LECTURE X. 125 



In the Linnaean genus Nereis also stands an 

 extremely minute species, so very small as to re- 

 quire the assistance of a microscope in order to be 

 distinctly examined. It has however been raised 

 to a kind of importance from having been sup- 

 posed by Linnasus the chief cause of. the luminous 

 appearance of sea-water by night ; since on tak- 

 ing up vessels full of sea-water, when the sea has 

 appeared most brilliant, this microscopic species of 

 Nereis has been observed in it in great plenty. 

 The Medusas however, and a great many other 

 marine animals, are of a phosphorescent nature, 

 and appear luminous during the night, so that the 

 Nereis noctiluca of Linnaeus cannot be considered 

 as the chief cause of the phenomenon. 



This circumstance induces me to mention one 

 of the new genera of marine mollusca lately insti- 

 tuted by the French naturalists, under the name 

 of Pyrosoma. It is described and figured in the 

 work entitled " Annales du Museum National 

 d'Histoire Naturelle." This animal, (for there is 

 only one species yet discovered) is of a lengthened 

 and tubular form, open at one extremity, and 

 closed at the other : the body is scattered over with 

 numerous soft papilla? or tubercles, and there is 



