Animals show remarkable diversity in body plan, symmetry, and complexity. They are broadly divided into invertebrates (lacking a backbone) and vertebrates (possessing a vertebral column).
Animals are classified based on the presence and type of body cavity (coelom):
| Type | Body Cavity | Example Phyla |
|---|
| Acoelomate | No body cavity | Platyhelminthes |
| Pseudocoelomate | False coelom (not fully lined by mesoderm) | Aschelminthes (Nematoda) |
| Coelomate | True coelom (completely lined by mesoderm) | Annelida, Mollusca, Arthropoda, Echinodermata, Chordata |
Coelomate animals are further divided based on embryonic development:
- Blastopore → mouth (first opening becomes the mouth)
- Cleavage: spiral and determinate
- Examples: Mollusca, Annelida, Arthropoda
- Blastopore → anus (mouth forms secondarily)
- Cleavage: radial and indeterminate
- Examples: Echinodermata, Chordata
- Simplest multicellular animals; no true tissues
- Body perforated by pores (ostia) and canals
- Filter feeders; sessile
- Skeleton of spicules (silica or calcium carbonate) or spongin fibres
- Example: Sycon, Spongilla
- Diploblastic (ectoderm + endoderm)
- Radial symmetry
- Possess nematocysts (stinging cells) for defence and prey capture
- Two body forms: polyp (sessile) and medusa (free-swimming)
- Examples: Hydra, jellyfish, sea anemones, corals
- Acoelomate, triploblastic
- Bilaterally symmetrical, dorsoventrally flattened
- No circulatory or respiratory system
- Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria
- Pseudocoelomate, triploblastic
- Cylindrical, unsegmented body with a complete digestive tract
- Many are parasitic
- Examples: Ascaris, Wuchereria (filarial worm)
- Coelomate, triploblastic
- Body divided into metameric segments
- Closed circulatory system; excretion by nephridia
- Examples: Lumbricus (earthworm), Nereis, Hirudo (leech)
- Coelomate; soft-bodied, usually with a shell
- Mantle secretes the shell; muscular foot for locomotion
- Open circulatory system (except cephalopods)
- Examples: Helix (snail), Mytilus (mussel), Octopus
- Largest animal phylum by number of species
- Chitinous exoskeleton and jointed appendages
- Segmented body; open circulatory system
- Classes: Insecta, Arachnida, Crustacea, Myriapoda
- Examples: insects, spiders, crabs, centipedes
- Deuterostomes; adults show pentaradial symmetry
- Unique Water Vascular System (Ambulacral system) — hydraulic system operating tube feet for locomotion and food capture
- Endoskeleton of calcareous ossicles
- Examples: starfish (Asterias), sea urchin, sea cucumber
Chordates are deuterostomes defined by four diagnostic features present at some stage of development:
- Notochord — flexible rod providing support
- Dorsal hollow nerve cord — develops into brain and spinal cord
- Pharyngeal gill slits — openings in the pharynx
- Post-anal tail — extension of body posterior to the anus
| Sub-phylum | Key Feature | Example |
|---|
| Urochordata (Tunicates) | Notochord only in larva | Herdmania |
| Cephalochordata (Lancelets) | Notochord persists throughout life | Amphioxus |
| Vertebrata | Notochord replaced by vertebral column | Fish, Amphibia, Reptilia, Aves, Mammalia |
- Skeleton entirely of cartilage
- No operculum; no swim bladder
- Placoid scales; internal fertilization
- Examples: sharks, rays
- Bony skeleton; operculum covers gills
- Swim bladder for buoyancy
- Examples: Labeo (rohu), salmon, tuna
- First tetrapods; moist, scaleless skin
- Larvae aquatic (breathe via gills); adults terrestrial (breathe via lungs and skin)
- Examples: frogs, toads, salamanders
- Dry, scaly skin; ectothermic
- Amniotic egg with leathery shell
- Examples: lizards, snakes, crocodiles, turtles
- Endothermic; body covered with feathers
- Forelimbs modified into wings; hollow bones
- Examples: Columba (pigeon), eagle, penguin
- Endothermic; body covered with hair/fur
- Mammary glands produce milk to nourish young
- Most are viviparous
- Examples: humans, whales, bats, dogs