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Snail Skeletal System

The skeletal system plays a key role in supporting the structural framework of the organism. The primary functions of a skeletal system include: supporting the body’s movement and protecting vital internal organs.

Snail belongs to the Mollusca phylum and falls under the invertebrate category. The structural framework of a snail consists of hard internal or external substances. However, the skeletal system of snails lacks bony structures.

Skeletal system of Snail

Snails, along with slugs and other similar species fall under the class of gastropods. Most of the gastropod species possess an outer shell. Snails too have an outer hard and non-bony shell. The type of skeletal system possessed by snails is described as the ‘Exoskeletal System’.

Exoskeleton

The type of external skeleton consisting of a hard encasement on the outer surface of the organism is called an exoskeleton. An important component of an exoskeleton is chitin. The percentage of chitin present in the exoskeleton of most arthropods is 30 to 50. Furthermore, the exoskeleton also includes calcium carbonate, which is known to strengthen the skeletal system.

Examples of exoskeletons are shells of insects, snails, crabs, etc.

The primary functions of an exoskeleton include:

  • Protecting the organism from the attack of predators.
  • Supporting the framework of the body.
  • Enabling contraction of muscles to promote body movement.

Exoskeleton of snail

  • Snails have a hard shell on their back which fills in as the skeleton. There are different shapes and sizes of shells. Spirally coiled shells are the most commonly observed. However, a few species such as limpets contain cone-shaped shells.
  • The shells give a wide surface region for the muscles to attach. Furthermore, the hard shells offer protection against predators and help the snail survive. These shells do protect the snails from the sun, mechanical stress, and water loss.
  • The shell of the snail consists of three separated layers significantly composed of (calcium carbonate) and around 2% of protein. The shells are not cellular structures. There are three layers in the shell of the snail - The outer proteinaceous periosteum, middle prismatic layer, and the inner nacre pearly layer. Among the three layers, the inner and outer layers are calcified and rich in calcium. The outer layer is composed of proteins and is non-calcified.
  • Mantle tissue is a piece of molluscan's body which are available in the shell. Mantle tissues are in direct contact with the shell. They discharge and precipitate out minerals like calcium and proteins for shell development. Accordingly, calcium cleans the shell while proteins function as a building block.
  • The mantle surrounds the internal organs and foot of the snail. It acts as a protective covering. In a few cases, the mantle also surrounds the shell, thereby offering extra protection to the organism.
  • Apart from the shell, the rest of the snail’s body is soft and smooth. The snail’s body consists of dark spots and a thick texture.
  • The snail moves with the help of its foot. The muscles of the snail produce small contractions because of which its foot exhibits a wavy movement. This type of movement is called gliding. To prevent friction, the foot also secretes a slimy and lubricious mucus.
  • The snails do not shed their exoskeleton. Consequently, the shell develops from base to top as the body expands.

Internal anatomy of snails

  • The body of snails does not contain any divisions.
  • The internal organs present in the snail include the heart, lungs, oesophagus, intestines, and gonads. All the internal organs make an organic mass that is protected by mantles.
  • The snails are referred to as pulmonary animals because their lung utilises the oxygen derived from the atmosphere.
  • Snails lack a brain. However, they contain nerve cells. All the nerve cells gather in a set of ganglia. The ganglia containing nerve cells emit certain neurosecretions that trigger fundamental activities. For example, neurosecretions stimulate the release of hormones.
  • Snails possess a sense of light. They can recognize changes in the levels of light intensity. It means that they will only be able to differentiate the difference between day and night.
  • Snails neither have an ear nor an ear canal. Thus, they lack hearing abilities. As compensation for this lack, they got a feature of incredible associative thinking. With associative thinking, they’ll be able to sense the place they are in and the objects around them.
  • Terrestrial snails are hermaphrodites i.e. every individual snail produces both male and female gametes. However, copulation takes place between two different snails.
  • The larvae of snails undergo a unique development process called torsion.

 

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