186 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



is digested. There are also cells, as in Hydra, which perform intra- 

 cellular digestion. 



Obelia gives rise to another type of polyp than the nutritive individ- 

 ual just described. This is the reproductive polyp, or gonangium that 

 grows out as a bud, expands into a knoblike central axis known as 

 the hlastostylc within a chitinous, closed vase, called the gonotheca. 

 On the sides of the blastostyle budlike structures, called medusa buds, 

 develop. These break off and swim away as tiny bisexual jellyfish, 

 or medusae, representing the sexual stage in the life history. A sperm 

 cell from one of these medusae fertilizes an egg from another, which, 

 after a developmental period, becomes a free-swimming ciliated larva, 

 called a planula. After a short time the planula settles down and 

 produces a new asexual colony of Obelia. Other related forms as the 

 jellyfish, Aurelia, possess a predominating free-swimming stage, while 

 the sessile, non-sexual generation is reduced. 



This life cycle is reminiscent of a similar condition in plants, which 

 also have an alternation of generations. During the maturation of the 

 sperm and egg cells, reduction division takes place in which the chro- 

 mosomes of the sex cells are reduced to half the body number. In 

 alternation of generations of plants, all the cells of the gametophytic 

 generation are haploid, but as in animals only the mature sex cells are 

 haploid, the body cells having the same number of chromosomes as 

 the body cells of the sexual generation. The end result accomplished 

 in both plants and animals is the same. 



SUGGESTED READINGS 



Curtis, W. C., and Guthrie, M. J., Textbook of General Zoology, 2nd ed., 



John Wiley & Sons, Inc., 1933, pp. 278-301. 

 Guyer, M. F., Animal Biology, Harper & Bros., 1931, pp. 197-206. 

 Hegner, R. W., College Zoology, The Macmillan Co., 1936. Ch. X. 



An authentic description of Hydra and its activities. 



