length of several yards, and which are covered all over with batteries 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 admitting 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
green and brown hydras of fresh-water ponds, the highly colored or glass-like jellyfish or medusæ, 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 regular alternation of a sexual generation with one or more generations reproducing asexually by budding or division—and by division of labor, an almost unlimited number of individuals with various functions, as we saw in physalia, may arise from a single polyp egg.