Evolution of Invertebrates: Symmetry & Specialization

Lesson Transcript
Instructor: Elizabeth Friedl

Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Invertebrates are animals without a backbone that can be found in various environments from the land to the sea. Explore invertebrate evolution through the development of symmetry, specialized tissues, and unique heads housing all sensory organs. Updated: 11/20/2021

What Are Invertebrates?

You might have it in your head that humans rule the planet. We are everywhere, that's for sure. But even with the more than 7 billion people on Earth, we've got nothing on the invertebrates. These are animals that don't have a backbone, and they constitute a whopping 97% of the animals found on Earth! You're likely very familiar with invertebrates already. Included in this group are things like worms and insects, but also spiders, sea anemones, clams, and even squid!

As you can see, invertebrates are quite diverse and cover all sorts of environments. Some of the earliest-known animal fossils are invertebrates, but they certainly have come a long way since then. The three main things that develop as invertebrate animals evolve are symmetry, specialization, and cephalization. Let's take a closer look at each of these to understand how invertebrates have evolved over time to become the incredible world-dominating animals that they are today.

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  • 0:01 What Are Invertebrates?
  • 0:57 Developing Symmetry
  • 2:25 Tissue Specialization
  • 4:04 A Good Head on Your Shoulders
  • 5:37 Lesson Summary
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Developing Symmetry

Simply put, if your body doesn't have a lot of specialized tissues and organs, it probably doesn't have much symmetry either. Take a look at yourself in the mirror to get a better idea of how this works. If you split yourself right down the middle you would have two halves that are mirror images of each other; in other words, you have bilateral symmetry ('bi' for 'two' and 'lateral' for 'side').

Most invertebrates have this type of symmetry, but others have radial symmetry, which is when all sides are the same. The term comes from the symmetry that 'radiates' from the center, like spokes on a wheel or slices of pizza. Things like sea stars and sea anemones have this type of symmetry. This serves these animals well because the environment around them is the same on all sides.

Some invertebrates, like sponges, do not have any symmetry at all. They are also the simplest of all animals and have no nerves, muscles, or organs. This relationship between body symmetry and body complexity makes sense if you think about it. If you have complex tissues and organs, you'll also need a blueprint of where those things belong. Imagine if your heart was in your thigh instead of your chest, or your eyes were in your feet instead of your head. What if your toes were on your ears, or just randomly scattered about your body?

These structures would be pretty useless if they were in the wrong place, so having designated areas of the body (the head, tail end, right side, and left side) allows them to go where they are best suited.

Tissue Specialization

We've already touched on specialization a little, but it's so important it deserves a bit of time to itself. Basically, as animals become larger and more complex, their tissues become specialized to form distinct body parts. Nerve cells are different from muscle cells because they perform different - or specialized - functions, just like kidney cells are different from lung cells for the very same reason. You don't need to get too far into the invertebrate world to see how specialization plays a role in the lives of animals.

Because really simple animals like sponges don't have any specialized tissues, they just hang out on the bottom and exist; but right down the street we might find some flatworms that have a head, a mouth, digestion organs, and even nerve cords. Some flatworms are free-living while others are parasitic, but without these specialized tissues they'd be stuck to the bottom like their neighbor the sponge.

Even more complex invertebrates exist, things such as crustaceans and insects, and these guys not only have more complex organs and tissues than their flatworm friends, but they also have segmented bodies and structures like pincers, stingers, antennae, and eyes. They can see, feel, fly, swim, and jump, but also eat a wider variety of diets and live in many different environments.

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