Nervous System of Medusoids 5 i 



pended, and the tension of the muscles of the 

 bell and velum is relaxed; but when the 

 potential energy of the cell is restored, external 

 stimuli produce another discharge of nervous 

 energy, followed as before by contraction of 

 muscular fibres. So long, therefore, as the 

 fundamental properties referred to are carried 

 on by the living matter of the system, and a 

 supply of potential energy is secured, regularly 

 recurrent contractions and relaxation of the 

 muscular fibres of the bell and velum take 

 place, whereby the swimming movements of the 

 animal are effected. 1 We thus come to appre- 

 ciate the force of Professor Lloyd Morgan's 

 oft-repeated remark that the instinctive be- 

 haviour of animals " depends upon the inherited 

 structure of their nervous system." 



It is, however, evident that unicellular beings 

 such as Amoeba, Stentor, and Euglena, al- 

 though they do not possess a nervous system, 

 nevertheless respond by appropriate purposive 

 movements to various forms of energy applied 

 to their body-substance or living protoplasm, 



1 Halliburton 's Handbook of Physiology, p. 191, Seventh 

 Edition. 



D 2 



