Survey of the Animal Kingdom 285 



tnedusoe- 

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Oi/a/T? 3perm 



Fig. 88. — Life cycle of Obelia, a coelenterate of the class Hydrozoa, showing 

 alternation of generations. A, Portion of a colony with its medusae and the ovum 

 and sperm. The sperm fertilizes the ovum to form the zygote, a, which divides 

 by mitosis to form two cells, b, then four cells, c, then a mass of cells known 

 as the blastula, d, and eventually the ciliated planula. B, Developing a new 

 colony from the attached planula. (From Curtis and Guthrie: Textbook of Gen- 

 eral Zoology, published by John Wiley & Sons, Inc.) 



