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THE BIOLOGY OF MARINE ANIMALS 



arched cuticular layer forming a concavo-convex lens. This overlies a layer 

 of tall columnar photogenic cells contained in a fibrous and cellular 

 sheath. In Sergestes prehensilis the cuticular lens is biconvex and is made 

 up of two distinct layers. Underneath the lens lies the photogenic tissue, 

 which is bounded by a reflector and a pigmented screen. A nerve penetrates 

 the back of the photophores in some of these forms and innervates the 

 light-cells (18). In addition to superficial photophores, some luminescent 

 shrimps {Sergestes) are provided with specially modified and luminous 

 liver tubules, known as the organs of Pesta. The light from these structures 

 shines through the body wall. Dennell (19) has observed the luminescence 



Cornea. 



Light 

 cells 



Reflector 



-Pigment 



Fig. 13.11. Photophore of a Euphausiid Meganyctiphanes norvegica. 



(After Dahlgren, 1916.) 



of many species of deep-sea shrimps, which were kept alive in the labora- 

 tory in chilled sea water for some time. 



The photophores of many deep-sea squids are striking objects (Fig. 

 13.12). Dahlgren traces all gradations from simple light-organs consisting 

 of a cup-shaped invagination of the epidermis, to highly differentiated 

 structures cut off from the overlying skin. In the cranchiid squids the photo- 

 phores consist of a mass of photogenic cells backed by a reflector and 

 lying in, or just beneath, the skin (Fig. 13.13(a)). In Leachia cyclura, for 

 example, the ocular photophores are spherical structures consisting of a 

 central cellular glandular mass lying within a reflector of connective tissue. 

 Where the integument extends across the organ it is provided with chroma- 

 tophores.In other oegopsid squids, such as Watasenia and Abraliopsis, the 

 photophores are grouped underneath the eye, at the tips of the tentacles 

 and as numerous small organs scattered over the surface of the body (Fig. 



