Diagnostic Characteristics

A handful of important shared characteristics make cnidarians unique among animals. Firstly every cnidarian species of is aquatic, and most are marine. Only a few cnidarians can survive in fresh water.

Symmetry is common among members of the Kingdom Animalia, and cnidarians are no exception. However, rather than the bilateral symmetry shared by arthropods and chordates, cnidarians have radial symmetry. Instead of a central line of symmetry, Cnidarians have a point of symmetry in the center of their bodies. Any feature on one side of this point is identical to a feature on the opposite side.
Cnidarian Morphology: Polyp vs. Medusa
Cnidarian Morphology: Polyp vs. Medusa

All cnidarians exist in one of two forms: the polyp or medusa. Polyps are sedentary tree-like cnidarians which anchor themselves to their substratum. A medusa is the free-swimming form of cnidarian- usually characterized by its bell-shaped body and dangling tentacles. Whether a medusa or polyp, all cnidarians are constructed the same way. They all have a coelenteron, a sac-like body space also known as the gastrovascular cavity (where gas exchange and digestion occur), the only opening of which functions as a mouth and anus. Cnidarians have only two cell layers (making them dioplastic): the epidermis- covering the exterior of the animal, and the gastrodermis- covering the surfaces inside the gastrovascular cavity. The space between these cell layers is filled with a jelly-like substance called mesoglea (this is the famed “jellyfish jelly” for which these cnidarians are colloquially named). This basic construction is universal for all cnidarians in both polyp and medusa stages.

The most defining characteristic of the phylum Cnidaria- the one for which it is named- is the development of cnidocytes, cells uniquely specialized for defense and the capture of prey. A cnidocyte will contain a cnida, an evertible capsule-like organelle. Cnidocytes specialized for stinging are called nematocysts. If an organism has cnidocytes, it is a cnidarian (the name of this phylum comes from the Greek word for nettle: cnide).

There are four classes of Cnidarians:
1. Class Hydrozoa: (includes Hydra, Obelia, fire corals)
Have unpartitioned guts and contain a shelf-like structure called the vellum at the base of the bell (as medusa)
Olindias formosa (Class Hydrozoa)
Olindias formosa (Class Hydrozoa)

2. Class Scyphozoa: (most true jellies)
Has a gut divided into four parts and lack a vellum (as medusa). Scyphozoa also have rhopalia, sensory organs that can detect light, provide a sense of gravity, and allow for taste of chemicals dissolved in water. Some open-ocean scyphozoans lack the polyp stage of development since there are no surfaces on which to anchor themselves.
Sea Nettles (Class Scyphozoa)
Sea Nettles (Class Scyphozoa)

3. Class Anthozoa: (includes sea anemones, true corals, and soft corals)
The gut is divided into more than four sections. Anthozoans lack the medusa stage in their life cycles.
Sea anemone (Class Anthozoa)
Sea anemone (Class Anthozoa)

4. Class Cubomedusa: (box jellyfish)
This class contains medusae with cube shaped bells, and includes some of the most venomous living organisms.
Box jellyfish (Class Cubomedusa)
Box jellyfish (Class Cubomedusa)

Acquisition and Digestion of food

All cnidarians are carnivores. They use arrays of cnidocytes on the tentacles surrounding their mouths to sting and capture prey. Cnidocytes contain cnidae, sack-like organelles which flip inside-out and attach a harpoon to any object in range.
Nematocyst before and after firing
Nematocyst before and after firing
Nematocysts inject powerful venom to immobilize and kill prey, while other types of cnidocytes have longer threads that stick to or entangle prey. Cnidocytes contain triggers which can be activated by either touch or chemical signals. The cnidarian itself does not trigger stinging cells to fire of communicate witht he cells; the cells do it on their own, according to their own stimuli (JE).

Video of Nematocyst Firing

The mouth and tentacles of cnidarians in their polyp form point upward- away from the surface they are anchored to. Since polyps cannot independently move they have to wait for their prey to swim or drift into their tentacles. The tentacles of a free-swimming medusa will sting any prey unlucky enough to come into contact with them as they trail behind the main body.

The tentacles, arranged in a ring around the mouths of both polyps and medusas, will push the incapacitated prey through the mouth into the gastrovascular cavity to be digested. Food in the gastrovascular cavity is broken down into pieces small enough to be digested in the food vacuoles of gastrodermal cells. With their flagella the gastrodermal cells help to both distribute nutrients and agitate the food-water mixture in the gastrovascular cavity. Any undigested remains are expelled through the mouth, acting as an anus.

One example of a predator cnidarian is the Portuguese Man of War (of the hydrozoan class) (16 AN). This cnidarian is a colony of four different specialized polyps and medusoids. (AN) These specialized cnidarians are physiologically integrated into each other, and cannot live independently. (AN) The Portugeuse Man of War is known for its painful sting and generally swarms in hundreds (AN)

Sensing the Environment

Cnidarians have a primordial but elegant sensory system. Primitive sensory receptors are distributed radially around the body, and relay electrochemical signals to a noncentralized nerve net (cnidarians do not have brains). Therefore, even though their sensations are not nearly as detailed as those of most vertebrates, cnidarians can detect and respond to stimuli from every direction.

The communications between most human neurons are via chemical synapses, extracellular gaps that let neurotransmitter chemicals cross. However, this process is too slow to ensure that the muscles around the edge of a medusa's bell contract simultaneously while swimming. The neurons which control this communicate by much faster electrical synapses across gap junctions. Gap junctions connect the cytoplasm of adjacent neurons, eliminating the extracellular transmission step. Neurons joined by gap junctions act more like wires than their chemically synaptic cousins: the signal decreases in magnatude as it passes from one cell to the next, and electrical synapses can occur in both directions over a gap juction. Signals loose energy to electrical resistance and can be transmitted in either direction.


Cnidarians in their polyp form are sessile- meaning that they do not locomote. They remain stationary on their stratum and must wait for their resources to come to them.

Medusas however can actively swim to hunt prey, or avoid predators and other hazards. They swim by jet propulsion, but most do so weakly, so they rely on currents to carry them long distances. (11DC) Through periodic changes in shape and contractions of the bell, coordinated by the nerve net, medusas swim in the ocean in a way no other animals do. Pennatulacean colonies move slowly across substrata by action of their inflatable peduncle- a stalk that attaches to the strata in the lower end and to the polyp body on the higher end. (4 VK) Cnidarians are diploblasic; they have cells originating from endoderm and ectoderm, but they lack mesoderm, the tissue layer from which true muscles develop. Cnidarian locomotion, therefore, is controlled by only the most primitive muscles. These simple muscles are composed of microfilaments arranged into contractile fibers in both the epidermis and gastrodermis.

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(T2).

Muscles only work best when pulling against a skeleton. And since cnidarians have jelly rather than bone, they use their gastrovascular cavity as a hydrostatic skeleton. If the medusa’s mouth is closed, the gastrovascular cavity contains a fixed amount of water and will resist changes in volume and shape just as a skeleton does.

Dispersal of dye behind swimming jellyfish. from on Vimeo.


Oxygen is passively diffused into cells as carbon dioxide is passively diffused out of cells. All cells in the epidermis and gastrodermis respire this way. Cnidarians lack a respiratory system or any respiratory specialization.Some Anthozoa have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a hydrostatic skeleton, to control the water pressure in the cavity without expelling undigested food.(15DO) .All living cells, and therefore tissues, must be within 1 mm of a surface which is soaked in oxygenated water (4 JSun).

Metabolic Waste Removal

Cnidarians excrete metabolic waste via diffusion from each individual cell into the surrounding water. Cnidarians lack an excretory system or any cell specialization for the facilitation of the removal of metabolic wastes.The main waste product of cell's internal processes is ammonia. (10-SC)


Cnidarians lack a circulatory system. The tasks of a circulatory system are performed by the surrounding water. Oxygen and nutrients diffuse into cnidarian cells from the water, and carbon dioxide and metabolic wastes are diffused from the cells out into the water.

Self Protection

The nematocysts of an Aglantha sp. (DPOD 20)

Cnidarians have developed several strategies to stay safe in today’s oceans and lakes. Nematocysts function not only in capturing prey, but also in repelling potential predators. Stings from certain cnidarians can be painful or even deadly to even the largest would-be predators.
Many jellies, of the cnidarian class Scyphozoa, have transparent bodies. Living in transparent water with a transparent body is a successful strategy to make visual detection by predators with less than perfect eyesight unlikely. Transparency is the simplest and often the ultimate cryptic coloration.

Corals, members of the cnidarian class Anthozoa, build calcium carbonate exoskeletons to protect their gelatinous bodies from the various dangers of the deep. When they die, their skeletons remain and other corals colonize and build their own skeletons on top of them, and in such a way erect a coral reef.

Osmotic Balance

Since the water-permeable cell membranes of cnidarian cells are in direct contact with the surrounding water, the cells must be isotonic with their environment. Marine cnidarians cannot survive in fresh water because their cells would rupture from the influx of water trying to equalize the external and internal salinities. And fresh-water cnidarians (hydras, of the class Hydrozoa) cannot survive in salt water because the water inside their cells would be expelled to increase the internal salinity to that of the surrounding water. Cnidarians have no system with which to regulate osmotic balance, making them especially sensitive to changes in salinity.

Temperature Balance

Cnidarians have no system to regulate temperature. They always take on the temperature of their environment. Rather than evolving a system of keeping warm, cnidarians living in cold climates live with slower metabolic rates than their warmer cousins.
The polyp, which comprises the coral reefs, have a sensitive temperature range of 23°C - 29°C (73°F - 84°). Since cnidaria cannot control their internal temperatures, global warming has had a drastic affect on the coral reefs (CH 6).


Members of Cnidaria (depending on the organism) can reproduce sexually, asexually, or both sexually and asexually. Although some cnidarians go through stages of sexual and asexual reproduction, a cnidarian is diploid in every stage of its life except as a gamete. Hydra and some medusae reproduce asexually by budding, a process involving outgrowths from a parent (7 J. Stein). One example of asexual reproduction is called strobilation. Strobilation starts with some morphological changes (i.e. reabsorbing of gonads) as well as neck formation. Simultaneously, constrictions occur in the neck formation and the number of constrictions increases, transforming the body into a series of small, equal-sized segments. The cnidarian's segmentation releases new, juvenile polyps called ephyras. Jellyfish, for example, reproduce via strobilation (6 AS). Medusae primarily reproduce sexually by producing gametes (sex cells). A male medusa gamete (sperm cell) fuses with a female medusa gamete (egg cell) to form a zygote. The zygote develops into a larva which then grows to be an adult medusa. Medusae can also reproduce through such asexual mechanisms as budding (10 MB).
The Life Cycle of a Jellyfish (AW)

Russel McAvoy

Review Questions:
1) Why would placing a marine cnidaria into a fresh water environment have such a drastic effect upon the cnidaria? (12 AL)
2) Explain the cnidocyte, the namesake of cnidaria. Describe its basic structure and discuss how specialized cnidocytes function as a means of capturing prey and defending against predators. (SW 7)
3) What type of symmetry do cnidaria have and what does that type of symmetry entail? (EK)
4) Explain how the cnidarian structure related to its respiratory system and metabolic waste system. (JE)
5) Describe the two main types of cindaria and the locomotion of the kind to which it applies. (AR 17)
6) Name three uses cnidarians have for tentacles. (CS 12)


1) Campbell, Neil A, and Jane B. Reece. Biology. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002.

2) "strobilation." Encyclopædia Britannica. 2009. Encyclopædia

3) Britannica Online. 24 Oct. 2009 <>.

4) Christian Brothers University (Arts and Sciences): Christian Brothers University. 24 October 2009

5) "Cnidaria." 24 Oct. 2009

6) Ramel, Gordon. "They Phylum Cnidaria" Oct. 21 2009. The Earth-Life Web. Oct. 25 2009 <>

7) "Cnidaria Outlines." 25 Oct. 2009 <>.

8) "Cnidarian."
. Web. 25 Oct. 2009. <>.
"Cnidarians." Web. 25 Oct. 2009. < tietjen/images/cnidarians.htm>.

9) "Cnidaria."
Tree of Life Web Project. Web. 25 Oct. 2009.

10) "Cnidarians." 31 Oct. 2009 "Jellyfish Cam" 2 Nov. 2009 <>

11) "Nematocysts." The JelliesZone - Jellyfish & Other Gelatinous Zooplankton//. Web. 06 Nov. 2009. <>.

12) "Cnidaria." Web. 9 Nov. 2009. <>.

13) "Gap Junction." Web. 9 Nov. 2009. <>.


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