Algae in Koi Ponds, Learn this lesson & have a clear water garden for life! The two layers of ectoderm which enclose the bell rudiment from outside now break leaving a marginal and circular shelf called velum. 6. 2. The tentacles in all cases are solid; the solid core or endoderm surrounded by a layer of ectoderm cells. Thus, an asexual polypoid generation alternates with a sexual medusoid generation. Planula. 1. The blastosyle is long, cylindrical and devoid of mouth and tentacles and enclosed in a transparent gonotheca, modified perisarce (Figs. 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. 1. Digestion is both extracellular and intracellular. The tentacles and the hypostome bear cnidocytes. Obelia is a classic example of alternation of generations in a hydrozoan life cycle. Welcome to BiologyDiscussion! 8. The medusae with a velum are known as craspedote and those without a velum are acraspedote. They soon hatch, and free-swimming "planula" larvae emerge from the female's mouth or brood pouch and set out on their own. Alteration of generations. When the bell tilts, the statoliths respond to the pull of gravity and stimulate the sensory bristles to which they are attached. The polypoid generation is asexual and produces polyps and blastostyles by asexual budding. The planula swims freely for a brief period and settles down on some submerged substratum by one end. Fertilisation usually takes place in open sea water where the gametes are set free. 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. D. Parenchymula is the larval stage in sponges. During this stage of life, Obelia are confined to substrate surfaces. Obelia has only one kind of nematocysts called basitrichous isorhizas in which the capsule is oval, butt is absent, the thread is open at the tip and has spines on its base. The free end forms a manubrium with a mouth and a circlet of tentacles. Some cells migrate into blastocoele, eventually filling it completely to form a solid gastrula known as stereo gastrula. Ciliated free swimming larva of obelia. The hydromedusa is craspedote. The Obelia colony is sexless, bears no gonads and develops by asexual process, i.e. Planula larva may be seen in masses on the oral arms of female medusae. 11. It is covered by a transparent gonotheca and contains an axis or blastostyle on which lateral buds form that develop into medusae or gonophores. Obelia. Both exumbrellar and subumbrellar surfaces of medusa are covered by ectoderm cells. Before publishing your Notes on this site, please read the following pages: 1. Polyp is the nutritive zooid of the colony. B. Planula is the larva of Obelia which is a coelenterate. 3. Mouth is absent. Specimen study of hydra, obelia, aurelia, sea-anemone, gargomia, scyphytoms - Duration: 17:25. After fertilization, the zygote develops into a blastula, which develops into a planula larva. 18. 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. 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. Difference # Aurelia: 1. Barrel-shaped and responsible for the nutrition of the colony. ONLINE BIOLOGY LESSONS IN BOTANY,ZOOLOGY,ZOOGEOGRAPHY, PHYSIOLOGY, AND MEDICINE 1. In this article we will discuss about the life cycle of obelia, explained with the help of suitable diagrams. Mesogloea is a thin jelly-like substance with no structure or cells. The tentacles are longer than hypostome, tapering and filiform. The ocelli consist of patches of pigment and photoreceptor cells organised either within a flat disc or a pit. Since sexes are separate, these are either testes or ovaries. Each colony of Obelia consists of a horizontal thread-like root called hydrorhiza which is attached to a weed or any substratum. 7. Yeast: Origin, Reproduction, Life Cycle and Growth Requirements | Industrial Microbiology, How is Bread Made Step by Step? 8. Product of fertilisation is the free swimming planula larva, which subsequently settles down to form the fixed asexual colony. Thus, a simple polyp or hydrula is formed which grows a hydrorhiza from its base, from which an Obelia colony is formed by budding. 14. 5. It has a cylindrical body attached to the axis of the hydrocaulus by its proximal end and free at its distal end. 7. A square mouth is present, at the apex of the manubrium. After that, the medusa stage sexually reproduces through the production of sperms and eggs. 5. 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 polyp colony reproduces asexually. Thus, a non-locomotory species becomes locomotory and an overcrowding of individuals within a limited area is avoided and, thereby, the species is successfully continued. The muscles coordinate the snake-like swimming movements of the medusa. Answer. The germ cells originate in the ectoderm of the manubrium quite early when the medusa itself remains attached to the blastostyle. The larva can swim from place to place with the help of cilia and, being aided by water current, it can travel a long distance. 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. or gonozooids. The perisarc around a blastostyle is a gonotheca, the blastostyle and gonotheca are together called a gonangium. Between the two ectoderm layers of the bell is a thin sheet of endoderm lamella except where the enteron lies. The endoderm cells have no muscle processes, they are ciliated epithelial cells, they are digestive. Privacy Policy3. Velum is characteristic of hydrozoan medusae but it is insignificant in Obelia. 2. It is with the lower ring that the statocysts are connected. On each side of the mesogloea is a nerve net composed of nerve cells and their fibres, the two nerve nets are inter-connected. The radial canals mark out four principal radii. Our mission is to provide an online platform to help students to share notes in Biology. After the attachment the cilia of the larva are lost and a mouth opens at the oral end. Its outer cell layer becomes the ectoderm and inner cell mass the endoderm. Between the ectoderm and endoderm is thick mesogloea forming the bulk of the bell of the medusa, manubrium and tentacles. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. 9. The bud grows larger and its coenosarc becomes like a vesicle which is attached to the blastostyle by a narrow stalk. 9. An Obelia colony exhibits polymorphism. This hydroid will form a planula larva. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. Thus, in the life history of Obelia, there is a regular alternation between fixed polypoid and free-swimming medusoid phases, both of them being diploid. 6. 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. Medusae buds are unisexual and free- living at maturity. Obelia is characterised by A. Ciliated free swimming planula larva B. Metagenesis C. Acraspedote medusa D. Statocysts present in medusoid stage (1) A and B (2) B only (3) A, B and D (4) A, B, C and D Sol. They form nematocyst and migrate actively to reach their final positions. The proximal end gradually narrows down and a disc appears for attachment. The tentacles are solid with a core of endoderm cells surrounded by a layer of ectodermal cells. Individual hydranthan in the mature colony is known as a gastrozooid, which has a medusa bud. 4. One individual possesses the power to reproduce the other by asexual reproduction, which again by sexual reproduction gives rise to the next generation. ADVERTISEMENTS: In this article we will discuss about the structure of obelia. 12. 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. However, the features of similarity between them are listed below: 2. The zygote undergoes complete or holoblastic and equal cleavage to form a single-layered blastula with a blastocoele. The planktonic planula metamorphoses into a benthic polyp (hydroid), which buds off further polyps and forms a colony. The mouth leads to a small, central, rounded or quadrangular chamber known as gastric cavity or stomach. The nervous system of medusa is more highly specialised than that of the polyp. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. 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. Which part of the male reproductive system store the sperm? The gonads mature after the medusae escape from the gonotheca. This organism shows statocysts, sensory structures that are used to detect gravity and thereby maintain vertical orientation. Polyps or hydranths (nutritive zooids); 2. The edge of the bell is produced inwards as a thin fold called velum. All these are lined by gastro dermis and serve the purpose of digestion and distribution of digested food. The endoderm lines the enteric cavity and the radial and circular canals. 11. Learn vocabulary, terms, and more with flashcards, games, and other study tools. 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. 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. From the edge of the bell numerous small solid tentacles hang downwards. Digestion is extracellular as well as intracellular in both the cases. Alternation of Generations and Metagenesis of Obelia: The best answers are voted up and rise to the top. The cnidoblasts are found in the basal part of the hydranth and in the coenosarc. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. 3. Medusa Buds. 5. Obelia has a worldwide distribution except the high-arctic and Antarctic seas. Pulsation will continue in the bell as long as any portion of the ring is intact. The tentacles are solid, each containing a core of vacuolated endodermal cells covered by ectoderm. The larvae are ciliated and free-swimming. At the growing ends of the main branches are immature club-shaped polyps. Striking as is the difference between polyp and medusa. 3. A cylindrical projection, the manubrium, hangs from the middle of the subumbrellar surface. The animal may then respond by muscular contractions to bring itself back into a horizontal position. Planula larva is without a blastopore and coelenteron. Cnidocytes are formed continuously in the bulb and migrate to the tentacles. Obelia is cosmopolitan in distribution, forming a whitish or light-brown plant-like fur in the sea; hence, the common name sea-fur is assigned to it. Log in Sign up. The medusa is circular and tiny umbrella-like in shape. 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 perisarc enclosing the polyp is termed hydro theca. They may be either in the form of pits or closed vesicles but in both cases, the walls contain sensory cells with bristles projecting into the lumen. 4. The endoderm of tentacles has cubical, vacuolated cells with thick walls. 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. Halfway between any two per-radii a radius of the second order or inter-radius may be taken. The outer layer of the body is composed of ciliated ectodermal cells and the inner layer is made of endodermal cells. The germ cells of Obelia do not arise in the gonads, they arise from interstitial cells of the ectoderm of the blastostyle where they may be seen in various stages of maturation, then they migrate into the medusa, then through the radial canals they take up their position in the ectoderm of the gonads. 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. 3. At maturity, small umbrella-like me dusae buds get detached from the blastostyle and escape to the sea water through an aperture at the free end of the gonotheca. 10. This stage of a scyphozoan polyp is known as scyphistoma larva, and it is about 1.25 cm in length. 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. This phenomenon is known as metagenesis. The planula larva swims about by means of the cilia of its ectodermal cells and after a brief period of free existence, settles down and fastens itself to the sea bottom by one pole. Formation of scyphistoma: The ciliated planula larva after a short free-swimming existence attaches itself to a substratum by its aboral end. Obelia is sedentary, marine colonial form found attached on the surface of sea weeds, molluscan shells, rocks and wooden piles in shallow water up to 80 metres in depth. 2. Digestive enzymes are secreted from the endoderm of bulbs. its outer layer of ectoderm cells becomes ciliated, and now it is called planula. The nerve rings connect with fibres innervating the tentacles, the musculature, and the sense organs. At the proximal end, the zooid is continuous with the coenosare. From hydrorhiza arises a vertical branching stem about 2.5 cm long which is known as a hydrocaulus. Furthermore, the muscular system is best developed around the bell margin and sub umbrella surface where the fibres form a radial and circular system. The mouth is homologous in both the cases; the mouth situated on the hypostome in polyp is homologous with the mouth situated on the manubrium of the medusa. 2. It is covered by a cup-shaped hydro theca. 20.12, 20.16). Horizontal movement is dependent upon water currents. The enteric cavity and the canals represent the enteron which distributes food. Disclaimer Copyright, Zoology Notes | Exclusive Notes on Zoology for Students, Life Cycle of Aurelia (With Diagram) | Phylum Cnidaria, Difference Between Obelia and Aurelia | Phylum Cnidaria, Structure of Aurelia (With Diagram) | Phylum Cnidaria. Planula larva – Obelia (cnidaria) b. Glochidium larva – Mussel (class pelecypoda) (phylum mollusca) c. Nauplius larva – Crab (class crustacea) (phylum arthropoda) d. Cysticercus larva – Taenia solium (class cestoda) (phylum platyhelminthes) A trimorphic colony, blastostyles for budding and medusae for disseminating gametes of sexual reproduction gives rise cnidocytes! 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