notes_ porifera, gates

Introduction to Porifera

Name Origin

  • "Porifera" means "bearing pores."

  • Comprises about 10,000 species.

  • Mainly found in marine environments.

Size and Shape

  • Size can range from a few millimeters to about 2 meters.

  • Various shapes, including:

    • Tubes

    • Fans

    • Cups

    • Other forms

  • Most sponges are asymmetrical in shape.

Characteristics of Sponges

  • Considered primitive animals.

  • Multicellular but lack tissues and organs.

  • Absence of circulatory, excretory, or nervous systems.

  • Continuous water flow provides nourishment and oxygen and removes waste.

Morphology

  • Basic Structure:

    • Hollow cylinders closed at the bottom (holdfast) and open at the top (osculum).

    • A central cavity known as the spongocoel.

  • Body Plan:

    • Two-layer body wall:

      • Outer layer (epidermis)

      • Inner layer (gastrodermis)

    • Mesohyl: jelly-like substance serving as an endoskeleton.

Special Cells and Functions

  • Choanocytes:

    • Flagellated cells that generate a water current and filter food.

  • Amoebocytes:

    • Totipotent (stem) cells that deliver nutrients and participate in reproduction.

  • Porocytes:

    • Tubular cells controlling water flow through the pores (ostia).

  • Sclerocytes:

    • Cells that secrete silica spicules, serving as structural elements.

Types of Canal Systems

  • Vary in complexity to allow larger sizes and efficient water circulation:

    • Asconoid: Simple design.

    • Syconoid: Moderate complexity.

    • Leuconoid: High complexity.

Skeletal Systems

  • The mesohyl contains a gelatinous matrix with suspended cells, including:

    • Sclerocytes: Produce spicules.

    • Spongocytes: Secrete spongin.

Classes of Porifera

  • Calcarea:

    • Marine, small sponges (less than 10 cm).

    • Have calcareous spicules without spongin.

  • Hexactinellida:

    • Exclusively marine, larger sponges (10-30 cm).

    • Known as glass sponges with siliceous spicules.

  • Demospongiae:

    • Comprises over 90% of porifera.

    • Includes sponges with various shapes and complex body systems.

Feeding Mechanism

  • Sponges feed through a process called filter feeding:

    • Water enters through small pores (ostia) created by porocytes.

    • The water is drawn into the spongocoel by choanocytes via the movement of their flagella.

    • Food particles (bacteria, microalgae) are trapped on the collar of choanocytes and phagocytosed.

    • Nutrients are then transported to other cells such as amoebocytes, which distribute them throughout the sponge.

    • The water exits through the osculum, removing waste products.

Reproduction

  • Asexual:

    • Budding (new sponge grows from the side of an adult).

    • Regeneration (new clones develop if the sponge is broken).

  • Sexual:

    • Broadcast spawning (fertilization occurs inside the sponge).

    • Hermaphroditic sponges produce both sperm and eggs.

Conclusion

  • Sponges demonstrate unique adaptations and reproductive strategies, representing some of the earliest forms of multicellular life, which is essential for understanding evolutionary biology.

Sponges are considered primitive animals due to:

  • No True Tissues or Organs:

    • Lack organized tissues and organs present in complex animals.

  • Simple Structure:

    • Basic body plan: porous outer layer and a central cavity.

  • Basic Reproduction:

    • Asexual reproduction (budding or regeneration) is simpler than higher animals' sexual reproduction.

  • Missing Systems:

    • Lack circulatory, excretory, and nervous systems, relying on water flow for nutrient acquisition and waste removal.

  • Ancient Origins:

    • Among the earliest multicellular life forms, critical for studying evolution.

Body Plan

  • Two-layer body wall:

    • Outer layer known as the epidermis.

    • Inner layer referred to as the gastrodermis.

  • The space between these layers contains the mesohyl, a jelly-like substance that serves as an endoskeleton.