24 



ANIMAL BIOLOGY 



Fig. 36. — Nettling Cell. 



II. discharged, and I. not discharged. 



nately elongating and rounding the foot. Can you dis- 

 cover other ways by which it moves ? Does the hydra 

 always stand upon its foot ? 



Lasso Cells. — Upon the tentacles (Fig. 35) are numer- 

 ous cells provided each with a thread-like process (Fig. 36) 



which lies coiled within the 

 cell, but which may be 

 thrown out upon a water 

 flea, or other minute animal 

 that comes in reach. The 

 touch of the lasso paralyzes 

 the prey (Fig. 37). These 

 cells are variously called 

 lasso cells, nettling cells, or 

 thread cells. The thread is 

 hollow and is pushed out by the pressure of liquid within. 

 When the pressure is withdrawn the thread goes back as 

 the finger of a glove may be turned back into the glove by 

 turning the finger outside in. 

 When a minute animal, or 

 other particle of food comes in 

 contact with a tentacle, how 

 does the tentacle get the food 

 to the mouth ? By bending 

 and bringing the end to the 

 mouth, or by shortening and 

 changing its form, or in both 



ways ? (Fig. 34, C.) Do the 

 neighboring tentacles seem to 

 bend over to assist a tentacle in 

 securing prey? (Fig. 34, C.) 

 Digestion. — The food parti- 



Fig. 37. — Hydra capturing a 

 cles break up before remaining vvate r flea 



