134 THE SENSES OF ANIMALS 



particles that act as balancing organs. Nerve cells are con- 

 centrated at the sense organs and convey impulses to the 

 rest of the nerve network. The sense organs are particularly 

 concerned with sending stimuli to the muscle cells that cause 

 the pulsations of the umbrella so that the jelly-fish swims 

 and avoids sinking in the water. In other kinds of jelly-fish 

 the nerve network is more highly modified than in Aurelia, 

 and in addition to the diffuse network some nerve cells are 

 concentrated into a definite nerve ring round the edge of the 

 umbrella. Jelly-fish such as these show a simple form 

 of the central nervous system characteristic of the higher 

 metazoa in which a concentration of nerve cells controls the 

 activities of the whole animal. 



Whether an animal has a nervous system consisting of a 

 diffuse network or of a central nervous system connected 

 with the different parts of the body by bundles of nerve 

 fibres, the stimuli from outside are transmitted to the nervous 

 system in essentially the same way by the sense organs, simple 

 or specialized. The nervous impulse is basically a physico- 

 chemical change, and must be started by a similar change at 

 the place where the nerve leaves the sense organ. The sense 

 organs are thus devices for translating stimuli from outside 

 into physico-chemical changes that can act upon the nerve 

 ending and start the change in the nerve fibre that trans- 

 mits the message. The organs of touch, sight, hearing, smell 

 and taste all depend upon similar actions for starting the 

 transmission of their messages to the rest of the animal 

 body; all the complicated special sense organs such as eyes 

 and ears are adapted for translating received stimuli of 

 light, sound, and so on into the necessary physico-chemical 

 changes. The sense of touch seems to be the simplest of the 

 senses, for pressure on a sensory cell is sufficient, probably 

 by the distortion it produces, to cause a minute change 

 sufficient to stimulate the nerve ending in contact with it. 



The stimuli received from the sense organs cause an 

 animal to react appropriately to changes in the environment 

 — to approach and take food, or to avoid an enemy, or to 

 move away from too great cold or heat and so on. Although 

 our senses are the same as some of those of other animals 

 we must not think that all animals are consciously aware 



