I 



SECRETION 53 



over the whole surface of the body (as in Chaetopterus) ; 

 restricted to various regions — narrow bands on the siphon 

 and mantle and a pair of triangular regions near the retractor 

 muscle in Pholas ; or more definitely localised, as in Cypridina, 

 which possesses on the upper lip above the mouth, a luminous 

 gland, made up of spindle-shaped cells each opening by a 

 separate pore with a kind of valve. 



Intracellular light-production is characteristic of the more 

 specialised types. In this category are the complex photogenic 

 organs of insects, some Crustacea, cephalopods and fishes. 

 In the fireflies the photogenic organ develops from the fat- 

 body and consists of a mass of granular luminescent cells 

 abundantly supplied with nerve fibres and tracheae, and 

 enveloped above by a layer of supposedly reflecting cells which 

 probably scatter incident light on account of the crystals of 

 xanthin, urates, or other purine derivatives contained in them. 

 In some of the shrimps (Sergestes), Cephalopods (Sepia), 

 and fishes (Stomias) the photogenic organs are still more 

 elaborate and diffusely scattered over the body. In these 

 forms the photogenic organ possesses not only a reflector 

 behind the photogenic cells, but in addition a cuticular lens, and 

 in one genus of fishes it is even endowed with an iris diaphragm 

 by which flash effects are produced. In general, photogenic 

 organs are richly innervated, and there is no special phyletic 

 or bionomic significance to be attached to the particular 

 structural arrangement. Both intracellular and extracellular 

 types occur within the Crustacea. 



For the production of light by the organism it has long been 

 known that oxygen and moisture are essential. Robert Boyle 

 (1665) showed that rotten wood infested with luminous fungi 

 ceases to glow if placed in a vacuum ; and Spallanzani lifted 

 the subject out of the domain of vitalism by showing that 

 the luminous material of medusae, if dried, would emit light 

 when water was added. Pyrosoma, Pholas, Phyllirhoe, fire- 

 flies, Pyrophorus, copepods, ostracods, pennatulids can all 

 be desiccated without destruction of the luminescent material. 

 Desiccated ostracods and copepods will luminesce in the 

 presence of moisture after being kept for several years. Though 



