56 



THE SIMPLER METAZOA 





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threads (flagella), two of which project from each cell. 

 The advantage in rate of movement resulting from the 

 union of cells is illustrated in rowing. Eight men in 

 a large rowing shell can go faster than one man in a single, 

 small shell. In reproduction, the sixteen cells fall apart, 

 and each one grows into a new colony. 



40. Volvox. — Volvox is a colony of hundreds of tiny 

 green cells embedded in a hollow gelatinous sphere. 

 Each cell has two flagella. For a time all the cells are 



alike and share equally in the 

 work of the colony. But in 

 reproduction only a few cells 

 take part. In the simplest 

 method, a few cells grow large 

 and escape into the hollow 

 sphere. There, they divide 

 and grow into new colonies. 

 Finally, the mother colony 

 breaks, and the daughter 

 colonies escape. 

 The more complex method is like the reproduction of 

 higher animals. Certain cells in the colony grow large 

 and escape into the hollow sphere. They are the egg 

 cells. Other cells of the colony enlarge and divide into 

 large numbers of slender, free-swimming cells called sperm 

 cells. The sperm cells escape into the hollow sphere and 

 swim about. One sperm enters an egg cell and unites 

 with it, forming a single cell, the fertilized egg cell, which 

 can develop a new colony. 



41. Division of Labor. — In gonium, the cells are alike in 

 form and function, but in volvox, we find that a few cells 

 have been changed in form in order better to perform the 

 special work of reproduction. This is the first step in the 

 division of labor. 



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Figure 55. — Volvox. 



