268 OCTOBER. 



swimming by means of cilia. This has been called a 

 planula. After swimming a while, it alters its form to 

 that of a pear, and presently adheres by its slender end 

 to a sea- weed or rock under water, hanging downward. 

 A depression now appears in the larger end, which 

 deepens and forms a mouth and stomach, and the little 

 planula has assumed a polype-form. Four tiny warts 

 now spring from the margin of the mouth, which 

 lengthen into tentacles ; four more then shoot in the inter- 

 spaces ; these eight increase to sixteen, then to thirty- 

 two, all at the same time acquiring great length. In 

 this stage, in which it is very common in our aquaria, 

 it has been supposed a new animal, and has been 

 named Hydra tuba. The space between the margin 

 and the mouth has widened into an " umbrella," and 

 the mouth has protruded into a polypite. The whole 

 is of a translucent white hue, and the body without 

 the tentacles is ordinarily about one-sixth of an inch 

 high. 



This stage sometimes lasts for years without further 

 change, except that creeping root-threads shoot from 

 the attached base, which send up at intervals buds that 

 grow into Hydros ; and buds break out from different 

 parts of the body itself, which likewise develop them- 

 selves, the form in both cases being exactly similar to 

 that of the present Hydra tula. Thus we frequently 



