Tectura virginea for comparison.
1: ctenidium contracted and withdrawn from sight into nuchal cavity.
2: red buccal mass shows pink through translucent head.
3: mantle cavity consists of nuchal cavity over head, and pallial groove (lacking pallial gills) around periphery of foot.
4: reddish band, on periphery of mantle-skirt, and corresponding pink ray on shell produced by it.
5: white repugnatorial glands set back from mantle edge and connected to it by narrow white necks; when attacked, glands emit viscid, slow-to disperse, white threads of (presumably) distasteful/toxic secretion.
6: mantle-skirt blue-green.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig