34 



Invertebrata. 



arises from the first bud before it has detached itself 

 from its parent. 



Later on in the season, eggs form within the Hydra, 

 beginning as modified cells of the inner layer. These 



FIG. 20. 



\ 



Sert-ularia alietina, one of the sea-firs ; a natural size, b magnified. The 

 figure shows the stem, or ccenosarc of the colony, with its polypites. 



burst through the surface (fig. 18, d), become free, and 

 in spring they shed their outer layer and proceed to 

 develop into new Hydrae, which in course of time give 

 rise to buds. 



Sub-Class 1 : Hydroida. All the lower coelenterate 

 animals are built on the pattern of the hydra, they 



