The hydrozoans contain sessile forms and swimming colonial forms like the Portuguese Man O’ War.The class Anthozoa includes all cnidarians that exhibit a polyp body plan only; in other words, there is no medusa stage within their life cycle. They have diverse colorful, radially symmetrical bodies which are known as the flower of the sea. The body wall consists of outer ectoderm and, inner endoderm or gastrodermis. The phylum Cnidaria is made up of four classes: Hydrozoa (hydrozoans); Scyphozoa (scyphozoans); Anthozoa (anthozoans); and Cubozoa (cubozoans).
They have calcareous skeletons which form the framework of the reefs and atolls in most tropical seas. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present. In this case, digestion is unique with extracellular digestion followed by intracellular digestion.They have a short and slender body with tentacles which encircle the mouth in one or two whorls. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. The body bears single or complicated sensory organs with eyespots called statocysts.They reproduce either asexual by external budding or sexual by the formation of ova and sperm.They show alternation of generation (metagenesis) in their life cycle, in which asexual colonial polypoid generation alternate with sexual free-swimming medusoid generation.Fertilization is internal or external and development is indirect with a larval stage. A prominent difference between the two classes is the arrangement of tentacles. is the phylum of animals that contains corals, jellyfish (sea jellies), sea anemones, sea pens, and hydrozoans. Therefore, Cnidaria is the most suitable name for his phylum. Cnidarians have separate sexes and have a lifecycle that involves morphologically distinct forms.
These animals also show two distinct morphological forms—medusoid and polypoid—at various stages in their lifecycle.Figure 1. In tropical seas, they form reefs by secreting calcium carbonate (CaCOThey are diploblastic animals with a radially symmetrical body.They are mostly marine organisms but some inhabit freshwater.Some species inhabit solitary and some form colony. In this case, the polyp stage is the dominant and medusa possesses true velum. Cnidocytes contain large organelles called (a) nematocysts that store a coiled thread and barb. Leuckart (1847) first coined the name Cnidaria. Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). Characteristics of Phylum Cnidaria. Copyright text 2020 by Biology EduCare. (credit “medusa”: modification of work by Francesco Crippa)This class includes jellies that have a box-shaped medusa, or a bell that is square in cross-section; hence, are colloquially known as “box jellyfish.” These species may achieve sizes of 15–25 cm.
Neurons may even be present in clusters called rhopalia. The larva settles on a suitable substratum and develops into a sessile polyp.Class Scyphozoa includes all the jellies and is exclusively a marine class of animals with about 200 known species.
In this case, tentacles function for food-engulfing, capturing the prey and defense.In nature, they have two distinct forms such as polyp and medusa (Polymorphism). Despite the simplicity of the nervous system, it coordinates the movement of tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.The phylum Cnidaria contains about 10,000 described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. Cnideraia are also known as Coelenterata. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma. The nerve cells show mixed characteristics of motor as well as sensory neurons. (credit a: modification of work by “Jimg944″/Flickr; credit b: modification of work by Mariana Ruiz Villareal)In the jellyfish, a mouth opening is present on the underside of the animal, surrounded by tentacles bearing nematocysts. They possess outer and inner tissue layers that sandwich a noncellular mesoglea. 1) Spongiaria 2) Cnidaria and 3)Ctenophora. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. In this case, mouth serves for ingestion and egestion.They are carnivorous and can digest all types of food. These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium. In between the two layers, a non-cellular mesoglea is present. Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers). Body Form: Body […] Cnidarians possess a well-formed digestive system and carry out extracellular digestion. A mouth opening, surrounded by tentacles, is present at the oral end of the animal. Cellular mesoglea region is present which consists of fibrous connective tissue and amoeboid cells.Gonads are originated from endoderm.
In some cases, the digestive system may extend into the pedalia.
Cubozoans exist in a polypoid form that develops from a planula larva.
These animals possess a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water.
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