STRUCTURE OF LUMINOUS ORGANS 79 



which enters the nerve, for the light of these organs is 

 tinder the control of the animal and may be flashed 

 "at will." 



All gradations in complexity of light organs may be 

 found from the condition in the shrimp Just described to 

 that found among the squid and fish. Figs. 29 and 30 are 



FiQ. 27. — Sectional view of photogenic organ of Sergestes prefienailis {after Terao). 

 bm, basement membrane; cs, connective strands of photogenic layer; hy^ hypodermia; 

 lit ht lit layers of lens; Ze. lens epitheUum; n, nerve; p/', photogenic cells; pu pigment layer; 

 r, reflector; th, theca. 



sections of two of the more complicated types found in 

 squid. The explanation given to the various structures is 

 that of Chun (1903) to whom we are indebted for a careful^ 

 histological investigation of these forms. It will be noted 

 that in addition to photogenic and lens tissues there are/ 

 various types of reflector cells and a line of pigment about! 

 the whole inner surface of the organ to effectively screen ^ 

 the animal's tissues from the light. In one form (Fig. y 



