CHAPTER XXV. 



OF THE CLASS OF POLYGASTRICA. 



1113. THE next class we shall consider is that of Polygas- 

 trica ; which includes the simpler kinds of Infusorial Animalcules. 

 Its true place in the Animal scale cannot be satisfactorily assigned, 

 until the internal organisation of the beings composing it has 

 been more thoroughly examined. At present, the general sim- 

 plicity of their character, and the absence of any other decided 

 type, leads us to rank them among the Radiata ; although few 

 of them present any distinct indications of the radiated confor- 

 mation. This class was formerly supposed to contain the sim- 

 plest members of the Animal Kingdom ; many tribes belonging 

 to it having been imagined to obtain their nutriment, by direct 

 absorption from the surrounding fluid. It is now known, how- 

 ever, through the researches of Ehrenberg and others, that most 

 of these possess an organisation of much greater complexity, 

 having a distinct mouth (usually surrounded by a fringe of cilia), 

 and internal cavities for the reception of food. 



1114. Wherever any decaying organised matter exists in^a 

 fluid state, and is exposed to air and warmth, it will speedily be 

 found peopled with minute inhabitants, of the most varied forms 

 and diversified movements, possessed of considerable activity, 

 and evidently endowed with an energetic system of nutrition. 

 They are, therefore, by no means so nearly allied to Vegetables, 

 as are those inactive and simple creatures the Sponges and their 

 allies. The cause of the spontaneous appearance of these Ani- 

 malcules, where no germs were previously suspected to exist, and 

 where it was not easy to suppose that they had been conveyed, 

 has been a matter of much speculation. Many have had recourse 

 to the supposition that the germs formed part, in a latent state, 

 of the living tissues of the animal and vegetable structures, from 



