Biology 11
Name: _______________________
Phylum Cnidaria (Cnidarians)
Ref: Heath 453-456
A. Introduction
•most cnidarians are found in marine environments, but some live in fresh water (eg.
Hydra)
•phylum Cnidaria gets its name from the stinging cell (cnidocyte) that is found all members
•the cnidocyte contains a stinging structure called a nematocyst
•all cnidarians are soft-bodied animals with stinging tentacles arranged around their mouth
B. Body Forms: Polyp and Medusa
•both forms consist of two cell layers (ectoderm or epidermis and endoderm or gastroderm)
separated by a jellylike material (mesoglea)
•To do:
•label the body forms below as either polyp or medusa
•also label the epidermis, gastroderm, mesoglea & gastrovascular cavity, cnidocyte &
nematocyst,tentacles, mouth/anus, body stalk (for polyp)
•indicate whether the form is motile or sessile
•cnidarians are classified according to which stage (polyp or medusa) predominates in the life
cycle (some cnidarians have an alternation of generations type of life cycle where a polyp
stage alternates with a medusa stage)
•examples of cnidarians in which polyp stage predominates: hydra, sea anemone, coral
•examples of cnidarians in which medusa stage predominates: jellyfish
C. Reproduction: Life Cycle of Jellyfish Aurelia (simplified version)
Label the following structures: medusa, polyp, egg, sperm, strobila, planula, young medusa
1. What is the planula? Larval form of jelly
2. What are the two body forms of the jellyfish shown in the diagram?
Polyp nand medusa
3. What kind of symmetry does Aurelia have? radial
4. Where does meiosis occur in the jellyfish life cycle? In the medusa
5. What happens between fertilization and the planula stage of the jellyfish?
Cleavage > (mitosis)> blastula> (Mitosis)> gastrula…el;ongates to form ciliated planula
6. Is the production of medusae from the polyp shown in the life cycle an example of sexual
or asexual reproduction? Explain your answer. Asexual> no gametes involved (diploid
polyp, diploid medusa therefore mitosis involved, therefore asexual)
7. What are the structures that aid mobility of the planula called? cilia
8. Describe how a medusa moves through the water. Ring of “muscle” around edge of medusa
contracts and relaxes, forcing water out of bell-shaped medusa and propels it
9. How is the planula like the larva of a sponge? Both are motile
D. Structure of Hydra
•Label the following structures: Gastrovascular cavity,Tentacle,Cnidocyte,Ectoderm,
Endoderm,Mesoglea,Ovary/Teste,Bud,Nematocyst,Mouth,Basal Disk
E. Form and Function in Cnidarians
1. Feeding and Digestion
•nematocysts located on tentacles release barbs containing posion to paralyze or kill prely
•tentacles push food through mouth & into gastrovascular cavity
•specialized cells in the gastroderm (endoderm) release enzymes that partially digest food
•food fragments taken up by gastroderm cells (via phagocytosis) & further digestion takes
place inside cells (intracellular digestion)
•nutrients transported throughout body by diffusion
•undigested wastes are expelled through mouth(therefore sac body plant)
Symbiotic Relationships (describe how many cnidarians are dependent on photosynthetic
symbionts): many have photosynthetic protists (algae/dinoflagellates) growing inside
gastroderm cells
Protists uses CO2 and other metabolic wastes(NH3) produced by cnidarian’s cells to produce
O2 and organic compounds such as carbohydrates and proteins. Protists uses some of these
products/ the remaining products released into cnidarian tissues.
2. Gas Exchange, Excretion, Internal Transport
•no specialized systems are necessary because most cnidarians are only a few layers thick
and individual cells can get nutrients and O2 and get rid of watses by diffusion through body
wall (with constantly flowing H20)
3. Nervous System
•cnidarians lack a centralized nervous system & brain
•have simple nerve net that coordinates movement of tentacles & body
•nerve net is concentrated around the mouth & is found throughout mesoglea
•specialized sensory cells in epidermis detect chemicals in food & touch
•movement is accomplished by epidermal (contractile) cells that contract under nervous
stimulation
4. Reproduction
•most cnidarians reproduce sexually and asexually.
•asexual is via budding (describe process): begins with swelling on side of polyp. Gradually
grows into a complete polyp and falls off. some polyps can bud off tiny medusae
•sexual (describe usual process) gametes released into H2O fertilization occurs in open
water of inside cnidarian with egg, develops into ciliated larvae and then adult
•some exhibit alternation of generations between polyp and medusa stages (although both
are 2 n ...ie. diploid)
F : How Cnidarians Fit into the World/Cnidarian Ecology
1. Relationships with other animals
a) Symbiotic(mutalism) relationships: Describe some examples of symbiotice relaitonships
that invlove sea anemones and how each organism benefits.
Protection for fish, shrimp: sea anemone provides food scraps/fish/shrimp clean and protect sea
anemone
b) Provide habitats for marine animals.Explain how corals and the reefs they form are
important for this reason. Many clonies attached together so caves , tunnels and deep channels
are created which provide habitat to many species
2. Importance to humans:
a)List some of the ways corals and the reefs they form are important to humans.
Home for food(fish) that we eat, home to organisms that produce shells and pearls that we
harvest
Reefs protect land form wave action that leads to erosion
Provide clues to oil deposits
Used to build houses (a good idea?), make jewellry and decorations
b) How are cnidarians used in medical research.
Cancer drugs ;corals produce chemicals to protect themselves from infection or being settled on
by other animals (can’t run away and they’re colonial)
Proteins in jellyfish , jellyfish mucus prevent tissue from drying out and prevents bacterial
inc\fection, used to keep eyes moist, put in ice cream
Nerve cell toxins help scietnists better understand how nervous systems work.