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Academic Script AIM- “To Study the Specimens of Hydra, Obelia, Aurelia, Seaanemone and Gorgonia” INTRODUCTION: All the above mentioned specimens follow under Phylum Coelenterata. The Coelenterates are colourful group of water dwelling, radially symmetrical and lower invertebrate animals including a large number of familiar and beautiful forms such as Hydra, Obelia, Aurelia, Sea-anemone, Gorgonia, etc. HYDRA Classification: Phylum: Coelenterata Tissue grade of organization, diploblastic and acoelomate. Class: Hydrozoa Hydroids containing medusa with velum. Order: Hydroidea Polypoid generation is well developed. Sub-order: Athecata Hydrotheca is absent. Genus: Hydra Habit and Habitat: Hydra is a solitary and sedentary fresh-water form, found attached to stones, rocks and weeds. Distribution: It is cosmopolitan, but most common in India, Canada and U.S.A. Comments: Cylindrical body is tube-like measuring approximately 1.3 cm in length. Anterior end contains mouth or hypostome, which is crowned with 6 to 10 filiform nematocyst bearing tentacles. Proximal end of the body contains pedal or basal disc or foot meant for attachment to the substratum. Body is diploblastic, consisting of outer ectoderm and inner endoderm. Body wall encloses a gastrovascular cavity extending into the tentacles. In female Hydra, ovary and buds are found on sides in mid position. In male Hydra 3 or 4 pairs of testes, are found anteriorly and a full grown bud with tentacles on side. Testes develop near the oral end and ovaries near the base. It reproduces sexually by fusion of sperms and ova. During asexual reproduction, lateral buds develop on the sides of the body which later on detach and develop into new Hydra. OBELIA Classification Phylum: Cnidaria Tissue grade of organization, diploblastic and acoelomate. This is sub-phylum of coelentereta Class: Hydrozoa Order: Hydroidea Sub-order: Leptomedusae Polyps athecate, i.e., not covered by hydrotheca. Medusa with gonads on manubrium. Genus: Obelia Habit and habitat: The hydroid phase of Obelia is a typical marine, small, branching, sedentary, and colonial form. It lives at 80 meters depth. Distribution: Obelia has a worldwide distribution except the high-arctic and Antarctic seas. Comments: The colony of Obelia looks like a delicate, mossy or fur-like growth, whitish or light brown. It consist horizontal and vertical filaments. The basal or horizontal anchoring portion of a colony is like a creeping root or complicated meshwork, encrusting over the surface of the substratum. It is called the hydrorhiza or rhizome (Gr., rhizo, root). The hydrorhiza gives off several slender, upright or vertical threads, called hydrocauli. Each hydrocaulus forms the main stem or axis of a colony and measures about 2 to 3 cm in height. The stem and the zooids of the colony are made up of a living hollow, cellular tube the coenosarc. Through its life cycle, Obelia take two forms: polyp and medusa. They are diploblastic, with two true tissue layers – an epidermis (ectodermis) and a gastrodermis (endodermis), with a jellylike mesoglea filling the area between the two true tissue layers. A gastrovascular cavity is present where the digestion starts and later becomes intracellular. They have incomplete digestive tracts where the food enters, is digested, and expelled through the same opening. During the polyp stage, the mouth is situated at the top of the body, surrounded by tentacles, whereas during the medusa stage, the mouth is situated at the distal end of the main body structure. Four gonads lie in this main body structure, or manubrium. When food is taken in through the mouth, it enters the manubrium. The food is then distributed through a canal system, consisting of four radial canals and an outer ring. Defense and the capture of prey are helped by unique stinging cells called cnidocytes that contain nematocysts, which are triggered by the cnidocil. AURELIA Classification: Phylum: Coelenterata Class: Scyphozoa Medusa well developed, hydroid reduced. Order: Semaeostomeae Free-swimming scyphomedusae with umbrella-like disk and oral lobes. Family: Ulmariidae Simple or branched radial canals and a ring canal. Genus: Aurelia (Jelly-fish) Habit and habitat: Aurelia is a solitary marine and medusoid jelly-fish often living in brackish water. It inhabits coastal water of all oceans in all zones and in abundant number, mostly living in warm and temperate latitudes. Distribution: Very common along the entire Atlantic coast and Pacific coast and only form reported in temperate regions. Comments: Commonly called as Jelly-fish measuring approximately 30 cm in diameter. Body is gelatinous, transparent, bluish, white, reddish or pinkish. Saucershaped body is distinguished into convex ex-umbrellar and concave sub-umbrellar surfaces. Various other structures seen are gastric filaments, sub-genital pits and velarium. Sub-umbrellar margin contains marginal tentacles having stinging cells and 8 marginal lappets having sense organs or tentaculocysts in 8 lobes. Mouth 4cornered. Each corner is drawn out into an oral arm along perradii. Inter-radial, adradial and per-radial gastrovascular canals open in circular canal. Jelly-fish feeds on molluscs, crustaceans, protozoans, nematodes, rotifers, diatoms, polychaetes and copepods. It swims by rhythmic contractions of umbrellar surfaces. Males and females are separate individuals. Life-cycle shows alteration of generation. In the life cycle of Aurelia, elongated larva metamorphose into a small trumpt-shaped or hydra like polyp, about 5mm high and without perisarc known as Scyphistoma. It represents the hydroid or polyp phase in the life cycle of Aurelia. Its proximal part is narrowed into a stalk like organ attached to the substratum by a basal disc. Tentacle buds are found at the region of mouth. In Aurelia, they are represented in larval stage only. The Scyphistoma feeds and grows upto 12mm and may survive several months in this stage. SEA ANEMONE Classification: Phylum: Coelenterata Class: Anthozoa, Actinozoa Solitary or colonial, exclusively marine. Order: Actiniaria Solitary or colonial anemone-like polyps with one siphonoglyph and no skeleton. Tentacles and septa paired and often in multiples of six. Examples: Fungia, Acropora, Oculina. Habit and habitat: The sea anemones live mostly attached to rocks, shells, seaweeds, submerged timbers and other strata from low tide marks to depths of 6,000. Distribution: It ranges to the Pacific and the North Atlantic coasts. External features: Brightly coloured mostly white, green, blue, orange, red or a combination of many colours but are usually brownish or yellowish. It has diameter of about one meter. Body is short, cylindrical, radially symmetrical and generally longer than broad when fully extended. Body is divided in to three regions- pedal disc, column and oral disc. The oral end of the body is flat or slightly convex, short and delicate. It bears numerous short hollow marginal tentacles. Many Sea anemones form symbiotic relationships with single-celled algae, whether zooxanthellae or zoochlorellae, that live within their cells. The sea anemone is considered to be the flower of the sea. Sea anemones look like plants, but they are really meat-eating animals. Sea anemones come in different sizes and many different colors. They spend most of their lives at one place. They can attach themselves to rocks. Some even bury themselves in the mud. GORGONIA Classification: Phylum: Coelenterata Class: Anthozoa Sub-class: Octocorallia Polyps with 8 tantacles, bioluminescent. Order: Gorgonaceae Horny corals, polyps short not reaching upto base. Sub-order: Scleraxonia Axial spicule is present. Family: Gorgonida Slender stem, thin, coenenchyme and partially retractile anthocodia. Genus: Gorgonia (Sea fan) Habit and habitat: Gorgonia is a colonial form inhabiting all seas from tide mark to over 4000 meters. Most of them are littoral. Some prefer warmer seas. Shallow water forms have fan- like structure. Deep sea forms branch in all directions. It forms one of the chief attractions of the submarine gardens of shallow tropical and subtropical waters. Distribution: Found in South Atlantic and West Indies, Indo-Pacific ocean, Malay Archipelago, Bermuda and Bahamas. Comments: Commonly called as Sea fan, whose graceful form and soft colouration constitute one of the chief attractions of ‘submarine’ gardens. It forms erect, yellowish or reddish arborescent branches in one plane connected by cross connections in a feathery manner and contour of the body becomes fan-like. Branches are united by numerous cross-branches, forming a network with meshes 2 to 5 mm wide, calcareous in nature. Base of the colony is expanded to form a hold fast organ called as pedal disk. Polyps or anthocodia emerge from branches and contain tentacles, mouth and gastrovascular cavity. Polyps occur in rows and on two sides only of the stem and its branches. In magnified portions, holes for polyps are seen. Skeleton consists of hornlike material called Gorgonin embedded in mesoglea. An axial rod extends throughout the colony and its branches. Axial rod consists of medulla and cortex. Gorgonin is proteinaceous consisting of tyrosine amino-acid. Sexes are separate. SUMMARY In this topic we have discussed some important specimens like- Hydra, Obelia, Aurelia, Sea-anemone and Gorgonia under phylum Coelenterata. The Coelenterates are all most marine, distributed generally throughout the seas. Aristotle considered Coelenterate as the intermediate between plants and animals and termed them Acalephae or Cnidae.