III. Cœlenterates.—The hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the Cœlenterata. The word "cœlenterate" (cœlom = 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.
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.)
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.
A jointed worm. The sandworm. Slightly reduced.
