98 THE VITAL FUNCTIONS. 



Ehrenberg forms of these the tribe of Cycloccela, 

 of which the Vorticella citrina, and the Stentor 

 polymoiyhus, are examples. Thus do we dis- 

 cover the same diversity in the structure of the 

 digestive organs of the several races of these 

 diminutive beings, as is found in the other classes 

 of animals. 



The Hydatina senta, one of the largest of the 

 infusoria, was found by Ehrenberg to possess a 

 highly developed structure with respect to many 

 systems of organs, which we should never have 

 expected to meet with so low in the scale of ani- 

 mals. As connected with the nutritive functions, 

 it may here be mentioned that the head of this 

 animalcule is provided with a regular apparatus 

 for mastication, consisting of serrated jaws, each 

 having from two to six teeth. These jaws are 

 seen actively opening and shutting when the 

 animal is taking its food, which consists of par- 

 ticles brought within reach of the mouth by 

 means of currents excited by the motions of the 

 cilia. 



Such are the simple forms assumed by the 

 organs of assimilation among the lowest orders 

 of the animal creation ; namely, digesting cavities, 

 whence proceed various canals, which form a 

 system for the transmission of the prepared nou- 

 rishment to different parts ; but all these cavities 

 and canals being simply hollowed out of the 

 solid substance of the body. As we ascend a 



