Deuterostomes, like tunicates and urchins, are amazing creatures that are classified together because they share aspects of their evolutionary pasts. In this lesson, you'll identify these characteristics and see what makes this group of animals unique.
What Are Deuterostomes?
Animals represent a vast and diverse part of our natural world. There are probably as many ways to classify them as there are different types of animals! We have an estimated 8.7 million different species of animals living on Earth, though most of them have yet to be identified. Where is a scientist to begin?
It sounds like a daunting task, but it's really not so bad. In fact, one of the best ways we can classify animals is by their body plans, which describe the combination and organization of an animal's individual body structures.
Most animals have true tissues, which are specialized tissues and cells. These develop to perform specific functions, like muscles, liver cells, or nerves. Some of these animals also have bilateral symmetry, so each side of the body is a mirror image of the other side. If you folded these animals in half, each side would fit perfectly over the other.
Finally, if the animals that have true tissues and bilateral symmetry also develop an anus first and mouth second during embryonic cell division, then they fit nicely into the category called deuterostome, which literally means 'second mouth.'
Deuterostomes include the echinoderms, so things like sea stars, sea urchins, and sand dollars, as well as chordates, which are things like humans, birds, and small marine creatures called lancelets and tunicates.
That's a pretty broad group! I bet you wouldn't have guessed that when we look at these body plan criteria, we are more similar to sea stars than we are to insects. I mean, at least insects have legs, eyes, and brains like we do. But it's true, although having true tissues and being bilaterally symmetrical will only get you so far. It's that last part, how we develop as embryos, that really sets the deuterostomes apart as a group.
If you've ever been to the beach, you've been lucky enough to see all the echinoderms that live there. Many of these animals, like sand dollars and sea stars, end up on shore. You don't usually see things like sea cucumbers and urchins, but they are common there as well.
Echinoderms get their name from their spiny skin. In fact, the word 'echin' means spine, and 'derm' means skin. Put these together, and you get spiny skin, which perfectly describes these animals! The spines are actually an external part of an internal feature called an endoskeleton. This internal skeleton sits just below the skin of echinoderms.
Echinoderms don't move very fast, and some of them don't even move at all. Echinoderms also have a unique feature called a water vascular system. This is a connection of water-filled canals inside the animal that eventually branch out into tube feet. Tube feet are appendages that help echinoderms move, eat, and 'breathe' through gas exchange.
I bet you have a question at this point, because when you look at a sea urchin, a sand dollar, or a sea star, it doesn't look bilaterally symmetric. And you're right! At this adult life stage, some echinoderms are radially symmetric, which means that, like the spokes of a wheel, the symmetry extends from a central point in the circle. However, these echinoderms are bilaterally symmetrical as larvae, so they still meet the deuterostome criteria, even though they change shape as they mature.
As mentioned before, because of their similar embryonic development, chordates and echinoderms can be classified together. But unlike echinoderms, chordates do not become radially symmetric as adults, and they don't have endoskeletons or water vascular systems.
But chordates are pretty special in their own right. This is because of four features that they all share at least as embryos and sometimes as adults. The four features that make a chordate a chordate are a dorsal hollow nerve cord, a notochord, pharyngeal slits, and a post-anal tail. Remember, not all of these features are retained through adulthood! Last time I checked I didn't have a tail anymore, but during embryonic development, it was there.
The major difference between chordates is whether they are vertebrates or invertebrates. We are vertebrates because we have a backbone made of vertebrae. In contrast, invertebrates do not have a backbone. We'll discover vertebrates in great detail in another chapter, so for now let's look at a couple of invertebrate chordates.
Animals such as lancelets and tunicates are examples of invertebrate chordates that share those four critical features with us. Tunicates are stationary and look like small sacs sitting on rocks and reefs. Lancelets, on the other hand, are mobile creatures, and they look like small blades moving through the water.
While these two animals may look nothing like you and me, it's been discovered that the genes that are responsible for important parts of the vertebrate brain appear in the same pattern and at the same end of the lancelet nerve cord. Do you see how we're really not that different when you look a little deeper?
It can be overwhelming to classify all of the millions of species of animals we have here on Earth. However, we can enlist some specific criteria to help make sense of how it all fits together.
Despite looking quite different, the deuterostomes represent a group of animals that all have true tissues, bilateral symmetry, and develop the anus before the mouth during embryonic cell division. Together, this combination and organization of body structures creates an animal's body plan, which can set it apart from other animals that look similar on the outside but may be quite different in their evolutionary past.
As we see with both the echinoderms and chordates, some of the rules only apply to certain life stages. Some echinoderms are only bilaterally symmetric as larvae, and some chordates lose their tails and pharyngeal slits as they mature into adults. However, because we all develop along the same lines, we're more similar than it might appear at first glance.
After this lesson, you'll be able to:
- Understand how body plans can be used to classify organisms
- Describe the criteria for characterization as a deuterostome
- Identify examples of echinoderms and chordates
- Explain how echinoderms and chordates meet the criteria of deuterostomes and describe their physical characteristics
- Differentiate between invertebrates and vertebrates