Animal Kingdom

When you observe the world around you, you'll notice many animals with diverse structures and forms. With over a million animal species identified to date, classification becomes crucial. Life can be defined as a physicochemical entity that exhibits growth, movement, irritability, and reproduction. Classification not only organizes these species but also aids in assigning a systematic position to newly discovered ones.

1.0What is Animal Kingdom?

The Animal Kingdom, also known as Kingdom Animalia, is one of the major biological classifications and includes all animals. It encompasses a vast diversity of organisms that share certain key characteristics.

2.0Classification of Animal Kingdom

Basis of Classification

Despite the structural and form differences among various animals, fundamental features are common across many species. These include the arrangement of cells, body symmetry, type of coelom, and patterns in digestive, circulatory, and reproductive systems. These features form the basis for classifying animals.

Levels of Organisation:

Although all members of the Animalia kingdom are multicellular, they do not all exhibit the same cellular organization pattern.

Cellular level: In sponges, cells are arranged as loose aggregates, with the division of labour among the cells but without forming tissues.

Tissue level: In coelenterates and ctenophores, cells that perform the same function are organized into tissues.

Organ level: In platyhelminthes, tissues group together to form organs.

Organ system level: In higher animals, organs are further organized into organ systems. This is seen in groups such as Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata.

Symmetry:

It refers to the arrangement of similar body parts on both sides of the body's main axis. Based on symmetry, animals are categorized into three types:

Germinal Layers:

Body Cavity or Coelom:

The presence or absence of a cavity between the body and gut walls is a key factor in classification. In animals, the coelom acts as a protective cushion for internal organs. In worms, it functions as a hydrostatic skeleton. Furthermore, the coelom allows for the movement of internal organs.

Segmentation:

• Metameric: Found in annelids, arthropods, and chordates. 
• The body is segmented externally and internally in these animals, with a serial repetition of certain organs. 
• For example, an earthworm's body exhibits this pattern, known as metameric segmentation, and this phenomenon is called metamerism.

3.0What are the 7 classifications of Animals?

Phylum – Porifera

• Members of this phylum are commonly known as sponges. They are predominantly marine animals and typically exhibit asymmetry. 
• These primitive multicellular animals have a cellular level of organization. Sponges possess a water transport or canal system, where water enters through minute pores (ostia) in the body wall, flows into a central cavity called the spongocoel, and exits through the osculum. 
• Examples include Sycon (Scypha), Spongilla (freshwater sponge), and Euspongia (bath sponge)

Phylum – Coelenterata (Cnidaria) 

• They are aquatic animals, primarily marine, that can be either sessile or free-swimming and exhibit radial symmetry. 
• The name "Cnidaria" comes from the cnidoblasts or cnidocytes (cells containing stinging capsules called nematocysts) found on their tentacles and bodies. Cnidoblasts are utilized for anchorage, defense, and capturing prey. 
• Examples include Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea pen), Gorgonia (sea fan), and Meandrina (brain coral).

Phylum – Ctenophora

• Ctenophores, or sea walnuts or comb jellies, are exclusively marine organisms characterized by radial symmetry and diploblastic tissue organization. 
• Their bodies feature eight external rows of ciliated comb plates that facilitate locomotion. Examples of ctenophores include Pleurobrachia and Ctenoplana.

Phylum – Platyhelminthes

• They have a dorsoventrally flattened body, which is why they are referred to as flatworms. 
• They are mostly endoparasites found in animals, including humans. 

Phylum – Aschelminthes

• The body of Aschelminthes is circular in cross-section, giving organ-level roundworms. These organisms can be free-living in aquatic and terrestrial environments or parasitic in plants and animals. 
• Roundworms exhibit an organ-system level of body organization. 
• Examples include Ascaris (roundworm), Wuchereria (filarial worm), and Ancylostoma (hookworm).

Phylum – Annelida

• They can be aquatic (both marine and freshwater) or terrestrial; they may be free-living or occasionally parasitic. 
• These organisms exhibit an organ-system level of body organization and possess bilateral symmetry. The body surface is clearly divided into segments or metameres, giving rise to the phylum Annelida. 
• Examples include Nereis, Pheretima (the earthworm), and Hirudinaria (the blood-sucking leech).

Phylum – Arthropoda ( Biggest phylum of Animal Kingdom)

• This is the largest phylum in the Animalia kingdom, encompassing insects. Arthropods account for more than two-thirds of all described species on Earth. 
• They exhibit an organ-system level of organization, possess bilateral symmetry, and are triploblastic, segmented, and coelomate. A chitinous exoskeleton protects their bodies. 
• Examples of economically significant insects include Apis (honey bee), Bombyx (silkworm), and Laccifer (lac insect).

Phylum – Mollusca

• Molluscs are the second-largest phylum in the animal kingdom. 
• They can be found in terrestrial, marine, or freshwater environments and possess an organ-system level of organization. 
• Examples include Pila (apple snail), Pinctada (pearl oyster) (devilfish), Aplysia (sea hare), Dentalium (tusk shell), and Chaetopleura.

Phylum – Echinodermata

• These animals possess an endoskeleton made of calcareous ossicles, which gives them the name Echinodermata. 
• All echinoderms are marine and exhibit an organ-system level of organization. While adult echinoderms display radial symmetry, their larvae are bilaterally symmetrical. 
• Examples include Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), and Ophiura (brittle star).

Phylum – Hemichordata

• It was previously classified as a subphylum within Chordata, but it is now recognized as a distinct phylum within the non-chordates. 
• Hemichordates possess a primitive structure in the collar region known as the stomochord, which is analogous to the notochord. 
• Examples of hemichordates include Balanoglossus and Saccoglossus.

Phylum – Chordata

• Animals belonging to phylum Chordata are fundamentally characterized by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Branchiostoma (Amphioxus or Lancelet).

Vertebrata

Class – Cyclostomata

• All living members of the class Cyclostomata are ectoparasites that attach to various fish. They possess an elongated body featuring 6 to 15 pairs of gill slits for respiration. 
• Cyclostomes have a circular, jawless mouth adapted for suction. Their bodies lack scales and paired fins, and the cranium and vertebral columns are made of cartilage. 
• Examples include Petromyzon (lamprey) and Myxine (hagfish).

Class – Chondrichthyes

• They are marine animals with streamlined bodies and have cartilaginous They are marine animals characterized by streamlined bodies and a cartilaginous endoskeleton. Their mouths are situated ventrally, and the notochord remains present throughout their lives. 
• Gill slits are distinct and lack an operculum (gill cover). The skin is tough and features tiny placoid scales. Their teeth, modified placoid scales, are oriented backward, and their jaws are notably powerful. 
• Examples include Scoliodon (dogfish), Pristis (sawfish), Carcharodon (great white shark), and Trygon (stingray).

Class – Osteichthyes

• It includes both marine and freshwater fish with a bony endoskeleton. Their bodies are streamlined, and the mouth is typically terminal. 
• They possess four pairs of gills, each covered by an operculum on either side. The skin is covered with cycloid or ctenoid scales, and they have an air bladder that helps regulate buoyancy. 
• Examples: Marine: Exocoetus (Flying fish), Hippocampus (Sea horse) Freshwater: Labeo (Rohu), Catla (Katla), Clarias (Magur) Aquarium:Betta (Fighting fish), Pterophyllum (Angelfish)

Class – Amphibia

• As the name suggests (from Greek, "amphi" meaning dual and "bios" meaning life), amphibians can inhabit both aquatic and terrestrial environments. 
• Most amphibians possess two pairs of limbs, and their bodies are divided into a head and trunk. Some may have a tail. 
• Their skin is moist and lacks scales, while their eyes are equipped with eyelids, and a tympanum is the ear. 
• Examples of amphibians include Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), and Ichthyophis (limbless amphibian).

Class – Reptilia

• The class name is derived from their creeping or crawling mode of locomotion (from the Latin words "repere" or "reptum," meaning to creep or crawl). 
• These animals are primarily terrestrial, and their bodies are covered with dry, cornified skin and epidermal scales or scutes. 
• Examples include Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), and Calotes (garden lizard).

Class – Aves

• The defining features of Aves (birds) include the presence of feathers, with most species capable of flight, except for flightless birds like the ostrich. Birds have beaks, and their forelimbs are adapted into wings. 
• The hind limbs typically have scales and are modified for walking, swimming, or grasping tree branches. 
• Examples of birds include Corvus (crow), Columba (pigeon), Aptenodytes (penguin), and Neophron (vulture).

Class – Mammalia

• They inhabit various environments, ranging from polar ice caps and deserts to mountains, forests, grasslands, and dark caves. 
• Some species have evolved to fly or thrive in aquatic habitats. 
• Examples of mammals include oviparous species like the platypus (ornithorhynchus) and viviparous species such as kangaroos (Macropus), flying foxes (Pteropus), camels (Camelus), monkeys (macaca), rats (Rattus), dogs (Canis), and cats (Felis).

4.0Animal Kingdom NEET Questions

1. In which phylum are nematocysts found?

   (a) Porifera                  (b) Cnidaria                  (c) Annelida            (d) Mollusca

Answer: (b) Cnidaria

2. What type of symmetry is exhibited by adult echinoderms?

    (a) Bilateral symmetry                                     (b) Radial symmetry

   (c) Asymmetry                                                 (d) None of the above

    Answer: (b) Radial symmetry

3. The respiratory organs of aquatic arthropods are:

(a) Gills                      (b) Lungs                      (c) Book lungs           (d) Trachea

    Answer: (a) Gills

4. Which group of animals is characterized by having a pseudocoelom?

(a) Annelids                (b) Nematodes              (c) Arthropods   (d) Platyhelminthes

    Answer: (b) Nematodes