134 



HISTOLOGY 



on each side of the body, near the siphon. The cells resemble fat cells 

 in form, the content of the vacuole being luciferase. 



The fishes are among the animals that produce light. The power is 

 found in some simple as well as some highly developed forms in this class. 

 As is true of the other classes, but few of the species and genera are pho- 



tr.e.c. 



lum. c. 



FIG. 122. Part of a vertical section through the luminous organ of Photinus marginalis. 

 lum.c., luminous cells; ref.c., reflector cells; tr., tracheae; ter.t., terminal twig of tra- 

 cheae, tracheoles not visible; tr.e.c., trachea! end-cells; tr.ep., tracheal epithelium. 

 X 1000. 



togenetic, in proportion to the great numbers that exist. The organs are 

 found in perhaps ten selachian forms and in very many more teleosts of 

 most of the larger groups. The number of light- producing teleost species 

 probably reaches into the hundreds. 



The selachians show the simplest forms, and we shall describe, as an 

 example, the light tissues of Spinax niger, a small Japanese shark which 

 glows brightly in the dark. The skin of this fish shows, morphologically, 



