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Introduction to Animals

Objectives

• Identify the characteristics that all animals share.

• Explain the essential functions animals must perform to survive.

• Describe some features of animal body plans.

What is an Animal?**

• Definition: Multicellular, heterotrophic, eukaryotic organisms.

• Categories:

  • Invertebrates: Animals that lack a vertebral column (e.g., worms, jellyfish, spiders).

    • More than 95% of animals are invertebrates.

  • Chordates: Phylum Chordata.

    • Exhibit key characteristics at some stage in their life cycle:

      • Dorsal hollow nerve cord.

      • A tail that extends beyond the anus.

      • Pharyngeal pouches (may develop into gills).

    • Most chordates are vertebrates, including fish, amphibians, reptiles, birds, and mammals.

    • Chordates without backbones are classified as invertebrates.

Characteristics of Chordates

• Common features include:

  • Dorsal hollow nerve cord

  • Notochord

  • Pharyngeal pouches

  • Tail extending beyond the anus.

  • Some chordates retain all these traits as adults, while others only exhibit them during embryonic development.

What Animals Do to Survive**

• Homeostasis:

  • Definition: Maintaining stable internal environment.

  • Mechanism: Feedback inhibition (negative feedback) regulates internal conditions.

  • Example: Shivering when cold to generate heat; sweating to cool when hot.

• Gathering and Responding to Information:

  • Receptor Cells: Available in the nervous system to detect external stimuli (sound, light, chemicals).

  • Invertebrates like sponges can respond with individual cells, while most chordates have complex nervous systems with a brain.

• Musculoskeletal System:

  • Muscles collaborate with supportive skeletal structures:

    • Earthworms utilize fluid pressure for movement (flexible skeleton).

    • Insects have external skeletons.

    • Vertebrates possess internal skeletons.

• Obtaining and Distributing Oxygen and Nutrients:

  • All animals require oxygen for cellular respiration.

  • Small animals can diffuse oxygen through their skin.

  • Larger animals use specialized respiratory systems (gills, lungs).

  • Animals ingest food for nutrients, requiring a digestive system, along with circulatory systems for transport.

• Waste Management and Reproduction:

  • Waste Elimination: CO2 expelled via respiratory systems, while complex animals handle other metabolic wastes.

  • Reproduction:

    • Most animals reproduce sexually for genetic diversity.

    • Some can reproduce asexually, leading to genetically identical offspring.

Animal Body Plans**

• Definition: Unique organization of body structures across different animal clades.

Components of Body Plans:

• Levels of Organization:

  • Cells differentiate into specialized tissues:

    • Epithelial tissue: Covers body surfaces.

    • Muscle tissue: Facilitates movement.

    • Nervous tissue: Sends signals.

    • Connective tissue: Includes bone and cartilage.

  • Tissues form organs; organs create systems that work together to maintain homeostasis.

• Body Symmetry:

  • Radial Symmetry: Equal halves can be achieved through multiple planes. (e.g., Sea anemone)

  • Bilateral Symmetry: One plane divides the organism into mirror image halves.

    • These animals possess defined anterior, posterior, dorsal, and ventral sides.

• Patterns of Embryological Development:

  • Begins with a zygote, forming a hollow ball of cells that eventually develops into a digestive tract.

  • Germ layers: Endoderm, Mesoderm, and Ectoderm.

• Body Cavities:

  • Fluid-filled spaces for organ accommodation; most complex animals have a coelom.

• Segmentation:

  • Bodies contain repeated segments that can specialize in functions.

• Cephalization:

  • Concentration of sense organs in the head region for improved sensory processing.

• Limb Formation:

  • Segmented animals typically possess external appendages for various locomotion styles.

• Complexity of Animal Body Plans:

  • Integrates morphological and developmental traits into functional units.

  • Key features affecting body plans: symmetry, tissues, body cavities.

Diversity of Invertebrates**

• Invertebrate Groups:

  • Account for 95% of animal species.

  • Occupy nearly all habitats and exhibit diverse morphological traits.

  • Key phyla include Lophotrochozoa, Ecdysozoa, and Deuterostomia.

Summary of Animal Evolution**

• Most animal phyla established by 500 million years ago, showing shared ancestry.

• Evolution reflects two major radiations leading to diverse species adapted to their environments.

Quick Summary

• All Animals: Multicellular, heterotrophic, eukaryotic organisms. Invertebrates (95%) lack vertebral column; Chordates have key life stage characteristics.

• Survival: Maintain homeostasis, respond to stimuli, obtain oxygen/nutrients, manage waste, and reproduce.

• Body Plans: Unique arrangement, symmetry (radial/bilateral), and distinct tissue organization.