CNIDARIA (Coelenterates): (pp. 99-132)

All Cnidarians possess radial symmetry, usually tetramerous radial symmetry.

Polymorphism with both polyp and medusa stage is common, but variable from one group to another. In some groups, only the polyp occurs and in other groups only the medusa occurs. In many groups, both occur.

All are aquatic and the overwhelming majority are marine.

They are probably not ancestral to the higher metazoa, although according to one author, "they are not evolutionarily distant from the early metazoan stock ".

All are carnivorous, but some have algal endosymbionts as well (both zoochlorellae and zooxanthellae occur).

Cnidarians are characterized by the tissue level of organization.

All undergo clonal growth at sometime in their life histories.

Body plan always consists of an outer epidermis, and inner gastrodermis, and an intermediate mesoglea. Epidermis and gastrodermis layers are one cell thick. Mesoglea is variable from very thick in anemones and most jellyfishes, to quite thin in some of the hydroids. Gastrovascular cavity and tentacles are always present.

Cnidocytes containing nematocysts are characteristic of (and limited to) this phylum.


Classification of Cnidaria: 3 ( or 4* ) classes

Class Hydrazoa: Hydroids. Usually both polyp and medusa.

Class Scyphozoa: Jellyfish. Medusa dominant, polyp reduced

*Class Cubozoa: Some authors consider the Scyphozoan order Cubomedusae ( "sea  wasps" and "box jellies" ) to be a separate fourth class, the Cubozoa.

Class Anthozoa: Anemones and corals. Polyp only.


Nematocysts: 25 different types have been described. Four main types are:

Nematocyst discharge appears to be caused by rapid influx of water into cnidocyte capsule. Mechanism is probably osmotically driven.


Polyp Body Plan: gastrovascular cavity, column, basal disk, oral disk (mouth on hypostome, tentacles) , nematocysts

Medusa body plan: body components are the bell, exumbrella, subumbrella, and mouth on a manubrium, tentacles, nematocysts, gastrovascular cavity, velum, radial canals (4), and ring canal.


Feeding behavior is stimulated by chemicals in water. Glutathione, a simple tripeptide released by injured prey, and even single amino acids may trigger it.

Nervous system: Cnidarians have true nerve cells arranged into nerve nets. A given species may have one, two, or more separate but functionally connected nets. Neurons are usually multipolar, but some may be bipolar as in higher organisms. There is no CNS, but the nerve ring of some medusas seems to coordinate some activities.

Reproduction and Development: asexual reproduction is by budding. Depending upon the species and the life histroy stage, budding may produce a polyp, a medusa, or a frustule, which is essentially a planula larva without cilia which separates from parent, crawls off a little way, and develops into a polyp (which may then bud-off more polyps). Polyps may bud-off either polyps or medusae. Medusae only bud-off more medusae; never polyps.

Sexual stage is usually the medusa, but in species that lack the medusa, the polyps produce gametes (Anthozoa and some Hydroids).

Most cnidaria are dioecious (gonochoristic)

Development proceeds from zygote through 2, 4, 8, etc. cell stages to blastula, gastrula, and then to a ciliated planula larva, which is bilaterally symetrical.


Class Hydrazoa: (pp. 133-151) There are about 3,000 species of Hydrazoans.

These have simplest body plan of all Cnidarians. Gastrovascular cavity is always simple. No cnidocytes in gastroderm.

Most are dimorphic, with polyp and medusa generally of equal importance although there are orders in which either polyp or medusa is much reduced, or even absent entirely.

Most are colonial with horizontal stolons connecting polyps. Rootlike stolons form a network of hydrorhizae which anchor colony to substrate. Continuous 3-layer perisarc (or coenosarc) covers the whole colony. Polyps often differentiate into gastrozooids and gonozooids. There are sometimes also specialized defensive polyps (dactylozooids). All of these may occur within a single colonial organism, a so-called polymorphic colony. The dactylozooids and most gonozoids do not feed.

A polyp-like Actinula larva develops from the planula in orders Narcomedusae and Trachymedusae. This develops into the adult medusa, so there is no polyp stage as such in these orders.

There are 10 orders of Hydrazoa, including:

Order Limnomedusae (examples: Gonionemus, Craspedacusta * also see footnote)

Order Leptomedusae (example: Obelia )

Order Anthomedusae (example: Hydra )

Order Milleporina and Order Stylasterina (the so called "Hydrocorals" that are reef - formers). The "fire corals" or stinging corals are millepores (not true corals).

Order Chondrophora which includes Velella with its chitinous air sacs & sail. Just as there are sessile medusae, Chondrophores seem to be single pelagic polyps, although some authorities consider them to be a complex colonial form.

Order Siphonophora which includes the infamous "Portuguese Man of War" (Physalia sp.) Individuals are actually colonies containing several types of polyps and several types of medusae simultaneously. Some medusae are modified for propulsion or as gas-filled floats. In some species there is a high degree of coordination of the colony. Stings are potentially fatal to man.

A representative Cnidarian: Hydra sp.:

Actually not terribly representative as it is a FW animal while the vast majority of Cnidarians are marine.

General biology as above for Cnidarians in general except for:

Excretion of excess water by active transport of salts into gastrovascular cavity which draws water out of gastroderm down the osmotic gradient. Water and salts then periodically emptied by contraction of the body column.

Mitotic production of new cells is continuous, with the division rate being higher at base of tentacles than at tip, and greater at the oral than at the aboral end. As a result, there is constant flow of cells from mouth to base, and along the tentacles. A hydra probably completly replaces all the cells in its body every few weeks.

Budding in Hydra only produces new polyps. Hydra lacks a medusa stage entirely.

Sexual reproduction usually occurs in fall and winter. In some species, the sexes are separate; other species are hermaphroditic. Ripe egg is held on the external surface of the polyp where it is fertilized. The embryo separates from the parent as a gastrula which is enclosed within a secreted "shell". Gastrula may continue development, or lie dormant until spring.


Another representative Cnidarian: Obelia sp.:

Obelia represents a colonial form which develops from a single polyp by a process of budding in which the buds fail to separate from the parent, but instead produce a series of branching, plant-like growths of polyps deployed along hollow vertical upright structures (stems) that are connected at their bases by horizontal hollow growths (stolons). Food captured and partially digested by one polyp is circulated through the hollow stems and stolons to all other polyps in the colony by the currents produced by flagellated gastrodermal cells.

Polyps or zooids are of two types: feeding polyps (gastrozoids) and reproductive polyps (gonozoids).

The entire colony, with the exception of the distal portions of the polyps, is enclosed in a thin protein and chitin covering, the perisarc. In the "thecate forms" the perisarc forms a goblet-like cup called the theca around each gastrozooid polyp.

Asexual reproduction produces more polyps and increases the size of the colony, and produces medusae by a process of budding in the gonothecae. Sexual reproduction occurs in the medusa stage. The sexes are separate and fertilization is external. A planula larva results.


SCYPHOZOANS (sy-fo-zo'-a) and ANTHOZOANS : (PP. 152-186)

Class Schyphozoa:

Schyphozoans are all marine, and include the larger jellyfishes. (Cyanea in polar waters can reach bell diameters of 8 ft. and have tentacles 130 ft. long)

Aurelia is a representative form. It may reach a diameter of 1 m. Aurelia has 4 trailing mouth lobes (oral arms) which each contain a ciliated groove and many nematocysts.

The scyphozoan gastric cavity is divided into 4 gastric pouches that contain a tentacle-like projection of gastrodermis called a gastric filament which contain numerous gland cells that secrete digestive enzymes. Unlike the hydrazoans, the gastric pouches contain nematocysts.

Scyphozoans lack the marginal nerve ring of the hydrazoa. Instead they have 8 marginal ganglia, one near each rhopalium, a complex of sensory structures (one statocyst, two simple eyes, and two chemosensory pits). There are 8 rhopalia, all located in the tentacular ring. There are two (or more) nerve nets, one which serves the whole surface of the animal, including the subumbrella, and one that serves only the subumbrella. The latter conducts impulses more rapidly and is concerned with swimming.

Reproduction in Aurelia, a representative scyphozoan:

The sexes are separate (dioecious). Sperm are released from mouth of male into the water and are filtered from the water by the female and fertilization is internal. Embryos develop into planula larvae in specialized brood pouches in the folds of the mouth lobes. Planulas are ultimately released and attach to overhanging rocks where they develop into polyps (scyphistoma). Scyphistomas bud-off additional polyps which ultimately undergo strobilation, a progressive constriction into a stack of saucer-like medusas (ephyras) which subsequently separate. Strobilation does not occur in the other Cnidarian classes.

The Cubomedusae are an order of scyphozoans (some rank them as a separate class, intermediate between the hydrazoa and scyphozoa) which are generally small and transparent. The bell is almost perfectly square in cross section. They have the most complex and sensitive rhopalial eyes of any Cnidarian (with a lens, retina, and thousands of sensory cells) and are strongly positively phototactic. Common names are "box jellies" and "sea wasps", named respectively for their square cross section and their powerful sting. They are the strongest swimmers among the Cnidaria and prey largely on fish and shrimp. Chironex, the most dangerous of any medussa known (tropical waters of north Australia) has a sting which causes intense pain and which has killed humans within 3 minutes.


Class Anthozoa: (about 6,500 spp.) These are all marine. There are no FW anthozoans.

No medussa stage in any anthozoan.

The order Actiniaria are the "sea anemones". All are large solitary polyps with the typical polyp organization of column, oral disk, mouth, tentacles, basal disk. While most anemones are sessile, a few can burrow into soft substrates and a few are pelagic, using a floating device secreted by the basal disk.

Since they are mostly sessile polyps, they lack the more complex nervous systems and structures seen in some of the medussae, but they do have at least one nerve net.

Mouth communicates to gastric cavity through a pharynx which has a ciliated groove called a siphonoglyph at either or both ends. Although external symmetry is radial, the internal symmetry is biradial or bilateral.

Mesoglea is a true cellular layer.

Gastrovascular cavity is divided by numerous septa (called mesenteries) which increase the surface area for absorption of the products of digestion.

Reproduction: (a) asexually by spontaneous binary fission or fragmentation of whole animal (very unusual for a fairly complex metazoan). or (b) sexually. Depending upon the species, the sexes may be separate or hermaphroditic. The gonadal tissue is in the gastric septa. Eggs and sperm usually ejected through mouth and fertilization is generally external, although some species fertilize internally and brood their young either internally, or attached to the outer surface.


Order Scleractinia are the "stony corals", which secrete a CaCO3 skeleton. All the reef-building strony corals harbor zooxanthellae. In one experiment, approx. 95% of carbon fixed by zooxanthellae was transferred to the coral. Most reef building corals live in less than 30 m. of water, and none live in water deeper than 90 m.The stony corals are "hexacorals", with tentacles and septa usually in multiples of six.

Subclass Alcyonaria (also called the octocorallia) ) or soft corals, sea whips, etc. These are "octocorals" with body parts in multiples of eight. All octocorals are colonial and usually polymorphic.

Order Gorgonacea (Gorgonians) are the "sea whips", "sea fans", etc. Precious red coral (consisting of fused orange or red calcareous spicules) used in jewelry comes from this group.

Order Pennatulacea or "sea pens" and "sea pansys"


Links: The Cnidaria Homepage

* Footnote: OK, Craspedacusta sowerbii, the only known FW jellyfish, is much more widely distributed than I had thought. It is now found throughout much of the United States and Eastern Canada. It has apparently also made its way to Europe as well. In West Virginia it is found in numerous artificial reservoirs and in the Ohio River. The map below shows its present distributiion in the US. The darker colored states are the states from which it has been reported.