The mantle cavity functions as a lung for respiration with atmospheric air. It is sealed off from the exterior by a thick mantle-collar which firmly embraces the body but allows it to extend-from/retract-into the shell. When immersed, as in this image, the cavity fills with water and the lung may be able to continue for a time obtaining oxygen from the water if well oxygenated.
1: mantle collar
2: body within mantle cavity.
3: air bubble, rest of cavity full of water.
When immersed, the body and shell become more translucent, sometimes, revealing internal organs such as;
4: anterior oesophagus.
5: posterior oesophagus.
6: salivary gland.
7: nerve ring and cerebral ganglia.
8: fold covering vas deferens.
9: buccal mass.
Shell heights 3.4 mm & 3.5 mm . Both in water. Salting on tidal River Dee, Flintshire, Wales. December 2018.