All I can say is that it is from the phyllum ctenophora, maybe genus Beroe. But it is hard with a picture taken outside water. The animal is too much "collapsed".

Thank you very much! There were many ctenophores in the water, but it was the only sample I found on the beach. In sea water the object was completely invisible (an interesting material).  What is the characteristic attribute, which indicates that the object is (part of) a ctenophore?

Ctenophoa body look a bit like jellyfish but the overall symetry is quite different. In most Ctenophora, you can see 8 comb rows on the body and it has a more or less radial symetry. When tentacles are present, but they seem to be absent on your sample (that is what makes me think of the genus Beroe), they are only two.

For more info : https://en.wikipedia.org/wiki/Ctenophora

 Thank you for the interesting information! You helped me a lot!

This is the shell of a mollusk: Cymbulia peroni Blainville, 1818

You are right! Thank you very much! Do you have an idea about the material of the shell? It is probably an interesting substance (relatively rigid and unvisible in sea water).

Best regards

Peter

Clarification: Cymbulia (Gastropoda; Opisthobranchia) is shell-less as an adult. What appears to be a "shell" is the cartilaginous pseudochonch. It is difficult to tell from the photograph, but when out of water, the wing-like parapodia (modified foot) collapse against the body, so it is hard to distinguish the parapodia from the pseudoconch.

Thank you for the interesting additional information. Only the pseudochonch of a the object is conserved, the parapodia are already lost. Is Cymbulia a frequent species?

This is a Cymbulia peroni Blainville, 1818 send you a photo from my book.

Nice pictures! Thank you very much.