250 ALTERNATE GENERATIONS. 
a degree that it remains united to them by a 
mere thread only, and it soon breaks off and dies. 
This is the signal for the breaking up of the whole 
pile in the same way by the deepening of the con- 
strictions ; but, instead of dying, as they part, 
they begin a new existence as free Medusx. Only 
the lowest portion of the body remains, and around 
its margin new tentacles are developed, corre- 
sponding to those which crowned the first little 
embryo. This repeats the whole history again, 
b 
G 
a 
Strobila of Aurelia flavidula: a, Scyphostoma reproduced at the base of a 
Strobila, all the disks of which have dropped off but the last, bb. 
as it grows up during the following season to 
divide itself anew into disks, like its predecessor. 
As each individual separates from the commu- 
nity of which it has made a part, it reverses its 
position, and, instead of turning the margin of 
the disk upward, it turns it downward, thus 
bringing the mouth below, and the curve of the 
disk above. These free individuals have been 
described under the name of Ephyra. This is 
the third phase of the existence of our Jelly-Fish. 
It swims freely about, a transparent, umbrella- 
