Classification in Animals

Animals are the eukaryotic, multicellular and heterotrophic organisms. Their cells do not have cell walls and most of them are mobile. Kingdom-Animalia is thus classified on the basis of the extent and type of body design.

1. Porifera
[Gk. porous–  pore; Ferre– to bear]

The characteristics possessed by poriferans are as follows:

(i) Animals that belong to this phylum are commonly called sponges.
(ii) These have minimal differentiation and division into tissues.
(iii) These organisms are acoelomate  (body cavity absent) with cellular level of body organisation.
(iv) They are mostly marine,  but few species are found in freshwater like Spongilla.
(v) They may be vase-like, rounded,  sac-like or branched.
(vi) They consist of innumerable holes or pores all over the body which lead to a canal system. This system helps in the circulation of water throughout the body to bring in food and oxygen.
(vii) Only the young forms or larvae are motile.  In the adult stage, they attach themselves to submerged solid objects and become stationary (sessile).
(viii) Their body is covered with a hard outside layer or skeleton.   

E.g.  Spongilla (freshwater sponge),  Sycon (Scypha), Euplectella (Venus’ flower basket), etc.


2. Coelenterata (Cnidaria)
[Gk. koilos – hollow; enteron– gut]

The characteristics possessed by coelenterates  are as follows:

(i) These animals show better body design differentiation than porifera.
(ii) These organisms are multicellular and acoelomate.  They have tissue level of organisation with a distinct division of labour.
(iii) They live in water and have tentacles (appendages) for locomotion.
(iv) Coelenterates have a central body cavity called coelenteron or gastrovascular cavity.
(v) The body is made up of two layers of cells. One makes up cells on the outside of the body and the other makes the inner lining of the body. Therefore,  these are called diploblastic animals.
(vi) The body of these animals shows radial symmetry.  In this type of symmetry, the body design is arranged in a circular pattern with a centre point.
(vii) Some species are colonial   (corals) while other have a solitary lifespan (Hydra).

E.g.–  Hydra, Metridium  (sea anemone), Aurelia(jellyfish) are examples of freshwater cnidarians.


3. Platyhelminthes
[(Gk. platys– flat; helmin– worm]

The    characteristics possessed by Platyhelminthes are as follows:

(i) These are triploblastic (body made up of three layers of cells), dorsoventrally flattened,  (flattened from the back through to the abdomen) leaf-like animals. They have tissue level of organisation. So,  these are also called flatworms.
(ii) They have bilateral symmetry i.e. the left and right halves of the body has the same design.
(iii) There is no true internal body cavity or coelom.
(iv) They are either free-living or parasitic animals  (living in the bodies of other animals) like a liver fluke and some are free-living like Planaria.

E.g.– Taenia solimn (pork tapeworm),  Dugesia (Planaria), Fasciola (liver fluke), etc.


4. Nematoda
(Nemathelminthes or Aschelminthes)
[Gk. nema– thread; helmin– worm]

The characteristics possessed by nematodes  are as follows:

(i) These are triploblastic, cylindrical worms with pseudocoelom  (no true body cavity or coelom).
(ii) Their body is bilaterally symmetrical.
(iii) They are parasitic worms causing diseases such as elephantiasis (filarial worms). These worms are also found in the intestines (roundworms or pinworms).
(iv) These are called roundworms because they appear circular in cross-section and are unsegmented.
(v) These are the first animals that have complete and straight alimentary canal. It consists of mouth and anus.

E.g–  Ascaris (giant intestinal  roundworm), Ancylostoma (hookworm), Wuchereria bancrofti (filarial worm), Enterobius (pinworm), etc.


5. Annelida
[Lt. annulus– Little ring; lidos– form]

The characteristics possessed by annelids are as follows:

(i) These are triploblastic animals having bilateral symmetry.
(ii) They are elongated, vermiform, cylindrical or dorsoventrally flattened organisms.
(iii) These animals are found in a variety of habitats i.e. freshwater, marine water as well as on land.
(iv) Annelids are the first animals to develop true body cavity or coelom, i.e. they are coelomates.
(v) There is extensive organ differentiation.  This differentiation occurs in a segmental fashion.   The segments are lined up one after the other from head to tail (i.e. metameric segmentation).
(vi) Locomotion is done with the segmentally,  arranged, paired, lateral appendages called parapodia or chitinous setae or chaetae.

E.g.– Nereis (sandworm), Hirudinaria (Indian cattle leech that sucks blood from its host), Pheretima Posthuma (earthworm), etc.


6. Arthropoda
[Gk. arthron– jointed; podos– foot]

The characteristics possessed by arthropods are as follows:

(i) Arthropoda is the largest group of animals.  These are found everywhere on earth.
(ii) These are triploblastic, bilaterally symmetrical and segmented animals.
(iii) Body is segmented and is grouped into three regions, i.e. head, thorax and abdomen.
(iv) Body consists of exoskeleton. It is lightweight, tough and made up of chitin.
(v) Body segments of these animals (somite) usually bear paired, lateral and jointed legs or appendages.
(vi) Body cavity or coelomic cavity is filled with blood (haemocoel).  The open circulatory system is present, i.e. blood does not flow in blood vessels.
(vii) In many arthropods, compound eyes are present with mosaic vision on the head.

E.g.–  Pariplaneta  (cockroach), Palaemon  (prawn), Palamnaeus (scorpion),  Aranea (spider),  Cancer (true crab), Scolopendra (centipede), butterflies, Musca (houseflies), etc.


7. Mollusca
(Lt. molluscus– soft or thin-shelled)

The characteristics possessed  by molluscs are as follows:

(i) Body of these animals is soft and has bilateral symmetry.
(ii) Little segmentation is present and animals are without appendages.
(iii) The coelomic cavity is reduced. Oysters and dans of this group produce pearls.
(vi) Open circulatory system is present. Kidney-like structures or a pair of metanephridia is present for excretion.
(v) The body is divided into an anterior head, a ventral muscular foot and a dorsal visceral mass.
(vi) Animals perform locomotion through their muscular foot.

E.g.– Pila (apple snail), Unio (freshwater mussel), Chiton, Octopus, etc.


8. Echinodermata  

[Gk. echinos– spine or hedgehog;  derma-skin]

The characteristics possessed by echinoderms are as follows:

(i) These are simple spiny-skinned animals with the star-like, spherical or elongated body.
(ii) These are triploblastic, coelomate and unsegmented. Their body has radial symmetry in adults, but bilateral in larvae.
(iii) These are exclusively free-living marine animals.
(v) They also have a peculiar water-driven tube system (ambulacral).   They possess tube feet (podia) for locomotion and exchange of gases.
(vi) Their digestive system is usually complete. Well-developed excretory organs are absent.
(vii) Body wall is covered with spiny hard calcareous (calcium carbonate) plates (ossicles). They form a rigid or flexible endoskeleton.

E.g. – Antedon (feather star), Holothuria (sea cucumber), Echinus (sea urchin), Asterias (star fish or star), etc.


9. Chordata

[Lt. chorda– string]

The characteristics possessed by chordates are as follows:

(i) Body of these animals is triploblastic, coelomate with bilateral symmetry.
(ii) All of them have a notochord  (a new feature of body design). Notochord is a long rod-like support structure that runs along the back of the animal. It separates the nervous tissue from the gut. It provides sire for muscle attachment.
(iii) They have dorsal nerve cord and paired gill pouches.
(iv) A post-anal rail present at some stage of life.
(v) A ventral heart is present with dorsal and ventral blood vessels and closed blood vascular system.
(vi) The complete digestive system is present.

Chordata is divided into two groups or sub-phylum:

(i) Protochordata (Acraniata)
(i) Vertebrata (Craniata)

[Gk. proto– first; chordata– notochord]

The  characteristics possessed by,   protochordates are as follows:

(i) They are triploblastic, have a bilaterally symmetrical body and a coelom.
(ii) They show the presence of notochord in some stages of life, i.e. tail of young larvae.
(iii) These are mostly marine animals.
(iv) Respiration occurs through gill slits.

E.g. – Balanoglossus, Herdmania, Amphioxus, etc.


The characteristics possessed by vertebrates are as follows: –

(i) They have a  true vertebral column and internal skeleton. It allows completely different distribution of muscle attachment points that help in movement.
(ii) Their body is bilaterally symmetrical, triploblastic, coelomic and segmented.
(iii) In these animals, complex differentiation of body tissues and organs is present.
(iv) They have a dorsal nerve cord and paired gill pouches.

Thus, vertebrates are considered the most advanced group of animals. These are further grouped into five classes:

(a)  Pisces
(b)  Amphibia
(c)  Reptilia
(d)  Aves
(e)  Mammalia

(i)Class-Pisces (Fishes) [Lt. Piscis– fish]

The characteristics possessed by Pisces are as follows:

(i) These are exclusively aquatic animals.
(ii) They have a streamlined body covered with scales. It ends in a muscular tail (which helps in movement).
(iii) Fins are present, but limbs are absent.
(iv) Respiration occurs through gills by taking oxygen dissolved in water.
(v) They are cold-blooded    (ectothermic) and have a two-chambered heart.
(vi) Most of them lay eggs, e.g. anglerfish, lionfish, rohu, Scoliodon;
(vii) Pisces are of two types, based on the nature of their skeleton,   i.e. Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony fishes).


(ii) Class- Amphibia [Gk. amphi– double; bios-life]

The characteristics possessed by amphibians  are as follows:

(i) In evolutionary terms, these are the first group among the chordates to live on land.
(ii) They are amphibious in nature, viz. can live on land as well as in water.
(iii) They lack scales and have mucus glands in their skin which secrete mucus that keep the skin moist.
(iv) Amphibians are cold-blooded and have a three-chambered heart.
(v) They have a distinct head and trunk.  Neck and tail may or may not be present.
(vi) Respiration can be either by gills in larva (in water) or by lungs in most adults (on land).
(vii) Generally, require water for the completion of their life cycle as the eggs are laid in water and the larvae are aquatic.
(viii) Limbs tetrapods (four-limbed) pentadactyl type (five-fingered). The digits do not possess claws.
(ix) They lay eggs (oviparous).

E.g.–   Rana (frog), Bufo (toad), Salamandra (salamander), Hyla (tree-frog), etc.


(iii) Class – Reptilia [Lt. reptare– to crawl]

The characteristics  possessed by reptiles are as follows:

(i) These animals possess dry skin, with scales and mostly live in warmer regions. They are cold-blooded.
(ii) These are the first class of terrestrial animals.
(iii) They lay eggs with tough covering on land, unlike amphibians that lay eggs in water.
(iv) Lungs are the respiratory organs.
(v) Reptiles bear two pairs of pentadactyl (five digits) limbs (snakes and some lizards have no limbs).
(vi) Most of them have three-chambered heart except crocodile, which has a four-chambered heart.

E.g.–  Chelone (turtle),  Chamaeleon, Naja(cobra), Hemidactylus (house wall lizard), Testudo (tortoise), Draco (flying lizard), etc.


(iv) Class- Aves [Lt. avis– bird]

The characteristics possessed by Aves are as follows:

(i) These are warm-blooded animals having a four-chambered heart.
(ii) Their body is divisible into head, neck, trunk and tail.
(iii) These are flying   animals, having an exoskeleton (outside covering)  of feathers.
(iv) They have forelimbs modified   into Passer (Sparrow) wings. Jaws are modified into horny toothless beak.
(v) Bones are light in weight because of air spaces (pneumatic bones) and body is streamlined to reduce air resistance during flight.
(vi) They breathe through lungs
(vii) Birds lay eggs (outside water). Their egg possesses a large amount of yolk and the shell covering the yolk is hard and calcareous.

E.g.– Ciconia ciconia (white stork),  Aythya fuligula (male tufted duck), Struthio camelus (Ostrich), pigeon, crow, sparrow, hummingbird (smallest).
Note:   Penguin. Ostrich (largest bird) and Kiwi are a flightless bird, i.e., they cannot fly.


(v) Class – Mammalia [Lt. mamma– breast]

The characteristics possessed by mammals are as follows:

(i) These are warm-blooded animals which have a four-chambered heart.
(ii) Females have mammary glands for the production of milk to nourish the young ones. Therefore,  called mammals.
(iii) These are known to be the most advanced group of animals with a very well-developed brain.
(iv) Sweat glands and oil glands are present in their skin and serve as lubricants and de-toxicants of the body. Skin is covered with hair.
(v) Mammalia is the first class to mark the presence of ear pinna (external fleshy ear).
(vi) Limbs are adapted for walking, running, swimming, burrowing and even flying (bats).
(vii) Most mammals produce live young ones (i.e. they are viviparous).  Some like Platypus and Echidna are oviparous. Kangaroos give birth to poorly developed young ones.
(viii) Respiration is through the lungs only.

E.g. Rattus  (rat), Homo sapiens (human), Felis (cat), Bat, Canis (dog), chimpanzee, Macropus (kangaroo), whale, dolphins (aquatic mammals).

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