A Guide to the Cnidarians

01
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Basic Anatomy

This anemone has tentacles and exhibits radial symmetry.
This anemone has tentacles and exhibits radial symmetry. Photo © Purestock / Getty Images.

Cnidarians are a diverse group of invertebrates that come in many shapes and sizes but there are some basic features of their anatomy that most share in common. Cnidarias have an internal sac for digestion which is called the gastrovascular cavity. The gastrovascular cavity has only one opening, a mouth, through which the animal takes in food and releases waste. Tentacles radiate outward from the rim of the mouth.

The body wall of a cnidarian consists of three layers, an outer layer known as the epidermis, a middle layer called the mesoglea, and an inner layer referred to as the gastrodermis. The epidermis contains a collection of different types of cells. These include epitheliomuscular cells which contract and enable movement, interstitial cells that give rise to many other cell types such as egg and sperm, cnidocytes which are specialized cells unique to cnidarians which in some cnidarians contain stinging structures, mucus-secreting cells which glandular cells that secrete mucus, and receptor and nerve cells which collect and transmit sensory information.

02
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Radial Symmetry

The radial symmetry of these jellyfish is apparant when they are viewed top-down.
The radial symmetry of these jellyfish is apparant when they are viewed top-down. Photo © Shutterstock.

Cnidarians are radially symmetrical. This means that their gastrovascular cavity, tentacles, and mouth are aligned such that if you were to draw an imaginary line through the center of their body, from the top of their tentacles through the base of their body, you could then turn the animal about that axis and it would look roughly the same at each angle in the turn. Another way to look at this is that cnidarians are cylindrical and have a top and bottom but no left or right side.

There are several sub-types of radial symmetry that are sometimes defined depending on the finer structural details of an organism. For example, many jellyfish have four oral arms that extend below their body and their body structure can therefore be divided into four equal parts. This type of radial symmetry is referred to as tetramerism. Additionally, two groups of cnidarians, corals and sea anemones, exhibit six- or eight-fold symmetry. These types of symmetry are referred to as hexamerism and octamerism, respectively.

It should be noted that cnidarians are not the only animals to exhibit radial symmetry. The echinoderms also display radial symmetry. In the case of the echinoderms, they possess five-fold radial symmetry which is referred to as pentamerism.

03
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Life Cycle - Medusa Stage

This medusa are free-swimming jellyfish.
This medusa are free-swimming jellyfish. Photo © Barry Winiker / Getty Images.

Cnidarians take on two basic forms, a medusa and a polyp. The medusa form is a free-swimming structure which consists of an umbrella-shaped body (called a bell), a fringe of tentacles that hang from the edge of the bell, a mouth opening located on the underside of the bell, and a gastrovascular cavity. The mesoglea layer of the medusa body wall is thick and jelly-like. Some cnidarians only exhibit the medusa form throughout their life while others first pass through other phases before maturing into the medusa form.

The medusa form is most commonly associated with adult jellyfish. Although jellyfish pass through planula and polyp stages in their life cycle, it is the medusa form that is most recognized with this group of animals.

04
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Life Cycle - Polyp Stage

This closeup of a colony of hydrazoans shows the individual polyps.
This closeup of a colony of hydrazoans shows the individual polyps. Photo © Tims / Wikipedia.

The polyp is a sessile form which attaches to the sea floor and often forms large colonies. The polyp structure consists of a basal disc that attaches to a substrate, a cylindrical body stalk, inside of which is the gastrovascular cavity, a mouth opening located on the top of the polyp, and numerous tentacles which radiate out from around the edge of the mouth opening.

Some cnidarians remain a polyp for their entire life, while others pass through the medusa body form. The more familiar polyp cnidarians include corals, hydras, and sea anemones.

05
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Cnidocyte Organelles

The tentacles of cnidarians have cnidocytes embedded within them. The cnidocytes of this jellyfish contain stinging nematocysts.
The tentacles of cnidarians have cnidocytes embedded within them. The cnidocytes of this jellyfish contain stinging nematocysts. Photo © Dwight Smith / Shutterstock.

Cnidocytes are specialized cells located in the epidermis of all cnidarians. These cells are unique to cnidarians, no other organism possesses them. Cnidocytes are most concentrated within the epidermis of the tentacles.

Cnidocytes contain organelles called cnidea. There are several types of cnidea which include nematocysts, spirocysts, and ptychocysts. The most notable of these is the nematocysts. Nematocysts consist of a capsule containing a coiled thread and barbs known as stylets. Nematocysts, when discharged, deliver a stinging venom that serves to paralyze prey and enable the cnidarian to ingest its victim. Spirocysts are cnidea found in some corals and sea anemones that consist of sticky threads and help the animal capture prey and adhere to surfaces. Ptychocysts are found in members of a group of cnidarians known as the Ceriantaria. These organisms are bottom dwellers adapted to soft substrates into which they bury their base. They eject ptychocysts into the substrate which help them establish a secure hold.

In hydras and jellyfish, the cnidocytes cells have a stiff bristle that projects out from the surface of the epidermis. This bristle is called a cnidocyl (it is not present in corals and sea anemones, which instead possess a similar structure called a ciliary cone). The cnidocyl serves as a trigger to release the nematocyst.

06
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Diet and Eating Habits

The mouth of a cnidarian is located on the top (polyp) or under the bell (medusa) and is surrounded by tentacles.
The mouth of a cnidarian is located on the top (polyp) or under the bell (medusa) and is surrounded by tentacles. Photo © Jeff Rotman / Getty Images.

Most cnidarians are carnivorous and their diet consists mainly of small crustaceans. They capture prey in a rather passive manner—as it drifts through their tentacles the cnidarian discharge stinging nematocysts that paralyze the prey. They use their tentacles to draw the food into their mouth and gastrovascular cavity. Once in the gastrovascular cavity, enzymes secreted from the gastrodermis break down the food. Small hair-like flagella that line the gastrodermis beat, mixing enzymes and food until the meal has been fully digested. Any undigestible material that remains is ejected through the mouth with a swift contraction of the body.

Gas exchange takes place directly across the surface of their body and waste is released either through their gastrovascular cavity or by diffusion through their skin.

07
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Jellyfish Facts and Classification

Jellyfish spend some of their life cycle as a free swimming medusa.
Jellyfish spend some of their life cycle as a free swimming medusa. Photo © James R.D. Scott / Getty Images.

Jellyfish belong to the Scyphozoa. There are approximately 200 species of jellyfish that are subdivided into the following five groups:

  • Coronatae
  • Rhizostomeae
  • Rhizostomatida
  • Semaeostomeae
  • Stauromedusae

A jellyfish begins its life as a free-swimming planula which after a few days drops to the sea floor and attaches itself to a hard surface. It then develops into a polyp which buds and divides to form a colony. After further development, the polyps shed tiny medusa which mature into the familiar adult jellyfish form which goes on to reproduce sexually to form new planulae and complete their life cycle.

The more familiar species of jellyfish include the Moon Jelly (Aurelia aurita), the Lion's Mane Jelly (Cyanea capillata) and the Sea Nettle (Chrysaora quinquecirrha).

08
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Coral Facts and Classification

Mushroom coral
Mushroom coral. Photo © Ross Armstrong / Getty Images.

Corals belong to a group of cnidarians known as the Anthozoa. There are many types of coral and it should be noted that the term coral does not correspond to a single taxonomic class. Some groups of corals include:

  • Alcyonacea (soft corals)
  • Antipatharia (black corals and thorny corals)
  • Scleractinia (stony corals)

Stony corals make up the largest group of organisms within the Anthozoa. Stony corals produce a skeleton of calcium carbonate crystals which they secrete from the epidermis of the lower part of their stalk and basal disc. The calcium carbonate they secrete forms a cup (or calyx) in which the coral polyp sits. The polyp can retract into the cup for protection. Stony corals are the key contributors to coral reef formation and as such provide the main source of calcium carbonate for the construction of the reef.

Soft corals do not produce calcium carbonate skeletons like those of stony corals. Instead, the contain tiny calcareous spicules and grow in mounds or mushroom shapes. Black corals are plant-like colonies that form around an axial skeleton that has black thorny structure. Black corals are found primarily in deep. tropical waters.

09
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Sea Anemones Facts and Classification

Jewel anemone
Jewel anemone. Photo © Purestock / Getty Images.

Sea anemones, like corals, belong to the Anthozoa. Within the Anthozoa, sea anemones are classified in the Actiniaria. Sea anemones remain polyps for their entire adult life, they never transform into the medusa form as jellyfish do.

Sea anemones are capable of sexual reproduction, though some species are hemaphroditic (a single individual has both male and female reproductive organs) while other species have individuals of separate sexes. Egg and sperm are released into the water and the resulting fertilized eggs develop into a planulae larva which attaches themselves to a solid surface and develop into a polyp. Sea anemones can also reproduce asexually by budding new polyps from existing ones.

Sea anemones are, for the most part, sessile creatures which means they remain attached to one spot. But if conditions grow inhospitable, sea anemones can detach from their home and swim off in search of a more suitable location. They can also slowly glide on their pedal disc and can even crawl on their side or by using their tentacles.

10
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Hydrozoa Facts and Classification

Crossota, a deep red medusa found just off the bottom of the deep sea. Alaska, Beaufort Sea, North of Point Barrow.
Crossota, a deep red medusa found just off the bottom of the deep sea. Alaska, Beaufort Sea, North of Point Barrow. Photo © Kevin Raskoff / NOAA / Wikipedia.

The Hydrozoa includes about 2,700 species. Many hydrozoa are very small and have a plant-like appearance. Members of this group include the hydra and the portuguese man-o-war.

  • Actinulida
  • Hydroida
  • Hydrocorallina
  • Siphonophora
  • Trachylina