In Depth Notes on Tardigrada and Protostome Lineages

Tardigrada

  • Species Count: Approximately 800 species.
  • Size: Size is around 100 μm.
  • Microscopy: Tardigrades observed using colorized scanning electron microscopy (SEM).

Animal Classification

  • Early animals: Sponges and cnidarians display traits of some of the earliest animals on Earth.
  • Classification into two groups: Protostomes and Deuterostomes, based on embryonic development.

Protostomes

  • Divided into two lineages:
    • Lophotrochozoa
    • Ecdysozoa (discussion for this week)

Ecdysozoan Protostomes

  • Components include:
    • Arthropoda (e.g., insects, crustaceans)
    • Nematoda (e.g., roundworms)
  • Growth mechanism: Molting (shedding of the exoskeleton/cuticle).
  • Exoskeleton characteristics:
    • Provides support and protection.
    • Non-living tissue; does not grow with the organism.
    • Organisms undergo ecdysis to continue growing.
  • Ecdysis: The process of molting involving a three-layered cuticle.

Post-Embryonic Development

Incomplete Metamorphosis

  • Definition: Juveniles resemble adults, differing only in size and sexual maturity.
  • Examples: Reptiles (including birds), mammals (including humans).

Complete Metamorphosis

  • Definition: Organisms experience significant transformations from juvenile (larval) stages to adult forms.
  • Examples: Aquatic invertebrates, insects (e.g., caterpillars to butterflies), amphibians (e.g., tadpoles to frogs).

Nematoda (Roundworms)

  • Size: Very small, usually less than 1 mm.
  • Habitats: Free-living or parasitic in soil, aquatic environments, or as parasites in plants and animals (including humans).
  • Model Organism: Caenorhabditis elegans (C. elegans) is commonly used in studies of cellular differentiation.
  • Notable Features:
    • Pseudocoelom and complete digestive tract.
    • Not segmented.

Arthropoda

  • Includes:
    • Insects
    • Chelicerates (e.g., spiders, scorpions)
    • Myriapods (centipedes, millipedes)
    • Crustaceans (e.g., lobsters, crabs)
  • Growth: Achieved through ecdysis.
  • Body Plan:
    • Comprised of segments termed tagmata, each with a pair of appendages.
    • Exoskeleton made of chitin.

Arthropoda Subphyla

  • Chelicerates:

    • Animals include horseshoe crabs, spiders, ticks.
    • Characterized by 2 tagma: cephalothorax and abdomen.
    • Have 6 pairs of appendages:
    • 1 pair of fanged chelicerae (for feeding and defense).
    • 1 pair of pedipalps.
    • 4 pairs of walking legs.

  • Myriapoda:

    • Includes millipedes and centipedes.
    • 2 tagma: head and trunk.
    • Centipedes have 1 pair of walking legs per segment (carnivorous).
    • Millipedes have 2 pairs of walking legs per segment (detritivores).

  • Crustacea:

    • Includes lobsters, shrimp, crabs, and barnacles.
    • 2 tagma: cephalothorax and abdomen.
    • Unique characteristic: possess 2 pairs of antennae.

  • Insecta:

    • Includes beetles, butterflies, bees, and many others (around 70,000 named species).
    • Most diverse group of arthropods.
    • 3 tagma: head, thorax, abdomen.
    • Typically have 3 pairs of walking legs and 2 pairs of wings.

Crayfish Anatomy

  • External Structures (Key):
    • Cephalothorax, rostrum, eyes, carapace, abdomen, segments (somites), antennules,
      and antennae.
  • Internal Structures (Key):
    • Pyloric stomach, cardiac stomach, gastric mill, digestive glands, heart, gills, mandibular muscles, green glands, mandibles, maxillae, maxillipeds, chelipeds, walking legs, swimmerets, uropods, telson.
  • Notable Mention: Identify each structure and function for practical understanding!