90 ELEMENTARY BIOLOGY. [CHAP. 



tion of their vitality; while their rapid propagation is, in 

 the main, due to their power of multiplying by division, 

 with extraordinary rapidity, when duly supplied with nou- 

 rishment. The majority are free and provided with nu- 

 merous cilia by which they are incessantly and actively pro- 

 pelled through the medium in which they live ; but some 

 attach themselves to stones, plants, or even the bodies of 

 other animals. A few are parasitic, and the bladder and 

 intestines of the Frog are usually inhabited by several spe- 

 cies of large size. 



The Bell-animalcules are Infusoria which are fixed, usu- 

 ally by long stalks, to water-plants, or, not unfrequently, to 

 the limbs of aquatic Crustacea. The body has the shape of 

 a wine-glass with a very long and slender stem, provided 

 with a flattened disc-like cover. What answers to the rim 

 of the wine-glass is thickened, somewhat everted, and richly 

 ciliated, and the edges of the disc are similarly thickened 

 and ciliated. Between the thickened edge of the cover, or 

 peristome, and the edge of the disc, is a groove, which, at 

 one point, deepens and passes into a wide depression, the 

 vestibulum. From this a narrow tube, the (Esophagus, leads 

 into the central substance of the body, and terminates ab- 

 ruptly therein ; and when faecal matters are discharged, they 

 make their way out by an aperture which is temporarily 

 formed in the floor of this vestibule. The outermost layer 

 of the substance of the body is denser and more transparent 

 than the rest, forming a cutiatla. Immediately beneath the 

 cuticle it is tolerably firm and slightly granular, and this 

 part is distinguished as the cortical layer ; it passes into the 

 central substance, which is still softer and more fluid. 



In the undisturbed condition of the Bell-animalcule, 

 the stem is completely straightened out ; the peristome is 

 everted, and the edges of the disc separated from the peri- 



