Cnidaria cont.

• Cnidarians exhibit two body plans during their life cycle: medusa and polyp
• Radial symmetry in cnidarians allows them to be divided into similar halves by passing a plane at any angle along a central axis
• Oral and aboral regions in cnidarians have 1 opening and 2 ends
• Cnidarians are diploblastic organisms with no specialized cells, featuring:

  • Epidermis covering the body
  • Gastrodermis lining the gastrovascular cavity
  • Mesoglea with ameobocytes for digestion, nutrient transport and storage, wound repair, and antibacterial defense
• Cnidae in cnidarians are complex intracellular organelles, with nematocysts being one type:

  • Nematocysts are tiny capsules composed of material similar to chitin, containing coiled threads that may bear barbs
  • Cnidae are abundant in tentacles and the gastrovascular cavity
• Three major categories of cnidae in cnidarians are:

  • Penetrant: Stenotele is the largest and most complex nematocyst, used for injecting poisonous fluid into prey
  • Glutinant: Ptychocysts are sticky and used to stick to prey, found in burrowing anemones
  • Volvent: Desmoneme is small and pear-shaped, coils around prey; smallest nematocysts
• Cnidae in C. adhaesivum are discharged for:

  • Wrapping around small objects
  • Sticking to surfaces
  • Penetrating surfaces
  • Secreting proteinaceous toxin
  • Discharged only once
• Cnidarians swim by jet propulsion with low pressure for low energy usage, or by waggling their base like sea anemones
• Cnidarians lack brains and CNS, instead having decentralized nerve nets with sensory neurons, motor neurons, and intermediate neurons acting as local coordination centers
• Cnidarians like Aurelia sp. have bipolar neurons running through conduction tracts for faster sending of impulses, with rhopalia serving as receptors for light, balance, touch, and chemical detection
• Statocysts in cnidarians sense tilt and acceleration, with organs lined with hairs detecting the movements of internal mineral grains called statoliths
• When a jellyfish tilts in the wrong direction, it rights itself by increasing the strength of the swimming movements on the side that is too low
• Ocelli are simple eyes that can detect sources of light:

  • Cubozoans have four sets of eyes, each with retinas, corneas, and lenses
  • They can clearly distinguish the direction from which light is coming as well as negotiate around solid-colored objects
  • Ocelli do not form images but detect the direction of light
• Feeding habits of Cnidarians include predation, absorption of dissolved organic matter, filter feeding, and having endosymbionts
• In digestion and excretion:

  • Gland cells release enzymes without flagella
  • Gastroderm cells absorb nutrients through intracellular digestion
  • Nutrient transport occurs by diffusion
  • Indigestible remains are expelled through the mouth
  • Wastes (ammonia) are removed by external and internal water currents
• Metagenesis (reproduction) in Cnidarians involves a succession of differently organized generations:

  • Alternation of asexually reproducing, sessile polyp, and sexually reproducing, free-swimming medusa or sessile polyp
  • Sexual reproduction in Cnidaria often involves a complex life cycle with both polyp and medusa stages
• Spawning in Cnidarians is generally driven by environmental factors like changes in water temperature and is triggered by lighting conditions such as sunrise, sunset, or the phase of the moon
• Larval formation in Cnidarians:

  • Fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere (blastula)
  • A depression forms at one end (gastrulation) and eventually becomes the digestive cavity
  • Larvae, called planulae, swim or crawl by means of cilia and eventually attach to a substrate if the species has a polyp stage
• Cnidarians can reproduce asexually by various means, in addition to regenerating after being fragmented:

  • Hydrozoan polyps only bud, while the medusae of some hydrozoans can divide down the middle
  • Scyphozoan polyps can both bud and split down the middle
  • Anthozoa can also split horizontally just above the base
  • Asexual reproduction makes the daughter Cnidaria a clone of the adult
• Respiration in Cnidarians:

  • No respiratory organs; both cell layers absorb oxygen and excrete carbon dioxide
  • Some Anthozoa have ciliated grooves on their tentacles to pump water out and into the digestive cavity, improving respiration after feeding