24 



BEGINNERS' ZOOLOGY 



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When the pressure is withdrawn the thread goes back as 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 



neighbouring tentacles seem to 



bend over to assist a tentacle in 



securing prey? (Fig. 34, C.) 

 Digestion. — The food parti- 



I'lG. 37. — HYDRA rapturing a 



cles break up before remaining wa ter flea. 



