Classification of Animals – Definition & Different Criteria of Classification

Classification of Animals: Animals show a great diversity of life on the earth. There are about 1.2 million species of animals that have been discovered and studied till now. It has been estimated that animals constitute the maximum percentage of all living organisms and exhibit a wide variety of different characteristics.

We have often seen that some animals live on the land while some live in water. Also, some eat grasses and plant products while some hunt other animals to obtain their food. Likewise, different criteria have been explained for the classification of animals. These mainly include their habitat, body complexity, feeding habits, reproduction, etc. Let’s read the article to study in detail the different bases for the classification of animals into different categories.

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What is Classification?

Classification is defined as the method of systematically arranging the different species of living organisms into closely related groups based on similarities and differences. It is the scientific ordering of organisms into different taxonomic groups based on their morphological, anatomical, and evolutionary relationship.

Criteria for the Classification of Animals

Animals exhibit several observable characteristics that make them differ from each other in certain features. Based on these features, the following main criteria have been established to classify the animals into different groups:
1. Based on habitat
2. Based on mode of Nutrition
3. Based on body structure and body complexity
4. Based on reproduction

Classification of Animals based on Habitats

Habitat is defined as the place on the earth where animals live. It can be said that habitat is the natural home of different organisms. Animals live in different places such as plains, deserts, mountains, lakes, rivers, and oceans. They have special body features to survive in their environment. Animals can be classified into the following categories on the basis of their habitat:

1. Terrestrial Animals: The animals that live on the land are called terrestrial animals. There are the following basic features of terrestrial animals:

I. The terrestrial animals have strong limbs (legs) for locomotion on the earth’s surface and to escape from predators.
II. Terrestrial animals have well-developed sense organs that help them to hunt other animals and to defend themselves from their predators.
III. They have strong claws, sharp teeth, and strong and heavy bones.
IV. Some examples of terrestrial animals include lions, dogs, bears, deer, elephants, camels, etc. are examples of terrestrial animals.

Fig: Some Terrestrial Animals

Adaptation in terrestrial animals: The land surface of the earth shows variation in certain environmental conditions that mainly include temperature, availability of water and food. The changes in the body features of the animals according to their surrounding environment are called adaptations. A few examples of adaptations in the terrestrial animals can be discussed as follows:

i. Desert animals have specific pads in their legs that make it easy to walk on the sand.
ii. Desert animals are adapted to store water in the body and live without drinking water for several days. Likewise, they can also store food.
iii. The skin of desert animals is thick and protects them not only from the sun but also prevents the loss of water from their bodies.
iv. Animals that live in the polar region that have low temperatures have thick fur that locks the heat to keep the body warm.
v. Some animals like bears, frogs, snakes, etc., become inactive during winter months when food is scarce. They go to sleep in the winter. This winter’s inactivity or sleep is called hibernation. These hibernating animals hide under the earth.
vi. Some animals move from one region to another region during the unfavourable conditions of the surroundings. It is called migration.

2. Aquatic Animals: The animals that live in the water are called aquatic animals. There are certain features that make them able to survive in the water. These features can be discussed as follows:

I. Aquatic animals have a streamlined body (the shape of the body that is tapering at both ends) to reduce friction while swimming.
II. The appendages (limbs) are modified into fins that facilitate swimming by removing the water according to the body weight.
III. The tail (tail fin) helps to maintain balance during swimming.
IV. Aquatic animals have gills for respiration instead of lungs. Gills are designed to extract oxygen from the water.
V. Fishes are the vertebrates that live in water.

Fig: Aquatic animals

3. Aerial Animals: The animals that can fly and spend a lot of time in the air are called aerial animals. Most of the birds are aerial animals. These animals have the following structural features:

i. The body is a streamlined shape to reduce friction with air during flight.
ii. The forelimbs are modified into wings.
iii. Aerial animals have hollow bones that reduce body weight and therefore contribute to smooth flying.
iv. Some birds, however, cannot fly due to their heavy body weight, such as ostrich, kiwi, emu, etc. These are called flightless birds.
v. Due to the unique feature of flying, aerial animals (birds) fly thousands of kilometres to warmer countries in search of food and to escape from the severe cold. This mass movement of birds is called migration. These migratory birds return to their homes after winter. Their wings are made up of thin layers of skin. Bats have very long webbed wings which help them to fly.

Fig: Aerial animals

4. Arboreal Animals: These animals spend most of their time on trees. Monkey, squirrel and tree lizard are some arboreal animals. The following body features help them to climb and live on the trees:

i. They have special claws and strong arms and legs, which help them to climb trees.
ii. Monkeys also use their tails to balance themselves and hold the branches. There are also some types of monkeys that have very long tails. They use this long tail to hold on to the branch of a tree when they swing from one tree to another.

Fig: Arboreal Animals

5. Amphibians: The word Amphibian is derived from the Greek word ‘Amphibios’, which means to live a dual life. Frogs, toads, and salamanders are examples of amphibians. The specific features of amphibians are as follows:

i. They have digit, webbed-limbs. The digits (finger-like structures) help them to move on the land surface, and the web found in between the digits help them to swim in the water.
ii. The strong forelimbs help them to hop on the land surface.
iii. The gaseous exchange occurs through moist and slimy skin, but the larval form has lungs for respiration as they live in the water until turned into an adult.

Classification of Animals based on Nutrition

Animals cannot prepare their food like plants and obtain their food from other sources. Therefore animals are called heterotrophs. The heterotrophs can be further classified into the following categories on the basis of their feeding habits:

1. Herbivores: The animals that feed upon grasses, plant leaves, and other plant products are called herbivores.

i. The herbivores have broad, flat premolars and molars for grinding and chewing the food.
ii. The canines are less developed as they do not need to tear the food.
iii. Monkeys, some birds, cows, buffalo, etc., are herbivores.

Fig: Herbivores

Types of herbivores:
i. Frugivores: These animals feed on fruits.
ii. Granivores: These are grain or seed-eating animals.
iii. Nectivores: These animals suck the flower nectar.
iv. Flavivores: The animals that consume leaves are called flavivores.

2. Carnivores: The animals that feed upon the flesh of other animals are carnivores. Lions, tigers, foxes, eagles, etc., are carnivores.
The animals that are being killed are called the prey, and the one who kills is called the predator. There are the following features of carnivores that help them to hunt and catch animals:

i. The strong legs of carnivores help them to run speedily to catch the prey.
ii. The strong, sharp, and pointed claws help to injure the prey.
iii. The sharp, pointed canines help to tear the flesh of the animals.

Fig: Carnivores

3. Omnivores: Animals that feed upon both plants and flesh of animals are called omnivores. Crows, bears, humans, etc., are examples of omnivores.

Fig: Omnivores

4. Parasites: These are the small animals that obtain Nutrition from another comparatively large and living animal called the host. Parasites can be of two types:

i. Ectoparasite
ii. Endoparasite

Ectoparasites are the animals that live on the body and obtain their Nutrition by sucking blood. Mosquitoes, lice, etc., are examples of ectoparasites.

Endoparasites live inside the body of the host and derive their Nutrition. Tapeworms, roundworms are examples of endoparasites. They remain attached to the host body through hooks and suckers.

Besides providing nourishment, the host also serves as a shelter for most of the parasites.

Fig: Ectoparasite and Endoparasite

5. Scavengers: These are also called decomposers. The animals that feed upon dead and decaying parts of plants and animals are called scavengers. They are called environment cleaners as they break down the dead parts of living beings and turn them into components that can be recycled. The animals that feed upon dead and decaying plants and plant parts are called detritivores. Hyenas, vultures are examples of scavengers.

Fig: Scavengers

Classification of Animals based on Body Structure

One of the important criteria to classify the animals is the body structure that mainly includes the presence and absence of the vertebral column (backbone). There are the following two groups of animals based on the presence and absence of vertebral column:

1. Invertebrates: The animals that lack the vertebral column are called invertebrates. There are the following common features of vertebrates:

I. Invertebrates have a flexible body due to the absence of hard and bony endoskeleton.
II. These are usually small in size. The body of advanced invertebrates is divided into the head, thorax, and abdomen.
III. Most invertebrates have an exoskeleton to protect the body. It may be a chitinous layer (in cockroaches), a hard shell (in snails).
IV. Sponges are the simplest invertebrates that have the whole body as tissue, and the body functions are carried along by water current that enters through the pores. The water current brings in food and oxygen and removes wastes and carbon dioxide when expelled out from the body.
V. Segmented worms, arthropods (insects, crabs, scorpion), molluscs (snail, oyster), echinoderms have the alimentary canal, heart, excretory organs to perform specific functions.
VI. Echinoderms (starfish, octopus) have a well-developed water vascular system to carry out locomotion, Nutrition, respiration, etc.
VII. Most invertebrates reproduce sexually. Some invertebrates can also reproduce asexually. This may occur by fission (divided into two or more daughter cells), budding ( a small organism attached to the parent body looking like bud), fragmentation (a small body part gives rise to a complete organism), etc.
VIII. Invertebrates like arthropods, molluscs, etc., are cold-blooded animals whose body temperature changes according to their surroundings.
IX. Examples of invertebrates include:
i. Sponges
ii. Unsegmented worms: tapeworm, roundworms, etc.
iii. Segmented worms: Earthworms, leech, etc.
iv. Arthropods: Cockroaches, millipede, centipede, mosquitoes, prawns, crabs, etc.
v. Echinoderms: starfish, sea cucumber, etc.
vi. Molluscs: Snails, oysters, octopus, etc.

Fig: An Overview of Invertebrates

2. Vertebrates: These are highly organised and advanced animals that have a vertebral column and bony endoskeleton that forms the internal framework of the body. Vertebrates are more complex animals that are categorised into the following classes in the increasing order of body complexity:

Vertebrates-chordates & Invertebrate-Non-chordates

The development of an adult from the larva is called metamorphosis. It can be said that the body undergoes several changes during metamorphosis.

i. A structure called notochord is present in the embryonic stage of highly organised animals (vertebrates) that converts into a vertebral column in an adult. Therefore the vertebrates are part of chordates.
ii. A few animals, though, have notochord in their embryos, but it becomes degenerated in the adult instead of forming a vertebral column. Hence these animals are not called vertebrates but are chordates. It can be concluded that all vertebrates are chordates, but not all chordates are vertebrates.
iii. All invertebrates are non-chordates because they neither have notochord nor have vertebral columns at any stage of their life cycle.

Summary

The earth is home to different types of animals. These animals live in different types of places such as deserts, mountains, grassland, water, etc. Besides this, animals also differ from each other in their feeding habits. They may be plant-eaters (herbivores), meat-eaters (carnivores), or omnivores (feed upon both plants and animals).

With respect to their habitats and feeding habits, there are certain adaptations in their specific body features. Moreover, these two criteria, body structure, is also considered as one of the main criteria for the classification of animals into different groups. Based on the body complexity, animals can be classified as invertebrates and vertebrates. This article is a sum-up of different classes of animals with respect to their place of living, Nutrition, and body structure.

Frequently Asked Questions (FAQs) on Classification of Animals

Q.1. What are the different types of animals based on habitat?
Ans: There are the following types of animals based on the habitat:
1. Terrestrial animals
2. Aquatic animals
3. Aerial animals
4. Arboreal animals
5. Amphibians

Q.2. What are carnivores?
Ans: The animals that feed upon animal flesh are called carnivores, such as lions.

Q.3. What are the five groups of animals with a backbone?
Ans: Fishes, amphibians, reptiles, birds, and mammals are the five groups of animals with a backbone.

Q.4. What are the five invertebrate groups?
Ans: Sponges, segmented worms, arthropods, molluscs, and echinoderms are the five main invertebrate groups.

Q.5. What are the examples of omnivores?
Ans: Humans, squirrels, dogs, pigs, etc., are examples of omnivores.

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