The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or

biradially symmetric, a general type of symmetry believed primitive for eumetazoans. They have achieved the tissue level of

organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur.

Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that

serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one

epidermal and the other gastrodermal, that help coordinate muscular and sensory functions.


Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating.

They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side,

and the tentacles originate on the rim of the umbrella.


Polyps, in contrast, are usually sessile. They have tubular bodies; one end is attached to the substrate, and a mouth (usually

surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case,

different individuals sometimes specialize for different functions, such as reproduction, feeding or defense.


Reproduction in polyps is by asexual budding (polyps) or sexual formation of gametes (medusae, some polyps). Cnidarian

individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and



If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. These tiny

organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective

deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned. A

nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into

the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the

tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or

in extreme cases, death.


We recognize four Classes of Cnidaria:


Class Hydrozoa


These organisms may exist as either polyps or medusae. Many species pass through both forms in their life cycles; in others,

one form or the other is suppressed. The majority of species are marine and colonial, but many species are solitary and some

live in fresh water.


Hydrozoans include such diverse forms as hydra, which are a group of species of solitary, fresh-water forms that live as polyps

throughout their lives. Reproduction by hydras can be the typical form for polyps, asexual by budding, or by sexual means in

which temporary gonads appear on the polyp itself.


Hydrozoans also include colonial species, called hydroids. Hydroids are actually colonies of polyps growing on a common

stalk. Colonies are formed by the asexual budding of members; these buds, unlike those of hydras, remain attached to the

parent. Some polyps in a typical hydroid specialize in reproduction; these produce medusae by budding. In this typical case, the

medusae, which are usually free-swimming, leave the colony, mature, and produce gametes for sexual reproduction. The

gametes are shed, fertilization takes place, and the zygote develops into a free-swimming, ciliated planula larva. After a time the

planula settles to the bottom and develops into a polyp, which by budding eventually forms a new colony. Not all hydroids

follow this pattern; in some, the medusa phase is compressed or reduced and gametes are shed directly from specialized











embryo —> planula —> polyp —> medusa —> gametes —> embryo


Some hydrozoans, such as the Portuguese man-of-war, form floating colonies that include both specialized medusae and

polyps. Others, called hydrocorals, have calcareous skeletons that resemble those of true corals.


Hydrozoan medusae all have a distinctive structure called a velum. This is a shelf or rim that projects inward around the margin

of the bell, partially closing the opening. Hydrozoan medusae swim by alternately constricting and relaxing muscles in the bell.

This causes water to shoot out the constricted opening of the bell, moving the animal by a sort of "jet propulsion."


Class Scyphozoa


The Class Scyphozoa includes most of the larger jellyfish. The medusae of members of this class lack a velum. In many species,

the rim of the bell contains sense organs, including statocysts that sense balance and orientation and photoreceptors that are

sensitive to light. Scyphozoans are often amply covered with nematocysts, so that a swimmer's encounter with one can be an

unpleasant experience. They feed in a variety of ways, but often involving prey capture by nematocysts on the arms and

transport of food items to the gastrovascular cavity by means of cilia. As in other cnidarians, digestion is intracellular.


Scyphozoans are frequently strikingly tetramerous (four-parted). Four gastric pouches connect with the gastrovascular cavity,

and the opening to that cavity may have four lobes. The peripheral sense organs are usually placed in pits or notches; these are

often present in multiples of four.


Scyphozoans are dioecious. Fertilization and early development usually take place in the gastrovascular cavity or on the lobes

near the gastrovascular opening. A ciliated planula larva forms; in most scyphozoans it settles and forms a hydra-like polyp.

This life stage, called a scyphistoma, reproduces asexually by budding. At first the product of budding is other scyphistomas,

but later they produce tiny medusae that break free and grow to form mature, sexual scyphozoan medusae.









embryo —> planula —> scyphistoma —> medusa —> gametes —> embryo




Class Cubozoa


Cubozoans are also known as box jellyfish, because in transverse section the bells appear to be square. Tentacles are located

at the corners of the square umbrella margin, and the base of each tentacle is distinctively flattened. The edge of the umbrella

turns inward to form a rim called a velarium, much like the velum of hydromedusae.


Cubozoans are considered by some to be a subclass of Scyphozoa. Effective predators and strong swimmers, they feed mostly

on fish. Their nematocysts are especially potent, sometimes resulting in the death of unfortunate human swimmers who

encounter them.





Class Anthozoa


The Class Anthozoa includes a variety of animals that have polyps with a flower-like appearance. In these forms, the

gastrovascular cavity is large. It is divided by walls or septa, which arise as folds from the body wall. These folds, along with

the mouth and pharynx, are usually arranged in a biradially symmetric pattern.


Anthozoans include sea anemones, a variety of corals, sea fans, and sea pens. Sea anemones are carnivorous polyps that are

quite large, ranging up to 200mm in length. They tend to be brightly colored. Most species live in warm water. They feed on

fishes, which are caught by means of the numerous nematocysts in their tentacles. These animals are known for their symbionts. 

These include species of fish that actually live among the tentacles of large anemones, somehow avoiding lethal contact with the

nematocysts. Other anemones have unicellular algae living within their tissues, from which they probably derive some nutrition.

Yet others have a symbiotic relationship with hermit crabs, which gather up the anemones and place them on the snail shells that

the crabs occupy. The anemones benefit from food particles dropped by the crab, and the crab gains protection from predators due

to the presence of the nematocyst-laden anemones.


The Class Anthozoa also includes many kinds of corals, including many reef-building species. Reefs are formed by the

calcareous skeletons of many generations of coral polyps. The polyps inhabit only the surface of the reefs. These reefs are

among the most productive environments of the world, housing thousands of species of fish and invertebrates, not to mention

plants and protists. Like some anemones, corals are inhabited by symbiotic algae called zooxanthellae. These photosynthetic

algae are essential for the coral, and corals generally do not live at depths to which light does not penetrate.