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Corals are characterized by their polypoid shape, sessility and slow growth and secretion of a solid skeleton made of calcium carbonate. Numerous coral individuals associate in intraspecific harmonic ecological interaction forming colonies with hundreds, thousands and even millions of beings. Water-permeated solid structures of these colonies, known as reefs, work as true ecosystems for other living beings. The biggest known coral colony is the Great Barrier Reef in the northeast coast of Australia. There are however many coral species whose individuals live alone and do not form colonies.

Jellyfish, hydra, corals and sea anemones are good examples. All of them are aquatic and most live in the sea.

Incomplete digestive system is that in which the digestive cavity has only one opening.

Cnidocytes are specialized cells present in coelenterates. They are found in the epidermis and contain toxic substances that can hurt, paralyze or even kill other animals.

Each cnidocyte has an internal capsule know as nematocyst where the actual urticating substance is stored. When a cnidocyte is excited it causes the nematocyst to expose a filament containing the toxic chemical. 

Cnidocytes and their nematocysts have the biological functions of defending the individual from external aggression and of helping to capture their prey.

Their nervous system is diffuse, there are no brain or ganglia.

These beings present ectoderm and endoderm, two germ layers. Animals with only two germ layers are called diploblastic animals.

The ectoderm gives birth to the epidermis and the endoderm originates the covering of the digestive cavity.

Morphologically, cnidarians classify as polyps or medusae. Polyps are cylindrical and medusae are circular convex, like an umbrella. Both shapes have tentacles.

In general polyps are sessile but some species, like hydra, can move by alternating contact points on the substrate and performing somersaults. Medusae can move expelling water jets by contraction of the body.

Some cnidarians alternate polypoid and medusoid forms in their life cycle.

Cnidarians have true tissue differentiation, they present distinct organized tissues in the body. Poriferans present only some dispersed specialized cells with no tissue differentiation. 

In the metagenesis of some coelenterates, like Aurelia and Obelia, there is alternation between polypoid and medusoid forms. The polyps reproduce asexually originating medusae that then liberate gametes. The male and female gametes unite under water to form the zygote that then develops into larva and later originates another polyp.

The form that reproduces asexually is the polyp. Polyps give birth to medusae by budding in Obelia and by strobilization in Aurelia.

Coelenterates are divided into three main classes: hydrozoans, scyphozoans and anthozoans. In hydrozoans the polypoid form predominates and examples are hydras, by-the-windsailors and Obelia. In scyphozoans the main phase is the medusoid and the best known example is the common jellyfish (Aurelia). In anthozoans there is only the polypoid form and corals and sea anemones are notable in this group.

Metagenesis is the type of life cycle in which there are two different forms of individuals of the same species, one haploid and the other diploid. In one of these stages gametogenesis occurs and fecundated gametes give birth to the zygote that then develops into the other form. Metagenesis is also known as alternation of generations or as diplobiont life cycle. (All plants, for example, present metagenesis.)

Sexual reproduction in corals and sea anemones have a larval stage and the larva is called planula.

Many marine animals are sessile or practically sessile, like sponges, corals and sea anemones. The mobile larval stage of their life cycle provides better spatial distribution of these species.

Hydras reproduce asexually by budding.

In echinoderms embryonic development is indirect, with ciliated larvae.

They present asexual and sexual reproduction.

These animals have a digestive cavity and they make extracellular and intracellular digestion. The extracellular digestion takes place within the digestive cavity.

Examples of representing beings: jellyfish, corals, sea anemones, hydra. Basic morphology: polyp or medusa. Type of symmetry: radial. Germ layers and coelom: diploblastics, acoelomate. Digestive system: incomplete. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: diffuse. Types of reproduction: asexual and sexual with larval stage and metagenesis.

Examples of representing beings: ascaris, hookworms, filaria, pinworms. Basic morphology: cylindric (round) body, not segmented. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, pseudocoelomates. Digestive system: complete. Respiratory system: respiration by diffusion. Circulatory system: circulating fluid within the pseudocoelom. Excretory system: excretory channels and excretory pore. Nervous system: ventral and dorsal ganglial chord, primitive cephalization. Types of reproduction: sexual. 

Nematodes reproduce sexually. The nematode sperm cell does not have cilia nor flagella and they move by amoeboid movement forming pseudopods.