390 RADIATA. 



inconceivable minuteness, yet varying greatly in 

 shape ; some appear a thin oval plate, some are glob- 

 ular, some eel-shaped, some pear-shaped, some bell- 

 shaped ; while others have no defined shape, their 

 outline perpetually changing. With respect to their 

 size and number, " Dr. Ehrenberg has described 

 Monads which are not larger than from ^^ to TtF |o 

 of a line,* and which appeared to be separated from 

 each other by intervals not greater than their dia- 

 meter. Each cubic inch of the water in which they 

 are found must contain, therefore, eight hundred 

 thousand millions of these animalcules, estimating 

 them to occupy but one fourth of its space." Some 

 of them are enclosed in transparent shells. A pow- 

 dery substance is abundantly found near Lake Letts- 

 naggsjon, in Sweden, which from its extreme fine- 

 ness resembles flour ; this has long been known to 

 the natives of the region where it is plentiful under 

 the name of Bergmehl, or mountain meal ; and is 

 used by them, mixed up with flour, as an article 

 of food, experience having taught them that it is 

 highly nutritive. On examination with the micro- 

 scope, this meal is found to consist entirely of the 

 shells of these animalcules, which have been accu- 

 mulating for ages at the bottom of the waters in 

 which the living animals are found, forming a stra- 

 tum of considerable thickness. 



The Rotifer a, or Wheel-bearers, are also par- 

 tially enclosed in a shell of exquisite delicacy and 

 transparency, which usually resembles in shape a 



* A line is the twelfth part of an inch. 



