Overview of Animal Characteristics and Development

Key Characteristics of Animals

• Multicellular Eukaryotes: Animals are composed of multiple cells that contain a nucleus and organelles, distinguishing them from prokaryotic organisms such as bacteria.

• Heterotrophic: Animals obtain their food by consuming other organisms, unlike plants, which produce their own food through photosynthesis.

• Movement: Almost all animals exhibit some form of movement at some stage in their life cycles, which is a significant characteristic that differentiates them from plants and fungi.

• Common Evolutionary Ancestry: Despite the vast diversity in lifestyles and appearances among different animal species, genetic studies show that all animals share a common ancestor.

Terminology Relevant to Animal Development

• Symmetry: Key feature for organizing body structure.

  • Asymmetry: No symmetry; cannot be divided into equal parts.

  • Bilateral Symmetry: Can be divided into 2 equal halves along one plane (e.g., humans, dogs).

  • Radial Symmetry: Organized around a central axis (e.g., jellyfish, starfish).

• Embryonic Stages:

  • Blastula: An early developmental stage of the embryo consisting of a hollow ball of cells.

  • Blastopore: The opening that forms during gastrulation; becomes either the mouth or anus in bilateral animals.

  • Germ Layers: Groups of cells in embryos that differentiate into specific tissues:

  • Ectoderm: Forms the outer skin and nervous system.

  • Endoderm: Forms the gut lining and associated organs.

  • Mesoderm: Forms connective tissues, muscles, and circulatory systems.

Gastrulation and Developmental Decisions

• Gastrulation: The process where the embryo develops from a blastula into a more complex structure with defined layers. During this process, cells undergo significant differentiation, leading to the formation of the animal's body plan.

• Cells in the embryo must make "career decisions" that determine their developmental pathways, ultimately leading to specialized tissues and organs.

Classification of Animals

• The animal kingdom comprises ~30 phyla, each categorized based on distinct evolutionary traits and characteristics.

• Fundamental questions for classifying animals include:

  • What type of symmetry does the organism exhibit?

  • Does the organism have specialized tissues?

  • Are they classified as protostomes (mouth first in development) or deuterostomes (anus first in development)?

Examples of Animal Phyla

• Sponges: No true tissues or symmetry.

• Cnidarians: Radial symmetry, stinging cells for predation.

• Flatworms, Annelids, Mollusks, Roundworms, Arthropods: Various body plans and developments from protostomes with bilateral symmetry.

• Echinoderms and Chordates: Deuterostomes with bilateral symmetry that have specialized tissues.

This breakdown establishes a foundational understanding of animal biology, emphasizing their unique characteristics and classification within the broader context of life on Earth, illustrating the evolutionary links among diverse animal forms.