LUMINOUS ORGANS 745 



however, special luminous organs are evolved for the production of the 

 photogenic materials. 



In extracellular biohiminescence, gland-like organs on the surface of the 

 body secrete a photogenic material which becomes luminous on contact with 

 the oxygen of the air or the sea-water. Such glands may be unicellular or 

 multicellular. This mechanism accounts for the luminescence of Coelente- 

 rates ; in the jellylish. Pelagia nociihica, for example, single gland-like cells 

 lie in the epidermis and stimulation, as by touching the animal, during the 

 evening but not during the daylight hours, produces the secretion of a 

 luminous mucus which spreads like a wave over it and can be rubbed away 



Fig. S!t8. — Section of the Light Organ in the Esca of the Anoi.er-fish, 



Sliowing luminous eiiithelium. L ; roflector layer, R ; ])iginent layer, P ; and the 

 0]iening of the lumen into a second cavity which commiuiicates with the outside, O 

 (after Brauer ; E. X. Harve\'"s Biohiminescence , Academic Press). 



with the finger (Dahlgren. 1915-17 ; Parker, 1920 ; Harvey, 1921 ; Moore, 

 1926) (Fig. 897). Such a spread indicates transmission of the stimulus by 

 a nerve-net ; the process is inhibited in the absence of Ca or K, and irri- 

 tability is markedly increased in the absence of Mg (Heymans and Moore, 

 1924). A somewhat similar luminous slime is produced by many worms ; in 

 the luminous earthworm it emerges from the mouth or anus or from dorsal 

 pores (Gates. 1925 ; Komarek, 1934), and in Polycha^tes the photogenic cells 

 are situated in association with mucous cells in the hypodermis {Chcefopferus 

 — Dahlgren, 1916) or in specific locations {e.g., in specialized nephridial 

 funnels in the transparent marine worm. Tomopteris — Meyer, 1929). Again, 

 a wave of light-production from the point of excitation indicates a spread 

 by nervous means. A similar slime is secreted by the clam, Pholas, luminous 

 Myriapods, and the colonial ascidian, Pyrosoma. CJlandular organs of a 

 more complex tyj^e are seen in Crustaceans in which granules are secreted 



