i88 MANUAL OF ELEMENTARY ZOOLOGY 



mesenteries, and a short ectodermal gullet, and gives rise to 

 the medusa, not by budding, but by dividing transversely 

 into slices {strobilisation), so that it appears for a while like 

 a pile of saucers. Each saucer then floats off, turns over, 

 and becomes a little jelly-fish or ephyra, which grows and 

 takes on the adult form. It seems likely that this curious 

 process arose by a polyp acquiring the habit of breaking off 

 its free end and sending it floating with the gonads to distri- 

 bute the species, and that then this came to be repeated 

 before the first slice was set free. In any case, while the 

 medusa of a hydroid probably represents a whole polyp 

 remodelled for a floating life, that of the hydra tuba is only 

 the top of a polyp remodelled, though the structure of 

 polyps is such that the result in the two cases is sub- 

 stantially the same. All these and other animals whose 

 structure is fundamentally that of a polyp or medusa are 

 known as Ccelenterata. 



