COORDINATION IN ANIMALS 195 



perform one of the functions of our ears, equilibration; while 

 some of the head appendages are particularly adapted to 

 receive sensations of touch. The senses of smell and taste are 

 also probably present, but here we are on less certain ground. 

 It is possible, perhaps probable, that environmental changes 

 which are without effect on the sense organs of the human 

 body, and so play no recognizable part in the 'world' of 

 Man, may stimulate receptors in lower organisms. 



The simplest form of sense organ in Vertebrates is a single 

 epithelial cell for the reception of stimuli, connected with a 

 nerve fiber for the conduction of the nerve impulse to a sen- 

 sory center. Usually, however, many associated cells are 

 arranged to respond and are aided by accessory structures for 

 intensifying the stimulus, protection, etc , so that the whole 

 forms a highly complex sense organ. (Figs. 108C, 112.) 



1. Cutaneous Senses 



Confining our attention to the Vertebrates we find that 

 practically the entire surface of the body constitutes a sense 

 organ, because the skin is permeated with a network of sen- 

 sory nerves. Certain regions are supplied with special tactile 

 organs, which may take the form of a regular system of sense 

 organs, such as the LATERAL LINE ORGANS of Fishes and Am- 

 phibians, or of groups of TACTILE CORPUSCLES as in Man. In 

 addition to pressure receptors, the whole surface of the human 

 body is provided with PAIN, HEAT, and COLD SENSE SPOTS. 



2. Sense of Taste 



In the higher Vertebrates the sense of taste is restricted 

 to the cavity of the mouth, particularly to the tongue, where 

 special receptors known as TASTE BUDS are in communication 

 with the brain by two of the cranial nerves; but in some Fishes 

 they are scattered quite generally, so that the whole body 

 surface is sensitive to such qualities as sweet, sour, and salt. 



