Overview
This lecture covers the diversity of animal life, key animal characteristics, and a detailed look at major protostome phyla such as sponges, cnidarians, flatworms, mollusks, annelids, nematodes, and arthropods.
Animal Characteristics & Classification
- Animals are multicellular, heterotrophic, and lack cell walls.
- Most animals are invertebrates without backbones.
- Animal features (like tissues, symmetry, and body cavity) reflect shared ancestry.
- Animals are classified by symmetry: none, radial (around a central axis), or bilateral (left/right sides).
- Germ layers form during embryonic development: ectoderm, endoderm, and (sometimes) mesoderm.
- Protostomes develop mouth first; deuterostomes develop anus first in embryos.
- Animals can have a true coelom, a pseudocoelom, or no coelom (body cavity).
- Digestive tract, segmentation, and reproductive strategies are key classification features.
Simple Invertebrates
- Sponges (Phylum Porifera) lack true tissues, are filter feeders, and often hermaphroditic.
- Cnidarians have radial symmetry, unique mesoglea (jelly-like layer), and stinging cells (nematocysts).
- Cnidarians reproduce both sexually and asexually.
Flatworms (Phylum Platyhelminthes)
- Flatworms are bilaterally symmetric, simple protostomes with no coelom.
- Planarians are free-living flatworms.
- Flukes and tapeworms are parasitic flatworms.
Mollusks
- Mollusks are soft, unsegmented protostomes with diverse forms and several organ systems.
- They have a complete digestive tract.
Annelids
- Annelids are segmented worms found in various environments.
- They possess several organ systems.
Nematodes (Roundworms)
- Nematodes are unsegmented worms with few organ systems.
- Some nematode species can cause diseases.
Arthropods
- Arthropods have jointed appendages, an exoskeleton, and specialized body segments.
- Their organ systems are complex, and they are the most diverse animal group.
- Arthropods are classified by mouthparts into chelicerates (spiders), myriapods (centipedes/millipedes), crustaceans (crabs), and insects.
Deuterostomes Preview
- Echinoderms are marine deuterostomes (covered briefly at end).
Key Terms & Definitions
- Protostome — animal whose mouth forms before the anus in embryonic development.
- Coelom — body cavity completely lined with mesoderm.
- Radial Symmetry — body plan arranged around a central axis.
- Bilateral Symmetry — body has distinct left and right sides.
- Mesoglea — jelly-like layer found in cnidarians.
- Hermaphrodite — organism with both male and female reproductive organs.
Action Items / Next Steps
- Review differences between protostome and deuterostome development.
- Study diagrams of animal body plans and organ systems.
- Complete assigned Mastering Concepts questions from section 21.1.