Sea Urchin
This creature has an endoskeleton like many animals. Its skeleton, known as the tests, is a rigid shell made of flat and fused calcium carbonate bones called calcareous ossicles. The test is divided into ten sections, going from the mouth, which is in the bottom, to the anus, which is at the top. Five of the sections with pierced holes on the tube feet are called ambulacral plates, and the other five without one are known as the interambulacral plates. The spines are usually circular and made to a fine point; certain urchins may have poisonous tips. The spines are attached to the muscles in the body wall, and since the sea urchin is shaped as a ball, it can move around in any direction.
Crinoids
Crinoids are marine organisms which have a skeleton made up of calcareous plates, which is similar to the previous echinoderms. The stalk ossicles form or rounded or pentagonal disk. The stem lacks muscles, but has elastic fibers connecting to the columns. However, the stem is capable of some movement, allowing to move against currents. They first appeared in the early in the middle of the Cambrian period and survive up until the present day, becoming abundant and important fossils from the Ordovician period onwards.
Starfish
The starfish is practically similar to the sea urchin, yet shaped differently. It has an endo-skeleton and similar calcium carbonate plates. It is closely related to the chordates. Since it consists of three different layers of skin and bone and it has stiff plates, it moves it arms in one generally direction. If it were to lose an arm, it can regenerate and the lost arm (if not eaten) can regenerate as well. This happens because the cells in the starfish are mostly totipotent, allowing the cells to differentiate into any cell. So it keeps its arms well place and tough.