Marine, streamlined body
Cartilaginous endoskeleton
Ventral mouth
Gill slits without operculum (gill cover)
Skin with placoid scales; teeth are modified placoid scales (backwardly directed)
Predaceous, powerful jaws
No air bladder, so swim constantly to avoid sinking
Poikilothermous (cold-blooded)
Internal fertilization, viviparous (most)
Examples: Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark), Trygon (sting ray - electric organ is absent, but has poison sting)

Marine & freshwater, streamlined body
Bony endoskeleton
Terminal mouth
Four pairs of gills covered by operculum
Cycloid/ctenoid scales
Air bladder present (buoyancy)
Poikilothermous
External fertilization, oviparous, direct development
Examples:
- Marine: Exocoetus (flying fish), Hippocampus (sea horse)
- Freshwater: Labeo (rohu), Catla (katla), Clarias (magur)
- Aquarium: Betta (fighting fish), Pterophyllum (angel fish)

Aquatic & terrestrial habitats
Body divisible into head & trunk (tail may be present)
Skin moist without scales
Two pairs of limbs
Tympanum represents ear
Alimentary canal, urinary, and reproductive tracts open into a common chamber called cloaca
Respiration by gills, lungs, and skin
Three-chambered heart (two atria, one ventricle)
Poikilothermous
Dioecious, external fertilization, oviparous, indirect development
Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibia)

Terrestrial animals, dry and cornified skin, epidermal scales/scutes
No external ear openings (tympanum present)
Limbs, if present, are two pairs
Three-chambered heart (except crocodiles with 4-chambers)
Poikilothermous
Internal fertilization, oviparous, direct development
Examples: Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator, Hemidactylus (wall lizard)
Snakes: Naja (cobra), Bungarus (krait), Vipera (viper)

Presence of feathers
Most can fly (except flightless birds like ostrich)
Beak present, no teeth
Forelimbs modified into wings
Hindlimbs adapted for walking, swimming, or clasping
Dry skin without glands (except oil gland at tail base)
Endoskeleton fully ossified (bony), long bones hollow (pneumatic)
Air sacs connected to lungs (supplement respiration)
Four-chambered heart
Homoiothermous (warm-blooded)
Internal fertilization, oviparous, direct development
Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture)