KB

Biol120 – Lecture 4: What Are Animals?

Learning Objectives

  • Understanding the basic characteristics of Animalia.

  • Understanding the basics of animal sexual cycle and reproduction.

  • Understanding cell division and development of the zygote.

  • Understanding the difference and significance of radial and bilateral symmetry.

  • Understanding the difference between protostome and deuterostome animals.

  • Understanding the different types of animal skeletons.

Characteristics of Kingdom Animalia

  • Multicellular; many specialized cells

  • HeterotrophicHeterotrophic: obtain organic nutrients from other organisms

  • Capable of locomotion (for food, reproduction, dispersal)

  • Cellular differentiation ➜ tissues ➜ organs ➜ organ systems

Levels of Organization

  • Cellular level: independent cells (e.g., Porifera)

  • Tissue level: coordinated cells; diploblastic & triploblastic animals

  • Organ level: different tissues integrated; typical of bilaterally symmetrical, triploblastic animals

Representative Abundance ("Typical" Animals)

  • Soil nematodes: 4.4×10204.4×1020 individuals (≈ 5757 billion per human)

  • Ants: 2.0×10162.0×1016 individuals; biomass ≈ 1212 megaton dry C

  • Bristlemouth (Cyclothone): 1.0×10151.0×1015 individuals; biomass ≈ 109109 tons

Reproduction & Early Development

  • Sexual cycle: egg ++ sperm → zygote

  • Cleavage: rapid mitotic divisions; cell number 2→4→8→16…2→4→8→16…, cell size decreases

  • Blastula: hollow ball; internal cavity = blastocoel

  • Gastrulation: invagination forms gastrula- Creates archenteron (primitive gut) and blastopore (initial opening)

    • Establishes germ layers

Germ Layers & Their Fates

  • Diploblastic: ectoderm + endoderm

  • Triploblastic: ectoderm + mesoderm + endoderm- Ectoderm → epidermis, nervous tissue

    • Mesoderm → muscles, blood, bones, reproductive organs, connective tissues

    • Endoderm → gut lining & associated organs

Protostomes vs Deuterostomes

  • Cleavage- Protostome: spiral, determinate

    • Deuterostome: radial, indeterminate

  • Blastopore fate- Protostome: becomes mouth

    • Deuterostome: becomes anus

  • Coelom formation- Protostome: mesoderm splits (schizocoely)

    • Deuterostome: mesodermal pouches pinch off gut (enterocoely)

  • Major groups- Protostomes: Mollusca, Annelida, Arthropoda, Nematoda, etc.

    • Deuterostomes: Echinodermata, Chordata

Body Cavities

  • Acoelomate: no body cavity (e.g., flatworms)

  • Pseudocoelomate: cavity not fully lined by mesoderm (e.g., nematodes)

  • Coelomate (eucoelomate): true coelom fully lined by mesoderm (e.g., annelids, chordates)

Symmetry

  • Asymmetry: no plane of symmetry (sponges)

  • Radial symmetry: multiple planes around longitudinal axis (cnidarians)

  • Bilateral symmetry: single sagittal plane; cephalization; "tube-within-a-tube" body plan; majority of animals

  • Pentaradial symmetry: five-fold; unique to adult echinoderms (larvae bilateral)

Phylogeny Snapshot

  • Metazoa splits into Parazoa (Porifera) & Eumetazoa (true tissues)

  • Eumetazoa ➜ Cnidaria + Bilateria

  • Bilateria divides into Protostomia & Deuterostomia

Quick Summary

  • Animals = multicellular, heterotrophic, motile, possess differentiated tissues/organs

  • Sexual reproduction forms zygote → cleavage → blastula → gastrula → germ layers

  • Triploblastic condition enables complex organs & bilateral symmetry

  • Protostome vs deuterostome development distinguishes major animal lineages

  • Body cavity type & symmetry are key anatomical traits for classification