312 



PHYSIOLOGICAL CONTROL IN ANIMALS 



Intensity of Response 



The intensity of response is influenced in two ways, either or both 

 of which may be operative in the same animal: these pertain to 

 summation and facihtation. Consider first Chaetopterus, an animal 

 which discharges luminous matter (extracellular luminescence). The 

 glow from this secretion lasts for as long as five minutes (Fig. 8). 

 Now, facilitation is not operative peripherally in the luminescent 

 responses of this animal, but with the protracted mode of response, 

 summation occurs readily at low frequencies of stimulation (above 2 



B 



M ill ;; ; ; ; 1 1 1 ) 1 1 1 



Fig. 9. Luminescent responses of Pyrosoma, a colonial pelagic tunicate. A, elec- 

 trical stimulation with 1 and 4 condenser shocks (the latter at 1 per second). 

 B, 2-second burst of electrical stimuli at 5 per second. C, response to flash 

 of light (pocket torch); approximate position of stimulation added to lower 

 hue. D, responses to weak (left) and stronger (right) tactile stimulation 

 (vibration). Amplification of A ten times that in B-D. Time scale 1 per 

 second. 



per minute). Increased intensity of response results, therefore, from 

 several or many stimuli, compared with one, and from short com- 

 pared with long interval between impulses (Nicol, 1952b,c, 1954b). 

 Similar response characteristics are encountered in Pyrosoma (Fig. 

 9). In this animal the response lasts some 4 seconds after a single 

 electrical stimulus. Facilitation is not operative, at least not at slow 

 frequencies of 1 per second. Prolonged stimulation at high frequencies, 

 however, results in augmentation of light intensity as the result of 

 summation of individual responses. Similarly, a strong mechanical 



