Basis of Classification

1. Levels of Organisation

• Animals exhibit various levels of organization: cellular, tissue, organ, and organ system.
• Cellular Level: Found in sponges where cells are loosely aggregated.
• Tissue Level: Present in coelenterates where cells with similar functions form tissues.
• Organ Level: Seen in higher phyla like Platyhelminthes where tissues group to form organs.
• Organ System Level: Found in Annelids, Arthropods, Molluscs, Echinoderms, and Chordates where organs collaborate to form functional systems.

2. Symmetry

• Animals can be categorized based on symmetry:
  • Asymmetry: No equal division along any plane (e.g., sponges).
  • Radial Symmetry: Division along a central axis results in identical halves (e.g., coelenterates, ctenophores, echinoderms).
  • Bilateral Symmetry: Division along one plane creates identical left and right halves (e.g., annelids, arthropods).

3. Diploblastic and Triploblastic Organisation

• Diploblastic: Animals with two embryonic layers (ectoderm and endoderm) and a mesoglea layer between (e.g., coelenterates).
• Triploblastic: Animals with three germinal layers, including mesoderm (e.g., Platyhelminthes to chordates).

4. Coelom

• The presence or absence of a body cavity (coelom) between the body and gut wall is crucial for classification.
• Coelomates: Animals with a lined coelom (e.g., annelids, mollusks, arthropods, echinoderms, hemichordates, chordates).
• Pseudocoelomates: Animals with a body cavity not lined by mesoderm (e.g., Aschelminthes).
• Acoelomates: Animals lacking a body cavity (e.g., Platyhelminthes).

5. Segmentation

• Some animals exhibit segmentation, with a serial repetition of organs.
• Example: Earthworms show metameric segmentation.

6. Notochord

• A rod-like structure on the dorsal side during embryonic development.
• Chordates: Animals with a notochord (e.g., chordates).
• Non-chordates: Animals without a notochord (e.g., Porifera to echinoderms).

Phylum – Porifera

Characteristics:

• Common Name: Sponges
• Habitat: Mostly marine
• Symmetry: Mostly asymmetrical
• Organization: Cellular level of organization
• Water Transport System:
  • Water enters through ostia (minute pores) in the body wall.
  • Central cavity: Spongocoel
  • Water exits through the osculum
  • Functions: Food gathering, respiratory exchange, waste removal
• Specialized Cells: Choanocytes or collar cells line the spongocoel and canals.
• Digestion: Intracellular
• Support Structure: Skeleton made up of spicules or spongin fibers
• Reproductive System:
  • Sex: Hermaphroditic (individuals produce both eggs and sperm)
  • Asexual Reproduction: Fragmentation
  • Sexual Reproduction:
    • Formation of gametes
    • Fertilization: Internal
    • Development: Indirect, involving a morphologically distinct larval stage
• Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge)

Phylum – Coelenterata (Cnidaria)

Characteristics:

• Common Name: Cnidarians
• Habitat: Mostly aquatic, primarily marine
• Symmetry: Radially symmetrical
• Organisation: Tissue level of organisation, Diploblastic
• Specialized Cells:Cnidoblasts or Cnidocytes
  • Contain stinging capsules or nematocysts
  • Used for anchorage, defense, and capturing prey
• Gastrovascular Cavity:
  • Central with a single opening (mouth on hypostome)
  • Functions for both extracellular and intracellular digestion
• Skeleton (in some): Calcium carbonate, e.g., corals
• Body Forms:
  • Polyp: Sessile, cylindrical form (e.g., Hydra, Adamsia)
  • Medusa: Umbrella-shaped, free-swimming (e.g., Aurelia or jellyfish)
• Reproductive Strategy:
  • Alternation of Generation (Metagenesis):
    • Polyps produce medusae asexually
    • Medusae from the polyps sexually (e.g., Obelia)
• Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral)

Phylum – Ctenophora

Characteristics:

• Common Names: Sea walnuts or Comb jellies
• Habitat: Exclusively marine
• Symmetry: Radially symmetrical
• Organisation: Tissue level of organisation, Diploblastic
• Ciliated Comb Plates:
  • Eight external rows aid in locomotion
• Digestion: Both extracellular and intracellular
• Bioluminescence:
  • Well-marked property of emitting light
• Reproduction:
  • Sexual Reproduction Only:
    • Fertilisation: External
    • Development: Indirect
• Examples: Pleurobrachia, Ctenoplana

Phylum – Platyhelminthes

Characteristics:

• Common Name: Flatworms
• Body Shape: Dorso-ventrally flattened
• Symmetry: Bilaterally symmetrical
• Organisation: Organ level of organisation
• Parasitism:
  • Mostly endoparasites, including those found in humans
  • Hooks and suckers present in parasitic forms
  • Absorption of nutrients directly through the body surface in some
• Osmoregulation and Excretion:
  • Flame cells for osmoregulation and excretion
• Reproduction:
  • Sexes not separate
  • Internal fertilisation
  • Development through multiple larval stages
• Regeneration:
  • High regeneration capacity in some members
• Examples: Taenia (Tapeworm), Fasciola (Liver fluke)

Phylum – Aschelminthes

Characteristics:

• Common Name: Roundworms
• Body Shape: Circular in cross-section
• Symmetry: Bilaterally symmetrical
• Organization: Organ-system level of body organization
• Habitat:
  • Free-living, aquatic and terrestrial
  • Some are parasitic in plants and animals
• Body Layers: Triploblastic
• Body Cavity: Pseudocoelomate
• Alimentary Canal:
  • Complete with a well-developed muscular pharynx
• Excretion:
  • The excretory tube removes body wastes from the body cavity through the excretory pore
• Reproduction:
  • Sexes are separate (dioecious)
  • Females are often longer than males
  • Internal fertilisation
  • Development may be direct (young ones resemble the adult) or indirect
• Examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm)

Phylum – Annelida

Characteristics:

• Common Name: Annelids
• Habitat:
  • Aquatic (marine and freshwater) or terrestrial
  • Free-living, sometimes parasitic
• Body Organisation: Organ-system level
• Symmetry: Bilateral symmetry
• Body Layers: Triploblastic
• Segmentation:
  • Metamerically segmented body with distinct rings or metameres
• Coelom: Coelomate animals
• Muscles:
  • Possess longitudinal and circular muscles for locomotion
• Special Features:
  • Aquatic annelids like Nereis have lateral appendages (parapodia) for swimming
  • Closed circulatory system
  • Nephridia (excretory organs) for osmoregulation and excretion
  • The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord
• Reproduction:
  • Dioecious (Nereis)
  • Monoecious (Earthworms and leeches)
  • Sexual reproduction
• Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood-sucking leech)

Phylum – Arthropoda

Characteristics:

• Common Name: Arthropods
• Size:
  • Largest phylum in Animalia
  • Over two-thirds of all named species on earth
• Body Organisation: Organ-system level
• Symmetry: Bilateral symmetry
• Body Layers: Triploblastic
• Segmentation: Segmented body into head, thorax, and abdomen
• Coelom: Coelomate animals
• Exoskeleton:
  • Body covered by chitinous exoskeleton
• Appendages:
  • Jointed appendages (arthros-joint, poda-appendages)
• Respiratory Organs:
  • Gills, book gills, book lungs, or tracheal system
• Circulatory System: Open circulatory system
• Sensory Organs:
  • Antennae, compound and simple eyes, statocysts or balancing organs
• Excretion:
  • Malpighian tubules for excretion
• Reproduction:
  • Mostly dioecious
  • Internal fertilisation
  • Mostly oviparous
• Development:
  • Direct or indirect development
• 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)

Phylum – Mollusca

Characteristics:

• Common Name: Mollusks
• Size:
  • Second largest animal phylum
• Body Organisation: Organ-system level
• Symmetry: Bilateral symmetry
• Body Layers: Triploblastic
• Coelom: Coelomate animals
• Body Structure:
  • Unsegmented body with distinct head, muscular foot, and visceral hump
• Shell:
  • Body covered by a calcareous shell
• Mantle:
  • Soft and spongy layer of skin forms a mantle over the visceral hump
  • Mantle cavity present between the hump and mantle
• Gills:
  • Feather-like gills in the mantle cavity for respiratory and excretory functions
• Head Region:
  • Anterior head region with sensory tentacles
• Mouth:
  • Mouth contains a file-like rasping organ called radula for feeding
• Reproduction:
  • Usually dioecious
  • Oviparous with indirect development
• Examples:
  • Pila (Apple snail)
  • Pinctada (Pearl oyster)
  • Sepia (Cuttlefish)
  • Loligo (Squid)
  • Octopus (Devil fish)
  • Aplysia (Seahare)
  • Dentalium (Tusk shell)
  • Chaetopleura (Chiton)

Phylum – Echinodermata

Characteristics:

• Common Name: Echinoderms
• Skeleton:
  • Endoskeleton of calcareous ossicles
• Size:
  • Marine animals
• Body Organisation: Organ-system level
• Symmetry:
  • Adults are radially symmetrical
  • Larvae are bilaterally symmetrical
• Body Layers: Triploblastic
• Coelom: Coelomate animals
• Digestive System:
  • Complete digestive system
  • Mouth on the ventral side
  • Anus on the dorsal side
• Water Vascular System:
  • Unique feature for locomotion, food capture, and transport, as well as respiration
• Excretory System:
  • Absent
• Reproduction:
  • Sexual
  • Usually external fertilisation
• Development:
  • Indirect development with free-swimming larva
• Examples:
  • Asterias (Star fish)
  • Echinus (Sea urchin)
  • Antedon (Sea lily)
  • Cucumaria (Sea cucumber)
  • Ophiura (Brittle star)

Phylum – Chordata

Characteristics:

• Common Name: Chordates
• Fundamental Characteristics:
  • Presence of notochord
  • Dorsal hollow nerve cord
  • Paired pharyngeal gill slits
• Symmetry: Bilaterally symmetrical
• Body Layers: Triploblastic
• Coelom: Coelomate animals
• Organisation: Organ-system level
• Additional Features:
  • Post anal tail
  • Closed circulatory system
• Subphyla:
  1. Urochordata (Tunicata):
     • Notochord present only in larval tail
     • Exclusively marine
     • Examples: Ascidia, Salpa, Doliolum
  2. Cephalochordata:
     • Notochord extends from head to tail
     • Persistent throughout life
     • Exclusively marine
     • Example: Branchiostoma (Amphioxus or Lancelet)
  3. Vertebrata:
     • Notochord present during embryonic period
     • Replaced by cartilaginous or bony vertebral column in adults
     • Ventral muscular heart with two, three, or four chambers
     • Kidneys for excretion and osmoregulation
     • Paired appendages (fins or limbs)

Class – Cyclostomata

Characteristics:

• Habitat:
  • Ectoparasites on fishes
• Body:
  • Elongated body
  • 6-15 pairs of gill slits for respiration
  • Sucking and circular mouth without jaws
  • Devoid of scales and paired fins
• Skeleton:
  • Cartilaginous cranium and vertebral column
• Circulation:
  • Closed type
• Habitat Details:
  • Marine but migrate for spawning to freshwater
  • After spawning, they die, and larvae return to the ocean
• Examples:
  • Petromyzon (Lamprey)
  • Myxine (Hagfish)

Class – Chondrichthyes

Characteristics:

• Habitat:
  • Marine animals
• Body:
  • Streamlined body
  • Cartilaginous endoskeleton
  • Mouth located ventrally
  • Persistent notochord throughout life
  • Tough skin with minute placoid scales
  • Teeth are modified placoid scales, backwardly directed
  • Powerful jaws for predation
  • Lack of air bladder, requiring constant swimming to avoid sinking
• Respiration:
  • Separate gill slits without operculum (gill cover)
• Circulation:
  • Two-chambered heart (one auricle and one ventricle)
• Special Adaptations:
  • Some have electric organs (e.g., Torpedo)
  • Some possess poison sting (e.g., Trygon)
• Temperature Regulation:
  • Cold-blooded (poikilothermous), lack the capacity to regulate body temperature
• Reproduction:
  • Sexes are separate
  • Males have pelvic fins with claspers
  • Internal fertilisation
  • Many are viviparous

Examples:

• Scoliodon (Dogfish)
• Pristis (Sawfish)
• Carcharodon (Great white shark)
• Trygon (Sting ray)

Class – Osteichthyes

Characteristics:

• Habitat:
  • Includes both marine and freshwater fishes
• Endoskeleton:
  • Bony endoskeleton
• Body:
  • Streamlined body
  • Mostly terminal mouth
• Gills:
  • Four pairs of gills covered by an operculum on each side
• Scales:
  • Skin covered with cycloid/ctenoid scales
• Buoyancy:
  • Presence of air bladder for buoyancy regulation
• Circulation:
  • Two-chambered heart (one auricle and one ventricle)
• Temperature Regulation:
  • Cold-blooded animals
• Reproduction:
  • Sexes are separate
  • External fertilisation
  • Mostly oviparous with direct development

Examples:

• Marine:
  • Exocoetus (Flying fish)
  • Hippocampus (Sea horse)
• Freshwater:
  • Labeo (Rohu)
  • Catla (Katla)
  • Clarias (Magur)
• Aquarium:
  • Betta (Fighting fish)
  • Pterophyllum (Angel fish)

Class – Amphibia

Characteristics:

• Habitat:
  • Can live in both aquatic and terrestrial habitats
• Limbs:
  • Most have two pairs of limbs
• Body Division:
  • Body divisible into head and trunk
  • Tail may be present in some
• Skin:
  • Moist skin without scales
• Sensory Organs:
  • Eyes with eyelids
  • Tympanum representing the ear
• Alimentary Canal:
  • Opens into a common chamber called cloaca
• Respiration:
  • By gills, lungs, and through the skin
• Heart:
  • Three-chambered heart (two auricles and one ventricle)
• Temperature Regulation:
  • Cold-blooded animals
• Reproduction:
  • Sexes are separate
  • External fertilisation
  • Oviparous with indirect development

Examples:

• Bufo (Toad)
• Rana (Frog)
• Hyla (Tree frog)
• Salamandra (Salamander)
• Ichthyophis (Limbless amphibian)

Class – Reptilia

Characteristics:

• Locomotion:
  • Creeping or crawling mode of locomotion
• Habitat:
  • Mostly terrestrial
• Skin:
  • Covered by dry and cornified skin
  • Epidermal scales or scutes
• Ear:
  • No external ear openings
  • Tympanum represents the ear
• Limbs:
  • Two pairs of limbs when present
• Heart:
  • Usually three-chambered
  • Four-chambered in crocodiles
• Temperature Regulation:
  • Poikilotherms
• Skin Shedding:
  • Snakes and lizards shed their scales as skin cast
• Reproduction:
  • Sexes are separate
  • Internal fertilisation
  • Oviparous with direct development

Examples:

• 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)

Class – Aves

Characteristics:

• Feathers:
  • Presence of feathers
  • Most can fly, except flightless birds (e.g., Ostrich)
• Beak:
  • Possession of a beak
• Limbs:
  • Forelimbs modified into wings
  • Hind limbs generally have scales and are modified for walking, swimming, or clasping tree branches
• Skin:
  • Dry skin without glands, except for the oil gland at the base of the tail
• Endoskeleton:
  • Fully ossified (bony)
  • Long bones are hollow with air cavities (pneumatic)
• Digestive Tract:
  • Additional chambers in the digestive tract: crop and gizzard
• Heart:
  • Completely four-chambered
• Temperature Regulation:
  • Warm-blooded (homoiothermous), able to maintain a constant body temperature
• Respiration:
  • Lungs for respiration
  • Air sacs connected to lungs supplement respiration
• Reproduction:
  • Sexes are separate
  • Internal fertilisation
  • Oviparous with direct development

Examples:

• Corvus (Crow)
• Columba (Pigeon)
• Psittacula (Parrot)
• Struthio (Ostrich)
• Pavo (Peacock)
• Aptenodytes (Penguin)
• Neophron (Vulture)

Class – Mammalia

Characteristics:

• Habitats:
  • Found in a variety of habitats: polar ice caps, deserts, mountains, forests, grasslands, and dark caves
  • Some adapted to fly or live in water
• Mammary Glands:
  • Presence of milk-producing glands (mammary glands) to nourish the young
• Limbs:
  • Two pairs of limbs adapted for walking, running, climbing, burrowing, swimming, or flying
• Skin:
  • Possession of hair
  • External ears or pinnae present
• Teeth:
  • Different types of teeth present in the jaw
• Heart:
  • Four-chambered heart
• Temperature Regulation:
  • Homoiothermous (maintain a constant body temperature)
• Respiration:
  • Respiration by lungs
• Reproduction:
  • Sexes are separate
  • Internal fertilisation
  • Viviparous with few exceptions
  • Direct development

Examples:

• 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)