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In some cells the rods form bundles or sheets which are inclined to one another 

 at various angles; in other cells the rods are all parallel. The iridiocysts overlap 

 one another so as to form from one to many layers. The closely laid rods 

 reflect and interfere with light waves , producing a sheen and iridescence which 

 is indescribably beautiful. Like the chromatophores , the iridiocysts are absent 

 from the integument which is not exposed to light, and from the lower surface 

 of the fins. The layer of iridiocysts is thickest above the ventral end of the 

 mantle and hides the dark colored organs in the viscera. The chromatophores 

 upon the lower side of the head, which is largely protected from light by the 

 siphon , are scattered , and the iridiocysts are replaced by a layer of short , 

 broad rods , or platelets , placed at right angles to the surface. 



