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You don't. Snails grow to their shells through a thin layer of skin. If you pick them from their shells you may very well kill or injure them.
An exoskelton is the layer on the hermit crabs actual body that they shed when they outgrow it. (similar as how a snake sheds their skin) A shell is that which the hermit crab takes as a home that normally if from snails.
snails use there thin layer of skin to diffuse any organic gasses.
The Bascal Cell Layer, the deepest layer where cells divide to produce new skin cells.
Epidermis.
follicle
Dermis
epidermis
layer, covering, coating, skin, surface, shell
The shell as is has a layer of thin skin !!
Epidermis
Analogous, Turtles and snails have no common ancestor and so they evolved independently of one another from different methods/structures. The Snails shell is secreted by specialised glands in the mantle whereas the turtle shell is modified skin