The Coelenterates are aquatic invertebrates having well developed body tissues. The majority are marine except very few forms adapted for existence in fresh water. They grow together as colonies, but solitary individuals also occur. Many are attached throughout life whereas others are free swimming.

The word Coelenterate (Coel, hollow, entron, gut) was originally applied to sponges and Coelenterates (hydrozoa and anthozoa). Because Coelenterates are characterized by the possession of stinging cells, the name Cnidaria (knide, nettle) is employed for them by some authors.

The individual coral animal or polyp is a simple jelly-like organism consisting of a bag-like body with a mouth at the top, surrounded by tentacles. The body wall consists of three layers; an inner, digestive endoderm and an outer ectoderm, which is sensory and bears stinging cells for protection and capture of prey. Between the ectoderm and endoderm lies a gelatinous layer; non-cellular in composition, called mesogloea. The body cavity of coelenterates is divided by radially-emplaced curtains of tissue called mesenteries. These serve to increase digestive and absorptive area.

Reproduction is either asexual (budding) or sexual (release of male and female gametes into the water). The budding process turns a solitary polyp into a compound individual; the continuation of the process results in a mass of polyps called a colony. Some corals reproduce by alternation of generations.

In coelenterates; two types of individuals exist, a fixed polyp and a free medusa.

These two forms (polyp and medusa) alternate successively where the polyp reproduce asexually to form a large number of medusa, each medusa reproduce sexually by the union of eggs and sperms to form zygote. The zygote grows into larva, which fix itself to a substrate and finally form a new polyp.

This alternation of generations is characteristic for class Hydrozoa. In class Scyphozoa the medusoid stage is dominant and the polypoid stage is very reduced. In class Anthozoa the medusoid stage is absent and the polypoid stage has become the sexual generation.

The coral polyp secretes a calcareous skeleton called corallite. This takes the form of a cup (calice) upon which the animal sits. There is a basal plate and radial walls (septa) extend from the periphery towards the center, these septa lie in-between the mesenteries and so reflect their number. Since the different coral orders have distinctive numbers of such mesenteries, the septa are of great taxonomic importance to the paleontologist.

As the polyp grows in size and the cup enlarges, more mesenteries are created and so more septa need to be inserted. The first formed septa (prosepta) are usually recognizable, being thicker and larger than the secondary (metasepta). All septa join the peripheral wall, which is called epitheca. As the polyp grows also, the coral secretes a horizontal floor called tabulae, this is secreted periodically as the polyp moves upward.

Hydrozoa are solitary (e.g. Hydra) or colonial (e.g. Obelia) organisms. Although most of them are of little importance in paleontology due to the absence of a hard skeleton, few kinds are rock builders having a calcareous hard part, which could be preserved as fossils. The skeletal structure of hydrozoa is not enough varied and distinctive to permit differentiation of species, which are useful in stratigraphic correlation or age determination. All recorded fossil hydrozoans are marine.

This group includes the fresh water hydras and the marine water Obelia.

Some hydroids secrete calcium carbonate in the form of a thin incrusting layer on a shell or other foreign surface to which the colony is attached, or it may build moderately thick laminated masses of irregular form. A thin layer of fleshy tissue covers the surface of the calcareous deposits and from this common colonial tissue the individual polyps rise.

Age: cretaceous.

These are colonial Hydrozoa which secrete massive calcareous structures of varied form. They commonly grow in association with other corals and together with calcareous algae, contribute significantly to the building of coral reefs in warm shallow seas in many places.

The skeletal structure consists of finely porous deposits with innumerable interconnecting minute passageways, penetrated by relatively straight-sided tubes of larger size with axes normal to the surface of the colony. The tubes are circular or stellate in cross section and are intersected by transverse partitions called tabulae.

Two distinct sizes of tubes are observed, the larger are called dactylopores and the smaller are called gastropores. The individual polyps are joined laterally by a thin layer of colonial fleshy tissue, which covers the skeletal deposits between the pores.

Age: Cretaceous – Tertiary – Recent.

Scyphozoans include jelly- fishes which lack hard parts and consequently their preservation as fossils is uncommon except as impressions in the pre-Cambrian and Cambrian rocks, (Idiacara fauna in Australia).

The Anthozoans are the most important coelenterates from the paleontologic viewpoint. They build a calcareous skeleton, which is significant in many rock formations.

The Anthozoa are distinguished by a radial compartmentation of the digestive cavity and the entire absence of a medusoid stage. They include solitary individuals but are mostly colonial and are exclusively marine. Although a majority secretes hard parts, several divisions are wholly soft-bodied and thus not suited for fossilization.

Alcyonarians are anthozoans having eight branched tentacles and eight mesenteries. They are exclusively colonial but the polyps are not directly joined with one another, as in many colonial corals, for they interconnect only by tubular passageways (stolons).

Newly formed polyps arise at various places along these stolons. The colonies are supported by a horny or calcareous skeleton. They include sea pens and sea fans.

Genus Heliopora is a representative of Alcyonarians. It contains wide tubular openings in which the polyps are lodged, these are separated by interspaces which consist of small tubules. Both sets of openings are intersected by transverse tabulae. The walls of the larger tubes are indented by 12-25 short inward projections, which are termed pseudosepta because their number and arrangement doesn’t conform to the eight mesenteries.

The red organ-pipe coral Tubipora also belongs to Alcyonaria. In this genus, the polyps arise singly from a basal fleshy mat containing stolons.

The tabulate corals are entirely Paleozoic, colonial (never solitary) organisms. Their corallites are small with no or very short septa. They have numerous horizontal elements called tabula.

- Abundant tabulae.

- Very short septa (spines).

- Mural pores connecting the corallites.

- They are elongated.

• In solitary species, the corallum is horn-shaped covered by thin calcareous skin called epitheca. The upper surface of the corallum where the skeletal elements filling the inside of the corallum are exposed is called calice. The skeletal elements inside the corallum are either vertical (septa and columella) or horizontal (tabulae and dissepiments).
• The corallite of rugose corals is wrinkled and hence the name rugosa and is filled by vertical plates called septa. In certain cases; some septa fail to develop leaving a space called fossula. There is no fossula in scleractinian corals due to different manner of septal arrangement as will be seen later.

Septa are arranged in a bilateral symmetry as follow:

cardinal side of alar and counter lateral septa.

the organism.

The columella has different types: -

• Solid: a central rod.

• Spongy: formed by the inner ends of septa.
• Papillose: may small rods.
• Lamellar: plate-like.

In Rugose corals, the individual secrets a calcareous skeleton called corallite. In colonial forms, the corallites together form a single skeleton (colony) called corallum; this may have the following forms:

are united by a zone of dissepiments.

Rugose corals are totally Paleozoic corals, which appeared in Middle Ordovician where solitary forms were dominant.

In Middle Devonian colonial forms appeared, they formed reefs together with tabulates and stromatoporoids.

Generally, Rugose corals disappeared at the end of Paleozoic where they were replaced by Scleractinian corals.

followed by metasepta added in cycles of 6, 12, 24, 48 …etc.

• The colonial forms are similar to those of rugose corals in addition to new forms termed

meandroid and hydnophoroid.