Animals

Invertebrate Classification

• Classified by:
  • Body plan (Symmetry)
  • Presence of a head
  • Method of digestion & absorption (Body cavity)

Body Plans

• Bilateral symmetry: Two similar halves (e.g., ants, land animals)
• Radial symmetry: Body parts arranged around a central point (e.g., sea anemones)
• Asymmetrical: No symmetry (e.g., sea sponges)

Advantages of Bilateral Symmetry

• Presence of ganglia (nerve cell groups) to control body movement
• Cephalization: Concentration of nerve tissue and sensory organs at the anterior end

Head or No Head

• Nerve systems (except in sponges) control body movements:
  • Some invertebrates lack a head and brain (simple)
  • Others have a group of nerve cells or a brain (complex)

Body Cavities

• Acoelomate: No body cavity (e.g., flatworms)
• Pseudocoelomate: Has a pseudocoelom (e.g., roundworms)
• Coelomate: True coelom present (e.g., earthworms)
• Importance: Allows gut movement without affecting body movement

Major Invertebrate Phyla

1. Porifera: Sponges
2. Cnidaria: Jellyfish, sea anemones, corals
3. Platyhelminthes: Flatworms
4. Nematoda: Roundworms
5. Annelida: Segmented worms
6. Mollusca: Clams, squids, snails
7. Arthropoda: Crustaceans, spiders, insects
8. Echinodermata: Sea stars, sea urchins
9. Chordata: Vertebrates and some non-vertebrates

Phylum: Porifera (Sponges)

• Characteristics:
  • No nerves or body cavity
  • Asymmetrical body plan
  • Filter feeders through collar cells

Phylum: Cnidaria

• Characteristics:
  • Complex tissues, no true body cavity
  • Nervous system with ganglia, radial symmetry
  • Two forms: Medusa (motile) and Polyp (sessile)

Phylum: Platyhelminthes (Flatworms)

• Characteristics:
  • Bilateral symmetry, simple brain
  • No coelom
  • Types: Turbellarians (planarians), flukes, tapeworms

Phylum: Nematoda (Roundworms)

• Characteristics:
  • Round, segmented, bilateral symmetry
  • Simple nervous system, pseudocoelom
  • Various types: hookworms, pinworms, Ascaris