572 THE BIOLOGY OF MARINE ANIMALS 



meridian, are produced by stimulation of nerve fibres, or by direct excitation 

 of the photocytes (14). 



Among polychaetes the polynoids have been studied rather extensively. 

 When a worm is stimulated mechanically the area touched begins to 

 flash, and the luminescence then spreads rapidly anteriorly and posteriorly 

 from this point. Electrical stimulation causes the whole worm to lighten, 

 and with alternating current separate responses corresponding to the 

 electrical pulses can be observed. Prolonged stimulation causes the light 

 to fade, but recovery occurs after several hours. When an animal is tran- 

 sected only the posterior fragment lightens, while the anterior portion 

 remains dark. Each scale receives a nerve which enters a ganglion and gives 

 off fibres which radiate outwards to the light cells. According to Bon- 

 homme (5), luminescence in Harmothoe is evoked through reflex pathways 

 involving peripheral tactile receptors and the ventral nerve cord. An elytrum 

 removed from the body gives a bright flash due to mechanical stimulation 

 of the severed nerve fibres but is no longer responsive to reflex tactile 

 stimulation. 



The luminescent responses of polynoids have been investigated in detail 

 by means of photo-electric recording (45, 46). An isolated scale subjected 

 to a single electrical shock usually responds by a series of repetitive flashes 

 which continue for a second or more (Fig. 13.27(a)). Flashing begins at 

 a frequency of five or more per second, then falls off to a steady level 

 which is maintained for some time. The initial flashes also increase rapidly 

 in intensity, owing to a rise in the level of excitation whereby the second 

 and third responses far exceed the first. Continued flashing, either from 

 mechanical or electrical stimulation (a single shock), is due to repetitive 

 discharge from the elytral ganglion, and the progressive increment in flash 

 intensity depends upon some facilitatory process in the neuro-effector 

 complex as well as summation of light intensity. 



Both the tentacles and the body of Polycirrus caliendrum luminesce 

 under stimulation. The response appears to be very localized, each tentacle 

 lighting independently of the others. In chaetopterids earlier workers 

 reported that luminescence proceeds over the surface of the animals 

 following stimulation. In Mesochaetopterus the light is sustained in the 

 middle body region, but flashes in alternate segments of the posterior 

 region. Electrical stimulation of the tentacles evokes a luminescent res- 

 ponse in other regions of the body, thus providing evidence for the trans- 

 mission of excitatory processes concerned with lighting through the ventral 

 nerve cord. 



The luminescent responses of Chaetopterus have also been analysed by 

 photoelectric methods (43, 44). Light-production is under nervous control, 

 and a single nervous impulse excites the glandular cells, leading to ex- 

 pulsion of a luminous secretion. Following stimulation the light intensity 

 rises to a maximum in about 10 sec and luminescence persists for 5-10 

 min (Table 13.3, Fig. 13.27(6)). Repetitive stimulation leads to augmenta- 

 tion of the response, due to summation of the contractile processes in the 



