32l? ZOOLOGY. 



would adhere to the finger, but when brought to the 

 light and closely examined with the aid of a lens, it 

 betrayed no visible substance; although the same 

 particle, when rubbed in the dark, could be divided 

 into many smaller luminous portions. 



In the great majority of cases, however, a phos- 

 phorescent sea would betray its history, by affording 

 to the tow-net numerous examples of luminous animals ; 

 sometimes fishes or shell-fish, but more commonly 

 molluscs or medusse. The fishes, shell-fish, and tuni- 

 cated molluscs have their luminous matter deposited 

 beneath a dense integument, and consequently do not 

 communicate it to the waters they infest; but this 

 does not apply to all the medusae, as some of them are 

 indebted for their phosphorescent quality to a peculiar 

 secretion, that covers their body in the form of a slime, 

 which is easily washed off, and, diffusing itself through 

 water, communicates to that fluid a luminous ap- 

 pearance, which may be entirely independent of the 

 actual presence of the animals from which it is derived : 

 and this, as I have elsewhere observed, may in a great 

 measure account for the occasional existence of a lu- 

 minous sea in which no tangible luminous bodies can 

 be detected. 



The greater number of the luminous marine animals 

 we noticed during this voyage have been described in 

 their proper places ; but there were some others, 

 medusae, captured under circumstances when cause and 

 effect were satisfactorily displayed, which I shall now 

 mention. The one species, which we captured in vast 

 numbers in the North Pacific, is circular, gelatinous 

 and transparent, and about the size of a dollar; its 

 upper surface convex, and marked with radiating 

 grooved lines. The centre of its inferior aspect is 

 concave, while the circumference is a comparatively 



