244. ALTERNATE GENERATIONS. 
themselves organic structure, and had begun to 
live and move. It swims by means of rapid 
contractions and expansions of its disk, thus 
impelling itself through the water, its tentacles 
floating behind it and measuring many times 
the length of the body. The disk is very con- 
Little Jelly-Fish, called Sarsia, the free Medusa of Coryne mirabilis, 
vex, as will be seen by the wood-cut; four tubes 
radiate from the central cavity to the periphery, 
