Skip Navigation

17.2: Overview of Invertebrates

Difficulty Level: At Grade Created by: CK-12
Turn In

Lesson Objectives

  • Describe general characteristics of invertebrates.
  • Outline major events in invertebrate evolution.
  • Give an overview of invertebrate classification.


bilateral symmetry
symmetry of a body plan in which there are distinct head and tail ends, so the body can be divided into two identical right and left halves
concentration of nerve tissue in one end of an animal, forming a head region
fluid-filled body cavity
complete digestive system
digestive system consisting of a digestive tract and two body openings (mouth and anus)
outer embryonic cell layer in animals
inner embryonic cell layer in animals
hydrostatic skeleton
type of internal support in an animal body that results from the pressure of fluid within the body cavity known as the coelom
incomplete digestive system
digestive system that consists of a digestive cavity and a single opening that serves as both mouth and anus
larva (plural, larvae)
juvenile stage that occurs in the life cycle of many invertebrates, fish, and amphibians and that differs in form and function from the adult stage
embryonic cell layer in many animals that is located between the endoderm (inner cell layer) and ectoderm (outer cell layer)
partial, fluid-filled cavity inside the body of some invertebrates
radial symmetry
symmetry of a body plan in which there is a distinct top and bottom but not distinct head and tail ends, so the body can be divided into two halves like a pie
division of an animal body into multiple segments


The majority of animals today are invertebrates. They have a wide range of physical traits and ways of life. Modern invertebrates include animals as different as the sponge and tarantula shown in Figure below. Why are both of these animals classified as invertebrates? What traits do they have common?

Examples of Invertebrates. Both a sponge (left) and tarantula (right) are invertebrates. Can you identify any traits they share?

Characteristics of Invertebrates

One trait the sponge and tarantula share is lack of a backbone. In fact, they don’t have any bones at all. These are defining traits of all invertebrates. Some invertebrates have a skeleton, but it isn’t made of bone. Many other traits of invertebrates show considerable diversity.


Invertebrates have one of two types of digestive system. They are called incomplete and complete digestive systems. Both are shown in Figure below. An incomplete digestive system consists of a digestive cavity with one opening. The single opening serves as both mouth and anus. A complete digestive system consists of a digestive tract with two openings. One opening is the mouth. The other is the anus.

Two Types of Digestive Systems in Invertebrates. On the left is an incomplete digestive system, found in a jellyfish; on the right is the complete digestive system of a roundworm. Invertebrates may have either of these two types of digestive system. Find the parts of each digestive system in each drawing. How do the two systems differ?


All invertebrates can move on their own during at least some stage of their life cycle. However, they may differ in how they move. Several ways are described below.

  • Some invertebrates are simply carried along by water currents. They cannot control their movement in a particular direction. An example is a jellyfish.
  • Other invertebrates can contract muscles to move independently of water currents or on solid surfaces. They can also control the direction in which they move. An example is a roundworm. It can move forward and to the left or right.
  • Still other invertebrates have specialized appendages for movement. For example, they may have jointed legs for walking or climbing or wings for flying. An example is an insect such as a fly.

Nervous System

Most invertebrates have a nervous system. The nervous system allows them to sense and respond to their environment. The simplest invertebrate nervous system is just a network of nerves that can sense touch (see Figure below). Most invertebrates have a more complex nervous system. It may include a brain and several different sense organs.

Nerve Net (Coral). The body of a coral is lined with a net of nerves that can detect touch. How might this ability be adaptive?


Most invertebrates reproduce sexually. Diploid adults produce haploid gametes (sperm and eggs). In some species, the same individuals produce both sperm and eggs. In other species, sperm and eggs are produced by separate male and female individuals. Fertilization occurs when a sperm and an egg fuse to form a diploid zygote. The zygote develops into an embryo and eventually into a new adult organism. On the way, it may pass through one or more larval stages. A larva (plural, larvae) is a juvenile, or immature, stage of an animal. It is generally quite different in form and function from the adult form of the species. For example, the larva may be able to swim freely, whereas the adult must remain permanently attached to a solid surface.

Some invertebrates can also reproduce asexually. This may occur by fission or budding. Fission takes place when an animal simply divides into two parts. Each part then regrows the missing part. The result is two whole organisms. Budding may take place when a parent forms a small bump, or bud. The bud remains attached to the parent while it develops into a new individual.

Invertebrate Evolution

Invertebrates evolved several important traits before vertebrates even appeared. These traits are now found in just about all animals.


The first animal trait to evolve was multicellularity. This was highly adaptive. Multiple cells could do different jobs. They could evolve special adaptations that allowed them to do their job really well. However, the first invertebrates still lacked tissues. Sponges represent the first organism at this stage of invertebrate evolution.


Living cnidarians, such as jellyfish, represent the next stage of invertebrate evolution. This was the evolution of tissues. It was the first step in the evolution of organs and organ systems. At first, invertebrates developed tissues from just two embryonic cell layers. There was an outer cell layer called ectoderm and an inner cell layer called endoderm. The two cell layers allowed different types of tissues to form.

Radial Symmetry

Another trait that evolved early on was symmetry. To understand symmetry, you need to see an animal that lacks symmetry. A sponge, like the one in Figure below, lacks symmetry. This means it cannot be divided into two identical halves. A symmetrical organism, in contrast, can be divided into two identical halves. Both the coral polyp and the beetle in Figure below have symmetry.

Symmetry in Invertebrates. Sponges lack symmetry. Radial symmetry evolved first. This was followed by bilateral symmetry. How do the two types of symmetry differ?

The coral polyp in Figure above has radial symmetry. This was the first type of symmetry to evolve. The coral has a distinct top and bottom but not distinct ends. It can be divided into identical halves like a pie, but not into right and left halves. Animals with radial symmetry have no sense of directions such as forward and backward or left and right. This makes controlled movement in these directions impossible.


Flatworms represent the next stage of invertebrate evolution. They evolved cephalization. This is the concentration of nerve tissue at one end of the body, forming a head region. This is highly adaptive. It allows central control of the entire organism. Cephalization was first step in the evolution of a brain.

Bilateral Symmetry

An outcome of cephalization was bilateral symmetry. This is demonstrated by the beetle in Figure above. With concentrated nerve tissue at the head but not at the tail end, the two ends of the body are distinct from each other. The animal can be divided down the middle to form identical right and left halves. It allows the animal to tell front from back and left from right. This is needed for controlled movements in these directions.


Ancestors of flatworms also evolved mesoderm. This is a third layer of cells between the ectoderm and the endoderm (see Figure below). Evolution of this new cell layer allowed animals to develop new types of tissues, such as muscle.

Three Cell Layers in a Flatworm. A flatworm has three cell layers.

Complete Digestive System

Early invertebrates had an incomplete digestive system. There was just one opening for the mouth and anus. Ancestors of modern roundworms were the first animals to evolve a complete digestive system. With a separate mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could also become specialized for different digestive functions. This led to the evolution of digestive organs.

Pseudocoelom and Coelom

Ancestors of roundworms also evolved a pseudocoelom. This is a partial body cavity that is filled with fluid. It allows room for internal organs to develop. The fluid also cushions the internal organs. The pressure of the fluid within the cavity provides stiffness. It gives the body internal support, forming a hydrostatic skeleton. It explains why roundworms are round and flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity, completely enclosed by mesoderm. It lies between the digestive cavity and body wall (see Figure below). Invertebrates with a true coelom include mollusks and annelids.

Cross Section of an Invertebrate with a Coelom. The coelom forms within the mesoderm.

Segmented Body

Segmentation evolved next. This is a division of the body into multiple segments. Both the earthworm and ant pictured in Figure below have segmented bodies. This trait increases flexibility. It permits a wider range of motion. All annelids and arthropods are segmented. Arthropods also evolved jointed appendages. For example, they evolved jointed legs for walking and “feelers” (antennae) for sensing.

Segmented Invertebrates. Earthworm (Annelid) and Black Ant (Arthropod). An earthworm consists of many small segments. An ant has three larger segments. Notice the ants jointed legs and “feelers.”


Some invertebrates evolved a notochord. This is the stiff support rod in a chordate. The first chordates were probably similar to modern invertebrate chordates. The sea squirt in Figure below is an example. Later, some invertebrate chordates evolved into vertebrates.

Notochord. A sea squirt is an invertebrate with a notochord.

Classification of Invertebrates

Eight major phyla contain the majority of invertebrate species.

Major Invertebrate Phyla

Table below gives an overview of the eight invertebrate phyla with the greatest number of species. The next chapter describes each phylum in greater detail.

Phylum (includes) Notable Characteristics Example
Porifera (sponges) multicellularity, specialized cells but no tissues, asymmetry, incomplete digestive system sponges
Cnidaria (jellyfish, corals) radial symmetry, true tissues, incomplete digestive system jellyfish
Platyhelminthes (flatworms, tapeworms, flukes) cephalization, bilateral symmetry, mesoderm, complete digestive system flatworm
Nematoda (roundworms) pseudocoelom, complete digestive system roundworm
Mollusca (snails, clams, squids) true coelom, organ systems, some with primitive brain snail
Annelida (earthworms, leeches, marine worms) segmented body, primitive brain earthworm
Arthropoda (insects, spiders, crustaceans, centipedes) segmented body, jointed appendages, exoskeleton, brain insect (dragonfly)
Echinodermata (sea stars, sea urchins, sand dollars, sea cucumbers) complete digestive system, coelom, spiny internal skeleton sea urchin

Protostomes and Deuterostomes

Most invertebrates (and higher animals) can also be placed in one of two groups based on how they develop as embryos. The two groups are called protostomes and deuterostomes. As shown in Figure below, organisms in the two groups have different ways of forming the coelom and mouth, among other differences.

Mollusks, annelids, and arthropods are protostomes. Echinoderms and chordates are deuterostomes. This distinction is important. Why does it matter? It shows that echinoderms are more closely related to chordates than are the other invertebrate phyla. This is not apparent based on other, more obvious traits.

Protostomes vs. Deuterostomes. In protostomes such as mollusks, the coelom forms within the mesoderm. In deuterostomes such as echinoderms, the coelom forms from a pouch of endoderm. How does the formation of the mouth differ in these two groups of animals?

Lesson Summary

  • The majority of living animals are invertebrates. Invertebrates lack a backbone. They may have an incomplete or a complete digestive system. They vary in how they move and in the complexity of their nervous system. Most invertebrates reproduce sexually. After hatching, many invertebrates pass through one or more larval stages that are different from the adult stage.
  • Many important traits evolved in invertebrates. They include: multicellularity, tissues and organs, radial and bilateral symmetry, cephalization, mesoderm, complete digestive system, coelom, segmented body, and notochord.
  • Eight invertebrate phyla contain most invertebrate species. Invertebrates (and higher animals) can also be placed in one of two groups based on how they develop as embryos.

Lesson Review Questions


1. Describe the range of variation in the nervous systems of invertebrates.

2. Distinguish among asymmetry, radial symmetry, and bilateral symmetry.

3. Define cephalization. What is its relationship to bilateral symmetry?

4. What is mesoderm? Name an invertebrate with mesoderm.

5. Define coelom. How is the coelom related to the hydrostatic skeleton?

6. What is segmentation? Why is it adaptive?

7. Describe evidence showing that echinoderms are more closely related to chordates than are other invertebrate phyla.

Apply Concepts

8. Create a diagram to show the life cycle of an invertebrate with a larval stage. Include simple sketches of the adult and larval stages of the animal.

9. Assume you have discovered a new invertebrate. It has a segmented body, a brain, and jointed appendages. In which phylum would you place it? Why?

Think Critically

10. Compare and contrast incomplete and complete digestive systems. Why is a complete digestive system more efficient?

11. Explain how invertebrate movement is related to body symmetry.

Points to Consider

This chapter presents an overview of invertebrate phyla. The next chapter describes invertebrate phyla in greater detail.

  • What questions do you have about invertebrate phyla now? For example, do you wonder where organisms in the different phyla live or what they eat?
  • Invertebrates evolved hundreds of millions of years ago. Which invertebrate phylum do you think has the greatest number of species today?

Opening image copyright RAGMA IMAGES, 2010. Used under license from Shutterstock.com.

For Table above, top to bottom,

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

Show Hide Details
Files can only be attached to the latest version of section
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original