Animals are fascinating, complex organisms with unique characteristics that set them apart from other life forms. From single-celled protozoans to massive blue whales, the animal kingdom showcases incredible diversity in form and function.

This section explores the defining traits of animals, their body plans, and classification systems. We'll also dive into animal reproduction and development, highlighting key stages from fertilization to organ formation. Understanding these concepts is crucial for grasping how animals evolved and thrive in various environments.

Defining Characteristics and Classification of Animals

Defining characteristics of animals

• Eukaryotic, multicellular organisms
  • Cells contain membrane-bound organelles (nucleus, mitochondria, endoplasmic reticulum) that perform specific functions
  • Specialized cells work together to form tissues (muscle), organs (heart), and organ systems (circulatory system)
• Heterotrophic nutrition
  • Obtain energy and nutrients by consuming other organisms (plants, animals, or microorganisms)
  • Lack rigid cell walls, allowing for greater flexibility and movement compared to plants and fungi
• Motility and sensory abilities
  • Most animals capable of movement at some stage of life cycle (swimming, flying, walking)
  • Sensory organs (eyes, ears, nose) detect changes in environment, enabling response to stimuli
• Sexual reproduction and embryonic development
  • Reproduce sexually by combining genetic material from two parents, with some exceptions (parthenogenesis in some insects)
  • Fertilized egg (zygote) undergoes complex embryonic development to form a multicellular organism

Body Plans and Adaptations

• Symmetry: Arrangement of body parts around a central axis
  • Radial symmetry: Body parts arranged around a central axis (e.g., jellyfish)
  • Bilateral symmetry: Body can be divided into left and right halves (e.g., humans)
• Exoskeleton: External supportive structure in some animals (e.g., insects, crustaceans)
• Metamorphosis: Dramatic change in body form during development (e.g., caterpillar to butterfly)
• Homeostasis: Maintenance of internal balance despite external changes
• Adaptation: Inherited trait that increases survival and reproduction in a specific environment

Levels of animal classification

• Kingdom Animalia divided into phyla based on body plan, symmetry, and embryonic development
  • Phylum: Broadest classification level within kingdom (Chordata includes all animals with a notochord)
• Each phylum divided into classes based on more specific characteristics
  • Class: Group of related orders (Mammalia includes all mammals, Aves includes all birds)
• Classes divided into orders, families, genera, and species
  • Order: Group of related families (Primates includes monkeys, apes, and humans; Carnivora includes cats, dogs, and bears)
  • Family: Group of related genera (Hominidae includes humans and great apes; Felidae includes all cat species)
  • Genus: Group of closely related species (Homo includes modern humans and extinct human species; Felis includes domestic cats and their wild relatives)
  • Species: Organisms capable of interbreeding to produce fertile offspring (Homo sapiens is modern humans; Felis catus is the domestic cat)

Animal Reproduction and Development

Stages of animal reproduction

1. Gametogenesis: Production of haploid gametes (eggs and sperm) through meiosis

2. Fertilization: Fusion of gametes to form a diploid zygote

3. Cleavage: Rapid cell division of zygote without significant growth

   • Blastula stage: Hollow ball of cells surrounding a fluid-filled cavity called the blastocoel

4. Gastrulation: Rearrangement of cells to form three germ layers

   • Ectoderm: Outer layer that gives rise to the nervous system and epidermis (skin)
   • Mesoderm: Middle layer that develops into muscles, skeleton, and other internal organs
   • Endoderm: Inner layer that forms the digestive system and associated glands (liver, pancreas)

5. Organogenesis: Development of organs from the three germ layers

6. Growth and differentiation: Cells specialize and tissues, organs, and systems develop and mature

Protostomes vs deuterostomes

• Protostome development ("mouth first")
  • Mouth develops from the blastopore, which is the first opening to form in the embryo
  • Coelom (body cavity) forms by splitting of the mesoderm
  • Spiral, determinate cleavage pattern, meaning early blastomeres are committed to specific cell fates
  • Examples include mollusks (snails, octopuses), annelids (earthworms), and arthropods (insects, crustaceans)
• Deuterostome development ("mouth second")
  • Anus develops from the blastopore, and the mouth forms later in development
  • Coelom forms from outpouchings of the archenteron, which is the primitive gut
  • Radial, indeterminate cleavage pattern, meaning early blastomeres are not committed to specific cell fates
  • Examples include echinoderms (sea stars, sea urchins) and chordates, which include all vertebrates (fish, amphibians, reptiles, birds, mammals)