PHOTOGENIC MARINE ANIMALS. 15 



types, especially those in tropical and subtropical waters. 

 Certain species are stated to give out a bright red light from the 

 nucleus, the photogenic granules formed in the blood-corpuscles 

 being oxidized by the oxygen dissolved in the blood-plasma. 

 Amongst the multitudes of Salpa (S. runcinata and S. spinosa) 

 examined for some weeks in the Outer Hebrides, no clear case 

 of luminosity was seen. A spark occasionally appeared in the 

 nucleus of some specimens, probably from the food. Doliolum, 

 on the other hand, is described by Giglioli as shining with a 

 greenish luminosity over the whole surface. 



So far as can be observed, there is nothing in the habits 

 of the luminous Tunicates to distinguish them from those of the 

 non-luminous. 



The literature of the so-called phosphorescent fishes extends 

 from the days of Aristotle and Pliny to modern times, Ehren- 

 berg, for instance, giving a list of about fifty fishes stated to be 

 luminous, though a considerable number are doubtful. Our 

 knowledge of the photogenic properties of living fishes, on the 

 other hand, is for the most part comparatively recent — in fact, 

 is mainly due to the voyage of the ' Challenger.' These luminous 

 fishes divide themselves into two groups, tiz. pelagic fishes and 

 deep sea or abyssal fishes (below one thousand fathoms). So 

 far as known, no littoral fish and few gregarious food-fishes 

 (Teleosteans) show this property in any marked degree during 

 life. 



In many of these luminous forms the mucous canal-system of 

 the head and body is largely developed, and the mucus which it 

 secretes is phosphorescent ; but in others special photogenic 

 organs are present, the distribution of which is subject to great 

 variety. De Kerville* has collected no less than nine modi- 

 fications of the disposition and structure of those special organs 

 of luminosity. In the first or primitive condition they appear as 

 numerous minute tubercles scattered over the skin, and covering 

 the sides of the body (Ex. Pachystomias) . 2nd. Larger but 

 less numerous nodules on the skin of the head and body (Xeno- 

 dermichthys). 3rd. Two rows of reddish or green eye-spots 

 ranged at regular intervals on the sides inferiorly, on the head, 

 at the base of the branchiostegal rays and the operculum 

 * ' Challenger,' vol. xiii. p. 162. 



