Don't forget that every single Mastigo specimen that I have is also lacking tentacles. Of course, it's only because of their poor construction so they get pulled off. Gonatopsis sure is adorable in paralarval form!
It is the starfish looking arrangement around what I understand to be the buccal mass that I have not noticed before. It may be that I just have not seen it opened like that since I mostly see live animals and don't really look much at the dead one
There is a buccal mass but not that starfish looking shape around it (either open or closed as in the Humboldt). I was sure I had not seen it before but then looked at my monitor's wallpaper - no starfish shape on O. briareus, just suckers in a circle around the not exposed beak. Then thinking about it, I am sure I have never see anything like that in the flesh, even with beak exposed so I would say from observation and personal photos the little guys we keep normally don't have it or it is very, very small and hidden behind the tiny suckers that encircle the beak area.
This is LittleBit (probably small vulgaris) all the ones I have for briareus show food in the way but it is the same as with hummelincki