Cnidarians, including jellyfish and corals, represent a significant leap in animal complexity. These creatures boast true tissues, a radially symmetrical body plan, and specialized cells called cnidocytes. Their unique features set them apart from simpler organisms like sponges.

Cnidarians come in two main body forms: the sessile polyp and the free-swimming medusa. They're divided into four classes, each with distinct characteristics. These animals have adapted to various aquatic environments, showcasing remarkable regeneration abilities and diverse reproductive strategies.

Phylum Cnidaria

Organization of Porifera vs Cnidaria

• Porifera (sponges)
  • Asymmetrical or radially symmetrical body plan
  • Lack true tissues and organs, instead have loosely organized cells
  • Possess a water canal system with choanocytes that filter feed by creating water currents (Leuconia)
  • Skeleton composed of spicules made of calcium carbonate or silica, or spongin protein fibers (bath sponge)
• Cnidaria (jellyfish, corals, and sea anemones)
  • Radially symmetrical body plan with oral and aboral ends
  • Possess true tissues organized into two layers (diploblastic), an epidermis and gastrodermis
    • Epidermis (outer layer) provides protection and contains cnidocytes
    • Gastrodermis (inner layer) lines the gastrovascular cavity aids in digestion and absorption
    • Mesoglea (non-cellular layer) separates epidermis and gastrodermis, providing support (jellyfish)
  • Gastrovascular cavity (coelenteron) serves as a digestive tract, distributing nutrients throughout the body
  • Specialized cells called cnidocytes contain nematocysts used for defense and prey capture (jellyfish tentacles)

Tissue development in Cnidaria

• Cnidarians are the first animals to develop true tissues, a key step in increasing animal complexity
  • Epidermis and gastrodermis are organized into functional layers with specific roles
  • Cells within each layer are connected by cell junctions, allowing communication and coordination
• Tissue organization enables specialization and division of labor among cells
  • Epidermal cells protect the animal from the environment and contain cnidocytes for defense (sea anemone)
  • Gastrodermal cells secrete digestive enzymes and absorb nutrients from the gastrovascular cavity
• Presence of a nervous system with nerve nets allows for coordination of body movements and responses to stimuli
  • Nerve nets consist of interconnected neurons that transmit signals throughout the body (Hydra)
• Development of tissues in cnidarians represents a significant increase in complexity compared to sponges

Polyp vs medusa body forms

• Polyp form
  • Sessile lifestyle, attached to a substrate
  • Cylindrical body with a mouth surrounded by tentacles used for feeding and defense
  • Mouth opens into a gastrovascular cavity that serves as a digestive tract
  • Examples include corals, sea anemones, and hydras
• Medusa form
  • Free-swimming lifestyle, moving through the water column
  • Bell-shaped body with tentacles hanging from the margin, used for swimming and prey capture
  • Mouth located on the underside of the bell, leading to the gastrovascular cavity
  • Examples include jellyfish like moon jellies and lion's mane jellies
• Some cnidarians exhibit both polyp and medusa forms in their life cycle, a phenomenon called metagenesis (Obelia)

Characteristics of cnidarian classes

• Hydrozoa
  • Exhibit both polyp and medusa forms in their life cycle
  • Can be colonial or solitary
  • Examples include Hydra, Obelia, and Portuguese man-of-war (Physalia)
• Scyphozoa
  • Predominantly exist in the medusa form
  • Have a complex life cycle that includes a planula larva, polyp, and medusa stage
  • Examples include moon jellyfish (Aurelia) and lion's mane jellyfish (Cyanea)
• Cubozoa
  • Exclusively exist in the medusa form, lacking a polyp stage
  • Possess a box-shaped bell with complex eyes and potent venom
  • Examples include box jellyfish (Chironex) and sea wasps (Carukia)
• Anthozoa
  • Exclusively exist in the polyp form, lacking a medusa stage
  • Includes both solitary and colonial species
  • Examples include corals (Acropora), sea anemones (Nematostella), and sea pens (Pennatula)

Adaptations and Reproduction

• Cnidarians possess remarkable regeneration abilities, allowing them to regrow lost body parts
• Many species exhibit hermaphroditism, with individuals capable of producing both male and female gametes
• Tentacles play a crucial role in prey capture, defense, and environmental sensing