1. Basis of Animal Classification
| Feature | Types | Examples / exam cue |
|---|---|---|
| Level of organisation | Cellular, tissue, organ, organ-system | Porifera; Cnidaria/Ctenophora; Platyhelminthes; Annelida onwards |
| Symmetry | Asymmetrical, radial, bilateral | Sponges; cnidarians & adult echinoderms; most animals |
| Germ layers | Diploblastic or triploblastic | Cnidaria/Ctenophora vs Platyhelminthes onwards |
| Coelom | Acoelomate, pseudocoelomate, coelomate | Platyhelminthes; Aschelminthes; Annelida onwards |
| Circulation | Open or closed | Arthropods/molluscs vs annelids/chordates |
High-yield rule Adult echinoderms are radial but triploblastic coelomates; their larvae are bilaterally symmetrical.
2. Acoelomate vs Pseudocoelomate vs Coelomate
| Feature | Acoelomate | Pseudocoelomate | Coelomate |
|---|---|---|---|
| Body cavity | Absent | Present but not lined by mesoderm | Present and lined by mesoderm |
| Mesoderm | Fills space between ectoderm and endoderm | Occurs as scattered pouches | Lines the true coelom |
| NCERT example | Platyhelminthes | Aschelminthes | Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata |
3. Porifera vs Cnidaria vs Ctenophora
| Feature | Porifera | Cnidaria | Ctenophora |
|---|---|---|---|
| Organisation / symmetry | Cellular; mostly asymmetrical | Tissue; radial | Tissue; radial |
| Key feature | Canal system; choanocytes; ostia and osculum | Cnidocytes with nematocysts; polyp and medusa | Eight rows of comb plates; colloblasts |
| Digestion | Intracellular | Extracellular and intracellular | Extracellular and intracellular |
| Reproduction | Hermaphrodite; internal fertilisation | Polyp-medusa alternation in some, e.g. Obelia | Hermaphrodite; external fertilisation |
| Examples | Sycon, Spongilla, Euspongia | Hydra, Obelia, Aurelia | Pleurobrachia, Ctenoplana |
4. Flatworms, Roundworms and Segmented Worms
| Feature | Platyhelminthes | Aschelminthes | Annelida |
|---|---|---|---|
| Body form | Dorsoventrally flattened | Circular in cross-section | Metamerically segmented |
| Coelom / organisation | Acoelomate; organ level | Pseudocoelomate; organ-system level | True coelomate; organ-system level |
| Digestive system | Incomplete in many | Complete; muscular pharynx | Complete |
| Excretion | Flame cells | Excretory tube and pore | Nephridia |
| Sexes | Usually hermaphrodite | Separate (dioecious) | Separate or united |
| Examples | Planaria, Fasciola, Taenia | Ascaris, Wuchereria, Ancylostoma | Nereis, earthworm, leech |
5. Arthropoda vs Mollusca vs Echinodermata
| Feature | Arthropoda | Mollusca | Echinodermata |
|---|---|---|---|
| Body plan | Segmented body with jointed appendages | Head, muscular foot and visceral hump | Spiny body; adult radial, larva bilateral |
| Covering / skeleton | Chitinous exoskeleton | Mantle usually secretes shell | Calcareous ossicle endoskeleton |
| Circulation | Open; haemocoel | Usually open | Water vascular system distinctive |
| Excretion / respiration | Malpighian tubules; gills, book lungs or tracheae | Kidneys; gills in aquatic forms | Excretory system absent; water vascular system assists respiration |
| Examples | Honey bee, prawn, scorpion, cockroach | Pila, Pinctada, Sepia, Octopus | Asterias, Echinus, Ophiura |
6. Non-chordates vs Chordates and Protochordates
| Feature | Non-chordates | Chordates |
|---|---|---|
| Notochord | Absent | Present at least in embryonic stage |
| Nerve cord | Ventral, solid and usually paired | Dorsal, hollow and single |
| Pharyngeal gill slits | Absent | Present at some stage |
| Heart / circulation | Usually dorsal; open or closed | Ventral; closed circulation |
| Chordate group | Notochord | Examples |
|---|---|---|
| Urochordata | Only in larval tail | Ascidia, Salpa, Doliolum |
| Cephalochordata | Head to tail, persistent through life | Branchiostoma (Amphioxus) |
| Vertebrata | Embryonic; replaced by vertebral column | Fishes to mammals |
7. Vertebrate Classes at a Glance
| Class | Covering / skeleton | Respiration & heart | Reproduction / examples |
|---|---|---|---|
| Cyclostomata | No scales or paired fins; cartilaginous | 6-15 gill slits; closed circulation | Petromyzon, Myxine |
| Chondrichthyes | Placoid scales; cartilaginous | 5-7 gill slits, no operculum; 2 chambers | Internal fertilisation; Scoliodon, Trygon |
| Osteichthyes | Cycloid/ctenoid scales; bony | 4 gill pairs with operculum; air bladder; 2 chambers | Usually external fertilisation; Rohu, Catla |
| Amphibia | Moist glandular skin | Gills/lungs/skin; 3 chambers | External fertilisation; Rana, Bufo |
| Reptilia | Dry cornified scales | Lungs; 3 chambers, except crocodile | Internal fertilisation; Calotes, Crocodylus |
| Aves | Feathers; pneumatic bones | Lungs with air sacs; 4 chambers | Oviparous; Columba, Corvus |
| Mammalia | Hair; mammary glands | Lungs; 4 chambers | Mostly viviparous; human, whale, bat |
Exceptions worth memorising Crocodile has a four-chambered heart. Whale and bat are mammals. Monotremes lay eggs, and marsupials give birth to very poorly developed young.