In the firefly there is no true lens, the light merely shining through the cuticle which is transparent over the light organ, whereas over the rest of the body it is dark and pigmented. In the deep sea shrimp, _Acanthephyra debelis_, with light organs scattered over the surface of the body, the cuticle covering the light organ forms a concavo-convex lens, behind which are the photogenic cells (Kemp, 1910). As may be seen from Fig. 26, the lens is made up of three layers which suggests that it may be corrected for chromatic aberration--a veritable "achromatic triplet." In an allied form, _Sergestes_ (Fig. 27), the lens is of two layers and double convex. Optical studies of these lanterns have been made by Trojan (1907). The course of the light rays is shown in Fig. 28. The lens of these organs is also bluish in color which suggests that they may serve also as color filters. Behind the photogenic cells is a mass of connective tissues through which enters the nerve, for the light of these organs is under the control of the animal and may be flashed "at will." [Illustration: FIG. 26.--Sectional view of photogenic organ of _Acanthephyra debilis_ (_after Kemp_). _n_, nerve; _s. l._, sheathing layer of cells; _g_, cone of refractive granules at end of nerve strand; _c_, cellular layer; _i. l._, _m. l._, _o. l._, inner, middle and outer layer of lens.] [Illustration: FIG. 27.--Sectional view of photogenic organ of _Sergestes prehensilis_ (_after Terao_). _bm_, basement membrane; _cs_, connective strands of photogenic layer; _hy_, hypodermis; _l_{1}_, _l_{2}_, _l_{3}_, layers of lens; _le_, lens epithelium; _n_, nerve; _ph_, photogenic cells; _pi_, pigment layer; _r_, reflector; _th_, theca.] [Illustration: FIG. 28.--Diagram of photogenic organ of _Nyctiphanes Conchii_, to show pathways of light rays arising in the light cell layer (_after Trojan_). _p_, pigment; _ri_, inner reflector; _lp_, light cells; _rf_, refractor; _f_, focus; _l_, lens; _A-A_, axis; _a_{1}-a_{4}_, _b_{1}-b_{4}_, light rays reflected from _ri_; _c_{1}-c_{4}_, light rays passing directly outward; _d_{1}-d_{9}_ and _e_{1}-e_{9}_, light rays which have passed refractor and lens respectively.]