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.