[Illustration: Sea anemones. One half natural size. The right hand specimen is expanded and shows the mouth surrounded by the tentacles. The left hand specimen is contracted. (From model at the American Museum of Natural History.)] III. Coelenterates.--The hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the _Coelenterata_. The word "coelenterate" (_coelom_ = body cavity, _enteron_ = food tube) explains the structure of the group. They are animals in which the real body cavity is lacking, the animal in its simplest form being little more than a bag. Some examples are the hydra, shown on page 179, salt-water forms known as hydroids, colonial forms which have part of their life free swimming as jellyfish; sea anemones and coral polyps, tiny colonial hydra like forms which build a living or secreted covering. IV. Worms.--The wormlike animals are grouped into _flatworms_, _roundworms_, and segmented or _jointed_ worms. (_a_) Flatworms are sometimes parasitic, examples being the tapeworm and liver fluke. They are usually small, ribbon- or leaf-like and flat and live in water. (_b_) Roundworms, minute threadlike creatures, are not often seen by the city girl or boy. Vinegar eels, the horsehair worm, the pork worm or trichina and the dread hookworm are examples. (_c_) Segmented worms are long, jointed creatures composed of body rings or segments. Examples are the earthworm, the sandworm (known to New York boys as the fishworm), and the leeches or bloodsuckers. [Illustration: A jointed worm. The sandworm. Slightly reduced.] [Illustration: The common starfish seen from below to show the tube feet. About one half natural size.] [Illustration: The crayfish, a crustacean. _A_, antenna; _M_, mouth; _E_, compound stalked eye; _Ch_, pincher claw; _C.P._, cephalothorax; _Ab_, abdomen; _C.F._, caudal fin. A little reduced.] V. Echinoderms.--These are spiny-skinned animals, which live in salt water. They are still more complicated in structure than the worms and may be known by the spines in their skin. They show radial symmetry. Starfish or sea urchins are examples. VI. Arthropods.--These animals are distinguished by having jointed body and legs. They form two great groups. The higher forms of the _Crustacea_ have only two regions in the body, a fused head and thorax, called the _cephalothorax_, and an abdominal region. A second group is the _Insecta_, of which we know something already. Crustacea breathe by means of _gills_, which are structures for taking oxygen out of the water, while adult insects breathe through air tubes called _trachea_. Two smaller groups of arthropods also exist, the _Arachnida_, consisting of spiders, scorpions, ticks, and mites, and the _Myriapoda_, examples being the "thousand leggers" found in some city houses. [Illustration: A common snail, a mollusk. (From a photograph by Davison.)]