by Madhu Prakash

With more than 9000 living species, the Cnidarian phylum is predominately composed of marine organisms: the exceptions being hydras and a few other fresh water hydrozoans. Cnidarians typically live in tropical and littoral areas as sessile organisms that dwell on rocky and coral formations. However there are some Cnidarians that are motile such as the jelly fish. The fossil record indicates that Cnidarians can be traced as far back as the Ordovician period. (ZJ- source 12)

Broad classification: Oldest clade of a Eumetrazoa, animals with true tissues, is a Radiata. In the Rediata there are the cnidaria, which are carnivores that have radial symmetry, a gastro vascular cavity, and cnidocytes.

Radial symmetry means that if you drew a line - any line - dividing the organism into two equal parts, each half would mirror the other. This is in contrast to bilateral symmetry, where an organism is only symmetrical when a line is drawn from front to back. (Matt B - Source 7)

What are cnidarians: Cnidarians are sea anemones, hydra, jellies, and coral animals
Some key characteristics of cnidarians, also, are that they are two cell layers thick. In between the two layers of cells is a gelitanous substance called mesoglea. In addition, cnidarians also have tentacles that can be used to capture prey.
On the outer layer (epidermis), cnidarians have specialized cells such as muscle cells to rovide movement, interstitial cells that give rise to sex cells, cnidocytes that enable cnidarians to sting, and nerve cells that collect and transmit sensory information. (AM)

Body Plan of cnidaria:
  • The cnidaria has only one opening, which is both the mouth and anus.
  • Gastro vascular cavity- Sac that has central digestive system.
When digestion occurs, enzymes are secreted into a central cavity. These are what break down food in the beginning. Then, these food particles are engulfed by food vacuoles in the gastrodermis, which passes the nutrients into the circulatory system. (PS Source 8)

  • The undigested food goes out of the anus/mouth.

Two types of body plan: sessile polyp and floating medusa. 33-05-HydraCnidocyte-L.gif33-04-CnidarianForms-L.jpg
Polyps: Has tentacles at end of the body and it captures prey.
Ex) hydra and sea anemone
Medula: has bell shaped body that helps it move in water in addition to passively moving in water.
    • Cnidocytes are unique cells on the tentacles that act as a defense. They have cnidae, which are capsule like organelles.
      • Cnidae called nemarocytes are stinging capsules

      • Nematocysts, and their enclosing Cnidocysts come in about 24 different forms, the differences play a functional role in the classification of the phylum. A basic Nematocyst is a capsule made of something like chitin within which rest a coiled thread. This thread can be shot out of the capsule to encounter prey items, or in some cases to repel predators. The Cnidocyst has either a modified flagellum called a Cnidocil, or a cone as a sensory trigger. If this trigger is touched the nematocyst thread is rapidly ejected. Nematocyst threads come in 3 basic types. The fundamental nematocyst is a thin tubular thread with barbs at the far end, though there may be barbs near the base as well. When the nematocyst is discharged, the barbs penetrate the skin of the prey and a toxin can be injected. (RW)
Some cnidarian groups have both poly and medusa stages in their life cycles. These animals are called polymorphic. Other cnidaria groups, like sea anemones and corals don't have medusas, and others don't have jellyfish, so their polyp stage is absent. Life cycles that have both polyps and medusae usually have a juvenile polyp stage that gives rise asexually to a medusa, which then reproduces sexually, thus both the polyp and medusa have a diploid number of chromosomes, but the gametes are haploid. (JP- Source 2)

Cnidarians do not have tissues, which are a series of cells that work together to perform one task, but they do have multiple cells that act as muscles and as a nervous system. Also, there is a ring of tentacles around their mouth which allows the food to be pushed into the body. (MM)

Muscles and nerves:
      • Have nerves and muscles of the simplest form
      • There is no centralization of the nervous system. (SP)
      • The epidermis, outer layer, contains many different types of cells (MB), and the gastrodemis, the inner layer, has filaments that are arranged in it.
      • The middle layer is called the mesoglea (MB)
      • The mesoderm is where the muscle tissues are formed
      • These epitheliomuscular cells contract and relax to enable movement (MB)
      • The gastro vascular acts as a fluid filled skeleton against which contractile cells can work
    • The cavity is fixed when the animal closes its mouth and the cell contraction causes animal to change shape
    • Cnidarians do not have a brain
    • Nerve net is for simple sensory receptors and is radially distributed around body
    • The nerve net conducts signals from sensory receptors to muscle cells. (SP)
    • Lets animal to respond to stimuli from all direction
    • The statocyst is a gravitational sensory organ. (SP)
    • The ocellus is a simple photoreceptor organ. (SP)

3 major classes of cnidaria:
Hydrozoa class:
    • Amimals that can alternate between polyp and medulla during its lifetime.
Ex) hydra- is in fresh water. It is a hydrozoa and it only stays in polyp form. When there are favorable environmental conditions they reproduce asexually by budding (splitting off parent).
In unfavorable conditions, they reproduce sexually.

Hydrozoa (JS)
Hydrozoa (JS)

Scyphozoa class:
    • This class is where medusa prevails
    • Medusas live mostly among planktons as jellies
    • Most scyphozoa has a small polyp stage, but jellies do not
Anthozoa class:
    • Sea anemones and corals and only occur as polyps
    • Coral animals secrete hard external skeleton made of calcium carbonate
    • Each generation builds on the skeleton remains of earlier generation
    • This is what we call a coral.

In Phylum Cnidaria, the cells of cnidaria beings aquire the necessities of respiration through simple diffusion. Each cell is on its own for respiration and ATP, molecule used for storing energy, on it own. (rj)

Sensing the Environment:
Most Cnidarians do not posses sensory structures. Only some medusae possess certain sensory structures, but they are overall very primitive. (AR)

Even though cnidarians do not posses a brain, most do poses a complex nerve network. This nerve network can allow cell communication across the entire organism, in some cases only taking milliseconds to occur. This allows reactions against predation or prey. Along with reactions to other organisms, many cnidarians have basic gravitational and light sensing structures. This allows basic orientation, sun compass navigation, and daily migration. (MS 15)

Acquiring and Digesting Food:
All cnidarians have Tentacles that have stinging cells at the tips of them which are called Cnidocytes used to capture prey or for self defense. The Cnidocysts contain something called a nematocyst which is a sharp coiled spring-like stinger. When the Cnidarian gets ready to attack the stinger uncoils, sharply juts through the Cnidocyte wall, and punctures the prey. The stinger contains a toxin that paralyzes the prey. (AP)
All Cnidarians are carnivorous, using their cnidae and tentacles to catch prey and defend themselves. A cnidae consist of a coiled tubule and venom; It allows members of this phylum, like jellyfish, to sting. Also, certain Cnidarians can absorb organic matter dissolved in nearby seawater. Lastly, many anthozoans have species of hydrozoan and scyphozoan, types of unicellular algae, present in their bodies. This symbiotic relationship allows the Cnidarians to derive reduced carbon. This common relationship is present in marine and freshwater Cnidarians. (AR)

A Cross section of a jellyfish (LPE)

This cross section of a jellyfish adds insight as to how the muscle contractions of jellyfish allow it to move (LPE)

With regards to locomotion, while most polyps do not move, medusas do have the ability to move. Medusas, such as jellyfish, are able to move by "swimming" around, although for a jellyfish this means opening and closing its bell (body). Jellyfish have a ring of muscle that surrounds their bells and when a jellyfish tightens this muscle, its bell closes. This results in any water that was inside the jellyfish to be pushed out, shooting the jellyfish forward. Then, as the muscle relaxes, the bell of the jellyfish is refilled with water. Although jellyfish can swim, the current often controls were they go. Jellyfish float, sink, and drift with ocean currents. (VN)

Metabolic waste removal
Cnidarians do not have an excretory system because the metabolic waste that they produce is able to diffuse out of the cnidarians into the water that surrounds them. This allows them to eliminate their metabolic waste without the need of an excretory system. (CP source 11)


A circulatory system is used to transfer Oxygen and nutrients to the rest of the body. In the case of cnidarians, all of the cells in the body are in direct contact with the surrounding water so this is not necessary. This is a result of it being only two layers thick. (RK)

Cnidarians have no heart. (MR; Source 16)
Self protection

Cnidarians have stinging cells known as nematocysts, or cnidae, at the end of the tentacles surrounding their mouths. These stinging cells allow some carnivorous Cnidarians to capture and stun prey,and allow other Cnidarians to protect themselves from predators. These nematocysts tend to look like a harpoon on coiled thread. When something touches the tentacle of a Cnidarian, or when a nerve impulse from a more complex Cnidarian tells it to "fire", the thread uncoils and the harpoon goes into the prey or predator. Most of these harpoons contain toxin which helps disable a prey or predator. (VN)

Osmotic balance
In cnidaria, there is no osmotic regulation at all. There is also no excretory system. (CM)

Temperature balance

Because cnidarians are only two cell layers thick, they have no form of temperature balance; their body temperatures are completely determined by the temperature of the water in which they live. (MR; Source 16)

Review Questions:

2. What are the three characteristics that define cnidaria? (IL)
3. What are the two types of body plans that a cnidaria can have and how do they defer? (RW)


Campbell, Neil A., and Jane B. Reece. "Chapter 33: Invertebrates." Biology. 6th ed. San Francisco: Pearson Education, Inc., 2002. Print
jellyfish picture:,r:0,s:0 (TB)

2. (MB) (AR) (AR) (used to define the word, cnidae)

"The Wonders of the Seas: Cnidarians." Oceanic Research Group. Web. 30 Oct. 2011. (AP)

4. "Biology of Animals & Plants - Sponges & Cnidarians" 30. Oct 2011
5. "The Nervous System: Organization." Web. 30 Oct. 2011. <>.
6. (AM) (JS)

7. (Matt B)


9. "Jellyfish." Jellyfish. Oracle ThinkQuest Education Foundation. Web. 6 Nov. 2011. <>. (VN)

10. "H2g2 - Cnidaria." H2g2 - The Hitchhiker's Guide to the Galaxy. Web. 06 Nov. 2011. <>. (VN)

11. "An Introduction to Zoology: Chapter 9 - Freethought Forum." Freethought Forum. Web. 06 Nov. 2011. <>. (CP)

12. Barnes, Robert. Invertebrate Zoology. 2nd. Philadelphia, PA: W.B. Saunders Company, 1969. Print.


14. (RK) (LPE)

"Appendix - The Nervous System and Sensory Capacities of Cnidaria (jellyfish, Sea Anemones, Corals and Their Relatives)." Angelfire: Welcome to Angelfire. Web. 13 Nov. 2011. <>.

16. "Cnidarians." National Zoo. Smithsonian Inst., n.d. Web. 27 Nov. 2011. < Animals/Invertebrates/Facts/cnidarians/default.cfm>.