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

• mesoglea

• middle layer

• jelly-like

• supports and gives shape

• gastrodermis

• lines gastrovascular cavity

• this is where digestion takes place

• 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

• Nervous Functions

• nerve net covers the epidermis

• responds to physical and chemical stimuli

• 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)

• Support

• hydrostatic skeleton

• water or body fluids fill the body

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

Obelia

Gonionemus

Hydra

• Class Staurozoa

• approx. 100 species, marine, cold water

• no medusa stage

• mouth surrounded by 8 tentacles, aboral side attached to substrate

• Class Scyphozoa

• marine, true jellyfish

• dioecious, alternation of generations

• Aurelia

• Class Cubozoa

• alternation of generations, tropical waters, extremely venomous

• box jellyfish

• Class Anthozoa

• marine, polyp only

• solitary or form colonies

• anemones and coral