Unit 4 explanation

Unit 4 — Amphibia

1. Big Picture / Definition

Amphibia (from Greek amphi = both, bios = life) are tetrapod vertebratesFour-limbed vertebrates adapted to land and water adapted to aquatic & terrestrial environments. Poikilothermic vertebrates with thin, glandular skin, undergoing metamorphosis from larval gill-breathing stage to adult lungs.

Key Orders:

• Apoda (Gymnophiona) — limbless caecilians
• Urodela (Caudata) — salamanders & newts (tailed)
• Anura (Salientia) — frogs & toads (tailless adults)

2. Diagnostic Characters

• Moist, glandular, scaleless skin with mucous & poison glands
• Respiration: gills (larvae), lungs (adults), skin (cutaneous)
• Mostly tetrapods (except Apoda)
• Three-chambered heart (2 atria, 1 ventricle)
• Excretion via mesonephric kidneys; ureotelic
• Reproduction mostly oviparous; external fertilization common
• Life cycle: egg → larva → juvenile → adult

3. External Morphology — Body & Skin

• Body: head + trunk; tail in Urodela & Apoda
• Skin layers:
  • Epidermis: thin, mucous & granular cells
  • Dermis: pigment cells, glands, collagen
• Chromatophores → color change & camouflage
• Cutaneous vascularization → effective gas exchange

Moist skin + capillary network allows cutaneous respiration, crucial during hibernation or diving.

4. Floating / Theme Toggle Button

Tap the floating moon icon at bottom right to switch between Light & Dark theme.

19. Diagram Guides

[Frog external diagram]

[Frog internal diagram]

[Tadpole diagram]

[Salamander diagram]

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