PHYSIOLOGICAL 425 



with the water; and here again Giesbrecht (1895) has shown that 

 the material can be kept dry for weeks without losing its virtue. 

 Jelly-fishes and sea-pens also illustrate the exudation of a secretion 

 which shines on the surface of the body ; and, to take a very different 

 instance, one of the Myriopods {Geophilus electricus) leaves a lumin- 

 ous trail on the ground. It is convenient, then, though perhaps 

 unimportant, to distinguish intra-luminescent and extra-luminescent 

 animals. 



Luminous Organs. — When the light is produced from an exuded 

 secretion, this usually comes from relatively simple glands, which 

 may be diffusely scattered or definitely arranged. But in some of 

 the instances of intra-luminescence there are very definite, often 

 very elaborate, luminous organs, well illustrated in various fishes, 

 higher crustaceans related to prawns, and in the fire-flies. Harvey 

 lays an interesting emphasis on the fact that these organs are 

 sometimes strangely like eyes. In front of the light-producing or 



Fig. 53. 



A Luminescent Deep-sea Fish (Argyropelecus), with numerous luminous organs 

 (LO) on the lower parts of the body, and prominent telescope eyes. 

 After Murray and Hjort. 



photogenic cells there may be a lens, sometimes triple; behind 

 them there may be a reflector; round the sides of the organ and 

 behind the reflector there is often a dark envelope shutting off the 

 light from the tissues of the animal itself; and, finally, there may 

 be a nerve, stimulating and controlling. Now many an eye has its 

 lens, its reflector, its pigmented choroid, and its optic nerve, so 

 that there is often a very striking resemblance between an organ 

 that produces light and one that perceives light. The probability 

 is that the resemblance is one of "convergence", and that there is 

 no homology between luminous organs and eyes. To solve analogous 

 problems, e.g. of focusing and darkening, somewhat similar methods 

 have been utilised in the course of evolution. No doubt there are 

 deep differences between luminous organs and eyes. Thus, as 

 Harvey puts it, the important transformation of energy in the 

 former is chemo-photic, in the latter photo-chemical. The nerve of 

 the luminous organ is of the stimulating efferent type, while that 

 of the eye is sensory and afferent. At the same time, the resemblance 

 between luminous organs and eyes is often striking, and it 

 becomes quaint in cases like the Stomiatid fishes (Anomalops and 



