84 LHE WORLD OF THE SEA. 
The Polypier hydraria are not true polypiers. Modern te- 
searches have discovered that these tree-like structures are for the 
most part a degraded and transitory form of the medusa. The 
AN ACTINARIA. 
(Gerardia Lamarckiz. ) 
medusa gives birth to the polypier, and in time the polypier will 
become a medusa. They are very common on our coast, and 
include the 7zdularia, the Campanularia, and the Sertularia. 
The 7ubularia indivisa is a very curious specimen of the class. 
Its numerous stems are horny and yellow, and marked with un- 
equal knots, like the stem of the oat straw. Its lower extremity 
