92 



BIOLOGY 



to become independent animals. In Figure C is shown another 

 cell much like Vorticella, possessing the same shape and similar 

 organs. In this animal, after the cells divide, they do not 

 separate but remain attached to a common stalk, and subse- 

 quently divide again and again, the result being a group of 

 similar cells connected by a branching stalk, D. This animal 

 is named Carchesium, and such a cluster is called a colony. 

 In this colony the members are independent, each carrying 

 on for itself all of the functions of life and each contracting 

 and expanding by itself independently of the rest. A third 

 species is found resembling Carchesium except in one respect. 



In this animal, Zoo- 

 thamnium, there is a 

 common muscle ex- 

 tending through the 

 stalk and its branches. 

 When this muscle con- 

 tracts ; all the members 

 of the colony contract 

 simultaneously. 



These three animals 

 are evidently closely re- 

 lated; but Vorticella is a 

 true unicellular animal, 

 Carchesium a cluster of 

 independent cells at- 

 tached together, and 



Zoothamnium a similar colony in which the members are not 

 wholly independent but have a vital connection. 



There are many other animals which are in a similar way 

 made up of colonies of cells, alike in structure and function. 

 Several of these are sho\m in Figure 40. In all cases the ani- 

 mals start their life as single cells which become colonies by 

 the method of incomplete division. All these are commonly 

 classed among unicellular animals and called Protozoa (Gr. 



FIG. 40. COLONIES OF UNICELLULAR OR- 

 GANISMS MADE UP OF SEVERAL CELLS 

 ATTACHED TOGETHER 



A, an animal with its pseudopodia protruding; in 

 the other specimens only the shell is visible. These ani- 

 mals belong to the group of Forminifera, whose shells 

 form chalk cliffs and limestone rocks. 



