How do shells form? Why are there so many types? Why do all the curly shells curl in the same direction?
In: Biology
Shells are secreted out through the skin of mollusks- they take on the shape of the animal that’s tucked inside it. Think of a slug as a snail that hasn’t grown its shell yet. I’m not sure if all shells curl in the same direction, I think they are just shaped that way because that’s how the mollusk inside it grew proximodistally
Mollusks like snails and clams have very delicate bodies, so they need protection from the elements and predators. As mollusks develop in the sea, their mantle tissue absorbs salt and chemicals. They secrete calcium carbonate, which hardens on the outside of their bodies, creating a hard shell. The shell stays attached to the mollusk but it is not actually part of its living body because it is made of minerals, not mollusk cells (unlike most animal structures). The mollusk continues to take in salt and chemicals from the sea and secrete calcium carbonate, which makes its shell grow even bigger. When a mollusk dies it discards its shell, which eventually washes up on the shore. This is how seashells end up on the beach.
All the curly shells don’t curl in the same direction. About 90% of gastropod species have shells that curl dextrally (right-handed) and 10% have a sinistral coil. Bivalve shells don’t have a handedness to their shells and neither do cephalopods.
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