Lecture 7 - Invertebrate Phyla: Overview of Sponges and Cnidarians

Introduction to Invertebrate Phyla

  • Transition from arthropods (insects) to other invertebrate phyla.

  • Overview of major invertebrate phyla excluding chordates (vertebrates).

  • Objective: Describe distinguishing features, general characteristics, and diversity within major invertebrate groups.

Major Invertebrate Phyla Overview

  • Discussion on major phyla, including variations and characteristics differentiating classes,

  • Focus on:

    • Porifera (sponges)

    • Cnidaria (jellyfish, anemones)

    • Platyhelminthes (flatworms)

    • Mollusca (mollusks)

    • Annelida (segmented worms)

    • Nematoda (roundworms)

  • Nematoda noted for high diversity but limited discussion due to morphological similarities.

Evolutionary Relationships and Characteristics

  • Animals evolved from ancestral colonial choanoflagellates (protozoa with single flagella and collar).

  • Morphological and genetic similarities to animals support this evolutionary claim.

  • Introduction to Porifera (sponges) and Cnidaria (eumetazoans) as early offshoots from the evolutionary tree.

Porifera: The Sponges

  • Classification: Traditionally categorized under parazoa, but outdated.

  • Key characteristics:

    • Lack of true tissues

    • No gastrulation during development

  • Cellular organization:

    • Composed of a mass of cells

    • Cellular level of organization with division of labor among approximately 10 cell types.

Characteristics of Sponges

  • Species Diversity: Approximately 6,000 species; significant fossil history.

  • Fossil evidence dates back to 565 million years; controversy around claims of 800 million year fossils.

  • Ecological Distribution: Mostly aquatic (marine and some freshwater).

Structural Features of Sponges

  • Body Organization: Filter feeding system through numerous pores (ostia) allowing water intake, exits through a single osculum.

  • Cell Types:

    • Choanocytes: Feeding cells with flagella that circulate water and capture food.

    • Amoebocytes: Mobile cells for transportation of nutrients and digestive processes.

    • Pinacocytes: Cells lining the outer surface, somewhat resembling an epidermis but lacking cell junctions.

    • Porocytes: Specialized cells forming ostia in basic sponges (esconoids).

  • Skeletal Structure:

    • Skeleton made of spongen (collagen), calcium, silica, or a combination.

Body Forms of Sponges

  • Three main types of body forms:

    • Asconoid: Simple structure; least efficient at filtering.

    • Syconoid: More complex; allows better water flow and food capture.

    • Leuconoid: Most complex; better food capture efficiency due to multiple chambers.

Functional Efficiency and Adaptations

  • Sponges have evolved various body designs for maximum filtering efficiency through their porous bodies.

  • Functionality relies on cell differentiation and division of labor.

  • Importance of skeletal structure in supporting sponge anatomy.

Cnidaria: The Jellyfish and Anemones

  • Characteristic features:

    • Presence of true tissues (ectoderm and endoderm).

    • Radial symmetry (typically).

  • Body Forms:

    • Polyp: Attached, cylindrical with tentacles around the mouth.

    • Medusa: Free-floating, umbrella-like, with tentacles oriented downward.

Key Features of Cnidaria

  • Nematocysts: Specialized stinging cells prominent in capturing prey; contain harpoon-like structures.

  • Two layers of tissue (diploblastic): epidermis and gastrodermis with a non-cellular mesoglea layer in between.

  • Gastrovascular cavity functioning as both mouth and anus (single opening).

  • Absence of mesoderm means muscles derived from the epidermis instead.

Cnidarian Classes

  • Hydrozoa: Primarily colonial polyps; have both medusa and polyp stages. Example: Hydra (freshwater, asexual reproduction).

  • Scyphozoa: True jellyfish; large, complex medusae predominating the life cycle.

  • Anthozoa: Exclusively polyp forms (e.g. corals and anemones) capable of sexual reproduction within polyp stage.

Conclusion

  • Overview of major invertebrate phyla provides foundation for understanding animal diversity and evolution.

  • Further exploration into Cnidarians and their ecological and biological significance in subsequent lectures.