Lecture on Phylum Cnidaria

Overview

• Focus on Phylum Cnidaria, specifically the classes:
  • Hydrozoa
  • Scyphozoa
  • Staurozoa
  • Cubozoa
  • Anthozoa
• Lesser known classes: Myxozoa and Polypodiozoa

Class Hydrozoa

• Characteristics:
  • Small predatory polyps
  • Can be colonial or solitary
  • Found in marine and freshwater environments

Example: Hydra

• Habitat: Freshwater, undersides of leaves in pools and streams
• Anatomy:
  • Cylindrical body with a basal disc and tentacles
  • Gastrovascular cavity for feeding
• Feeding: Uses cnidocytes to capture prey
• Movement:
  • Sessile but can float or "loop"/"somersault"
• Reproduction:
  • Asexual (budding) and sexual (gonads, sperm, and eggs)
• Regeneration:
  • Capable of morphallaxis (tissue regeneration)
  • Biologically immortal, does not age

Other Hydrozoans

• Turritopsis dohrnii ("Immortal Jellyfish"):
  • Not true jellyfish, part of Hydrozoa
  • Medusa stage, capable of reverse metamorphosis
  • Biologically immortal, escaping death by transdifferentiation
• Obelia:
  • Colonial with base, stalk, and zooids
  • Zooids specialized for feeding or reproduction
• Siphonophores (e.g., Apolemia, Portuguese man o' war):
  • Colonies of polyps and medusae
  • Can be very large (longer than a blue whale)

Other Notable Hydrozoans

• Hydrocorals (fire corals and rose corals):
  • Resemble true corals but are hydrozoan colonies
• Chondrophores, deep red jellyfish, air ferns
  • Various unique hydrozoans with distinctive features

Conclusion

• Hydrozoans exhibit a wide variety of life forms and reproductive strategies
• They are found in diverse environments and possess unique adaptations, including biological immortality in some species
• Next, exploration of other classes within Phylum Cnidaria