570 



THE BIOLOGY OF MARINE ANIMALS 



cutting the sea-pen in various ways, as long as a connecting bridge is left 

 between the pieces to form a nervous pathway. 



With electrical stimulation usually several shocks are necessary to evoke 

 a response, and with continued stimulation successive flashes increase 

 progressively in intensity up to some plateau level (Fig. 13.26(a), (&)). In 

 part, this increase in intensity of consecutive flashes results from facilita- 

 tion at the neurophotocyte-junctions, each impulse in a series recruiting 



TABLE 13.4 

 Intensity of Light Emitted by Some Marine Animals 



(Various sources) 



additional photocytes (cf. p. 385). Following prolonged stimulation the 

 animal passes into a hyper-excitatory state, in which it may continue 

 flashing repetitively long after stimulation has ceased (Fig. 13.26 (c)). 

 Presumably the neurones of the nerve-net, when a sea-pen is in this con- 

 dition, are capable of maintained repetitive discharge (17, 47, 48). 



In ctenophores strong tactile stimulation excites a luminous response 

 in all the ciliary rows. If one pole is stimulated a luminous wave travels 

 towards the opposite pole, and if the middle of a row is stimulated luminous 

 waves proceed along the meridian towards both poles. The luminescent 

 response is readily fatigued but recovers after a period of rest. Trans- 

 mission of excitation takes place in a non-polarized nerve-net accompany- 

 ing each meridional canal. Transection of the canal blocks the passage of 

 the luminescent wave. Tactile receptors for the luminescent response are 

 limited to the eight radial canals. In addition, photoreceptors are believed 

 to establish connexions locally with the photocytes and to be responsible 

 for inhibiting luminescence when the ctenophore is illuminated. 



Electrical stimulation of a whole meridian evokes repetitive brief flashes 



