FIG. 79.—SAND WORM × ⅔ (Nereis). ] Why is the bilateral type of structure better adapted for development and higher organization than the radiate type of the starfish? The earthworm’s body is a double tube; the hydra’s body is a single tube; which plan is more advantageous, and why? Would any other colour do just as well for an earthworm? Why, or why not? The _sandworm_ (Nereis) lives in the sand of the seashore, and swims in the sea at night (Fig. 79). It is more advanced in structure than the earthworm, as it has a distinct head (Fig. 80), eyes, two teeth, two lips, and several pairs of antennæ, and two rows of muscular projections which serve as feet. It is much used by fishermen for bait. If more easily obtained, it may be studied instead of the earthworm. [Illustration: FIG. 80.—HEAD OF SANDWORM (enlarged). ] =There are four classes in the branch Vermes=: 1) the _worms_, including sandworms and leeches; 2) the _roundworms_, including trichina, hairworms, and vinegar eels; 3) _flatworms_, including tapeworm and liver fluke; 4) _rotifers_, which are microscopic aquatic forms. The =tapeworm= is a flatworm which has lost most of its organs on account of its parasitic life. Its egg is picked up by an herbivorous animal when grazing. The embryo undergoes only partial development in the body of the herbivorous animal, _e.g._ an ox. The next stage will not develop until the beef is eaten by a carnivorous animal, to whose food canal it attaches itself and soon develops a long chain of segments called a “tape.” Each segment absorbs fluid food through its body wall. As the segments at the older end mature, each becomes full of eggs, and the segments become detached and pass out of the canal, to be dropped and perhaps picked up by an herbivorous animal and the life cycle is repeated. The =trichina= is more dangerous to human life than is the tapeworm. It gets into the food canal in uncooked pork (bologna sausage, for example), multiplies there, migrates into the muscles, causing great pain, and encysts there, remaining until the death of the host. It is believed to get into the bodies of hogs again when they eat rats, which in turn have obtained the cysts from carcasses.