The such as snails, sea slugs, oysters,

The animal kingdom ranges from simple organisms like sponges to complex organisms like humans. We will look at some defining characteristics of animals as well as examples of both invertebrates and vertebrates.

Characteristics of Animals

Out of all six kingdoms, the animal kingdom is most likely the one that you already know the most about. After all, we are a part of this vast group. While the characteristics of animals vary greatly, there are a few things that they all share. Animals are eukaryotic, multicellular organisms that do not have cell walls, get their nutrients by ingestion, and are capable of movement.

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With so many shared qualifications, it may be hard to imagine all the possible differences between the more than 1.3 million living species of animals. Because they are made of eukaryotic cells – those with a nucleus and membrane-bound organelles – they are part of the Eukarya domain. ‘Multicellular’ means that they are made of many cells – not just one cell like their unicellular protozoan ancestors.

Unlike plant cells, animal cells do not have cell walls.
Animals and plant cells

Unlike the other kingdoms, animal cells do not have cell walls. While cell walls help protect cells, they also limit flexibility – something that is essential to the animal cells since these organisms are capable of movement.

This movement is also permitted by the presence of both muscles and nerves.As for reproduction, most animals reproduce sexually, though some species are capable of asexual reproduction. Sexual reproduction involves the formation of gametes – egg and sperm – as well as the union of these two cells to create genetically unique offspring. Most animals have sperm that are capable of movement due to the flagella, while the larger egg is not capable of movement.Scientists are able to study embryos and have found that different animal species have very similar early stages of development – even though the mature organisms may be nothing alike. We can see below that the early embryos of several species look relatively the same but the mature organisms are completely different.

In the early stages of development, animals species look the same.

comparison of embryos

There are four main characteristics that are used to classify animals: symmetry, body cavity, tissue, and vertebral column. Let’s quickly take a look at some of these before we look at some examples of animals. The first characteristic is symmetry. Animals generally fall into two categories – they either have radial or bilateral symmetry. Organisms like sea stars have radial symmetry, while organisms such as humans have bilateral symmetry – as we can see here:

The two categories of symmetry in animals are radial and bilateral.


The next thing is body tissue. The more complex an organism is, the more distinctive layers of tissue it has. There are three main layers:

  • Ectoderm – the outermost layer
  • Endoderm – the innermost layer
  • Mesoderm – the middle layer

As for vertebral column, there are two key groups: invertebrates and vertebrates.

Invertebrates are animals without a backbone, while vertebrates are animals with a backbone. As we look at the evolution of animals, we will first look at the more simple invertebrates and then consider the more complex vertebrates.

Evolution of Invertebrates

Let’s look at some key evolutionary characteristics of invertebrates. Most of the pattern of evolution is from simple to more complex. Examples of simple invertebrates include things like mollusks and nematodes. Mollusks have a soft body and protective shell, such as snails, sea slugs, oysters, and squids. Nematodes are simple worms – not earthworms.

Nematodes are organisms like hookworms and roundworms that often cause illnesses in humans. Nematodes are actually the most widespread group of animals, meaning they live in many different places.Arthropods are more complex invertebrates that have a segmented body, hard exoskeleton, and jointed appendages. This group is often called the ‘insects’ but contains more than just bugs. There are more than 1 million different identified species of arthropods ranging from horseshoe crabs to spiders to centipedes to butterflies.

Examples of arthropods

Another important group of invertebrates is the echinoderms.

The name means ‘spiny skin,’ which is easy to remember due to some classic examples including starfish and sea urchins.

Evolution of Vertebrates

Remember that vertebrates are organisms that have a backbone. Like the invertebrates, the vertebrates have evolved to be more complex over time. Let’s look at a few key groups of vertebrates, starting with the simplest forms of vertebrates – the lampreys. These organisms have a simple body plan and mouth. They are mostly parasites and latch on to other animals.

The development of a jaw and mineralized skeleton led to the next group – the cartilaginous fish. Examples of cartilaginous fish include sharks and rays. They are called cartilaginous fish because, while their skeletons are mineralized, they are made of cartilage and not hard bone like our skeletons. However, bony fish – such as trout, sunfish, perch, and tuna – do have skeletons made of bone and are evolutionarily more advanced than cartilaginous fish.The next major adaptation is that of legs. Amphibians – such as frogs and salamanders – were the first group of vertebrates to develop legs. The evolution of the amniotic egg – that is, an egg with its own source of food – was an advantage for reptiles such as turtles and snakes.

This allowed the eggs to be laid out of the water, which increased the available environments for organisms like reptiles and birds.The last major evolutionary change that we will look at was the ability to produce milk. All species of mammals are capable of milk production.

This nutrient-rich beverage increased care of young and therefore the overall survival of young. Mammals have other shared characteristics, such as having fur or hair, having highly efficient respiratory and circulatory systems, and being warm-blooded – meaning they internally regulate body temperature. There are currently more than 5,000 different species of mammals on Earth, including humans.

Lesson Summary

Animals are a vast group of organisms but do share some common characteristics. Remember that all animals have eukaryotic cells – those with a nucleus and membrane-bound organelles – as well as cells without a cell wall. This lack of a cell wall, along with nerves and muscles, allows for movement. All animals ingest food and are considered heterotrophic. Most of the reproduction is sexual, permitting a great amount of diversity in the animal kingdom.

Animals are classified on four key characteristics: symmetry, body cavity, tissue, and vertebral column. We focused mostly on the vertebral column and grouped organisms as either invertebrates – lacking a backbone – or vertebrates – having a back bone. We took a look at some organisms with different adaptations for both the invertebrates and vertebrates.

Learning Outcomes

Following this lesson, you should be able to:

  • Describe the characteristics that those in the animal kingdom share
  • Explain the four characteristics used to classify animals
  • Summarize the evolution of vertebrates and invertebrates

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