=The muscular coat= beneath, and much thicker than the skin, consists of two layers: an outer _layer runs around the body_ just beneath the skin, and an inner, thicker _layer of fibers runs lengthwise_. The worm crawls by shortening the longitudinal muscles. As the bristles (_setæ_) point backward, they prevent the front part of the body from slipping back, so the hinder part is drawn forward. Next, the circular muscles contract, and the bristles preventing the hind part from slipping back, the fore portion is pushed forward. Is the worm thicker when the hinder part is being pulled up or when the fore part is being thrust forward? Does the earthworm pull or push itself along, or does it do both? Occasionally it travels backward, _e.g._ it sometimes goes backward into its hole. Then the bristles are directed forward. The right and left halves of the body are counterparts of each other, hence the earthworm is _bilaterally symmetrical_. The lungs and gills of animals must always be kept moist. The worm _cannot live long in dry air_, for respiration in the skin ceases when it cannot be kept moist, and the worm smothers. Long immersion in water is injurious to them, perhaps because there is far less oxygen in water than in the air. Darwin wrote a book called “Vegetable Mold and Earthworms.” He estimated that there were fifty thousand earthworms to the acre on farm land in England, and that they bring up eighteen tons of soil in an acre each year. As the acids of the food tube act upon the mineral grains that pass through it, the earthworm renders _great aid in forming soil_. By burrowing it makes the soil more _porous_ and brings up the subsoil. Although without eyes, the worm is sensitive to light falling upon its anterior segments. When the light of a lantern suddenly strikes it at night, it crawls quickly to its burrow. Its sense of touch is so keen that it can detect a light puff of breath. Which of the foods kept in a box of 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. [Illustration: FIG. 79.--SAND WORM × ²⁄₃ (Nereis).]