NERVOUS SYSTEM AND BEHAVIOUR 



425 



produced are components of the normal feeding reaction, and are called 

 up in succession through facilitation. The graded responses which can be 

 elicited in the disc are the result of interneural facilitation between parts 

 of the nerve-net, each impulse paving the way for entry of its successors 



Sphincter 

 muscle 



Perfect 

 mesentery 



Retractor 

 muscle 



Parietal 

 muscle 



Fig. 10.4. Diagrammatic Vertical Section through the 

 Sea-anemone Metridium. (From Pantin (82).) 



Circular 

 muscle 



_ Ciliated 

 siphonoglyph 



Parietal 

 muscle 



Seconds 



Fig. 10.5. Facilitation in the Marginal Sphincter of Calliactis 

 parasitica as the Result of Repetitive Electrical Stimuli 

 Figures refer to intervals (in seconds) between successive shocks. (From Pantin, 1935.) 



into other sectors of the disc. Conduction in the nerve-net of the disc, 

 therefore, involves facilitation between parts of the nerve-net, in addition 

 to neuro-muscular facilitation. Apparent decremental conduction in the 

 disc is due to decrease in number of impulses as a volley of the latter 

 spreads out from the point of stimulation. Besides facilitated fast retractor 



M.A. 



■14* 



