70 BIOLOGY: GENERAL AND MEDICAL 



throughout the entire cell, all parts of which are affected 

 by the disturbance. 



When vorticella is touched, the irritation is immedi- 

 ately transmitted to the pedicle, which contracts suddenly 

 and violently, withdrawing the animal from the harmful 

 influence. In Carchesium, disturbance of one individual 

 may result in contraction of all the stalks so that the 

 whole colony is withdrawn from the source of stimulation. 



When a tentacle of one of the hydroids discovers some 



PIG. 17. Vorticella. a, Extended; 6, contracted; c, section of the stalk showing 

 the elongate contractile (muscle) fibre in its interior. (Verworn.) 



object useful for food, the stimulation is quickly imparted 

 to other tentacles in order that it be firmly grasped and 

 brought to the oral opening. 



In the higher animals whose movements must be 

 precise and well-coordinated, this simple transmission of 

 irritable reaction from cell to cell is inadequate because 

 too slow to meet the requirements, and is replaced by 

 specialized cells, nerve fibres, greatly elongated in form, 

 and able to establish immediate communication between 



