Not a Holothuroid, but most probably a member of the Phylum Sipuncula under the genus Aspidosiphon or Phascolion. There are quite a few species which appear around the Turkish coast under these two genera and their respective subgenera but, determining the exact species from these images will neither be wise nor prudent, at least from a taxonomic point of view.

Thank you very much Abhishek Mukherjee

The specimen was sampled from carapace of a sea turtle. It could have come from anywhere, as sea turtles travel long distances in the Mediterranean and Atlantic. If you have a suggestion for an identification key that includes these regions as a starting point, I would greatly appreciate it.

A conference paper available on RG entitled "A new illustrated guide to identify Mediterranean sipunculans" by Louis Ferrero-Vicente might aid you in your venture.

Regards

Sipunculus sp.