Cnidaria cnidaria2.jpg

(Moon Jelly- a memeber of the Scyphoza class) [13 SV]
Kingdom: Animalia
Super-Phylum: Radiata
Phylum: Cnidaria
Classes: Hydrozoa, Scyphozoa, Anthozoa
-Grace Rehnquist
edited by: Meru Nangia
Yasheka Sharma, Hanna Zhu,
Ethan Richman, Josh Czik. SS, Kevin Nayer
Rachel Kornetsky, Becca Levenson, Hilary Stepansky,
Denny Poliferno, Sarah Vlach, Arielle Reiter, Omer Zaidi,
Jesse Carmen, NK, Vonai Moyo, Sam Blatchford, Brittany Marcus-Blank, Sarah Schwarzschild

Overview and Diagnostic Characteristics (Defining Characteristics)

The phylum Cnidaria, is a part of the larger super-phylum Radiata, which includes animals with radial symmetry, animal exhibits no clear left and right side, and can be cut evenly down many radial planes, and diploblastic embryos. Diploblastic embryos or animals only have two germ levels (which form the tissues and organs of the body), the ectoderm, which covers the surface of the embryos, and the endoderm, which is the innermost germ level that lines the digestive tube.There are three main classes of the phylum cnidarian; hydrozoa, scyphozoa, and anthozoa. Hydrozoans are hydras, Portuguese man-of-war, Obelia and some corals. Most hydrozoas live in a marine habitat (oceans), with a few that live in freshwater. The scyphozoa class includes jellies, sea wasps, and sea nettle. Examples of anthozoan are sea anemones, most coral and sea fans. Both scyphozoans and anthozoans only live in marine habitats. A fourth goup of cnidaria is the class Cubozoa (although some scientists consider this to be an order of scyphozoa). They resemble the jellyfish, but they have a square shape and, as a result, are called "box jellies." The tentacles extend from each corner of the box shape. The Cubozoa are very adept at catching fish, and their nematocysts (see "self-protection" section) releases a strong toxin (4)[HZ]. This simple body construction has two different variations, the sessile polyp and the floating medusa. Polyps are cylindrical forms that are attached to the substratum by the aboral end, top, of the body and extend to the cnidarian tentacles. Examples of the polyp form of the cnidaria are the sea anemones and hydras. A medusa is a mouth down version of the polyp. Medusa's have a gelatinous layer, in between the epidermis, outermost layer of skin, and the gastrodermis, the most inner layer of skin, called mesoglea. The gelatinous form of the mesoglea gives medusa's the name jellies. Some cnidarians are strictly polyps and others are strictly medusas, however they are a few that can pass through both stages in their life cycles.
Hydras, unlike most other hydroaoans, which alternate between the polyp and the medusa form, and the class anthozoa, for example, exist only as the polyp form, while the life cycle of the class Scyphozoa exists predominantly in the medusa form (1). [HZ]
One site claims that cnidarians are the simplest organisms at the tissue grade of organization, meaning they possess groups of cells that together have specific functions. Cnidarians are essentially bags made of two cell layers: the outer ectoderm, or epidermis, which contains the cnidocysts, the stinging cells that are characteristic of the phylum, and the inner endoderm, or gastrodermis, which lines the gut of the organism. (8) (SS)
external image cnidariaDiagram.jpeg
(top left picture-sessile polyp, bottom right picture-floating medusa)

Cnidaria have two life stages as displayed by the picture below. In the medusa stage (e.g. jellyfish) the body shape is more "umbrella-like" because its mouth is at the bottom. The second life stage is a polyp, which is attached to the surface of the organism. It is hollow with a mouth surrounded by tentacles at the top. (16) (BMB)

Graphic of the two life stages of Cnidaria
Graphic of the two life stages of Cnidaria

Acquiring and Digesting Food

Cnidarians are carnivores, which means they eat other animals. To capture their prey, cnidarians use tentacles which are arranged in a ring around their mouth. The tentacles have groups of cnidocytes, cells that function in defense and capture of prey. Cnidarians inject their prey with a poison from the nematocyst and paralyze their prey. (3. Sharma) After, they push the food from their mouth into their gastrovascular cavity, which is where digestion commences. The gastrovasular cavity is a single opening which acts as both a mouth and anus. When a cnidarian closes its mouth, the volume of the cavity is fixed, and the contraction of the cells in the cavity cause the animal to change shape. The acquiring and digestion of food in medusa's becomes more specific. Like other cnidarians the process begins in the gastrovascular cavity, but it is then completed in the gastrodermal cells, cells that line the gastrovascular cavity, in food vacuoles. Flagella, a tail like structure that projects from cell bodies, which are located on these gastrodermal cells are used to keep the gastrovascular cavity agitated and move nutrients throughout the body.

Sensing the Environment

Since cnidarians lack a brain, they use their non centralized nerve net, a simple type of nervous system, which is associated with the simple sensory receptors, which are separated radially through out the body, to respond to their environments. The nerve net, a loosely organized network of nerve cells that allow cnidarians to detect stimuli. Statocysts are sensory cells that help cnidarians determine the direction and help orient the organism. And the ocelli, or eyespots, help the cnidarians detect light. (3. Sharma)
Some species of Cnidaria have the ability to sense tilt and acceleration. They are able to do so due to hair lined chambers called statocysts that act as balance organs. If the organism falls out of balance, the statocysts help it right itself and regain its balance. (5) (KN)


sharonapbio-taxonomy » Animalia-Cnidaria
The life circle of cnidaria (OZ)

The cnidarian class of hydrozoa reproduce asexually, independent of sexual process, through the process of budding. A formation grows on the parent hydrozoa, which contains all genetic material of the animal, pinches off from the parent and lives independently. The only time that hyrdrozoans reproduce sexually is when the environmental conditions deteriorate.

For Cnidarians that have both the polyp and the medusa stages, they alternate between the two forms in their sexual life cycle. The polyp reproduces asexually, and the medusa reproduces sexually. Starting with the larval stage, a cnidarian larva, called a planula, attaches to a solid surface and undergoes metamorphosis to become a polyp. The polyp then reproduces asexually to produce more polyps that will form a colony. Certain environmental conditions or the colony's size may cause the polyps to start producing medusae asexually. The medusae then drift off, and they produce eggs and sperms that fuse to produce planulae (5) [HZ].

An example of a Cnidarian that reproduces both asexually and sexually is the jellyfish. The males' sperm and the female's egg are relseased into the water. When they fuse, a larvae is formed and attaches itself to a rock (or other object) and develops into a polyp. The polyp then asexually reproduces by divding itself to form a colony that looks like a stack of saucers. Eventually, each new jellyfish develops tentacles and swims away to repeat the cycle (14) (AR).


Medusa cnidarians are free-swimming animals, whose movements are coordinated by their nerve net. Sessile polyps are usually in the form of colonies and do not move. Sessile means attached permanently and not free moving.
Some cnidarians move using a hydrostatic skeleton. , in the polyp form of Cnidarians. Also, the circular and longitudinal muscles that use water in the gastrovascular cavity help propel the cnidarian through water, where the water pressure causes the body to elongate. The muscles expand and contract to allow movement.

Whereas the Medusa's move by contracting muscles to force water out of
the body and move forward, also known as jet propulsion. (3.Sharma)

external image image?id=72139&rendTypeId=35
A medusa, which moves by propulsion through muscle contraction, and a polyp which is stationed in a colony. (VM)

Metabolic Waste Removal

The gastrovascular system plays an important role in the digestion and dispersion of food and the removal of metabolic waste, as the gastrovacular system serves two separate functions, digestion and transport(ER)(6). Undigested food is egested, or excreted, from the cnidarians body through the gastrovascular cavity.

Self Protection

Cnidarians use the same cells, cnidocytes, that they use in capturing their prey for warding off predators. All cnidarians have tentacles which have stinging cells at the tips. "Cnidarian" comes from "stinging creature". The cells are called cnidocytes, and contain a stinging capsule called the nematocyst, which contains an inverted thread. When a stimulus, something external that affects a behavior, is triggered by a touch or be certain chemicals, the thread shoots out from the cell and puncture the predator. Most cnidarians' nematocyst contains a toxin, which is used to disable their prey. The stimulus that causes the nematocyst to uncoil may be due to a direct stimulus to the cnidocyte or may be due to a nerve impulse sent by the animal, telling the nemocyst to fire. Nematocysts may only be used once before the cnidocyte dies. Generally, nematocysts are meant to subdue prey or protect the cnidarian, and are therefore not harmful to humans. However, there are some species of jellyfish whose stings can be extremely painful or even fatal to humans. [2 Nangia]
Firing sequence of the cnida in a hydra's nematocyst ( (Nangia)
Firing sequence of the cnida in a hydra's nematocyst ( (Nangia)


There is no respiratory system in a cnidaria, but both cell layers have the ability to absorb oxygen, and expel carbon dioxide.
The Cnidara is able to absorb gasses through its cell layers. Because the cells are in contact with the outer environment, there is no need for additional transport within the organism. Each cell is able to take what it needs from the outside environment. (RJS)


Although there is no organ specialized for circulation, cnidaria use diffusion to circulate nutrients through its body. (7) [JAC] They are able to do this through the space in the middle of their body called the gastrovascular cavity, where nutrients are sucked in, digested, and distributed throughout the organism. This can be seen in the diagram below, which shows the gastrovascular cavity of a cnidaria (10 BL). Radial canals and the gastrovascular system of cnidaria is used for the generation of pressure to move gases
or food around. These pressure generating systems are primarily used for locomotion but in cnidaria they are used for circulation. (HS 11)

Osmotic Balance

Osmotic balance is keeping the correct amount of water pressure within ones body. To maintain this balance cnidarian's remove salt from their cells. It is crucial to maintain osmotic balance in its cells because If there is too much salt (hypertonic) in the organsims body the cells will shrivel up and die and if there is too little salt(hypotonic) in the cells the cell will burst.(1SJB)

Temperature Balance

The temperature of the cnidarians change with the temperature of their environment. They have the ability to do this because they are always in constant contact with their environment.

Cnidarians are well known for their ability to regenerate. If they lose a head or a foot it can easily be regenerated. This ability to regenerate also makes it possible for asexually reproduction. Cnidarians have a specialized type of cell (called interstitial, pluripotent cell) that is able to transform into other cell types (nerve cells, etc) that enables them to regenerate (9). (RK) The cells of Cnidarians do not proliferate (grow and divide); instead, they seem to reorganize themselves in order to cover and fix any gap or injury that has been sustained (12) (DP).

Review Question

1. How do the cells in cnidaria help to ward off predators? [Jesse Carmen]
2. How do Cnidaria obtain food and what organs are used in the process? (NK
3. What allows for cnidarians' ability to regenerate? (VM)
4. What are statocysts responsible for? How do the hairs help their function? (DJ)
5. Explain the alternation of generations that occurs with the polyp and the medusa stages of cnidarians (SES).
[1] Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
[2] "CNIDARIANS: SIMPLE ANIMALS WITH A STING!" Wonders of the Seas. 5 June 2007. Oceanic Research Group. 30 Nov. 2008 <>.

[3] Cnidarians, Worms, and Mollusks. <,%20Worms%20and%20Mollusks.ppt>
[4] Myers, P. and J. Burch. 2001. "Cubozoa." Animal Diversity Web. December 06, 2008 <>.
[5] Fautin, Daphne G., and Sandra L. Romano. "Cnidaria." Tree of Life Web Project. Dec 6, 2008 <>.
[6] Moore, J. 2001. An Introduction to the Invertebrates. Cambridge: Cambridge Univ. Press. ISBN 0521779146
[7] "Phylum Cnidaria." Animals - Phylum Cnidaria. 1999. Electric Rodent Web Design. 7 Dec. 2008 <>.
[8] McGinley, Mark, ed. "Cnidaria." The Encyclopedia of Earth. 10 Dec. 2007. Boston University. 7 Dec. 2008 <>.
(9) "Cnidaria." New World Encyclopedia. 2 July 2006. 7 December 2008
(10) "circulation." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 07 Dec. 2008
(11) Aird, William C. Endotheliam Biomedicine. Cambride Medicine. 1126.
12. Holstein, T. W., E. Hobmayer, and U. Technau. Developmental Dynamics. Wiley-Liss, Inc., 2003. Cnidarians: An evolutionarily conserved model system for regeneration? 31 Oct. 2002. 9 Dec. 2008 <>.
(13) "Cnidaria" 13 Dec. 2008.<>
(14) "Cniderians." MSN Encarta. 13 Dec 2008. <>.
(15)"Cnidarian". 15 Dec 2008 <>
16) Davey, Keith. "Cnidaria." Life on Australian Seashores. 2000. MESA. 17 Dec. 2008 <>.