THE PROTOZOA 



13 



to be found in most elementary text-books of zoology, 1 and any 

 further account of it will be omitted here, since Vorticella, the 

 next unicellular form to be 



studied, amply illustrates the mito 



complexity of differentiation 

 and life -history possible in 

 these simple forms ; also it 

 is so very commonly met ' 

 with by the student of pond- 

 life that a study of it is in- 

 evitable. 



Type : Vorticella, the Bell 

 Animalcule. 



Habitat. In examining 



General water - weeds or 



Appearance. other ob j ects 



taken from a pond, there are 



very frequently to be seen 



with the naked eye, minute, 



colourless, bell-shaped bodies, 



that constantly seem to float 



away from the surface of the 



weed, and then, on the 



slightest vibration of the 



water, to be suddenly drawn 



back again by a delicate 



thread which contracts into 



a thick, spirally- coiled stalk. This organism is Vorticella, the 



Bell Animalcule. 



There are three chief points in which Vorticella is seen at 

 once to differ from Amoeba: it is permanently fixed by 

 this contractile stalk ; the body does not exhibit the same 

 curious irregular changes of shape as in Amoeba, for although the 

 protoplasm contracts and expands, it never sends out irregular 

 pseudopodia but keeps its definite shape ; and, finally, round 

 the free margin of the " bell," there is a circle of fine cilia, 

 which by their vibrations keep the surrounding water in con- 

 stant motion. 



1 See Elementary Biology, by T. Jeffery Parker. 



FIG. 4. 



Vorticella, attached to a Water-weed. 



A , An extended bell ; B, a contracted 

 bell ; C, a bell dividing by simple 

 fission ; nu, macro-nucleus ; con. vac, 

 contractile vacuole ; /. vac, food 

 vacuoles ; con. fibre, contractile fibre 

 of stem. 



