Anterior of animal to right of image, right of animal towards top of image.
The inhalant current of oxygen-bearing seawater enters the nuchal cavity from the left (1) and passes between the lamellae of the ctenidium (2).
Exhalant water currents pass along the pallial groove on either side of the body to exit at the mid-point of the posterior (3).
The pedal-retractor muscle, a U of white muscle bundles (4) separated by narrow gaps (5), attaches the body/foot to the shell.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig