Terrestrial Snails

Snail on a branch.

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Terrestrial snails, also known as land snails, are a group of land-dwelling gastropods that have the ability to breathe air. Terrestrial snails include more than just snails, they also include slugs (which are very similar to snails except they lack a shell). Terrestrial snails are known by the scientific name Heterobranchia and are also sometimes referred to by an older (now deprecated) group name, Pulmonata.

Terrestrial snails are one of the most diverse groups of animals alive today, both in terms of their variety of form and the sheer number of species that exist. Today, there are more than 40,000 living species of terrestrial snails.

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What Does a Snail's Shell Do?

Snail in grass

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A snail's shell serves to protect its internal organs, prevent water loss, provide shelter from cold, and protect the snail from predators. A snail's shell is secreted by glands in its mantle rim.

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What is the Structure of a Snail's Shell?

Small snail on leaf.

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The shell of a snail consists of three layers, the hypostracum, the ostracum and the periostracum. The hypostracum is the innermost layer of the shell and lies closest to the snail's body. The ostracum is the middle, shell-building layer and consists of prism-shaped calcium carbonate crystals and organic (proteid) molecules. Finally, the periostracum is the outermost layer of a snail's shell and it consists of conchin (a mixture of organic compounds) and is the layer that gives the shell its color.

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Sorting Snails and Slugs

Snail on leaf.

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Terrestrial snails are classified in the same taxonomic group as terrestrial slugs because they share many similarities. The scientific name for the group that includes terrestrial snails and slugs is called the Stylommatophora.

Terrestrial snails and slugs have less in common with their marine counterparts, the nudibranchs (also called the sea slugs or sea hares). Nudibranchs are classified into a separate group called the Nudibranchia.

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How Are Snails Classified?

Snail on leaf stem.

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Snails are invertebrates, which means they lack a backbone. They belong to a large and highly diverse group of invertebrates known as the mollusks (Mollusca). In addition to snails, other mollusks include slugs, clams, oysters, mussels, squids, octopuses, and nautiluses.

Within the mollusks, snails are classified into a group called the gastropods (Gastropoda). In addition to snails, gastropods include terrestrial slugs, freshwater limpets, sea snails, and sea slugs. An even more exclusive group of gastropods has been created that contains only air-breathing land snails. This subgroup of gastropods is known as the pulmonates.

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Peculiarities of Snail Anatomy

Snail on leaf.

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Snails have a single, often spirally coiled shell (univalve), they undergo a developmental process called torsion, and they possess a mantle and a muscular foot used for locomotion. Snails and slugs have eyes on the top of tentacles (sea snails have eyes at the base of their tentacles).

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What Do Snails Eat?

Close up of snail's mouth.

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Terrestrial snails are herbivorous. They feed on plant material (such as leaves, stems, and soft bark), fruits, and algae. Snails have a rough tongue called a radula that they use to scrape bits of food into their mouths. They also have rows of tiny teeth made of chiton.

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Why Do Snails Need Calcium?

Snail on aloe plant.

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Snails need calcium to build their shells. Snails obtain calcium from a variety of sources such as dirt and rocks (they use their radula to grind bits from soft stones such as limestone). The calcium snails ingest is absorbed during digestion and is used by the mantle to create the shell. 

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What Habitat Do Snails Prefer?

Snail on branch.

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Snails first evolved in marine habitats and later expanded into freshwater and terrestrial habitats. Terrestrial snails live in moist, shady environments such as forests and gardens.

A snail's shell provides it with protection from changing weather conditions. In arid regions, snails have thicker shells that help them retain their body moisture. In humid regions, snails tend to have thinner shells. Some species burrow into the ground where they remain dormant, waiting for rain to soften the ground. In cold weather, snails hibernate.

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How Do Snails Move?

Close up of snail on branch.

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Terrestrial snails move using their muscular foot. By creating an undulating wave-like motion along the length of the foot, a snail is able to push against a surface and propel its body forward, albeit slowly. At top speed snails cover a mere 3 inches per minute. Their progress is slowed by the weight of their shell. In proportion to their body size, the shell is quite a load to carry.

To help them move, snails secrete a stream of slime (mucus) from a gland located at the front of their foot. This slime enables them to glide smoothly over many different types of surface and helps to form a suction that helps them cling to vegetation and even hang upside down.

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Snail Life Cycle and Development

Snails on leaf.

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Snails start life as an egg buried in a nest a few centimeters below the surface of the ground. Snail eggs hatch after about two to four weeks depending on the weather and environmental conditions (most importantly, temperature and soil moisture). After hatching, the newborn snail sets out on an urgent search for food.

The young snails are so hungry, they feed on the leftover shell and any nearby eggs that have not yet hatched. As the snail grows, so does its shell. The oldest part of the shell is located at the center of the coil while the most recently added parts of the shell are at the rim. When the snail matures after a few years, the snail mates and lays eggs, thus completing the full life cycle of a snail.

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Snail Senses

Snails on a leaf.

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Terrestrial snails have primitive eyes (referred to as eyespots) that are located on the tips of their upper, longer pair of tentacles. But snails don't see in the same way we do. Their eyes are less complex and provide them with a general sense of light and dark in their surroundings.

The short tentacles located on a snail's head are very sensitive to touch sensations and are used to help the snail build a picture of its environment based on feeling nearby objects. Snails don't have ears but instead use their bottom set of tentacles to pick up sound vibrations in the air.

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The Evolution of Snails

Snail on branch

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The earliest known snails were similar in structure to limpets. These creatures lived in shallow seawater and fed on algae and they had a pair of gills. The most primitive of the air-breathing snails (also called pulmonates) belonged to a group known as the Ellobiidae. Members of this family still lived in water (salt marshes and coastal waters) but they went to the surface to breathe air. Today's land snails evolved from a different group of snails known as the Endodontidae, a group of snails that were in many ways similar to the Ellobiidae.

When we look back through the fossil record, we can see various tendencies in how snails changed over time. In general, the following patterns emerge. The process of torsion becomes more prominent, the shell became increasingly conical and spirally coiled, and there is a tendency among pulmonates towards the entire loss of a shell.

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Estivation in Snails

Snail shell in leaves.

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Snails are usually active in the summer, but if it gets too warm or too dry for them, they enter a period of inactivity known as estivation. They find a safe place—such as a tree trunk, the underside of a leaf, or a stone wall—and suction themselves onto the surface as they retreat into their shell. Thus protected, they wait until the weather becomes more suitable. Occasionally, snails will go into estivation on the ground. There, they go into their shell and a mucous membrane dries over the opening of their shell, leaving just enough space for air to get inside allowing the snail to breathe.

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Hibernation in Snails

Snail in shell on branch

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In late fall when temperatures drop, snails go into hibernation. They dig a small hole in the ground or find a warm patch, buried in a pile of leaf litter. It is important that a snail finds a suitably protected place to sleep to ensure its survival through the long cold months of winter. They retreat into their shell and seal its opening with a thin layer of white chalk. During hibernation, the snail lives on the fat reserves in its body, built up from a summer of eating vegetation. When spring comes (and with it rain and warmth), the snail wakes and pushes the chalk seal to open the shell once again. If you look closely in spring, you may find a chalky white disc on the forest floor, left behind by a snail that has recently come out of hibernation.

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How Large Do Snails Grow?

Snail on sidewalk

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Snails grow to a variety of different sizes depending on the species and individual. The largest known land snail is the Giant African Snail (Achatina achatina). The Giant African Snail has been known to grow to lengths of up to 30cm.

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Snail Anatomy

snail on rock

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Snails are very different from humans so when we think about body parts, we're often at a loss when relating the familiar parts of a human body to snails. The basic structure of a snail consists of the following body parts: foot, head, shell, visceral mass. The foot and head are the parts of the snail's body that we can see outside its shell, while the visceral mass is located within the snail's shell and includes the snail's internal organs.

A snail's internal organs include a lung, digestive organs (crop, stomach, intestine, anus), a kidney, a liver, and their reproductive organs (genital pore, penis, vagina, oviduct, vas deferens).

A snail's nervous system is made up of numerous nerve centers that each control or interpret sensations for specific parts of the body: cerebral ganglia (senses), buccal ganglia (mouthparts), pedal ganglia (foot), pleural ganglia (mantle), intestinal ganglia (organs), and a visceral ganglia.

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Snail Reproduction

Snail on branch

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Most terrestrial snails are hermaphroditic which means that each individual possesses both male and female reproductive organs. Although the age at which snails reach sexual maturity varies among species, it may be up to three years before snails are old enough to reproduce. Mature snails begin courtship in early summer and after mating both individuals lay fertilized eggs in nests dug out of moist soil. It lays several dozen eggs and then covers them with soil where the stay until they are ready to hatch.

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The Vulnerability of Snails

Snail on flowers

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Snails are small and slow. They have few defenses. They must retain enough moisture so their tiny bodies don't dry out, and they must obtain enough food to give them the energy to sleep through the long cold winter. So despite living in tough shells, snails are, in many ways, quite vulnerable.

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How Snails Protect Themselves

Tiny snail on mushroom

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Despite their vulnerabilities, snails are quite clever and are well adapted to deal with the threats they face. Their shell provides them with good, impenetrable protection from weather variations and some predators. During the daylight hours, they usually hide. This keeps them out of the way of hungry birds and mammals and also helps them conserve moisture.

Snails aren't too popular with some humans. These little creatures can to quickly eat their way through a carefully tended garden, leaving a gardener's treasured plants all but bare. So some people leave poisons and other snail deterrents around their yard, making it very hazardous for snails. Also, since snails don't move quickly, they are frequently in danger of crossing paths with cars or pedestrians. So, be careful where you step if ​you're walking on a moist evening when snails are out and about.

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Snail Strength

Close up of snail on leaf

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Snails can haul up to ten times their own weight when crawling up a vertical surface. When gliding along horizontally, they can carry up to fifty times their weight.

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Klappenbach, Laura. "Terrestrial Snails." ThoughtCo, Aug. 27, 2020, thoughtco.com/guide-to-terrestrial-snails-130415. Klappenbach, Laura. (2020, August 27). Terrestrial Snails. Retrieved from https://www.thoughtco.com/guide-to-terrestrial-snails-130415 Klappenbach, Laura. "Terrestrial Snails." ThoughtCo. https://www.thoughtco.com/guide-to-terrestrial-snails-130415 (accessed June 2, 2023).