cnidarians final2

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CHAPTER 13 The Cnidar ians

(Radiate Animals)


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13.1. Position and

Contributions

A. Position in AnimalKingdom

1. Both phyla Cnidaria

and Ctenophora make

up the radiate animals.

2. Other eumetazoans

have bilateral

symmetry or their

radial symmetry is

derived from a bilateral

ancestor.

3. Neither Cnidaria nor

Ctenophora have

advanced beyond

tissue level oforganization although


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B. Bio logical Contr ibut ions

1. Both phyla have two well-defined germ layers: ectoderm and

endoderm; mesoderm may be derived from ectoderm.

2. There is an internal body cavity: the gastrovascular cavity.

3. Extracellular digestion occurs in the gastrovascular cavity;

gastrodermal cells accomplish cellular digestion.


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B. Bio logical Con tr ibut ions

1. Both phyla have two well-defined germ

layers: ectoderm and endoderm;

mesoderm may be derived from

ectoderm.

2. There is an internal body cavity: the

gastrovascular cavity.

3. Extracellular digestion occurs in the

gastrovascular cavity; gastrodermal cells

accomplish cellular digestion.

4. Most have tentacles, which are

extensible projections for food capture.

5. Radiates are the simplest animals with

nerve cells; there is no central nervous

system.


sense organs: statocysts and ocelli.


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B. Bio logical Contr ibut ions

1. Both phyla have two well-defined germlayers: ectoderm and endoderm; mesodermmay be derived from ectoderm.

2. There is an internal body cavity: thegastrovascular cavity.

3. Extracellular digestion occurs in thegastrovascular cavity; gastrodermal cellsaccomplish cellular digestion.

4. Most have tentacles, which are extensibleprojections for food capture.

5. Radiates are the simplest animals withnerve cells; there is no central nervoussystem.


sense organs: statocysts and ocelli. 7. Locomotion is by muscular contraction or

ciliary comb plates.

8. The polyp and medusa forms allow widerecological possibilities.

9. Unique features include nematocysts,colloblasts and ciliary comb plates.


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13.2. Phylum Cnidaria

A. Cnidarian Life History

1. Over 9,000 species are in thephylum Cnidaria.

2. Cnidaria have specialized cells(cnidocytes) that contain aspecialized stinging organelle, thenematocyst.

3. Nematocysts are only formed and

used by Cnidarians.

4. Cnidarians originated close to thebase of the metazoan lineage.

5. Today, they are most common inshallow marine environments, some

are freshwater but none areterrestrial.

6. Some ctenophores, molluscs andflatworms eat hydroids and use thestinging nematocysts in their owndefense.

7. Some live symbiotically; algae inreef-building corals are critical to


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B. Characteristics of

Cnidaria

1. All are aquatic and mostlymarine.

2. Radial or biradial

symmetry forms oral and

aboral ends.

3. The two body types arethe free-swimming medusae

and the polyps.

4. Some have an

exoskeleton or endoskeleton

of chitinous, calcareous or

protein components.


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B. Characteristics of Cnidaria

1. All are aquatic and mostly marine.

2. Radial or biradial symmetry forms oral and aboralends.

3. The two body types are the free-swimming medusaeand the polyps.

4. Some have an exoskeleton or endoskeleton ofchitinous, calcareous or protein components.

5. They have a diploblastic body, with two layers:epidermis and gastrodermis; some are triploblastic withan ectomesoderm.

6. The gastrovascular cavity has a single openingserving as both mouth and anus.

7. Special stinging cells called nematocysts are inepidermis or gastrodermis and abundant on tentacles.

8. The nerve net may include some sensory organs.

9. The muscular system has an outer layer oflongitudinal fibers and an inner layer of circular fibers.

10. Reproduction is either asexual or sexual; a planulalarva may be present.

11. There is no excretory or respiratory system.

12. There is no coelomic cavity.


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1. All are aquatic and mostly marine.

2. Radial or biradial symmetry forms oral and aboralends.

3. The two body types are the free-swimming medusaeand the polyps.

4. Some have an exoskeleton or endoskeleton ofchitinous, calcareous or protein components.

5. They have a diploblastic body, with two layers:epidermis and gastrodermis; some are triploblastic withan ectomesoderm.

6. The gastrovascular cavity has a single openingserving as both mouth and anus.

7. Special stinging cells called nematocysts are inepidermis or gastrodermis and abundant on tentacles.

8. The nerve net may include some sensory organs.

9. The muscular system has an outer layer oflongitudinal fibers and an inner layer of circular fibers.

10. Reproduction is either asexual or sexual; a planulalarva may be present.

11. There is no excretory or respiratory system.



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C. Form and Function

1. Cnidaria have two basic body

plans.

2. A polyp is a hydroid form.

a. Polyps are an adaptation to

a sedentary life.

b. The body is tubular with the

mouth directed upward andsurrounded by tentacles.

c. The aboral end is attached

to a substrate by a pedal disc.

d. In colonial forms, the polyps

may be specialized for feeding,reproduction or defense.

e. Sea anemones and corals

are all polyps with no medusa

stage.


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3. A medusa is bell or umbrella-

shaped.

a. The mouth is directed

downward; tentacles may extenddown from the rim of the umbrella.

b. In tetramerous symmetry, body

parts are arranged in fours.

c. Medusae dominate the life oftrue jellyfish, but many have a

polypoid form at the same time.

4. Hydrozoans often have life histories

featuring both stages.

5. Both stages have retained the sac-

like body plan typical of the phylum.

6. Both have three body layers;

medusa has a much thicker mesoglea

than do polyps.


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D. Nematocysts: Stinging Organelles

1. Over 20 different types of nematocysts have been described; they are

important in taxonomy.

2. Nematocysts are tiny capsules made of chitin-like material and containing a

coiled filament. 3. A little lid oroperculum covers the end of the capsule.

4. The inside of the thread may have tiny barbs or spines.

5. The cnidocyte is the cell that produces the nematocyst; it develops from a

cnidoblast.

6. Except in Anthozoa, a modified cilium called a cnidocil functions as a trigger.

7. Both small organic molecules and vibrations sensitize anthozoan cnidocytes.

8. After a nematocyte is discharged, its cnidocyte is absorbed and another

develops.

9. Some lack barbs or poison and rapidly recoil, grasping and holding prey.


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10. Mechanism of NematocystDischarge

a. The cell can generate a highosmotic pressure of 140

atmospheres within thecnidocyte.

b. The osmotic pressure falls asthe hydrostatic pressureincreases.

c. When stimulated, the highinternal osmotic pressure causeswater to rush into the capsule.

d. The operculum opens andrapidly releases the increased

hydrostatic pressure, launchingthe thread.

e. At the everting end of thethread, the barbs point backwardto anchor.

f. Poison may be injected when itpenetrates the prey.


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E. Nerve Net

1. Two nerve nets, one at the base

of epidermis and one at the base

of gastrodermis, interconnect.

2. Nerve impulses move acrosssynapses by neurotransmitters.

3. Unlike higher animals, cnidarian

nerves have neurotransmitters on

both sides of the synapses

allowing transmission in either

direction. 4. Cnidarian nerves lack the

myelin sheath on axons.

5. There is no central nervous

system, but ring nerves serve as

marginal sense organs of

medusae.

6. In some species, a fast network

coordinates swimming

movements, a slower one

coordinates tentacles.

7. This nerve net pattern is alsofound in the digestive systems of


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F. Class Hydrozoa

1. Hydra: A Freshwater Hydrozoan (Fig. 13-6)

a. Hydra are solitary polyps and one of a

few cnidarians found in freshwater.

b. The body is a cylindrical tube; the aboral

end has a basal or pedal disc for

attachment.

c. A ring of hollow tentacles encircles the

mouth.

d. The mouth opens to a gastrovascular

cavity.

e. Undifferentiatedinterstitial cells can

develop into cnidoblasts, sex cells, buds, or

nerve cells, but not epitheliomuscular cells. f. Gland cells on the adhesive disc secrete

an adhesive and sometimes a gas bubble

for floating.

g. Hydras have nematocysts that: 1)

penetrate prey and inject poison; 2) recoiland entangle prey; and 3) secrete an


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H. Class Anthozoa (Fig. 13-21)

1. Sea Anemones

a. They attach to shells, rocks, timber, etc. by pedal discs;some burrow in mud or sand.

b. A crown of tentacles surrounds the flat oral disc.

c. A slit-shaped mouth leads into a pharynx.

d. The siphonoglyph is a ciliated groove that creates thewater current into the pharynx.

e. Currents carry in oxygen and remove wastes, and maintainfluid pressure for a hydrostatic skeleton.

f. When in danger, water is rapidly expelled through pores asthe anemone contracts to a small size.

g. Most anemones can glide slowly on pedal discs; some canswim with limited ability.

h. Escape reactions occur in response to extracts frompredators (e.g. sea stars, nudibranchs).

i. Most harbor symbiotic algae; some have a mutualisticrelationship with hermit crabs.

j. Some damselfishes shelter in sea anemones and have skin

mucus that protects them from triggering nematocysts.


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2. Zoantharian Corals

a. Members of the order Scleractinia are also known as true or stony corals.

b. They are miniature sea anemones that live in calcareous cups they havesecreted. (Fig. 13-26)

c. Instead of a pedal disc, they secrete a limey skeletal cup with scleroseptaprojecting up into the polyp.

d. A sheet of living tissue forms over the coral surface, connecting allgastrovascular cavities. (Fig. 13-28)

3. Coral Reefs

a. Coral reefs have great productivity, rivaled only by tropical rainforests.

b. Living plants and animals are limited to the top layer above the calciumcarbonate deposits.

c. Hermatypic corals and coralline algae form most coral reefs. (Fig. 13-28)

d. These corals require full salinity of seawater and warmth and light, limitingthem to waters between 30 degrees north and south.

e. Microscopic zooanthellae are photosynthetic and begin the food chain andrecycle phosphorus and wastes.

f. These habitats support a diversity of corals and fish.

g. Few nutrients enter or leave the system.

h. Nutrients from fertilizer and sewage threaten coral reefs with excessivealgal growth.

Fig 13 1a


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Fig. 13.1a

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