The asymmetry of the body of the snail is very conspicuous, at least so far as certain organs are concerned. The foot on which the animal crawls and the head have preserved their bilaterality; but the visceral mass of the animal, contained in the spirally wound shell, lying on the middle of the upper surface of the foot, is twisted into a spiral form. Many of the organs of one side of the body are atrophied. The gill, the kidney, the reproductive organ, and one of the auricles of the heart have completely, or almost completely, disappeared. The cause of this loss seems to be connected with the spiral twist of the visceral mass. One of the consequences of the twisting has been to bring the organs of the left side of the body around the posterior end until they come to lie on the right side, the organs of the original right side being carried forward and there atrophying. There is another remarkable fact connected with the asymmetry of the snail. In some species, _Helix pomatia_, for example, the twist has been toward the right, _i.e._ in the direction which the hands of a watch follow when the face is turned upward toward the observer. Individuals twisted in this direction are called dextral. Occasionally there is found an individual with the spiral in the opposite direction (sinistral), and in this the conditions of the internal organs are exactly reversed. It is the left set of organs that is now atrophied, and the right set that is functional. Such changes appear suddenly. Organs of one side of the body that have not been functional for many generations may become fully developed. Moreover, Lang has shown that when a sinistral form breeds with a normal dextral form, or even when sinistral forms are bred with each other, the young are practically all of the ordinary type. An attempt has been made to connect these facts with the mode of development of the mollusks. It is known that the eggs of a number of gasteropod mollusks segment in a perfectly definite manner. A sort of spiral cleavage is followed by the formation of a large mesodermal cell from the left posterior yolk-cell. From this mesodermal cell nearly all the mesodermal organs of the body are formed. Thus it may appear that the spiral form of the snail is connected with the spiral form of the cleavage. In a few species of marine and fresh-water snails the cleavage spiral is reversed, and the mesoderm arises from the right posterior yolk-cell. It has been shown in several cases that the snail coming from such an egg is twisted in the reverse direction from that of ordinary snails.