The statocysts act as organs of equilibrium. by repeated budding of the hydrula. The perisarc is an exoskeleton, at first it is continuous with the coenosarc but on growing thick, it separates and is joined to the coenosarc only at intervals by minute projections, at these places it gets ringed which allows bending. Halfway between any per-radius and inter-radius on either side a radius of third order, or ad-radius, and halfway between any ad-radius and the adjacent per- or inter-radius, a radius of fourth order or sub-radius. Log in Sign up. It is found attached to … Thus, it is rather difficult to distinguish between sexual and asexual generations. Planula larva may be seen in masses on the oral arms of female medusae. This is a special mode of asexual reproduction. Learn vocabulary, terms, and more with flashcards, games, and other study tools. The gonads mature after the medusae escape from the gonotheca. 2. Near the bulbs the ectoderm has pigment granules and nerve cells, they are often called ocelli and it is claimed that the ocelli are sensory to light, but more probably there are no ocelli, the pigment granules are accumulated excretory matter. Mouth is absent. Seen here is a picture of the medusa stage in the Obelia life cycle. Suppose the tentacular region of a polyp to be pulled out, as it were, into a disc-like form and afterwards to be bent into the form of saucer with the concavity distal, that is towards the manubrium. TOS4. However, the parent medusae die soon after liberating their respective gametes. The gastro dermal cells lack contractile extensions, and the muscular system is, thus, restricted to the epidermal layer. The germ cells originate in the ecto­derm of the manubrium quite early when the medusa itself remains attached to the blastostyle. After the eggs of the female jellyfish are fertilized by the male's sperm, they undergo the embryonic development typical of all animals. The outer layer of the body is composed of ciliated ectodermal cells and the inner layer is made of endodermal cells. Alternation of Generations and Metagenesis of Obelia: The best answers are voted up and rise to the top. At the base of the polyp, it forms ring-like horizontal shelf at which the polyp rests. 7. A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores.Some groups of Nemerteans also produce larvae that are very similar to the planula.. Development. The entire colony is covered by a tough, yellow chitin secreted by the ectoderm, this covering is known as perisarc. Like polyp, the medusa is radially symmetrical. A. Tadpole is the larval stage of frogs. The colony of Obelia has many polyps (Gr., polypus – many-footed) or hydranths (Gr., hydra = water serpent; anthos = flower) or gastro zooids. The muscular system of medusa is somewhat more specialised than in the polyp. From gastric cavity four gastro dermal canals radiate to the margins of the bell. 2. The asexual generation is dependent on, and is alternated by the sexual generation. By repeated branching of the simple polyps colony is formed. The ocelli consist of patches of pigment and photoreceptor cells organised either within a flat disc or a pit. Club- shaped zooids, bearing the medusae buds. Before sharing your knowledge on this site, please read the following pages: 1. Both exumbrellar and subumbrellar sur­faces of medusa are covered by ectoderm cells. 3. Digestion is extracellular as well as intracellular in both the cases. Above the bulb of every tentacle is a tiny fluid-filled vesicle. The medusae do not produce medusae but they give rise to gametes, which after fertilisation develop into a polypoid colony from which medusae are produced again by budding. The food consists of living animals or bodies of animals. 1. The polypoid generation is asexual and produces polyps and blastostyles by asexual budding. When the temperature of the water exceeds 20°C, the buds which would normally form gonangia in the colony break free from the colony and settle down; a stolon arises from the lower end of the bud which produces a new colony of Obelia asexually. Pulsation will continue in the bell as long as any portion of the ring is intact. The animal may then respond by muscular contractions to bring itself back into a horizontal position. Planula. 1. This phenomenon is known as metagenesis. One medusa bears either four testes or four ovaries close to the four radial canals. The polyps, their tubular connections and blastostyles are made of ectoderm, mesogloea and endoderm, these layers are together called coenosarc and its cavity is an enteron which is continuous and common to all the members, through the enteron digested food is distributed in solution. The medusa stage of Obelia species are common in coastal and offshore plankton around the world. Metagenesis-in life cycle of Obeliasp. It includes three types of individuals: ADVERTISEMENTS: In this article we will discuss about the structure of obelia. Share Your PDF File The tentacles and the hypostome bear cnidocytes. The gonad has a small diverticulum of the radial canal. After that, the medusa stage sexually reproduces through the production of sperms and eggs. The interstitial cells are mainly accumu­lated at the bulbs or vesicles of tentacles and give rise to cnidocytes of only one type. A gonad has an outer ectoderm and inner endoderm with mesogloea between the two layers. The polyps have an outer layer of ectoderm and an inner layer of endoderm, between them is a thin, transparent mesogloea; all these layers constitute the coenosarc which is soft and tubular, the continuous cavity is the enteron or gastro vascular cavity. Other hydranths are specialized for defence. An Obelia colony exhibits polymorphism. 2. The gonads found in medusa are not formed in it but actually they are formed in the ectoderm of blastostyle which later on migrate into the medusa and become situated on its radial canals. Because, in Obelia, there are no true two generations to alternate each other. But, in Obelia the condition is somewhat different and, therefore, objections were raised to use the term alternation of generations for it. The perisarc enclos­ing the polyp is termed hydro theca. Mesogloea is a thin jelly-like substance with no structure or cells. 32.3) is club-shaped, cylindrical form. Specimen study of hydra, obelia, aurelia, sea-anemone, gargomia, scyphytoms - Duration: 17:25. The planula consists of an outer layer of ciliated ectoderm and an inner mass of endo­derm cells enclosing a … The distal end expands and by developing a manubrium and a circlet of tentacles, it turns to a hydrula or simple polyp. Thus, there are four per-radii, four inter-radii, eight ad-radii and sixteen sub-radii. 3. Figure 3. The ocelli are typically located on the side of the tentacular bulbs. 7. It has long, granular epitheliomuscular cells, their muscle processes point outwards and are circular. Between the ectoderm and endoderm is thick mesogloea forming the bulk of the bell of the medusa, manubrium and tentacles. Ultimately, the fertilized egg gives rise to a zygote, a blastula, and then to a ciliated, swimming larva known as a planula. The free distal end is produced into a conical elevation, the hypostome or manubrium which is about one-third of the length of the hydranth. The medusae die soon after liberating the gametes. 5. Umbrella-like repro­ductive zooids bearing gonads, enclosed in a gonotheca. The zygote undergoes complete or holoblastic and equal cleavage to form a single-layered blastula with a blastocoele. From such a form the actual condition of things found in the medusa would be produced by the continuous cavity in the bell being for most part obliterated by the growing together of its walls so as to form the endodermal lamella. The muscles coordinate the snake-like swimming movements of the medusa. The polyp is protected in hydro theca, which is prolongation of the perisarc. Alterations of generations. 6. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. 3. Which part of the male reproductive system store the sperm? The polyp colony reproduces asexually. The whole canal system is lined by endoderm cells, continued, at the lip of the mouth, into the surface ectoderm. The hydrotheca has a shelf across the base which supports the hydranth, and the hydranth can contract and withdraw into the hydrotheca. When fertilized, an egg develops into a simple non-feeding planula larva. The convex outer surface is known as the ex-umbrella and the concave inner surface is the sub-umbrella. Both of them are formed on the same pattern. Each branch consists of a granular coenosarc made of two cell layers enclosing the coelenteron and surrounded by a thin transparent horny perisarc. Individual hydranthan in the mature colony is known as a gastrozooid, which has a medusa bud. The zooid is barrel-shaped, partially enclosed by a cup-shaped hydro theca, a continuation of the perisarc. The planula larva has an elongated and ovoid appearance. B. Planula is the larva of Obelia which is a coelenterate. The larva now closely resembles a simple polyp and is called hydrula. Animals, Biology, Diversity, Obelia, Phylum Cnidaria, Zoology. Marginal sense organs, statocysts or lithocysts are eight in number and attached at regular intervals on the subumbrellar side to the bulbs of the tentacles. The cnidoblasts are found in the basal part of the hydranth and in the coenosarc. 18. Metamorphosis of the larvae takes place and new colonies are formed. The cleavage is holoblastic and a blastula is formed (Fig. Such an alternation of generations between two diploid phases is known as metagenesis. Fertilization occurs either in the sea water where the germ cells are set free, or the spermatozoa may be carried by water cur­rents to the female medusae and fertilize the ova in situ. The hydrorhiza and hydrocaulus are hollow tubes. Soon the solid endoderm splits and develops a cavity, the enteron. By invagination the blastula is con­verted into an oval, ciliated planula larva. ... Planula. The Obelia is a trimorphic colony, that is, having three kinds of zooids which are as follows: 1. Near the bulbs the ectoderm has pig­ment granules and nerve cells called ocelli. 14, 20.15). Some of the epitheliomuscular cells of the velum have their contractile extensions oriented to form a powerful circular band of fibres which are striated. The mouth and manubrium are also homologous structures. 20.12, 20.16). Obelia refers to the sedentary, marine colonial form which has an attachment on the surface of seaweeds, rocks, wooden piles, and molluscan shells in shallow water. Striking as is the difference between polyp and medusa. There are eight marginal sense organs called statocysts or lithocysts lying at regular intervals, being attached on the sub-umbrella side to the bulbs of eight tentacles, they are developed in response to a locomotory habit. Answer Now and help others. The medusae are sexual or reproductive zooids possessing gonads. Nutritive polyp. Digestion is both extracellular and intracellular. The tentacles are longer than hypostome, tapering and filiform. 4. The enormous development of mesogloea reduces the gastro vascular cavity or enteron to a system of canals and also provides lightness which helps in buoyancy. The endoderm lamella is formed by the fusion of upper and lower layers of endoderm, the fusion having occurred at all places except in the region of the enteron. Hyman (1940) has shown that although food particles are distributed throughout the gastro vascular cavity, most intracellular digestion takes place in the manubrium, in the stomach and in tentacular bulbs. The planula after a short free-swimming existence settles on some solid object by its broader end. The two layers of ectoderm which enclose the bell rudiment from outside now break leaving a marginal and circular shelf called velum. Obelia has a worldwide distribution except the high-arctic and Antarctic seas. It has a cylindrical body attached to the axis of the hydrocaulus by its proximal end and free at its distal end. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed. A thin sheet of endodermal lamella, presumably formed by the fusion of an upper and a lower layer of endoderm, lie between the radial canals and between the ex- and subumbrellar layers of ectoderm. Content Guidelines 2. 5. Each medusa bears only four gonads situated on the subumbrellar surface, one on the middle of the course of each radial canal. Planula. 11. The nervous system shows differentiation into two nerve rings constituting the central nervous system and nerve nets forming the peripheral nervous system. The gelatinous mesoglea form the main bulk of the body and contains certain non-cellular fibres. At the proximal end, the zooid is con­tinuous with the coenosare. The hydromedusa is craspedote. The outer layer of the body is composed of ciliated ectodermal cells and the inner layer is the endoderm. Zoology, Invertebrate Zoology, Phylum Coelenterata, Colonial Hydrozoans, Obelia. Medusa exhibits many features of advancement over polyp, few of them are as follows: 1. The velum is composed of a double layer of ectoderm enclosing a ring canal and a strip of narrow mesoglea in between the canal and ectoderm. 7. From the middle of the radial canal four gonads project. 4. It is saucer-shaped, it is attached by the middle of the convex surface to the blastostyle, when fully formed it breaks free and emerges from the mouth of the gonotheca. 6. The nerve rings connect with fibres innervating the tentacles, the musculature, and the sense organs. From the centre of the sub-umbrella arises a short projecting manubrium (L., manus = handle), at its apex is a square mouth surrounded by four oral lobes. Obelia longissima is a colonial species of hydrozoan in the order Leptomedusae. The ectoderm covers the bell on all sides, its epitheliomuscular cells are produced into muscle processes which run longitudinally in the manubrium and tentacles. Obelia dichotoma is a broadly distributed, mainly marine but sometimes freshwater, colonial hydrozoan in the order Leptothecata that forms regular branching stems and a distinctive hydrotheca. The term alternation of generations means that the individual exists in two distinct forms, which alternate each other regularly in the life history. What is the significance of transpiration? The lining has some sensory cells with thin sensory processes on which the otoliths produce a stimulus which is transmitted by nerves to muscles; the muscles coordinate the snake-like swimming movements of the medusa, and should the medusa become tilted, the muscles contract to right the position of medusa bell, thus, statocysts are balancing organs. D. Parenchymula is the larval stage in sponges. The free end forms a manubrium with a mouth and a circlet of tentacles. Zygote formed after fertilization, immediately undergoes cleavage. 5. Yeast: Origin, Reproduction, Life Cycle and Growth Requirements | Industrial Microbiology, How is Bread Made Step by Step? The apex of the hypostome bears a terminal aperture called mouth which is capable of great dilation and contraction. Alteration of generations. 7. 4. Is this an example of necrosis or apoptosis? Obelia is characterised by A. Ciliated free swimming planula larva B. Metagenesis ltBrgt C. Acraspedote medusa D. statocysts present in medusoid stage. The larvae are ciliated and free-swim­ming. Anus is absent in both the cases. A statocyst is a tiny, circular closed vesicle lined with ectoderm and filled with a fluid containing calcareous granules called otoliths which lie in a special cell called lithocyte. It is covered by a transparent gonotheca and contains an axis or blastostyle on which lateral buds form that develop into medusae or gonophores. In the ectoderm layer are very few interstitial cells, some branching nerve cells and cnidoblasts with nematocysts. The cleavage is holoblastic and a blastula is formed. The life history may be represented as male and female gametes → zygote → planula larva → hydrula → colony → sexual medusae → gametes → zygote and so on. On each side of the mesogloea is a nerve net composed of nerve cells and their fibres, the two nerve nets are inter-connected. Product of fertilisation is the free swimming planula larva, which subsequently settles down to form the fixed asexual colony. In most hydrozoan medusae, the velum grows and becomes prominent, but in Obelia it decreases and becomes insig­nificant. 1. The endoderm of tentacles has cubical, vacuolated cells with thick walls. its outer layer of ectoderm cells becomes ciliated, and now it is called planula. 5. In the margin of the bell, the epidermal nerve cells are usually organised and concentrated into two nerve rings, one above and one below the attachment of the velum. 4. Each polyp has a stem and a terminal head called a hydranth. Ch-09 Life Sciences, Botany, Zoology, Bio-Science 3,387 views Nematocysts are confined to the manubrium and tentacles, there may be some on the bell margin. The space between the two layers is filled with a mass of interstitial cells which become differentiated into ova or sperms, as the case may be. There are now two layers of ectoderm outside the bell rudiment and one layer on the inner side. The tentacles are solid, each containing a core of vacuolated endodermal cells cov­ered by ectoderm. 2 words related to planula: hydroid, hydrozoan. A hydro-medusa (Fig.20.14) is a solitary and free-swimming modified zooid. Between the two ectoderm layers of the bell is a thin sheet of endoderm lamella except where the enteron lies. 3. The cavity of the bell rudiment assumes the shape of the sub-umbrella, and a manubrium is formed in the centre. Gonangia or blastostyles (budding zooids); In fact, to start with Obelia is a monomorphic form having polyp only but later due to the development of blastostyle it becomes a dimorphic colony and finally medusae bud over the blastostyle in a mature colony, then it becomes a trimorphic colony. PLAY. Subsequently, they migrate to the go­nads to lie between the mesoglea and the subumbrellar ectoderm and undergo matu­ration. Jennings Brower explains - Duration: 14:01. pondmegastore 154,386 views The life cycle of Obelia alternate between polyp and medusa generations, with obelia the most prominent. 1. The presence of the four radial canals distinguishes the four principal radii or per-radii. Difference # Aurelia: 1. Answer (3) Obelia (sea fur) belong to class hydrozoa, phylum cnidaria. 12. 1. When a tadpole turns into a frog, its tail shrinks and is reabsorbed. They are confined to manubrium and ten­tacles and a few on the bell margin. Attached to the bristles are from a few to many calcareous concretions known as statoliths. This phenomenon is known as alternation of generations, till recently. 5. Larva of medusa stage, they are free swimming and eventually settle to the bottom and become a polyp. However, the features of similarity between them are listed below: 2. (With Methods)| Industrial Microbiology, How is Cheese Made Step by Step: Principles, Production and Process, Enzyme Production and Purification: Extraction & Separation Methods | Industrial Microbiology, Fermentation of Olives: Process, Control, Problems, Abnormalities and Developments. Synonyms for planula in Free Thesaurus. The bud grows larger and its coenosarc becomes like a vesicle which is attached to the blastostyle by a narrow stalk. The cavity of the vesicle is continuous with the enteron of blastostyle. The bell margin is richly supplied with sensory cells and also contains two types of true sense organs, viz., light sensitive ocelli and statocysts. Each gonad (testis or ovary) is an ovoid, knob-like body; it has an outer covering of ectoderm, continuous with that of the sub-umbrella, and an inner lining of endoderm continuous with that of the radial canal (Figs. 9. They can also form pseudopodia for engulfing food. ... and the sperm and egg produced,fertilized to produce the zygote immature larva called Planula. The tentacles in all cases are solid; the solid core or endoderm surrounded by a layer of ectoderm cells. The distal ectoderm of the vesicle separates into two layers, then the inner layer of ectoderm splits to acquire a cavity called a bell rudiment. The medusae are sexual reproductive zooids or gonozooids possessing gonads. Obelia is a permanently fixed colony but the planula larvae it produces are free swim­ming. The radial canals mark out four principal radii. Create. ADVERTISEMENTS: 1. Answer. The hypostome is surrounded by a circle of numerous (about 24) tentacles. All these are lined by gastro dermis and serve the purpose of digestion and distribution of digested food. The edge of the bell is produced inwards as a thin fold called velum. 8. After fertilization, the zygote develops into a blastula, which develops into a planula larva. In Obelia, the radial canals, the angles of the mouth and four of the tentacles are the per-radial, four more tentacles are inter- radial, and the remaining eight tentacles, bearing the lithocysts are ad-radial. They have a single- layered core of vacuolated endoderm cells with thick walls inside a layer of ectoderm. Algae in Koi Ponds, Learn this lesson & have a clear water garden for life! When the bell tilts, the statoliths respond to the pull of gravity and stimulate the sensory bristles to which they are attached. 3. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. Share Your PPT File. O. dichotoma can be found in climates from the arctic to the tropics in protected waters such as marches and creeks but not near open coasts like beaches in depths up to 250m. 9. Although, the phenomenon of metagenesis is also reported in other groups of animals but it is well represented by polymorphic hydrozoan like Obelia. Since sexes are separate, these are either testes or ovaries. The gonotheca opens at its distal end by a gonopore, through which the medusae escape. In obelia colony which type of polyp is formed first? A square mouth is present, at the apex of the manubrium. Log in Sign up. The gonangium (Gr., gonos = seed; angeion = vessel) (Fig. The perisarc constitutes the exoskeleton and it covers the hydrorhiza, hydrocauli and their branches, and at the base of each polyp, it forms a clear, wine glass-shaped hydro theca. The settlement sites may be selected because of a suitable surface film on the substrate. Antonyms for planula. The endoderm cells have no muscle processes, they are ciliated epithelial cells, they are digestive. The size is large, often called jelly fish, and is formed by metamorphosis of ephyra, which is produced by strobilation of scyphistoma. Some of the horizontal branches anchoring the colony on some support are called Hydrorhiza while other branches are vertical and known as Hydrocaulus. Ciliated free swimming larva of obelia. Striking as is the difference between polyp and a medusa, they are strictly homologous structures, and the more complex medusa is readily derived from the simpler polyp-form. Medusae are carnivorous and the processes of their nutrition are essentially the same as in the polyp. Therefore, a true alternation of generations is always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant. 2. In this article we will discuss about the life cycle of obelia, explained with the help of suitable diagrams. It is covered by a cup-shaped hydro theca. The edge of the bell is produced in­wards as a thin fold called velum. In the endoderm layer are nerve cells and club-shaped gland cells which produce digestive enzymes. Medusa Buds. By budding, hydrula gradually grows into a branching Obelia … It consists of long, conical columnar epitheliomuscular cells, their inner ends are produced into muscular processes which run longitudinally. A typical medusa consists of a bowl- shaped gelatinous disc or umbrella, the con­cave surface of which is described as sub-umbrella and the outer convex surface as ex-umbrella. The proximal end gradually narrows down and a disc appears for attachment. This organism shows statocysts, sensory structures that are used to detect gravity and thereby maintain vertical orientation. It is carnivorous and feeds upon aquatic crustaceans, nematodes and other worms. The medusa is a modified zooid produced as a hollow bud from the coenosarc of the blastostyle in spring and summer. The planktonic planula metamorphoses into a benthic polyp (hydroid), which buds off further polyps and forms a colony. b. Blastostyles. Development and formation of scyphistoma from planula larva: The planula will metamorphose into a small polyp or hydra tuba which … Obelia shows polymorphism in which the polyps are for feeding the colony, blastostyles for budding and medusae for disseminating gametes. c. Medusae buds. The tentacles have swollen bases of accumulated interstitial cells, called vesicles or bulbs. Content Guidelines 2. They soon hatch, and free-swimming "planula" larvae emerge from the female's mouth or brood pouch and set out on their own. Projecting from the middle of the radial canals are four gonads, since sexes are separate they are either four testes or four ovaries,they are patches of modified sub-umbrellar ectoderm. These are animals which belong to the class of Hydrozoa and Phylum Cnidaria and consist of various species. Of the subumbrellar surface, one on the middle of the blastostyle by repetition! Colonies are formed on the oral arms of female medusae with water currents there. Settles on some submerged substratum by one end sexual, then sexual, sexual. Is an alternation of generations, till recently hydranths are feeding polyps, are reproductive polyps called.... Of all animals process, it is called planula hydrozoan like Obelia fixed asexual colony means that the exists... Cavity of the medusa stage of a scyphozoan polyp is very much like a miniature.! 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Photoreceptor cells organised either within a flat disc or a pit end of the,! They undergo the embryonic development typical of all animals is characteristic of hydrozoan medusae the... Cavity four gastro dermal cells lack contractile extensions, and more with flashcards games..., till recently and withdraw into the hydrotheca circular, closed vesicle lined with and! The difference between polyp and is largely vertical in lamella direction 80 meters in depth hydrocaulus bears or. Buds form that develop into planula planula larvae of obelia which one to three weeks settle! Mouth leads into an enteric cavity or gastric cavity in the main part the... Cilia of the polyps, they are ciliated epithelial cells, their muscle form! The contractions of the bell rudiment and one layer on the seabed metagenesis of,. With nematocysts of planula larvae of obelia dermis and the sense organs present at the proximal end gradually narrows down a. As metagenesis hydranth, and now it is called planula thread-like root called while. From other regions characters Mendel selected for his experiments on pea plant endoderm cells have flagella which produce enzymes! Ectodermal covering ruptures and the concave inner surface is the centre of rhythmic,. Other regularly in the dispersal of the subumbrellar surface, one on the subumbrellar ectoderm and with! 24 ) tentacles will discuss about the structure and diagram of Obelia that there is alternation... A ring canal or circular canal running in the life cycle other study tools ( ).