Animals are the multicellular eukaryotic organisms which belong to the kingdom Animalia.

Criteria for classification 
Animals are classified on the basis of different features.
    •  Cellular or tissue level of body organization
    •  Body symmetry
    •  Type of body cavity called as coelom
    •  Presence or absence of segmentation
    •  Presence or absence of a backbone.

Classification of animal kingdom
Classification of the kingdom Animalia includes Invertebrata and Vertebrata.

Invertebrata:  It includes group of animals that do not possess a vertebral column. Invertebrata is classified into different phyla such as Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca and Echinodermata.
a)  Porifera are multicellular organisms exhibiting minimal level of tissue organization. They lack nervous system. Porifera get their name from two words, “pori,” meaning “holes,” and “fera,” meaning “bearing”.   Porifera includes Sycon, Spongilla and Euplectella.  e.g.  Sponges.

• Porifera are primitive, multicellular, asyymetrical, marineorganisms exhibiting minimal level of tissue organisation.
• Body is supported by skeleton made up of spicules or sponging fibres.
• Water vascular system is responsible for nutrition, respiration and excretion. 
• They lack nervous system. 
• Poriferans are hermaphrodites and reproduce both by asexual and sexual means of reproduction.
•  Fertilisation is internal. They exhibit indirect development. 
• Porifera gets its name from two words, “pori,” meaning “holes,” and “fera,” meaning “bearing”.
• Porifera includes Sycon, Spongilla and Euplectella.e.g.   Sponges.

b) Coelenterates are radially symmetrical organisms which live in marine habitat except for hydra.  Some are solitude and some are colonial.  Coelenterates get their name from two Greek words - “koilos,” meaning “hollow,” and “enteron,” meaning “intestine”.  e.g. Corals,  Hydra. 

• Coelenterates are aquatic, marine, sessile or free-swimming, diploblastic organisms.
• Coelenterates are radially symmetrical organisms exhibiting tissue level of organization.
• Some are solitude and some are colonial. 
• Some coelenterates possess skeleton made up of calcium carbonate. Coelenterates possess tentacles with cnidoblasts. Hence, the other name of the phylum is cnidaria.
• Coelenterates possess central gastrovascular cavity. They exhibit extracellular and intracellular digestion.
• Two basic body forms of coelenterates are polyp and medusa.
• Coelenterates get their name from two Greek words - “koilos,” meaning “hollow,” and “enteron,” meaning “intestine”.
• Coelenterates include Corals, Hydra etc.

c) Platyhelminthes are either free living or parasitic.  They are triploblastic animals. Platyhelminthes get their name from two Greek words - “platy,” meaning “flat,” and “helminthes,” meaning “worms”.  e.g. Planaria. 

• Blind sac plan includes animals having just a single pore opening to the exterior in their digestive system. Food is taken in and the undigested food leave the body through a common pore.
• Division of labour is exhibited by the cells. 
• Planaria is a flat worm. Planaria exhibits asexual mode of reproduction by regeneration process. It can regenerate in whatever plane it is cut. But the only condition is that the cut piece of the organism should have atleast 20 % of  neoblasts, the adult stem cells. If the longitudinal cut is given from the head to tip and just it is held ( not cut at the tail region), planaria develops two heads and can survive. Regeneration is also called as morphallaxis 

d) Nematoda are bilaterally symmetrical, triploblastic parasitic worms.  Nematoda get their name from two Greek words - “nema,” which means “thread,” and “ode,” which means “like”.  Nematodes can be free living or parasitic.  

• Nematoda are bilaterally symmetrical, triploblastic parasitic worms with a cylindrical body.
• Nematodes are unsegmented round worms which are considered to be pseudocoelomate organisms.
• Nematodes possess complete alimentary canal.
• Sexes are separate in nematodes and they exhibit sexual dimorphism. Eggs are protected by thick walls to overcome unfavourable conditions.
• e.g. Ascaris, Ancyclostoma, Dracunculus etc.

e) Annelida are bilaterally symmetrical, triploblastic, schizocoelomates with segmented body.  Annelida get their name from the Latin word “anellus,” which means "little ring".  Annelids are characterised by the presence of a circulatory system.  e.g. Earthworm. 

• Annelidans are segmented, aquatic or terrestrial, free living or parasitic in nature. 
• Annelidans are bilaterally symmetrical, triploblastic, schizocoelomates with segmented body. 
• They exhibit organ-system level of organization. 
• Locomotion is by longitudinal muscles, circular muscles and setae. 
• Closed type of circulatory system is seen. 
• Nephridia help in osmoregulation and excretion. 
• Nervous system comprises of nerve ganglia connected to nervecords.
• Annelids are both dioecious and monecious exhibiting sexual reproduction.
• Annelids are characterised by the presence of a circulatory system. 

f) Arthropoda are bilaterally symmetrical, triploblastic animals with true coelom. Arthropoda means “joint legs”.  This phylum gets its name from the Greek words arthron, meaning “joint”, and podos, meaning “foot”.   

• Arthropoda are bilaterally symmetrical, triploblastic animals with true coelom. 
• These are segemented organisms with organ system level of organisation.
• Presence of jointed appendages and chitinousexoskeleton.
• Arthropods respire through different structureslike gills, book gills, book lungs and trachea.
• They possess open circulatory sytem. Malpighian tubules help in excretion. 
• Sensory structures are present. 
• Sexes are separate. Fertilisation is internal. Mostly oviparous with direct or indirect development.  

g)  Molluscs are bilaterally symmetrical, triploblastic gastropods with reduced coelom. Mollusca is derived from a Latin word, which means “thin-shelled and soft”.

• Locomotion in molluscs is by means of a muscular foot.  
• Water molluscs breathe through their gills, while land molluscs have lungs
• Circulatory system in molluscs is open. 
• Snail is a small molluscan  Snail is protected by its external skeleton called as shell. Snails are found in wet places. Entire body except the foot lies inside the shell. When disturbed, snail even retracts the foot in to the shell. Snail moves with the help of single muscular foot. The under surface of the muscular foot is lubricated with mucus. The muscular foot glides over the surface. The rhythmic contractions and relaxations of this muscular foot brings about pulling action in the body. The secreted mucus also reduces the risk of injury from sharp objects. 

h) Echinodermata are triploblastic animals with true coelomic cavity.  Echinodermata are spiny skinned organisms which get their name from the Greek words “echinos,” meaning protective “spines,” and “derma,” meaning “skin”. Skeletons of echinoderms are hard calcium carbonates. They exhibit radial symmetry.

• Echinodermata are triploblastic animals with true coelomic cavity. Echinodermata are spiny skinned organisms which get their name from the Greek words “echinos,” meaning protective “spines,” and “derma,” meaning “skin”. Skeletons of echinoderms are hard calcium carbonates. 
• The organisms belonging to the Phylum Echinodermata are called as echinoderms and exhibit many peculiar characters.
• Echinoderms exhibit pentameral radial symmetry in the body organisation.
• Echinoderms have special type of water vascular system with tube feet.
• Echinoderms are eucoelomates.
• Echinoderms are deuterostomes.
• Larvae of  echinoderms are motile, bilaterally symmetrical in form. 
• Starfish, brittle star belong to this phylum.

Protochordata:  These are the organisms belonging to the phylum Chordata, and are primitive chordates.  Protochordates possess a notochord during their early stage of development.  The notochord is a long rod-like support that runs all along the back of the animal separating the nervous tissue from the gut. e.g.Balanoglossus, Herdmania.

Vertebrata: These are the most advanced group of animals with true vertebral column and strong endoskeleton.  Vertebrates are grouped into different classes based on bilateral symmetry, notochord, dorsal nerve cord, paired gill pouches, triploblastic, and coelomate. These classes are Pisces, Amphibia, Reptilia, Aves and Mammalia.
a) Pisces includes all fish. These are aquatic cold blooded organisms with a spindle-shaped body covered by scales.

• Fish are the aquatic vertebrates which respire through structures called as gills. This is called as branchial respiration.
• Gills are present on either side of the head and are supplied by rich blood vessels. Fish obtain oxygen dissolved in water. During respiration, water enters the body through mouth, passes through gills and comes out of the operculum.
• Exchange of gases takes place in the gills of fish supplied by numerous blood vessels. They accept oxygen into the body and expel out carbon dioxide.
• Fish is a lower vertebrate with a two chambered heart exhibiting single mode of circulation. Impure blood is pumped to gills for oxygenation. This oxygenated blood from gills is supplied directly to the body tissues without sending to the heart. Blood passes through the heart only once hence called as single circulation.
• Skeleton may be cartilaginous or bony in nature.
• Fish exhibits locomotion by the process of swimming. Muscles and bones bring about locomotion in fish. Fish swim with the help of their fins of which tail fin acts as a rudder. The tail flaps from side to side to make them swim in the right direction.  The undulating movements of the tail also helps the  fish move by bending their bodies from one side to another in quick succession. This produces a thrust that helps it move forward.
• Fish reproduce by laying eggs.

b) Amphibians are cold-blooded animals. Amphibians can live both on land and in water. They are the first vertebrates to have four limbs, each with five digits and are called tetrapods. 

• Amphibians are the only organisms which exhibit cutaneous, branchial and pulmonary respiration in during different stages of their life cycle. Adult frogs can breathe through their skin. Skin is moist and slimy helping the animal to respire through skin under water as well as on land. This is termed to be cutaneous respiration. Adult frogs while on land respire through lungs. Lungs are the respiratory organs which help in exchange of gases. This is termed to be pulmonary respiration.
• Amphibians have a three chambered heart which includes two atria and one ventricle. Here both oxygenated and deoxygenated blood gets mixed up with each other. Double circulation is seen in the organisms possessing this type of heart. Lungs are involved in purifying the blood.
• Amphibians exhibit camouflage.The phenomenon of colour changing in frogs was brought about by different factors.
• Amphibians mostly jump and walk with their limbs when on land and can also swim in water. 
• Tails are almost absent in adult amphibians. 
• Amphibians reproduce by laying eggs and exhibit external fertilisation. 
• Tadpoles, the larval stages of frog live in water. They respire through their gills as they do not possess well developed lungs. This is termed to be branchial respiration .Tadpoles can thrive only in water. Tadpoles possess long tails.

c) Reptilia are cold-blooded animals. They have four limbs with five fingers or toes each, and hence, are called pentadactyle tetrapods. 

• Respiration is through lungs. Lungs are involved in purifying the blood. 
• Hearts are three-chambered except for crocodiles. It is a three chambered heart which includes two atria and one ventricle. Here both oxygenated and deoxygenated blood gets mixed up with each other. Double circulation is seen in the organisms possessing this type of heart.
• They have cloaca, a single opening to pass out, urine, faeces, sperms or eggs.
• Reptiles also reproduce by laying eggs.  
• Some reptiles also have the capability of reproducing by parthenogenesis.   

d) Aves are warm-blooded animals. Birds bear three clawless digits.

• Their hind limbs are strong and are developed for walking. Fore limbs are modified into wings which help them in flight.
• Bones are hollow. Body is covered by feathers.
• Respiration is through lungs.
• They have four-chambered heart.
• Birds can generate lot of heat through their metabolic reactions. Only little amount of food what they eat is converted into body mass. Rest of it is utilised inorder to generate heat to maintain body temperature constant.
• Birds are oviparous and lay eggs which hatch into chicks. 

• Coelom in mammals is divided into peritoneal, pleural and pericardial cavities. 
• Breathing is through lungs.
• Four-chambered heart is present. Mammalian heart is made up of four chambers which include two atria and two ventricles. The organisms with this type of heart exhibit double circulation. The deoxygenated blood is completely separated from the oxygenated blood completely. The atria and ventricles are separated by many types of valves which prevent the mixing up of pure blood from that of impure blood.
• Mammals give birth to young ones through different modes. Mammals like platypus lay eggs. Mammals have milk-producing glands called as mammary glands to nourish their young ones.