Phylum Cnidaria: Characteristics and Structure

Phylum Cnidaria

• Members of this phylum include sea anemones, jellyfish, and corals, which exist as polyps, medusa, or alternate stages.
• Polyps have a base with a mouth and tentacles extended upwards, while medusas float with a mouth and tentacles extending downwards.
• Cnidaria are the simplest of the true metazoans, with cells organized into tissues on a radial plan.

Characteristics of Cnidaria

• Cnidaria have a life cycle with alternating generations of different kinds, with polyps giving rise to medusas, which reproduce sexually to produce polyps.
• Individuals can be solitary or colonial, and are of distinctly different but essentially homologous types, including polyp and medusa.

Classification of Cnidaria

• There are three classes of Cnidaria: Hydrozoa, Scyphozoa, and Anthozoa.
• Class Anthozoa includes corals, which have a polyp stage only, with no medusoid stage in the life cycle.

Class Anthozoa

• Corals in this class have a polyp stage only, with no medusoid stage in the life cycle.
• Hard corals secrete aragonitic skeletons, which are tube-shaped with walls that extend upwards as the polyp grows.
• The skeleton is called a corallite, and can be either solitary or colonial.
• Anthozoans are grouped into three subclasses: Ceriantipatharia, Octocorallia, and Zoantharia.

Subclass Zoantharia

• This subclass includes solitary corals living on various substrates and at depths of 0-2750m.
• They have a soft basal tissue that secretes an aragonitic cup or corallum, which is short and horn-shaped.
• The corallum has an outer wall (epitheca) with numerous radially arranged septa between which lies the mesentaries.

Order Rugosa

• This order includes solitary and colonial corals that show bilateral symmetry.
• They are an almost exclusively Palaeozoic group, with solitary forms building straight, curved, conical, or coni-cylindrical coralla.
• Colonial species built dendritic or massive compound coralla of many closely packed septate corallites.

Septal Growth

• The septa are thin vertical plates arranged in a characteristic biradial pattern, which develops through the ontogeny of the corals.
• Septal growth begins with a single proseptum, which soon becomes separated into cardinal and counter prosepta.
• Two other pairs of prosepta follow, resulting in six primary septa.

Rejuvenescence

• In solitary cylindrical forms, the corallites are often constricted at regular intervals, leaving a broad or narrow shelf where the septate older calice is exposed.
• This constriction is thought to represent periods of famine during which the polyp resorbed its own tissue to stay alive and shrank away from the edges.
• When food supply increases, growth begins again, resulting in rejuvenescence.

Evolution and Geologic History of the Rugosa

• The guiding principle controlling the evolution of the rugosa was the development of a strong and firm skeleton.
• The oldest known tetracorals are found in rocks of lower Ordovician age, and the rugosa attained its maximum abundance and number of species in Mid Palaeozoic seas.
• They declined to extinction at the end of the Permian.

Order Tabulata

• The tabulate corals are an extinct, exclusively Palaeozoic group of compound corals.
• They are always colonial, never solitary, and usually have small corallites with prominent tabulae, but reduced or absent septa.

Form of Corallum

• The corallum is built up by individual polyps, which may or may not be directly connected to each other.
• Cerioid forms have polygonal corallites all in contact, while cateniform colonies have elongated corallites joined end to end in a wandering palisade.
• Fasciculate tabulates have cylindrical corallites which may be dendroid or phaceloid and may be provided with connecting tubules.

Evolution and Ecology of the Tabulata

• The earliest tabulata are known in the Chazyan (oldest of three epochs of the Ordovician) of North America.
• By Trenton (Mid Ordovician) times, they had spread to many parts of the world and the first Heliolithina and Halysitina had appeared.
• The Favositina became dominant from the Middle Devonian to the end of the Permian.

Cnidaria as Reef Builders

• Large tabulates are found in coral-stromatoporoid reefs and were relatively important, though they were not really frame builders since they had no proper means of attachment.
• Smaller tabulates tended to occur in deeper waters and fasciculate genera usually lived in quieter environments.
• Tabulata are not normally found in very high energy environments and sometimes occur in useful marker bands.

Learn about the members of Phylum Cnidaria, including corals, sea anemones, and jellyfish, and their distinct characteristics such as polyp and medusa stages.