Phylum Porifera (1)

Phylum Porifera

General Characteristics

  • No Hox genes present.

  • Lack "true" tissues; not classified as diploblastic or triploblastic.

  • Do not form a true gastrula; no true endoderm and ectoderm.

  • Possess specialized cells performing "tissue-like" functions that are not homologous to others in the animal kingdom.

  • Key specialized cells include choanocytes and amoebocytes.

  • Nutrient and oxygen intake occurs through water filtration.

  • Recommended video: Amazing footage of sponges pumping! (YouTube)

Poriferan Morphology

Body Plan

  • Characterized as asymmetrical.

  • Cylindrical shape with a large, internal cavity known as the spongocoel.

  • Water enters the spongocoel through structures called porocytes (also called ostia).

  • Water is expelled through the osculum, the large opening at the top of the sponge.

  • Mesohyl, a non-living substance, exists between the outer cell layer and the spongocoel.

Cellular Structure

  • Choanocyte cells:

    • Line the spongocoel to create a directional water flow.

    • Feature a flagellum that beats rhythmically to facilitate water movement.

  • Amoebocyte cells:

    • Located within the mesohyl, behave similarly to stem cells, capable of differentiating into other cell types.

Poriferan Physiology

Nutritional and Excretory Processes

  • Lacking a true digestive system.

  • Choanocytes have a mucus layer that traps food particles, which are engulfed via phagocytosis.

  • Engulfed food is placed in vacuoles and passed to amoebocytes for digestion.

  • Nutrients are diffused to other cells throughout the sponge.

  • No specialized respiratory or circulatory systems; function mainly through diffusion.

  • All cells in the spongocoel can perform gas exchange, absorbing O2, releasing CO2, and excreting nitrogenous waste.

Communication and Response

  • Absence of a nervous system.

  • Intercellular communication facilitates contraction of spongocoel and coordination among choanocytes.

  • Cells can respond to environmental changes.

Poriferan Reproduction

  • Sponges can reproduce asexually through fragmentation or budding.

  • Capable of sexual reproduction; monoecious organisms.

  • Amoebocytes differentiate into oocytes for egg production.

  • Choanocytes differentiate into sperm cells, which are released into water through the osculum.

  • Sperm enter other sponges through ostia to fertilize eggs in the mesohyl.

  • Early larval development occurs in the mesohyl; larvae eventually develop flagella.

  • Mature larvae dislodge from mesohyl, swim out of the osculum, and float in the water column.

  • Larvae eventually settle on a substrate and continue development to adulthood.