Perhaps one-half of all the known species of molluscs are snails and slugs (fig. 108). Snails are either aquatic or terrestrial in habit, but in either case they (the true pulmonate snails) breathe not by means of gills, as do most of the other molluscs, but by means of a so-called "lung." This lung is a sac with an external opening on the right side of the body and with its inner surface richly furnished with fine blood-vessels. The exchange of gases between the blood and the outer air takes place through the thin walls of the blood-vessels. Most snails which live in the water, as the pond-snails and the river-snails, have to come occasionally to the surface to breathe. These fresh-water and land-molluscs which possess a lung-sac instead of gills constitute the order Pulmonata. The pulmonate pond- and land-snails and slugs are vegetable feeders and where they occur in large numbers do much injury to vegetation. While the common pond-snails have but one pair of feelers, at the base of which are found the eyes, most of the land-snails and slugs have two pairs of "horns," the eyes being on the tips of the second pair. The lung-sac, besides serving as a breathing organ, also enables the snail to rise or sink according as the animal varies the size of the sac and consequently the amount of air in it. All the Pulmonata are hermaphroditic, each individual producing both sperm- and egg-cells. The eggs of the pond-snail "are laid in gelatinous transparent capsules, half an inch to an inch in length, flattened and linear or oblong in outline. After a few snails have been kept a short time in a small vessel of water with their appropriate food, these egg-capsules may be looked for on the bottom and sides of the vessel or closely adherent to the stems or leaves of plants placed in the water. They are so transparent as to be easily overlooked." Young snails may be reared from these eggs. There are other snails common in ponds, also called, like the pulmonate forms, pond-snails, which have gills and no lung-sac. These pond-snails belong to a different order of molluscs, and live on the bottom of the pond, crawling about in the soft mud and feeding on animal instead of vegetable food. The shells of the various kinds of snails vary much. In many of the land-snails the spiral is not spire-shaped or conical, but is flat. In some the whorls of the spiral run from left to right (dextral) when the shell is looked at with apex held toward one, while in others the whorls run from right to left (sinistral). [Illustration: FIG. 109.--Three Pacific Coast nudibranchs; _Doris tuberculata_ (in lower left-hand corner), _Echinodoris_ sp. (upper one), and _Triopha modesta_ (at right). (From living specimens in a tide-pool on the Bay of Monterey, California.)]