Origin of Nerve Cells 37 



of protoplasm, inherent mnemic and instinctive 

 elements. Part of the energy set free by stimuli 

 acting on the palpocils and the living substance 

 of their cylindrical cells passes to corresponding 

 nerve cells, and by means of their instinctive 

 elements is transformed into purposive nerve 

 force, which extends to groups of muscle cells, 

 causing them to contract in a co-ordinate 

 manner, and thus to give rise to instinctive 

 movements of certain parts of the animal's body. 

 It is, therefore, through the instrumentality 

 of a system consisting of a sensory organ, 

 nerve cell, and muscular fibres, that the 

 automatic or instinctive movements of the 

 animals to which we have referred take 

 place. 



Between the bases of the cylindrical cells of 

 Hydra are a number of ovoid cells, some of 

 which become developed into germ cells, others 

 are so much elongated at one end as to form a 

 fibre armed at its base with a sharp spine. 

 (Fig. 2.) By a process of introversion of the 

 wall of the cell this fibre comes to assume a 

 spiral form within the cell, with one of its ends 

 projecting outwards on the surface of the 



