326 THE VOYAGE OF H.M.S. CHALLENGER. 



the control of the animal, and may singly or in groups emit light apparently at the will 

 of the Phylliro'e. 



A stUl higher form of phosphorescent organ is met with in Pyrosoma} Here we find 

 groups of cells which are spherical and destitute of nuclei (?). These groups of cells are 

 in connection with a stout nerve and are glandular, secreting a fatty substance which, 

 under the influence of nervous stimulation, is burnt, and emits a brilliant light. It is 

 obvious that these organs of Pyrosoma are in every respect similar to the " simple 

 ©cellar phosphorescent organs " described above from Opostomias micripnus. 



Further stages in the development of phosphorescent organs are found in fishes, 

 where also their development reaches to the highest point among aquatic animals. 



The ocellar phosphorescent organs are comparable to the elements in Euphausia 

 which have been considered as accessory eyes by Claus,^ but which according to Sars^ are 

 not sense-organs at all, but phosphorescent. They are spherical, and divided by a stout 

 ring situated in the surface tangentially in the outer third, into two portions, an outer 

 and an inner one. The outer is occupied by a transparent cellular structure, and covered 

 by a watch-glass-shaped cuticle. The lower portion is surrounded by a layer of red 

 pigment, and filled with roundish cells similar to those in the phosphorescent organ of 

 Pyrosoma. In the centre of the proximal portion a bundle of vertical threads is found, 

 and a lens is situated between the two portions. 



No organs comparable to the highly diiferentiated phosphorescent apj^aratus with 

 reflectors in fishes have been found in other animals. 



The phosphorescent organs of insects are not comparable in any way to those of 

 fishes. 



The mode of development of the phosphorescent organs in aquatic animals, as repre- 

 sented by those of living adult forms, is shown in the annexed scheme. 



1 Paolo Panceri, Gli organi luminosi e la luce del Pirosomi e delle Foladi, AUi Accad. Sci. Fis. e Mat., vol. v. 

 No. 13, 3 pis.; also Panceri, Gli organi luminosi e la luce dei Pirosomi, Rendiconti Accad. fis. e math., xi., fasc. 2, 1872, 

 p. 43. 



* C. Clans, Ueber einige Schizopoden und niedrige Malacostraken, Zeitschr. f. wiss. ZooL, Bd. xiii. p. 446. 



^ G. 0. Sars, Report on the ScMzopoda of the Challenger Expedition, Zool. Chall. E.xp., pt. xxxvii. p. 70, pi. xi. 



