NATURE AT SEA. 



75 



length of several yards, and which are covered all over with bat- 

 teries of poison cells, a touch from which is like the sting of a 

 dozen nettles ; the reproducers are the very small polyps at the 

 bases of the tentacles; while the locomotor float represents a 

 polyp or hydra which has become modified the most of all. 

 This float, like a miniature sail, may be raised or reefed by ad- 

 mitting or expelling the air, and if punctured it collapses like a 

 toy balloon. 



The physalias are hydrozoa that is to say, they belong to 

 that large class of marine forms which include, with the little 



FIG. 7. SHIP IN PHOSPHORESCENT WATERS. 



green and brown hydras of fresh- water ponds, the highly colored 

 or glass-like jellyfish or medusae, and those numerous hydroid 

 colonies or branching stocks which often remind one more of 

 small shrubs or some vegetable growth than of a community of 

 animals. Many hydrozoa possess marvelously complicated life 

 histories. By " alternation of generations" that is, by the regu- 

 lar alternation of a sexual generation with one or more genera- 

 tions reproducing asexually by budding or division and by divi- 

 sion of labor, an almost unlimited number of individuals with 

 various functions, as we saw in physalia, may arise from a single 

 polyp egg. 



