324 THE VOYAGE OF H.M.S. CHALLENGER, 



phorescent organs of fishes was that of a gland, which produced luminous slime, and that 

 to these glandular structures other more highly differentiated elements were added in the 

 course of development. It may be assumed that the glands which produce luminous 

 slime are, like other glands, supplied with nerves, and that their secretion is subject to 

 nervous influences. These nervous elements do not seem to undergo any particular 

 modification, even in the most highly developed phosphorescent organs, except that they 

 increase in size. The phosphorescent gland was originally, as the lowest forms show, 

 attached outside to the skin, and the dermis, or the scale which underlies it, may be 

 further developed, so as to form a reflector. The lower portion of the gland, like the 

 nerves which supply it, remains unchanged, but the superficial portion is sometimes 

 highly modified, and here the typical phosphorescent cells are developed, and the 

 nerves which supply them become modified, special ganglion cells being often 

 developed in their course. Leydig comes to the conclusion that these organs have 

 nothing to do with the slime-canal system. The various suborbital organs above 

 described, and also the ocellar organs on the lateral line of Halosaurus, show 

 clearly that in these cases at least, the organs in question have been developed in 

 connection with the slime-canal system.^ The histological facts ascertained are not in all 

 cases sufficient to allow of a conclusion regarding the function of these organs, but it is 

 quite certain that some of them are phosphorescent, and there are good reasons for 

 assuming that the remainder are so likewise. It is quite certain that none of them are 

 eyes. Most of the organs seem to be defensive, inasmuch as their position precludes 

 their illuminating anything that lies within the field of vision of the fish, so that their 

 only use can be to frighten away its enemies. This particularly applies to the meta- 

 meric organs on the body. The organs on the barbels may have the function of luring 

 other fish, whilst the large and formidable suborbital organs may be regarded as 

 aggressive, inasmuch as they are used for the purpose of illuminating the field of 

 vision and facilitating the capture of the prey. The large eyes of many deep-sea fishes 

 show that there must be some light in the depths inhabited by them, the source of 

 which can only be sought for in phosphorescent organs. 



h. The Typical Phosphoeescent Cells. 



The clavate cells with their highly refracting oval vesicle may be regarded as 

 the most highly difi"erentiated elements to be found in the phosphorescent organs of 

 fishes. I am not aware that cells of this kind have been described before, although 

 the luminous elements of Phylliroe bucephala, discovered by Panceri,^ resemble them to 



' Dr. Giinther arrived at the same conclusion from a general consideration of the distribution of the luminous 

 organs. 



2 Paolo Panceri, Atti Accad. Sci. Fis. e Mat. Napoli, vol. v. No. 14. 



