Animal Taxonomy and Classification

Introduction

• Animal taxonomy involves identifying, naming, and classifying animals.
• Carolus Linnaeus (1707-1778) is recognized as the founder of taxonomy.
• There are approximately 1.2 million identified animal species, with estimates of 9-10 million total species on Earth.
• The importance of classification lies in assigning a systematic position to newly discovered species.

Taxonomy vs. Systematics

• Taxonomy focuses on describing, naming, and classifying organisms.
• Systematics studies biological diversity and evolutionary relationships between organisms.
• Taxonomic groups are now based on hypotheses about evolutionary relationships derived from systematics.

Classification

• The classification of animals is hierarchical, with progressively smaller and more specific groups within each level.
• For example, a species of nematode, Caenorhabditis elegans, is classified as follows:
  • Kingdom: Animalia
  • Phylum: Nematoda
  • Class: Chromadorea
  • Order: Rhabditida
  • Family: Rhabditidae
  • Genus: Caenorhabditis
  • Species: elegans

Body Symmetry

• Body symmetry refers to the balance of an organism's body parts.
• Three axes are considered:
  • Anterior-posterior (head-tail) axis
  • Dorsal-ventral (back-belly) axis
  • Left-right axis
• Cross sections bisecting the anterior-posterior axis are used to categorize symmetry:
  • Bilateral symmetry: A single plane divides the body into two mirror images.
  • Radial symmetry: Many planes can divide the body equally.
  • Asymmetry: No plane can divide the body into equal halves.

Coelom

• A coelom is a body cavity present in most animals, located between the intestinal canal and the body wall.
• The presence of body cavities plays a role in classification.

Germ Layers

• Germ layers are the primary tissues that form during embryonic development.
• Diploblastic animals: Have two embryonic layers:
  • External ectoderm
  • Internal endoderm
  • Mesoglea present between ectoderm and endoderm
• Triploblastic animals: Have three embryonic layers:
  • External ectoderm
  • Middle layer of mesoderm
  • Internal endoderm
  • Mesoglea is absent. Instead, the mesoderm is present between the endoderm and ectoderm.

Lateral Line System

• The lateral line system (LLS) is a sensory organ found in aquatic vertebrates.
• It detects movement, vibration, and pressure changes in the surrounding water.
• Hair cells, modified epithelial cells, respond to displacement caused by motion and transduce these signals into electrical impulses through excitatory synapses.

Basis of Classification

• Several key characteristics are used to classify animals:
  • Body symmetry
  • Nature of coelom
  • Patterns of digestive, circulatory, and reproductive systems
  • Arrangement of cells in germ layers