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Ch. 26.3
hydras, jellyfish, sea anemones, and coral
Found all over the world
Can live individually or in colonies
What is a Cnidarian
Soft bodies, carnivorous, stinging tentacles
Simplest animals to have body symmetry
Name comes from STINGING CELLS (cnidocytes)
used for defense and capturing prey
contain NEMATOCYST  poison-filled stinging structure that
contains a coiled dart
Form and Function
Only a few cells thick
Tissues function for feeding and movement
RADIAL symmetry
Life cycle has 2 parts:
POLYP – cylindrical body with armlike tentacles; mouth points
MEDUSA – motile, bellshaped body; mouth on the bottom
Both have a body wall that surrounds the GASTROVASCULAR CAVITY
Digestion takes place in the GASTRODERM
Pulls prey into its mouth after stinging them
Food enters and wastes leave the body via the gastrovascular cavity
Digestion is EXTRACELLULAR (outside of the cells)
food can be absorbed in the GASTRODERM
Respiration, Circulation, and Excretion
Nutrients are transported via diffusion
Specialized sensory cells (NERVE NET)
Found throughout the body
OCELLI – eyespots that detect light
HYDROSTATIC SKELETON – circular muscles and longitudinal muscles that
enable movement via contraction and water pressure
Medusas move by jet propulsion (from muscle contractions)
Reproduce both sexually and asexually
asexually by BUDDING (identical offspring)
Female releases eggs into the water; male releases sperm
Zygote forms a free swimming larva and then develops into a polyp
Groups of Cnidarians
JELLYFISH – Class Scyyphozoa (cup animals) live as medusas
HYDRAS – Class Hydrozoa. Use nematocysts to sting prey
lack a medusa stage
SEA ANEMONES and CORALS – Class Anthozoa (flower animal). Only have
polyp stage
Ecology of Corals
Coral live in areas based on temperature, water depth, and light intensity
Corals in reefs need lots of light
Rely on algae for survival (symbiosis)
Coral reefs are being damaged from logging, farming, mining, water
pollution, overfishing
changes ecological balance
Rising temperatures and global warming causes coral bleaching