Snail Skeletal System

The skeletal system plays a significant role to support the structural framework of the organism. Supporting the body’s movement and protecting vital internal organs are the primary functions of a skeletal system. Snails belong to the phylum Mollusca and come under the category of invertebrates. The snail's structural framework consists of hard internal or external substances. However, the snail's skeletal system lacks bony structures.

The skeletal system is needed to actually support the body, protect internal organs, and enable an organism to move. There are three different shapes of the skeleton that satisfy these basic functions: hydrostatic skeleton, exoskeleton, and endoskeleton. In terms of classified species, gastropods (snails and slugs) are by far the most numerous molluscs, accounting for a large part of the molluscan population. 

The Gastropoda class includes about 40,000 species of snails, slugs, and similar species. This class is primarily a marine group, but it also contains many molluscs of freshwater and land. Most gastropods have a shell, but some have lost their shells through evolution, like slugs and nudibranchs. Gastropods usually run along a foot that can be modified to swim.

The shell on the snail's back serves as the skeleton for it, as well as protection and camouflage, like most molluscs. The shell of a gastropod is a shell that is part of a gastropod or snail's body. The shell of the gastropod is an external skeleton or exoskeleton that serves not only for muscle attachment but also for predator and mechanical damage protection. The shell also offers essential protection against the sun and drying out in land snails and in some freshwater snails and in marine snails. Most shells of the gastropod are spiral and usually the coiling is right-handed. Only some gastropods are shell-less (slugs), but there's a shell in most gastropods. In almost all cases, the shell consists of one piece and is typically spirally coiled, though some groups, such as the different families and limpets, have simple cone shells as adults.

Snails, slugs, and other similar species come under the gastropods class. Most gastropod species consist of an outer shell. Therefore, Snails too have a hard and non-bony outer shell. The skeletal system type of snails is referred to as the ‘Exoskeletal System’.

Exoskeleton

The exoskeleton is an external skeleton formed by a hard enclosure on an organism's surface. Crabs and insect shells, for example, are exoskeletons. This type of skeleton provides defense against predators, supports the body, and allows movement through muscle contraction. Like vertebrates, inside the exoskeleton muscles must cross a joint. Muscle shortening changes the relationship between the exoskeleton's two segments. Arthropods like crabs and lobsters have exoskeletons consisting of 30% to 50% chitin, a polysaccharide glucose derivative that is a strong but flexible material. The epidermal cells secrete chitin. The exoskeleton is further reinforced in organisms such as the lobster by adding calcium carbonate. Since the exoskeleton is acellular, arthropods need to shed their exoskeletons periodically because the exoskeleton does not grow as the organism grows.

The primary functions of an exoskeleton are,

• To protect the organism from the predator attack.

• To support the framework of the body.

• To enable contraction of muscles and promote body movement.

Snails and slugs are known as gastropods collectively. Not every gastropod has a shell, but most of them do. Snails have on their back a hard shell that serves as a skeleton. The shell on the snails ' back is the outer skeleton or exoskeleton. They provide muscle attachment with a wide surface area. They also play the role of camouflage and mechanical stress and predators’ protection. Shells protect them from sun and water loss, especially inland snails. In fact, every gastropod has a shell, but it has reduced and eased its mobility during embryonic development.

The gastropod shell consists of three differentiated layers consisting mainly of calcium (calcium carbonate) and approximately 2% of protein. These shells are non-cellular structures, unlike structures of the animal body. Mantle tissue is a part of the body of mollusks in the shell. Mantle tissue contacts the shell directly. For shell formation, they secrete and precipitate minerals such as calcium and proteins. Proteins, therefore, act as the building block while the shell is polished by calcium. The exoskeleton never sheds, so the shell grows as the body enlarges from bottom to top. The three shell layers include an inner nacre layer, a prism layer, and a protein periosteum.

Among these, calcium is rich and calcified in the inner and middle layers, while the outer layer is uncalcified and consists of proteins. Shells also vary in form and size. Spirally coiled shells are typically observed, but shells are cone-shaped in some varieties such as limpets.

External Anatomy

Let's divide the snails' body into the shell and the soft body that holds it to analyze the snails ' external anatomy. The former is a solid spiral-shaped structure carried on the back, consisting mainly of calcium carbonate, made of a single piece. The shell's central layer, called ostracum, has two layers of the same substance's crystals, calcium carbonate. The Hipostracum is below and the most superficial layer is the periostracum, which is made up of many proteins.

Depending on the species, a land snail's shell can be very different in size and shape. Some of them, while others are round, are cone-shaped. They all have a spiral design, however, caused by the manner in which land snails produce and their shells grow. This structure protects the snail against the environment and even against predators. It consists of calcium carbonate that makes it strong and remains so long a time as the snail consumes calcium food. Its surface with fringe designs can show different colors, but they are usually brown or yellow. The shell protects the animal's body and internal organs and has a side opening, usually the right one. 

The rest of the body is soft, with a viscous texture and dark spot colors. The foot has a wave-like movement produced by muscular contractions that make the snail "glide" while the foot secretes slippery mucus that reduces the friction on the surface in which it moves. This mucus is the "trace" left on the ground by the mollusk as it moves. The head has one to two pairs of tentacles at one end of the body (retractable and provided with tactile receptors), with eyes at the tips. The lower pair is working to smell like olfactory organs.

It also has an outer tissue skin fold that covers the internal organs and usually also covers the shell and the cavity of the mantle. You may not always see their tentacles because they can be retracted by all land snails.

Some species of land secrete a layer of mucus that blocks the shell's entrance when hardened and is called an epiphragm. They hide in the shell when snails feel danger around them. When the weather is hot and dry, snails spend a long time in their shell. Otherwise, they might dry out their moist bodies. 

Snails vary in color and size. The largest are members of the Achatinidae family, which can reach a length of up to 11.8 inches and a diameter of up to 5.9 inches for the species Achatina Achatina.

Internal Anatomy

Inside, there are no divisions in the snails ' bodies. The internal organs, including gonads, intestines, heart and esophagus create a mantle-protected organic mass. They are pulmonary animals, meaning they have a lung that is specialized in using the oxygen obtained from atmospheric air-breathing.

They don't have a brain like dogs or humans, on the other hand. Nerve cells instead concentrate in a set of ganglia and emit neurosecretion that triggers necessary actions such as hormone release. The ganglia interconnect at high speed with bundles of nerve fibers that carry the signals. Although this is a rudimentary brain, associative thinking has excellent abilities.

Snails ' sense of sight is useful but only detects changes in light intensity to recognize whether it's night or day; they can move up or down their tentacles to enhance their viewing ability. They are almost deaf, though, because they have no ears or ear canal. To compensate for this lack of hearing, they have excellent associative thinking that helps them remember where they were or where the objects around them are.

Most land snails are hermaphrodites because they each have reproductive organs that are both male and female. A snail's mouth is just below the tentacles at the bottom of its head. The radula forms the snail mouth structure. It is similar to an elongated sack with several rows of small teeth inside it that help scrap the food instead of chewing; then the food is passed through the esophagus and other digestive tract organs. They have the anus in the lower part of their soft body.

Most land snails are hermaphrodites because each has reproductive organs that produce both eggs and sperm, both male and female. They can fertilize themselves, but they usually copulate with each other.

The mantle is a protective layer covering the foot and certain internal organs. In some cases, additional protection is also found to cover the shell. During larval development, there is a unique process in the gastropod molluscs called torsion. The body moves from the back to the front region, causing a rotation so that the cavity of the mantle, which includes the anus, shell, and visceral mass, rotates about 180 degrees and is placed suddenly above the head, and the shell appears to be back.