SENSES OF ANIMALS 303 



in the larval stage over winter, hiding in the mud at the 

 bottom of the ponds. One of the urodeles, the axolotl, 

 usually continues in its larval condition throughout life. 

 The drying up of the water causes it to assume the adult 

 form when it is known as an amblystoma. Cold and lack 

 of food may prolong the larval life of some species several 

 months. 



Among the higher vertebrates the developing young, up 

 to the time when it assumes the characteristic form of 

 the parent, is called the embryo. 



21. SENSES OF ANIMALS. 



Sense of Feeling. There are probably no animals, 

 however simple in structure, which do not respond to cer- 

 tain kinds of stimuli. While it is true that none of the 

 protozoa or sponges and but few of the worms and mol- 

 lusks can see or hear, yet the sense of feeling seems to be 

 present in all animal life. The amoeba can distinguish 

 between a particle of sand and a bit of flesh. The Para- 

 mceeium brought from darkness into bright light responds 

 by vigorous movements, owing doubtless to the fact that 

 the protoplasm of all parts of the body is sensitive to the 

 light rays, but the animal can not be said to see. The 

 more complex the nervous system the more acute is the 

 sense of feeling, which is the only sense with organs 

 widely distributed over the body. The most sensitive 

 parts in the Hydra and sea anemone (Fig. 342) are the 

 tentacles. The loss of a leg or wing or even of a portion 

 of the body of an insect seems to cause the creature little 

 or no pain. With a pair of sharp scissors one may 

 cut a third of the abdomen off the mosquito while it is 



