=The Branch Polyps= (sometimes called _Cœlenterata_).—The hydra is the chief _fresh-water representative_ of this great branch of the animal kingdom. This branch is characterized by its members having only one opening to the body. The polyps also include the salt water animals called _hydroids_, _jellyfishes_, and _coral polyps._ =Hydroids.=—Figure 40 shows a _hydroid_, or group of hydra-like growths, one of which eats and digests for the group, another defends by nettling cells, another produces eggs. Each hydra-like part of a hydroid is called a _hydranth_. Sometimes the buds on the hydra remain attached so long that a bud forms upon the first bud. Thus three generations are represented in one organism. Such growths show us that it is not always easy to tell what constitutes an individual animal. [Illustration: FIG. 42.—The formation of many free-swimming jellyfishes from one fixed hydra-like form. The saucer-like parts (_h_) turn over after they separate and become like Fig. 43 or 44. Letters show sequence of diagrams. ] _Hydroids_ may be conceived _to have been developed_ by the failure of budding hydras to separate from the parent, and by the gradual formation of the habit of living together and assisting one another. When each hydranth of the hydroid devoted itself to a special function of digestion, defence, or reproduction, this group lived longer and prospered; more eggs were formed, and the habits of the group were transmitted to a more numerous progeny than were the habits of a group where members worked more independently of one another. As the _sponge_ is a simple example of the devotion of _special cells to special purposes_, the hydroid is a primitive and simple example of the occurrence of _organs_, that is of _special parts of the body set aside for a special work_. [Illustration: FIG. 43.—A JELLYFISH. ] How many mature hydranths are seen in the hydroid shown in Fig. 40? Why are the defensive hydranths on the outside of the colony? Which hydranths have no tentacles? Why not? [Illustration: FIG. 44.—A JELLYFISH (medusa). ] =Jellyfish.—Alternation of Generations.—Medusa.=—With some species of hydroids, a very curious thing happens.—The _hydranth that is to produce the eggs falls off_ and becomes independent of the colony. More surprising still, its appearance changes entirely and instead of being hydra-like, it becomes the large and complex creature called _jellyfish_ (Fig. 43). But the _egg of the jellyfish_ produces a small _hydra-like animal_ which gives rise by budding to a _hydroid_, and the cycle is complete.