ANIMAL KINGDOM | Chapter 4 | Biology | NEET

Animal Kingdom

  • Millions of species of Animals have been described and it becomes more necessary to classify them to assign a systematic position.
  • Animals are classified on the basis of the arrangement of cells, body symmetry, nature of coelom, pattern of digestive, circulatory and reproductive system.

  • Incomplete digestive system has one opening but complete digestive system has two opening- mouth and anus.
  • Open circulatory system- blood is pumped out of heart and cells and tissue are directly bathed in it.
  • Closed circulatory system- blood is circulated through arteries, veins and capillaries.
  • The animals in which cells are arranged in two embryonic layer, external ectoderm and internal endoderm are called diploblastic. Eg. Porifera and Cnidaria.

  • The animals in which developing embryo has a third germinal layer, mesoderm besides ectoderm and endoderm are called triploblastic. Eg. Platyhelminthes, Chordates.
  • The body cavity which is lined by mesoderm is called coelom. Animals possessing coelom are called coelomate (Annelida, Chordates, Mollusca). In some animals cavity is not lined by mesoderm but scattered as pouches in between ectoderm and endoderm, are called pseudo-coelomates (Aschelminthes). The animals in which body cavity is absent are called acoelomate (Platyhelminthes). 

  • In some animals, body is externally and internally divided into segments with serial repetition as in earthworm, called metameric segmentation. 


Phylum- Porifera

  • Members of this phylum are commonly known as sponges. Mostly marine, asymmetrical and have cellular level of organization.
  • They have water transport or canal system. Water enters through minute pores, Ostia into central cavity Spongocoel, from where it They are aquatic, mostly marine, sessile, free swimming, radially symmetrical animals. goes out through Osculum.
  • Nutrition, respiration, and excretion are performed by the pathway of the water transport system.
  • Skeleton made up of spicules or spongin fibres.
  • Egg and sperms are produced by same organism (hermaphrodite). Asexual reproduction by fragmentation and sexual reproduction by gametes formation.
  • Fertilisation internal and development is indirect.
  • Example- Sycon, Spongilla

Phylum- Cnidaria (Coelenterate)

  • They are aquatic, mostly marine, sessile, free swimming, and radially symmetrical animals. 
  • They exhibit tissue level of organization, diploblastic, coelomate with single opening.
  • They show two types of body called polyp and medusa.
  • Polyp is sessile, fixed, and cylindrical, without gonads. Example: Hydra, Adamsia. Medusa is free swimming, umbrella like having gonads like Aurelia and Jelly fish.
  • Some cnidarians exhibits both forms (Obelia). Polyp produce medusa asexually and medusa produce polyp sexually.

Phylum- Ctenophora

  • Commonly known as the Comb Jellies or Sea Walnuts.
  • Exclusively marine, diploblastic, radially symmetrical, with tissue level of organization.
  • Body bears eight ciliated comb plates which help in locomotion. 
  • Bioluminescence (to emit light) is present in Ctenophores.
  • Are Hermaphrodite, fertilisation is external, development indirect.
  • Examples- Ctenoplana, Pleurobranchia.

Phylum- Platyhelminthes (The Flatworms)

  • Dorso-ventrally flattened body, bilaterally symmetrical, triploblastic, acoelomate with organs levels of organization.
  • Hooks and sucker are present in parasitic forms. Flame cells help in osmoregulation and excretion.
  •  Fertilisation is internal, development is indirect. They are hermaphrodite.
  • Examples- Taenia, Planaria, Fasciola.

Phylum- Aschelminthes (The Round Worm)

  • They may be free-living, aquatic, terrestrial or parasitic in plants or animals.
  • Bilaterally symmetrical, triploblastic, pseudo coelomate. 
  • Alimentary canal is complete with well-developed muscular pharynx.
  • They are Dioecious. Females are longer than male. 
  • Examples- Ascaris (round worm), Wucheriria(filarial worm), Ancyclostoma.  

Phylum- Annelida

  • Aquatic or terrestrial, bilaterally symmetrical, segmented with organ system level of organization.
  • Aquatic Annelids like Nereis possesses lateral appendages parapodia, for swimming. Nephridia help in osmoregulation and excretion. 
  •  Neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
  • Dioecious (Nereis) or monocious (earthworm, leech) 
  • Example- Pheretima (earthworm), Hirunidaria (Blood sucking leech).

Phylum- Arthropoda

  • Largest phylum of animals which includes insects. They have organ system of organization. They are triploblastic, coelomate, bilaterally symmetrical with chitinous exoskeleton.
  • Body consists of head, thorax and abdomen, jointed appendages (jointed feet). Respiratory organs are gills, book lungs or tracheal system with open circulatory system. 
  • Excretion through malpighian tubules, sense organs antenna or eyes. Fertilisation internal, mostly oviparous.
  • Example- Economically important Apis (honey bee), Bombyx (silk worm). Vectors Anopheles, Ades, Culex (mosquito). Living fossils – Limulus (king crab)

Phylum- Mollusca

  • Terrestrial or aquatic, organ level of organization, bilaterally symmetrical, triploblastic and coelomate.
  • Body divided into head, muscular foot and visceral hump. Unsegmented and covered with calcareous shell. 
  • Feather like gills are present between hump and mantle.
  • Mouth contains file like rasping organ for feeding called radula.
  • Example- Pila, Octopus.

Phylum- Echinodermata (The Spiny Skinned Animals)

  • Endoskeleton of calcareous ossicles, marine with organ system of organization. 
  • Triploblastic, coelomate, presence of water vascular system help in locomotion, capture of food and respiration.
  • Sexes are separate, fertilisation is external and development is indirect.
  • Examples- Asterias (Star fish), Cucumaria (Sea cucumber), Antedon (Sea lily).

Phylum- Hemichordata

  • Worm-like marine animals with organ system of organization, bilaterally symmetrical, triploblastic and coelomate animals.
  • Body is cylindrical, composed of anterior proboscis, a collar and a long trunk.
  • Open circulatory system, respiration by gills, excretory organ is proboscis glands.
  • Sexes are separate, fertilisation external, indirect development. 
  • Example-Balanoglossus, Saccoglossus.

Phylum- Chordates

  • Presence of notochord, have dorsal hollow nerve chord and paired pharyngeal gill slits.
  • Bilaterally symmetrical, triploblastic, coelomate with organs system levels of organization.
  • Closed circulatory system, ventral heart, post-anal tail is present.

  • In Urochordata, notochord is present only in larval tail. In Cephalochordate it extends from head to tail and persists throughout the life.
  • Vertebrata possesses notochord in embryonic period which is replaced by vertebral column in the adults. 

  • Sub-phylum Vertebrata is further divided into two division Agnatha( lacks jaw) and Gnathostomata (bears jaw).
  •  Gnathostomata is further divided into two super class- Pisces( bears fins) and Tetrapoda (bears limbs).

Class- Cyclostomata (Circular mouthed fishes)

  • They are ectoparasites on some fishes. They have sucking and circular mouth without jaws. 
  • Body devoid of scales, gill slits for respiration, cranium and vertebral column is cartilaginous. 
  • Circulation is closed type. They are marine but migrate to fresh water for spawning and die after few days. Larva return to seas after metamorphosis.
  • Example- Petromyzon (Lamprey), Maxine (Hag fish).

Class- Chondrichthyes (The Cartilaginous Fish)

  • They are marine, streamlined body, have cartilaginous endoskeleton, cold blooded, tough skin with minute placoid scales.
  • Gill slits are separate without operculum.
  • They have powerful jaw and are predators.
  • Air bladder is absent, hence to avoid sinking swims constantly. Heart is two chambered, cold blooded (Poikilothermous).
  • Sexes separate. Males have pelvic fins which bear claspers. Internal fertilisation, many are viviparous.
  • Electric organ is present in Torpedo and Poison sting in Trygon.
  • Example- Scoliodon (Dog fish), Carcharodron (great white shark).

Class- Osteichthyes (The body fish)

  • Marine and freshwater both have bony endoskeletons. Streamlined body with four pairs of gills covered by operculum.
  • Skin is covered with scales, the air bladder is present, and the heart is two-chambered and cold-blooded.
  • Sexes are separate, fertilization external, oviparous, and development direct.
  • Example-  Marine– Hippocampus (Sea horse), Exocoetus (Flying fish). Fresh water– Labeo (Rohu), Catla,Clarias (Magur).

Class- Amphibia

  • Can live in aquatic as well as terrestrial habitat
  • Two pairs of limbs
  • Moist skin without scales
  • Respiration by gills, lungs, or skin.
  • Heart three-chambered, cold-blooded.
  • Oviparous
  • Examples- Rana (frog), Salamander, Hyla

Class- Reptilia

  • Mostly terrestrial animals.
  • Limb two pairs if present.
  • Dry and cornified skin having scale or scute.
  • Respiration by lungs.
  • Heart three-chambered, Except Crocodile 4-chambered heart.
  • Oviparous
  • Examples- Chamelion, Crocodilus, Naja

Class- Aves

  • Presence of feathers for flying.
  • The forelimb is modified into wings.
  • Skin is dry without glands. Long bones are hollow with air cavities. 
  • Respiration by lungs.
  • The heart is four-chambered and warm-blooded.
  • Oviparous
  • Examples- Columba, Pavo, Ostrich.

Class- Mammals

  • Mostly terrestrial, a few can fly and live in water.
  • Two pairs of limbs
  • The skin possesses hairs. The mammary gland is present to produce milk.
  • Respiration by lungs.
  • Heart Four Chambered.
  • Viviparous or oviparous
  • Examples- Platypus(oviparous), Camel, Dog, Blue whale. 


The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom. Besides the fundamental features, there are many other distinctive characters which are specific for each phyla or class.

Porifera includes multicellular animals which exhibit cellular level of organisation and have characteristic flagellated choanocytes. The coelenterates have tentacles and bear cnidoblasts. They are mostly aquatic, sessile or free-floating. The ctenophores are marine animals with comb plates. The platyhelminths have flat body and exhibit bilateral symmetry. The parasitic forms show distinct suckers and hooks. Aschelminthes are pseudocoelomates and include parasitic as well as non-parasitic roundworms.

Annelids are metamerically segmented animals with a true coelom. The arthropods are the most abundant group of animals characterised by the presence of jointed appendages. The molluscs have a soft body surrounded by an external calcareous shell. The body is covered with external skeleton made of chitin. The echinoderms possess a spiny skin. Their most distinctive feature is the presence of water vascular system. The hemichordates are a small group of worm-like marine animals. They have a cylindrical body with proboscis, collar and trunk.

Phylum Chordata includes animals which possess a notochord either throughout or during early embryonic life. Other common features observed in the chordates are the dorsal, hollow nerve cord and paired pharyngeal gill slits. Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess jaws (Gnathostomata). Agnatha is represented by the class, Cyclostomata. They are the most primitive chordates and are ectoparasites on fishes. Gnathostomata has two super classes, Pisces and Tetrapoda. Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces. The Chondrichthyes are fishes with cartilaginous endoskeleton and are marine. Classes, Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are thus grouped under Tetrapoda. The amphibians have adapted to live both on land and water. Reptiles are characterised by the presence of dry and cornified skin. Limbs are absent in snakes. Fishes, amphibians and reptiles are poikilothermous (coldblooded). Aves are warm-blooded animals with feathers on their bodies and forelimbs modified into wings for flying. Hind limbs are adapted for walking, swimming, perching or clasping. The unique features of mammals are the presence of mammary glands and hairs on the skin. They commonly exhibit viviparity.

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