Above are two sea urchins of the same species sitting side by side. They look nothing alike because the one on the right is still alive. The left one is simply the skeleton left behind from a recently deceased individual. This skeleton is called a test.
Unlike insects and crustaceans, echinoderms ("spiny-skinned" animals such as urchins, sea stars and sea cucumbers) do not have an external skeleton. Like ours, theirs is on the inside, although covered only by a very thin sheet of skin tissue.
An echinoderm’s skeleton varies with each class of animals found within this phylum, and before moving on to the test we’ll need to see exactly what this "skeleton" is, since it is unlike anything else found in the animal kingdom.
Although all echinoderms have a skeleton, not all have a test. The skeleton is basically a collection of small calcium-based structures called ossicles. In animals like sea cucumbers these ossicles may be barely present and the result is a smooth, soft outer body that appears to have no trace of a skeletal structure.
Sea stars have many more ossicles but these are loosely arranged and embedded into the subdermal layer of tissue. The result of this is a more firm but still flexible body with a skeleton that has interactive, but not necessarily connected, parts.
In sea urchins, members of a group of animals known as Echinoidea, we see the ossicles are fused together into a connected structure that we can now call a test. While the ossicles of a sea star are varied, consisting of flat plates, rods, crosses and other shapes and held loosely together simply by a layer of connective tissue, the urchin’s ossicles are made up solely of flattened plates. All of the plates are fused together, making the skeletal structure immovable, but much more solid and protective.
Looking at the test of your average sea urchin you can see it forms bands which run from the top of the animal down to the bottom. These bands exist because there are two different kinds of calcareous plates that make up the test. One type of plate forms a band known as an ambulacral area and the other type forms an interambulacral area. If you recall an earlier diary called How Tube Feet Work, another term for the suction-cup feet these animals use to move with is ambulacrae. Thus the ambulacral areas are parts of the test that have tiny holes that the tube feet can use to extend out of the test, and the interambulacral areas are more solid and don’t contain these foot holes.
Notice that this animal still adheres to my rules about echinoderms from that earlier essay, one of which was that all members of this group have a body plan divided into five, or multiples of five, parts. Count the bands in the urchin tests above and you will find each one has twenty sections. Ten ambulacral and ten interambulacral areas. Below is a live urchin with all of its tube feet protruding from the ambulacral area’s hundreds of tiny holes:
One feature of the test that is easily noticed is the small raised bumps all over the structure. Look at the first photo again and you’ll see what these bumps are for. They are attachment points for the spines. The spines are actually greatly modified ossicles that grow from these tubercles and are attached using a kind of ball and socket joint like the one found in our shoulder. As our ball joint allows movement of our limbs, so the tubercle joint allows movement of the spines. Because the connection is actually tissue, the spines fall off soon after the animal dies. And because the spines are part of the internal skeletal structure they are still covered by a very thin layer of tissue, so they are not external parts of the body.
As in starfish, the mouth of the sea urchin (called an "Aristotle’s Lantern", and discussed in this diary) is in the center of the bottom side of the body. This is called the oral surface, while the top of the body is the aboral surface. When you look at the test you will see two openings in the skeleton. A small hole at the top, called a periproct and a larger opening underneath called the peristome. The large peristome gives the mouth plenty of room to move as the animal feeds, while the periproct provides an opening aborally for wastes and gametes to be discharged into the surrounding water.
Although sea urchins are very common throughout the world, the tests are rarely found by beachcombers. This is because although the plates of the skeleton are fused, the test itself is very fragile. Most are pounded into sand long before they ever reach the shore. Another type of animal in the class Echinodea are sand dollars, which basically look like flattened-out sea urchins with very tiny spines. The test of a sand dollar is much thicker and more likely to survive a trip through the waves and be found washed ashore. Sand dollars will have an upcoming diary all their own.
Fun Fact: The word "urchin" is derived from the Latin term for "hedgehog".
Other diaries in this series can be found here.