translate English to French: They usually have no external shell, but when there is one, the animal is incapable of retracting into it, as snails can.
Elles nâ€™ont gÃ©nÃ©ralement pas de coquille externe, mais le cas Ã©chÃ©ant, lâ€™animal ne peut sâ€™y recroqueviller comme le font les escargots.