Sand Dollars revisited

Last weekend whilst Xanthe was at Tournament of the Mind I took the boys for a walk along the beach at Casuarina Coastal Reserve behind the hospital. The beach is a classic tropical beach - empty, gentle slope and sand as far as the eye can see. We were amazed at how many sand dollars we found, and we ended up bringing 30 home.
This weekend Tish is on short night shifts - that is she goes in at 9pm not 7pm - we all decided to go for a walk on the beach before she had to get some sleep.
We were not really expecting to have a repeat of the previous weekend so I had not taken the shell box - what a mistake....

In total we found over 220 of them along a relatively short stretch of beach.
Some of them were quite large..

and as you can see from the last one have an attractive pink fringe.

The reason for the high numbers became obvious as we reached one part of the beach. We had thought that they lived off shore and that the shells we were finding were washed up after the animal inside dies. Whilst we were walking this morning though we started to find several much darker ones that were much more brittle, we assumed that they were young individuals that had not had time to lay down enough calcium carbonate to form their shells, and whilst this is undoubtably true it is only half the story.
Tish was picking one up when she noticed lots of tracks in the wet sand, and at the end of each one was a raised circular mound of sand...

on closer inspection we realised that these were live sand dollars! There were literally hundreds of them of varying sizes.
We carefully dug up one of the larger ones...

....and realised that the darker coloured ones were in fact the live ones! If you enlarge the photo above you can see that the edge has a ring of fine cilia or hairs. I assume that they move through the wet sand by using these as little paddles.
What amazing beasts they are.