2_Arthropods _ what are arthropods

Understanding Vertebrates and Invertebrates

Key Differences

  • Vertebrates

    • Possess an internal skeleton (endoskeleton)

    • Have vertebrae/backbones arranged in a spinal column

    • Provide support against gravity

    • Can achieve large body sizes in terrestrial environments

  • Invertebrates

    • Lack vertebrae and an internal skeleton

    • Typically have an external support structure (exoskeleton)

Examples

  • Vertebrates: Includes fish, amphibians, reptiles, birds, and mammals.

  • Invertebrates: All other animals, including arthropods, mollusks, and cnidarians.

Arthropods

Definition

  • The term "arthropod" comes from the Greek words "arthron" (joint) and "podos" (foot).

  • Refers to organisms with jointed appendages.

Characteristics

  • Jointed Appendages: Used for locomotion and modified for antennae, mouthparts, and reproductive organs.v

  • Species Diversity: Approximately 1,200,000 known species; constitute over 60% of all named organisms.

  • Examples: Crabs, lobsters, spiders, scorpions, millipedes, centipedes, insects.

Common Traits of Arthropods

Body Structure

  • Bilateral Symmetry: Can be divided into left and right halves along the anterior-posterior axis.

  • Segmented Body Plan: Bodies consist of segments often fused into regions (tagmata) with specific functions.

  • Nerve Cord: Ventral nerve cord differs from vertebrates which have dorsal nerve cords.

  • Open Circulatory System: Lack a network of blood vessels; a single dorsal blood vessel circulates blood through body cavities.

Exoskeleton

  • Made of cuticle and epidermis.

  • Cuticle Layers:

    • Epicuticle: Waxy layer that prevents water loss and acts as a pathogen barrier.

    • Exocuticle: Hardened through tanning (crosslinking proteins), offers protection and support.

    • Endocuticle: Remains pliable and provides flexibility.

Molting Process (Ecdysis)

Steps in Molting

  1. Apolysis: Separation of old cuticle from epidermal cells, stimulating replication due to ecdysteroid molting hormone.

  2. Digestive Fluid Release: Enzymatic breakdown of endocuticle layer; metabolites reabsorbed by epidermal cells to form new cuticle.

  3. Ecdysis: New cuticle formed, arthropod sheds old one.

  4. Expansion of New Cuticle: Increased internal pressure by contracting muscles or swallowing air/water allows stretching of the soft cuticle before hardening.

Vulnerability

  • Newly shed arthropods are vulnerable to predation and desiccation until the cuticle hardens.

Conclusion

  • Understanding the characteristics of vertebrates and invertebrates provides insight into the diversity of life, particularly within arthropods, which demonstrate unique adaptations and a complex evolutionary history.