Kingdom Animalia: Phylum Cnidaria
By Thomas Molyneux

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Examples of Cnidarians. On the left is an image of a Sea Anemone or Apitasia. On the right is an image of a Jellyfish or Aurelia. (LW) (10)

Diagnostic/Defining Characteristics

Cnidarians are relatively simple creatures with a limited body structure- these organisms lack even a mesoderm (a middle layer of cells that during embryonic development, develop the muscles, skeleton, kidneys, and most of the circulatory system)! Because cnidarians lack a mesoderm, they only have two layers of cells- an inside layer (endoderm) and an outside layer (ectoderm). However, they are still a diverse phylum, with over 10,000 living species; most of whom are marine. The basic body plan of cnidarians follows a structure of a sac with a central digestive compartment called the gastrovascular cavity. This basic body layout has two variations: the attached polyp and the floating medusa. The polypoid is the configuration of corals and anemones, with the tentacles and mouth facing up and the other side attached to a substrate or connected to a colony of other creatures of the same species. In the medusoid form, the orgnism is essentially facing down , with the mouth and tentacles pointing down. Most cnidarians alternate between these two forms in an alternation of generations. (LW) (3) Cnidarians can undergo both variations, or sometimes only experience one.

Polyps are cylindrical bodies that stick to a surface on one end and have their gastrovascular cavity opening on the other- picture an uncapped bottle. Around the opening are extended tentacles to catch prey.

Medusas are almost an upside-down version of polyps- instead of mouth-up, it is mouth-down. Furthermore, unlike polyps, medusas are not attached- they move freely in water through passive drifting and bodily contractions.
As stated in the text above and shown in picures above, the polyp has a nearly identical body plan as the medussa except the polyp has been flipped over and attached to some surface, while the mudusa is free floating. Their tissues and cavities are in the same layers, but their mouth/anus and tentacles are on opposite sides. (SD)(18)
As stated in the text above and shown in picures above, the polyp has a nearly identical body plan as the medussa except the polyp has been flipped over and attached to some surface, while the mudusa is free floating. Their tissues and cavities are in the same layers, but their mouth/anus and tentacles are on opposite sides. (SD)(18)

Cnidarians are also divided into three major classes: hydrozoa, scyphozoa, and anthozoa

Most hydrozoan species are marine organisms, (although some are freshwater), and most have both polyp and medusa stages, where the polyp stage is colonial. Examples include hydras, some corals, Portuguese man-of-war, and Obelia.
Most hydrozoans alternate between the polyp and medusa stages. Most of them are also colonial; some form delicate colonies, some form huge colonies that look like "true corals," and some form pelagic (floating) colonies that are often confused with jellyfish. (SM)(4)

All scyphozoan species are marine-based, whereas most have a reduced polyp stage polyp and are free swimming. Scyphozoan medusas can grow up to 2 meters in diameter. Examples include jellies, sea wasps, and sea nettles.
The Scyphozoa are found in all oceans and are found living from the sea surface to the abyss. The medusae of the Scyphozoa are unique in being much larger or more complex than the polyp. Also unique to the Scyphozoa is that they do not have a velum, a shelf of tissue pointing inward from the edge of the bell. (RL) (6)
Scyphozoan (true Jelly Fish) can range in size from 12 millimeters to two meters long. The largest, Cyanea arctica, may have tenticles over 40 meters long. (MLK)(19)

All anthozoan species are marine, lack medusa stages, and are fixed to a surface. Many others are also colonial. Anthozoa individuals have a sac like body that is partitioned into separate sections called septa. Many harbor symbiotic algae in their that supplement their diet.(KL)(2) Examples of Anthozoa include sea anemones, most corals, and sea fans.

Diagram of the basic structure of a cnidarian (ZXU) (1)

Acquiring and Digesting Food

In cnidarians, the gastrovascular cavity functions as the mouth, where tentacles work to capture food, and push it into the gastrovacular cavity for digestion. Cnidarians are carnivores, and frequently catch prey through the use of unique cells called cnidocytes, which contain cnidae. Cnidae are organelles capable of inverting. This is relevant towards catching prey as certain cnidae, called nematocysts, are stinging capsules, and are extremely effective at injecting poison into prey. When prey is injected with poison, the goal is usually to paralyze the prey, so it is easier to guide into the gastrovascular cavity. Other cnidocytes, instead of injecting prey, have longer threads to wrap around their pray and entangle it to capture it. Cnidarians can also be omnivores, and all are filter feeders. (CSR 15)

Digestion begins in the gastrovascular cavity, but occurs mostly within individual cells. Since cnidarians are only two cell layers thick, the interior cells of the endoderm receive the nutrients that get put into the gastrovascular cavity. Once food is put in the cavity, the cells of the interior freely absorb food particles. Digestion really takes place within these cells, as food vacuoles are used to break down food internally. Flagella on these endodermal cells keep the gastrovascular cavity contents agitated, and help distribute nutrients properly among the cells. The extracellular digestion that does occur takes place in the coelenteron, where digestion is aided by enzyme-producing cells in the gastrodermis. Nutritive-muscular cells then take in the peptides, carbohydrates and fats broken down in the coelenteron through phagocytosis and pinocytosis. (CSR 16)

Most Cnidarians contain photosynthetic algea in their cells. Occasionally the products of these cells will escape into the Cnidarians cells and be consumed by it. In many species it is the major food source. This is so central to most Cnidarians that many species die with out this symbiosis. (MC 11)

Sensing the Environment

Cnidarians lack any sort of brain or centralized nervous system. However, they have what are called nerve nets. The nerve net serves as a receptor system throughout the body- simply put, cells are able to react to stimuli equally, and then communicate to all other cells through a grouped cellular process. Extremely simple nerves and muscles in cnidarians act as their way of sensing their environment- any one stimulus for a cell can cause an equal reaction across the body through the net, adjusting each other cells behavior, and the behavior of the entire body as a whole. The nerve system is more pronounced in the medusa stage. In addition, cnidarians can have ocelli, patches of photoreceptor cells and pigment. These patches help cnidarians sense light. Ocelli are either in the form of a flat disk or a pit. (ORS 7) Cnidarians may also have statocysts, which keep them upright by sensing gravity, balance, and orientation.(ORS 8,9)


Locomotion in cnidarians is dependent on body type. If the cnidarian is in polyp form, the body is affixed to a surface, and the only moving part of the organism will be the tentacles- which move through basic muscles within the cells on the tentacles.

In medusa body types, locomotion comes either from organisms riding water currents as a means of transport, (just floating by), or through gradual bodily contractions. The well-known “pump” of a jellyfish comes from this cnidarian method of transport. When the gastrovascular cavity is closed, its volume is fixed and it can act as a stiff water bubble for the rest of the body to work against. Using this rigid part of the body, the animal changes shape by contracting certain cells, allowing it to propel itself its environment. These movements are coordinated by the nerve net, and are carried out by bundles of microfilaments (muscle thread) arranged on the cell layers to act as a fiber of muscle to contract in unison.
Since some Cnidaria are composed of almost 95% water, they are almost the same density of water as well. This makes it much easier for them to move around and eliminates the need for bouyancy-regulating organs common to other aquatic animals. (KS)


Since cnidarians are only two cells thick, all cells of cnidarian organisms are free to respire as needed directly with the environment. For example, all levels of oxygen are not determined by a bodily process, but on a cellular level. There, the levels of oxygen in the surrounding environment that’s accessible to the organism and the ability of the organism to use oxygen are what control levels of respiration- each cell is really responsible for its own self.

In Cnidaria, the gas exchange occurs at either the gastrovascular or epithelial surface. (CC) (13

Cnidaria lack a sophisticated respitatory system. They do not have any respiratory organs. This causes both of the cell layers to absorb their own oxygen and excrete their own carbon dioxide. To improve respiration after feeding, some Anthozoas use the ciliated grooves on their tentacles to pump water out of and into their digestive cavity without using their mouth. This allows cnidarias to control the water pressure within their cavity without having to expel any indigested food. (12)(MF)

Metabolic Waste Removal

For cnidarians, not only does the gastrovascular cavity act as a mouth opening, it is also an anus. The opening for letting in nutrients and the opening where waste leaves are the exact same place. In this case, waste removal is the opposite of feeding- cells individually release their waste, which collects into the gastrovascular cavity, and is then passed out through the anus/mouth of the organism.


The circulatory system for cnidarians is different than that of say a bird or person- there is no internal system of blood or a heart- the fluid through which waste is expelled and nutrients absorbed is just the surrounding water to the organism! Think about it- all the cells have the ability to directly interact with their surrounding environment for gaining nutrients or releasing waste. In this fashion, the water becomes the circulatory system, and the flow of it is the pulse of life for cnidarians.

Self Protection

As explained for catching prey, cnidarians have special cells that contain unique organelles that effectively stab other organisms. Besides stinging and paralyzing prey, this also makes for an intimidating defense mechanism. The slightest touch of a cell can cause a receptor site to be triggered, causing a discharge of a nemocyst, which acts almost as a harpoon to pierce other organisms and inject poison. Many of these poisons are incredibly lethal- a number of humans die each year due to jellyfish stings or the like- the paralyzing effects of nemocysts are often enough to kill within minutes.

Nematocyst cells may only be used once, and take several days to reform. Because of this cnidarians often required two stimuli before a nematocyst cell is discharged, to prevent unnecessary use. These stimuli include both contact with the Nematocyst cell and detection of chemical signals nearby. LJ (20)

Image of a Portuguese man-of-war's cnidocyte before the stinging nematocyst is discharged (AK) (17)

Osmotic Balance

The diffusion of water into cnidarians is again, controlled by each individual basis of each cell. Since each cell is in contact with the aquatic environment surrounding it, each cell can properly and autonomously control its own water level through simple diffusion. As each cell concerns itself with only its own issues, the organism as a whole regulates its osmotic balance through cell self-interest.
Meaning marine cnidarians cannot survive in fresh water bodies because their cells would burst from the water trying to equalize the internal and external salt content of the body of water, and fresh water cnidarians, such as hydras, cannot survive in salt water because the water inside their cells would be expelled to increase the internal salt content to that of the surrounding water. Cnidarians have no system with which to regulate osmotic balance, making them especially sensitive to changes in salinity.(ZS)

Temperature Balance

The balance of temperature for cnidarians is completely dependent on the surroundings of the cnidarian. Whatever temperature the water is that the organism is in, the cnidarian is at that temperature. Since cnidarians only have two cell layers to perform bodily tasks, thermoregulation is relatively irrelevant, as the cells are probably warmed by the environment.

Cnidarian individuals may be monoecious (have both male and female reproductive systems) or diocious (produce only one type of gamete). Polyps are capable of asexual budding and sexual reproduction. During asexual reproduction, new individuals of polyps arise from tissues that were budded off from the parent. Polyps form a colony when the new individuals remain physically attached to each other. During sexual reproduction, separate male and female medusae produce gametes that join together through external fertilization to produce polyps. The process is complex and occurs during the medusa stage of the life cycle. (MT).

Some Cnidaria experience strobilation, where a larva swims until it finds a good spot, and grows into a polyp. The polyp then grows normally for a while, before absorbing its tentacles and splitting horizontally in a group or juvenile medusa. The polyp then regrows and will continue to strobilate periodically. Cnidaria can also produce asexually by splitting and regenerating full body parts from the fragments. (JS) (21)

Review Questions

1) Since cnidarians are only two cell layers thick, how does the cnidarian ensure that it's body obtains the nutrients it needs to survive?(YA)
2) Compare the thermoregulation of Cnidarians to some of the other kingdoms and phylums in the wiki-space, to which is it most similar? Most different from? why? (GR)
3) What are some mechanisms Cnidarians use to attack their prey? (LW)
4) Explain how Cnidarians sense the environment and why a system of nerve nets might be beneficial. Include how the structure of their body provides a function towards this. (NG)
5) Describe briefly the circulatory system of a Cnidarian, and explain why it makes sense that the circulatory fluid is what it is. (NI)
6) Compare and contrast the polyps stage vs the medusa stage and describe the advantages and disadvantages of each. Which stage is more efficient and why? (CW)
8) How do Cnidarians regulate their temperature? To what extent are they dependent on the environment? (RG)

(Original Page Edit) Campbell, Neil A., and Jane B. Reece. "Unit 5 The Evolutionary History of Biological Diversity, Chapter 33 Invertebrates, Section 'Radiata', Subsection 'Phylum Cnidaria: Cnidarians Have Radial Symmetry, a Gastrovascular Cavity, and Cnidocytes'" Biology. Sixth ed. San Francisco: Benjamin Cummings, 2002. 648-50. Print.

1. (ZXU)
3. (LW)
4. (SM)
5. (MT)
6. (RL)
9. (ORS)
10. (LW)
11. (MC)
12. Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings (ZS)
13. (CC)
14. (MF)
15. (CSR)
16. (CSR)
18. (SD)
19. (MLK)
21. (JS)