Most live in shallow tropical to temperate water environments all around the world. They largely live in unconsolidated sediment on organic detritus and when they die, their bodies (also called a "test") frequently wash up on beaches and such..
They come in a wide diversity of shapes and forms. If you live in colder type northern climates, you think of sand dollars as looking like this:
|Image by Jani Oravisjärvi
Lunules REDUCE lift either by interfering with hydrodynamic flow or by "relieving" pressure from the under surface!! i.e., they INTERRUPT and LOWER the coefficient of lift!
The lunules of Mellita reduce lift by providing channels to bleed off excess pressure from the oral surface. This is shown by the clear flow of water (which was marked with dye)...through the lunules.Telford used three different sea urchin tests as models, and each one had its own specific modifications and stories..
Subtle modifications, such as the absence of tube feet in these areas are also present.
After seeing how lunules and notches can affect and help these animals from attaining "lift-off" one has to wonder then, what happens with critters like these rotulid sand dollars with the odd notches and lunules below??
|Image by Bellwizard via Flickr
But how do the urchins WITHOUT lunules solve this problem?? Go here to see all about sand dollar weight belts!