Short Description of Phylum Coelenterata 4.36/5 (94)

By Pratiksha Pokhrel in Biology November 10, 2015

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Short Description of Phylum Coelenterata

Coelenterata/Cnidaria

(Gr. Kilos-cavity/hollow + enteron- intestine)
(Gr. Knide-Stinging cell or nettle)

The coelenterates are simplest metazoans and have tissue grade of body organization. All of them possess a single large internal cavity known as gastro-vascular cavity or coelenteron, which performs the function of intestine that is related with digestion and circulation of digestion.
They are commonly called stinging animals.
Study of coelenterates is called Cnidology.
Peyssones recognized coelenterata as animals.
Term Coelenterata and Cnidaria were given by Leuckkart and Hyman respectively.

General characteristics of Coelenterata

Aquatic, mostly marine and some are fresh water forms (e.g. Hydra).
May be solitary (e.g. Hydra) or colonial (Obelia).
Many are sedentary (Corals). A few are active swimming or free-floating (e.g. Aurelia).
Body is cylindrical, cup-shaped or flat.
Bilaterally symmetrical (Sea anemone) or radially symmetrical (mostly) with central gastro-vascular cavity.
Multicellular with tissue grade of body organization.
Body wall is diploblastic i.e. outer-ectoderm (epidermis) and inner layer-endoderm (gastrodermis).
Between them, there is a gelatinous non cellular layer called mesoglea is present.
They are acoelomate animals.
Body possess an internal hollow cavity, called coelenteron, which performs the function of intestine.
Blind sac body plan is present.
Mouth is encircled with tentacles, having nematocysts.
Mouth opens to gastrovascular cavity but anus is absent. Mouth helps in both intake of food and for elimination of indigestible matter.
Tentacles helps in food capturing, its ingestion and also for defense.
Tentacles bear nematocyst which are the organ of defense, adhesion and food capturing.
Nutrition is holozoic.
Digestion is intracellular or extracellular.
They do not possess skeletal structures but coral polyps secrete calcareous skeleton.
These skeletons are generally composed of minerals like calcium carbonate.
Locomotion takes place by smooth muscle fibers and tentacles. Some like coral and sea anemone remain fixed on substratum.
Respiratory, excretory and circulatory organs are absent.
Respiration and Excretion takes place through general body surface.
NOTE: They are ammonotelic since their excretory material is ammonia.
Nervous system is primitive consisting of a diffuse nerve cells.
NOTE: Nervous system is present but brain is absent.
Ocelli and Statocyst are the sensory organ.
Reproduction is both asexual and sexual.
NOTE: Asexual reproduction occurs by budding (leads to extensive colony formation) and fission. Sexual reproduction occurs by fusion of gametes.
Regeneration power is well developed. (This is due to interstitial cells).
Exhibit polymorphism i.e. two types of individual occur in their life cycle. They are- asexual form- polyp and sexual form- medusa.
Life cycle shows an alternation of generation or metagenesis. Asexual polypoid form alternates with the medusoid form.
Development is indirect having free-swimming ciliated larva.
Body consists of swelling knob like structure called cnidoblast cells which secrete hypnotoxin chemical that help in offence, defence and paralyzing the prey.
Examples: Hydra, Obelia, Aurelia, Metridium

Polymorphism

It is the process of possessing two or more than two morphologically different types of individuals in the colony or body of same species. The different individuals are called zooids. There are 3 kinds of zooids. They are: Polyp, Blastostyle and Medusa. Each kind of zooid is specialized to perform a specific function.
Physalia is an example of polymorphic colonies.

What is coral?

Deposition of calcium carbonate by coelenterates are called corals.
It is an exoskeleton formed by secretion of polyp of Hydrozoa and anthozoa. Most of the corals belong to class Anthozoa and few belong to class Hydrozoa.
Corals are marine, mostly colonial, branched, polypoid coelenterates looking like sea anemone and living in a secreted skeleton of their own.

Things to Remember

Nematocyst (present inside cnidoblast cells) are activated by touch with foreign body.
Sensory and nerve cells appeared first time in this phylum.
Tentacles are soft present around mouth. Tentacles may be solid or hollow.
Solid- Obelia
Hollow- Hydra
Mode of digestion is first extracellular and then intracellular. Pathway of intracellular digestion is first acidic and then alkaline.
The form may be monomorphic (polyp only, e.g. Hydra) or dimorphic (having polyp and medusa in larval form, e.g. Jelly fish) or polymorphic (having polyp, blastostyle and medusa).
The common larval forms in the case of indirect development are:
Planula- Obelia
Ephyra- Aurelia
Cnidoblasts are most abundant on tentacles and absent on pedal disc.
Epidermis contains 7 types of cells namely: Epitheliomuscle cells, Glandulo-muscle cells, interstitial cells, Cnidoblast or nematocysts, Germ cells, Sensory and Nerve cells.
Gastroderm contains 5 types of cells: Epithelio-muscle cells (or nutritive cells), Endothelio-gland cells, Interstitial cells, Sensory cell and nerve cell.
Hydra
The name Hydra was given by Linnaeus.
Most common species of Hydra is Hydra vulgaris.
Growth zone of Hydra is located just below the tentacles.
Pedal disc contains a glandular zone, which secretes substance helpful for attachment and gas bubblies for floating.
Hypostome is surrounded by 6-10 tentacles.
Types of locomotion: Gliding, floating, climbing, walking, and somersaulting. Walking method in Hydra is that of cuttle fish so also called cuttlefish movement.
Exclusively carnivorous and feeds on animal like water fleas.
Prefers cool or cold water, it disappears from surface at temperature more than 20-25°C.
Regeneration of tentacles in Hydra takes place by epimorphosis.
- If Hydra is broken into pieces then every fragment will grow into complete Hydra. Hydra regenerates by morphallaxis.