LECTURE X. 
125 
In the Linnasan 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 Linneeus 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 Medusee 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 Linnagas 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 Py rosoma. It is described and figured in the 
work entitled “ Annales du Museum National 
d’Histoire Naturelle.” Tins 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 papillae or tubercles, and there is 
