1. Classification of Animals:

The animals can be classified on the following basis:

(i) Levels of Organisation

(a) Cellular Level: In case of cellular level organisation, a single cell is responsible for all the metabolic activities.

Cellular level organisation is present in unicellular animals and some of the multicellular animals.

(b) Tissue Level: In case of tissue level organisation, a group of cells is responsible for all the metabolic activities, e.g., Coelenterates.

(c) Organ Level: In case of organ level organisation, some specialised organs are present for some specific functions, e.g., Platyhelminthes.

(d) Organ System Level: In organ system level organisation, complex organ systems are present for various functions, e.g., Mollusca, Chordate.

(ii) Symmetry

(a) Some of the animals are almost asymmetrical. Their body cannot be divided into two equal halves from any plane, e.g., sponges.

(b) Radial Symmetry: In case of radial symmetry, any plane passing through the central axis divides the body into two identical halves, e.g., Coelenterates, Ctenophores, Echinoderms, etc.

(c) Bilateral Symmetry: In case of bilateral symmetry, the body can be divided into two identical halves only through a single plane, e.g., Annelida, Arthropoda, etc.

Diplobalstic And Triploblastic Organisation
When the cells are arranged in two embryonic layers, the animal is called a diploblastic animal (ectoderm and endoderm).

(d) Mesoglea which is an undifferentiated layer is present between the ectoderm and endoderm. Example: Coelenterates.

(iii) When the cells are arranged in three embryonic layers, the animal is called triploblastic animal.

(a) The three layers are ectoderm, mesoderm, and endoderm. Examples: Platyhelminthes to Chordates.

(iv) Coelom is the body cavity, which is lined by mesoderm is called coelom.

(a) Coelomates (Eucoelomates): If coelom is present, the animal is called coelomate, e.g., Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates and Chordates.

(b) Pseudoceolomates: If the body cavity is not lined by mesoderm but the mesoderm is present as scattered pouches in between the ectoderm and endoderm, the animal is called pseudo coelomate, e.g., Aschelminthes.

(v) Segmentation / Metamerism

(a) The body of some animals is externally and internally divided into segments with serial repetition of at least some organs. For example, the body of the earthworm shows metameric segmentation.

(b) This phenomenon is called metamerism.

(vi) Notochord

(a) Notochord is a mesodermally derived rod-like structure. It is formed on the dorsal side during embryonic development in some animals.

(b) If notochord is present, then the animal comes under chordates.

2. Phylum Porifera (Gr: porus - pore; ferre - to bear):

(i) They are also referred to as sponges.

(ii) Sponges are primitive, multicellular organisms.

(iii) They exhibit cellular grade of organisation.

(iv) They are diploblastic, asymmetrical, or radial.

(v) The inner gastral layer encloses a large cavity called spongocoel which opens to the exterior by the mouth opening called osculum.

(vi) The surface of the body is pierced by large number of minute openings called ostia.

(vii) Collar cells: The inner gastral layer is made up of collar cells or choanocytes.

(viii) Skeleton: Sponges possess an internal skeleton in the form of needle like structure called spicules or in the form of fibres called spongin fibres, made up of a protein like spongin.

(ix) Asexual reproduction takes place by budding.

(x) The sexual reproduction takes place by production of gametes. Sexes are not separate (Hermaphrodite)

(xi) Embryo Development: Indirect development and fertilisation is external.

(xii) Example: Sycon (Scypha), Spongilla (Fresh water sponge), Euplectella (Venus flower Basket) and Euspongia (Bath sponge)

3. Phylum Coelenterata (Gr: coel - cavity; enteron - intestine) or Cnidaria:

(i) They are characterised by the presence of stinging cells called Cnidoblast and a cavity called coelenterons justifying the name Cnidaria and Coelenterata.

(ii) They are exclusively aquatic and marine.

(iii) They are radially symmetrical, diplobalstic animals, Tissue level of organisation.

(iv) Cnidoblast: The ectoderm of Coelenterates is provided with special type of cells called cnidoblasts or stinging cells, which are used for defence are paralyzing the prey. They are found in large numbers around the mouth and over the tentacles.

(v) Body form: It is of two types:

(a) Polyp: Cylindrical shape like in Hydra.

(b) Medusae: Umbrella shaped like in Jelly Fish.

(vi) Asexual reproduction takes place by budding or spores.

(vii) The sexual reproduction takes place by production of gametes. Sexes are not separate.

(viii) Embryo Development: Indirect development and Fertilisation is external.

(ix) Some exhibit alternation of generation, i.e., metagenesis, a phenomenon where the asexual generation (polyp) alternates with the sexual generation (medusa).

(x) Some common examples are Hydra, Aurelia (Jelly Fish- Medusae form), Adamsia (Sea anemone- Polyp form), Pennatula (Sea pen), Gorgonia (Sea Fan). Meandrina (Brain coral), Physalia (Portuguese man-of-war), Obelia (Both polyp and medusa form)

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4. Phylum – Ctenophora:

(i) These are commonly known as sea walnuts or comb jellies.

(ii) These are exclusively marine animals.

(iii) The body is radially symmetrical and diploblastic.

(iv) Tissue level organisation is present.

(v) There are eight eternal rows of ciliated comb plates on the body. These rows help in locomotion.

(vi) Bioluminescence is well-developed in ctenophores.

(vii) These are hermaphrodite animals. Reproduction is only through sexual means.

(viii) Fertilisation is external and development is indirect.

(ix) Example: Pleurobrachia and Ctenoplana.

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5. Phylum: Platyhelminthes (Gr: platy - flat: helminthes - worms):

(i) They are characterised by flat body justifying the name Platyhelminthes.

(ii) Most of them are free living, a few of them are end parasitic.

(iii) They are bilaterally symmetrical, triploblastic, acoelomate animals. Organ level of organisation

(iv) Body shape: They have a dorsi-ventrally flattened body.

(v) Excretory system: The flames cells absorb the excretory product from the body. They empty it into the excretory canals which is eliminated out through the excretory pore. Some members like Planaria possess high regeneration capacity.

(vi) They are mostly hermaphrodites. The fertilisation is internal.

(vii) The development is indirect with larval stages.

(viii) Example: Tapeworm, Liver fluke

6. Phylum: Aschelminthes or Nemathelminthes (Gr: askos - sac-like; heliums - worm):

(i) They are characterised by thread like bodies justifying the name Nematoda.

(ii) They are bilaterally symmetrical, triploblastic, pseudo coelomate animals. Organ system level of organisation.

(iii) Body shape: They are cylindrical, vermiform (worm like), unsegmented.

(iv) Digestion system: Digestive tract is complete with mouth and anus at the opposite ends. There is presence of muscular pharynx.

(v) Excretory system: Longitudinal excretory canals or tubes which open outside by means of an excretory pore.

(vi) Sexes are separate. Males are smaller than females and exhibit sexual dimorphism.

(vii) Fertilisation is internal. Development direct or indirect.

(viii) Example: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm)

7. Phylum: Annelida Or Annulata (L: annullus –ring):

(i) They are all bilaterally symmetrical, triploblastic and coelomate animals.

(ii) Body is divided into several segments or metameres. The successive segments of the body are separated externally by ring like constrictions called annuli.

(iii) In earthworms, locomotion is by contraction and relaxation of the longitudinal and circular muscles.

(iv) In Nereis, it is by hallow, unjointed lateral outgrowths called parapodia.

(v) Circulatory system: It is of closed type. Blood is coloured due to the dissolved respiratory pigment haemoglobin in the plasma. Heart is seen for the first time.

(vi) Nervous system: Arises from the double ventral nerve cord which extends along the ventral body wall. It bears segmental ganglia in each segment.

(vii) Excretory system: It includes segmentally arranged tubules called nephridia.

(viii) Most of the annelids are bisexual (hermaphrodite or monoecious) i.e,. testes and ovaries are developed within a single individual.

(ix) Development may be direct or indirect.

(x) Example: Nereis, Pheretima (Earthworm), Hirudinaria (Blood sucking Leech)

8. Phylum: Arthropoda (Gr: arthros - joint; podos - appendages):

(i) This is the largest phylum in the animal kingdom.

(ii) It constitutes about $80 %$ of all known animal species in the animal kingdom.

(iii) They are bilaterally symmetrical, triploblastic, coelomate animals.

(iv) Body covering: It is in the form of a chitinous exoskeleton.

(v) Body division: It is differentiated into three distinct regions namely head, thorax, and abdomen. In some cases, head is fused with thorax and is called cephalothorax.

(vi) Body cavity: It is called haemocoel and contains colourless blood (haemolymph).

(vii) The respiratory organ differs in different arthropods. In aquatic forms, it is gills, in terrestrial forms, it is book lungs or tracheae.

(viii) Circulatory system: It is of open type. It consists of a dorsal heart and colourless blood.

(ix) Nervous system: Arises from the double ventral nerve cord which extends along the ventral body wall. It bears segmental ganglia in each segment.

(x) In most arthropods, the excretory structures are the Malpighian tubules.

(xi) Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.

(xii) Sexes are separate. Fertilisation is internal. Development is direct or indirect with larval stage.

(xiii) Example: (i) Economically important insects - Apis, Bombyx, Laccifer (ii) Vector - Anopheles, Culex, Aedes. (iii) Gregarious pest - Locusta (Locust). (iv) Living fossil - Limulus (king crab)

9. Phylum: Mollusca (L: Mollis-soft):

(i) This phylum is the second largest phylum among invertebrates next to Arthropoda. They are characterised by soft body justifying them name Mollusca.

(ii) They are bilaterally symmetrical triploblastic and coelomate animals.

(iii) Body is soft an unsegmented and is differentiated into $3$ distinct region namely head, visceral mass, and foot.

(iv) Mantle: The visceral hump is covered by a fleshy fold of skin called mantle. The space between the mantle and the visceral hump is called the mantle cavity. The mantle secretes the shell.

(v) Shell: It is the protective exoskeleton covering the soft body. It is calcareous in nature.

(vi) Buccal cavity contains a characteristic rasping tongue with transverse rows of teeth called radula.

(vii) Respiratory system: The respiratory organ varies according to their habitat. In aquatic forms it is ctenidia (gills). In terrestrial forms, it is in the form of pulmonary sac or lung.

(viii) Circulatory system: It is open type. Heart is present on dorsal side. Blood as respiratory pigment haemocyanin.

(ix) Nervous system: It includes $3$ paired ganglia.

(x) Excretory system: It includes $1$ or $2$ pairs of sac-like metanephridia (protonephridia).

(xi) Sexes are separate. Fertilisation is internal or external. Development may be direct or indirect with larval stages.

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

10. Phylum: Echinodermata (Gr: echino - hedgehog: derma - skin):

(xiii) They are characterised by spiny skin hence justifying the name Echinodermata.

(xiv) They are triploblastic, eucoleomate, unsegmented animals.

(xv) Body symmetry: Adult echinoderms have a combination of radial symmetry. Here the body can be divided into identical halves and along five planes, and it is also called penta-radial symmetry or pentamerous radial symmetry.

(xvi) Water vascular system: It includes a system of canals within the body in which the sea water circulates.

(a) The water vascular system includes a ring canal in the central part of the body.

(b) From which arise $5$ tube (radial) canals running at the centre of each arm.

(d) Tube feet are essentially used for creeping or crawling type of locomotion.

(xvii) Sexes are separate. No sexual dimorphism. Fertilisation is mostly external.

(xviii) Echinoderms are known for autotomy and regeneration.

(xix) Example: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star)

11. Phylum Hemichordata: (Hemi - half):

(i) This phylum has a small group of animals which look like worms. They are generally marine.

(ii) Sister phylum is Echinodermata. Connecting link between chordates and non-chordates.

(iii) Notochord is present only in half of the body. Hence, called Hemichordate.

(iv) Organ system level organisation is present.

(v) Body is bilaterally symmetrical, triploblastic and coelomate.

(vi) The body is cylindrical and consists of anterior proboscis, a collar, and a long trunk.

(vii) Proboscis gland is the excretory organ.

(viii) These are dioecious animals. Fertilisation is external and development is indirect.

(ix) Example: Balanoglossus and Saccoglossus.

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.
Coelenterata (Cnidaria)	Tissue	Radial	Absent	Absent	Incomplete	Absent	Absent	Cnidoblasts present.
Ctenophora	Tissue	Radial	Absent	Absent	Incomplete	Absent	Absent	Comb plates for locomotion.
Platyhelminthes	Organ & Organ system	Bilateral	Absent	Absent	Incomplete	Absent	Absent	Flat body, suckers.
Aschelminthes	Organ-system	Bilateral	Pseudo coelomate	Absent	Complete	Absent	Absent	Often worm-shaped, elongated.
Annelida	Organ-system	Bilateral	coelomate	Present	Complete	Present	Absent	Body segmentation like rings.
Arthropoda	Organ-system	Bilateral	coelomate	Present	Complete	Present	Present	Exoskeleton of cuticle, jointed appendages.
Mollusca	Organ-system	Bilateral	coelomate	Present	Complete	Present	Present	External skeleton of shell usually present.
Echinodermata	Organ-system	Radial	coelomate	Present	Complete	Present	Present	Water vascular system, radial symmetry.
Hemichordata	Organ-system	Bilateral	coelomate	Present	Complete	Present	Present	Worm-like with proboscis, collar and trunk.

12. Chordata:

(i) Fundamental characters of phylum Chordata

(a) Presences of notochord: They are characterised by the presence of rod like supporting notochord extending throughout the length of the body.

(b) Presence of dorsal tubular nerve cord: They are characterised by the presence of dorsal, nerve cord lying above the notochord.

(c) Presence of paired pharyngeal gill slits: They are characterised by the presence of paired pharyngeal gill slits at some stage of their lifetime.

(d) Presences of ventral heart: They are characterised by the presences of a ventral heart i.e., heart is situated on the ventral side of the alimentary canal in the body cavity.

(e) Presence of post anal tail.

(ii) Body cavity: They are coelomates, Triploblastic body, and bilaterally symmetrical animals.

(iii) Phylum Chordata is divided into three sub-phyla:

(a) Urochordata (Gr: aura - tail): Here, the notochord is confined only to the tall region of the larva.

It is represented by primitive chordates like Ascidia, Herdmania, commonly called ‘sea squirt’.

(b) Cephalochordate (Gr: cephlaon - head): Here, the notochord is well developed extending from anterior to posterior region of the body.

Anteriorly, it extends beyond the nerve cord. Hence the name cephalochordate.
It includes primitive chordates like Amphioxus.
Since the above two subphyla include primitive chordates it is also called Protochordata.

(iv) Vertebrata or Craniata: Here, the notochord is replaced by vertebral column.

(v) The Sub-phylum Vertebrata is divided into two divisions namely:

(a) Division I- Agnatha: Absence of jaws, monorhina (single nostril).

They are called jawless vertebrates. One of the most primitive type of vertebrates.
The only existing living class of Agnatha is Cyclostomata.

(b) Division II-Gnathostomata: Presence of jaws, amphirhina (two nostrils).

They are called as jawed vertebrata. It includes advanced vertebrates.

13. Class – Cyclostomata (Cyclo - round or circular; stoma - mouth):

(i) All the existing members of the class Cyclostomata are ectoparasites on some fishes.

(ii) The body is devoid of scales and paired fins.

(iii) There are $6 - 15$ pairs of gill slits for respiration.

(iv) A sucking and circular mouth is present.

(v) Cranium and vertebral column are composed of cartilage.

(vi) They are marine animals but migrate to freshwater for spawning. They die within a few days of spawning. The larvae return to the ocean after metamorphosis.

(vii) Example: Petromyzon (Lamprey) and Myxine (Hagfish).

Comparative Characteristics Of Various Classes In Gnatostomata:

Characteristics	Superclass Pisces	Class Amphibia	Class Reptilia	Class Aves	Class Mammalia
Habitat	Aquatic	Amphibious	Terrestrial	Terrestrial	Terrestrial
Exoskeleton	Scales (dermal)	Usually absent	Scales (epidermal)	Feathers	Hair
Respiratory organs	Gills	Lungs (in adult), Gills (in larvae)	Lungs	Lungs	Lungs
Locomotor organs	Fins	Limbs	Limbs	Limbs	Limbs
Heart	Two chambers	Three chambers	Three chambers	Four chambers	Four chambers
Cranial Nerves	10 pairs	10 pairs	12 pairs	12 pairs	12 pairs
Mode of reproduction	Oviparous, Viviparous	Oviparous	Oviparous (Cleidoic eggs)	Oviparous (Cleidoic eggs)	Mostly Viviparous
Fertilisation	External, Internal	Internal	External	Internal	Internal
Examples	Cartilagenous Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray), electric organs (e.g., Torpedo), poison sting (e.g., Trygon) Bony Fishes Marine – Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater – Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium – Betta (Fighting fish), Pterophyllum (Angel fish)	Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).	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)	Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture)	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)

Difference Between Chordates And Non- Chordates (Invertebrates)

Chordates	Non-chordates
1. They possess a stiff elastic notochord or vertebral column forming the body axis.	1. The notochord or vertebral column is absent.
2. They possess dorsal hollow tubular nerve cord.	2. The nerve chord is ventral and solid
3. They possess paired pharyngeal gill clefts at some stage in their life history.	3. The gills may be present or absent. When present do not arise from pharynx
4. They possess ventral heart.	4. They possess dorsal heart
5. They are all provided with tail, which represents the post anal region of the body.	5. Tail may be absent or present. When present need not represent the post anal region of the body.
6. They are coelomates	6. They may be acolemates, pseudo coelomates and eucoelmates.
7. They may be acellular, diploblastic and triploblastic. 8. They show bilateral, radial, biradial and asymmetric nature	7. They may be acellular, diploblastic and triploblastic. 8. They show bilateral, radial, biradial and asymmetric nature

Difference Between two classes of Superclass - Pisces:

Class: Chondrichthyes (Gr: chondros –cartilage ; ichthiys –fish)	Class : Osteichthyes (Gr: Osteos-bone)
This class includes cartilaginous fishes like sharks, and rays.	This class includes bony fishes like Mackerel, sardines, sear fish, sea horse, pipe fish, Clarias, anabas, freshwater aquarium fishes etc.
1. They are marine in distribution.	1. They are fresh water as well as marine in distribution.
2. The Endoskeleton is cartilaginous	2. The Endoskeleton is bony.
3. The scales are embedded in the skin. They are microscopic placoid scales.	3. The scales over the skin are large, they may be cycloid or ctenoid.
4. The gill slits from $5$ pairs to $7$ pairs, they are naked (i.e; without the operculum).	4. The gill slits vary from $4$ pairs or less than $4$ pairs. They are covered by an operculum.
5. The caudal fin is heterocercal in nature.	5. The caudal fin is either homocercal or diphycercal in nature.
6. The mouth and nostril are ventral in position	6. The mouth and nostrils are terminal in position.
7. The alimentary canal opens outside by cloacal aperture.	7. The alimentary canal opens outside by anus.
8. The males are provided with a pair of claspers (copulatory organs).	8. The males are not provided with claspers.
9. Viviparous development. Fertilisation is internal.	9. Oviparous development. Fertilisation is external.
10. Air bladder is absent.	10. Air bladder is present.

Important Questions on Animal Kingdom

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