32 INTRODUCTION TO ZOOLOGY chap. 



and Plumulmia, the polyps grow from one side of the stem 

 only. Obelia is a very common genus, forming a fur-like 

 growth on the wooden piles of piers and wharfs. 



Fig. 13. — Sertularia pmnila. 

 S, Natural .size ; A, enlarged ; p, polyp ; c, horny cup. 



Those with Bougainvillea is a little marine form which is 

 no horny found attached to rocks and shells below the 



Cip- surface in the lowest tide pools, many colonies, 

 each 1 or 2 inches long, growing together, and so forming 

 minute, branched, tree-like growths (Fig. 13, A). The 

 particular point of interest about this genus is the specialisa- 

 tion of some of the polyps to form bell-shaped individuals 

 which alone form reproductive cells. These peculiarly modified 

 individuals break away from the colony, and swim off like 

 minute jelly-fish, and so serve the purpose of dispersing the 

 genus far and wide. 



A reference to Fig. \i, B and C, will make clear the 

 main characters of the two kinds of individuals. The 

 ordinary nutritive polyp is very like a Hydra with a short 

 tubular body and a ring of tentacles surrounding the mouth 

 cone. 



, The reproductive swimming bells, or Medusae, 



as they are called, are shown in Fig. 14 in dif- 

 ferent stages. They arise as lateral buds, assume a bell or 

 goblet shape, and develop a mouth-tube, which hangs down 

 inside the " bell." 



