Echinodermata
Reminder: Clicking on the picture will take you to the site where I originally found it.
Perhaps we should start with a definition. Echino means spiny, and if you say "echiny" you can get it to rhyme. Derm means skin, and you may have heard this before when discussing a dermatologist. Put it all together and you get the echinoderms, or spiny-skinned animals.
Underneath that spiny skin, there is a calcium-based skeleton. This houses a body which has five parts. Humans and most animals have two-part bodies (left and right halves), but echinoderms are divided into five parts, or multiples of five. This is weird because, when they hatch as larvae, they have only two part bodies.
Echinoderms have unusual feet. Their feet work like hydraulic suction cups. They are called tube feet, and are very effective at clinging onto rocks and solid objects.
The echinoderms peaked in prominance in the early days of life on earth. Most fossil evidence includes crinoid columns, or the stalks from ancient plant-shaped echinoderms.
Let's meet a few modern groups:
Starfish and Brittle-stars
There are a couple of people who insist you call these critters sea-stars instead of starfish. They want to emphasize the fact they aren't fish. If you've ever held one or seen one up close, there is no mistake made. Not only do these guys not look like fish, they are hard to mistake for anything else.
The starfish is the most familiar of the echinoderms. They have the characteristic tube-feet and spiny skin. They have bodies which have a five part body plan. They also have the ability to regrow limbs, so it's not impossible to see a six-legged sea star.
Most starfish belong to the class Asteroidea. Aster is Latin for star. Pretty basic, eh? The bat star (pictured) is only one of the many starfish in this class.
There is another class which closely resembles the starfish, and that is the brittle stars. They belong to the class Ophiuroidea, and they're easy to identify. Picture a starfish which is all leg and nearly no body. You've got a brittle star.
The name ophiur means snake, and the arms of the brittle stars are very snake-like. They are fast moving for echinoderms and move more than their starfish cousins.
Basket-stars are also grouped with the brittle-stars. They have legs which branch out and resemble a plant more than an animal. These are very delicate and beautiful.
Starfish have a wild way of eating. If a starfish pries open a clam shell, it pushes its stomach out of its mouth and into the shell. The starfish digests the clam and then draws the stomach back in. If humans could do that, lunchrooms would never be the same. Kinda nasty, eh?
Sea urchins and sand dollars
Another class in the echinodermata family is the Echinoidea. There are two familiar characters in this class. The first is the sea urchin.
Sea urchins are spiny to protect themselves from predators. If you get through the spines, you'll notice the characteristic spiny skin. Underneath there is an exoskeleton called a test. The spines aren't connected to the test securely, and fall off when the urchin dies.
Most urchins can be safely picked up by humans, but there is a species off the coasts of Florida which has sharp, poisonous spines. Sea urchins will eat almost anything they come across, but they have many critters trying to eat them. Fish and some mollusks fins urchins tasty, so those spines come in handy. Apparently, some humans also find sea urchins tasty.
Sea urchins feed on dissolved material in the water. This makes them targets for pollutants. They also live near coral reefs, so their habitat is in danger.
Closely related to the urchins are the sand dollars. Actually, they roughly resemble spineless urchins.
Sand dollars are aptly named. They live their lives in the shallows on the edge of the oceans. They get their food by swallowing sand and extracting small plants and animals. The live in sand, they swallow sand (and spit it out again), and they resemble sand when cleaned and dried.
Generally speaking, flat, small sand dollars are just called sand dollars. There are larger, thicker echinoderms called sea biscuits. There is a very thick one called a sea gopher, and it mosst closely resembles a spineless urchin. Some sand dollars have slits in them.
Sea cucumbers
Sea cucumbers are so strange, they're almost fun. They do resemble cucumbers, to an extent, but they are securely in the echinoderm phylum.
Like all echinoderms, sea cucumbers have spines, but they're under the skin. The skin is soft and somewhat rubbery. In many ways, sea cucumbers resemble worms. They have a mouth at one end and an anus at the other. They still have the 5-part bodies though.
Picture a sea urchin, but then stretch it out to form a column. Now let that column fall over sideways, and you have a sea cucumber. THe sea cucumber has a couple advantages to its body that other echinoderms lack. First, the distance between the mouth and anus is large, which is always an advantage. Next, the cucumber shape makes it more mobile. Although the tube feet of a sea cucumber are symmetrically arranged over its body, most cukes (my nickname) always use the same feet to move around on.
If a sea cucumber feels threatened, it has a rather weird defense. It can spew out its internal organs and move away. The cucumber can regrow the parts, and the predator is distracted by an onslaught of tasty organs. Wouldn't it be strange if humans could do that?
Sea cucumbers have been known to grow as large as 90 centimeters (nearly 3 feet) in warm waters, but generally don't get that big in cooler climates. Some burrow in sand, and most stay in the shallow reef areas. All belong to the class Holothuroidea.
Crinoids and related groups
Way back in the fossil record, echinoderms were prominant. Most echinoderm fossils come in the form of crinoid columns. These date back as far as the Cambrian era.
There are still modern crinoids on the earth. Actually, they're in the oceans, but you get the picture. Modern crinoids are known as sea lilies or feather stars.
These creatures faintly resemble the sea anemone in their sessile beauty. They are starfish-like though. Their arms are branched and they are used to gather food from the water. There is tiny cilia which propel the dissolved food down to arms to the mouth.
The modern sea lilies and feather stars belong to the class Crinoidea. Other closely related classes existed in the past.
A new class was discovered in 1986. It is called Concentricycloidea but if the number of syllables lost you, you can call them sea daisies. These look like flat little daisy heads on the ocean floor. No mouth or arms have been discovered, so scientsts think they absorb food through their skin.
All the way back to the animal page,
see sponges,
near the cnidarians,
meet the mollusks,
advance to the arthropods,
wade through the worms,
commune with the chordates or
over to the overview.