Because that is where they live, that way they can burrie themselves in the sand to get protection from predators.
Sand dollars breathe through a process known as diffusion, using their tube feet.
Click on the link for a picture on a different website.
I think mostly in the northern hemisphere.
You are misidentifying the animals as sand dollars (Mellita), when in fact they are 6-holed sea urchins.
No it would die immediately as they are salt water creatures.
Sand dollars are commonly found on many beaches around the world from the Carribean to New Zealand. However, they are often found in California too and many are reported to be found on the beaches near Fort Bragg.
They both or found in the ocean
Yes they live on the coast.
I found one that is 4.6" wide in Olympia, Washington
At first it was not known that the sand dollars are living things. People thought (myth) it was like a large coin and that it was the coin/dollar used by the sea mermaids. It was found on beach sands. hence the name sand dollars.
In the sea, the sand dollars are found in groups on a soft muddy or sandy sea bed.
I found these sand dollars in that littoral zone.