In-Depth Notes on Animal Phylogeny and Classification

Animalia Overview
  • Kingdom Animalia includes nearly 2 million described species.
  • Majority are invertebrates; about 67,000 are vertebrates.
  • Evolutionary diversification allowed for feeding on other organisms and mobility.
Definition of an Animal
  • Eukaryotic, multicellular organisms without cell walls unlike plants or fungi.
  • Specialized tissues and organ systems facilitate nutrient, oxygen delivery, and waste removal.
  • All animals are heterotrophs, requiring consumption of other organisms for energy.
Animal Characteristics
  • Oxygen is metabolized through aerobic respiration.
  • Most animals exhibit motility during some life stage.
  • Presence of sensory and nervous systems enables reaction to stimuli.
  • Reproductive strategies include both asexual and sexual means:
    • Sexual reproduction involves haploid gametes (eggs and sperm) forming diploid zygotes, leading to a multicellular embryo.
Origins of Animals
  • Common ancestor likely a colonial flagellated protist existing 700 million years ago, resembling today's choanoflagellates.
Key Innovations in Animal Evolution
  • Tissue Structure: Divides animals into Parazoa (sponges with no tissues) and Eumetazoa (true tissues).
  • Body Symmetry: Asymmetrical (sponges) vs. symmetrical (eumetazoans). Types:
    • Radiata: radial symmetry
    • Bilateria: bilateral symmetry with cephalization (anterior head development).
  • Body Cavities:
    • Acoelomates: No body cavity (e.g., flatworms).
    • Pseudocoelomates: Fluid-filled space between gut and body wall (e.g., roundworms).
    • Coelomates: Body cavity completely lined by mesodermal peritoneum (e.g., mollusks).
  • Developmental Patterns: Divided into two lineages: Protostomes and Deuterostomes.
Developmental Patterns in Animals
  • Cleavage:
    • Protostomes have spiral cleavage; deuterostomes have radial cleavage.
    • Determinate cleavage in protostomes vs. indeterminate cleavage in deuterostomes.
  • Mouth and Anus Formation:
    • Protostomes: Blastopore becomes mouth; anus forms elsewhere.
    • Deuterostomes: Blastopore becomes anus; mouth forms separately.
  • Mesoderm and Coelom Formation:
    • Protostome mesoderm formation via schizocoelom.
    • Deuterostome mesoderm forms from enterocoelom.
  • Segmentation: Body parts as repeating units observed in both lineages (e.g., vertebrae).
Classification of Animal Phyla
  • Major Clades:
    • Parazoa: Phylum Porifera (sponges, ~8,000 species).
    • Eumetazoa: Radially symmetric Cnidaria (~11,000 species) and Ctenophora (~150 species).
    • Bilateria: Vast diversity including Lophotrochozoa (e.g., Mollusca, Annelida), Ecdysozoa (e.g., Arthropoda, Nematoda), and Deuterostomia (Chordata).
Eumetazoans with Radial Symmetry
  • Cnidaria: Simple tissue structure; has specialized cells (cnidocytes) for capturing prey.
  • Ctenophora: Transparent comb jellies; move via cilia, capturing prey with sticky tentacles.
Bilaterally Symmetric Animals
  • Possess more complex tissue structures and organ systems.
  • Most display coeloms or pseudocoeloms; sensory organs concentrated anteriorly.
Lophotrochozoan Protostomes
  • Include various phyla characterized by a lophophore for feeding (Ectoprocta, Brachiopoda).
  • Mollusks exhibit diverse forms and functions.
Ecdysozoans (Protostomes)
  • Characterized by an exoskeleton that is periodically shed (ecdysis).
  • Major phyla include Nematoda (roundworms) and Arthropoda (insects, arachnids).
Conclusion on Animal Phylogeny
  • Through studying anatomical and developmental traits, understanding animal phylogeny reveals the evolutionary relationships among species, elucidating the development of diverse forms and functions in the animal kingdom.