ECTODERM 



95 



are such that the arrival of an impulse which is not strong enough 

 to jump across makes it easier for the next impulse to do so, a 

 phenomenon known as facilitation. The result is that a strong 

 stimulus affects more muscles than does a weak one, and in 

 particular it affects muscles at a greater distance, as is very 

 obvious when a tentacle is touched. A further result of facilitation 

 is that a repeated weak stimulus may have the same effect as a 

 single stronger one. Both the sense cells and the muscles vary in 

 their sensitivity from time to time, so that the animal does not 

 always react in the same way to the same stimulus. A well-fed 



Sensory cell 



Nucleus of musculo - epitheUal cell 



Fig. 63. — Sensory cell protruding through an epitheliomuscular cell of 

 Hydra oligactis. X 600. — Redrawn from McConnell. 



hydra, for instance, cannot easily be induced to give the feeding 

 reaction (p. 97), and repeated stimulation induces fatigue. This 

 primitive type of nervous system allows of much less precise 

 movements than that in which a particular sense organ is pre- 

 ferentially connected with a particular muscle by means of a 

 nerve. So far as is known, the sense cells are not specialised to 

 serve particular senses, but this, as we shall see later, is largely true 

 even of the sense cells (as distinct from the sense organs) of man. 

 A description of the behaviour of Hydra condensed for a text- 

 book necessarily gives the impression that it is a somewhat rigid 

 matter of responses to stimuh, but a little observation of the 

 creature under the microscope will show that there is both much 

 spontaneity and much individual variation. Indeed, a great 



