damp earth disappeared first? What is indicated as to a sense of taste?
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 color 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.
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Fig. 79.—Sand Worm × 2/3 (Nereis).
There are four classes in the branch Vermes: 1) the earthworms, including sandworms and leeches; 2) the roundworms, including trichina, hairworms, and vinegar eels; 3) flatworms, including tapeworm and liver fluke; 4) rotifers, which are mere specks in size.
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Fig. 80.—Head of Sandworm (enlarged).
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
