2.2 Phylum Cnidaria - Spot-Bodied Animals
Learning Objectives1. Describe the characteristics of the phylum Cnidaria.2. Explain the structure and functions of cnidocytes.3. Analyze the advantages of alternation of generations.4. Understand the ecological and environmental importance of cnidarians.
What is a cnidarian?• Characteristics:
• radial symmetry
• cnidocytes (stinging cells)
• mesoglea (jelly-like layer) between the epidermis and gastrodermis
• gastrovascular cavity
• nerve net
What are cnidarians made of?
• Tissues perform functions
• body organized into 3 layers:
• epidermis, mesoglea, gastrodermis
• Epidermis
• outer layer
• contains cnidocytes
• cnidocytes
• stinging cells
• used for protection and food gathering
• when triggered, they discharge a nematocyst
• nematocyst is harpoon-like structure with toxin
How do Cnidarians Work?
• Respiration
• rely on water to bring oxygen in and wastes out
• gasses diffuse between water and cells
• Feeding
• carnivores
• tentacles surround the mouth
• tentacles paralyze prey and bring into mouth
• digestion takes place in gastrovascular cavity
• Reproduction
• mostly dioecious
• gametes are released for external fertilization
• two body forms
• medusa - jellyfish
• polyp - coral
• alternation of generations
• medusas produce polyps (sexually), polyps produce medusas (asexually)
What are the different types of Cnidarians?• Over 9,000 species
• 5 classes
• Class Hydrozoa
• small, mostly marine
• alternate generations
• statocysts help determine gravity
• Obelia, Gonionemus, Hydra
• Class Staurozoa
• approx. 100 species, marine, cold water
• no medusa stage
• mouth surrounded by 8 tentacles, aboral side attached to substrate