^^^^ THE NEW EVOLUTION Wf 



and in this way build up more or less extensive 

 colonies (figs. 59, Gy, p. iii; figs. 69-72., p. 12.7). 



There are very many variations of this process. 

 For instance some sponges, including the sponges of 

 fresh water, and most of the fresh water polyzoans, 

 produce as they grow internal buds consisting of 

 masses of cells encased in a tough and usually more or 

 less complicated shell. After the animal dies these 

 little seed-like structures are liberated through the 

 decomposition and disintegration of the body and 

 drift away, to grow into a new animal, or colony, if 

 and when conditions become favorable. 



In most polyzoans the tentacles, alimentary canal 

 and nervous system of the individuals from time to 

 time disintegrate, and a new set of these organs may 

 be formed through internal budding from the per- 

 sistent body wall of such partially disintegrated 

 polypides. 



In the insects in which the metamorphosis is in- 

 complete, as for instance the grasshoppers, mantles 

 (fig. 19, p. 33) and bugs (fig. ^3, p. 33), the body of 

 the young grows gradually into the body of the adult, 

 and the new organs, such as the wings, are formed as 

 simple outgrowths. 



In those insects, as the beetles, wasps, butterflies 

 and flies, in which the young are very different from 

 the adults a few of the systems of organs, such as the 

 reproductive, the nervous and the circulatory, persist 

 from the larva to the adult, but the other organs dis- 

 integrate, being later entirely rebuilt from special 

 groups of cells called imaginal disks. 



