Basis of Classification
Animals are classified based on:
- Level of organisation: Cellular, tissue, organ, organ system.
- Symmetry: Asymmetrical, radial, bilateral.
- Body cavity: Acoelomate, pseudocoelomate, coelomate.
- Segmentation: Metameric or non-segmented.
- Notochord: Present (Chordata) or absent (Non-chordata).
Body Plans
Cell-tissue level: Cells arranged as tissues (Cnidaria). Organ level: Tissues grouped into organs (Platyhelminthes). Organ system level: Organs work in systems (Annelida to Chordata).
Phylum Porifera
- 'Pore-bearing' (pores = ostia).
- Aquatic, mostly marine, sessile.
- Cellular level of organisation, asymmetrical.
- Body with canal system (water flow for nutrition and respiration).
- Skeleton of spicules (calcium/silica) or spongin fibres.
- Examples: Sycon, Spongilla (freshwater), Euspongia (bath sponge).
Phylum Cnidaria (Coelenterata)
- Aquatic, mostly marine.
- Radial symmetry, tissue level organisation.
- Nematocysts (stinging cells) on tentacles for capture and defence.
- Diploblastic (two germ layers: ectoderm and endoderm).
- Polymorphism (some exist as polyp and medusa forms).
- Examples: Hydra, Aurelia (jellyfish), Corals, Obelia.
Phylum Platyhelminthes
- Flatworms with dorsoventrally flattened body.
- Bilateral symmetry, triploblastic, acoelomate.
- Organ level organisation.
- Many are parasitic, with hooks and suckers.
- Flame cells for excretion.
- Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria.
Phylum Aschelminthes (Nematoda)
- Roundworms, pseudocoelomate.
- Bilateral symmetry, triploblastic.
- Complete digestive tract (mouth and anus).
- Mostly parasitic, some free-living.
- Examples: Ascaris (roundworm), Wuchereria (filarial worm), Ancylostoma (hookworm).
Phylum Annelida
- Segmented worms (metameric segmentation).
- Bilateral symmetry, triploblastic, coelomate.
- Closed circulatory system.
- Nephridia for excretion.
- Examples: Earthworm (Pheretima), Leech (Hirudinaria), Nereis.
Phylum Arthropoda
- Largest phylum (over 80% of all known species).
- Jointed appendages, chitinous exoskeleton.
- Bilateral symmetry, triploblastic, coelomate.
- Open circulatory system (blood = haemolymph).
- Body divided into head, thorax, abdomen (in insects).
- Examples: Cockroach (Periplaneta), Butterfly, Scorpion, Crab, Millipede.
Important Classes
- Insecta: 3 body parts, 6 legs, 2 antennae, wings often present.
- Arachnida: 2 body parts, 4 pairs of legs (Spiders, Scorpions).
- Crustacea: Mostly aquatic (Crabs, Prawns, Daphnia).
Phylum Mollusca
- Second largest phylum after Arthropoda.
- Soft-bodied, muscular foot, visceral mass, mantle (secretes shell).
- Bilateral symmetry, coelomate.
- Radula (rasping organ) in mouth.
- Examples: Pila (snail), Unio (pearl oyster), Octopus, Sepia (cuttlefish).
Phylum Echinodermata
- Spiny-skinned (endoskeleton of calcareous ossicles).
- Aquatic, marine, free-living.
- Radial symmetry (pentaradial in adults).
- Water vascular system with tube feet for locomotion.
- Examples: Starfish (Asterias), Sea urchin (Echinus), Sea cucumber, Brittle star.
Phylum Hemichordata
- Worm-like, with proboscis, collar, trunk.
- Stomochord (not true notochord).
- Examples: Balanoglossus.
- Now considered separate from Chordata by many taxonomists.
Phylum Chordata
Four Key Characteristics
- Notochord: Rod-like structure at some stage.
- Dorsal hollow nerve cord.
- Pharyngeal gill slits at some stage.
- Post-anal tail.
Subphyla
Urochordata (Tunicates): Notochord present only in larval tail. Example: Ascidia, Salpa.
Cephalochordata: Notochord extends to head. Example: Branchiostoma (Amphioxus/Lancelet).
Vertebrata: Notochord replaced by vertebral column in adults.
Vertebrata Classes
Cyclostomata: Jawless, scaleless. Examples: Lamprey, Hagfish.
Pisces (Fish): Aquatic, gills, scales, fins.
- Chondrichthyes (Cartilaginous): Shark, Ray.
- Osteichthyes (Bony fish): Rohu, Carp, Salmon.
Amphibia: Dual life (water and land), moist skin. Examples: Frog, Salamander.
Reptilia: Dry scaly skin, lay amniotic eggs on land. Examples: Lizard, Snake, Turtle, Crocodile.
Aves (Birds): Feathers, forelimbs as wings, warm-blooded, beak. Examples: Pigeon, Eagle, Sparrow.
Mammalia: Mammary glands, hair, warm-blooded, diaphragm. Examples: Human, Whale, Bat, Elephant.
Comparison Table: Non-Chordates vs Chordates
| Feature | Non-Chordates | Chordates |
|---|---|---|
| Notochord | Absent | Present |
| Nerve cord | Ventral, solid | Dorsal, hollow |
| Pharyngeal slits | Absent | Present |
| Post-anal tail | Absent | Present |
| Heart | Dorsal | Ventral |
Worked Examples
Example 1: Which phylum has the largest number of species? Give examples. Solution: Arthropoda (cockroach, butterfly, crab, scorpion). Jointed appendages, exoskeleton, open circulation.
Example 2: Differentiate between cartilaginous and bony fish. Solution: Cartilaginous: skeleton of cartilage, no air bladder. Bony fish: skeleton of bone, air bladder present.
Common Mistakes
- Porifera vs Cnidaria: Porifera has canal system, Cnidaria has nematocysts.
- Pseudocoelom vs true coelom: Pseudocoelom (Aschelminthes) is not lined by mesoderm. True coelom (Annelida and above) is lined by mesoderm.
- Hemichordata vs Chordata: Hemichordata lacks true notochord.
- Amphibia vs Reptilia: Amphibians need water for reproduction; reptiles lay amniotic eggs on land.
ISC Exam Focus
- Theory (70%): Phylum characteristics, chordate features, classification basis.
- Application (30%): Identifying organisms' phylum, distinguishing features.
- ISC frequently asks: "Give the characteristic features of ..." and "Differentiate between ...".
Self-Test Questions
Q1: Write four key features of Chordata. Answer: Notochord, dorsal hollow nerve cord, pharyngeal gill slits, post-anal tail.
Q2: Name the phylum with jointed appendages and give two examples. Answer: Arthropoda. Examples: Cockroach, Butterfly, Crab.
Q3: Differentiate between Pseudocoelom and Coelom. Answer: Pseudocoelom: not fully lined by mesoderm. Coelom: completely lined by mesoderm.
Q4: Give two examples each of Pisces, Amphibia, and Reptilia. Answer: Pisces: Shark, Rohu. Amphibia: Frog, Salamander. Reptilia: Lizard, Snake.
Q5: What are the characteristic features of Mammalia? Answer: Mammary glands, hair, warm-blooded, diaphragm, four-chambered heart.
Q6: Which organisms are called 'Amphibians of the animal kingdom' and why? Answer: Amphibia (frogs). They live in water as larvae and on land as adults.
