Cnidaria 06_jellyfish_cnidaria_sp.jpg
Cnidaria is the taxonomic group that includes a vast variety of mainly marine organisms such as jellyfish, hydras, sea anemones, and coral. They belong to the Radiata group and the broader taxonomic group known as Eumetazoa. Radiata are distinguished by their radial symmetry and diploblastic embryos. Organisms with radial symmetry are shaped like a pie or barrel and have rotational symmetry about the center of the body. Organisms with diploblastic embryos have two germ layers in their embryonic state. Humans have three germ layers in their embryos, an ectoderm, mesoderm and endoderm; however, Cnidaria only have an ectoderm and endoderm. Between the ectoderm and endoderm is the mesoglea, which contains collagen-like fibers that form the jelly in jellfish and act as a glue holding the two germ layers together in other cnidarians. These germ layers differentiate during embryonic growth to become different cells in the body. Cnidaria gets its name from the word "Cnidos", which is greek for stinging nettle. All Cnidaria have nematocysts, which are cells that inject venom into prey in order to capture it, or they are used as a defense mechanism to ward off predators.external image 013_Life_cycle_of_a_jellyfish.jpg

Imagine if this life cycle started with planula larva, cycled up to medua, reproduced some more separate planula larvae, and then reverted into being a polyp.
The Turritopsis Nutricula jelly fish, originating in the Carribean (but spreading like crazy) can do that. It will not die of natural causes, because if it gets too 'old' it just reverts to being a polyp, over and over again. It's immortal! (more or less).

Cnidarians disdinguising feature is their cnidocytes, which are specialized cells that are used for capturing prey. Their bodies are made of mesoglea, a gelatinous substance, which is layered in between to layers of epithelial tissue, which are one cell-layer thick. Cnidaria have a very simple digestive tract known as the gastrovascular cavity. The gastrovascular cavity is located at the center of the sac-like body and is accessed by a single opening. This opening serves as the mouth and anus. Cnidarians also have tentacles with stinging cells. These cells are used to subdue prey, and some contain toxins irritating or poisonous to humans. Cnidarians exist as one of two types. The first is a polyp, which has a barrel shape and attaches to the sea floor via its body. Extending upwards off of the body are the tentacles. Some species of polyps can propagate vegetatively, meaning that they reproduce asexually through methods such as budding (an outgrowth of the parent separates to form a new individual). This leads to the formation of clones and colonies of polyps. The other type of Cnidaria is called a medusa. A medusa is very similar to a polyp, but is not fixed to the ground and instead floats freely. The central body is the top of the medusa and then tentacles hang down from the central body. Medusas move through the water either by passive drifting or by contracting the central body, which is shaped like a bell. The muscles and nerves within a Cnidarian are very simple, consisting of two layers of cells. The outermost layer, known as the epidermis and the innermost layer, the gastrodermis form bundles of cells known as microfilaments which contract to act as muscle tissue. These contractions are controlled by a nerve net, a very simple “brain”. Cnidaria feel their surroundings via nerve cells distributed about their bodies that sense surroundings and send the message to the nerve net.
The impulses triggered by the stimulus radiate in all directions. This is caused by the bipolar nature of the neurons, which means that they send impulses in two directions.


Medusae swim by weak jet propulsion and are also carried by the currents. Polyps are generally sedentary, rarely moving from the sea floor that it attaches to. When provoked, Sea anemones are able to move by many means. The can use their tentacles (pedal disks) to move slowly away, which attach them to the surfaces they live on, or they can completely come loose and then roll away. Some anemone species are to to swim by paddling their tentacles or flexing their columns. Pennatulacean colonies are able to move across soft substrata using their inflatable peduncles (stalks that attach to the strata in the lower end and to the polyp body of the higher end).

Basic structure of a cnidarian.
Basic structure of a cnidarian.
external image Cnidaria.jpg

Cnidaria are carnivorous organisms meaning that they consume only meat. They use their tentacles to capture their prey and then transfer the prey to the opening of the gastrovascular cavity. The food is digested and any remaining undigested food is released back out of the mouth/anus. To aid in capturing prey, the tentacles are covered with stingers called cnidocytes. Within cnidocytes are tiny pouches called cnidae, which sometimes release a poison that causes a sting. The cnidae that release the poison are called nematocysts. Nematocysts are a phylum of Cnidaria that use toxins to capture prey or as a defense mechanism. They inject toxins that can even be toxic on man, but most of the time they are just used to stun.Cnidaria are mostly considered to be passive predators. Instead of hunting their prey, they simply wait for it to wander into them, allowing their tentacles to do the rest of the work. Also, some Cnidaria possess the ability to absorb dissolved organic matter right from the water in which they live.

The mouth of a Cnidaria, located on the polyp or under the meduza and surrounded by tentacles.
The mouth of a Cnidaria, located on the polyp or under the meduza and surrounded by tentacles.

Classes of Cnidaria
The phylum Cnidaria can be divided into three classes. The first class is Hydrozoa, which exist as both polyps and medusas in their lifetimes. Almost all Hydrozoa are live in water, and a few live in freshwater. Hydrozoans reproduce asexually, meaning they do not need a partner to reproduce. However, they will only reproduce asexually when conditions are favorable. When conditions in their environment are unfavorable, they will reproduce sexually, and the zygote offspring will not be born until the living condition improve. Sexual reproduction yields free swimming, ciliated larvae. In the typical life cycle of the cnidaria, the male and female medusae release their eggs into the sea where fertilization takes place, and a planula matures. The planula then transforms into the polyp, which then asexually reproduces to create additional polyps. The second class of Cnidaria is Scyphozoa, which are predominantly medusas and are commonly known as jellyfish. In the coastal areas closer to land, Scyphozoa are sometimes polyps during a short stage and then become medusas.[Scyphozoans display a four-part symmetry and have an internal gelatinous material called mesoglea. Scyphozoans have no durable hard parts, including no head, no skeleton and no specialized organs for respiration or excretion. RS.] The final class of Cnidaria is Anthozoa, which is composed of sea anemones and coral. Anthozoa are only found as polyps and build hard skeletons made of calcium carbonate. These skeletons of calcium carbonate are the hard formations known as coral.

[RS]jellyfish.gif external image Sea%20Anemone.jpg
sea anemones [anthoza]
An example of Scyphozoa, a class of Cnidaria. Scyphozoa are often considered to be the "true jellyfish," as are the ones above. (SW)

Review Questions:
1) Explain the difference between the polyp stage and the medusa stage of Cnidaria. (KNS)
2) What is the substance that makes up Cnidaria's body? (MSR)
3) In what ways do Cnidaria protect themselves from predation? (MSR)
Sources: (Sarah Fleming)
"Cnidarian (invertebrate) :: Locomotion -- Britannica Online Encyclopedia." Encyclopedia - Britannica Online Encyclopedia. Web. 26 Oct. 2009. <>.(Walker K.) (KTD) (Jake Schwartz) (Sara Waugh) (Jesse Landy) [BY], Leo Schwartz) (AG) (DG) (Alyssa Zisk) (JJF) (Ali Kirsch) (Jackson Murphy) (Alysse) (Donna McDermott) (Liz Daley)