Biol120 - Lecture 5: Introduction to Animals

Learning Objectives

  • Understanding the level of organisation in animal bodies.

  • Understanding the difference between Parazoa and Eumetazoa.

  • Understanding the key characteristics and diversity of Cnidaria.

  • Understanding key characteristics of free-living Platyhelminthes.

Distinguishing Animal Phyla

  • Identify phyla by: body plan, organisational level, key adaptations, diversity in form/function.

Levels of Organisation

  • Multicellular, no tissue differentiation → Porifera.

  • Differentiated tissues, no organs → Cnidaria.

  • Tissues organised into organs → Platyhelminthes.

Subkingdom Parazoa – Porifera (Sponges)

  • No true tissues; asymmetric.

  • Skeleton of spicules: Calcarea (CaCO33), Silicea (SiO22).

  • Water flow system: pores → spongocoel → osculum; choanocytes drive current & capture food.

  • Cell types: choanocytes (feeding), amoebocytes (transport, spicule formation, totipotent).

  • Reproduction: asexual gemmules (resist desiccation, extreme cold/low O22​, high G-forces); sexual larvae.

Subkingdom Eumetazoa ("true" animals)

  • Possess tissues, usually organs, defined symmetry, embryonic germ layers.

  • Vast majority of animal diversity (≈1.51.5 million described spp.).

Phylum Cnidaria

  • Radial symmetry; diploblastic (epidermis + gastrodermis, separated by mesoglea).

  • Tissue level organisation; single opening (mouth/anus) to gastrovascular cavity.

  • Specialized cells: cnidocytes with nematocysts (harpoon-like, toxin-injecting).

  • 22 body forms:

    • Polyp – sessile, mouth upward, thin mesoglea.

    • Medusa – free-swimming, mouth downward, thick mesoglea.

  • Life cycle (e.g., Obelia): asexual polyp colony → sexual medusa → planula larva → new polyp.

  • Major groups: Medusozoa (jellyfish, hydrozoans, siphonophores), Anthozoa (anemones, corals), Myxozoa (unicellular parasites).

Phylum Platyhelminthes (Flatworms)

  • Bilateral symmetry; triploblastic; dorsoventrally flattened.

  • Acoelomate: no body cavity, mesoderm forms parenchyma; diffusion replaces circulatory/respiratory systems; protonephridia remove excess water.

  • Organ-level complexity; obvious cephalisation (brain, sensory organs).

  • Digestive system: single opening; branching gastrovascular cavity.

  • Musculature: outer circular, inner longitudinal, dorsoventral layers → controlled locomotion.

  • Reproduction: asexual (fragmentation, budding) & sexual; simultaneous hermaphrodites.

  • Classes: Turbellaria (free-living), Cestoidea (tapeworms), Trematoda (endoparasitic flukes), Monogenea (ectoparasitic flukes).

Key Takeaways

  • Porifera: multicellular but tissue-less; filter-feeders with spicules.

  • Cnidaria: radial, diploblastic, cnidocytes, alternation of polyp/medusa.

  • Platyhelminthes: bilateral, triploblastic, organ systems, acoelomate, diverse life strategies.