288 



THE VOYAGE OF H.M.S. CHALLENGER. 



phosphorescence, clouds of light pass over the body. This may be compared to the 

 changing of the colour in Sepia, where the different hues travel over the body like 

 shadows of clouds. 



e. Development. 



Remarkable structures, like those represented in PI. LXXII. fig. 35, are occasionally 

 met with, which seem to indicate that these organs multiply by division. As the fish 

 grows in size, and the area to be occupied by these organs extends, it appears that they 

 multiply, and that they are not formed spontaneously in the skin of the adult fish at all. 

 No indications of a spontaneous formation of these organs have been observed. 



From which organs in the skin these phosphorescent apparatus were originally 

 developed phylogenetically it is difficult to say. It appears most probable that they were 

 developed from small slime-glands of the skin, such as are found in all fish. The slime 

 of some Batrachians is luminous, and so the slime produced liy the small glands in 

 the skin of some fishes may have hy chance become slightly phosphorescent. This may 

 at a certain time, particularly when the fish in question took up its abode in great 

 depths, have become advantageous to them, and with tlie demand for a luminous slime 

 the glands would naturally have been modified in the course of time, so as to produce 

 slime more and more luminous, and afterwards, the duct leading from the gland out- 

 wards may have been closed and the luminous slime retained. The slender cells forming 

 the upper stratum of these organs are newly formed coenogenetic structures, and indicate 

 that these organs are already very highly differentiated. 



These regular, ocellar, simple phosphorescent organs without a pigment coat are of 

 particular interest, as from them all the other forms may be supposed to have developed. 

 There is no doubt that these organs are by far the simplest of their kind hitherto 

 observed, and they appear to have retained the original character, which has been 

 much changed and further modified in the cases of the other kinds of regular ocellar 

 phosphorescent organs. 



Their phylogenetic position is elucidated in the following table : — 



Composite, with reflector. 



Regular ocellar phosphorescent organs. - 



Composite, without reflector. 



Simple, with pigment coat. 



Simple, without pigment. 



Small slime-glands of the skin. 



