CNIDARIA

Different Body Forms of The Phylum Cnidaria: Polypoid (left) and Medusoid (right).

Despite their simplicity, cnidarians have their own unique features, and are very successful when viewed in terms of their diversity and abundance. They were also among the first multicellular life to evolve on earth, having arisen at least 650 mil ears ago. Their most obvious unique feature is their highly specialized stinging cells (nematocysts).

The Structure Of A Nematocyst

These vary in shape and function, but usually houses a coiled thread-like sting that can be rapidly discharged in attack or defence to penetrate the skin of its target to release toxins into its body.

Diagramatic presentation of the puncturing of the prey's integument.

Characteristics:

1. Symmetry radial or biradial about an oral-aboral axis. No head or segmentation.

2. Body of two layers of cells, an external epidermis and an inner gastrodermis, with a varying amount of mesoglea in between. Nematocysts in either or both layers.

3. Skeleton limy, or none. Muscle fibers in epithelia.

4.Mouth surrounded by soft tentacles and connecting to a saclike digestive (gastrovascular) cavity that may be branched or divided by septa. No anus.

5. No circulatory, respiratory, or excretory organs.

6. A diffuse network of unpolarized nerve cells in body wall, but no central nervous system. Some with eyespots or statocysts.

7. Reproduction commonly with a sexual budding in the attached (polyp) stage and with sexual reproduction by gametes in the medusa stage. Monoecious or dioecious. Some with simple gonads, but no sex ducts. Cleavage holoblastic. A ciliated planula larva.Mouth forms from blastopore.

8. Cnidocyte cells containing nematocysts (stinging and paralizing organelles) present in all and employed extensively for food capture and defense.

9. All aquatic, chiefly marine.

10,000 species


The phylum is divided into four classes. listed below.

HYDROZOA

Hydrozoa are the least familiar of the four classes because most species are small. In their complex life cycle they alternate between two types of body plan. In the asexual fase it consists of a tubular body that is connected to the substratum at the lower end and bears a ring of tentacles around the mouth at the opposite end. These types of individuals are called polyps. They asexually bud off the second stage wich resemble a miniature jellyfish and is called a medusa. They are umbrella-shaped with a pulsing bell that propels them forward, on through the water,and a central trailing malibrum that carries the mouth below the bell. Medusa are the sexual fase in the life cycle of the hydrozoa. They produce sperm and ovaries (eggs) that fuse to create planktonic larvae. Ultimately these setle and give rise to polyps. A few species have solitary polyps, but in most cases the polyps devide repeatedly to form whole colonies often featherlike in appearance. An extreme example of the division of labour in a colony occurs in the bluebottle: One individual forms the float, others serve as defensive, stinging tentacles, others are for digestion and some solely engage in reproduction. Characteristics: Hydroids and some medusae. No stomodeum. Gastrovascular cavity lacks partitions and nematocysts. Mesoglea noncellular. Medusa usually small and with velum (craspedote). Chiefly in shallow salt waters. Colonial or solitary. 3700 species.

HYDROIDA

Sea firs or hydroids

Hydroids form colonies consisting of numerous individuals (polyps) and are often tree or feather-like. Hydroida Polyp

Body Of Hydra (longitudinal section)

The polyps are crowned with a ring of stinging tentacles around the mouth (used for defence and to capture prey-microscopic animals). In some species the colony has an external skeletal sheath (perisarc) and each polyp is protected by a cup-like housing(hydrotheca). The amount of teeth on the margin and the shapes of the hydrotheca identify species. Some polyps are modified into single stinging tentacles and are housed in tubular nematothecae. Sac-like reproductive structures on the colony (gonothecae) form medusa (a miniature jellyfish-like animal) which reproduce sexually, bringing forth larvae that heralds the next generation of polyps.

Hydroid life cycle, showing the structure of a hydroid colony.

Characteristics: Polyp generation well developed. Solitary or colonial animals. Usually budding of small , free medusae that bear ocelli and ectodermal statocysts.

MILLEPORINA

Allpora

Characteristics: Polyps minute and dimorphic (short, plump gastrzooids-tubular individuals with a mouth- and slender dactylozooids-a single massive tentacle for defence and prey capture).

SIPHONOPHORA

Bluebottles

Definitely the most unusual order of the class Hydrozoa are the floating forms that consist of colonies of highly specialised individuals. The structure of a siphonophore colony. One individual is modified to form a float. It has a gas gland that inflates it with a mix of carbon monoxide. Gastrozooids take in and digest food and from here it is distributed through the whole colony. Dactilozooids cosists of a single massive tentacle for defence and capture of prey. Gonozooids concern themselves only with reproduction.

SCYPHOZOA

Bell-shaped jellyfish

Scyphozoa has developed the medusa stage of the life cycle to the expense of the polyp stage wich exists only as a short-lived larva stage. The class consists of jellyfish, some of which attain remarkable sizes exceeding a meter in diameter. They are bell-shaped gelatinous creatures with a simple body structure (described as a medusa).

Side view of schyphozoan medusa with section.

Scyphozoan medusa in oral view.

A highly diagramatic section through the bell of a scyphozoan, showing the primitive arrangement of the organs.

Aurelia, a scyphozoan medusa. Viewed from above they are round and their organs radiate outward from a single stomach, so that they have a radial symmetry. Appart from their brief larval stage, jellyfish are planktonic, swimming by pulsing their bodies and jetting water from beneath their bells. Most jelyfish are carnivorous, stunning prey with stinging cells (nematocysts) on the tentacles that fringe the bell, and passing it to the frilly mouth (manubrium) hich hangs down from the centre of the bell. Jellyfishes are an important source of food for some turtle species.

CUBOZOA

Box-shaped jellyfish

Cubozoa has developed the medusa stage of the life cycle to the expense of the polyp stage wich exists only as a short-lived larva stage. The class consists of jelly fish, some of which attain remarkable sizes exceeding a meter in diameter. The jellyfish in this class are box-shaped gelatinous creatures with a simple body structure (described as a medusa). Viewed from above they are round and have only four very long tentacles and their organs radiate outward from a single stomach, so that they have a radial symmetry. Appart from their brief larval stage, jellyfish are planktonic, swimming by pulsing their bodies and jetting water from beneath their bells. Most jelyfish are carnivorous, stunning prey with stinging cells (nematocysts) on the tentacles that fringe the bell, and passing it to the frilly mouth (manubrium) hich hangs down from the centre of the bell. Jellyfishes are an important source of food for some turtle species. This class includes one species wich probably has the the most venomous toxin of all animals - the sea wasp.

ANTHOZOA

Anemones are a very famaliar example of the Anthozoa. Their life cycle consists only of a polyp stage with a much larger and complex polyp than the Hyrdozoa. Their gastric cavaties are devided verically by sheets that extend inwards from the body wall and increases the total surface area for digestion. While anemones are solitary many anthozoans also form colonies of polyps as are found in the sea fans. Corals are also colonial anthozoans. They secrete a hard calcium carbonate skeleton that supports the relatively thin skin of living tissue.

OCTOCORALLIA

The structure of a polyp of an octorillian coral.

ALCYONACEA

Soft corals

Soft corals lack internal skeletons, but form colonies of polyps, each carrying eight distinctive feathery tentacles(pinnate). They are micro carnivores, using their tentacles with nematocysts (stining cells) to capture planktonic animals. Tendroniphyta : A soft coral.

GORGONACEA

Sea fans

Sea fans form tree like colonies, often arranged like a fan. Consisting of a stiff central rod of gorgonin (a horn-like protein), covered with small polyps, each carrying eight feathery tetacles.

PENNATULACEA

Sea pens

Sea Pens have a fleshy body, covered with polyps and a soft, unbranced peduncle that anchors the colony in mud or sand. Ptilosarcus gurneyi : Orange sea pen.

ZOANTHARIA

ACTINIARIA

Sea anemones

Simplistic solitary animals, lacking a hard skeleton but supported be internal hydrostatic pressure.

Longitudinal section of a sea anemone showing its structure.

Cross section of the sea anemone at the level of the pharynx.

Body is hollow and cylindrical, attached at the base by a flat adhesive disc. The mouth is encircled by tentacles, armed with stinging cells (nematocysts) that are harmless to humans. Prey captured with the tentacles is stuffed through the mouth into the digestive cavity. Reproductionn can be sexual or by simple division of the body.

CORALLIOMORFA

Corynactis

ZOANTHIDEA

Zoanthids

Anemone-like creatures with their polyps forming upright, hollow collumns crowned by tentacles around the mouth. Unlike Anemones, they are colonial(their polyps are joined at their bases by a sheet-like coenchyme) forming carpets on tropical and subtropical shores. They capture tiny prey, but like corals, most depend on symbiotic microscopic alge(zooxanthellae) in their tissues for much of their nutrition.

SELERACTINIA

Corals

Corals consist of anemone-like individuals (polyps) which produce a limestone skeleton. A longitudinal section of a coral showing the structure of a scleractinian polyp within its skeletal cup. Some are solitary, but most form colonies with massive skeletons (accumulating to form reefs of over 1000m thick). The surface view of a living coral colony. The surface of the skeleton forms small craters or projections (corallites), each housing one polyp. Usually the upper surface of each corallite is divided by vertical radiating plates (septa), which may be joined by small bridges (synapticula). Colonial corals house symbiotic single-celled algae (zooxanthellae) in their tissues. The algae gain fertilising nitrogen, in return the provide fod and help build the skeleton. This association limits the reef corals to sunlit warm waters.