The diverse world around us, with millions of biotic and abiotic components that make up our ecosystem, is nothing less of a wonder. Of them, the biotic or the living community can be further subdivided into two broad groups- plants, and animals. In this discussion, we explore the kingdom animalia and the various phyla that it houses, along with their distinctive features.

Kingdom Animalia

Animals are multicellular, eukaryotic life forms, characterized by heterotrophic mode of nutrition and inhabit all types of ecosystems including terrestrial, freshwater and marine. They can be further segregated into two groups based on the presence of notochord- chordates and invertebrates. Chordates constitute only 5% of the animal kingdom while the invertebrates claim the remaining majority. Vertebrates form a subphylum under chordates and are characterized by the presence of a vertebral column or backbone. With an exception of sponges, all animal cells are organized into tissues and in majority, the tissues organize to form well-defined organs and organ systems for carrying out essential bodily functions. Most animals are also diploid in nature and reproduce sexually by the formation of gametes.

The following are the major phyla that constitute the animal kingdom and their respective features that distinguish them from one another.

Phylum Porifera

Sponges, a multicellular, primitive animal with cellular grade organization, exemplify the phylum Porifera.
They are free-living, aquatic life forms with no fixed body shape or plane of symmetry. A new sponge can be regenerated from even a few detached cells from a pre-existing sponge.
They mostly thrive in marine ecosystems and are characterized by a body wall formed by two layers of loosely arranged cells with a mesenchyme between them. Inside the body wall, a large cavity (spongocoel) is present, containing mostly a number of small canals to supply oxygen and food, and to eliminate toxic waste matters.

Example: Spongilla (freshwater sponge)

Phylum Cnidaria

The members of phylum cnidaria, the coelenterates, are radially symmetrical and possess tissue level organization (their tissues do not form organs).

The organisms’ bodies generally have two main forms, polyp, and medusa. The former is fixed and is cylindrical in structure, while medusa is freely swimming and umbrella-shaped. This form of polymorphism can differ among the members of this phylum and can have multiple varieties. Usually, when both the forms are found in the same organism, they alternate in the life cycle, a phenomenon termed as alternation of generation.

Example: Adamsia (Sea anemone), Meandrina (Brain coral)

Phylum Platyhelminthes

The members of phylum Platyhelminthes are bilaterally symmetrical animals with organ-system level of organization.
Flatworms, the members of phylum Platyhelminthes, lack body cavities (acoelomate) and are devoid of proper circulatory and respiratory organs. Parenchyma, a type of connective tissue, packs the organs of the flatworms’ bodies.
The animals do not possess anus but have a mouth.
The mode of reproduction is internal. Extensive precautions are also taken to ensure minimum incidences of self-fertilization as the animals are hermaphroditic in nature.

Example: Fasciola (liver fluke)

Phylum Aschelminthes

They are also triploblastic organisms with a pseudocoelom located between the gut and the body wall. The cavity is not lined by mesodermal epithelium, and thus, despite the presence, the members of aschelminthes are classified as pseudocoelomates or organisms lacking true body cavity.
The body wall is formed of a hard, resistant cuticle, a muscle layer, and epidermis. The latter can be both cellular and syncytial.
Opposite reproductive organs are present on different organisms and fertilization is internal. The mode of reproduction is sexual.
Among the various members of the phylum, nematodes are of most importance. They can inhabit both aquatic and terrestrial and are parasitic in nature.

Example: Wucheraria bancrofti (filarial worm)

Phylum Annelida

The members of phylum Annelida are triploblastic, i.e. they develop from the three germ layer organization and have bilaterally symmetrical bodies. They are also in possession of a true coelom or body cavity, which is lined by a layer of mesodermal epithelium.
The body is elongated, flattened or cylindrical in shape and ring-like grooves, called annuli (from which the name of the phylum is derived) divides the body into segments.
The animals of this phylum also have a proper closed circulation.

Example: Hirudinaria (leech), Pheretima (earthworm)

Phylum Arthropoda

With the largest phylum under animal kingdom, the arthropods exhibit organ-system level of organization with bilaterally symmetrical, triploblastic bodies. They also possess true body cavity or coelom (coelomates).
All members of this phylum are characterized by the presence of an exoskeleton composed of chitinous cuticles. To keep up with the growth of the organism, the cuticle sheds from time to time and is replaced by a new one.
The body of arthropods are subdivided into head, thorax, abdomen and jointed appendages.
They have an open circulatory system, which connects to the body cavity called haemocoel. The respiratory and the excretory organs vary from organisms to organisms.

Example: Apis (honey bee), Periplanata (cockroach)

Phylum Mollusca

The second largest phylum under the animal kingdom, Mollusca includes common aquatic animals like snails, slugs, octopuses, oysters etc.
They possess organ-level organization in their systems and their body is bilaterally symmetrical, triploblastic (made up of three germ layers) and coelomate (containing true body cavity, lined by mesodermal epithelium). The body is covered by calcareous shells and consists of a distinct head portion, a visceral hump, and muscular feet.
Nephridium acts as an excretory organ for the removal of wastes from the body. With the exception of cephalopods, all other mollusks have open circulation.

Example: Pila (apple snail), Dentalium (tusk shell)

Phylum Echinodermata

Triploblastic, coelomate animals belonging to phylum Echinodermata are marine and inhabit the bottom of the sea (banthoic). The adult organisms are radially symmetrical while at the larval stage they exhibit bilateral symmetry.
The most distinctive feature of the phylum Echinodermata is the presence of a water vascular system in their bodies. They can swim freely and the tube feet are contractile appendages serving the functions of locomotion, respiration and capturing of food, besides keeping the body attached to the substratum.
The mode of replication is sexual and fertilization occurs externally.

Example: Echinoidea (sea urchin), Antedon

Phylum Protochodata

Protochordates, also called hemichordates, are exclusively marine animals with organ system level of organization.
Their bodies are bilaterally symmetrical, triploblastic and consist of a true body cavity or coelom. The body is cylindrical in shape and consists of an anterior proboscis, a long trunk, and a collar. The notochord is hollow and originates from the endodermal germ layer.
The mode of reproduction is sexual and fertilization takes place externally. The circulatory system is open.

Example: Ptychodera, Balanoglossus (tongue worm)

Phylum Chordata

Animals belonging to phylum Chordata are triploblastic, exhibit bilateral symmetry and organ system level of organization, along with a true coelom. The most significant feature of this phylum is the presence of notochord, a dorsal, hollow nerve cord, and paired pharyngeal slits.
The subphylum Vertebrata falls under this phylum; the animals belonging to this group possess a backbone or vertebral column from which the name is derived. The other two subphylas of Chordata are urochordata and cephalochordata.
This phylum includes birds (Aves), reptiles (Reptilia), amphibians (Amphibia), fishes, and mammals (Mammalia).

Example: Scoliodon (dog fish), Betta (fighter fish), Bufo (toad), Draco

(flying lizard), Corvus (crow), Panthera leo (lion), Homo sapiens (Human).

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FAQs on Animal Kingdom: Classification, Features & Examples

1. What is the primary basis for classifying animals in the Animal Kingdom?

The primary basis for classifying animals involves fundamental features of their body plan and organisation. Key criteria include:

Levels of Organisation: Whether the cells are arranged as a loose aggregate (cellular level), in tissues (tissue level), organs (organ level), or organ systems (organ system level).
Symmetry: The arrangement of body parts. Animals can be asymmetrical (e.g., sponges), radially symmetrical (e.g., coelenterates), or bilaterally symmetrical (e.g., humans, insects).
Germ Layers: Whether the embryo has two germ layers (diploblastic - ectoderm and endoderm) or three (triploblastic - ectoderm, mesoderm, and endoderm).
Coelom: The nature of the body cavity. Animals can be acoelomates (no cavity), pseudocoelomates (false cavity), or coelomates (true cavity).
Segmentation: The presence of repeated body segments, as seen in annelids and arthropods.
Notochord: The presence or absence of a supportive rod-like notochord during embryonic development, which distinguishes chordates from non-chordates.

2. What is the difference between diploblastic and triploblastic organisation in animals?

The main difference lies in the number of embryonic germ layers from which all body tissues and organs are formed.

Diploblastic Organisation: In these animals, the cells are arranged into two embryonic layers: an external ectoderm and an internal endoderm. An undifferentiated layer called mesoglea may be present between them. This is characteristic of simpler animals like those in Phylum Coelenterata (e.g., Hydra, jellyfish).
Triploblastic Organisation: These animals have a third germ layer, the mesoderm, situated between the ectoderm and endoderm. The mesoderm gives rise to complex organs and systems like muscles, the heart, and excretory organs, leading to a more complex body plan. This is found in all phyla from Platyhelminthes to Chordata (e.g., flatworms, humans).

3. What are the major phyla covered in the Animal Kingdom chapter as per the NCERT syllabus?

The NCERT Class 11 Biology syllabus systematically covers eleven major phyla of the Animal Kingdom. These are studied based on their increasing complexity. The phyla are:

Phylum Porifera (e.g., Sponges)
Phylum Coelenterata (or Cnidaria) (e.g., Hydra, Jellyfish)
Phylum Ctenophora (e.g., Comb jellies)
Phylum Platyhelminthes (e.g., Flatworms)
Phylum Aschelminthes (or Nematoda) (e.g., Roundworms)
Phylum Annelida (e.g., Earthworms)
Phylum Arthropoda (e.g., Insects, Crabs)
Phylum Mollusca (e.g., Snails, Octopus)
Phylum Echinodermata (e.g., Starfish)
Phylum Hemichordata (e.g., Balanoglossus)
Phylum Chordata (e.g., Fish, Birds, Mammals)

4. Explain the different types of body symmetry found in animals with examples.

Symmetry refers to the arrangement of body parts around a central axis. In the Animal Kingdom, there are three main types:

Asymmetry: The body cannot be divided into two equal halves through any plane. This is common in simple animals like sponges (Phylum Porifera).
Radial Symmetry: The body can be divided into identical halves by any plane passing through the central axis, similar to cutting a pie. This is advantageous for sessile or slow-moving animals to respond to stimuli from all directions. Examples include coelenterates (like jellyfish) and echinoderms (like starfish).
Bilateral Symmetry: The body can be divided into identical right and left halves through only one specific plane (the sagittal plane). This allows for streamlined movement and leads to cephalisation (development of a head), where sensory organs are concentrated. This is the most common type of symmetry, found in animals from Platyhelminthes to Chordata, including humans.

5. What is the evolutionary significance of developing a true coelom in animals?

The development of a true coelom (a body cavity lined by mesoderm) was a major evolutionary leap. Its significance lies in several key advantages:

Organ Protection: The coelomic fluid acts as a shock absorber, protecting internal organs from mechanical shocks and impacts.
Space for Organ Development: It provides ample space for internal organs to grow, develop, and function without being constrained by the body wall.
Hydrostatic Skeleton: In some animals, like earthworms, the fluid-filled coelom acts as a hydrostatic skeleton, aiding in locomotion.
Independent Organ Movement: It allows the internal organs to move and function independently of the outer body wall. For instance, the gut can undergo peristalsis without interfering with body movement.
Circulation: The coelomic fluid can help in the transport of gases, nutrients, and waste products, supplementing or forming a circulatory system.

6. How does the level of organisation (cellular, tissue, organ) correlate with an animal's complexity and phylum?

The level of organisation is a direct indicator of an animal's evolutionary complexity and is closely tied to its phylum. The correlation is as follows:

Cellular Level: This is the simplest level, where cells are just loose aggregates and do not form tissues. There is a minimal division of labour. This is the hallmark of Phylum Porifera (sponges).
Tissue Level: Here, cells that perform a similar function are grouped into tissues. This represents a step up in complexity. This level is characteristic of Phylum Coelenterata and Phylum Ctenophora.
Organ Level: In this organisation, different tissues are grouped together to form organs, each specialised for a particular function. This is first seen in Phylum Platyhelminthes (flatworms).
Organ System Level: This is the most complex level, where various organs work together in a coordinated manner to form organ systems (e.g., digestive system, circulatory system). This is found in all higher phyla, from Aschelminthes to Chordata, and allows for the highest degree of functional efficiency and complexity.

7. Why is the notochord considered a fundamental character for classifying animals into Chordata and Non-chordata?

The notochord is a flexible, rod-like structure derived from the mesoderm, present during the embryonic stage of all chordates. It is considered a fundamental character because its presence or absence represents a major divergence in the evolutionary pathway. Non-chordates are animals that never possess a notochord at any stage of their life. In contrast, Chordates are defined by the presence of a notochord at some point in their life cycle. In vertebrates, a sub-phylum of Chordata, this notochord is replaced by a cartilaginous or bony vertebral column (backbone) in the adult stage. Therefore, this single feature forms the primary distinction for the most advanced phylum in the animal kingdom.

8. How does segmentation in Annelida (like earthworms) differ from that in Arthropoda (like insects)?

While both phyla exhibit segmentation, there are key differences. The segmentation in Annelida is known as metamerism, where the body is divided into a series of similar segments (metameres) both externally and internally, with serial repetition of at least some organs. In arthropods, while the body is also segmented, it shows a more advanced feature called tagmatisation. Here, segments are fused and specialised into distinct functional body regions called tagmata (e.g., head, thorax, and abdomen in an insect). This specialisation allows for more complex functions and adaptations, such as specialised appendages for feeding, walking, or flying, making it a more advanced form of segmentation compared to the more uniform segments of an earthworm.