This comes in two halves held together by 10 rare-earth magnets.  It's quite solid when together, and then you can pull it apart to get at the interesting cross-section.  (It is slightly fiddly to put together, but by no means a puzzle.)

Plankton drifts. Nekton swims.
 

Nekton metal print with magnets, 2 x 3 x 4½" - $250

Also available on Shapeways in a variety of fine plastics.