3. The =Polyps= are the only many-celled animals having a single opening into the body, serving for both mouth and vent. They are radiate in structure, and usually possess tentacles. 4. The =Echinoderms= are marine animals of more or less radiate appearance, having a food tube in the body separate from the body wall. The following groups are plainly bilateral: that is, dorsal and ventral surfaces, front and hind ends are different. 5. The =Vermes= have usually a segmented body but lack jointed legs. 6. The =Arthropods=[3] have an external skeleton and jointed legs. Footnote 3: Insects and crustaceans. 7. The =Molluscs= have soft bodies, no legs, no skeleton, but usually a limy shell. 8. The =Vertebrates= have an internal skeleton of bones, and usually two pairs of legs. [Illustration: FIG. 247.—A SNAIL. (Which branch? Why?) ] CHAPTER XI BATRACHIA The theory of evolution teaches that animal life began in a very simple form in the sea, and that afterward the higher sea animals lost their gills and developed lungs and legs and came out to live upon the land; truly a marvellous procedure, and incredible to many, although the process is repeated every spring in countless instances in pond and brook. In popular language, every cold-blooded vertebrate breathing with lungs is called a reptile. The name reptile is properly applied only to lizards, snakes, turtles, and alligators. The common mistake of speaking of frogs and salamanders as reptiles arises from considering them only in their adult condition. Reptiles hatch from the egg as tiny reptiles resembling the adult forms; frogs and salamanders, as every one knows, leave the egg in the form of tadpoles (Fig. 248). The fact that frogs and salamanders begin active life as fishes, breathing by gills, serves to distinguish them from other cold-blooded animals, and causes naturalists to place them in a separate class, called batrachia (twice breather) or amphibia (double life). TADPOLES SUGGESTIONS.—Tadpoles may be studied by placing a number of frog’s eggs in a jar of water, care being taken not to place a large number of eggs in a small amount of water. When they hatch, water plants (_e.g._ green algæ) should be added for food. The behaviour of frogs may be best studied in a tub of water. A toad in captivity should be given a cool, moist place, and fed well. A piece of meat placed near a toad may attract flies, and the toad may be observed while catching them, but the motion is so swift as to be almost imperceptible. Live flies may be put into a glass jar with a toad. Toads do not move about until twilight, except in cloudy, wet weather. They return to ponds and brooks in spring at the time for laying eggs. This time for both frogs and toads is shown by trilling. All frogs, except tree frogs, remain in or near the water all the year. [Illustration: