This phenomenon is called rphism is very common in coelenterates and the class Hydrozoa provides the best example of polymorp. 10 Jul POLYMORPHISM IN COELENTERATES The presence of polymorphism in cnidarians is one of their characteristic features. It is defined as. In Hydrozoan coelenterates polymorphic tendency is well developed. The order Siphonophora organisms are exhibiting this tendency to a maximum extent.

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These kinds of nematocysts are small and pear-shaped. Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top. Class Anthozoa – inc. The axial skeleton composed of calcareous spicules or horn-like material, gorgonin is present. They are studded with nematocysts and help in protection. Write about the distribution of nematocysts.

In Velella and Porpitathere is a single large central gastrozooid with a mouth, around which are arranged concentric rows of gonozooids and dactylozooids.

It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very complex. Holomycota Cristidiscoidea Zoosporia Opisthosporidia True fungi. In order Siphonophorasuch as Diphyes, Halistemmia, Stephalia and Physalia, zooids are so much modified that they appear like organs of a single body i than individuals of a colony.


Some, however, omit either the polyp or the medusa stage. Development Genes and Evolution.

The two types of zooids are, Gastrozooids are concerned with feeding. Mechanism of Defense The discharge or explosion of nematocysts takes place when cnidocil is stimulated by food, prey or enemy. The outer end of the cnidoblast projects freely beyond the epidermal surface as a tiny, pointed hair-like process called cnidocil or trigger. Spiral dactylozooids for protection 3. Recently, Moser has revived the poly-organ theory.

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They may be also called as feelers or palpons. These kinds of nematocysts are oval or elongated in shape. Further subsidence of the land till it sinks completely out of site results in the formation of an atoll Fig.

Coelenterata: General Characters, Tissues and Corals | Metazoa

Medical Journal of Australia. The tentacle bears nematocysts. Vogt and Gegenbaur proposed this theory. These are solitary or colonial polypoid forms and live in a skeleton of calcium carbonate secreted by themselves. In Diphyes colonies are linear with one or more nectophores located at the apical end. Medusae often trap prey or suspended food particles by swimming upwards, spreading their tentacles and oral arms and then sinking.

Corals are animals belonging to the phylum Coelenterata. Coral reefs form some of the world’s most productive ecosystems. On this tentacle many nematocysts are present.

Absorption may take a few hours, and digestion within the cells may take a few days. Chaetognatha arrow worms Gnathostomulida jaw worms. Gastrozooids for feeding 2. Anthozoan larvae either have large yolks or are capable of feeding on planktonand some already have kn algae that help to feed on.

The life history exhibits the phenomena of alternation of generations or metagenesis in which the asexual polypoid, sessile generation alternates with sexual medusoid, free-swimming generations. This is proposed by ‘Huxley’ and others. The dactylozooids are also called palpons or feelers. The lower end of the colony usually consists of a basal plate for the attachment with some objects.

Common coral reef cnidarians include both Anthozoans hard corals, octocorals, anemones and Hydrozoans fire corals, lace corals. From its base along tentacle will arise.

Coral reefs are very hard structures and are an important source of mortar, cement, lime etc as they contain enough amount of CaCO3, therefore their rocks can be used for making roads and houses etc. Cristidiscoidea Zoosporia Opisthosporidia True fungi. Cnidarians form an animal phylum that are more complex than spongesabout as complex as ctenophores comb jelliesand less complex than bilaterianswhich include almost all other animals.


Following it, radial folds develop which secrete sclerosepta. Though the exact mechanism of discharge and enzymes involved are not known, but is very much evident that the response is wholly local without the involvement of nervous system.

Coelenterata or Cnidaria are radially symmetrical diploblastic animals with only epidermis and gastrodermis, between these two layers is a jelly-like mesogloea which is originally structure less but in higher forms, it becomes fibrous and has wandering amoebocytes. The Alcyonacea includes soft corals like Alcyonium.

Gonophore bearing gonads, which may be either male, producing sperms, or female producing ova. Siphonophora show the highest degree of polymorphism which is not found anywhere else in the animal kingdom. Gastrozooids catch the prey and poolymorphism it. It looks-like a single individual. Two [9] or Three [11] [12].

Pneumatophore or float is a bladder-like modified medusa filled with mixture of gases and helping the colony to float on the surface. The Great Barrier Reef is the contribution of all different kinds of coral growth over the years. Discuss the mechanism of defense in Cnidaria. But a homology exists between the two in their basic features. Not all cnidarians reproduce sexuallywith many species having complex life cycles of asexual polyp stages and sexual medusae.