M Q Reproduction in 



Multicellular Animals 



EACH KIND of animal and plant must re- 

 produce rapidly enough to compensate for 

 the death rate of its population; failing this, 

 extinction is inevitable. Accordingly, the trail 

 of evolution is strewn with thousands of 

 extinct species, known today only by the fos- 

 silized remnants of their former being. 



With few exceptions, multicellular animals 

 reproduce sexually, although quite a few can 

 also multiply asexually. Primary attention 

 will be given to the sexual methods, with 

 only a brief discussion of the asexual proc- 

 esses. 



REPRODUCTIVE ORGANS 



Most multicellular animals possess well- 

 developed gonads. These essential reproduc- 

 tive organs produce the gamete cells, which 

 are destined to convey part of the protoplasm 

 of each parent to the zygote and other cells 

 of the next generation. 



Typically the testes, or sperm-producing 

 gonads, are distinct from the ovaries, which 



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produce the eggs. However, in rare cases (for 

 example, certain mollusks), eggs and sperm 

 are generated within the same gonad, al- 

 though not usually at the same time. 



One very simple type of gonad is possessed 

 by Hydra (Fig. 21-1). During the sexual period 

 this primitive animal develops both testes 

 and ovaries, which, however, are merely tem- 

 porary organs. The gonads first appear as 

 small swellings at the surface of the body, 

 the testes near the upper end, and the ovaries 

 nearer the foot. Each gonad represents a 

 local accumulation of germ cells (p. 476), cov- 

 ered externally by an ectodermal epithelium. 

 In the testes (Fig. 21-1) the germ cells divide 

 repeatedly, forming a large number of very 

 small flagellated sperm. Eventually these 

 sperm escape into the surrounding water 

 when the epithelial wall of the ripe testes 

 begins to disintegrate. 



In the ovary (Fig. 21-1) the germ cells also 

 divide several times, but in this case the re- 

 sulting cells are actively amoeboid and begin 

 devouring each other in cannibalistic fashion. 



