CHAPTER LXII 

 REPRODUCTION IN ANIMALS 



378. Aquatic invertebrates. Among the invertebrate ani- 

 mals that is, those having no backbone living in the 

 water, such as sponges, corals, starfish, clams, and crayfish, 

 fertilization usually takes place outside the body of the parent. 

 In the cases of many, however, the developing egg cell may be 

 protected by some portion of the mother's body, as when the 

 young hatch in the mantle cavity of the clam. 



379. Reproduction in fishes. Among the fishes, the female 

 gametes are usually deposited in quiet places at the bottom of 

 the sea, near shore, or in quiet pools of rivers. Then the male 

 fish swims over the eggs, dropping out a quantity of fluid con- 

 taining the sperm cells. These swim about in the water, fer- 

 tilization taking place in much the same way as in the rock- 

 weed (see p. 299). The fluid containing the sperms is called 

 milt, or semen. A sperm cell of a fish is illustrated in Fig. 1 56, ^. 

 As soon as the nucleus of the egg has fused with the nucleus 

 of the male gamete, the combined nucleus begins to divide, and 

 thus the development of a new fish is started. 



The female gamete of the fish contains a small amount of 

 food material in addition to the protoplasm. While the devel- 

 opment is under way the young fish lives on this accumulated 

 food. In some species of fish the adults swim about in the 

 neighborhood of the developing fry and protect them against 

 possible destruction by other fish. In most species, however, 

 the sperm and eggs are thrown out by the adult males and 

 females, and then left to themselves. Thus exposed, thousands 

 of eggs are destroyed before they have a chance to develop 

 into fish. Of course, thousands are also destroyed in the case 



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