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Phylum Cnidaria
A. 2 body forms
1. Medusa: jellyfish
a. motile
b. umbrella shaped
c. tentacles and mouth underneath
2. Polyp: corals
a. sessile
b. tubelike
c. Tentacles and mouth on top
Some cnidarians exist as either a medusa or polyp but many will alternate
between the two during their life cycle.
Radial symmetry
The unique feature of the Cnidaria is that they have stinging cells, called
nematocysts. Each of these cells contains a thread-like sting that is discharged
in attack or defense.
Class Hydrozoa: Hydra
Classs Scyphozoa: Jellyfish
Class Anthozoa: Corals
Class Hydrozoa
Hydra, Portuguese man-of-war
No Medusa Stage
Most are found in lakes, rivers, ponds
Some are Marine; ex: Portuguese man-of-war
Attach to rocks or other plants through their basal disk
Brown or white
Move by a tumbling or somersaulting action. (pg 642)
1. Asexually: budding; Polyps produce buds, they will grow into a
new polyp, then they will eventually separate from the old
polyps body and attach
themselves to rocks or plants and live independently.
2. Sexually Reproduction: Mostly marine hydrozoas.
a. The medusas release sperm or eggs into the water. The gametes fuse
and produce zygotes that develop into free-swimming larvae (planulae).
The planulae will settle and attach to the bottom of the ocean floor and
develop into a polyp. Each polyp will produce multiple buds (called a
colony). These buds do not separate from the body (as it does in asexually
reproduction). They will stay attached and eventually turn into male and
female medusas. These medusas will leave the polyps to grow and mature.
When feeding, hydras extend their body to maximum length and then slowly
extend their tentacles. Despite their simple construction, the tentacles can
extend four to five times the length of the body. Once fully extended, the
tentacles are slowly maneuvered around waiting for a suitable prey animal to
touch a tentacle. Once contact has been made, nematocysts on the tentacle
fire into the prey and the tentacle itself coils around the prey. Within 30
seconds, most of the remaining tentacles have already joined in the attack to
control the struggling prey. Within two minutes, the tentacles will surround
the prey and move it into the mouth. Within ten minutes, the prey will be
enclosed within the gastrovascular cavity and digestion will have started. The
hydra is able to stretch its body wall considerably in order to digest prey more
than twice its size. After two or three days, the undigestible remains of the
prey will be discharged by muscular contraction through the mouth.
Hydra Structure
A: Tentacle
B. Budd/new hydra forming
C. Mesoglea
D. Basal Disk
E. Cnidocyte (stinging cells)
F. Nematocyst (used for defense)
G. Gastrovascular cavity (Help Digest)
H. Endoderm (inner cell layer)
I. Ectoderm (outer cell layer)
J. Sperm sac
K. Mouth
Class Scyphozoa
Can range from a thimble to as large as a queen-size mattress.
Feeding: predators; use their tentacles to capture and sting
their prey. Their nematocysts can be extremely toxic.
Reproduction: sexually. Alternation of polyps and medusas (as
discussed in Hydrozoa)
Dominant form is the Medusa
Class Anthozoa
Largest class
Consists of only polyp form
Ex: sea anemones, coral
Reproduce: asexually: budding. Sexually be releasing eggs and sperm into the
ocean where fertilization will occur.
Most species live in warm water and they are brightly colored.
Feeding: They feed on fishes, which are caught by means of the numerous
nematocysts in their tentacles. These animals are known for their symbionts.
These include species of fish that actually live among the tentacles of large
anemones, somehow avoiding lethal contact with the nematocysts. Other
anemones have unicellular algae living within their tissues, from which they
probably derive some nutrition. Yet others have a symbiotic relationship with
hermit crabs, which gather up the anemones and place them on the snail
shells that the crabs occupy. The anemones benefit from food particles
dropped by the crab, and the crab gains protection from predators due to the
presence of the nematocyst-laden anemones.
CORAL GRIEF: Global warming isn't just affecting cold areas. Coral reefs are found
mainly in warm waters. Tiny animals called coral polyps build the reefs. But if the water
gets too warm, the polyps can die. That's already starting to happen at the Great
Barrier Reef, Earth's largest coral reef.