Key Notes: Animal Kingdom | Additional Study Material for NEET PDF Download

1. Levels of Organisation

• All members of animalia are multicellular but all of them do not exhibit the same pattern of organisation of cells.

2. A complete digestive system has two openings, mouth and anus.

• Platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus, and is hence called incomplete.

3. Circulatory system may be of two types

• Open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and
• Closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).

4. Symmetry

• Sponges are mostly asymmetrical, i.e., any plane does not divide them into equal halves.
• When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry. Coelenterates, ctenophores and echinoderms have this kind of body plan
• Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.

5. Diploblastic and Triploblastic Organisation

• Two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., coelenterates.
  — An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm
• Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (Platyhelminthes to chordates).

6. Coelom

• Between the body wall and the gut wall → The body cavity
• Lined by mesoderm
• Animals possessing coelom → coelomates,
  — e.g., annelids, molluscs, arthropods, echinoderms, hemichordates and chordates
• if the body cavity is not lined by mesoderm, (the mesoderm is present as scattered pouches) → Pseudocoelom
  — The animals possessing them are called pseudocoelomates, → e.g., Aschelminthes
• The animals in which the body cavity is absent are called acoelomates, → e.g., Platyhelminthes

7. Segmentation

• In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs.
• This pattern called metameric segmentation
• Phenomenon is known as metamerism.
  — For example, in earthworm

8. Notochord

• Mesodermally derived rod-like structure
• Formed on the dorsal side during embryonic development
• Animals with notochord are called chordates
• Animals which do not form this structure are called non-chordates, e.g., porifera to echinoderms.

9. Phylum – Porifera

• Commonly known as SPONGES.
• Generally marine
• Mostly ASYMMETRICAL ANIMALS
• Primitive multicellular animals
• CELLULAR LEVEL OF ORGANISATION.  
• Sponges have a water transport or canal system.
  (i) Water enters through minute pores (ostia) in the body wall
  (ii) A central cavity, SPONGOCOEL,
  (iii) Water goes out through the osculum.
• This pathway of water transport is helpful in food gathering, respiratory exchange and removal of waste.
• Choanocytes or collar cells line the spongocoel and the canals.
• Digestion is intracellular.
• A skeleton made up of spicules or spongin fibres.
• Sexes are not separate (hermaphrodite),
  — i.e., eggs and sperms are produced by the same individual.
• Sponges reproduce
  (i) asexually by fragmentation
  (ii) sexually by formation of gametes.
  (iii) Fertilisation is internal
  (iv) development is indirect having a larval stage
  (v) Larva is morphologically distinct from the adult.
• Examples
  (i) Sycon (Scypha),
  (ii) Spongilla (Fresh water sponge)
  (iii) Euspongia (Bath sponge).

10. Phylum – Coelenterata (Cnidaria)

• Aquatic, mostly marine,
• Sessile or free-swimming,
• Radially symmetrical animals
• The name cnidaria is derived from the cnidoblasts or cnidocytes
  (i) Cnidoblasts or Cnidocytes (contain the stinging capsules or nematocytes) present on the tentacles and the body.
  (ii) Cnldoblagts are imd for anchorage, defence and to capture prey
• Cnidarians exhibit TISSUE LEVEL OF ORGANISATION
• Diploblastic.
• Central gastro-vascular cavity with a single opening, mouth on hypostome.
• Digestion is extracellular and intracellular.
• Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
• Cnidarians exhibit two basic bodyforms called polyp and medusa
• Polyp is a sessile and cylindrical form like Hydra, Adamsia, etc.
• Medusa is umbrella-shaped and free-swimming like Aurelia or jelly fish.
• Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis). i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
• Examples:
  — Physalia (Portuguese man-ofwai),
  — Adamsia (Sea anemone),
  — Pennatula (Sea-pen),
  — Gorgonia (Sea-fan) and
  — Meandrina (Brain coral).

11. Phylum - Ctenophora

• Commonly known as SEA WALNUTS OR COMB JELLIES
• Exclusively marine,
• Radially symmetrical,
• Diploblastic organisms
• Tissue level of organisation.
• The body bears eight external rows of ciliated comb plates, which help in locomotion
• Digestion is both extracellular and intracellular.
• Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.
• Sexes are not separate.
• Reproduction takes place only by sexual means.
• Fertilisation is external with indirect development.
• Examples: Pleurobrachia and Ctenoplana.

12. Phylum - Platyhelminthes

• Dorso-ventrally flattened body, hence are called flatworms
• Mostly ENDOPARASITES found in animals including human beings.
• Bilaterally symmetrical,
• Triploblastic and acoelomate animals
• ORGAN LEVEL OF ORGANISATION.
• Hooks and suckers are present in the parasitic forms.
• Absorb nutrients from the host directly through their body surface.
• Specialised cells called FLAME CEIIS help in osmoregulation and excretion.
• Sexes are not separate.
• Fertilisation is internal and development is through many larval stages.
• Some members like Planaria possess high regeneration capacity.
• Examples
  — Taenia (Tapeworm),
  — Fasciola (Liver fluke).

13. Phylum – Aschelminthes

• Body is circular in cross-section, hence, the name roundworms
• May be free living, aquatic and terrestrial or parasitic in plants and animals.
• ORGAN-SYSTEM LEVEL OF BODY ORGANISATION.  
• Bilaterally symmetrical,
• Triploblastic
• PSEUDOCOELOMATE ANIMALS.  
• Alimentary canal is complete with a well developed muscular pharynx.
• An excretory tube removes body wastes from the body cavity through the excretory pore.
• Sexes are separate (dioecious), i.e., males and females are distinct.
• Females are longer than males.
• Fertilisation is internal
• Development may be direct (the young ones resemble the adult) or indirect. Examples:
  — Ascaris (Round Worm),
  — Wuchereria (Filaria worm),
  — Ancylostoma (Hookworm).

14. Phylum – Annelida

• Aquatic (marine and fresh water) or terrestrial;
• Free-living, and sometimes parasitic.
• They exhibit organ-system level of body organisation a
• Bilateral symmetry.
• Triploblastic,
• METAMERICALLY SEGMENTED  
• Coelomate animals.
• Their body surface is distinctly marked out into segments or metameres
  hence, the phylum name Annelida (Latin, annulus : little ring)
• They possess longitudinal and circular muscles which help in locomotion.
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
• A closed circulatory system is present.
• Nephridia (sing. nephridium) help in osmoregulation and excretion.
• Neural system consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.
• Nereis, an aquatic form, is dioecious,
• earthworms and leeches are monoecious.
• Reproduction is sexual.
• Examples :
  — Nereis,
  — Pheretima (Earthworm) and
  — Hirudinaria (Blood sucking leech).

15. Phylum – Arthropoda

• Largest phylum
• includes insects.
• Over two-thirds of all named species on earth are arthropods
• Organ-system level of organisation.
• Bilaterally symmetrical,
• Triploblastic,
• Segmented and Coelomate animals.
• The body covered by chitinous exoskeleton.
• The body consists of head, thorax and abdomen.
• They have jointed appendages (arthros-joint, poda-appendages).
• Respiratory organs are gills, book gills, book lungs or tracheal system.
• Circulatory system is of open type.
• Sensory organs like antennae, eyes (compound and simple), statocysts or balance organs are present.
• Excretion takes place through malpighian tubules.
• They are mostly dioecious.
• Fertilisation is usually internal.
• They are mostly oviparous.
• Development may be direct or indirect.
• Examples:
  — Economically important insects
  Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect)
  — Vectors
  Anopheles, Culex and Aedes (Mosquitoes)
  — Gregarious pest
  Locusta (Locust) Living fossil – Limulus (King crab).

16. Phylum – Mollusca

• Second largest animal phylum
• Terrestrial or aquatic (marine or fresh water)
• Organ-system level of organisation.
• Bilaterally symmetrical, triploblastic and coelomate animals.
• Body is covered by a calcareous shell
• unsegmented with a distinct head, muscular foot and visceral hump.
• A soft and spongy layer of skin forms a mantle over the visceral hump.
• The space between the hump and the mantle is called the MANTLE CAVITY in which feather like gills are present.
  — They have respiratory and excretory functions.
• The anterior head region has sensory tentacles. The mouth contains a file-like rasping organ for feeding, called RADULA
• They are usually dioecious and oviparous with indirect development. 
• Examples:
  — Pila (Apple snail),
  — Pinctada (Pearl oyster),
  — Sepia (Cuttlefish),
  — Loligo (Squid),
  — Octopus (Devil fish),
  — Aplysia (Seahare)
  — Dentalium (Tusk shell) and
  — Chaetopleura (Chiton).

17. Phylum – Echinodermata

• Have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied).
• All are marine
• Organ-system level of organisation.
• The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
• They are triploblastic and coelomate animals.
• Digestive system is complete
  — mouth on the lower (ventral) side
  — anus on the upper (dorsal) side.
• The most distinctive feature → the presence of water vascular system
  helps in locomotion, capture and transport of food and respiration.
• An excretory system is absent.
• Sexes are separate.
• Reproduction is sexual.
• Fertilisation is usually external.
• Development is indirect with free-swimming larva.
• Examples:
  — Asterias (Star fish),
  — Echinus (Sea urchin),
  — Antedon (Sea lily),
  — Cucumaria (Sea cucumber) and
  — Ophiura (Brittle star).

18. Phylum – Hemichordata

• Placed as a separate phylum under non-chordata.
• Small group of worm-like marine animals
• Organ-system level of organisation.
• Bilaterally symmetrical, triploblastic and coelomate animals.
• The body is cylindrical
  — composed of an anterior proboscis, a collar and a long trunk
• Circulatory system is of open type.
• Respiration takes place through gills.
• Excretory organ is proboscis gland.
• Sexes are separate.
• Fertilisation is external; Development is indirect.
• Examples: Balanoglossus and Saccoglossus.

19. Phylum – Chordata

• Fundamentally characterised by the presence of
  (i) a notochord,
  (ii) a dorsal hollow nerve cord and
  (iii) paired pharyngeal gill slits

Chordata

• Bilaterally symmetrical,
• Triploblastic,
• Coelomate
• Organ-system level of organisation.
• Possess a post anal tail
• Closed circulatory system.

20. Phylum Chordata is divided into three subphyla

• Urochordata or Tunicata,
• Cephalochordata
• Vertebrata.

21. Subphyla Urochordata + Cephalochordata → Protochordates

• Exclusively marine.

22. In Urochordata

Notochord is present only in larval tail

• Examples: Urochordata – Ascidia, Salpa, Doliolum;

23. In Cephalochordata

Notochord extends from head to tail region and is persistent throughout their life.

• Examples: Cephalochordata – Branchiostoma (Amphioxus or Lancelet).

24. The members of subphylum Vertebrata

• Possess notochord during the embryonic period.
• The notochord is replaced by a cartilaginous or bony vertebral column in the adult.
• “all vertebrates are chordates but all chordates are not vertebrates”
• Besides the basic chordate characters,
  (i) vertebrates have a ventral muscular heart with two, three or four chambers,
  (ii) kidneys for excretion and osmoregulation and
  (iii) paired appendages which may be fins or limbs.

25. Class – Cyclostomata

• All are ectoparasites on some fishes.
• Elongated body bearing 6-15 pairs of gill slits for respiration.
• Cyclostomes have a sucking and circular mouth without jaws
• Their body is devoid of scales and paired fins.
• Cranium and vertebral column are cartilaginous.
• Circulation is of closed type.
• Cyclostomes are marine but migrate for spawning to fresh water.
• After spawning, within a few days, they die.
• Their larvae, after metamorphosis, return to the ocean.
• Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

26. Class – Chondrichthyes

• Marine animals
• Streamlined body
• Cartilaginous endoskeleton
• Mouth is located ventrally.
• Notochord is persistent throughout life.
• Gill slits are separate and without operculum (gill cover).
• The skin is tough, containing minute placoid scales.
• Teeth are modified placoid scales which are backwardly directed.
• Their jaws are very powerful.
• These animals are predaceous.
• Due to the absence of air bladder, they have to swim constantly to avoid sinking.
• Heart is two-chambered (one auricle and one ventricle).
• Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon).
• They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature.
• Sexes are separate.
• In males pelvic fins bear claspers.
• They have internal fertilisation and many of them are viviparous
• Examples:
  (i) Scoliodon (Dog fish),
  (ii) Pristis (Saw fish),
  (iii) Carcharodon (Great white shark),
  (iv) Trygon (Sting ray).

27. Class – Osteichthyes

• both marine and fresh water fishes
• Bony endoskeleton.
• Streamlined Body.
• Mostly terminal mouth
• Four pairs of gills ; covered by an operculum on each side.
• Skin is covered with cycloid/ctenoid scales.
• Air bladder is present which regulates buoyancy.
• Heart is two chambered (one auricle and one ventricle).
• They are cold-blooded animals.
• Sexes are separate.
• Fertilisation is usually external.
• They are mostly oviparous and development is direct.
• Examples:
  (i) Marine – Exocoetus (Flying fish), Hippocampus (Sea horse);
  (ii) Freshwater – Labeo (Rohu), Catla (Katla), Clarias (Magur);
  (iii) Aquarium – Betta (Fighting fish), Pterophyllum (Angel fish).

28. Class – Amphibia (Gr., Amphi : dual, bios, life)

• Live in aquatic as well as terrestrial habitats
• Most of them have two pairs of limbs.
• Body is divisible into head and trunk.
• Tail may be present in some.
• The amphibian skin is moist (without scales).
• The eyes have eyelids.
• A tympanum represents the ear.
• Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca which opens to the exterior.
• Respiration is by gills, lungs and through skin.
• The heart is three chambered (two auricles and one ventricle).
• These are cold-blooded animals.
• Sexes are separate.
• Fertilisation is external.
• They are oviparous and development is indirect.
• Examples:
  (i) Bufo (Toad),
  (ii) Rana (Frog),
  (iii) Hyla (Tree frog),
  (iv) Salamandra (Salamander),
  (v) Ichthyophis (Limbless amphibia)

29. Class – Reptilia

• Creeping or crawling mode of locomotion (Latin, reptum, to creep or crawl).
• Mostly terrestrial animals
• Body is covered by dry and cornified skin, epidermal scales or scutes
• Do not have external ear openings.
• Tympanum represents ear.
• Limbs, when present, are two pairs.
• Heart is usually three-chambered, but four-chambered in crocodiles.
• Reptiles are poikilotherms.
• Snakes and lizards shed their scales as skin cast.
• Sexes are separate.
• Fertilisation is internal.
• They are oviparous and development is direct.
• Examples:
  (i) Chelone (Turtle),
  (ii) Testudo (Tortoise),
  (iii) Chameleon (Tree lizard),
  (iv) Calotes (Garden lizard),
  (v) Crocodilus (Crocodile),
  (vi) Alligator (Alligator).
  (vii) Hemidactylus (Wall lizard),
  (viii) Poisonous snakes
  – Naja (Cobra), Bangarus (Krait), Vipera (Viper). 

30. Class – Aves

• The characteristic features → Presence of feathers 
• Most of them can fly except flightless birds (e.g., Ostrich).
• They possess beak
• The forelimbs are modified into wings.
• The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches.
• Skin is dry without glands except the oil gland at the base of the tail.
• Endoskeleton is fully ossified (bony)
• Long bones are hollow with air cavities (pneumatic).
• The digestive tract of birds has additional chambers, the crop and gizzard.
• Heart is completely four chambered.
• They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature.
• Respiration is by lungs.
• Air sacs connected to lungs supplement respiration.
• Sexes are separate.
• Fertilisation is internal.
• They are oviparous and development is direct.
• Examples:
  (i) Corvus (Crow),
  (ii) Columba (Pigeon),
  (iii) Psittacula (Parrot),
  (iv) Struthio (Ostrich),
  (v) Pavo (Peacock),
  (vi) Aptenodytes (Penguin),
  (vii) Neophron (Vulture).

31. Class – Mammalia

• Found in a variety of habitats – polar ice caps, deserts, mountains, forests, grasslands and dark caves.
• Adapted to fly or live in water.
• The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.
• They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying
• The skin of mammals is unique in possessing hair.
• External ears or pinnae are present.
• Different types of teeth are present in the jaw.
• Heart is four chambered.
• They are homoiothermous.
• Respiration is by lungs.
• Sexes are separate
• Fertilisation is internal.
• They are viviparous with few exceptions
• Development is direct. 
• Examples:
  (i) Oviparous
  - Ornithorhynchus (Platypus);
  (ii) 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)