CHAPTER XVI 

 THE CONTINUITY OF LIFE 



No UNDERSTANDING of animal life is possible 

 without an appreciation of the ways in 

 which the perpetuation of the species is 

 assured. Continuity of life from one generation to 

 the next is brought about by three apparently quite 

 different processes. 



In the first place, there is the usual sexual repro- 

 duction. Secondly, many animals are reproduced by 

 females only, through the development of unfertilized 

 eggs. In the third place, an animal may divide in 

 two, or may produce buds which grow into new 

 animals; these may separate from the parent, or may 

 remain attached to it. 



Many different kinds of animals, such as all 

 sponges, all coelenterates, many flatworms, some 

 jointed worms or annelids, the phoronids, the poly- 

 zoans, some tunicates, the cephalodiscids, some brit- 

 tle-stars and starfishes, and a few crustaceans, either 

 form buds which become detached and develop into 

 independent animals, or, usually at an early stage, 

 divide into two or more parts, each of which develops 

 into an independent animal. 



Many of these types, especially the sponges, the 

 coelenterates, the polyzoans and the tunicates, besides 

 producing buds which become detached, also produce 

 buds which remain attached to the parent animal. 



