What Classifies as an Animal? Exploring the Characteristics and Classification

Have you ever wondered what makes an animal an animal? Sure, we all know cats, dogs, and cows are all animals but what about a jellyfish or a sponge? Believe it or not, those too can be classified as animals. So, what exactly makes something an animal?

According to science, animals are multicellular organisms that are heterotrophic and move spontaneously during at least one stage of their life cycle. In simple terms, they are living organisms that can’t produce their own energy and have the ability to move on their own. This can range from a tiny insect to a massive mammal like a blue whale.

It’s fascinating when you think about it, the vast array of creatures that fall under the classification of animals. From birds that can fly thousands of miles to creatures that can survive in the harshest of environments. There’s no doubt about it, animals have captivated humans since the dawn of time, and will continue to do so for generations to come.

Animal Kingdom Classification

The animal kingdom is a diverse group of living organisms that includes more than 2 million species. Animals are divided into various categories based on different characteristics such as body structure, mode of nutrition, and habitat. The classification of the animal kingdom is important for understanding the diversity of animals and their relationships with other organisms.

Types of Animal Kingdom Classification

  • Phylum: The highest level of classification is phylum. This categorizes animals into specific groups based on their shared physical characteristics. For example, arthropods are a phylum that includes insects, spiders, and crustaceans.
  • Class: The next level of classification is class, which groups animals based on their overall body structure. Birds and mammals are part of a class, while insects and fish are part of another class.
  • Order: Further down the classification hierarchy, we have orders. This groups animals based on their shared characteristics and ancestry. For example, primates are part of an order that also includes apes and monkeys.

The Importance of Animal Classification

The classification of animals is essential for understanding their morphological, ecological, and evolutionary characteristics. It helps scientists to group animals that are related to one another and to identify the similarities and differences between them. Additionally, it helps us to understand the relationships between animals and their environment and to identify new species that have yet to be discovered.

The Animal Kingdom Table

Phylum Examples
Arthropoda Insects, spiders, crustaceans
Chordata Vertebrates such as fish, birds, and mammals
Mollusca Oysters, clams, snails, and squids
Annelida Earthworms, leeches
Cnidaria Jellyfish, coral

The table above shows some of the major phyla in the animal kingdom and some examples of organisms that belong to each group. This classification system helps us to understand the diversity of the animal kingdom and how organisms are related to one another.

Definition of an animal

Animals are living organisms that belong to the kingdom Animalia. As per the biological classification system, animals are categorized into various groups based on their physical and biological characteristics. Some of the defining characteristics of animals include their ability to move, reproduce sexually, and consume and digest organic matter for energy.

What classifies as an animal?

  • Animals have cells that lack a cell wall and contain a nucleus.
  • They are heterotrophic, meaning they consume other organisms for energy.
  • Animals are capable of movement, even if they don’t move from place to place.

The diversity of animals

There is a vast diversity among animals in terms of their size, shape, habitat, behavior, and evolutionary lineage. Scientists have estimated that there could be between 5 and 10 million different animal species on Earth, with new species being discovered every year. Some examples of animal phyla include Arthropoda (insects, crustaceans, arachnids), Chordata (vertebrates and their relatives), Cnidaria (jellyfish, coral), and Mollusca (snails, cephalopods).

Animal diversity also extends beyond the confines of the Earth. Several missions have been sent to different planets and moons in our solar system to search for signs of life, which could include animals. However, so far, no conclusive evidence has been found yet.

The importance of animals

Animals play critical roles in our ecosystems as pollinators, prey, and predators. They also provide essential goods and services to humans, such as food, clothing, and medicine. However, the rapid loss of animal species due to habitat destruction, climate change, poaching, and other human activities is threatening the stability of our planet’s ecosystems and the benefits they provide. Therefore, it is vital to not only protect and conserve endangered animals but also to appreciate and take responsibility for the vital role they play in our world.

Animal Classification Characteristics
Vertebrates Animals with backbones, including fish, amphibians, reptiles, birds, and mammals.
Invertebrates Animals without backbones, including insects, crustaceans, mollusks, and worms.
Protozoa Single-celled organisms that can move around and typically live in aquatic environments.

By understanding the diverse spectrum of animal life on our planet, we can better appreciate the wonders and intricacies of the natural world, and hopefully, take steps to conserve and protect them for generations to come.

The Role of Taxonomy in Animal Classification

Animal classification is a way of organizing animals into groups based on shared characteristics. Taxonomy, the science of classification, plays a crucial role in this process. Taxonomy helps scientists organize and classify animals based on their physical and genetic characteristics. This classification system has several benefits including:

  • Better understanding of the diversity of living organisms
  • Improved communication between scientists about different organisms
  • Ability to identify and describe new species

Without taxonomy, it would be impossible to categorize and study the vast number of species on our planet. Taxonomy provides a framework within which biologists can compare and contrast different animals and understand how they relate to one another. This has led to many important discoveries in evolution, genetics, ecology, and medicine.

Taxonomy also has an important practical application in conservation biology. By understanding the different groups of animals and their relationships to one another, conservationists can identify which species are most at risk of extinction and develop strategies to protect them.

One of the most important aspects of taxonomy is the hierarchical system used to classify animals. This system starts with the largest group, the kingdom, and becomes progressively more specific, with each group containing more closely related organisms. The current system of classification includes the following groups:

Category Description
Kingdom Grouping of organisms based on similar characteristics
Phylum Includes organisms with similar body plans
Class Groupings within a phylum based on shared characteristics
Order Groupings within a class based on similar features
Family Groupings within an order based on common features
Genus Groupings within a family based on genetic similarity
Species Groupings within a genus that share many traits and can interbreed

Taxonomy is an essential part of animal classification, providing a framework for understanding the relationships between different species and how they evolved. This, in turn, helps us to better understand and appreciate the diversity of life that exists on our planet.

Characteristics of Vertebrate Animals

Vertebrate animals are a diverse group of organisms that possess a number of defining characteristics.

  • Backbone: The most defining characteristic of vertebrates is the presence of a backbone or spinal column, which gives these animals their common name – vertebrates. The backbone is made up of a series of vertebrae that provide support and protection for the central nervous system.
  • Jawed Vertebrates: The majority of vertebrates have jaws, which are essential for feeding on a variety of food sources. This group includes fish, amphibians, reptiles, birds, and mammals.
  • Internal Organs: Vertebrates have a well-developed set of internal organs, including a heart, lungs, liver, and kidneys, that are vital for survival. These organs work together to maintain homeostasis in the body.

In addition to these defining characteristics, vertebrates also display a number of other traits that distinguish them from other groups of animals, such as:

  • Endoskeleton: Vertebrates have an internal skeleton made up of bone or cartilage, which provides structural support and protection for the body.
  • Sensory Systems: Vertebrates possess well-developed sensory systems, including eyes, ears, and a sense of smell, that allow them to detect and respond to stimuli in their environment.
  • Breathing: Vertebrates use different methods of breathing, depending on the group to which they belong. Fish breathe through gills, while amphibians breathe through their skin and lungs. Reptiles, birds, and mammals breathe through their lungs.

To help classify vertebrates further, scientists have developed a system that groups them into five major classes:

Class Examples
Fish Salmon, sharks, trout, tuna
Amphibians Frogs, toads, salamanders
Reptiles Alligators, lizards, snakes, turtles
Birds Eagles, penguins, sparrows
Mammals Cats, dogs, humans, whales

Each of these classes has its own unique characteristics and adaptations that allow them to survive in different environments and fulfill different ecological roles.

Characteristics of invertebrate animals

Invertebrate animals are animals that do not have a backbone or vertebral column. They are found in a wide array of phyla, ranging from sponges to mollusks. These animals make up the vast majority of the animal kingdom and are characterized by several unique features.

Five subtypes of invertebrate animals:

  • Sponges
  • Cnidarians
  • Flatworms
  • Roundworms
  • Arthropods


Sponges are the simplest and most primitive of all invertebrate animals. They have no true tissues or organs, but are composed of loosely connected specialized cells known as choanocytes. These organisms filter food particles from water that flows through their pores, and are known for their ability to regenerate lost parts.


Cnidarians range from simple polyps to complex jellyfish. These animals have specialized stinging cells, known as nematocysts, which they use to capture prey and defend themselves. Cnidarians can be found in a number of environments, including freshwater, saltwater, and even coral reefs.


Flatworms are soft-bodied animals that are often found in aquatic environments. Many species are free-living, while others are parasitic. Flatworms have a simple nervous system, a digestive system with a single opening, and are hermaphroditic, meaning they have both male and female reproductive organs.


Characteristics Example
Long, cylindrical body Caenorhabditis elegans
Two openings for digestive system Ascaris lumbricoides
Live in a variety of habitats Necator americanus

Roundworms are also called nematodes and are one of the most diverse and abundant groups of invertebrate animals. They are characterized by a long, cylindrical body that is tapered at both ends. Most roundworms have two openings for their digestive system, one at the mouth and one at the anus. They live in a variety of habitats, including soil, freshwater, and marine environments.


Arthropods are one of the largest and most diverse groups of invertebrates. They are characterized by their segmented bodies, jointed appendages, and exoskeletons made of chitin. Arthropods include a wide range of animals, such as insects, spiders, crustaceans, and centipedes. They play a vital role in the ecosystem, serving as pollinators, decomposers, and predators of other animals.

Evolutionary history of animals

Animals are a diverse and fascinating group of organisms that have evolved over millions of years. Their evolutionary history can be traced back to the earliest forms of life on Earth, which were single-celled organisms like bacteria and archaea. Over time, these simple organisms gave rise to more complex forms of life, including eukaryotes, which are cells that have a distinct nucleus and other membrane-bound organelles.

Following the emergence of eukaryotes, animals went through a series of evolutionary changes that led to the great diversity of species we see today. This evolution was driven by a number of factors, including changes in the environment, competition with other organisms, and mutations that gave rise to new traits and features.

Key milestones in animal evolution

  • Cambrian explosion: Around 540 million years ago, there was a sudden burst of diversification in animal life, during a period known as the Cambrian explosion. This saw the emergence of many new animal phyla, or major groups of life, including arthropods, molluscs, and chordates.
  • Vertebrate evolution: A major milestone in animal evolution was the emergence of vertebrates, or animals with backbones. This group includes fish, amphibians, reptiles, birds, and mammals, and first appeared around 500 million years ago.
  • Mammal evolution: The evolution of mammals is a complex and fascinating story that spans millions of years. Mammals first appeared around 200 million years ago, and over time, they evolved a range of features that allowed them to live in a variety of environments and fill many different ecological niches.

Divergence and convergence in animal evolution

One of the most interesting aspects of animal evolution is the way in which different groups of animals have converged or diverged over time. For example, the wings of birds, bats, and insects all serve the same purpose of allowing these animals to fly, but they evolved independently from one another. On the other hand, birds and mammals both evolved warm-bloodedness, or endothermy, as a way to maintain a stable body temperature, despite evolving from different ancestors.

Other examples of convergence and divergence can be seen in the ways different animals have adapted to particular environments or ecological niches. For instance, dolphins and sharks both have streamlined bodies and fins that allow them to swim efficiently through water, but they diverged from a common ancestor hundreds of millions of years ago.

Animal classification

Due to the vast diversity of animal life, scientists have developed a classification system to help organize and categorize different species. This system is based on a hierarchy of categories, ranging from the most general, or kingdom, to the most specific, or species. The current system places animals in the kingdom Animalia, which is then further divided into smaller groups based on their evolutionary relationships, physical characteristics, and other traits.

Kingdom Phylum Class Order Family Genus Species
Animalia Chordata Mammalia Primates Hominidae Homo sapiens

The classification system is constantly evolving, as scientists discover new species and learn more about the relationships between different groups of animals. However, it remains an important tool for studying and understanding the evolutionary history of animals.

Similarities and Differences between Animal Species

As a diverse group of living organisms, animals come in many different forms, sizes, and shapes. However, despite these differences, there are also many similarities that link different animal species together. Here are some of the main similarities and differences between animal species that help us classify them:

Physical Characteristics

  • Animals can be classified based on various physical characteristics such as whether they have fur, feathers or scales.
  • Mammals, for example, are a class of animals that share several characteristics including the ability to nurse their young, possess hair or fur, and are warm-blooded.
  • Reptiles, on the other hand, are a class of animals that are characterized by the presence of scales and a cold-blooded metabolism.

Evolutionary Relationships

Another way to classify animals is by looking at their evolutionary relationships. Scientists use a system of classification based on common ancestry, known as taxonomy, to group organisms together.

  • For instance, humans share a recent common ancestor with other primates such as chimpanzees, gorillas and orangutans. Thus, they are classified under the order Primates.
  • Similarly, dogs, foxes, wolves, and coyotes all belong to the dog family, Canidae.

Habitat and Ecology

Animals can also be classified based on their habitat and ecology. This means considering their interactions with other living organisms and the environment they live in.

  • Marine animals, for example, are adapted to survive in the ocean environment, which has its own unique set of physical and chemical conditions.
  • Terrestrial animals, on the other hand, have adapted to live on land and deal with its own set of challenges that include obtaining water, food, and shelter.

Anatomical Structures

Anatomy is another way to classify animals. This is because animals with similar anatomical structures often share a common ancestry and evolutionary history.

Characteristic Examples
Vertebrates (animals with backbones) fish, reptiles, amphibians, birds, and mammals
Invertebrates (animals without backbones) insects, spiders, crustaceans, mollusks, and worms

Overall, classifying animals is important because it helps us gain a deeper understanding of how different species relate to one another. Knowing about these similarities and differences is also essential for researchers to help conserve and manage animal populations and their habitats.

FAQs: What Classifies as an Animal?

Q: What is an animal?
A: An animal is a living organism that is capable of movement and sensory response.

Q: Do humans count as animals?
A: Yes, humans are considered to be animals because they meet the criteria of being living organisms that can move and sense their environment.

Q: Are insects animals?
A: Yes, insects are classified as animals because they are living organisms that move and respond to their environment.

Q: What about plants?
A: Plants are not considered to be animals because they cannot move, nor do they possess the ability to sense their environment in the same way that animals do.

Q: What distinguishes animals from other living things?
A: The ability to move and sense their environment is the primary distinguishing characteristic of animals as compared to other living things.

Q: Are there any types of animals that can’t move?
A: Yes, some animals such as coral or sponges are sessile, meaning that they are anchored to one spot for their entire lives. However, they still meet the criteria for being classified as animals.

Q: Are viruses considered to be animals?
A: No, viruses are not considered to be animals because they are not living organisms – they require a host in order to reproduce and do not possess the ability to move or sense their environment.

Closing Thoughts

Well, now you know what classifies as an animal! It’s important to remember that the ability to move and sense the environment are the key characteristics that differentiate animals from other living things. Thanks for reading, and we hope to see you again soon!