There are some annelids with scales, but the organism is not obviously segmented.  As far as I know, the scaled annelids have organic scales whereas the sea cucumbers would have calcareous (calcite) scales.  This means the annelid scales would tend to dissolve somewhat in bleach, whereas echinoderm scales would dissolve readily in acid.

How big is it?

Any information on substrate? (was it on a rock, on another organism...)  There are some bizzare parasites that lose most of the distinguishing features of their phylum as adults, so if it was living on something else, it could be a job for parasitology.

Monoplacophorans are limpet-like, not like this.

Some groups of sea cucumbers have lost the tube feet, but I don't know if any of them look like this.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

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