Phylum Cnidaria: Characteristics and Structure

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Questions and Answers

What is the characteristic of Cnidaria cells?

Organized into tissues with a spiral plan

Not organized into tissues at all

Properly organized into tissues with a radial plan (correct)

Properly organized into tissues with a bilateral plan

What is the life cycle of Cnidaria characterized by?

No alternation of generation

Random alternation of generation

Alternation of generation with different kinds (correct)

Successive generation with similar kinds

What is the characteristic of Tabulates corals?

They are solitary and have well-developed septae

They are colonial and have well-developed tabulae

They are colonial and have poorly developed septae (correct)

They are solitary and have poorly developed tabulae

What is the characteristic of Rugose corals?

They can be either colonial or solitary and have well-developed septae in sets of four Signup and view all the answers

What is the characteristic of Anthozoa class?

They have a polyp stage only, with no medusoid stage in the life cycle Signup and view all the answers

How many classes are there in Phylum Cnidaria?

3 Signup and view all the answers

What is the shape of solitary Rugosans?

Cup or cone shape Signup and view all the answers

When did Tabulates corals become extinct?

At the end of the Palaeozoic Signup and view all the answers

What type of skeleton do hard corals secrete?

Aragonitic skeleton Signup and view all the answers

What is the term for the tube-shaped skeleton of hard corals?

Corallite Signup and view all the answers

What is the characteristic of solitary hard corals?

They are not attached to any other corallite Signup and view all the answers

How many classes are Anthozoans grouped into?

3 Signup and view all the answers

What is the shape of the corallum in solitary Zoantharia?

Short and horn-shaped Signup and view all the answers

What is the characteristic of the septa in Rugosa corals?

They are inserted in four loci Signup and view all the answers

What is the common name for solitary Rugosa corals?

Horn-corals Signup and view all the answers

What is the basis of classification for the Rugosa?

Skeletal elements Signup and view all the answers

What was the guiding principle controlling the evolution of the rugosa?

Towards the development of a strong and firm skeleton Signup and view all the answers

When did the oldest known tetracorals appear?

In rocks of lower Ordovician age Signup and view all the answers

During which period did the rugosa reach its maximum abundance?

In Mid Palaeozoic Signup and view all the answers

What is the characteristic feature of the tabulate corals?

They are always colonial Signup and view all the answers

What is the characteristic feature of the cerioid forms of tabulate corals?

Polygonal corallites all in contact Signup and view all the answers

What is the characteristic feature of the coenenchymal tabulates?

They have a common mass of complex tissues Signup and view all the answers

What type of rocks are fossil rugosa most abundant in?

Calcareous rocks Signup and view all the answers

When did the rugosa become extinct?

At the end of the Permian Signup and view all the answers

Which of the following septa is not among the primary six prosepta?

Lateral Signup and view all the answers

What is the pattern of the septa in the corals?

Biradial Signup and view all the answers

What happens to the polyp during periods of famine?

It resorbs its own tissue Signup and view all the answers

What is the purpose of the metasepta?

To support the coral's structure Signup and view all the answers

What happens to the corallite after the prosepta are developed?

Metasepta and minor septa are added Signup and view all the answers

What is the result of the polyp's growth during periods of increased food supply?

The corallite expands again Signup and view all the answers

Where do the metasepta appear in the cardinal quadrant?

On each side of the cardinal septum Signup and view all the answers

What is the significance of the constricted bands in the coral's structure?

They represent periods of famine Signup and view all the answers

When did the Tabulata first appear?

Chazyan epoch Signup and view all the answers

Which group became dominant in the Silurian and Devonian?

Favositina Signup and view all the answers

What type of environment did larger Tabulata typically inhabit?

Coral-stromatoporoid reefs Signup and view all the answers

What is characteristic of Ordovician and Lower Silurian tabulates of smaller sizes?

They formed association with early solitary Rugosa Signup and view all the answers

What happened to the Favositina in the Carboniferous?

They lost their dominance and became subordinate to the Syringoporina Signup and view all the answers

What is true about Tabulata in general?

They are not normally found in very high energy environments Signup and view all the answers

Study Notes

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.

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Description

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