FIG. 78.—Side view, showing setæ, nephridia pores, and reproductive openings. ] =Habits.=—The earthworm is _omnivorous_. It will eat bits of meat as well as leaves and other vegetation. It has also the advantage, when digging its hole, of _eating the earth_ which must be excavated. Every one has noticed the fresh “casts” piled up at the holes in the morning. As the holes are partly filled by rains, the casts are most abundant after rains. The chief _enemies_ of the earthworm are moles and birds. The worms _work at night_ and retire so early in the morning that the very early bird has the advantage in catching worms. Perhaps the nearest to an intelligent act the earthworm accomplishes is to _conceal the mouth of its hole_ by plugging it with a pebble or a bit of leaf. Worms _hibernate_, going below danger of frost in winter. In dry weather they burrow several feet deep. =The muscular coat= of the body wall is much thicker than the skin. It consists of two layers: an outer _layer of fibres which run around the body_ just beneath the skin, and an inner, thicker _layer of fibres which run 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 the 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 it, perhaps because there is far less oxygen in water than in the air. Darwin wrote a book called “Vegetable Mould 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? [Illustration: