296 Descriptive Zoology. 



currents maintained by the cilia. There is also a con- 

 tractile vacuole, which is near the vestibule and empties 

 into it. The vorticella has a C-shaped nucleus. 



How Vorticella protects Itself. When a vorticella is 

 disturbed it is at once jerked up close to its mooring 

 by the coiling of its stalk. At the same time the body 

 changes its shape. The disk is drawn in, the rim of the 

 bell turns in, the cilia are inclosed, and the body becomes 

 pear-shaped or even spherical, there being no opening left 

 at the free end. After the disturbance has ceased, the 

 stalk elongates, the bell opens, the disk protrudes, the 

 cilia extend, and the operations of active life are all 

 resumed. 



Development of Vorticella. Vorticella multiplies by 

 longitudinal division. For some time two bells are attached 

 to the stalk, but one finally breaks loose and swims away 

 by means of its cilia. Later it becomes attached and 

 develops a stalk. 



GENERAL CHARACTERISTICS OF PROTOZOA. 



1. The protozoans are the simplest animals. Most of 

 them are one-celled. As a group, they are the smallest 

 of animals. 



2. They are the most numerous, in individuals, of any 

 branch of animals. It is not true, as commonly believed, 

 that every drop of water swarms with animal life. One 

 would find few animalcules in ordinary well or spring 

 water. But they usually abound in stagnant water. There 

 is room for vast numbers of these lowly forms which 

 occupy so little space. 



3. They multiply the most rapidly of all animals. In 

 most cases multiplication consists simply of division into 



