses with lassoes. 

 The lasso is a long rope with a loop at the end, which 

 the Indian skillfully throws over the horse's head. So 

 the hydra throws out its long, rope-like fingers, and 

 catches little animals which are swimming near it. 

 Sometimes it gets hold of things which are so strong, 



Fig. 82. Fig. 83. 



Thread-Cell. Thread-Cell. 



they might tear the lassoes and get away again. But 

 the hydra has another weapon at hand. Some of the 

 cells of the outer skin are oval, or egg-shaped, and if 

 you look through their walls you will see what appears 

 to be a long coiled thread with two hooks at its base, 

 lying snugly within each of the cells (Figs. 82, 83). 

 These oval cells are called " thread-cells," and the hy- 

 dra has many thousand of them in its tentacles, or 

 fingers. This thread, or spring, darts out of the cell 

 whenever the hydra wants it, and sticks itself into the 



