PTEROPODA. 239 



The first of these naturalists, who often passed whole 

 nights in contemplating myriads of these punctual mollusks 

 when floating on the sea, invariably observed that they 

 began to appear with the drawing on of twilight, and that 

 different species came to the surface with tlie utmost regu- 

 larity at certain hours of the night. He was, in consequence, 

 induced to believe that they inhabited particular zones, as 

 it were, or different depths of ocean, and that they occupied 

 as many different periods of time in making their way to 

 the surface. When the nights were calm, especially in tro- 

 pical regions, the surface of the ocean was darkened with 

 swarms of these active creatures, bat when day broke, not 

 one was to be seen. They vanished with the first gleam of 

 light on the horizon, like those tiny people 



" "Whose midnight revels. 

 By a forest side or fountain, some belated peasant sees, 

 Or dreams he sees, while overhead the moon sits arhitress, 

 And nearer to the earth wheels her pale com-se." 



The depth of the Pteropod''s descent is said to be governed 

 by the intensity of light upon the surface of the water ; 

 they are '' spirits of twilight/' as Mr. Reeve has poetically 

 expressed it, '' whose downward flight is measured by the 

 opacity of their native element." 



