MANUAL OF NATURAL HISTORY. 365 



ants of all parts of the world, but their especial 

 element is the boundless expanse of waters. Here 

 myriads glide about free to go wherever they will, 

 and, masters over their movements, roam as they list 

 from place to place. Some again, remain more or 

 less in one locality, while to others the power of 

 free motion is denied, and they have, accordingly, to 

 spend the allotted period of their existence fixed in 

 one spot. Most inhabit salt, but a few are found in 

 fresh water. 



But innumerable as they are in our times, they 

 are shewn by geological explorations to have been 

 even still more abundant during former periods of 

 the earth's history, their remains being very nu- 

 merous in certain strata. Such are the fossil 

 madrepores of the chalk system, in which, also, the 

 large isolated masses of flint that so frequently 

 occur, represent, it is believed, old-world sponges. 

 Abundant occupation for palaeontologists is supplied 

 by the Foraminifera of ancient eras, the number 

 of extinct species already known being very con- 

 siderable. The " Bergmehy too, or mountain-meal 

 of Sweden, exhibited to the microscopic eye of 

 Ehrenberg the skeletons of primitive animalcules, 

 which had finished their career long ere man had 

 made his appearance on the scene. 



To most of these curious beings it is a matter of 

 comparative indifference if a portion of their frame 

 is lopped off, as they remain quite as lively after 

 the amputation as before. Many indeed, propagate 

 their kind after this fashion, little portions, or buds, 



