Olá, Alessandro! Platyhelminthes is very unlikely. My best guess is protozoa. It looks somewhat like a gregarine but those usually are parasites in the guts of invertebrates.

Could it be an acoelomorph/acoel?

Thanks a lot Jon Norenburg .

Not like any acoel I've ever seen and I would expect to be able to see the statocyst.

i am not convinced about the protozosn affinity

I also have doubts about protozoan affinity because I can't place it any better than that, but I have even greater doubt about affinity to common metazoans. I see no multicellular features but if this is an image of a poorly preserved organism, that might not be surprising. For instance, if poorly preserved, I cannot rule out gnathostomulid or gastrotrich, though the image does not resemble any that I have encountered or seen pictures of. For acoel or the latter two it is easy enough to go directly to relevant experts - e.g., Ulf Jondelius, Rick Hochberg, Wolfgang Sterrer. I don't have suggestions for marine protozoans on tip of my tongue but should be easy enough to find.

In my opinion, it is a badly preserved Ciliate. I've seen similar looking "objects" in my samples.

Thanks everyone for the answers Jon Norenburg Jon Norenburg Kristian E Moller Oleksandr Holovachov . Unfortunately, it's not very well preserved. If I get some more I will come back to the question with a better picture.

Cheers

Looks like a Ciliate.