Animal Kingdom

Key Concepts

Basis of Classification

Animals are classified based on fundamental features:

1. Levels of Organization:
   • Cellular level: Cells are loose aggregates (e.g., Sponges).
   • Tissue level: Cells perform the same function in tissues (e.g., Coelenterates).
   • Organ level: Tissues grouped into organs (e.g., Platyhelminthes).
   • Organ system level: Organs associate to form functional systems (e.g., Annelids, Arthropods, Chordates).
2. Circulatory System:
   • Open type: Blood is pumped out of the heart; tissues are bathed in it.
   • Closed type: Blood circulates through a series of vessels.
3. Symmetry:
   • Asymmetrical: Cannot be divided into equal halves (e.g., most Sponges).
   • Radial symmetry: Any plane passing through the central axis divides the body into identical halves (e.g., Coelenterates, Ctenophores, Echinoderms).
   • Bilateral symmetry: Body divided into identical left and right halves in only one plane (e.g., Annelids, Arthropods).
4. Diploblastic and Triploblastic Organization:
   • Diploblastic: Cells in two embryonic layers (ectoderm and endoderm) with a middle mesoglea (e.g., Coelenterates).
   • Triploblastic: Three germinal layers (ectoderm, mesoderm, endoderm) (e.g., Platyhelminthes to Chordates).
5. Coelom (Body Cavity):
   • Acoelomates: No body cavity (e.g., Platyhelminthes).
   • Pseudocoelomates: Cavity not lined by mesoderm (e.g., Aschelminthes).
   • Coelomates: Cavity lined by mesoderm (e.g., Annelids to Chordates).
6. Segmentation: Metameric segmentation (metamerism) is found in animals like earthworms.
7. Notochord: A mesodermally derived rod-like structure on the dorsal side. Present in Chordates, absent in Non-chordates.

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Classification of Animals (Non-Chordates)

1. Porifera (Sponges): Cellular level, water canal system (ostia, spongocoel, osculum), choanocytes (collar cells), internal fertilization.
2. Coelenterata (Cnidaria): Tissue level, radial symmetry, cnidoblasts (stinging cells), two body forms: Polyp (sessile) and Medusa (free-swimming). Exhibit metagenesis (e.g., Obelia).
3. Ctenophora (Sea Walnuts): Exclusively marine, 8 rows of ciliated comb plates, bioluminescence.
4. Platyhelminthes (Flatworms): Dorso-ventrally flattened, acoelomate, flame cells for excretion, high regeneration capacity (e.g., Planaria).
5. Aschelminthes (Roundworms): Circular cross-section, pseudocoelomate, separate sexes (dioecious).
6. Annelida: Metamerically segmented, closed circulatory system, nephridia for excretion, parapodia in aquatic forms (Nereis).
7. Arthropoda: Largest phylum, chitinous exoskeleton, jointed appendages, malpighian tubules for excretion, open circulatory system.
8. Mollusca: Second largest phylum, soft body with calcareous shell, muscular foot, visceral hump, radula for feeding.
9. Echinodermata: Spiny-bodied, spiny skin, water vascular system, adults are radially symmetrical, larvae are bilaterally symmetrical.
10. Hemichordata: Worm-like marine animals, body divided into proboscis, collar, and trunk.

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Phylum Chordata

Characterized by a notochord, dorsal hollow nerve cord, and paired pharyngeal gill slits.

• Subphyla:
  • Urochordata: Notochord in larval tail only.
  • Cephalochordata: Notochord from head to tail, persistent.
  • Vertebrata: Notochord replaced by vertebral column.
• Classes of Vertebrata:
  1. Cyclostomata: Jawless, ectoparasites on fishes, circular mouth.
  2. Chondrichthyes: Cartilaginous fishes, placoid scales, lack air bladder (e.g., Sharks).
  3. Osteichthyes: Bony fishes, operculum over gills, air bladder present.
  4. Amphibia: Dual life (land and water), moist skin, three-chambered heart.
  5. Reptilia: Creeping/crawling, dry cornified skin, three-chambered heart (four in crocodiles).
  6. Aves (Birds): Feathers, pneumatic (hollow) bones, four-chambered heart, warm-blooded.
  7. Mammalia: Mammary glands, hair on skin, four-chambered heart, mostly viviparous.