LUMINESCENCE 575 



squid whose light-organs are overlaid by chromatophores, as Leachia, 

 Watasenia and Abraliopsis. The firefly squid Watasenia scintillans occurs 

 in large numbers off the Japanese coast in spring and early summer, and 

 has been available for examination alive. The large brachial light-organs of 

 this species emit short flashes, lasting some 30 sec, with varying periodicity. 

 Luminescence of other photophores (ocular, mantle series) varies in in- 

 tensity from time to time, but may persist for 20 min or more. Mechanical 

 and electrical stimulation causes these organs to luminesce strongly. 

 Chromatophoral movements are probably involved in control of light- 

 emission from photophores of Watasenia. The pigment cells of cephalopods, 

 it will be recalled, are activated by muscle fibres (p. 508), and contraction 

 and expansion of the pigment cells can control emission of light from 

 underlying photophores. Whether light-production is controlled directly 

 by the nervous system, as well, has not been clearly established. 



Indirect control of lighting by chromatophores also occurs in certain 

 teleosts, for example Coelorhynchus and Hymenocephalus, in which light 

 shines through the skin from light-organs situated at deeper levels. The 

 amount of light emitted can be increased or decreased by concentration or 

 dispersion of chromatophores. These structures are effectors per se, 

 and are subject to various degrees of nervous control in teleosts (24). 



Some of the photophores which have been described above reveal a 

 pattern of optical structure remarkably similar to that of an eye, and this 

 has given rise to the suggestion that a device for regulating the focus may 

 be present in some photophores. The photophores of both Nyctiphanes 

 and Abraliopsis are complex structures in which the lenses are invested by 

 a circular ring of tissue that could have a focusing function, but the idea 

 is wholly conjectural, and it is uncertain whether muscle is even present in 

 those regions. 



Effect of Ionic Environment on Luminescence 



Anisosmotic solutions and unbalanced salt solutions have been tested 

 on many luminescent species. Dilute sea water or fresh water usually 

 evokes luminescence, and fresh water was a favoured means of causing 

 animals to luminesce (coelenterates, polychaetes and many others). 

 Hyperosmotic sea water is without effect on Chaetopterus. Isosmotic 

 solutions of single salts have effects as follows. K+ excites luminescence 

 in Pe/agia, Cavernu/aria, ctenophores, Chaetopterus and polynoids (pro- 

 longed glow). Na+ causes luminescence in Chaetopterus, polynoids 

 (quick flashes) and Ophiopsila (Na + in excess). Ca ++ produces a state of 

 hyperirritability in Pelagia, ctenophores and polychaetes. Mg ++ has a 

 narcotizing effect. Ca ++ and Mg ++ counteract the excitatory effect of 

 Na+ but not K+ in excess (polynoids). By using the inert substance choline 

 chloride it can be shown that K+ excites only in excess (>0-05 m KC1). 

 The excitatory effect of Ca ++ may be due to absence of Mg ++ . In ex- 

 periments of this kind the several ions may be acting at several loci, 

 namely peripheral receptors, nerves and photogenic cells. In polynoid 



