540 THE BIOLOGY OF MARINE ANIMALS 



species the light produced by chaetopterids and certain syllids is very 

 striking. There are few luminescent lamellibranchs and gastropods, but 

 many deep-water cephalopods are particularly noteworthy in the brilliance 

 of their displays and the complexity of their light-organs or photophores. 

 The specialization and widespread distribution of light organs in this group 

 are equalled only by teleost fishes, in which a great many different species, 

 especially those from mesopelagic waters, bear numerous photophores. 

 Only the ophiuroids among the echinoderms are luminescent. Light- 

 production has appeared sporadically in several crustacean groups, and 

 complex light organs and glandular structures occur in ostracods, cope- 

 pods, euphausiids, mysids and deep-sea decapods. There are many diffi- 

 culties in the way of discerning light-production in marine animals, and 

 many more luminous animals are still to be reported. An adventitious 

 mode of luminescence may here be mentioned. This concerns certain trans- 

 parent pelagic animals, such as small Crustacea, that appear luminous as 

 the result of ingesting phosphorescent food, which then shines through 

 their body wall. 



Luminous species occur sporadically in the littoral and sublittoral fauna, 

 under stones, on weeds and hidden in burrows and crevices. 



The fiery sparks those tangled fronds infold, 

 Myriads of living points; th'unaided eye 

 Can but the fire, and not the form, descry. 



Crabbe 



They are particularly abundant in the surface plankton, and in the bathy- 

 pelagic fauna. The remarkable phosphorescence of the surface of the sea is 

 frequently due to dinoflagellates such as Nocti/uca, Peridinium and Gony- 

 aulax, while medusae, ctenophores, euphausiids and copepods and 

 Pyrosomae also produce bright displays. 



F. T. Bullen, in The Cruise of the Cachelot, has given a vivid description 

 of one such occurrence — 



On the way, we one night encountered that strange phenomenon, a "milk sea." 

 It was a lovely night, with scarcely any wind, the stars trying to make up for the 

 absence of the moon by shining with intense brightness. The water had been more 

 phosphorescent than usual, so that every little fish left a track of light behind him, 

 greatly disproportionate to his size. As the night wore on, the sea grew brighter and 

 brighter, until by midnight we appeared to be sailing on an ocean of lambent 

 flames. Every little wave that broke against the ship's side sent up a shower of 

 diamond-like spray, wonderfully beautiful to see, while a passing school of por- 

 poises fairly set the sea blazing, as they leaped up and gambolled in its glowing 

 waters. ... In that shining flood the blackness of the ship stood out in startling 

 contrast, and when we looked over the side our faces were strangely lit up by the 

 brilliant glow. 



The eggs, early developmental stages and larvae of some light-producing 

 organisms are also luminescent. For example, segmentation stages of 

 ctenophore eggs produce light on stimulation, and ophiuroid plutei, 

 Chaetopterus trochospheres, euphausiid larvae and copepod nauplii are 

 luminescent (20). 



