THE HYDRA AND JELLY-FISH. 



31 



attaches itself to rocks, dead shells, or sea-weeds, and then 

 assumes a Hydra form, with often twenty-four very long 

 tentacles. This stage was originally described as a distinct 

 animal under the name of Scyphistoma. In this Scyphisto- 

 ma stage (Fig. 26) it remains about eighteen months. 

 Toward the end of this period the body increases in size and 

 divides into a series of cup-shaped disks. These saucer- 

 like disks are scalloped on the upturned edge; tentacles 

 bud out, and the animal assumes the Strobila stage (Fig. 

 27). Finally the disks separate, the upper one becomes 



FIG. 39. Aurelia, one-third of the natural size. 



detached and dies, but the others swim away in the Epliyra 

 form (Fig. 28) when about a fifth of an inch in diameter, 

 and toward the middle or end of summer each becomes an 

 adult Aurelia (Fig. 29). 



In the wonderful changes of the Aurelia we have what is 

 called a metamorphosis ; it is like the change of a caterpillar 

 into a chrysalis and afterward a butterfly. The Scyphisto- 

 ma may be compared to the caterpillar, the tStrobila to the 

 chrysalis, and the free jelly-fish to the butterfly. 



