This is a tubeworm taken out of its chitinous tube for dissection.
The gray part is the cavity or trophosome. The muscle is between
the gray cavity and the red plume.
The cavity of a tubeworm has been cut open to reveal the large, brownish mass of bacteria. The curved red tube on the right side
of the photo is the plume of the worm. The tan flap between the
plume and the cavity is the muscle.