444 GENERAL BIOLOGY 



hormones, therefore, appear to be an important part of the 

 self-regulating mechanism. 



Sense organs. — The exigencies of animal existence 

 demand that the functions of relation to the outside world 

 should be performed as speedily as possible — that stimuli 

 should be quickly received and transmitted and translated 

 into useful acts. 



Now the stimuli are of various sorts. Some, like the pull 

 of gravity or the change of temperature act constantly or 

 slowly, and there may be no special sense organs for their 

 reception. Others of mechanical, chemical and vibratory 

 nature have caused the development of sense organs of the 

 most specialized sorts. 



1. Mechanical stimuli produce the sensation of touch. 

 Special receptors for mechanical stimuli are scattered about 

 over the bodies of all the higher animals. They abound 

 in the tips of outgrowing organs that are specially exposed 

 to contact with external objects. The earlike-lobes at the 

 front end of a planarian, and the prostomium of an earth- 

 worm are very sensitive to touch. In our own skin tactile 

 organs are least numerous in the middle of our backs and 

 most abundant in the tips of fingers and tongue. 



2. Chemical stimuli give rise to sensations of taste and 

 smell. Some sort of discernment of the difference between 

 edible and inedible substances is well-nigh universal among 

 animals. Since food is limited in quantity and distribution, 

 there is need that it should be recognized. In an aquatic 

 organism doubtless these two classes of sensation are not very 

 different in kind ; indeed they are not always sharply distin- 

 guished in ourselves. One may bite of an onion and not be 

 very sure whether his impression of the thing is mainly of 

 its taste or its smell. Volatile particles may travel through 

 the air and so may reach the olfactory organ from a distance. 

 Hence the sense of smell is less exclusively subservient to food 



