This group includes the "spiny skinned" animals, such as starfish (not really fish at all -- you know, the star-shaped organisms with the sucker feet), sea urchins, sand dollars, crinoids, and more obscure forms. Echinoderms are called that because of calcite plates (Echino = plates, or spines; derm = skin). The term skin is used loosely here, as we refer to the outer covering of the body, which has developed within it calcite plates, forming a kind of "embedded skeleton." You may know about sea urchins. Have you ever seen photographs of sea otters munching on sea urchins they dive down to the bottom to retrieve? And, on some sandy beaches, sand dollars can be found, often broken, as they are usually rather delicate and thin. You are probably less familiar with the crinoids, because they are not well-known today, although they can be very prolific on some sea bottoms. Crinoids are peculiar animals, consisting of a "root" holding them to the bottom (hence, the name holdfast), a long "stem-like" column, and a head structure called the calyx, where the mouth is surrounded by a delicate set of radiating feathery arms that serve to bring food particles toward the mouth. Fossil crinoids are very important, and form part of massive sea floor deposits known from the Paleozoic Era.
We can start with crinoids, so common in many Paleozoic limestones, and still around today, but not well known. Living ones are called "sea lilies" or "feather stars." First, to some typical fossils -- the following photograph shows the proceeds from dumping out a box full of crinoid columnal pieces:
Individual pieces show the radial symmetry characteristic of this group. Smaller pieces of columnals look like "poker chips." To emphasize how common crinoid columnals can be, here are more:
Now, from what sort of organism do those round, cylindrical, sectioned, pieces come? Well, the crinoid animal is unique. It is like an animal that wants to be a plant. It is an animal, in fact, but it has the holdfast to keep it stuck to the bottom, a long stem-like column (that's where the pieces above come from), and way up on top, a head (the calyx), with all the feathery arms. Strange animal indeed. But, they don't all have the column, or "stalk" -- there is a diversity of forms in related echinoderms, ranging from primitive ones, with many small plates of calcite in a kind of spiral, but low structure, to the advanced long crinoids with the extensive columnals.
The following museum photograph shows a nice complete fossil of a stalked crinoid:
Image Source: University of California, Museum of Paleontology, UCMP .
That's the holdfast (but not permanent, if you look at that video!) at the bottom, the long "stalk," and at the top, the calyx, with the arms radiating out. You rarely find a complete one like this, because crinoids usually come apart after death. Even the specimen below might be considered unusual, as it shows, at least, parts of the crinoid in living position, but much of it has been scattered into pieces before burial on the sea bottom:
Echinodermata is a diverse group. We go from elongate crinoids to flat sand dollars:
You see the radial symmetry, if you look closely for faint outlines near the center. Sand dollars and sea urchins and some other echinoderms show a kind of combination symmetry, where there is the five-rayed radial symmetry, but superimposed with it is bilateral symmetry. In the specimen above, visualize a vertical line extending down and across the specimen -- this divides the specimen into mirror image halves, left and right.
Here's an older fossil sand dollar:
This specimen nicely shows the many plates of calcite that make up the skeleton. You can also make out the five-rayed radial symmetry if you focus your eyes at center right to find the center of the radial symmetry, from which you can trace out radial divisions (two are very faint). For a blobbier close cousin to sand dollars, look to the sea biscuits, which fossilize well after filling with mud and becoming rather solid:
Look at the biggest one, on the right, to see the five-rayed symmetry easily.
The sand dollars and sea biscuits above were covered in life by a carpet of tiny, movable calcite spines, but it is the sea urchins who steal that show:
The spines, among other things, offer a level of protection. If you touch a sea urchin, the spines will move in reaction. In fact, the spines along the bottom help a little bit with locomotion, which is mostly done with suction cup feet, similar to the ones possessed by starfish (Again, not fish at all). Fossil sea urchins are identified by their more round shape, as compared to the sand dollars and sea biscuits:
Images Source: Wikipedia
Spines often fall off after death, so they aren't usually present on fossils.