Animal Kingdom and Classification - Animalia Phylum, Subphylum

Animal kingdom is one of the important R.H Whittaker’s classification system. He classified organisms into five kingdoms based on the cell structure, source of nutrition, and body design. The multicellular eukaryotic organisms were put under the kingdom animalia. Animals are differentiated based on their cellular structure, body functions and structure, presence of coelom, etc.

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Animal Kingdom 

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Organisms are divided into different phyla within the Animal Kingdom (Kingdom Animalia) according to their symmetry, organization, body structure, and other characteristics.The Kingdom Animalia is the largest among the five kingdoms. Animals are multicellular eukaryotes and they lack chloroplast and a cell wall, unlike plants. 

• Every year, roughly 10 thousand species are discovered and till now 2 million species have been discovered on Earth.
• Their mode of nutrition is heterotrophic, which means that they depend on plants and other animals directly or indirectly for food.

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Classification of Animal Kingdom

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Animal kingdom is classsified on the basis of phyla, class, order, family and genus. Vertebrates and invertebrates are the two sub-divisions of kingdom animalia. 

Invertebrates

Animals which lack vertebral column is called invertebrates. Animals such as porifera, molluscs, annelids, arthropoda, and cnidarians are some of the examples of invertebrates. Following are some generalised characteristics of invertebrates: 

• Absence of vertebral column.
• Radial or bilateral symmetry are observed.
• Both external and internal fertilization takes place.
• Presence of open type of circulatory system.
• Respiration in organisms occurs through gills or trachea or body surface.

Vertebrates

The presence of vertebral column which protects the spinal cord in organisms is the characteristic feature of vertebrates. Mammals, fish, reptiles, birds, and amphibians are some of the examples of vertebrates. Following are few generalised characteristics of vertebrates: 

• Organisms lack vertebral column.
• Bilateral symmetrical organisms are present.
• Closed type of circulatory system is present.
• Presence of notochord.

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Phylum in Animal Kingdom

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Kingdom Animalia is known to be the largest among the five kingdoms and phylum has the second highest rank in the taxonomy hierarchy. Based on the fundamental features the Animal Kingdom comprises ten different subphyla namely:

1. Porifera
2. Coelenterata (Cnidaria)
3. Platyhelminthes
4. Nematoda
5. Annelida
6. Arthropoda
7. Mollusca
8. Echinodermata
9. Hemichordata
10. Chordata

Phylum Porifera 

Phylum Porifera includes organisms with pores and are typically referred to as sponges. The following are the features of Porifera: 

• Multicellular organisms with a rigid outer skeleton that are non-motile.
• These organisms have a porous body. 
• Pores on the body form a canal system that assists in the circulating substances.
• Not differentiated into head and tail and lack a well-developed organ or organ system.
• Include marine habitat.
• Examples of phylum Porifera are Spongilla and Sycon.

Phylum Coelenterata (Cnidaria)

The word Coelenteratais originated from the Greek term “kilos,” which implies hollow-bellied. The features of Coelenterata include:

• It has a hollow body cavity.
• Their body is differentiated into two ends.
• It consists of all aquatic animals.
• Their body comprises two layers of cells - inner and outer linings.
• It stays in colonies (Corals) and solitary (Sea anemones).
• Examples of phylum Coelenterata are – Hydra and Jellyfish.

Phylum Platyhelminthes

Platyhelminthes are invertebrates that have a soft body. These are commonly called flatworms. The features are as follows:

• They have dorsoventrally flattened body.
• Complex and differentiated body structure.
• Tissues are triploblastic and differentiated from three layers of cells. 
• They do not have a coelom or true internal cavity. 
• These organisms have a bilateral symmetry.
• They are either parasitic (liver flukes) or free-living (Planaria). 
• Examples of phylum Platyhelminthes are -Tapeworm and Planaria.

Phylum Nematoda

Phylum Nematoda comprises nematodes or roundworms. Their key features are as follows:

• They have a cylindrical body.
• Bilaterally symmetrical and triploblastic.
• Nematoda have a pseudocoelom or a false body cavity.
• These are parasitic and cause diseases like elephantiasis and ascariasis.
• Ascaris and Wuchereria are examples of the phylum Nematoda.

Phylum Annelida

Annelids are generally called segmented or ringed worms. Their key features are:

• Have a segmented cylindrical body.
• Their body is distinguished by head and tail.
• Bilaterally symmetrical and triploblastic.
• They possess a true body cavity.
• Their habitat includes marine, freshwater, and land.
• Examples of phylum Annelida include – Earthworm and Leech.

Phylum Arthropoda

Animals that have jointed legs belong to this phylum. It is known to be one of the largest phylum in the animal kingdom.

• They are bilaterally symmetrical.
• Bear jointed appendages, exoskeleton, and a segmented body.
• They have a well-differentiated organ and organ system.
• They have an open circulatory system but lack differentiated blood vessels.
• Examples of phylum Arthropoda are – Spiders, butterflies, and mosquitoes.

Phylum Mollusca

Phylum Mollusca is the second-largest phylum in animal kingdom. The following are the prominent features: 

• These organisms are bilaterally symmetrical and triploblastic.
• They have a less segmented body.
• Well-developed organ and organ system.
• They possess an open circulatory system. 
• Limbs are present.
• Examples of phylum Mollusca are - Snails and Octopuses.

Phylum Echinodermata

Echinodermata originated from the Greek words echinos, signifying hedgehog and derma, denoting skin. Therefore, echinoderms are spiny-skinned animals. Their key features include:

• These organisms are radially symmetrical and triploblastic.
• They maintain a true coelom.
• Echinodermata Have a rigid skeleton structure of calcium carbonate. 
• They are free-living marine animals.
• Sea urchins and Starfish are examples of phylum echinodermata.

Phylum Hemichordata

Hemicordata are a worm-shaped phylum in animal kingdom. The features of the phylum Hemichordata include:

• The body is soft, fragile, and separated into a proboscis.
• The epidermis is single-layered.
• These are marine organisms and have an organ-system level of organization.
• Hemichordata contain an open circulatory system.
• They respire through gills as they are marine.
• They include separate sexes i.e. male and female.
• External fertilization takes place. 

Phylum Chordata

Animals, birds, and human beings fall under phylum chordata. They have the following attributes:

• They are bilaterally symmetrical, triploblastic, and classified at the organ-system level.
• They have a notochord and a nerve cord.
• Chordates have a closed-type circulatory system.
• Urochordata, vertebrata, and cephalochordata are the sub-phyla of phylum chordata.

Things to Remember

• Phylum in animal kingdom is known as the second highest ranks in the toxonomical heirarchy.
• Platyhelminthes have a flat body and exhibit bilateral symmetry. 
• Annelids have a true coelom and are metamerically segmented organisms.
• Arthropods are the most abundant group of animals characterized by jointed appendages.
• Phylum Chordata includes animals that possess a notochord throughout or during early embryonic life.