LUMINESCENCE 



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and Astronesthes, the photophores are numerous and are arranged in 

 rows or groups along the body (Figs. 13.8, 13.18). Sometimes there is a 

 large cheek organ and tentacular light organ as well (Photonectes, Gram- 

 matostomias). These structures consist of a mass of photogenic gland cells, 

 often backed by a reflector, and sheathed in a pigmented screen. Towards 



Fig. 13.16. A Luminescent Shark Spinax niger, with the Luminous 

 Areas Lighted Up. (From a drawing by Horsfall in Dahlgren, 1917.) 



Pigment 

 layer 



Light 

 cells 



Fig. 13.17. Light Gland of the Shark Spinax niger. 

 (After Johann (35), and Dahlgren, 1917.) 



the free surface there is a lens for concentrating the light produced (Fig. 

 13.19). In certain other forms, such as Cyclothone and Gonostoma, the 

 photophores are similar in structure but are provided with a long duct 

 leading to the exterior (Fig. 13.20). Morphologically the various structures 

 are derived from the epidermis. In the midshipman Porichthys notatus, a 

 shallow-water fish of the west coast of North America, Greene has traced 

 the development of the photophores from free epithelial buds which are 



