J. A. C. NICOL 301 



ous system is already defined (Peters, 1905; Yatsu, 1912; Enders, 

 1909). 



With the highly sensitive multiplier phototubes now available, the 

 luminescent responses of some of these forms may be amenable to 

 photoelectric recording. They afford additional instances of direct 

 responses to environmental changes before the advent of nervous con- 

 trol and provide experimental material for the study of excitation and 

 effector activity at the cellular level. 



Luminescence and Nerve Net 



Many coelenterates, both pelagic and benthic, are luminescent. It 

 is characteristic of these animals that their responses to environmental 

 changes are controlled by a nerve net. This is to a large extent a 

 meshvvork of discrete neurones, across which two-way conduction 

 takes place. The quality and magnitude of response are governed by 

 various factors, prominent among which is neural facilitation. This is 

 a selective condition governing behavior by which seriated impulses 

 bring about responses which are out of all proportion to the appar- 

 ent effect of a single impulse. 



Luminescent responses in certain pennatulids will illustrate the 

 functioning of the nerve net. The response in these animals takes the 

 form of a flash of light which sweeps over the surface. In sea pens 

 Pennatida, a stimulus apphed at any point excites a wave or waves 

 which sweep away from the affected area. Similarly, simultaneous 

 stimulation at the two extremities excites convergent waves. Con- 

 firmatory experiments, showing that conduction is nonpolarized, have 

 been carried out on Renilla, in which animal it has been shown that 

 complex cuts, producing devious pathways, still allow transmission 

 (Panceri, 1872a; Parker, 1920a, b). 



In many coelenterates several impulses are necessary to evoke a 

 response owing to the intervention of facilitation. Harvey noted that 

 Cavermilaria usually failed to respond to a single shock, but gave a 

 flash after three shocks in rapid succession. In Renilla it is found that 

 several shocks are necessary to elicit a luminous wave, and with 

 continued stimulation the consecutive flashes increase progressively in 

 intensity (Fig. 1). Both the aforementioned features (ineffectiveness 



