Slugs and snails are very similar gastropods belonging to the mollusk phylum in the animal kingdom. Because they produce such ample amounts of slime and mucous, slugs and snails are the only mollusks able to live on land. Though they may look identical, these animals do have a few biological differences.
Differences in Shell
The long body and tentacles of slug is very similar to those of a snail, but the main physical difference between the two species is the shell. The snail uses its shell as an external attachment to protect and cover its body. The slug, however, has a small inner shell, which lies underneath the skin. Slugs and snails both breathe through an air hole on their mantle, called a pneumostome. On snails, the pneumostome lies is juxtaposed next to the shell.
Slugs are extremely sensitive to even the slightest temperature change. Slugs find shelter to protect them from extremely cold weather and will forage underground to hide from the heat and prevent dehydration. They can also live under water for short bouts of time, but may drown after a few hours. Slugs take one about one year to reach sexual maturity and have a lifespan of only two years.
There are hundreds of varieties of snails in America alone. Of these, some are able to live on land, while others can live in water. Aquatic and land snails breathe differently --- the land snail receives air through the breathing hole near its mantle, whereas the water snail uses a tube to collect air from the water surface. A snail shell grows in a spiral coil and matures as the snail matures.
Snails require two years to reach sexual maturity. Some species of garden snails can live up to 25 years.
Both snails and slugs are hermaphrodites, which means they contain male and female reproductive organs. They can lay between 80 to 100 eggs, up to six times a year. Eggs can be self-fertilized and are typically found hidden beneath rocks, near plants or under the soil. Both gastropods are considered garden-variety pests, as they can damage and eat leaves and fruits from plants. They also leave a trail of mucous as they travel.