TISSUES OF LIGHT-PRODUCTION 



137 



lU. C, 



A much greater variety of light-organs is to be found in the teleost 

 fishes. Space forbids a description of all the types, several of the most 

 important of which will be given. 



The single light-organ on the angling fin ray of Gigantactus, a deep- 

 sea fish, is a simple type. It consists of an imagination of the stratified 

 epithelium into the connective tissues of the bulb on the end of the ray. 

 This invagination consists of a deep round sac at the fundus, an inter- 

 mediate vestibule, which is wide but short, and a tube leading from the 

 vestibule to the exterior (Fig. 124). 



The deep sac is the important structure. The epithelium which lines 

 it is still stratified, but during its proliferation each of its cells secretes the 

 light substance, and when finally cast off at the surface it degenerates 

 and the light substance is thrown into the lumen. 



From the lumen it must be slowly forced out into the vestibule and 

 from the vestibule it must pass into the surrounding w r ater through the 

 tube. It is probably oxidized in the vestibule and made to give up its 

 light, so that only the combustion products pass into the water. The 

 organ thus acts as an external or superficial tissue of photogenesis, a 

 comparatively rare form, especially among the higher animals. The 

 greater part of these teleost light- 

 organs, however, are internal, 

 meaning that the secretion is not 

 discharged, but is used in situ hi 

 or near the cells that produced it, 

 as was the case in Spinax. 



Passing over the simplest 

 forms that show but a few 

 specific cells inclosed proximally 

 by a pigment layer, we shall 

 examine the type of luminous 

 organ found on Chauliodus. 



This consists of a proximal, CUp- FIG. 125. A light-organ of Chauliodus. lu.c., 

 Shaped pigment mantle (Fig. 125) luminous cells; pg., pigment layer; /., lens. 

 ,. *, ,. ,, , . i , r (After A. BRAUER.) 



lined distally by a single layer of 



columnar cells whose small basal ends contain a nucleus and whose long 



distal ends are filled with the secretion. 



Held in the hollow of this cup-shaped gland layer is the solid mass 

 of cells which, in life, are transparent and refractive and act as the lens. 

 This mass has an outer layer somewhat separated from the rest by its 

 columnar arrangement but functionally a part of it. 



Outside the lens is the connective-tissue layer of the skin. Its rela- 

 tion to the remainder of the organ is, functionally, that of a cornea. 

 Morphologically the lens and gland cells came from the epithelium and 



