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Animal Kingdom

Animalia kingdom includes multicellular, eukaryotic, and heterotrophic organisms and their cells lack cell walls.

Animalia kingdom includes multicellular, eukaryotic, and heterotrophic organisms and their cells lack cell walls.

Basis of classification:

Kingdom Animalia is further classified into different phyla based on fundamental features. Such as the arrangement of cells, body symmetry, nature of coelom, and patterns of digestive, circulatory, or reproductive systems.

Levels of organization:

  1. Cellular level of organization – Porifera

  2. Tissue level of organization – Coelenterata, Ctenophora

  3. Organ level of organization – Platyhelminthes

  4. Organ-system level of organization – Aschelminthes to Chordates.

Circulatory System:

  1. Open type: Blood is pumped out of the heart and the cells and tissues are directly bathed in it.

  2. Closed type: Blood is circulated through a series of vessels.


  1. Asymmetrical: The body of an organism does not divide into two equal halves at any plane. e.g., Sponges.

  2. Radial symmetry: Any plane passing through the central axis of the body divides the organism into identical halves. e.g., Coelenterata, Ctenophora, Echinodermata.

  3. Bilateral symmetry: If the body of an organism can be divided into identical left and right halves in only one plane. e.g., Annelids and Arthropods and most of the chordates.

Germinal layers:

  1. Diploblastic: Cells arranged in two embryonic layers i.e. external ectoderm and internal endoderm. (Mesoglea may be present in between ectoderm and endoderm) e.g., Coelentrates. (Cnidarians).

  2. Triploblastic: Three layers present in the developing embryo i.e., ectoderm, endoderm, and mesoderm. e.g., Chordates.

Coelom (Body cavity):

  1. Coelom: The body cavity which is lined by mesoderm is called coelom. Animals possessing coelom are called coelomate animals. e.g., Annelids to Chordates.

  2. Pseudocoelom: If the body cavity is not lined n=by mesoderm, instead, the mesoderm is present in scattered pouches between ectoderm and endoderm. e.g., Aschelminthes.

  3. Acoelom: The body cavity is absent. e.g., Platyhelminthes. Metamerism: If the body is externally and internally divided into segments with a serial repetition of at least some organs. This phenomenon is called metamerism. e.g., Earthworm.


It is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. e.g., Chordates.


  • Members of this phylum are commonly known as sponges.

  • They usually marine and asymmetrical.

  • Have cellular level of organisation.

  • Food gathering, respiratory exchange and removal of wastes occurs through water canal system.

  • Digestion is intracellular.

  • Ostia (minute pores on body), spongocoel (body cavity) and osculum help in water transport.

  • Spongocoel is lined by choanocytes (collar cells).

  • The body is supported by a skeleton made up of spicules and spongin fibres.

  • Sexes are not separate hence they are hermaphrodites.

  • Fertilisation internal, development is indirect (i.e., has a larval stage distinct from adult stage)

  • e.g., Sycon (Scypha), Euspongia, Spongilla.

(a) Sycon (b) Euspongia (c) Spongilla


  • Also called Cnidarians because they have cnidoblasts or cnidocysts.

  • They usually marine and radially symmetrical.

  • Have tissue level of organisation, diploblastic animals.

  • Food gathering, anchorage and defense occurs through cnidoblasts present on tentacles.

  • Digestion is extracellular and intracellular.

  • They have gastro-vascular cavity and an opening, mouth on hypostome.

  • Body wall composed of calcium carbonate (corals).

  • Exhibit two body forms; polyp and medusa. e.g., Hydra, Aurelia.

  • Alternation of generation between body forms called Metagenesis occurs in Obelia.

  • e.g., Physalia, Adamsia, Pennatula, Gorgonia, Meandrina.


  • Also called as sea walnuts or combjellies.

  • They are exclusively marine, radially symmetrical.

  • Have tissue level of organisation and are diploblastic animals.

  • Digestion is both extra and intracellular.

  • Body has eight external rows of ciliated comb plates for locomotion.

  • They Show Bioluminescence (the property of living organism to emit light).

  • Sexes are not separate, only sexual reproduction occurs.

  • External fertilization, Indirect development.

  • e.g., Ctenoplana, Pleurobranchia.



  • They have dorso-ventrally flattened body, hence are called ‘flat worms’.

  • They are endoparasites found in animals.

  • Bilaterally symmetrical, triploblastic, acoelomate animals.

  • Absorb nutrients through body surface.

  • Parasite forms have hooks and suckers.

  • ‘Flame cells’ help in osmoregulation and excretion.

  • Sexes are not separate.

  • Fertilisation is internal, development is indirect with many larval stages.

  • Planaria has high regeneration capacity.

  • e.g., Taenia, Fasciola.

Taenia Soliumn


  • Also called ‘round worms’.

  • May be free living, parasitic, aquatic or terrestrial.

  • They have organ system level of organisation, are bilaterally symmetrical, triploblastic, pseudocoelomate animals.

  • Alimentary canal complete (has muscular pharynx).

  • Wastes removed through excretory pore.

  • Sexes separate (dioecious), shows sexual dimorphism.

  • Females are longer than males.

  • Fertilisation is internal, development is direct or indirect.

  • e.g., Ascaris, Wuchereria, Ancyclostoma.

Ascaris lumbricoides


  • They are aquatic or terrestrial, free-living or parasitic organisms.

  • Their body surface distinctly marked out into segments or metameres.

  • Bilaterally symmetrical, triploblastic, organ-system level of organisation and have metamerically segmented body.

  • Have longitudinal and circular muscles for locomotion.

  • Nereis (dioecious and aquatic annelid) has lateral appendages called parapodia for swimming.

  • Have nephridia for osmoregulation and excretion.

  • e.g., Earthworm (Pheretima) and Leech (Hirudinaria) which are hermaphrodites (i.e., monoecious).


Nereis and Leech


  • Largest phylum of kingdom Animalia.

  • They are bilaterally symmetrical, triploblastic and organ system level of organisation, coelomate animals.

  • Locomotors are represented by jointed appendages.

  • Body divisible into head, thorax, abdomen and has a chitinous exoskeleton.

  • Respiration is done by gills, book gills, lungs or tracheal system.

  • Excretion through malpighian tubules.

  • Sensory organs are Antennae, eyes; organs of balance called Statocysts.

  • Sexes are separate, fertilization is internal, development is indirect or direct.

  • They are mostly oviparous.

  • e.g., Apis, Bombyx, Anopheles, Locusta, Limulus.



  • Second largest phylum of Animalia.

  • They are bilaterally symmetrical, triploblastic and organ system level of organisation, coelomate animals.

  • Body divisible into head, muscular foot and visceral hump and is covered by calcareous shell.

  • It is unsegmented over visceral hump.

  • Mantle: Soft and spongy layer of skin; Mantle cavity: Space between visceral hump and mantle.

  • Respiration and excretion are by feather like gills in mantle cavity.

  • Head has sensory tentacles.

  • Radula, file-like rasping organ for feeding.

  • They are oviparous, dioecious, have indirect development.

  • e.g., Pila, Pinctada, Octopus.



  • They are spiny bodied organisms.

  • Exclusively marine, radially symmetrical in adult but bilaterally symmetrical in larval stage.

  • Organ system level of organisation.

  • Digestive system complete. Mouth is ventral, anus on dorsal side.

  • Food gathering, respiration, locomotion carried out by water vascular system.

  • Excretory system is absent.

  • Fertilisation is external, development is indirect (free swimming larva).

  • e.g., Asterias, Cucumaria.


  • This phylum consists of a small group of worm-like marine animals with organ-system level of organisation.

  • They are bilaterally symmetrical, triploblastic and coelomate animals.

  • The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk.

  • Circulatory system is of open type.

  • Respiration takes place through gills.

  • Excretory organ is proboscis gland.

  • Sexes are separate. Fertilization is external. Development is indirect.

  • Examples: Balanoglossus and Saccoglossus.




Animals belonging to phylum Chordata are fundamentally characterized by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.

Comparison of chordates and non-chordates

Phylum Chordata is divided into three subphyla:

Urochordata or Tunicata: In Urochordata, notochord is present only in larval tail.

E.g., Ascidia, Salpa, Doliolum.

Cephalochordata: in Cephalochordata, notochord extends from head to tail region and is persistent throughout their life.

E.g., Branchiostoma (Amphioxus or Lancelet).


  • The members of subphylum Vertebrata possess notochord during the embryonic period.

  • The notochord is replaced by a cartilaginous or bony vertebral column in the adult. Thus, all vertebrates are chordates but all chordates are not vertebrates.

  • Vertebrates have a ventral muscular heart with two, three or four chambers.

  • Kidneys for excretion and osmoregulation.

  • Paired appendages which may be fins or limbs.

The subphylum Vertebrata is further divided as follows:

Class – Cyclostomata

  • All living members of the class Cyclostomata are ectoparasites on some fishes.

  • They have an elongated body bearing 6-15 pairs of gill slits for respiration.

  • Cyclostomes have a sucking and circular mouth without jaws.

  • Their body is devoid of scales and paired fins.

  • Cranium and vertebral column are cartilaginous.

  • Circulation is of closed type.

  • Cyclostomes are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.

  • E.g., Petromyzon (Lamprey) and Myxine (Hagfish).

Class – Chondrichthyes

  • They are marine animals with streamlined body and have cartilaginous endoskeleton.

  • Mouth is located ventrally.

  • Notochord is persistent throughout life.

  • Gill slits are separate and without operculum (gill cover).

  • The skin is tough, containing minute placoid scales.

  • Teeth are modified placoid scales which are backwardly directed.

  • Due to the absence of air bladder, they have to swim constantly to avoid sinking.

  • Heart is two-chambered (one auricle and one ventricle).

  • Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon).

  • They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature.

  • Sexes are separate. In males pelvic fins bear claspers.

  • They have internal fertilisation and many of them are viviparous.

  • Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).


(a) Scoliodon (b) Pristis

Class -Osteichthyes

  • It includes both marine and fresh water fishes with bony endoskeleton.

  • Their body is streamlined.

  • Mouth is mostly terminal.

  • They have four pairs of gills which are covered by an operculum on each side.

  • Skin is covered with cycloid/ctenoid scales.

  • Air bladder is present which regulates buoyancy.

  • Heart is two chambered (one auricle and one ventricle). They are cold-blooded animals.

  • Sexes are separate. Fertilisation is usually external. They are mostly oviparous and development is direct.

  • Examples: Marine Exocoetus (Flying fish), Hippocampus (Sea horse); FreshwaterLabeo (Rohu), Catla (Katla), Clarias (Magur); AquariumBetta (Fighting fish), Pterophyllum (Angel fish).

(a) Hippocampus (b) Catla

Class – Amphibia

  • As the name indicates (Gr., Amphi: dual, bios: life), amphibians can live in aquatic as well as terrestrial habitats.

  • Most of them have two pairs of limbs.

  • Body is divisible into head and trunk.

  • Tail may be present in some.

  • The amphibian skin is moist (without scales).

  • The eyes have eyelids. A tympanum represents the ear.

  • Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca which opens to the exterior.

  • Respiration is by gills, lungs and through skin.

  • The heart is three chambered (two auricles and one ventricle). These are

  • cold-blooded animals.

  • Sexes are separate. Fertilisation is external. They are oviparous and development is direct or indirect.

  • Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).


(a) Salamandra (b) Rana

Class – Reptilia

  • The class name refers to their creeping or crawling mode of locomotion (Latin, repere or reptum, to creep or crawl).

  • Their body is covered by dry and cornified skin, epidermal scales or scutes.

  • They do not have external ear openings. Tympanum represents ear.

  • Limbs, when present, are two pairs.

  • Heart is usually three-chambered, but four-chambered in crocodiles. Reptiles are poikilotherms.

  • Snakes and lizards shed their scales as skin cast.

  • Sexes are separate. Fertilisation is internal. They are oviparous and development is direct.

  • Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator). Hemidactylus (Wall lizard), Poisonous snakes – Naja (Cobra), Bangarus (Krait), Vipera (Viper).

(a) Chameleon (b) Crocodilus (c) Chelone (d) Naja

Class -Aves

  • The characteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich).

  • They possess beak instead of teeth.

  • The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches.

  • Skin is dry without glands except the oil gland at the base of the tail.

  • Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic).

  • The digestive tract of birds has additional chambers, the crop and gizzard.

  • Heart is completely four chambered. They are warm-blooded homoiothermous) animals, i.e., they are able to maintain a constant body temperature.

  • Respiration is by lungs. Air sacs connected to lungs supplement respiration.

  • Sexes are separate. Fertilisation is internal. They are oviparous and development is direct.

  • Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio

    (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

(a) Neophron (b) Struthio (c) Psittacula (d) Pavo

Class – Mammalia

  • They are found in a variety of habitats – polar ice caps, deserts, mountains, forests, grasslands and dark caves.

  • The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.

  • They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying.

  • The skin of mammals is unique in possessing hair.

  • External ears or pinnae are present.

  • Different types of teeth are present in the jaw.

  • Heart is four chambered. They are homoiothermous.

  • Respiration is by lungs.

  • Sexes are separate and fertilisation is internal. They are viviparous with few exceptions and development is direct.

  • Examples: Oviparous - Ornithorhynchus (Platypus); Viviparous - Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion).

(a) Ornithorhynchus (b) Macropus (c) Pteropus (d) Balaenoptera

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