Animal Kingdom: Basis Classification

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:23 PM IST

The Animal Kingdom is a vast and diverse group containing species from simple sponges to complex animals. The Animal Kingdom is a group of living things that includes all animals. These are multicellular organisms that cannot make their food (heterotrophic) and depend on other living things for energy. The Classification of the Animal Kingdom is based on the possession of certain key characteristics, such as bilateral body symmetry, notochord, segmentation, and coelom. The Animal Kingdom chapter in Biology deals with the taxonomy of the animal kingdom, grouping animals according to common characteristics and relationships during evolution.

This Story also Contains

Basis Of Classification Of Animal Kingdom
Levels of Organization in Animals: From Cells to Organ Systems
Types of Body Symmetry in Animals: Asymmetrical, Radial, and Bilateral
Germ Layers in Animal Development
Coelom in Animals: Coelomate, Pseudocoelomate, and Acoelomate
Animal Segmentation: Segmented and Non-Segmented Body Plans
Notochord in Animals
Different Phyla of Animal Kingdom
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The Basis of Classification of the Animal Kingdom

The basis of classification into the animal kingdom helps understand the diversity of animals and their evolutionary relationships. Animals are also grouped by the presence or absence of a notochord, which separates Chordates from Non-Chordates. Animal cells are eukaryotic and do not have rigid cell walls. This article includes the Basis of Classification of the Animal Kingdom, 11 phyla of the kingdom Animalia, and key features like body symmetry, type of organisation level, body cavity, and segmentation. These features help scientists understand how animals are related and how they evolved.

Basis Of Classification Of Animal Kingdom

The Basis of Classification of the Animal Kingdom is a fundamental aspect of biology that organizes animals into hierarchical categories based on shared characteristics and evolutionary relationships.In the animal classification kingdom, organisms are grouped into various phyla, such as Chordata for vertebrates and Arthropoda for invertebrates, to reflect their distinct anatomical and physiological traits. The levels mentioned for the basis of classification of the animal kingdom are as described below:

Level of organization
Body symmetry
Germ layers
Body cavity (coelom)
Segmentation
Presence or absence of notochord

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Levels of Organization in Animals: From Cells to Organ Systems

Every single animal in the animal kingdom is multicellular. However, the degree of cell organisation varies among different animals. Animals can range from basic cell aggregations to complex cell arrangements that give rise to tissues and organs.

Level of Organisation	Description	Examples
Cellular Level	Animals with loosely organized cells where each cell is responsible for all life functions, including breathing, nutrition, and excretion.	Porifera (e.g., Sponges)
Tissue Level	Groups of similar cells perform specific functions; certain metabolic activities are carried out by designated tissues, such as those for digestion or respiration.	Coelenterates and Ctenophores
Organ Level	Groups of tissues that work together to perform specific functions; specialized organs are present for various purposes like respiration, digestion, and excretion.	Platyhelminthes and Aschelminthes
Organ System Level	More complex arrangements where organs form systems to perform specific physiological functions; each system is dedicated to a particular function.	Arthropods, Echinoderms, Chordates

Types of Body Symmetry in Animals: Asymmetrical, Radial, and Bilateral

If a figure can be split into two equal halves, it is said to be symmetrical. Animals can have radial symmetry, bilateral symmetry, or asymmetry.

Type of Symmetry	Description	Examples
Asymmetry	Animals with nearly asymmetrical shapes; no plane can divide the body into two equal parts.	Sponges
Bilateral Symmetry	Only one plane can divide the body into two identical halves; most animals exhibit this type of symmetry.	Annelids, Arthropods, Molluscs, Echinoderms, Hemichordates, Chordates (note: echinoderms can show bilateral or radial symmetry depending on development stage)
Radial Symmetry	The body can be divided into identical halves by any plane passing through the central axis; appears circular from any angle.	Coelenterates, Ctenophores, Echinoderms

Germ Layers in Animal Development

The layers of cells in an embryo are called germinal layers. These layers are known as the germinal layers because they are where many organs originate or germinate. Organs can develop from endoderm, mesoderm, or ectoderm, depending on the kind.

Type	Description	Examples
Diploblastic	Animals with cells organized into two embryonic layers: ectoderm and endoderm. A non-differentiated layer called mesoglea lies between them.	Coelenterates, Ctenophores
Triploblastic	Animals with cells organized into three embryonic layers: ectoderm, mesoderm, and endoderm.	Platyhelminthes, Annelids, Arthropods, Chordates

Coelom in Animals: Coelomate, Pseudocoelomate, and Acoelomate

Different animals may or may not have a coelom or body cavity. Mesoderm lines the coelom. Animals are divided into three groups based on this feature.

Type	Description	Examples
Coelomates	Animals that possess a true body cavity (coelom) lined by mesoderm.	Molluscs, Arthropods, Chordates
Pseudocoelomates	Animals with a false cavity (pseudocoelom) that is not lined by mesoderm and have patches of mesoderm.	Aschelminthes
Acoelomates	Animals that lack a true body cavity (coelom).	Platyhelminthes

Animal Segmentation: Segmented and Non-Segmented Body Plans

In certain species, the body is segmented both internally and externally, with at least some organs repeated serially. For example, the body of an earthworm has a pattern known as metameric segmentation.

Eg: Phylum Annelida (e.g., earthworms, leeches)

Notochord in Animals

The notochord is a rod-like structure located on the dorsal (back) side of the animal's body. The notochord originates from the embryo's mesoderm and is mesodermally generated. In most animals, the notochord forms during embryonic development and vanishes later. Even in later phases in some species, it could still be rudimentary. Animals are classified as chordates if a notochord is present. Non-Chordate refers to an animal lacking a notochord, such as echinoderms and Porifera. The notochord is a structure seen in hemichordates and chordates.

Different Phyla of Animal Kingdom

The animal kingdom classification system organises living organisms into hierarchical categories, starting from broad groups like kingdoms and phyla down to specific species. In animal kingdom classification, animals are divided into various phyla based on their body structure, reproductive methods, and genetic relationships, with Chordata and Arthropoda being two of the largest groups. Advances in molecular biology have significantly impacted animal kingdom classification, allowing scientists to refine classifications based on genetic data rather than solely on physical traits. They progress from the most basic to the most complex in the chart classifying animals. The animal kingdom is classified into 11 different phyla namely: Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata. Understanding animal kingdom classification is essential for biologists as it provides a framework for studying the diversity of life and the evolutionary relationships among different species.

Phylum	Common Name	Characteristics	Examples
Porifera	Sponges	- Basic multicellular animals - Bodies covered with pores (Ostia) - Canal system for circulation of water and nutrients - Supported by spicules - Minimal tissue differentiation	Spongilla, Sycon
Cnidaria	Cnidaria	- More body differentiation - Single opening for ingestion and egestion - Diploblastic with two germ layers - Can live solitary or in colonies	Jellyfish, Sea Anemones, Hydra
Ctenophora	Comb Jellies	- Marine animals - Radially symmetric and diploblastic - Bioluminescent - Eight rows of ciliated comb plates for locomotion	Pleurobrachia, Ctenoplana
Platyhelminthes	Flatworms	- Dorsoventrally flattened bodies - Triploblastic with three germ layers - Acoelomates with bilateral symmetry - Presence of flame cells for excretion	Planaria, Liver Fluke, Tapeworm
Aschelminthes	Roundworms	- Triploblastic and bilateral symmetry - Cylindrical body shape - Pseudocoelomates (no true coelom) - Complete alimentary canal	Ascaris, Wucheria
Annelida	Segmented Worms	- Found in various environments (land, freshwater, marine) - Triploblastic and bilaterally symmetrical - True coelomates with segmented bodies	Earthworms, Leeches
Arthropoda	Jointed-Legged Animals	- Largest phylum in the animal kingdom - Body divided into head, thorax, abdomen - Open circulatory system - Chitinous exoskeleton	Crabs, Spiders, Mosquitoes
Mollusca	Molluscs	- Second-largest animal phylum - Coelomate and bilaterally symmetrical - Organ-system level organization - Presence of radula for feeding	Octopus, Snails, Mussels
Echinodermata	Spiny-Skinned Animals	- Marine animals with calcareous endoskeletons - Triploblastic with a coelomic cavity - Radial symmetry in adults but bilateral in larvae	Starfish, Sea Urchins
Hemichordata	Acorn Worms	- Marine animals resembling worms - Triploblastic and bilaterally symmetrical - Open circulatory system	Balanoglossus, Saccoglossus
Chordata	Chordates	- Presence of notochord, dorsal hollow nerve cord, paired pharyngeal gill slits - Coelomate with organ-system level organization	Amphioxus (Lancelet), Ascidia (Urochordata)

The animal classification kingdom has evolved over time, with advancements in genetic research leading to the re-evaluation of certain classifications based on molecular data rather than just physical characteristics.

Also Read:

Notochord	Earthworm Scientific Name
Digestive system of Earthworms	Vermicomposting
Reproductive System of Earthworm	Earthworm - Morphology and anatomy

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Frequently Asked Questions (FAQs)

1. What is the basis of classification of Animal Kingdom?

The basis of classification of the Animal Kingdom is how animals are built and how their bodies work. Scientists look at:

Body structure (simple or complex)
Symmetry (shape of the body)
Body cavity (space inside the body)
Germ layers (layers formed in early growth)
Segmentation (body divided into parts)
Notochord (a rod-like structure in some animals)

2. What is the meaning of metamerism?

The repeating of similar bodily parts is known as metamerism. Metamerism is also called segmentation. The Annelids, which include earthworms, leeches, and tubeworms exhibit this sort of development. Additionally, Arthropods, which include crustaceans, insects, etc exhibit it in a more developed form.

3. Pseudocoelom is the characteristic feature of which phylum?

Pseudocoelom is the characteristic feature of the phylum Aschelminthes.

4. What characteristics are present in all chordates?

The characteristic features of all chordates are:

Presence of notochord.
Presence of dorsal nerve cord.
Triploblastic.
Post anal tail is present
Pharyngeal slits are present.

5. Mention the advantages of the Basis of Classification.

The basis of classification has made it simpler to study animals. Learning about organisms and comparing one set of animals to another for a better understanding are becoming simpler.

6. What are the major classifications of the animal kingdom?

The major classifications of the animal kingdom include various phyla, such as Porifera, Coelenterata (Cnidaria), Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata.

7. Why is metamorphosis considered in animal classification?

Metamorphosis, a dramatic change in body form during development, is considered in classification as it reflects complex life cycles and adaptations. It's particularly important in classifying insects and amphibians, helping to understand their evolutionary strategies and ecological roles.

8. What is the importance of body wall composition in animal classification?

Body wall composition (e.g., presence of chitin, calcium carbonate, or keratin) is significant in classification as it reflects adaptations to different environments and lifestyles. It helps in distinguishing between major groups like arthropods, mollusks, and vertebrates.

9. How does the presence of specialized feeding structures affect animal classification?

Specialized feeding structures (like filter-feeding apparatus or venom-injecting fangs) are important in classification as they reflect adaptations to specific diets and ecological niches. They help in understanding the diversity within animal groups and their evolutionary strategies.

10. What role does the level of cephalization play in animal classification?

Cephalization, the concentration of sensory and neural structures in a head region, is significant in classification. Higher levels of cephalization often indicate more advanced nervous systems and behaviors, helping to distinguish between more primitive and complex animal groups.

11. How does the type of larval stage influence animal classification?

Larval stages are important in classification, especially for invertebrates. The type of larva (e.g., trochophore, veliger, or nauplius) can indicate evolutionary relationships between different groups, even if the adult forms look very different.

12. What is the primary basis for classifying animals into different groups?

The primary basis for classifying animals is their body organization and complexity. This includes factors like cell layers, body symmetry, presence of body cavities, and segmentation. Classification helps us understand evolutionary relationships and organize the diversity of animal life.

13. What is the significance of body temperature regulation in animal classification?

Body temperature regulation (ectothermic or endothermic) is an important classification criterion, especially for vertebrates. It reflects metabolic adaptations and often correlates with activity levels and habitat preferences. This feature helps separate major groups like reptiles from mammals and birds.

14. How does the presence of specialized organs influence animal classification?

Specialized organs (like hearts, kidneys, or complex eyes) indicate evolutionary advancements and adaptations. Their presence or absence helps in distinguishing between simpler and more complex animal groups, reflecting the overall organization and functionality of the body.

15. How does the type of excretory system affect animal classification?

The excretory system type (protonephridia, metanephridia, or kidneys) is significant in classification as it reflects the complexity of body organization and adaptations to different environments. It helps in distinguishing between more primitive and advanced animal groups.

16. What role does the digestive system complexity play in animal classification?

Digestive system complexity (from simple sacs to complex, specialized organs) is important in classification. It reflects evolutionary advancements and adaptations to different diets. This feature helps in understanding the relationships between different animal groups and their ecological roles.

17. What is the significance of a coelom in animal classification?

A coelom is a fluid-filled body cavity that develops within the mesoderm. Its presence or absence is important in classification as it affects body organization and organ systems. Animals are classified as acoelomates (no coelom), pseudocoelomates (false coelom), or coelomates (true coelom) based on this feature.

18. How does the presence or absence of a true coelom differ from a pseudocoelom in classification?

A true coelom is completely lined by mesoderm, while a pseudocoelom is partially lined. This distinction is crucial in classification as it reflects different levels of body organization. True coelomates (like vertebrates) are generally more complex than pseudocoelomates (like roundworms).

19. What role does the type of body cavity play in animal classification?

The type of body cavity (acoelomate, pseudocoelomate, or coelomate) is crucial in classification. It reflects the complexity of body organization and often correlates with the sophistication of organ systems, helping to distinguish between major animal groups.

20. What is the importance of the presence or absence of a coelom in classifying invertebrates?

The presence or absence of a coelom is crucial in classifying invertebrates. Acoelomate, pseudocoelomate, and coelomate body plans reflect different levels of complexity and often correlate with the sophistication of organ systems.

21. How does the presence of a complete digestive system influence animal classification?

A complete digestive system (with separate mouth and anus) versus an incomplete one is an important classification criterion. It reflects the level of body organization and often correlates with feeding habits and lifestyle, helping to separate more primitive from advanced animal groups.

22. How does the type of respiratory system influence animal classification?

The respiratory system type (gills, lungs, trachea, or body surface) is important in classification as it reflects adaptations to different environments. It helps distinguish aquatic from terrestrial animals and indicates the complexity of body systems, aiding in grouping animals into appropriate phyla.

23. What is the importance of reproductive strategies in animal classification?

Reproductive strategies (sexual or asexual, internal or external fertilization) are significant in classification. They reflect evolutionary adaptations and often correlate with habitat and lifestyle. This feature helps in understanding relationships between different animal groups and their survival strategies.

24. Why is the presence or absence of a skeleton important in animal classification?

The presence and type of skeleton (endoskeleton, exoskeleton, or hydrostatic skeleton) is significant in classification as it reflects an animal's body support system and evolutionary adaptations. It helps distinguish major groups like vertebrates (with internal skeletons) from many invertebrates.

25. What role does the circulatory system play in animal classification?

The circulatory system (open or closed, with or without a heart) is a key factor in classification. It reflects the complexity of body organization and often correlates with body size and activity level. This feature helps separate major animal groups and understand their physiological adaptations.

26. How does the nervous system complexity affect animal classification?

Nervous system complexity (from simple nerve nets to complex brains) is crucial in classification. It reflects evolutionary advancements and often correlates with an animal's behavior and adaptability. This feature helps distinguish between more primitive and advanced animal groups.

27. How does the number of embryonic germ layers affect animal classification?

The number of embryonic germ layers (ectoderm, mesoderm, endoderm) is a fundamental criterion in animal classification. Animals are grouped as diploblastic (two germ layers) or triploblastic (three germ layers). This distinction reflects the complexity of body organization and organ development.

28. What is the significance of body wall layers in animal classification?

The number and organization of body wall layers are fundamental in classification. The distinction between diploblastic (two layers) and triploblastic (three layers) animals reflects basic body organization and is crucial in separating major animal groups.

29. What role does the type of mouth opening play in animal classification?

The type of mouth opening (e.g., protostome vs. deuterostome development) is a fundamental criterion in classifying animals. It reflects early embryonic development and separates two major lineages of animals, indicating deep evolutionary relationships.

30. How does the mode of embryonic development influence animal classification?

The mode of embryonic development (e.g., radial or spiral cleavage) is a fundamental criterion in animal classification. It reflects deep evolutionary relationships and helps separate major animal lineages, even when adult forms look very different.

31. What is the significance of the presence or absence of tissues in animal classification?

The presence of true tissues is a key feature distinguishing most animals from simpler organisms like sponges. This criterion reflects the level of body organization and is fundamental in separating the most basic animal phyla from more complex ones.

32. How does the presence or absence of a notochord influence animal classification?

The notochord is a flexible rod-like structure that provides support in some animals. Its presence is a key characteristic of chordates, including vertebrates. Animals without a notochord are classified as invertebrates. This distinction helps separate major animal groups and understand their evolutionary history.

33. How does the presence of a notochord at some life stage affect animal classification?

The presence of a notochord, even if only during embryonic stages, is a defining feature of chordates. This criterion helps classify animals into one of the major phyla, including vertebrates, and understand their evolutionary relationships to other groups.

34. How does segmentation influence animal classification?

Segmentation, the division of the body into repeating units, is a key feature in some animal groups. It affects classification because it reflects internal organization and evolutionary relationships. Segmented animals (like earthworms and arthropods) are grouped separately from non-segmented ones.

35. What is the importance of segmentation patterns in animal classification?

Segmentation patterns (e.g., homonomous or heteronomous) are significant in classifying certain animal groups. They reflect body organization and specialization, helping to distinguish between different types of segmented animals like annelids and arthropods.

36. How does the presence of metameric segmentation affect animal classification?

Metameric segmentation, the repetition of body units, is a key feature in classifying certain animal groups. It reflects a specific body organization strategy and helps distinguish groups like annelids and arthropods from other invertebrates.

37. How does the presence of a hydrostatic skeleton influence animal classification?

A hydrostatic skeleton, using fluid pressure for support, is characteristic of certain invertebrate groups. Its presence helps classify animals like cnidarians and annelids, reflecting their body organization and movement strategies.

38. What role does body symmetry play in animal classification?

Body symmetry is crucial in animal classification as it reflects internal organization and evolutionary adaptations. Animals can be radially symmetric (like jellyfish), bilaterally symmetric (like humans), or asymmetric (like sponges). Symmetry often correlates with an animal's lifestyle and habitat.

39. What is the significance of radial versus bilateral symmetry in animal classification?

Radial symmetry (body parts arranged around a central axis) versus bilateral symmetry (left and right sides) is a fundamental distinction in classification. It often correlates with lifestyle, movement patterns, and evolutionary history, helping to separate major animal groups.

40. How does the presence of a true head influence animal classification?

The presence of a true head, with concentrated sensory and neural structures, is significant in classification. It reflects a higher degree of cephalization and often correlates with more complex behaviors, helping to distinguish between different animal groups.

41. How does the type of body covering influence animal classification?

Body covering (e.g., scales, feathers, fur) is significant in classification as it reflects adaptations to environment and lifestyle. It's particularly important in classifying vertebrates and helps understand their evolutionary relationships and ecological adaptations.

42. What is the importance of cell type specialization in animal classification?

The degree of cell type specialization is crucial in classification. More complex animals have a greater variety of specialized cells, tissues, and organs. This feature helps distinguish between simpler animals like sponges and more complex organisms.

43. How does the presence of a water vascular system influence animal classification?

The water vascular system, unique to echinoderms, is a key feature in classification. Its presence immediately identifies an animal as belonging to this phylum, demonstrating how unique adaptations can be crucial in separating major animal groups.

44. What role does the type of nervous system organization play in animal classification?

Nervous system organization (e.g., nerve net, ladder-like, or centralized) is crucial in classification. It reflects the complexity of body control and behavior, helping to distinguish between more primitive and advanced animal groups.

45. What role does the presence of a mantle play in animal classification?

The mantle, a specialized tissue layer in mollusks, is a key feature in classification. Its presence immediately identifies an animal as a mollusk, demonstrating how unique structures can be crucial in defining major animal groups.

46. How does the presence of a shell or exoskeleton affect animal classification?

The presence and type of shell or exoskeleton is important in classification, particularly for invertebrates. It reflects adaptations for protection and support, helping to distinguish groups like mollusks and arthropods from other invertebrates.

47. What is the significance of body size range in animal classification?

While not a primary classification criterion, body size range can be significant. It often correlates with metabolic rates, life strategies, and ecological roles. Extreme size differences can indicate separate evolutionary paths within a group.

48. How does the mode of locomotion affect animal classification?

Locomotion mode (e.g., swimming, flying, walking) is considered in classification as it reflects adaptations to different environments. While not a primary criterion, it can help understand evolutionary trends and ecological roles within animal groups.

49. How does the presence of a closed circulatory system differ from an open system in classification?

A closed circulatory system (blood confined to vessels) versus an open system (blood flows freely in body cavity) is an important distinction. It often correlates with body size and metabolic rates, helping to separate more advanced animals like vertebrates from many invertebrate groups.

50. How does the type of respiratory pigment affect animal classification?

The type of respiratory pigment (e.g., hemoglobin, hemocyanin) is considered in classification, particularly for more advanced animals. While not a primary criterion, it can reflect evolutionary relationships and adaptations to different environments.

51. What role does the presence of a closed circulatory system play in animal classification?

A closed circulatory system, where blood is confined to vessels, is characteristic of more complex animals. Its presence helps classify animals into more advanced groups, reflecting higher metabolic rates and more sophisticated body organization.

52. What role does the presence of specialized sensory organs play in animal classification?

Specialized sensory organs (e.g., complex eyes, statocysts) are important in classification as they reflect evolutionary advancements. Their presence and complexity help distinguish between more primitive and advanced animal groups.

53. How does the type of skeletal system affect animal classification?

The type of skeletal system (hydrostatic, exoskeleton, or endoskeleton) is significant in classification. It reflects different strategies for body support and protection, helping to distinguish between major animal groups and understand their adaptations.

54. How does the type of excretory product influence animal classification?

The type of excretory product (e.g., ammonia, urea, or uric acid) can be considered in classification, especially for vertebrates. It reflects adaptations to different environments and water availability, helping to understand evolutionary relationships.

55. How does the mode of reproduction influence animal classification?

The mode of reproduction (sexual, asexual, or both) is considered in classification. It reflects evolutionary strategies and adaptations to different environments, helping to understand the diversity within and between animal groups.

56. What role does the type of body fluid play in animal classification?

The type of body fluid (e.g., hemolymph or blood) and its composition are considered in classification. While not a primary criterion, it can reflect adaptations to different environments and metabolic needs, aiding in understanding evolutionary relationships.