IX.] THE BELL-ANIMALCULE. 87 



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

 main, due to their power of multiplying by division, with ex- 

 traordinary rapidity, when duly supplied with nourishment. 

 The majority are free and provided with numerous cilia by 

 which they are incessantly and actively propelled 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 species of large size. 



The Bell-animalcules are Infusoria which are fixed, 

 usually 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 vesti- 

 bulum. From this a narrow tube, the oesophagus, leads into 

 the central substance of the body, and terminates abruptly 

 therein ; and when fsecal 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 sub- 

 stance of the body is denser and more transparent than the 

 rest, forming a cuticula. Immediately beneath the cuticle it 

 is tolerably firm and slightly granular, and this part is dis- 

 tinguished 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 peristome ; the 

 vestibule gaping widely and the cilia working vigorously. 

 But the least shock causes the disc to be retracted, and the, 



