Introduction to Animalia
All creatures are members of the Kingdom Animalia, also called Metazoa. This Kingdom doesn't contain prokaryotes (Kingdom Monera, which includes bacteria, blue-green algae) or protists (Kingdom Protista, which includes unicellular eukaryotic organisms). All members of Animalia are multicellular, and all are heterotrophs (that is, they calculate directly or laterally on other organisms for their ailment). Most ingest food and digest it in an internal depression.
It's estimated that around 9 or 10 million species of creatures inhabit the earth; the exact number isn't known and all estimates are rough. Creatures range in size from no further than many cells to organisms importing numerous tons, similar to blue jumbos and giant squid. By far most species of creatures are insects, with groups similar as molluscs, crustaceans, and nematodes also being especially different. By this measure, our own group, the invertebrates, is fairly inconsequential from a diversity perspective.
R.H. Whittaker organized organisms into five kingdoms. He classified organisms supported cell structure, mode, source of nutrition and body design. The five kingdoms proposed by Whittaker are Monera, Protista, Fungi, Plantae, and Animalia. Let us learn about the animal kingdom, i.e., Kingdom Animalia.
Kingdom Animalia
Kingdom Animalia constitutes all animals. Amongst the five kingdoms, the most important kingdom is Animalia. Animals are multicellular eukaryotes. However, like plants, they do not possess chlorophyll or a cell membrane. Therefore, members of the Animalia exhibit a heterotrophic mode of nutrition. Kingdom Animalia has been classified into ten different subphyla supported their body design or differentiation.
The different phylum of the Animalia are as follows:
Porifera
Coelenterata (Cnidaria)
Platyhelminthes
Nematoda
Annelida
Arthropoda
Mollusca
Echinodermata
Hemichordata
Chordata
Phylum Porifera
Porifera means organisms with holes. They are commonly known as Sponges. Features of the Porifera are:
Non-motile, multicellular organisms with a hard outer skeleton.
Have a porous body.
Pores on the bodies create a canal system that helps in the circulation of drugs.
Their body does not include a typical head and tail; well-developed organs or organ systems are absent.
Include marine habitat.
Example of phylum Porifera includes- Spongilla, Sycon.
Phylum Coelenterata (Cnidaria)
The term Coelenteratais derived from the Greek word “kilos” which suggests hollow-bellied. Their features are:
Have a hollow body cavity.
The body is differentiated into two ends.
Includes all aquatic animals.
The body is formed of two layers of cells: inner and outer linings.
They are either found solitarily (Sea anemone) or in colonies (corals).
Example of phylum Coelenterata includes – Hydra, Jellyfish.
Phylum Platyhelminthes
Platyhelminthes are commonly known as flatworms. Their features are:
Dorsoventrally flattened body.
Complex and have differentiated body structure.
They are triploblastic, i.e. tissue is differentiated from three distinct cell layers.
Do not have a real internal cavity or coelom.
Have bilateral symmetry.
Either free-living (Planaria) or parasitic (liver flukes).
Example of phylum Platyhelminthes includes -Tapeworm, Planaria.
Phylum Nematoda
Phylum Nematoda consists of nematodes or roundworms. Their features are:
Nematodes have a cylindrical body.
Bilaterally symmetrical and triploblastic.
Have pseudocolour, a false body cavity.
Parasitic and causes diseases such as elephantiasis, ascariasis.
Example of phylum Nematoda includes – Ascaris, Wuchereria.
Phylum Annelida
Annelids are commonly referred to as segmented or ringed worms. They have the following features:
Have a segmented cylindrical body.
The body is differentiated into the head and tail.
Bilaterally symmetrical and triploblastic.
Have a true body cavity.
Habitat: marine, freshwater, and land.
Example of phylum Annelida includes – Earthworm, Leech.
Phylum Arthropoda
Arthropod means jointed legs. Animals that have jointed appendages belong to the present phylum. This is the most important phylum within the Animalia. Other features are:
They are bilaterally symmetrical.
Have jointed appendages, exoskeleton, and a segmented body.
Have a well-differentiated organ and organ system.
Have an open cardiovascular system, but don't have differentiated blood vessels.
Examples include – butterflies, spiders and mosquitoes.
Phylum Mollusca
Phylum Mollusca consists of an outsized group of animals. Features are:
Bilaterally symmetrical and triploblastic.
Less segmented body.
Well-developed organ and organ system.
Typically, an open circulatory system is present.
Limbs are present.
Example of phylum Mollusca includes- Snails and octopus.
Phylum Echinodermata
The term Echinodermata springs from the Greek words, ‘echinos’ meaning hedgehog and derma meaning skin. Thus, echinoderms are spiny-skinned animals.
Radial symmetry and triploblastic.
Have true coelom.
Have a hard calcium carbonate skeleton structure.
Free-living marine animals.
Example of phylum Echinodermata includes- Sea urchins, starfish.
Phylum Hemichordata
The characteristics of the phylum Hemichordata are as follows:
Soft and fragile body divided into a proboscis.
The epidermis is single-layered.
Includes marine animals (worm-like) with an organ-system organization level.
They have an open circulatory system.
They respire through gills since they are marine.
Sexes are separate and fertilization is external.
Development is direct.
Phylum Chordata
The Chordates possess the following characteristics:
Body bilaterally symmetrical, with an organ-system organisation level and are triploblastic.
They possess a notochord and a nerve cord.
The circulatory system is closed type.
Phylum Chordata is often divided into the subsequent sub-phyla:
Urochordata
Cephalochordata
Vertebrata
Hence Animalia is the largest kingdom amongst the five kingdoms consisting of all animals. Animals are multicellular eukaryotes; don't possess a cell membrane or chlorophyll like plants, and share an equivalent mode of nutrition, that is, the heterotrophic mode. Among the other prominent characteristics of this kingdom include body symmetry, cell arrangement, extent of organization, presence/absence of notochord, etc.
FAQs on Animal Kingdom
1. What are the fundamental characteristics that define the Animal Kingdom?
All organisms in the Kingdom Animalia share a set of core characteristics that distinguish them from plants, fungi, and other life forms. The primary features are:
- Multicellularity: Their bodies are composed of multiple cells.
- Eukaryotic Cells: Their cells have a true nucleus and membrane-bound organelles.
- No Cell Wall: Unlike plant cells, animal cells lack a rigid cell wall.
- Heterotrophic Nutrition: They obtain nutrients by ingesting other organisms, as they cannot produce their own food.
- Mobility: Most animals are capable of movement at some stage in their life cycle.
- Nervous System: They possess a nervous system that allows them to respond to external stimuli.
2. What are the main bases for animal classification as per the NCERT syllabus for 2025-26?
The classification of the Animal Kingdom is based on several fundamental features that reflect the evolutionary relationships among different groups. The primary criteria include:
- Levels of Organisation: Whether the body is organised at a cellular, tissue, organ, or organ system level.
- Symmetry: The arrangement of body parts. It can be asymmetrical (e.g., sponges), radial (e.g., jellyfish), or bilateral (e.g., humans).
- Body Cavity (Coelom): The presence or absence of a body cavity between the body wall and the gut wall. Animals can be acoelomates (no coelom), pseudocoelomates (false coelom), or coelomates (true coelom).
- Germ Layers: Whether the embryo has two germ layers (diploblastic) or three (triploblastic).
- Segmentation: The division of the body into a series of repetitive segments.
- Notochord: The presence or absence of a supportive rod-like structure called a notochord.
3. What is the difference between bilateral and radial symmetry in animals?
Symmetry refers to how an animal's body can be divided into identical halves. The two main types are:
- Radial Symmetry: The body can be divided into two identical halves by any plane passing through the central axis. This is common in sedentary or slow-moving animals like jellyfish (Phylum Coelenterata) and starfish (Phylum Echinodermata), allowing them to sense their environment from all directions.
- Bilateral Symmetry: The body can be divided into identical left and right halves by only one specific plane. This is associated with active movement and the development of a head (cephalisation), as seen in arthropods, molluscs, and all vertebrates.
4. Why is a true coelom considered an important evolutionary advantage for animals?
A true coelom, or a fluid-filled body cavity lined by mesoderm, represents a significant evolutionary advancement for several reasons. It provides space for internal organs to develop, grow, and function without being constrained by the movements of the body wall. The coelomic fluid can also act as a hydrostatic skeleton, aiding in movement for soft-bodied animals like earthworms. Furthermore, it allows for the development of an efficient circulatory system to transport nutrients and waste, enabling animals to grow larger and become more complex.
5. List the major non-chordate phyla with one defining feature for each.
The non-chordates are a diverse group of animals lacking a notochord. The major phyla are:
- Porifera: Possess a porous body with a water canal system for feeding and respiration. Example: Sponges.
- Coelenterata (Cnidaria): Have tentacles with stinging cells called cnidoblasts. Example: Hydra, Jellyfish.
- Platyhelminthes: Dorso-ventrally flattened body, commonly called flatworms. Example: Tapeworm.
- Nematoda: Cylindrical, unsegmented body; many are parasitic. Example: Ascaris (roundworm).
- Annelida: Exhibit true segmentation (metamerism). Example: Earthworm.
- Arthropoda: Have jointed appendages and a chitinous exoskeleton. Example: Butterfly, Spider.
- Mollusca: Soft-bodied animals, usually with a calcareous shell. Example: Snail, Octopus.
- Echinodermata: Spiny-skinned marine animals with a water vascular system. Example: Starfish.
- Hemichordata: Worm-like animals with a proboscis, collar, and trunk. Example: Balanoglossus.
6. How does the water transport system in Phylum Porifera support its life functions?
The water transport or canal system is a unique feature of sponges (Phylum Porifera) and is essential for their survival. Water enters the body through minute pores called ostia, flows into a central cavity called the spongocoel, and exits through a larger opening called the osculum. This continuous water flow facilitates several vital functions that more complex animals perform with distinct organ systems. It helps in gathering food particles, carrying out respiratory exchange of gases, and removing waste products, effectively acting as the sponge's circulatory, respiratory, and excretory system.
7. What are the three fundamental characteristics of all organisms in Phylum Chordata?
All animals belonging to Phylum Chordata share three fundamental features, which are present at some stage of their life cycle:
- The presence of a notochord, a flexible rod-like structure that provides support.
- A dorsal, hollow nerve cord located above the notochord.
- Paired pharyngeal gill slits, which are openings connecting the pharynx to the outside.
8. Why are all vertebrates considered chordates, but not all chordates are vertebrates?
This distinction arises from the classification hierarchy within Phylum Chordata. All vertebrates are chordates because they possess the three fundamental chordate features (notochord, dorsal nerve cord, pharyngeal gill slits) during their embryonic development. However, in vertebrates, the notochord is replaced by a cartilaginous or bony vertebral column (backbone) in the adult stage.
Phylum Chordata includes two other subphyla, Urochordata and Cephalochordata, which are invertebrates. These organisms possess a notochord but it never develops into a vertebral column. Therefore, the presence of a vertebral column is the specific feature that defines a vertebrate, making it a sub-group within the larger Phylum Chordata.
9. Why is Phylum Arthropoda the most successful and largest phylum in the Animal Kingdom?
Phylum Arthropoda's immense success and diversity, making up over 80% of all known animal species, can be attributed to several key evolutionary innovations:
- Exoskeleton: A hard, protective outer covering made of chitin that provides support, prevents water loss, and offers defence against predators.
- Jointed Appendages: Their limbs are jointed, allowing for a wide range of specialised movements like walking, swimming, jumping, and feeding.
- Segmentation: Their bodies are segmented and often grouped into functional units (head, thorax, abdomen), allowing for specialisation of different body regions.
- Developed Sensory Organs: They possess complex sensory organs, including antennae and compound eyes, which enhance their ability to interact with the environment.
