368 MANUAL OF NATURAL HISTORY. 



ing been ascertained to consist almost entirely of 

 their extinct remains. The Swedish-earth, eaten by 

 the inhabitants mixed with flour, tinder the name of 

 " Bergmehl," is also entirely composed of their ske- 

 letons, which, in the course of ages, have been accu- 

 mulating so as to form thick and extensive beds. 



The Polygastrica are either naked or entirely soft 

 and gelatinous, or they are protected by a thin 

 glassy shell, which often varies in form, sometimes 

 constituting a dorsal shield, as in Euplcea, and some- 

 times resembling a bivalve shell, as in the Naviculce. 

 Their organs of progression consist either of delicate 

 filaments, called cilia, with which their bodies are 

 covered, or of stiff, moveable, bristle-like organs, or 

 little hooks, which also serve to attach them to 

 foreign bodies. The reproduction of these minute 

 organisms is by means of buds, which sprout from 

 the surface of their bodies, and become, in the course 

 of time, like the parent animals, or by free gem- 

 mules, contained in the interior of the mother, whose 

 globular body bursts, and the little ones come forth 

 at the sacrifice of their parent's life. They also in- 

 crease by spontaneous fissure, so that an old animal 

 may thus renew his youth, by becoming two or more 

 young ones. 



I CLASS.— INFUSORIAL-ANIMALCULES 



(Polygastrica). 



Intestinal canal replaced by a number of small, 

 interior cavities ; multiply by spontaneous division 

 of their bodies. 



