What are animals? A look at the different types and classification of animals.

The definition of animals includes many-celled organisms that differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials (such as proteins), in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation.

In addition, animals are distinguished from plants by their ability to move spontaneously and rapidly respond to external stimuli. Animals also require more complex food materials than plants do. The higher level of organization within an animal’s body is another distinguishing factor. Finally, domesticated animals are those that have been bred by humans over many generations, while wild animals have not undergone this selective process.

Characteristics of Animals

All animals are eukaryotic, meaning they have cells with a membrane-bound nucleus. Animals also have tissues and organs, which single-celled organisms like bacteria or protists do not have. Cells in animal tissues may be held together by cell junctions. Animal cells also have a plasma membrane and a cytoskeleton.

The majority of animals are motile, meaning they can move at will. This allows them to search for food and escape predators. Some common methods of locomotion include running, walking, jumping, hopping, slithering swimming, gliding and flying.

Most animals reproduce sexually; the eggs and sperm of animals are divided into haploid cells while they develop. In addition to sexual reproduction, some animals can also reproduce through parthenogenesis- a process where the animal produces fertile eggs without mating . All animals have a nervous system that helps them respond to their environment as well as an integumentary system which protects them from pathogens and parasites . Lastly, all animals have a muscular system that helps them move around.

History of Animal Classification

Animal classification has come a long way in the past few centuries. It all started in 1735 with the publication of Linnaeus’ Systema Naturae, which is considered to be the first modern system of classification. This work laid the groundwork for how we classify animals today.

Linnaeus’ method was based on shared characteristics, and he created two-word scientific names for each animal species that included the genus and species name. This approach is still used today and is known as the Linnaeus Method.

Taxonomy is so important because it helps us understand how all living organisms are related to one another within the animal kingdom. There are nine branching categories for classification, which can be summarized as follows: Domain – Kingdom – Phylum – Class – Order – Family – Genus – Species

How Animals Are Classified

There are nine animal phyla, and each phylum is further divided into classes. The table below lists the largest animal phyla and their respective classes.

  • Porifera (sea sponges)
  • Cnidaria (jellyfish, anemones, corals)
  • Arthropoda: Includes insects, spiders, crabs, lobsters and shrimps
  • Nematoda: Roundworms and flatworms are both nematodes
  • Annelida: Includes earthworms, leeches and marine worms
  • Mollusca: Snails and clams are two examples of mollusks
  • Platyhelminthes (flatworms)
  • Echinodermata (such as starfish, sea urchins and sea cucumbers)
  • Chordata: Includes vertebrates such as fish, amphibians, reptiles, birds and mammals

There are many different types of animals in the world, and each one is classified into a specific family. For example, cats belong to the family Felidae, while dogs are in the family Canidae. Each animal family is then divided into smaller groups, called genera. Cats, for instance, are subdivided into lions, tigers, leopards, and cheetahs.

Each genus contains animals that have very similar features and are closely related. For example, all lions share many characteristics–they have big manes around their necks, they are the biggest cats in the world, and they live in Africa. This is because lions belong to the genus Panthera.

The largest animal phyla are Arthropods (insects), Chordates (vertebrates), Annelida (earthworms) and Mollusca (snails).

Evolution of Animals

The history of animals is a long and complex one, with many different types and classifications of creatures. The first fossils of multicellular organisms were found in South Australia about 600 million years ago during the Neoproterozoic era, and the biota came and went after flourishing for 40 million years before starting to flourish again during the Cambrian period. Pre-Cambrian life is also called “Pre-Cambrian” because they existed long before the start of the Cambrian period which is when we see most of our modern animals on Earth.

One of the most famous pieces in Paleontology are the fossils in the Burgess Shale, containing diverse animals from trilobites to echinoderms to brachiopods and more. The Ediacaran fauna are a group of early animals that lived during the pre-Cambrian period; these are some of our earliest known animal specimens. The earliest sponge-like organisms appeared 541 million years ago during the Cambrian period.

The Phanerozoic eon is divided into periods: Paleozoic, which precedes the Cambrian period, and then Cambrian followed by Mesozoic and Cenozoic periods. The Cambrian explosion was a rapid period of evolution, and the fossils in the Burgess Shale are some of the most famous pieces in Paleontology.

One of the major changes during the Cambrian period was when animals started to venture onto land- this happened about 530 million years ago during the Late Cambrian or Early Ordovician periods. The Silurian era was dominated by jawless and jawed fish, with the first freshwater fish appearing. Invertebrates were the most common animals in the Ordovician period, with a few primitive fish species. The mass extinction that affected both invertebrates and fish happened during the Ordovician-Silurian extinction event.

The Silurian era also saw an increase in arthropods as well as amphibians; terrestrial animals evolved during this time period. Land animals evolved internal gas exchange systems, waterproof external layers, skeletal systems (endo- or exoskeletons), and a form of reproduction that does not involve water.” Fish were the dominant vertebrate during the Devonian period; one of the main reasons for their success was their ability to move farther inland. Amniotes (a group including reptiles, birds, and mammals) began to dominate during the Carboniferous period. The first tetrapods to appear were amphibians, not terrestrial animals. The Devonian extinction was the third of four major mass extinctions.

The Carboniferous period is also notable for the many invertebrates that became more prevalent in freshwater and marine habitats. The largest land invertebrate is Arthropleura, which grew up to 8 feet long and was used for defense against predators like saber-toothed cats. Amniotes continued to thrive and diversify during the Permian period; one of the most successful groups were synapsids, ancestors of mammals. However, at the end of the Permian period a massive extinction event occurred which wiped out 90% of all species on Earth.”

Types of Vertebrates

There are over 60, 000 species of vertebrates on Earth, ranging from the tiny Pygmy Shrew to the massive Blue Whale. Vertebrates are a diverse group of animals, and can be divided into several different subgroups.


Fish are the most common vertebrates on Earth, and can be found in all of the world’s oceans. There are over 30,000 species of fish, making them the most diverse group of vertebrates on the planet. Fish come in all shapes and sizes, from the tiny 5 cm (2 in) long pygmy goby to the massive 16 m (52 ft) long whale shark.

Fish are an important part of the global food chain and provide a vital source of protein for billions of people around the world.


Amphibians are animals that live in water, but can also live on land. There are three main groups of amphibians: frogs and toads, salamanders, and caecilians. Amphibians are ectothermic, meaning that they rely on the environment to regulate their body temperature. They can breathe both through their skin and through their lungs.

There are over 5,000 species of amphibians, ranging in size from the salamander to the giant salamander. 


Reptiles are cold-blooded, meaning that they have to bask in sunlight to warm up. They also regulate their body temperature by using the environment for heat and cooling down with water. Reptiles have a three-chambered heart and blood is pumped throughout the body.

There are over 4,000 species of reptiles, ranging in size from the gecko to the saltwater crocodile.


Birds are warm-blooded animals that have feathers, wings and evolved over time to fly. They have a four chambered heart, which makes it possible for them to pump oxygenated blood throughout the body and also generate heat. Birds can control their body temperature by using their wings to increase or decrease the flow of blood.

There are over 9,000 species of birds, ranging in size from the hummingbird to the ostrich


Mammals are animals that have hair, sweat glands and a four-chambered heart. Their blood is oxygenated throughout their body to provide more energy for the system. The temperature of mammals can be changed by sweating and panting, which causes the blood to cool.A mammal’s four-chambered heart makes it possible for them to pump oxygenated blood throughout the body.

There are over 5,000 species of mammals, ranging in size from the tiny Etruscan shrew, which weighs less than 1 gram (0.035 ounces), to the blue whale, which can weigh up to 190 metric tons (209 short tons).