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* 
