318 



APPLIED BIOLOGY 



tral, highly contractile fiber (the simplest muscle known), 

 which frequently shortens (contracts) and coils the stalk into 

 a close spiral (Fig. 94, c). The slightest jar of the micro- 

 scope, the touch of another organism, or even a current of 

 water will cause sudden contraction. 



Vorticella multiplies by division (Fig. 94, d, e). One of 

 the two individuals thus formed leaves the stalk and becomes 



free swimming (Fig. 94, /). 

 Soon it settles down, grows 

 a stalk of its own, and be- 

 comes a full-grown vorticella, 

 ready to reproduce by di- 

 vision. 



If we imagine that the vor- 

 ticellas formed by division 

 remain on branches of the 

 same stalk, then division re- 

 peated many times would 

 produce a tree-like colony 

 with individual animals in 

 the same positions as are the 

 terminal buds on branches of 

 plants. 



279. Volvox: Colonial Ani- 

 mal or Plant ? The most re- 

 markable of colonial organisms is the beautiful Volvox (Fig. 

 95), which lives in ponds of fresh water. The colonies, 

 which are visible to the unaided eye, are hollow spheres of 

 a transparent gelatinous material in which are set numerous 

 individuals (single cells), each with two flagella. Each in- 

 dividual cell has a nucleus, a chlorophyll-body, and a red 

 spot. The combined lashing of all the flagella causes the 

 colony to roll through the water; hence the name Volvox 

 was made from a Latin word meaning to roll. 

 The eight dark spheres represented in the figure are new 



FIG. 95. Colony of Volvox. Each 

 zooid has two cilia. Eight young 

 colonies are shown in the interior 

 of the hollow sphere, a, colonies 

 just beginning to form from certain 

 zooids at the surface. (From 

 Parker.) 



