Classification of Animal Kingdom

Kingdom Animalia is one of the divisions of The Five Kingdom Classification Model. The organism under this kingdom is multicellular eukaryotes without any cell walls. They are heterotrophs and depend on plants for food.

Their digestion process is done in the internal cavity stores fat. There is ingestion of food inside the body or nutrition is holozoic. They have a fixed growth cycle with a definite shape or size for each phase or cycle.

They can locomote or follow a clear movement and follow sexual reproduction between male and female. This is the only reproduction method possible in these organisms. We are going to see the classification of animal kingdom now.

This kingdom includes eleven main phylum divisions – Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata.

They are all equally crucial under this kingdom. The division in kingdom Animalia is on the basis of criteria like level of organization, symmetry, coelom, segmentation, and notochord. These criteria decide their classification and features.

They also determine their subgroups further. We will look at all the eleven phylum divisions of Animal Kingdom. Let’s look at them in detail.

 

Basis of Animal Kingdom Classification

1. Levels of Organization

All the organisms under kingdom Animalia are multicellular but have a different pattern of cell organization. The spores have loose cell aggregates or cellular levels of the organization. The cells divide their own work.

On the other hand, coelenterates have a complex cell arrangement and have a tissue level organization. This is because cells with the same function form a tissue. The organ level organization is present in Platyhelminthes and other higher species.

The tissues together create an organ to perform a particular function. Annelids, Arthropods, and Molluscs functional systems for different functions; this is the organ level of organization. However, this level is very complex in some organisms.

Platyhelminthes have a single opening as the mouth and anus. But a proper digestive system has two different openings. The circulatory system has other parts as well like arteries, veins, and capillaries.

2. Patterns of Organ System

a. Digestive System 

The framework of the digestive system in an organism is either Complete or Incomplete. The incomplete digestive system has one opening of intake and release.

This means you eat and release toxic from one opening only. In the case of a complete digestive system, there are two separate openings for purposes. 

b. Circulatory System 

The circulatory system is also of two types i.e., open and closed circulatory system.  In an open circulatory system, the heart pumps out the blood, and cells and tissue get washed in it.

But in the case of a closed circulatory system, there are proper vessels for blood circulation that are veins and arteries. 

3. Symmetry

The organisms under animal kingdom have different symmetry as well. The sponges are asymmetrical in most cases. The centre does not divide them into equal halves. And when the line divides an organism into two equal halves, it becomes radical symmetric.

Sponges are asymmetrical. Any line that passes through does not divide them into equal halves. Coelenterates and Echinoderms have this kind of body symmetry. When the body has identical left and right halves, they are bilaterally symmetric. Annelids and Arthropods are examples of this. 

3. Diploblastic and Triploblastic Organisation

The cell arrangement in different embryonic layers also differentiates the organism in this kingdom. When the cell arrangement is in two embryonic layers, external and internal endoderm. They become diploblastic animals.

Mesoglea is an undifferentiated layer between the endotherms. And cell arrangement in three different endoderm layers is a triploblastic animal. 

4. Coelom

The cavity presence and absence are other criteria to classify animals under this kingdom. 

The body cavity is coelom and is by mesoderm. Animals with coelom become coelomates like molluscs, annelids, and chordates. And some animals don’t have a body cavity but they do have mesoderm.

They become pseudocoelomates like Aschelminthes. And the animals without a cavity are acoelomates like pseudocoelomate. 

5. Segmentation

The external and internal body in many organisms has different segments with the repetition of a few organs. Earthworms have a metameric segmentation and the pattern is metamerism. 

6. Notochord

The notochord is a rod-like structure that supports the embryonic development in animals. Animals with notochords are chordates and animals with them are non-chordates. Examples of non-chordates are Porifera and Echinoderms. 

Hierarchy in Animal Kingdom Classification 

As mentioned above, there are many differences in different organisms on the basis of their feature, size, and more. But other than that they also have a certain place in the animal kingdom hierarchical framework.

The common elements help in the division of these levels. Carolus Linnaeus recognized the five-kingdom classification and the hierarchical framework with levels moving from highest to lowest. 

Kingdom 

This is the umbrella level of all organisms. All of them come under it and incase of kingdom Animalia, all animals belong to one kingdom. 

Phylum 

The Kingdom further divides into smaller phylums. Chordata is an example of phyla that has notochord as a common element in all the organisms. 

Class 

The phylums may divide into classes. In Chordata, Aves is one of the class and they all have feathers to fly. 

Family 

The class further divides into families and they have more than one genus. Felidae is a term for cat family that includes cheetah, cat, etc. 

Genus 

The family has two divisions – genera and animals having the same genus are the same in all ways. Panthera is a genus under Felidae that has cheetah, tigers, jaguar, etc. 

Species 

The species has only one kind of animal like a cheetah. 

Classification of Animal Kingdom

1. Phylum – Porifera

• These organisms have holes. 
• They have cellular levels of the organization and are multicellular. 
• They are attached to a solid support as they are non-motile. 
• There minimal differentiation of body design with tissue division. 
• A common name for them is sponges. 
• They have an asymmetrical structure and are mostly aquatic. 
• The sponges have a water transport system as they live in water.
• Water enters the body through pores into a central cavity called spongocoel. 
• This transport system allows food gathering, respiration, and waste removal. 
• Spongin fibres support the body structure. 
• One individual produces both the eggs and the sperms.
• They can reproduce by fragmentation or by gamete formation. 
• They have internal fertilization and the larva is distinct from adults. 
• Some examples are – Sycon, Spongilla, and Euspongia.

2. Phylum – Coelenterata 

• They are present on the tentacles of the body.
• They are mostly aquatic with radical symmetry for free swimming. 
• These have a tissue level of organization and a gastrovascular single cavity. 
• They are also diploblastic i.e. cell arrangement in two layers.
• They live in colonies called corals. 
• Some of them prefer living alone like a hydra. 
• Some cnidarians have a calcium carbonate skeleton. 
• They have two forms of body – polyp, and medusa. 
• The polyp is a sessile and cylindrical shape like a hydra.
• The medusa type is umbrella-shaped and can swim freely like jellyfish. 
• Cnidarians have an alternation of generation. The polyps reproduce asexually while the medusae reproduce asexually. 
• The digestion system is extracellular and intracellular.
• Examples are – Aurelia, Pennatula, Gorgonia, and Meandrina.

3. Phylum – Ctenophora

• A common name for them is ea walnuts or comb jellies.
• They are aquatic and diploblastic. They also have a radical symmetry body structure. 
• They have a tissue-level organization of cells.
• Their body has comb plates externally for locomotion 
• The digestion system is extracellular and intracellular.
• Bioluminescence is a type of ctenophores.
• They follow only sexual reproduction methods.
• The process of fertilization is external and indirect. 
• Examples are Pleurobrachia and Ctenoplana.

4. Phylum – Platyhelminthes

• They are the most complex in all the groups. 
• They have a bilaterally symmetric body.
• These are triploblastic and have a tissue level organization. 
• They are flatworms as they have a flat body top to bottom. 
• They are free-living or parasites. 
• The parasites are hooks and suckers. And free living is planarians.
• Endoparasite is a common parasite found on animals and humans. They often absorb minerals from the host’s body surface.
• They are Acoelomate as there is no internal cavity for organs. 
• Flame cells are present for excretion.
• The reproductive parts are not separate.
• The process of fertilization is internal and indirect. 
• Some examples are Platyhelminthes and Tapeworm.

5. Phylum – Aschelminthes

• They have a cylindrical shape and are not flat. 
• They have an organ system level cell arrangement 
• These are triploblastic with a body cavity presence. 
• They may be marine or terrestrial and free-living or parasites. 
• They are common in causing diseases like elephantiasis 
• These are roundworms because of their circular cross-section. 
• They waste removal is by an excretory tube through an excretory pore.
• The males and females are separate or dioecious. Mostly females are longer.
• The fertilization process is internal and direct embryo development. 
• Aschelminthes is an example of this. 

6. Phylum – Annelida

• They are marine or terrestrial.
• They can be free-living or parasitic. 
• Annelida has a segmented body surface thus justifies the name. 
• They have an organ system level cell arrangement. 
• They have a body cavity to fit in true organs. 
• These have a bilateral symmetry structure and are triploblastic.
• For locomotion, they have circular muscles.
• The marine annelids have lateral appendages for swimming and a closed circulatory system.
• Nephridia facilitate excretion in their body. 
• Nereis is a marine annelid that is dioecious but earthworms are monoecious. 
• They follow sexual reproduction.
• Examples are Nereis and Hirudinaria.

7. Phylum – Arthropoda

• They are the largest classification of the animal kingdom. 
• Exoskeletons – a hard external shell cover the segmented body and joint legs. 
• They have an organ level of cell arrangement. 
• Their body structure is bilateral symmetry and triploblastic. 
• Their internal cavity is full of blood. 
• The body structure includes the head, thorax, and abdomen.
• The circulatory system is open without proper blood vessels.  
• Gills act as the respiratory organ or tracheal system. 
• antennae, eyes, and statocysts are present sensory organs. 
• Malpighian tubules facilitate excretion. 
• The fertilization is internal and they are dioecious.
• Embryo development may be direct or indirect.
• Examples are – scorpion and prawns.

a. Arachnids

Arachnida is a class of Arthropoda which includes – 

• Spiders
• Harvestmen
• Mites
• Ticks

b. Crustaceans

A large group of arthropods is crustaceans, who have gills and pairs of antennae. They are –

• Crabs
• Lobsters
• Crayfish
• Shrimp

c. Insects

This class of arthropods has three-part bodies, six legs, eyes, and two antennae. They are – 

• Bees
• Wasps
• Beetles
• Mosquitoes
• Grasshoppers
• Butterflies

8. Phylum – Mollusca

• They are the second-largest animal phylum and are terrestrial or aquatic.
• They have an organ system level cell organization. 
• They have bilateral symmetric structures and are triploblastic.
• The circulatory system is open and a kidney-like organ for waste excretion. 
• The radula is present for feeding. 
• They are mostly dioecious and have indirect embryo development. 
• A calcareous shell covers the entire unsegmented body.
• Examples are octopus and snails.

9. Phylum – Echinodermata

• Echinoderms have an endoskeleton of calcium carbonate.
• They are marine animals and free living.
• They have an organ level cell arrangement. 
• They are triploblastic with a cavity for true organs. 
• They have radial symmetry but the larvae are bilaterally symmetric. 
• The vascular system is water-driven for locomotion, transportation, and respiration. 
• They are triploblastic and have cavities for true organs.
• They have a well developed digestive system and no excretion system.
• The reproduction process is sexual with internal fertilization and indirect development.
• Some examples are Starfish, Sea urchin, and Sea lily.

10. Phylum – Hemichordata

• Before it came under phylum Chordata. But now it is a different phylum all together. 
• It has worms like marine organisms with an organ level cell arrangement. 
• They are bilaterally symmetric with triploblastic organization and cavity for true organs. 
• Their circulation system is open, and gills facilitate respiration. 
• An excretory system is present and the sex organs are different. 
• The fertilization is external and indirect development. 
• Some examples are Balanoglossus and Saccoglossus.

11. Phylum – Chordata

• The main feature of Chordata is the presence of a notochord, hollow nerve cord, and paired pharyngeal gill.
• They are bilaterally symmetric with an internal cavity for true organs. 
• They have an organization level cell arrangement. 
• Their circulatory system is closed and has a tail and post. 
• There are three subphylums – Urochordata, Cephalochordata, and Vertebrata.
• The first two are marine and are protochordate. 
• Urochordata have notochord present in the larval tail only. 
• Cephalochordata have notochord in the entire body.
• Some examples are Ascidia, Salpa, Doliolum, and Amphioxus.
• All chordates have a notochord, dorsal nerve cord, are triploblastic, and coelomate.

a. Vertebrata

• The animals under this classification have vertebral columns and internal skeleton.
• There is a proper distribution of muscle attachment for movement. 
• They have a notochord during the embryonic stage. Later cartilaginous replace it in the adult phase. 
• All chordates are not necessarily vertebrates.
• They have a muscular heart with 4 chambers, a proper excretion system, and proper limbs. 
• They are bilaterally symmetric and triploblastic. 
• They have internal cavities for true organs with complex body tissues and organs. 

Difference between  Chordates and Non-chordates
Chordates	Non-chordates
Presence of Notochord	Absence of Notochord
Dorsal, hollow, and single nervous system	Ventral, solid, and double nervous system
Presence of gill slits	Absence of Gill slits
Ventral Heart	Dorsal Heart
Tail is present	Tail is absent

Difference Between All the Phylums – Animal Kingdom 

Phylum	Level of Organisation	Symmetry	Coelom	Digestive System	Segmentation	Blood  CirculationSystem	Respiration
Porifera	Cellular	Asymmetric	Absent	Absent	Absent	Absent	Absent
Coelenterata	Tissue level	Radial	Absent	Incomplete	Absent	Absent	Absent
Ctenophora	Tissue level	Radial	Absent	Incomplete	Absent	Absent	Absent
Platyhelminthes	Organ level	Bilateral	Present	Incomplete	Absent	Absent	Absent
Aschelminthes	Organ level	Bilateral	Present	Complete	Absent	Absent	Absent
Annelida	Organ level	Bilateral	Present	Complete	Present	Present	Absent
Arthropoda	Organ level	Bilateral	Present	Complete	Present	Present	Present
Mollusca	Organ level	Bilateral	Present	Complete	Absent	Present	Present
Echinodermata	Organ level	Radial	Present	Complete	Absent	Present	Present
Hemichordata	Organ level	Bilateral	Present	Complete	Absent	Present	Present
Chordata	Organ level	Bilateral	Present	Complete	Present	Present	Present

Conclusion

This article is a biology article for competitive exams like UPSC, RRB, SSC, and more. This article covers the classification of Animal Kingdom, Its divisions, and its distinctive features.

We looked at Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata at large. This has a high chance of appearing in the UPSC Biology and Science exam.

It is because it is an intermediate level topic. The chances of it coming in UPSC Mains is higher than Prelims. But again if you are a science aspirant then this article will help you build your basic understanding of the subject.

All the UPSC aspirants must read this article before appearing for the exams.