The bud (or reproductive hydranth) of the hydroid does not produce a hydroid, but a jellyfish; the egg of the jellyfish does not produce a jellyfish, but a hydroid. This is called by zoologists, _alternation of generations_. A _complete individual_ is the life from the germination of one egg to the production of another. So that an “individual” consists of a hydroid colony fixed in one place together with all the jellyfish produced from its buds, which may now be floating miles away from it in the ocean. Bathers in the surf are sometimes touched and stung by the long, streamer-like tentacles of the jellyfish. These, like the tentacles of the hydra, have nettling cells (Fig. 41). [Illustration: FIG. 45.—CORAL POLYPS (tentacles, a multiple of _six_). Notice hypostome. ] The umbrella-shaped free-swimming jellyfish is called a _medusa_ (Fig. 44). =Coral Polyps.=—Some of the salt water relatives of the hydra produce buds which remain attached to the parent without, however, becoming different from the parent in any way. The _coral polyps_ and _corallines_ are examples of _colonies_ of this kind, possessing a common stalk which is formed as the process of multiplication goes on. In the case of coral polyps, the separate animals and the flesh connecting them secrete within themselves a hard, _limy, supporting structure known as coral_. In some species, the coral, or stony part, is so developed that the polyp seems to be inserted in the coral, into which it withdraws itself for partial protection (Fig. 45). The _corallines_ secrete a smooth stalk which affords no protection, but they also secrete a coating or sheath which incloses both themselves and the stalk. The coating has apertures through which the polyps protrude in order to feed when no danger is near (Fig. 46). The red “corals” used for jewelry are bits of stalks of corallines. The corallines (Figs. 47, 48) are not so abundant nor so important as the coral polyps (Figs. 45, 49). [Illustration: FIG. 46.—RED CORALLINE with crust and polyps (_eight_ tentacles). ] [Illustration: FIG. 47.—SEA FAN (a coralline). ] [Illustration: FIG. 48.—ORGAN PIPE “Coral” (a coralline). ] Colonies of coral polyps grow in countless numbers in the tropical seas. The coral formed by successive colonies of polyps accumulates and builds up many islands and important additions to continents. The Florida “keys,” or islands, and the southern part of the mainland of Florida were so formed. [Illustration: FIG. 49.—UPRIGHT CUT through coral polyp × 4. _ms_, mouth; _mr_, gullet; _ls_, _ls_, fleshy partitions (mesenteries) extending from outer body wall to gullet (to increase absorbing surface); _s_, _s_, shorter partitions; _mb_, _fb_, stony support (of lime, called coral); _t_, tentacles. ] [Illustration: FIG. 50.—SEA ANEMONE. ]