PHYLUM
COELENTERATA
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Animals belonging to the phylum coelenterata are commonly called coelenterates or Cnidarians. They are more advanced than sponges in having true tissues.
It includes about 9000 species.
They are aquatic. Most of them are marine. Hydra is a fresh water form
Important characteristic features
They are sessile or free swimming.
They exhibit a tissue level of organisation.
Acoelomates
Radially symmetrical. But sea anemone is biradially symmetrical.
Body wall: Diploblastic (outer ectoderm, inner endoderm and middle mesoglea)
Body cavity: A central gastro vascular cavity or coelenteron which opens out through mouth or hypostome. Anus is absent. Body cavity does both digestion and circulation of food.
Coelenterates are holozoic and carnivorous in nutrition.
Digestion: it is intracellular and extracellular. Mouth is surrounded by many finger like structures called tentacles. It is used for food capture and defence.
Cnidoblasts: Certain ectodermal cells are modified into stinging cells or cnidodoblasts or cnidocytes, hence the name Cnidaria. Each cnidoblast possesses a stinging capsule called nematocyst containing poisonous fluid.
Cnidoblasts are defensive cells used to kill or paralyse other prey and enemies. They are mainly found on the tentacles.
The nematocyst poison of Physalia is a neurotoxin
Phylum includes two distinct types of individual.
Polyp: sedentary, with a tubular body and mouth surrounded by circlet of tentacles. It represents the asexual stage. Eg: Hydra and Adamsia.
Medusa: free floating with a soft, transparent bell like body with tentacles at the margin.
Umbrella like.
Mouth at the centre of concave surface which is directed downward.
It represents the sexual stage.
Reproduction: asexually and sexually.
Asexual reproduction: It takes place by budding in polyp and gives rise to medusa.
During sexual reproduction, medusa liberates gametes into water. After fertilisation; the zygote develops into a larva.
Larvae of cnidarians are ciliated and free swimming. They are called planula. The planula grows into sessile polyp.
Coelenterates exhibit alternation of generation in their life cycle. In coelenterates which exist in both forms (polyp and medusa), the asexual polyp generation alternates with sexual medusa generation. Eg: Obelia
Polymorphism: many colonial cnidarians exhibit this phenomenon where the individuals (zooids) of the colony show morphological and functional differentiation.
Eg: Halistemma, Physalia etc.
Examples of common coelenterates are
Hydra, Obelia (sea fur), Aurelia (jelly fish), Physalia (potuguese man of war), Adamsia (sea anemone), Vallela (sail by wind), Fungia (mushroom coral), Astraea (star coral), Pennatula (sea pen), Gorgonian (sea fan), Madrepora (stag horn coral), Meandrina (brain coral), Corallium (red coral), Porpita.
Classification of coelenterata
Class I: Hydrozoa (both polyp and medusa form occur. eg: Hydraobelia, Physalia, etc)
Class II: Scyphozoa (only medusa form occurs. eg: Aurelia)
Class III: Anthozoa (only polyp form occurs. eg: Adamsia, Fungia, Astraea, Pennatula, Gorgonia, Madrepora, Meandrina, Corallium,etc.
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