334 



ELEMENTARY BIOLOGY 





animals we find that much more is suppHed for the young. 

 The lobster and crayfish mothers carry the eggs about on their 

 abdominal legs, or swimmerets, and even the young em- 

 bryos until they are able to care for themselves. Among the 

 insects there are some that abandon their eggs as soon as 

 laid, whereas others provide shelter and food for the young. 



Most fish leave their 

 eggs in the water with- 

 out further attention. 

 There are a few fishes, 

 like the stickleback, that 

 prepare a rather rough 

 protection, or nest, for 

 the eggs (Fig. 157). 



Some toads carry their 

 eggs about in the mouth 

 until they are hatched. 

 Among the reptiles 

 and birds the egg begins 

 its development inside 

 the parent's body, and 

 receives a large amount 

 of food and a protective 

 covering. Most reptiles and some birds leave their eggs to be 

 hatched by the heat of the sun, or at ordinary temperatures. 

 Most of the common birds, however, build more or less elabo- 

 rate nests and care for the fledglings and for the eggs, besides 

 supplying heat for the hatching. The feeding of the young 

 birds by the parents is a very interesting operation to obser\^e, 

 and it shows a very complex development of instincts. 



388. Infancy among mammals. When we come to the 

 mammals, the dependence of the young upon the parents is 

 carried even farther. Not only does the egg develop inside 

 the body of the mother until it has acquired the general form 

 characteristic of the species, but it is nourished by the parent 





Fig. 158. Wallaby and young 



The babies are not only protected and kept warm in 



the njarsitpbim, or pouch, but are also nourished by 



a milky secretion produced by glands in the lining 



of the pouch 



