36 Instinct and Intelligence 



Palpocils, projecting as they do from the 

 outer free surface of cylindrical cells, are well 

 adapted to receive, and transmit any tactile 

 impressions made on them to the protoplasmic 

 contents of the cell of which they form a part. 

 In this way the palpocils with their cells con- 

 stitute a rudimentary sensory organ or receiver 

 of energy from the outer world ; they act some- 

 what after the fashion of a trigger, and when 

 stimulated discharge a part of the potential 

 energy stored in the living substance of the 

 cylindrical cells. These cells are constantly 

 kept primed with energy derived from the 

 metabolic processes carried on by their proto- 

 plasmic elements. Energy thus released from 

 the cells becomes manifest in definite move- 

 ments of the Hydra's body or tentacles; be- 

 cause the deep or attached part of these cells is 

 prolonged into a system of nerves and con- 

 tractile structures. 



Beneath the ectoderm of Hydra nerve cells 

 have been detected. These cells are in direct 

 communication with in fact, are a part of 

 the living substance of cylindrical cells, and 

 possess therefore, in common with other kinds 



