4.1 Basis of Classification
Over a million animal species are known, so a shared framework is essential. Animals are grouped using fundamental features that cut across appearances.
1. Levels of organisation
- Cellular level — cells loosely arranged (Porifera).
- Tissue level — cells form tissues (Coelenterata, Ctenophora).
- Organ level — tissues form organs (Platyhelminthes onward).
- Organ-system level — organs form systems (Aschelminthes onward).
2. Symmetry
- Asymmetrical — no plane divides into equal halves (most sponges).
- Radial — any plane through the central axis gives equal halves (Coelenterates, Ctenophores, adult echinoderms).
- Bilateral — only one plane divides into identical left & right halves (most animals).
3. Germ layers
- Diploblastic — two layers: ectoderm + endoderm, with mesoglea between (Coelenterates).
- Triploblastic — three layers: ecto-, meso-, endoderm (Platyhelminthes → Chordates).
4. Coelom (body cavity)
| Type | Body cavity | Examples |
|---|
| Coelomate | True coelom lined by mesoderm | Annelids, arthropods, molluscs, echinoderms, hemichordates, chordates |
| Pseudocoelomate | Cavity not lined by mesoderm (mesoderm in scattered pouches) | Aschelminthes (round worms) |
| Acoelomate | No body cavity | Platyhelminthes |
5. Segmentation & 6. Notochord
Metameric segmentation — body externally & internally divided into serial segments (Annelida). Notochord — a rod formed on the dorsal side during embryonic development; animals with it = Chordates, without it = Non-chordates.
4.2 Porifera → Aschelminthes
| Phylum | Key features | Symmetry / layers / coelom | Examples |
|---|
| Porifera (sponges) | Cellular level; water canal system (ostia→spongocoel→osculum); choanocytes (collar cells) line canals; spicules/spongin skeleton; hermaphrodite; indirect development | Mostly asymmetrical; "porocyte" body | Sycon, Spongilla, Euspongia |
| Coelenterata / Cnidaria | Tissue level; cnidoblasts (stinging cells) for defence/prey; central gastro-vascular cavity (coelenteron) with one opening; two forms — polyp (fixed) & medusa (free), with metagenesis | Radial; diploblastic | Hydra, Adamsia, Physalia, Aurelia, Pennatula |
| Ctenophora (comb jellies) | Marine; 8 external comb plates (ctenes) for locomotion; bioluminescent; digestion both extra- & intracellular | Radial; diploblastic | Pleurobrachia, Ctenoplana |
| Platyhelminthes (flatworms) | Organ level; dorso-ventrally flattened; mostly endoparasites; flame cells for osmoregulation/excretion; hooks & suckers in parasites | Bilateral; triploblastic; acoelomate | Taenia (tapeworm), Fasciola (liver fluke), Planaria |
| Aschelminthes (round worms) | Organ-system level; circular in cross-section; many parasitic; sexes separate (dioecious) with sexual dimorphism | Bilateral; triploblastic; pseudocoelomate | Ascaris, Wuchereria (filaria), Ancylostoma (hookworm) |
Choanocytes = the signature cell of sponges. Cnidoblasts = the signature cell of cnidarians. Flame cells = the signature cell of flatworms.
4.3 Annelida → Echinodermata → Hemichordata
| Phylum | Key features | Examples |
|---|
| Annelida | Metameric segmentation; coelomate; closed circulation; nephridia for excretion & osmoregulation; longitudinal & circular muscles; Nereis has parapodia | Nereis, Pheretima (earthworm), Hirudinaria (leech) |
| Arthropoda (largest phylum) | Jointed appendages; chitinous exoskeleton; body = head, thorax, abdomen; open circulatory system; respiration via gills/book gills/book lungs/trachea; sensory & compound eyes | Apis (bee), Bombyx (silkworm), Laccifer (lac insect), Anopheles, Locusta, Limulus (king crab, a living fossil) |
| Mollusca (2nd largest) | Soft body covered by a mantle; usually a calcareous shell; muscular foot; radula (rasping feeding organ); gills for respiration | Pila (apple snail), Pinctada (pearl oyster), Sepia, Loligo, Octopus, Aplysia (sea hare), Dentalium, Chaetopleura |
| Echinodermata | Spiny endoskeleton of calcareous ossicles; unique water vascular system (tube feet) for locomotion, food capture & respiration; marine; adults radial but larvae bilateral | Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star) |
| Hemichordata | Worm-like marine animals; have a rudimentary stomochord in the collar (not a true notochord); proboscis–collar–trunk body | Balanoglossus, Saccoglossus |
Water vascular system = the unique identity of echinoderms. Radula = molluscs. Metameric segmentation + nephridia = annelids.
4.4 Chordata — fundamental features & lower groups
Four features define Chordata (at least at some life stage):
NotochordDorsal hollow nerve cordPharyngeal gill slitsPost-anal tail
| Subphylum | Notochord | Examples |
|---|
| Urochordata (Tunicata) | Present only in the larval tail | Ascidia, Salpa, Doliolum |
| Cephalochordata | Extends from head to tail, persists throughout life | Branchiostoma (Amphioxus / lancelet) |
| Vertebrata | Notochord replaced by a vertebral column in the adult; have a cranium | Fishes → Mammals |
All vertebrates are chordates, but all chordates are not vertebrates. Vertebrates additionally have a backbone and a skull.
4.5 Vertebrata — the seven classes
| Class | Hallmarks | Examples |
|---|
| Cyclostomata | Jawless; circular sucking mouth; no scales/paired fins; ectoparasites on fish; cartilaginous skeleton | Petromyzon (lamprey), Myxine (hagfish) |
| Chondrichthyes | Marine; cartilaginous endoskeleton; placoid scales; ventral mouth; no air bladder so must swim; internal fertilisation | Scoliodon (dogfish), Pristis (sawfish), Carcharodon (great white), Trygon (sting ray) |
| Osteichthyes | Bony endoskeleton; cycloid/ctenoid scales; air bladder regulates buoyancy; operculum over gills; mostly external fertilisation, oviparous | Labeo (rohu), Catla, Clarias (magur), Exocoetus (flying fish), Hippocampus (sea horse) |
| Amphibia | Live on land & water; moist skin (no scales); two pairs of limbs; 3-chambered heart; external fertilisation, oviparous | Rana (frog), Bufo (toad), Hyla (tree frog), Salamandra, Ichthyophis (limbless) |
| Reptilia | Creeping; dry, scaly skin; mostly 3-chambered heart (crocodile 4); cold-blooded; internal fertilisation; lay shelled eggs on land | Chelone (turtle), Testudo (tortoise), Naja (cobra), Crocodilus, Hemidactylus (wall lizard) |
| Aves (birds) | Feathers; forelimbs → wings; beak; 4-chambered heart; warm-blooded; bones pneumatic (air-filled); oviparous | Corvus (crow), Columba (pigeon), Struthio (ostrich), Neophron (vulture) |
| Mammalia | Mammary glands (milk); hair/fur; external ears (pinna); 4-chambered heart; warm-blooded; mostly viviparous | Ornithorhynchus (egg-laying platypus), Macropus (kangaroo), Pteropus (flying fox), Balaenoptera (blue whale), Panthera, Homo |
Warm-blooded (homeothermic): only Aves & Mammalia. 4-chambered heart: Aves, Mammalia, and crocodiles. Everything else is cold-blooded.