|Sea urchin, about 1 cm. across|
In the aquarium, he burrowed into the sand quickly, but later he was out in the open riding this snail, and carrying pieces of shell and sand grains, possibly as camouflage. It would have worked better if he had ever stopped waving those spines about.
Tonight, cleaning the tank, I found him back on the Turkish towel. He dropped off onto my fingertip, and attempted to glue himself there. Adventurous little beastie!