An Introduction to Invertebrates

Vocabulary

  • Metabolic characteristics of major phyla:

    • Porifera (sponges)

    • Cnidaria

    • Lophotrochozoa

    • Ecdysozoa

    • Deuterostomia

  • Physical characteristics of major phyla:

    • Porifera

    • Cnidaria

    • Lophotrochozoa

    • Ecdysozoa

    • Deuterostomia

  • Comparative analysis of physical and metabolic characteristics among different genera in each group.

  • Importance of structural roles within each group.

Understanding Invertebrate Diversity

  • Invertebrates make up over 95% of known animal species and inhabit nearly every environment on Earth.

  • Morphological diversity includes organisms ranging from microscopic size to species exceeding 18 m in length.

Evolutionary Relationships

  • Classification based on evolutionary relationships helps to comprehend the extensive diversity of invertebrates:

    • Common ancestor of all animals

    • Split into Eumetazoa, Porifera, Cnidaria, Lophotrochozoa, Ecdysozoa, and Deuterostomia

    • Within Eumetazoa:

    • Bilateria: Animals displaying bilateral symmetry, three tissue layers

      • Lophotrochozoa: Exhibits the most versatile body plans

      • Ecdysozoa: Contains the majority of animal species

      • Deuterostomia: Inclusive of both invertebrate and vertebrate species.

Details on Major Invertebrate Groups

Phylum Porifera (Sponges)

  • Characteristics:

    • Approximately 5,500 known species.

    • Habitat and feeding: Sessile filter feeders; lack true tissues.

    • Body plan:

    • Structure resembles a sac, with pores for water circulation.

    • Water enters through spongocoel, exits via the osculum.

    • Cell types:

    • Choanocytes: Flagellated cells that capture food through phagocytosis.

    • Mesohyl: Gelatinous region separating two cell layers.

    • Amoebocytes: Totipotent cells for digestion, nutrient transport, and skeletal fiber production.

    • Reproduction: Hermaphroditic, often functioning first as one sex then the other (sequential hermaphroditism). Zygotes develop into flagellated larvae.

Phylum Cnidaria

  • Characteristics: 10,000 species, radially symmetrical, diploblastic (two germ layers) and possess a gastrovascular cavity.

  • Examples: Corals, jellies, and hydras.

  • Body Plan Variations:

    • Polyp: Sessile form adhering to surfaces; aboral end opposite the mouth.

    • Medusa: Free-swimming jellyfish form.

  • Feeding:

    • Predators utilizing tentacles with cnidocytes for capturing prey.

    • Cnidocytes contain specialized nematocysts that eject stinging threads.

  • Nervous System: Non-centralized nerve net allowing responsive movement with sensory structures.

Clades of Cnidaria:

  • Medusozoa: Include all cnidarians producing a medusa stage.

  • Anthozoa: Exclusively polyp forms including sea anemones and corals.

  • Ecological Impact: Coral polyps create reefs, crucial habitats for marine biodiversity but are threatened by environmental changes.

Phylum Lophotrochozoa

  • Defined by structural diversity including 18 phyla. Notable examples:

    • Platyhelminthes (Flatworms): Dorsoventrally flattened, primarily parasites and free-living.

    • Rotifers: Microscopic, complex organ systems, known for parthenogenesis.

    • Brachiopods and Ectoprocts: Feature lophophores for feeding alongside unique morphologies.

    • Mollusca: Includes snails, clams, squid comprising diverse habitats (marine, freshwater, terrestrial).

Phylum Ecdysozoa

  • Noted for high species richness, characterized by a tough outer cuticle shed during molting or ecdysis.

  • Major groups:

    • Nematoda (Roundworms): Abundant in many habitats, significant plant and animal parasites.

    • Arthropoda: Over 1 million species, featuring exoskeletons, segmented bodies, jointed appendages.

    • Subgroups: Myriapods (centipedes, millipedes), Chelicerates (spiders, scorpions), and Pancrustaceans (insects, crustaceans).

Phylum Deuterostomia

  • Includes both invertebrate and vertebrate groups (e.g., Hemichordata, Echinodermata, and Chordata).

  • Shared Developmental Patterns:

    • Radial cleavage leading to the formation of the anus from the blastopore.

Key Invertebrate Groups in Deuterostomia:

  • Echinodermata:

    • Marine animals with calcareous endoskeletons, a water vascular system.

    • Exhibit radial symmetry in adulthood with bilateral symmetry in larvae.

    • Examples: Sea stars (Asteroidea), brittle stars (Ophiuroidea), sea urchins (Echinoidea), feather stars (Crinoidea), sea cucumbers (Holothuroidea).

  • Chordata: Contains vertebrates and invertebrates encapsulating lancelets and tunicates.

Concluding Remarks

  • Invertebrate diversity exemplifies ecological, morphological, and evolutionary complexity, crucial for understanding broader biological principles and ecosystems.