746 



THE EYE IN EVOLUTION 



and when ejected into the sea-water, appear as a luminous cloud (Fig. 895). 

 In Copepods the photogenic cells are in small groups; in the Ostracod, 

 Cypridina, there is a complex gland of 4 types of cell near the mouth from 

 which granules are ejected by muscular action (Okada, 1926 ; Takagi, 1936) ; 

 a similar mechanism is found in the deep-sea shrimps and squids (Harvey, 

 1931). In these the operative mechanism is neuro-muscular. Finally, in 

 some bathypelagic fishes such as Malacocephalus or Gigantactis, similar 

 luminescent granules (which may be bacterial) are expelled on the ventral 

 surface of the body from sac-like organs when the fish is excited (Fig. 898). 



The intracellular production of bioluminescence is more widespread, and, 

 again, may be effected either by single cells or elaborate organs equipped 



with secretory cells, a lens and cornea, 

 light-absorbing and light -reflecting struc- 

 tures, the whole resembling in many ways 

 a well-formed eye. Such organs are called 

 PHOTOPHORES. The luminous brittle-stars 

 and the nemertean worm, Emplect enema, 

 have single light-producing cells scattered 

 over their entire surface (Kanda, 1939). 

 The Arthropods, however, show more 

 specialized photophores as are seen parti- 

 cularly in shrimps, consisting of large 

 granular light -producing cells lying under- 

 neath an epithelial lens and upon a 

 reflecting layer (Fig. 899) (Vallentin and 

 Cunningham, 1888 ; Terao, 1917). Organs 

 of a somewhat similar type, consisting of 

 photogenic cells, a lens and a reflector 

 surrounded by pigment, frequently occur in Molluscs, and also in many 

 deep-sea Fishes arranged along the ventro -lateral aspect of the body. 

 The photophores of Insects are equally elaborate. In the fire-fly, 

 Lampyris, for example, the luminous organ is situated ventrally in the 

 posterior part of the abdomen ; it consists of a layer of light -producing cells 

 lying under the surface epithelium, backed by a layer of light -reflecting cells 

 which owe their optical property to small particles of urates, while an 

 abundance of oxygen is provided by a rich supply of tracheae (air tubes) 

 equipped with end-cells which act as minute pumps or valves (Fig. 900) 

 (Hess, 1922). All these photophores are well supplied with nerves and 

 apjoear to be under nervous control except in some fishes ; studying the 

 luminous organs of the Californian stinging fish, Porichthys, Greene and 

 Greene (1924) failed to find any nerves and demonstrated that they were 

 under hormonal control, the whole animal remaining alight and glowing for 

 over an hour after a subcutaneous injection of adrenalin. It is noteworthy, 



Fig. 899. — Section of a Photophore 

 OF THE Decapod Hkrimf, Se rue f<Tjiti 



PREHEySILIX, 



Showing the leri.s layers, Lj to L, ; 

 photogenic cells, Ph ; reflector, K ; 

 and pigment, P (after Terao ; E. N. 

 Harvey's Bioluminescence, Academic 

 Press). 



