386 



THE BIOLOGY OF MARINE ANIMALS 



shocks (intervals of 0-5-1-5 sec), a contraction is produced in the sphincter 

 muscle of the oral disc. There is no response to the first stimulus, whereas 

 responses to subsequent stimuli gradually increase, resulting in a staircase 

 effect. The strength of the contraction is independent of the strength of 

 the electrical stimulus, above threshold, and the whole system appears to 

 work as one functional unit. For facilitation to occur the stimuli must be 

 spaced within 3 sec of each other, and by increasing the frequency of 

 stimulation a more rapid increase of the response is obtained (Figs. 9.10, 

 9.11). Since lengthening the interval results in a small increment or stair- 

 case it follows that facilitation is subject to temporal decay, and quickly 

 falls from the initial level obtaining at the moment of stimulation. The 



Fig. 9.10. Responses of Calliactis Sphincter to Electrical Stimulation 



(a) Paired shocks at intervals of 2, 1 and 0-5 sec; (b) same after 60 min in sea water 

 + 0-4 m CaCl 2 (95:5); (c) above, response of sphincter in sea water to stimuli at 1/sec; 

 below, after 30 min in sea water + 0-4 m CaCL (95:5) (from Ross and Pantin (108).) 



neuromuscular junction is sensitive to changes in the ionic environment, 

 calcium enhancing and magnesium depressing neuromuscular facilitation 

 (108). 



The coelenterate nervous system is organized as a diffuse, often non- 

 polarized network, in which through-conduction tracts occur. Stimulation 

 of one such tract in the mesenteries of Calliactis produces impulses which 

 are propagated throughout the system to the entire sphincter muscle. 

 Each impulse produces a transitory change at the neuromuscular junction, 

 and facilitates the passage of subsequent impulses. The increase in mag- 

 nitude of response is brought about by recruitment of muscle fibres, as 

 facilitation progressively brings more of these into activity (91). 



A comparable process of facilitation occurs in the circular muscles of 

 jellyfish {Amelia, Cyaned). These muscles, responsible for the swimming 



