TISSUES OF LIGHT-PRODUCTION 



127 



of Calliteuthes reversa, is a well-defined and easily understood structure 



found on the outer integument of this dibranchiate cephalopod. Its 



secretion is used in situ in the tissues, 



the organ being inside the integument, 



instead of being thrown out as in Pholas 



(Fig. 117). Its origin is not known, and 



it may be either ectodermal or mesoder- 



mal. 



Like most other organs of this nature, 

 it may be described as a number of layers 

 formed in a limited region and arranged 

 on a distinct proximodistal axis. The 

 first proximal layer consists of a coat of 

 pigment, thin and black, which covers 

 the inner end of the organ like a cap. 

 We know nothing of its use except that 

 it probably absorbs superfluous light. 



The well defined and thick layer of 

 tissue lying distad of the thin pigment 

 layer is composed of the very peculiar 

 connective tissue which has been differ- 

 entiated to reflect light. The cells are 

 spindle-shaped, and lie packed with their 

 ends interlocked and their long axis at 

 right angles to the direction of the light 

 ray that comes from the nearest light cells. The substance which they 

 have in their bodies to reflect light is possibly some urate in a very 

 finely divided state. 



Still distad of this tapetum or reflector is the photogenic layer, com- 

 posed of a single row of columnar cells lying in a row that conforms to 

 the shape of the last two layers. It is as thick as the many-layered re- 

 flector, but smaller by necessity of its distal and inner position. Its 

 shape, as that also of the last two layers, may be roughly likened to a 

 horseshoe. The cells which compose it are long, thin gland cells, each 

 with a small nucleus in the proximal part, and the secretion region and 

 storage region in the distal cytoplasm. The blood supply is evidently 

 connected in some way with the reflecting tissue. 



Distal from the light-gland is found the lens-tissue. This is rather 

 remarkable for its apparent lack of homogeneity. It is made up of a 

 number of heavy fibers that anastomose into a reticular mass. This 

 mass forms two rounded and connected areas, the smaller of which lies in 

 the concavity on the distal side of the light tissue. This small portion 

 is directly continuous with the far larger outer or distal part, which is 



re/ 



P9- 



FIG. 117. Section through main axis 

 of light organ of Calliteuthes reversa. 

 lu.c., luminous cells; /., lens; ref., 

 reflector; pg., pigment; in., integu- 

 ment. (After C. CHUN.) 



