The gastrula elongates and. Obelia: Structure and Locomotion | Zoology. The interstitial cells are mainly accumu­lated at the bulbs or vesicles of tentacles and give rise to cnidocytes of only one type. The planula is set free after a time from the oral arms, after a brief free-swimming existence it sinks, loses its cilia, the blastopore closes, and it gets fixed to some object by its aboral end. The edge of the bell is produced inwards as a thin fold called velum. Tentacles help in catching and conveying the prey to the mouth. 20.12, 20.16). After the attachment the cilia of the larva are lost and a mouth opens at the oral end. Alternation of Generations and Metagenesis of Obelia: The best answers are voted up and rise to the top. The mouth and manubrium are also homologous structures. 2. The convex outer surface is known as the ex-umbrella and the concave inner surface is the sub-umbrella. They have a single- layered core of vacuolated endoderm cells with thick walls inside a layer of ectoderm. The perisarc around a blastostyle is a gonotheca, the blastostyle and gonotheca are together called a gonangium. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. The entire colony is covered by a tough, yellow chitin secreted by the ectoderm, this covering is known as perisarc. The hydranths are feeding polyps, they feed by capturing minute animals and larvae. The outer layer of the body is composed of ciliated ectodermal cells and the inner layer is the endoderm. It is clearly evident from the life history of Obelia that there is an alternation of polypoid and medusoid generations. The vertical branches towards the base are further branched and all the branches end in zooids. The hypostome is surrounded by a circle of numerous (about 24) tentacles. When fertilized, an egg develops into a simple non-feeding planula larva. On mature colonies there are individual hydranths called gastrozooids, which can be found expanded or contracted, to aid in the growth of this organism by feeding; the reproductive polyp gonozooids have medusa buds. 5. The hydrocaulus bears zooids or polyps on either side in a cymose formation. In this article we will discuss about the life cycle of obelia, explained with the help of suitable diagrams. All these are lined by gastro dermis and serve the purpose of digestion and distribution of digested food. 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. Therefore, a true alternation of generations is always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant. 2. They can also form pseudopodia for engulfing food. or gonozooids. 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. Medusae buds are unisexual and free- living at maturity. The hydromedusa is craspedote. PLAY. c. Medusae buds. Antonyms for planula. 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. Answer Now and help others. 3. The gonangium (Gr., gonos = seed; angeion = vessel) (Fig. 14, 20.15). These are radial canals, opening into a ring canal or circular canal running in the margin of the bell. Obelia longissima is a colonial species of hydrozoan in the order Leptomedusae. By repeated branching of the simple polyps colony is formed. Our mission is to provide an online platform to help students to share notes in Biology. The bud grows larger and its coenosarc becomes like a vesicle which is attached to the blastostyle by a narrow stalk. The me­dusae with a velum are known as craspedote and those without a velum are acraspedote. 2. Extracellular digestion occurs in the main part of the gastro vascular cavity and is purely proteolytic. Some cells migrate into blastocoele, eventually filling it completely to form a solid gastrula known as stereo gastrula. Both are diploblastic with outer epidermis (ectodermal) and inner gastro dermis (endodermal). The polyp colony reproduces asexually. 2. Answer. 8. Barrel-shaped and responsible for the nutrition of the colony. 7. 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. Nutritive polyp. The term alternation of generations means that the individual exists in two distinct forms, which alternate each other regularly in the life history. A square mouth is present, at the apex of the manubrium. Planula larva may be seen in masses on the oral arms of female medusae. Obelia is a permanently fixed colony but the planula larvae it produces are free swim­ming. 2. The cavity of the vesicle is continuous with the enteron of blastostyle. 2. The enteron has a fluid and its lining is flagellated. 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. It is with the lower ring that the statocysts are connected. Medusa Buds. It is found attached to … When the bell tilts, the statoliths respond to the pull of gravity and stimulate the sensory bristles to which they are attached. The ocelli consist of patches of pigment and photoreceptor cells organised either within a flat disc or a pit. 6. B. Planula is the larva of Obelia which is a coelenterate. Planula larva is without a blastopore and coelenteron. TOS4. The ectoderm of the ex-umbrella is devoid of musculature. The resulting embryos develop into planula larvae which one to three weeks later settle on the seabed. The larvae are ciliated and free-swim­ming. In most hydrozoan medusae, the velum grows and becomes prominent, but in Obelia it decreases and becomes insig­nificant. The cleavage is holoblastic and a blastula is formed (Fig. The food consists of living animals or bodies of animals. 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. They soon hatch, and free-swimming "planula" larvae emerge from the female's mouth or brood pouch and set out on their own. This is a special mode of asexual reproduction. Ciliated free swimming larva of obelia. The edge of the bell is produced in­wards as a thin fold called velum. Each polyp has a stem and a terminal head called a hydranth. They are confined to manubrium and ten­tacles and a few on the bell margin. Polyps or hydranths (nutritive zooids); 2. One individual possesses the power to reproduce the other by asexual reproduction, which again by sexual reproduction gives rise to the next generation. The contractions of the muscular system, particularly of circular fibres produce pulsation of the bell. Medusa swims freely on the surface water. 5. Our mission is to provide an online platform to help students to discuss anything and everything about Zoology. Metagenesis-in life cycle of Obeliasp. 3. Disclaimer Copyright, Share Your Knowledge At the base of the polyp, it forms ring-like horizontal shelf at which the polyp rests. A distinct alternation of generations or metagenesis is present in the life history of Obelia. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. by repeated budding of the hydrula. 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. At the proximal end, the zooid is con­tinuous with the coenosare. 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. Zoology, Invertebrate Zoology, Phylum Coelenterata, Colonial Hydrozoans, Obelia. The gastrula elongates and. Development and formation of scyphistoma from planula larva: The planula will metamorphose into a small polyp or hydra tuba which … The ocelli are typically located on the side of the tentacular bulbs. After the eggs of the female jellyfish are fertilized by the male's sperm, they undergo the embryonic development typical of all animals. 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. Specimen study of hydra, obelia, aurelia, sea-anemone, gargomia, scyphytoms - Duration: 17:25. 1. The muscles coordinate the snake-like swimming movements of the medusa. The zooid is barrel-shaped, partially enclosed by a cup-shaped hydro theca, a continuation of the perisarc. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed. 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 tentacles and the hypostome bear cnidocytes. Each polyp is very much like a miniature Hydra. 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. Sometimes, the sperms are carried into the female medusae with water currents and there they fertilize the eggs in situ. The gonotheca opens at its distal end by a gonopore, through which the medusae escape. 8. 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. Welcome to NotesOnZoology.com! In the sub-umbrella, the muscle processes of the ectoderm are so large in proportion to the epithelial part that they almost form muscles only. They are dioecious—testes and ovaries are borne by separate individuals. The nervous system of medusa is more highly specialised than that of the polyp. 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. The statocysts act as organs of equilibrium. Cnidocytes are formed continu­ously in the bulb and migrate to the tentacles. The muscle processes form a striated circular muscle and some radial muscles in the sub-umbrella, they bring about locomotory movements. its outer layer of ectoderm cells becomes ciliated, and now it is called planula. 1. Obelia is characterised by A. Ciliated free swimming planula larva B. Metagenesis ltBrgt C. Acraspedote medusa D. statocysts present in medusoid stage. The features of similarity between them are listed below: 2 the muscle,... Embryo in urochordates mass the endoderm of tentacles fibres, the enteron lies in shape made of two cell enclosing... 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