The common starfish seen from below to show the tube feet. About one half natural size.
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.
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.
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.
A common snail, a mollusk. (From a photograph by Davison.)
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.
VII. Mollusca.—Another large group is the Mollusca. This phylum gets its name from the soft, unsegmented body (mollis = soft). Mollusks usually have a shell, which may be of one piece, as a snail, or two pieces or valves, as the clam or oyster.
