II HABITS AND NATURAL HISTORY OF THE FROG 33 



Stimuli that irritate the surface of the body are gotten rid 

 of in different ways. If the eye is touched, it is quickly drawn 

 into the head and covered by the lower eyelid. Curiously 

 enough, the same action is performed if the nose is touched 

 or any part of the head near the eyes. Stimuli on the right 

 side cause the right eye to wink, or if the stimulus is on the 

 left side, the left eye responds. The fore foot is often 

 brought forward to remove the stimulus, the foot on the 

 side stimulated being always employed. Stimuli applied to 

 the side of the body often cause the hind foot to be brought 

 forward to the stimulated spot. There is also a twitching 

 of the muscles of the side of the body near a stimulated spot 

 which reminds one of the twitchings produced by the skin 

 muscles of a horse. If the tip of the urostyle is irritated, the 

 heels of both hind legs are brought up to that point. A frog 

 may be caused to repeat these reactions many times, as a 

 rule, but after a while it attempts to avoid further persecu- 

 tion by hopping away. 



Seasonal Changes. — The frog undergoes an unusual 

 amount of change in relation to the different periods of the 

 year. One reason for this is the fact that it is a cold-blooded 

 creature and cannot maintain itself in the same condition 

 winter and summer, as, for instance, is done by man. Its 

 condition changes markedly with reference to differences in 

 temperature to which it adapts itself. Another reason for 

 periodic changes is the ripening of the reproductive cells, 

 which, especially in the female, makes extensive draughts 

 upon the stored-up nutriment of the body. Then there are 

 the changes correlated with the recurrence of the period of 

 sexual activity, such as the development of the nuptial excres- 

 cences on the thumb of' the male, the occurrence, in some 

 species, of papillae on the back and sides of the female, and 

 the breeding instincts, which appear only at this time. 



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