Revision: Class 11 >> Animal Kingdom NEET (UG) Animal Kingdom

Small pores through which water enters the body of a sponge are called ostia.

A large opening through which water exits the body of a sponge is called an osculum.

Special types of cells present in sponges that help in generating water current and food collection are called collar cells.

Coelom: Coelom is the internal body cavity between visceral organs and body wall in which well-developed organs can be accommodated. In some animals pseudocoelom is present (e.g. nematodes) which is a body cavity not lined by mesoderm. True coelom is lined by mesoderm and first appears in annelids. Examples of animals in which true coelom is present are Earthworm, cockroach, snail, all chordates such as birds, humans, etc.

A cylindrical body form in coelenterates, such as Hydra, which is usually attached to a surface, is called a Polyp.

An umbrella-shaped body form in coelenterates, such as Aurelia (jellyfish), is called a Medusa.

• Meaning - Heterotrophic, multicellular eukaryotes; cells lack a cell wall; cannot produce their own food.
• Nutrition - Mode of nutrition is holozoic (food obtained by ingestion); food is digested in an internal cavity; food is stored as glycogen or fat.
• Reproduction - By sexual reproduction - copulation of male and female followed by embryological development.
• Other features - Follow a definite growth pattern; higher forms have elaborate sensory and neuromotor mechanisms (nervous system and sensory organs); most are capable of locomotion.
• Cell structure - Has a nucleus and membrane-bound organelles; no chloroplasts (hence cannot photosynthesise); no cell wall (unlike plants and fungi).

• The animal kingdom is classified based on level of organisation, symmetry, and body cavity.
• Cellular-level organisms are asymmetrical and belong to Porifera.
• Tissue-level organisms show radial symmetry and include Cnidaria and Ctenophora.
• Organ/organ system level organisms show bilateral symmetry and are further classified based on the body cavity.
• Based on body cavity:
  Acoelomates → Platyhelminthes,
  Pseudocoelomates → Aschelminthes,
  Coelomates → Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata.

• Classification is based on features like cell arrangement, symmetry, coelom, and organ systems (digestive, circulatory, reproductive).
• Levels of organisation:
  Cellular (Porifera) → Tissue (Cnidaria) → Organ (Platyhelminthes) → Organ system (higher animals).
• Types of symmetry:
  Asymmetrical (sponges), radial (Cnidaria, Ctenophora), bilateral (Annelids, Arthropods).
• Types of circulatory system:
  Open (blood flows freely) and closed (blood flows through vessels).
• Body organisation becomes more complex from simple to higher animals with specialised organs and systems.

• Animals are classified based on the number of germ layers formed during embryonic development.
• Diploblastic animals have two layers: ectoderm and endoderm, with mesoglea in between.
• Examples of diploblastic organisms include Porifera, Cnidaria, and Ctenophora.
• Triploblastic animals have three layers: ectoderm, mesoderm, and endoderm.
• Mesoderm is present between the ectoderm and the endoderm and helps in the formation of complex organs.
• Triploblastic animals include Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata.

• The body cavity (coelom) is the space between the body wall and the gut, and is important for classification.
• True coelom is lined by mesoderm, while the absence or incomplete lining leads to other types.
• Acoelomates lack a body cavity; examples include Porifera, Cnidaria, Ctenophora, and Platyhelminthes.
• Pseudocoelomates have a body cavity not fully lined by mesoderm; example: Aschelminthes.
• Coelomates have a true coelom; examples include Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata.
• This classification helps in understanding body organisation and the evolutionary advancement of animals.

• Meaning - Sponges; primarily marine (except Spongilla - freshwater); body is asymmetrical; cellular level of organisation.
• Water canal system - Water enters through ostia (pores) → spongocoel (body cavity) → exits through osculum; helps in feeding, respiration and waste removal; canals lined by choanocytes (collar cells).
• Digestion - Intracellular; body supported by spicules and spongin fibres.
• Reproduction - Hermaphrodite (produces both eggs and sperm); asexual by fragmentation; sexual by gamete formation; fertilisation is internal with indirect development (distinct larval stage).
• Examples - Sycon, Spongilla (freshwater), Euspongia (bath sponge), Euplectella (Venus' Flower Basket).

• Body Shape: Cylindrical (Polyp) or umbrella-like (Medusa)
• Habitat: Mostly marine; few are freshwater dwellers
• Body Structure: Radially symmetrical and diploblastic
• Special Feature: Tentacles with cnidoblasts used for capturing prey and injecting toxins
• Examples: Hydra, Adamsia (Sea anemone), Physalia (Portuguese man-of-war), Aurelia (Jellyfish), Corals

• Ctenophores (comb jellies) are exclusively marine and radially symmetrical organisms.
• They are diploblastic and show tissue-level organisation.
• The body has eight rows of ciliated comb plates used for locomotion.
• They show bioluminescence, i.e., the ability to emit light.
• Reproduction is sexual, with external fertilisation and indirect development; examples include Pleurobrachia and Ctenoplana.

• Body Shape: Slender, flat like a leaf or strip (called flatworms)
• Habitat: Mostly endoparasitic; few are free-living and aquatic
• Body Structure: Acoelomate, bilaterally symmetrical, and triploblastic
• Reproduction: Hermaphrodite – both male and female systems in one body
• Examples: Planaria, Liver fluke, Tapeworm

• Body Shape: Long, thread-like or cylindrical (called roundworms)
• Habitat: Free-living (aquatic or terrestrial) or endoparasitic
• Body Structure: Triploblastic, pseudocoelomate, non-segmented, with a tough cuticle
• Reproduction: Unisexual – male and female are separate
• Examples: Ascaris (intestinal worm), Filarial worm, Loa loa (eye worm)

• Body Structure: Long, cylindrical, metamerically segmented
• Habitat: Mostly free-living (marine, freshwater, or terrestrial); some are ectoparasites
• Body Type: Triploblastic, bilaterally symmetrical, and eucoelomate
• Locomotion: Movement with setae, parapodia, or suckers
• Reproduction: Hermaphrodite or unisexual
• Examples: Earthworm, Leech, Nereis

• Body Structure: Segmented, triploblastic, eucoelomate, and bilaterally symmetrical
• Appendages: Jointed appendages with a chitinous exoskeleton
• Habitat: Found in all types of environments – aquatic, terrestrial, and aerial
• Reproduction: Unisexual
• Examples: Crab, spider, cockroach, butterfly, honey bee

• Body Structure: Soft, non-segmented, triploblastic, eucoelomate body
• Symmetry: Bilateral (except in some, like snail)
• Body Division: Head, foot, and visceral mass covered by a mantle
• Shell: Hard, calcareous shell (may be external, internal, or absent)
• Reproduction: Unisexual
• Examples: Bivalve, Snail, Octopus

• Body Structure: Triploblastic, eucoelomate, radially symmetrical in adults and bilaterally symmetrical in larvae
• Habitat: Exclusively marine
• Locomotion: Tube feet are used for movement and capturing prey; some are sedentary
• Skeleton: Made of calcareous spines and ossicles
• Reproduction: Mostly unisexual
• Examples: Starfish, sea urchin, sea cucumber, brittle star

• Body Structure: Divided into proboscis, collar, and trunk.
• Notochord: Present only in the proboscis region, hence the name hemichordates.
• Habitat: Marine animals that live in burrows in sand.
• Respiration: Possess one to many pharyngeal gill slits.
• Reproduction: Mostly unisexual, though some may be hermaphrodite.
• Examples: Balanoglossus, Saccoglossus.
• Evolutionary Importance: Balanoglossus is considered a connecting link between non-chordates and chordates.

• Notochord: Replaced by vertebral column
• Head: Well developed
• Brain Protection: Enclosed in a cranium
• Endoskeleton: Cartilaginous or bony
• Jaw Type: Some are jawless (Agnatha), others have jaws (Gnathostomata)

• Mouth: Jawless, with sucker
• Skin: Soft, without scales
• Appendages: Paired appendages absent
• Endoskeleton: Cartilaginous
• Lifestyle: Mostly ectoparasitic
• Examples: Petromyzon, Myxine

• Chondrichthyes are marine fishes with a cartilaginous skeleton and streamlined bodies.
• They have a ventral mouth, separate gill slits, and no operculum.
• Skin has placoid scales, modified as teeth; no air bladder is present.
• They are cold-blooded with a two-chambered heart; some have electric organs or poison stings.
• Sexes are separate, fertilisation is internal, and many are viviparous.
• Examples: Scoliodon, Pristis, Carcharodon, Trygon.

• Osteichthyes are bony fishes found in both marine and freshwater habitats.
• They have a bony endoskeleton, a terminal mouth, and gills covered by an operculum.
• Skin has cycloid or ctenoid scales and an air bladder for buoyancy.
• They are cold-blooded with a two-chambered heart and separate sexes.
• Fertilisation is mostly external and they are oviparous.
• Examples: Labeo, Catla, Exocoetus, Hippocampus.

• Life Stages: Aquatic larva (with aquatic respiration); adult lives on land and water
• Appendages: Two pairs, digits without claws
• Skin: Moist, without scales
• Respiration: Both aquatic and aerial in adults
• Sensory Organs: Tympanum present, external ear absent, eyelids present
• Examples: Frog, Toad, Salamander

• Movement: Creeping (body cannot be fully lifted)
• Temperature Regulation: Cold-blooded (Poikilotherms)
• Skin: Dry and scaly
• Neck: Present
• Digits: With claws
• External Ear: Absent
• Examples: Tortoise, Lizard, Snake

• Temperature Regulation: Warm-blooded (Homeotherms)
• Body Structure: Spindle-shaped to reduce air resistance
• Forelimbs: Modified into wings
• Exoskeleton: Feathers; digits with scales and claws
• Jaws: Modified into beak
• Neck: Present
• Examples: Peacock, Parrot, Duck, Penguin

• Unique Feature: Mammary glands present
• Temperature Regulation: Warm-blooded
• Body Division: Head, neck, trunk, tail
• Digits: Provided with nails, claws, or hooves
• Exoskeleton: Hair or fur
• Examples: Elephant, Human, Kangaroo, Dolphin