The animal kingdom displays an immense diversity in structure and form, with over a million species described to date. This vast variety necessitates classification to facilitate study and assign systematic positions to newly discovered species.

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Basis Of Classification

Despite the wide differences among animals, certain fundamental features are shared by various groups. These common characteristics provide the basis for animal classification. Key features used include the arrangement of cells, body symmetry, presence or absence of a body cavity (coelom), and patterns of organ systems like digestion, circulation, and reproduction.

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Levels Of Organisation

Although all animals are multicellular, the complexity of cell arrangement varies, leading to different levels of organisation:

• Cellular Level: Cells are arranged as loose aggregates, with some basic division of labour among them (e.g., Porifera/Sponges).
• Tissue Level: Cells performing similar functions are organized into tissues (e.g., Coelenterates, Ctenophores).
• Organ Level: Tissues are grouped together to form organs, each specialized for a particular function (e.g., Platyhelminthes).
• Organ System Level: Organs associate to form functional organ systems, where each system carries out a specific physiological function (e.g., Annelids, Arthropods, Molluscs, Echinoderms, Chordates).

Organ systems show varying degrees of complexity across different animal groups.

• Digestive System:
  • Incomplete: Has a single opening that serves as both mouth and anus (e.g., Platyhelminthes).
  • Complete: Has two separate openings, a mouth and an anus.
• Circulatory System:
  • Open type: Blood is pumped out of the heart, and cells and tissues are directly bathed in the blood.
  • Closed type: Blood circulates within a network of blood vessels (arteries, veins, capillaries).

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Symmetry

Animals can be classified based on the arrangement of their body parts relative to a central axis or plane:

• Asymmetrical: The body cannot be divided into two equal halves by any plane passing through the center (e.g., most Sponges).
• Radial Symmetry: Any plane passing through the central axis divides the body into two identical halves (e.g., Coelenterates, Ctenophores, adult Echinoderms).
• Bilateral Symmetry: The body can be divided into identical left and right halves by only one specific plane (e.g., Annelids, Arthropods, Platyhelminthes, Chordates).

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Diploblastic And Triploblastic Organisation

Based on the number of embryonic germ layers present during development, animals are classified as:

• Diploblastic: Cells are arranged in two primary embryonic layers: an external layer called the ectoderm and an internal layer called the endoderm. An undifferentiated jelly-like layer called mesoglea is found between these two layers (e.g., Coelenterates).
• Triploblastic: The developing embryo possesses a third germinal layer, the mesoderm, situated between the ectoderm and the endoderm (e.g., Platyhelminthes to Chordates).

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Coelom

The presence or absence of a body cavity between the body wall and the gut wall is a crucial classification feature. The body cavity is called a coelom if it is lined by the mesoderm.

• Coelomates: Animals that possess a true coelom (body cavity lined by mesoderm) (e.g., Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, Chordates).
• Pseudocoelomates: Animals where the body cavity is not lined by mesoderm. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm. This cavity is called a pseudocoelom (e.g., Aschelminthes).
• Acoelomates: Animals in which a body cavity (coelom) is completely absent (e.g., Platyhelminthes).

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Segmentation

In some animals, the body is divided externally and internally into a series of segments, with repeated organization of certain organs. This pattern is called metameric segmentation, and the phenomenon is known as metamerism (e.g., Earthworm).

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Notochord

The notochord is a rod-like structure derived from the mesoderm. It forms on the dorsal side during embryonic development in some animals.

• Chordates: Animals that possess a notochord at some stage of their embryonic development.
• Non-chordates: Animals that do not form a notochord (e.g., animals from Porifera to Echinodermata).

Classification Of Animals

Based on the fundamental features discussed, the Animal Kingdom (Animalia) is broadly classified into various phyla and classes.

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Phylum – Porifera

Common Name: Sponges.

Habitat: Generally marine, but some are freshwater.

Symmetry: Mostly asymmetrical.

Organisation Level: Primitive multicellular animals with cellular level of organisation.

Distinctive Feature: Presence of a water transport or canal system.

• Water enters through small pores (ostia) in the body wall into a central cavity (spongocoel).
• From the spongocoel, water exits through a large opening (osculum).
• This system aids in food collection, gas exchange, and waste removal.

Specialized Cells: Choanocytes (collar cells) line the spongocoel and canals.

Digestion: Intracellular (occurs within cells).

Support: Body is supported by a skeleton composed of spicules or spongin fibres.

Sexes: Not separate (hermaphrodites); same individual produces eggs and sperm.

Reproduction: Asexual by fragmentation; sexual by gamete formation.

Fertilisation: Internal.

Development: Indirect, involving a larval stage that is morphologically different from the adult.

Examples: *Sycon* (Scypha), *Spongilla* (Fresh water sponge), *Euspongia* (Bath sponge).

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Phylum – Coelenterata (Cnidaria)

Habitat: Aquatic, mostly marine; can be sessile (attached) or free-swimming.

Symmetry: Radially symmetrical.

Organisation Level: Tissue level of organisation.

Germ Layers: Diploblastic.

Distinctive Feature: Presence of cnidoblasts or cnidocytes (containing stinging capsules or nematocysts) on tentacles and the body. Used for anchorage, defense, and prey capture.

Body Cavity: Have a central gastro-vascular cavity.

Digestive System: Incomplete, with a single opening (mouth) on a raised area called the hypostome. Digestion is both extracellular and intracellular.

Skeleton: Some cnidarians (e.g., corals) have a skeleton made of calcium carbonate.

Body Forms: Exhibit two basic body forms:

• Polyp: Sessile, cylindrical form (e.g., *Hydra*, *Adamsia*).
• Medusa: Umbrella-shaped, free-swimming form (e.g., *Aurelia* or jelly fish).

Alternation of Generation (Metagenesis): Some cnidarians (e.g., *Obelia*) exist in both forms and show alternation of generation, where polyps reproduce asexually to produce medusae, and medusae reproduce sexually to form polyps.

Examples: *Physalia* (Portuguese man-of-war), *Adamsia* (Sea anemone), *Pennatula* (Sea-pen), *Gorgonia* (Sea-fan), *Meandrina* (Brain coral), *Hydra*, *Aurelia*.

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Phylum – Ctenophora

Common Names: Sea walnuts or comb jellies.

Habitat: Exclusively marine.

Symmetry: Radially symmetrical.

Germ Layers: Diploblastic.

Organisation Level: Tissue level of organisation.

Distinctive Feature: Body bears eight external rows of ciliated comb plates which are used for locomotion.

Digestion: Both extracellular and intracellular.

Bioluminescence: Well-marked property of emitting light.

Sexes: Not separate.

Reproduction: Only by sexual means.

Fertilisation: External.

Development: Indirect.

Examples: *Pleurobrachia*, *Ctenoplana*.

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Phylum – Platyhelminthes

Common Name: Flatworms, due to their dorso-ventrally flattened body.

Habitat: Mostly endoparasites in animals (including humans), but some are free-living.

Symmetry: Bilaterally symmetrical.

Germ Layers: Triploblastic.

Coelom: Acoelomate (lack a body cavity).

Organisation Level: Organ level of organisation.

Parasitic Adaptations: Hooks and suckers are present in parasitic forms. Some absorb nutrients directly through their body surface from the host.

Excretion and Osmoregulation: Specialized cells called flame cells help in these functions.

Sexes: Not separate (mostly hermaphroditic).

Fertilisation: Internal.

Development: Through many larval stages.

Regeneration: Some members like *Planaria* have a high regeneration capacity.

Examples: *Taenia* (Tapeworm), *Fasciola* (Liver fluke).

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Phylum – Aschelminthes

Common Name: Roundworms, due to their circular body in cross-section.

Habitat: Free-living (aquatic or terrestrial) or parasitic in plants and animals.

Symmetry: Bilaterally symmetrical.

Germ Layers: Triploblastic.

Coelom: Pseudocoelomate (have a false body cavity).

Organisation Level: Organ-system level of organisation.

Digestive System: Complete, with a well-developed muscular pharynx.

Excretion: An excretory tube removes body wastes from the pseudocoelom through an excretory pore.

Sexes: Separate (dioecious); males and females are distinct, with females often being longer.

Fertilisation: Internal.

Development: May be direct (young resemble adult) or indirect.

Examples: *Ascaris* (Roundworm), *Wuchereria* (Filaria worm), *Ancylostoma* (Hookworm).

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Phylum – Annelida

Habitat: Aquatic (marine and freshwater) or terrestrial; free-living or sometimes parasitic.

Symmetry: Bilaterally symmetrical.

Germ Layers: Triploblastic.

Coelom: Coelomate (possess a true body cavity).

Organisation Level: Organ-system level of organisation.

Distinctive Feature: Body surface is distinctly divided into segments or metameres, exhibiting metameric segmentation. The name Annelida comes from the Latin 'annulus', meaning 'little ring'.

Locomotion: Possess longitudinal and circular muscles. Aquatic forms like *Nereis* have lateral appendages called parapodia for swimming.

Circulatory System: Closed type.

Excretion and Osmoregulation: Performed by structures called nephridia.

Neural System: Consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.

Sexes: *Nereis* (aquatic) is dioecious (separate sexes); earthworms and leeches are monoecious (hermaphroditic).

Reproduction: Sexual.

Examples: *Nereis*, *Pheretima* (Earthworm), *Hirudinaria* (Blood sucking leech).

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Phylum – Arthropoda

Size: This is the largest phylum in the animal kingdom, including insects. Over two-thirds of all described species on Earth are arthropods.

Symmetry: Bilaterally symmetrical.

Germ Layers: Triploblastic.

Coelom: Coelomate (though the coelom is often reduced, and the body cavity is filled with blood and called a haemocoel).

Organisation Level: Organ-system level of organisation.

Distinctive Features:

• Body is covered by a tough, external skeleton made of chitin (chitinous exoskeleton).
• Body is typically divided into three regions: head, thorax, and abdomen.
• Presence of jointed appendages ('arthros' = joint, 'poda' = appendages).

Respiration: Various organs for respiration, including gills, book gills, book lungs, or a tracheal system.

Circulatory System: Open type.

Sensory Organs: Include antennae, compound or simple eyes, and statocysts (balancing organs).

Excretion: Takes place through malpighian tubules.

Sexes: Mostly dioecious (separate sexes).

Fertilisation: Usually internal.

Development: May be direct or indirect. Mostly oviparous (egg-laying).

Examples:

• Economically important insects: *Apis* (Honey bee), *Bombyx* (Silkworm), *Laccifer* (Lac insect).
• Vectors (disease carriers): *Anopheles*, *Culex*, and *Aedes* (Mosquitoes).
• Gregarious pest: *Locusta* (Locust).
• Living fossil: *Limulus* (King crab).

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Phylum – Mollusca

Size: This is the second largest animal phylum.

Habitat: Terrestrial or aquatic (marine or freshwater).

Symmetry: Bilaterally symmetrical (although some show torsion leading to asymmetry).

Germ Layers: Triploblastic.

Coelom: Coelomate (though the coelom is significantly reduced, mainly around the heart).

Organisation Level: Organ-system level of organisation.

Distinctive Features:

• Body is typically covered by a calcareous shell (though absent in some).
• Body is unsegmented.
• Body is divided into a distinct head, a muscular foot, and a visceral hump (or mass).
• A soft and spongy layer of skin forms a mantle over the visceral hump.
• The space between the hump and the mantle is the mantle cavity.

Respiration and Excretion: Feather-like gills are present in the mantle cavity and perform these functions.

Sensory Structures: The anterior head region has sensory tentacles.

Feeding: The mouth contains a file-like rasping organ called a radula, used for feeding.

Sexes: Usually dioecious (separate sexes).

Reproduction: Oviparous (egg-laying).

Development: Indirect development with larval stages.

Examples: *Pila* (Apple snail), *Pinctada* (Pearl oyster), *Sepia* (Cuttlefish), *Loligo* (Squid), *Octopus* (Devil fish), *Aplysia* (Sea-hare), *Dentalium* (Tusk shell), *Chaetopleura* (Chiton).

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Phylum – Echinodermata

Common Name: Spiny bodied animals, referring to the presence of an endoskeleton of calcareous ossicles.

Habitat: Exclusively marine.

Organisation Level: Organ-system level of organisation.

Symmetry: Adults are radially symmetrical, but larvae are bilaterally symmetrical.

Germ Layers: Triploblastic.

Coelom: Coelomate (possess a true body cavity).

Digestive System: Complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side.

Distinctive Feature: Presence of a water vascular system. This system is used for locomotion, capturing and transporting food, and respiration.

Excretory System: Absent.

Sexes: Separate.

Reproduction: Sexual.

Fertilisation: Usually external.

Development: Indirect, involving a free-swimming larva.

Examples: *Asterias* (Star fish), *Echinus* (Sea urchin), *Antedon* (Sea lily), *Cucumaria* (Sea cucumber), *Ophiura* (Brittle star).

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Phylum – Hemichordata

Note: Hemichordata was formerly considered a subphylum of Chordata but is now classified as a separate phylum under non-chordates.

Description: A small group of worm-like marine animals.

Symmetry: Bilaterally symmetrical.

Germ Layers: Triploblastic.

Coelom: Coelomate.

Organisation Level: Organ-system level of organisation.

Body Structure: Cylindrical body composed of an anterior proboscis, a collar, and a long trunk.

Distinctive Feature: Presence of a rudimentary structure in the collar region called stomochord, which is similar to a notochord.

Circulatory System: Open type.

Respiration: Takes place through gills.

Excretory Organ: Proboscis gland.

Sexes: Separate.

Fertilisation: External.

Development: Indirect.

Examples: *Balanoglossus*, *Saccoglossus*.

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Phylum – Chordata

Fundamental Characteristics: Animals in this phylum are defined by the presence of:

1. A notochord (present at least during embryonic life).
2. A dorsal, hollow nerve cord.
3. Paired pharyngeal gill slits.

General Features:

• Symmetry: Bilaterally symmetrical.
• Germ Layers: Triploblastic.
• Coelom: Coelomate.
• Organisation Level: Organ-system level of organisation.
• Presence of a post-anal tail (at some stage).
• Circulatory System: Closed type.

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Comparison of Chordates and Non-chordates:

S.No.	Chordates	Non-chordates
1.	Notochord present.	Notochord absent.
2.	Central nervous system is dorsal, hollow and single.	Central nervous system is ventral, solid and double.
3.	Pharynx perforated by gill slits.	Gill slits are absent.
4.	Heart is ventral.	Heart is dorsal (if present).
5.	A post-anal part (tail) is present.	Post-anal tail is absent.

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Phylum Chordata is divided into three subphyla:

1. Urochordata (Tunicata)
2. Cephalochordata
3. Vertebrata

Urochordata and Cephalochordata are collectively known as protochordates. They are exclusively marine.

• Urochordata: Notochord is present only in the larval tail. Examples: *Ascidia*, *Salpa*, *Doliolum*.
• Cephalochordata: Notochord extends from the head to the tail region and persists throughout life. Examples: *Branchiostoma* (Amphioxus or Lancelet).

Members of the subphylum Vertebrata possess a notochord during the embryonic stage, but in the adult, it is replaced by a cartilaginous or bony vertebral column.

This is why the statement "All vertebrates are chordates, but all chordates are not vertebrates" is true (Vertebrates are a subgroup of Chordates).

Besides the basic chordate characteristics, vertebrates have additional features:

• A ventral muscular heart with 2, 3, or 4 chambers.
• Kidneys for excretion and osmoregulation.
• Paired appendages, which can be fins or limbs.

The subphylum Vertebrata is further divided into classes based on whether they possess jaws (Gnathostomata) or lack jaws (Agnatha), and whether they bear fins (Pisces) or limbs (Tetrapoda).

Subphylum Vertebrata is divided as follows:

Division: Agnatha (Lack jaw)

• Class 1: Cyclostomata

Division: Gnathostomata (Bear jaw)

• Super Class: Pisces (Bear fins)
  • Class 1: Chondrichthyes
  • Class 2: Osteichthyes
• Super Class: Tetrapoda (Bear limbs)
  • Class 1: Amphibia
  • Class 2: Reptilia
  • Class 3: Aves
  • Class 4: Mammals

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Class – Cyclostomata

Group: Agnatha (jawless vertebrates).

Life Style: All living members are ectoparasites on some fishes.

Body: Elongated body with 6-15 pairs of gill slits for respiration.

Mouth: Sucking and circular mouth, without jaws.

Appendages/Scales: Body lacks scales and paired fins.

Skeleton: Cranium and vertebral column are cartilaginous.

Circulation: Closed type.

Reproduction and Life Cycle: Marine, but migrate to fresh water for spawning. They die within a few days after spawning. The larvae undergo metamorphosis and return to the ocean.

Examples: *Petromyzon* (Lamprey), *Myxine* (Hagfish).

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Class – Chondrichthyes

Group: Gnathostomata, Superclass Pisces (jawed, fin-bearing).

Habitat: Marine animals.

Body: Streamlined body.

Endoskeleton: Made of cartilage (cartilaginous endoskeleton).

Mouth: Located ventrally.

Notochord: Persistent throughout life.

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

Skin and Scales: Tough skin containing minute placoid scales. Teeth are modified, backwardly directed placoid scales.

Jaws: Very powerful, making them predaceous.

Buoyancy: Absence of an air bladder means they must swim constantly to avoid sinking.

Heart: Two-chambered (one auricle, one ventricle).

Thermoregulation: Cold-blooded (poikilothermous) - cannot regulate body temperature.

Special Organs: Some have electric organs (*Torpedo*); some possess a poison sting (*Trygon*).

Sexes: Separate. In males, pelvic fins bear claspers.

Fertilisation: Internal.

Development: Many are viviparous (give birth to live young).

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

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Class – Osteichthyes

Group: Gnathostomata, Superclass Pisces (jawed, fin-bearing).

Habitat: Includes both marine and freshwater fishes.

Endoskeleton: Made of bone (bony endoskeleton).

Body: Streamlined.

Mouth: Mostly terminal.

Gills: Four pairs of gills covered by an operculum on each side.

Skin and Scales: Skin covered with cycloid or ctenoid scales.

Buoyancy: Presence of an air bladder, which helps regulate buoyancy.

Heart: Two-chambered (one auricle, one ventricle).

Thermoregulation: Cold-blooded (poikilothermous).

Sexes: Separate.

Fertilisation: Usually external.

Reproduction: Mostly oviparous (egg-laying).

Development: Direct.

Examples: Marine – *Exocoetus* (Flying fish), *Hippocampus* (Sea horse); Freshwater – *Labeo* (Rohu), *Catla* (Katla), *Clarias* (Magur); Aquarium – *Betta* (Fighting fish), *Pterophyllum* (Angel fish).

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Class – Amphibia

Meaning: The name means "dual life" (Greek: Amphi = dual, bios = life), indicating they can live in both aquatic and terrestrial habitats.

Limbs: Most have two pairs of limbs.

Body Division: Divisible into head and trunk; a tail may be present in some.

Skin: Moist skin, without scales.

Eyes: Possess eyelids.

Ear: Represented by a tympanum.

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

Respiration: By gills (in larval stages), lungs, and through the skin (cutaneous respiration).

Heart: Three-chambered (two auricles, one ventricle).

Thermoregulation: Cold-blooded (poikilothermous).

Sexes: Separate.

Fertilisation: External.

Reproduction: Oviparous (egg-laying).

Development: Indirect, involving a larval stage (e.g., tadpole).

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

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Class – Reptilia

Meaning: Name refers to their creeping or crawling mode of locomotion (Latin: repere or reptum = to creep or crawl).

Habitat: Mostly terrestrial animals.

Skin: Body covered by dry and cornified skin, with epidermal scales or scutes.

External Ear: Absent; tympanum represents the ear.

Limbs: When present, usually two pairs (except in snakes).

Heart: Usually three-chambered, but four-chambered in crocodiles.

Thermoregulation: Cold-blooded (poikilothermous).

Skin Shedding: Snakes and lizards shed their scales as a skin cast.

Sexes: Separate.

Fertilisation: Internal.

Reproduction: Oviparous (egg-laying).

Development: 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).

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Class – Aves

Common Name: Birds.

Characteristic Features: Presence of feathers and most can fly (except flightless birds like Ostrich).

Body Structure:

• Possess a beak.
• Forelimbs are modified into wings for flight.
• Hind limbs typically have scales and are adapted for walking, swimming, or perching.

Skin: Dry skin, except for the presence of an oil gland at the base of the tail.

Endoskeleton: Fully ossified (bony). Long bones are often hollow with air cavities (pneumatic bones), which reduces weight for flight.

Digestive Tract: Additional chambers present, the crop (for storing food) and gizzard (for grinding food).

Heart: Completely four-chambered.

Thermoregulation: Warm-blooded (homoiothermous) - capable of maintaining a constant body temperature.

Respiration: By lungs, supplemented by air sacs connected to the lungs.

Sexes: Separate.

Fertilisation: Internal.

Reproduction: Oviparous (egg-laying).

Development: Direct.

Examples: *Corvus* (Crow), *Columba* (Pigeon), *Psittacula* (Parrot), *Struthio* (Ostrich), *Pavo* (Peacock), *Aptenodytes* (Penguin), *Neophron* (Vulture).

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Class – Mammalia

Habitat: Found in a wide variety of habitats, including polar regions, deserts, mountains, forests, grasslands, and caves. Some are adapted for flight or aquatic life.

Unique Characteristic: Presence of milk-producing glands (mammary glands) for nourishing their young.

Limbs: Two pairs of limbs, adapted for various functions like walking, running, climbing, burrowing, swimming, or flying.

Skin: Unique in possessing hair.

External Ear: Presence of external ears or pinnae.

Teeth: Different types of teeth are present in the jaw (heterodont dentition).

Heart: Completely four-chambered.

Thermoregulation: Warm-blooded (homoiothermous).

Respiration: By lungs.

Sexes: Separate.

Fertilisation: Internal.

Reproduction: Mostly viviparous (give birth to live young), with a few exceptions.

Development: 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).

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Salient distinguishing features of the different phyla in the animal kingdom are summarized in the following table:

Phylum	Level of Organisation	Symmetry	Coelom	Segmentation	Digestive System	Circulatory System	Respiratory System	Distinctive Features
Porifera	Cellular	Various	Absent	Absent	Absent	Absent	Absent	Body with pores and canals in walls; Choanocytes present.
Coelenterata (Cnidaria)	Tissue	Radial	Absent	Absent	Incomplete	Absent	Absent	Cnidoblasts present.
Ctenophora	Tissue	Radial	Absent	Absent	Incomplete	Absent	Absent	Comb plates for locomotion; Bioluminescence.
Platyhelminthes	Organ & Organ-system	Bilateral	Absent	Absent	Incomplete	Absent	Absent	Flat body; Hooks and suckers often present in parasitic forms; Flame cells for excretion.
Aschelminthes	Organ-system	Bilateral	Pseudocoelomate	Absent	Complete	Absent	Absent	Often worm-shaped, circular in cross-section.
Annelida	Organ-system	Bilateral	Coelomate	Present	Complete	Present (Closed)	Absent (usually diffusion/skin)	Body segmentation (metamerism) like rings; Nephridia for excretion.
Arthropoda	Organ-system	Bilateral	Coelomate (reduced)	Present	Complete	Present (Open)	Present (Gills, book lungs, trachea)	Chitinous exoskeleton; Jointed appendages; Malpighian tubules for excretion.
Mollusca	Organ-system	Bilateral (some asymmetry)	Coelomate (reduced)	Absent	Complete	Present (Open)	Present (Gills/ctenidia, lungs)	Calcareous shell usually present; Mantle; Radula.
Echinodermata	Organ-system	Radial (Adult) / Bilateral (Larva)	Coelomate	Absent	Complete	Present (Reduced)	Present (Tube feet, papulae)	Water vascular system; Spiny skin (calcareous ossicles).
Hemichordata	Organ-system	Bilateral	Coelomate	Absent	Complete	Present (Open)	Present (Gills)	Worm-like body with proboscis, collar, trunk; Stomochord.
Chordata	Organ-system	Bilateral	Coelomate	Present (in some)	Complete	Present (Closed)	Present (Gills, lungs, skin)	Notochord; Dorsal hollow nerve cord; Pharyngeal gill slits; Post-anal tail.

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