Relatives - Close and Not So Close

Seastars have a 5 part body plan. 5 (or more) rays radiate out from the center. Water filled suction tube feet provide locomotion.

Brittlestars are a bit different. They have 5 rays that radiate out from a distinct central disc. They do not have suction tube feet. Their tube feet are pointed and they use their highly flexible arms to move.

Sea Urchins are related as well. Look up through an empty urchin shell and you will see a distinct star pattern based on the number 5. They again have suction tube feet, but also impressive spines. A sand dollar is just a very flat urchin.

Finally - Sea Cucumbers are also related. They have 5 long muscles that run the length of the body (there is that star pattern again).

All of these are Echinoderms, the spiny skinned animals. They share the 5 part body plan and some other basic characteristics. Sort of like how all of the animals that have a backbone are vertebrates. Each of these groups are about as related to each other as we are to the other vertebrate groups: us to fish, fish to birds, birds to amphibians, etc., or, for echinoderms: seastars to sea urchins, sea urchins to brittle stars, brittle stars to sea cucumbers, etc.

A cool thing about echinoderms? They have "catch collagen." They can take their connective tissue and make it hard or soft, catch or flow. Soft bodied sea cucumbers can soften up to flow into a space, then harden up again. One can slough off chunks of it's body when threatened. Another simply dissolves into a mucky pile. Seastars can lock into place, or form a hard arched bridge over the clam they are pulling open. Sea urchins harden the connective tissue at the base of the spine right as your foot drives down onto it, to hold the spine rigid during times of attack. They relax the tissue then to move the spine when needed. Catch collagen - a great shared characteristic of the echinoderms.

Aloha,

Mark