Phylum Coelenterata is a group of animals belong to kingdom Animalia and belongs to sub-kingdom Eumetazoa. The animals of this phylum have attained tissue grade level of organization and show some very distinct characteristic features from animals of other groups. This article explains the various characteristic features exhibited by the animals of this phylum.
Coentrates or Cnidarians are exclusively aquatic animals that live mostly in freshwater or marine water. This phylum includes 11,000 living species of animals. Some of the fossil forms of this phylum are known to have existed even 791 million years back. The word Coelenterata is derived from two Greek words- Koilos = hollow, enteron = intestine. The body wall of these animals include certain protective cells called cnidoblast cells and hence it has got its other name 'Phylum Cnidaria'. Coelenterates are multicellular, eukaryotic and heterotrophic organisms with the tissue grade level of organization. The phylum name Coelenterata is coined by Leuckart and the term Cnidaria is coined by Barnes.
Animals of Phylum Coelenterata (Courtesy:-wikipedia.org)
Characteristic features of Phylum Coelenterata or Cnidaria
Coelenterate animals are exclusively aquatic and most of the animals are marine. These animals are accustomed to the free-living or sedentary mode of life. Some of these animals live in a colonial form or in a solitary way. The size of these animals ranges from 5-10 mm. long as in Hydra to Lion's mane jellyfish measuring 75 m. long. Examples of this Phylum:-corals, hydras, jellyfish, sea pens, sea whips, Portuguese men-of-war, sea anemones, and sea fans, etc.
The chief characteristic feature of these animals is they possess a sac-like body with a cavity enclosed inside it. This cavity present inside the body is known as coelenteron or gastrovascular cavity.
These animals show radial symmetry as they are cylindrical in shape. In radial symmetry, the body can be cut into two equal halves in any plane or angle.
These are multi-cellular organisms which have attained tissue grade level of organization and they exhibit distinct division of labor.
The body wall of these animals is said to be diploblastic as it includes two germ layers-outer ectoderm layer and inner endoderm layer. In between these two layers, a non-cellular layer filled with a slimy material called mesoglea is present. This substance is secreted by both ectoderm and endoderm. This layer binds both the layers of the body wall and acts as endoskeleton.
These animals possess a number of finger-like projections around their mouth called tentacles. These tentacles are useful for capturing food, locomotion and also for of-fence and de-fence.
The body wall of these animals possess epithelial cells, muscle cells, glandular cells, nerve cells and cnidoblast cells or nematocyst cells or stinging cells. These stinging cells present in the body wall will help in paralyzing the prey while collecting food.
These animals possess only one opening at the anterior end called mouth and it lacks anus. The coelenteron or gastrovascular cavity acts as a stomach for digesting and absorbing digested food.
Digestion of food occur inside the gastrovascular cavity. The digestion is both extracellular and intracellular. One part of the digestion occurs in the gastrovascular cavity and another part of digestion occurs inside the cells with the help of enzymes. Coelenteron serves the double purpose of digestion and circulation. It is lined with endodermal cells.
No respiratory organs are present in these animals. They respire through general body surface by diffusion method.
Nervous system is absent in these animals but its body wall contains nerve cells loosely arranged in the form of diffused network.
Excretory organs are absent in these animals and they excrete the metabolic wastes from the body cells or tissue by diffusion.
Some of the animals of this phylum endoskeleton are found in the form of hard calcareous corals.
Animals of this phylum exist in two forms-polyps representing asexual phase and medusa representing sexual phase. Polyps lead a solitary or colonial life but are always fixed. Medusa, on the other hand, is free living. Polyp forms adopt a budding method in Asexual reproduction. Medusae reproduce sexually by sperms and ova.
Reproduction in these animals occur through asexual and sexual methods. The asexual method is by budding or fission and sexual reproduction by gonads.
Fertilization in these animals occur either externally or internally.
Alternation of generations can be seen in these animals where asexually reproducing diploid polyp stage alters with haploid sexually reproducing medusoid stage. Planula larval stage can be seen in the life cycle of these animals.
As both male and female sex organs are present in the same body of these organisms are described to be hermaphrodite.
Important points to remember about Phylum Coelenterata or Cnidaria
Coelenterates possess a sac-like body with a gastrovascular cavity inside their body.
Coelenterates possess cylindrical body with radial symmetry.
Coelenterates show tissue grade level of organization and division of labour.
Nematocyst cells or stinging cells are present in the mouth and are meant for offence and defence.
Digestion in these animals is both extracellular and intracellular.
Coelenterates reproduce both asexually and sexually.
Alternation of generations can be seen in the life cycle of these organisms.
Interesting article. The marine life forms under Phylum Coelenterata were evolved in precambrian ages itself and that is a fascinating area of the research and study by the scientists to know the patterns of the early life forms on the Earth. The precambrian period is estimated from the birth of Earth (approximately 4500 million years back) to 542 million years ago. These marine animals were already evolved then in some form and now they must have gone many transformations with time. The scientists compare their fossils belonging to that time and their present structure to visualise the changes happened in that long geologic duration to understand the evolutionary mechanism more clearly. From that angle these life forms are very important to the researchers who are working in the area of finding origin of life and evolutionary mechanisms on Earth.
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