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



Like the organs without pigment coat, these are probably incited to action in groups 

 and not singly, although there may be more individuality in their performance than in 

 that of the less highly differentiated organs without pigment coat. 



The function of the pigment coat is probably to shade the light at the sides, and to 

 concentrate it in one direction, which, of course, would be of advantage to the fish. Pos- 

 sibly the pigment is only to be considered as a residue of the chemical reactions which 

 go on in the organ. 



e. Development. 



I do not doubt that these organs have been developed from the simple phosphorescent 

 organs without pigment coat, and that they in this way represent the second stage in the 

 development of the ocellar phosphorescent organs of fishes. Not only does their structure 

 indicate a much greater differentiation, but also their distribution over the surface of 

 the fish is more sharply defined and not so irregular as in the case of the former organs. 



3. Composite, ocellar, regular phosphorescent organs, without reflector. 



a. Distribution. 



Organs of this kind have been found by me in Opostomias micripnus, Echiostoma 

 barhatum, Astronesthes niger, and Pachystomias microdon. 



These organs invariably appear in two rows on each side of the body, one lateral and 

 one ventral. The rows extend forwards on the lower side of the head, a row on the pro- 

 jecting crest of a fold below the operculum being especially conspicuous. 



Ussow ^ has described the distribution of these organs over the body very carefully. 

 The organs in one row are from 2*5 to 5 mm. apart according to the size of the species. 

 Large specimens possess a much greater number of them than small ones. 



The organs which Ussow calls " augenahnliche Organe " in Astronesthes martensii,'-^ 

 Stomias barbatus,^ and Chauliodus sloani* belong to this group. 



Some of Leydig's " augenahnliche Organe," which he found in Ichthyococcus ovatus ^ 

 and in Ichthyococcus palmerise,^ likewise belong to this group. 



In all the fishes in which these organs have been found, they are distributed over the 

 surface of the body in the same way, always occupying four lines. 



In this respect the composite organs differ much from the two preceding kinds, which are 

 scattered much more irregularly over the surface and which never occur in distinct rows. 



1 M. Ussow, loc. at., pp. 108, 109. 2 ]^];_ Ussow, loc. cit., p. 89, pi. ii. fig. 9. 



3 M. Ussow, loc. cit, p. 91, pi. ii. fig. 10. * M. Ussow, loc. cit, p. 93, pi. ii. fig?. 6, 7. 



* F. Leydig, Die augenalmUcliea Orgaue der Fische, p. 22, pi. vi. fig. 33. ° F. Leydig, loc. cit, p. 25. 



