Picture: [RN]
The hollow space between a shell's whorls on the side opposite to the shell tip (apex) is called the shell navel or umbilicus. In different snail species, it can be wide open, very narrow, or even covered by the columellar part of the apertural lip.
Picture: [RN]
Upper left: A garden banded snail (Cepaea hortensis), whose navel is covered by the apertural lip and thus is closed. Upper right: An Eastern heath snail (Xerolenta obvia) with a wide open, perspectivic navel. Below: An incarnate snail (Monachoides incarnatus) with a narrow navel.
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