ALTERNATE GENERATIONS. 249 



body, where a digestive cavity is already formed. 

 At this time the constrictions have deepened, so 

 that the margins of all the successive divisions of 

 the little Hydroid are very prominent, and the 

 whole animal looks like a pile of saucers, or of 

 disks with scalloped edges, and the convex side 

 turned downward. Its general aspect may be 

 compared to a string of Lilac-blossoms, such as 

 the children make for necklaces in the spring, in 



Strobila of Aurelia flavidula. 



which the base of one blossom is inserted into the 

 upper side of the one below it. In this condition 

 our Jelly-Fish has been called Strobila. 



While these organic changes take place in the 

 lower disks, the topmost one, forming the summit 

 of the pile and bearing the tentacles, undergoes 

 no such modification ; but presently the first con- 

 striction dividing it from the rest deepens to such 

 11* 



