LUMINOSITY. . XXV 



temperature of the water, and mixing iron, tan, or gall nuts has also been 

 found to induce this change of colour (see vol. ii, p. 167). The yellow 

 colour is- due to incipient albinism, for the chromatophores which normally 

 contain dark ingredients are here, filled with yellow pigment, or the first 

 step towards an entire loss of colouring matter when they become albinos. 



Dr. Stark observed that if fish are kept in glass vessels of various colours 

 such occasions a tendency to their assuming the colour of the vessel in 

 which they reside, which would aid them in concealment. Mr. Nesbit 

 concluded that light merely modifies an existing pigment which is absent in 

 pink or albino fish, but which it is powerless to manufacture. 



The colour of the flesh may be influenced, as in other groups of animals, 

 by the food consumed, minute crustaceans, as gammari are reputed to colour 

 that of the salmonidse : while white flesh is occasionally the ' result of 

 insufficient food. Ill-health similarly has an etiolating effect. 



LUMINOSITY. 



Another subject closely connected with the tegumentary system is the 

 existence of certain metameric organs. There are some pelagic or deep-sea 

 fishes, as Argyropelecus (plate cix, fig. 1), Stemoptyx, Ichthyococcus, Mauro- 

 licus, Gono'stoma, Ghauliodus, Stomias, &c, which possess luminous organs 

 of a circular form, some being as impressions, others as slight prominences 

 of the skin. A British form exists in the " pearl-sides" of Yarrell, Maurolicus 

 Pennantii (plate cix, fig. 2), which is Pennant's Argentine. These brilliant 

 spots may be present along the entire length of the abdominal surface or 

 even . be seen on the head, but their uses have been subject to much 

 speculation. M'Culloch remarked that, considering at 800 or 1000 feet 

 depth the light of the sun ceases to be transmitted in the ocean, can animal 

 luminousness be a substitute for that light ? May it not be the means of 

 enabling its possessor to discover its prey or for its fellows to find one 

 another ? Perhaps a partial confirmation of this view exists in the fact that 

 fishes living at great depths or dark places, as in the Mammoth Caves of 

 Kentucky or the recently discovered caverns in Algeria, are found to be 

 destitute of eyes ; but they have no luminous organs, while the deep-sea 

 forms which possess them' have large eyes. This difference would seem 

 to len<J strength to the view that these organs may b*e for the use of 

 the individuals which are furnished with them, for where no eyes exist these 

 organs arc absent. 



A few years ago it was advanced that these spots were accessory eyes, 

 possessing a species of lens posterior to the cornea, and a chamber behind 

 the lens -containing fluid and having a dark lining, which may serve as a 

 re.tina : the second form shows a simple glandular structure from which light 



