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Snail skeletal system is made of a hard shell which is present on the outer side of the snail’s body and is known as the exoskeleton. Unlike in the vertebrates, it has the internal or external non-bony substance as the skeleton of the invertebrates. It belongs to the Gastropoda class and is categorized under the largest Mollusca phylum among the invertebrate species.
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Snail Skeletal System
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Like several Gastropoda species, the snail has an outer non-bony hard shell that forms its skeletal system. It is known as the Exoskeletal System.
Exoskeleton
The hard outer shell on the back of a snail serves as the external skeleton or Exoskeleton. Among the various shapes and sizes, the most commonly observed ones are the spirally coiled shells. Though some snail species like the limpets carry cone-shaped shells.
Snail Skeletal System
Snail shells provide a wide surface for its muscles to attach. The hard shells form a protective camouflaging layer that saves the snail from the eyes of predators. It also provides protection from the sun, water loss and mechanical stress.
The shells are non-cellular structures and consist of three different layers. It is composed primarily of calcium carbonate and almost 2% of protein. The outer layer of the shell is termed as proteinaceous periosteum, the middle one is the prismatic layer and the inner one is called the nacre pearly layer. The inner and outer layers of the shells being rich in calcium content are calcified while the outer layer is non-calcified as it is rich in protein. Though the color of the shell is generally yellow or brown, however with its fringe designs, the surface of the shell looks colorful.
The mantle tissue or the body of the snail is directly attached to the shell. Calcium and proteins released from the mantle tissue help to make the shells strong. The released calcium acts as a cleaning agent for the shell and protein serves the purpose of a building block. A snail’s internal organs and foot are covered and protected by the mantle. In some of the species, the mantle also covers the shell thereby providing additional protection to the snail.
Excluding the shell, the remaining body of the snail is soft and viscous. The body texture is thick and has dark spots on it. It moves through its foot exhibiting a wavy movement, also known as gliding. Small contractions produced by the snail’s muscles are the reason for such wavy movements. The foot, by secreting a slimy and lubricated mucus avoids any kind of friction produced while moving on the ground.
The snail’s head, situated at one end of the body, has tentacles with its eyes placed at the tips. The tentacles have tactile receptors and can be retracted. Whenever the snail senses danger it hides its body inside the shell. Also, in extremely hot and dry weather conditions, they stay inside the shell to avoid their moist bodies from drying up.
Snails never shed their shell or the exoskeleton. Therefore, as the body of the snail grows, the formation of the shell occurs from the base to the top.
Things to Remember
- The skeletal system of a snail does not consist of any bony structures.
- The external skeletal system of a snail is made up of a hard, non-bony shell and is called the Exoskeleton.
- The exoskeleton helps to protect the snail from predators and extreme environmental conditions (hot and dry weather), support the structural framework and stimulate muscle contraction for body movement.
- Snail shells vary in shape and size. The common ones include spirally coiled shells, but there are some with cone-shaped shells as well.
- The shells are made up of three layers which are mostly composed of calcium carbonate and 2% of protein.
- The body of the snail or more commonly known as the mantle tissue is attached to the shell. Minerals like calcium and protein are released from the mantle tissue which further helps in the development of the shell.
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Sample Questions
Ques. Describe the structure of the skeletal system of snails. (2 marks)
Ans. Snails and slugs together belong to the Gastropod class. Though all Gastropods do not have a shell, yet there are some which do have one and snails fall in that category. Their outer hard shell plays the role of a skeleton in snails. This outer shell is the external skeleton or also known as the exoskeleton.
Ques. What is the movement of snails called and why? (2 marks)
Ans. Snails move around with the help of their feet. They move in a wavy manner and so the movement is called gliding. The small muscular contractions produced by the snail result in its foot moving in a wavy manner. Also, a slimy lubricating mucus is secreted through the foot which helps to avoid any kind of friction.
Ques. What is the purpose of the hard shell on the back of snails? (2 marks)
Ans. The hard shell on the back of snails acts as the skeleton. It is the external skeleton or also known as the exoskeleton. The shell has a wide surface area where the muscles attach. It also serves the purpose of camouflaging and protecting from predators, mechanical stress and water loss.
Ques. What is mantle tissue in snails and what is its function? (2 marks)
Ans. A piece of the snail’s body that is present in the shell is called mantle tissue. It is directly attached to the shell and releases calcium and proteins which help in the development of the shell.
Ques. A snail’s shell is made up of how many layers? (2 marks)
Ans. A snail’s shell is made up of three individual layers and they are composed of primarily calcium carbonate and 2% of protein. The outer layer of the shell is called the proteinaceous periosteum, the central one is the prismatic layer and the inner one is called the nacre pearly layer. The outer and inner layers are rich in calcium and are calcified whereas the central layer consisting of protein composition is uncalcified.
Ques. Why are land snails called hermaphrodites? (2 marks)
Ans. All land snails have both male and female reproductive organs and can produce eggs and sperms. Though self-fertilization is possible, the copulation process also occurs between two different snails.
Ques. What is associative thinking in snails? (2 marks)
Ans. Snails are deaf and do not have an ear or ear canal. However, this disability of the snails is counterbalanced by the associative thinking ability. Through this ability, they know the location where they are and what all objects are there around them.
Ques. Describe the shape and color of the snail’s shell. (2 marks)
Ans. The shell of a snail varies in shape and size. There are the ones that are spirally coiled. These are the most commonly observed shells. Then there are a few snail species that have cone-shaped shells. Limpet would be a good example of such a snail species. The surface of the shells exhibit a fringe pattern and show different colors, but mostly the colors are yellow or brown.
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