Notes on Porifera and Cnidaria

KINGDOM ANIMALIA

  • Divided into 36 phyla.
  • 9 phyla account for over 96% of all animal species:
    • Porifera (Sponges)
    • Cnidaria (Jellyfish, Coral, Anemones)
    • Platyhelminthes (Flatworms)
    • Annelida (Segmented Worms)
    • Mollusca (Snails, Bivalves, Octopi)
    • Nematoda (Roundworms)
    • Arthropoda (Insects, Spiders, etc.)
    • Echinodermata (Starfish, Urchins, etc.)
    • Chordata (Vertebrates)

THE SIMPLEST INVERTEBRATES

  • Phylum Porifera and Cnidaria are considered the simplest invertebrates.
  • Porifera (Sponges):
    • Lack true tissues
    • Exhibit an asymmetrical body plan
  • Cnidarians (Jellies & Anemones):
    • Diploblastic (two tissue layers)
    • Radial body symmetry
    • No true organs

PHYLUM PORIFERA

  • Only animal group without true tissues, operating at a cellular level.
  • Composed of few cell types and lacks muscle and nervous tissue.
  • Sponges are primarily marine and sessile, having simple or complex porous structures that filter water for nutrients.

PHYLUM PORIFERA: ANATOMY

  • Choanocytes:
    • Flagellated and collared filtering cells lining the interior.
    • Resemble choanoflagellates; generate water currents for feeding.
  • Porocytes:
    • Tube-shaped cells allowing water flow into the sponge's internal chamber.
  • Food particles are captured via microvilli through phagocytosis.
  • Food digested in vacuoles of choanocytes and amoebocytes which assist in digestion.
  • Sponges can filter excessive amounts of water—up to 100 L per day.

PHYLUM PORIFERA: STRUCTURAL SUPPORT

  • Two types:
    • Spongin: Tough fibrous protein, providing structure.
    • Spicules: Needle-like structures of calcium carbonate or silica, providing structural classification (bony, glassy, or spongin).

PHYLUM PORIFERA: REPRODUCTION

  • Sponges reproduce both asexually (fragmentation or budding) and sexually.
  • Sexual reproduction involves sperm released into the water, entering female sponges for fertilization, resulting in larvae released through an osculum.

PHYLUM PORIFERA: ECOLOGY

  • Sponges are ancient and simple, with few natural predators.
  • Spicules and toxic substances act as defense mechanisms.
  • Sponges can regenerate and reassemble from damaged parts.

PHYLUM CNIDARIA: CHARACTERISTICS

  • Bell-shaped predatory animals; primarily aquatic.
  • Notable for cnidocytes, specialized stinging cells for capturing prey.

PHYLUM CNIDARIA: CNIDOCYTES

  • Each cnidocyte has a nematocyst, a spirally coiled thread that discharges upon contact.
  • Nematocysts can entangle, stick, or penetrate prey with barbed spines, which can also release toxins.

PHYLUM CNIDARIA: ANATOMY

  • Composed of two tissue layers:
    • Outer epidermis (ectoderm)
    • Inner gastrodermis (endoderm)
  • Gastrovascular Cavity:
    • Serves as a hydrostatic skeleton, aiding in movement and structure.
    • Mesoglea: jelly-like substance between layers.

PHYLUM CNIDARIA: FUNCTIONAL ANATOMY

  • Epidermis features muscle fibers allowing contraction and coordination of movements.
  • Two body forms exist:
    • Polyp: Tentacles directed upwards.
    • Medusa: Tentacles directed downwards, larger mesoglea compared to polyp.

PHYLUM CNIDARIA: LIFE CYCLE

  • Many species have life cycles involving both polyp and medusa forms.
  • Polyp stages can reproduce asexually and produce medusa forms that reproduce sexually.

PHYLUM CNIDARIA: DIVERSITY

  • Jellyfish (dominant medusa phase) and hydrozoans (dominant polyp phase).
  • Sea Anemones:
    • Sessile polyps with vibrant colors and no medusa stage.
  • Corals:
    • Resemble anemones encased in calcium carbonate.
    • Some solitary, most in colonies, with symbiotic algae enhancing their energy acquisition.