The aboral side of the shell (test) has the off-centered petalidium, as seen clearly here in a dead specimen which has lost its spines. Note also that, unlike urchins which have 5, sand dollars have only 4 gonopores, visible around the star-shaped pattern in the center.
The oral (under) side of the test is flat. It has the mouth in the middle, with tracks leading to it. The anus is near the edge of the shell (at the bottom in this photo).
Although no external ridges are visible around the mouth from the outside (see previous photo), this broken piece shows that 5 radiating ridges around reinforce the test on the inside. Photo by Dave Cowles, July 2010.
Live Dendraster excentricus on sand in an aquarium at Shannon Point Marine Center. Found subtidally in Bellingham Bay. Diameter 6 cm. (Photo by: Dave Cowles, August 2006)
Eccentric Sand Dollar (Dendraster excentricus), Ganges Harbour, Salt Spring Island, British ColumbiaSource: http://en.wikipedia.org/wiki/File:Eccentric_sand_dollar.jpg
