210 



ANIMAL BIOLOGY 



Q^ 





B 



that we judge the capacity of the sense organs of other animals 

 merely by comparing them with our own. This is a safe procedure 

 only in the case of sense organs that more or less correspond 



in structure to those which we possess. 

 In the Crayfish, for example, we find com- 

 plex sense organs which, without doubt, 

 are eyes, and others which are ears, or 

 at least 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 pres- 

 ent, but here we are on less sure ground. 

 It is, indeed, almost certain that environ- 

 mental 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. But Man's ingenuity 

 Fig. 145. — Diagram of . . . 



stages in the differentiation has in certain cases devised apparatus to 

 of sense cells. A, primitive minimize his limitations: witness the radio 



sensory neuron of Hydra- receiver> ( Figs< 54 63 ) 



like animals; B, sensory . N ° 



neuron of a Mollusc; C, The simplest form of sense organ in 



primary sensory neuron of Vertebrates is a single epithelial cell for 



a Vertebrate. In each case ^ rece p t j on f stimuli, connected with a 

 the sensory surface is rep- Jl . 



resented below, and there- nerve fiber for the conduction of the 

 fore the nerve impulse nerve impulse to a sensory center, 

 passes upward. (From Usually, however, many associated cells are 



arranged to respond and are aided by ac- 

 cessory structures for intensifying the stimulus, protection, etc., so 

 that the whole forms a highly complex sense organ. (Figs. 145, 150.) 



1. Cutaneous Senses 



Confining our attention to the Vertebrates, we find that prac- 

 tically the entire surface of the body constitutes a sense organ, be- 

 cause the skin is permeated with a network of sensory nerves. 

 Certain regions are supplied with special pressure receptors, 

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

 as the lateral line organs of Fishes and Amphibians, or of 

 groups of tactile corpuscles as in Man. In addition to pressure 



