180 



ANARTHROPODA 



tion is ready to divide again within twenty-four hours if 

 the conditions are favorable. After the formation of a 

 hundred or more generations they seem to become ex- 

 hausted and must then be rejuvenated by a process called 

 conjugation. This is the union of two individuals along 

 their ventral surfaces by means of which a mutual ex- 

 change of protoplasm occurs. They then separate, and 

 each is ready to divide again. 



Bell Animalcule (Vorticella). This is another infusorian 

 common in stagnant water. It is easily recognized under 



the low power of the 

 microscope by its 

 bell-shaped body 

 attached to a stem 

 which occasionally 

 contracts into a 

 spiral. The cilia 

 around the mouth 

 are used to form 

 a vortex drawing 

 the food into its 

 gullet leading 

 blindly into the 

 soft protoplasm. 



Amoeba. This 

 genus includes sev- 



FIG. 210. Photograph of a living bell animalcule, 

 enlarged two hundred diameters. 



eral species of the class Rhizopoda (root-footed), which are 

 regarded as being the lowest in the scale of animal life. 

 Seldom does one become large enough to be seen with the 

 naked eye, but it is easily distinguished with a magnifica- 

 tion of one or two hundred diameters. In some regions 



