B. Planula is the larva of Obelia which is a coelenterate. What are synonyms for planula? 1. A cylindrical projection, the manu­brium, hangs from the middle of the subumbrellar surface. Obelia has a worldwide distribution except the high-arctic and Antarctic seas. The whole canal system is lined by endoderm cells, continued, at the lip of the mouth, into the surface ectoderm. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. During this stage of life, Obelia are confined to substrate surfaces. 3. Before publishing your Notes on this site, please read the following pages: 1. Both are carnivorous; the food is captured and ingested with the help of tentacles. After the attachment the cilia of the larva are lost and a mouth opens at the oral end. The blastosyle is long, cylindrical and devoid of mouth and tentacles and enclosed in a transparent gonotheca, modified perisarce (Figs. The gonangium (Gr., gonos = seed; angeion = vessel) (Fig. TOS4. Obelia is a permanently fixed colony but the planula larvae it produces are free swim­ming. 4. The convex outer surface is known as the ex-umbrella and the concave inner surface is the sub-umbrella. Planula finally grows into a simple hydrula stage which by repeated bud­ding, produces obelia colony. Individual hydranthan in the mature colony is known as a gastrozooid, which has a medusa bud. Ultimately, the fertilized egg gives rise to a zygote, a blastula, and then to a ciliated, swimming larva known as a planula. Halfway between any two per-radii a radius of the second order or inter-radius may be taken. 3. The basic pattern of the planula larva is same as in Obelia, except that the blastopore is not completely closed and is represented by a mere slit. Obelia longissima is a colonial species of hydrozoan in the order Leptomedusae. 32.3) is club-shaped, cylindrical form. Algae in Koi Ponds, Learn this lesson & have a clear water garden for life! Endoderm cells have flagella which produce a current in the enteron. Gonotheca, blastostyle and the gonophores together form a gonangium. 2. Answer. 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. Privacy Policy3. They have a single- layered core of vacuolated endoderm cells with thick walls inside a layer of ectoderm. Thus, it is rather difficult to distinguish between sexual and asexual generations. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. The ocelli consist of patches of pigment and photoreceptor cells organised either within a flat disc or a pit. The perisarc enclos­ing the polyp is termed hydro theca. Therefore, a true alternation of generations is always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant. The free end forms a manubrium with a mouth and a circlet of tentacles. c. Medusae buds. From the edge of the bell numerous small solid tentacles hang downwards. Privacy Policy3. There are now two layers of ectoderm outside the bell rudiment and one layer on the inner side. In the endoderm layer are nerve cells and club-shaped gland cells which produce digestive enzymes. The planktonic planula metamorphoses into a benthic polyp (hydroid), which buds off further polyps and forms a colony. 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. The stomach, radial canals and circular canal of medusa are homologous with the gastro-vascular cavity of the polyp. The zooid is barrel-shaped, partially enclosed by a cup-shaped hydro theca, a continuation of the perisarc. Obelia is a classic example of alternation of generations in a hydrozoan life cycle. From gastric cavity four gastro dermal canals radiate to the margins of the bell. ADVERTISEMENTS: In this article we will discuss about the structure of obelia. 6. 8. 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. Furthermore, the muscular system is best developed around the bell margin and sub umbrella surface where the fibres form a radial and circular system. A hydro-medusa (Fig.20.14) is a solitary and free-swimming modified zooid. Larva of medusa stage, they are free swimming and eventually settle to the bottom and become a polyp. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. Mouth is absent. Alterations of generations. The medusa is a modified zooid produced as a hollow bud from the coenosarc of the blastostyle in spring and summer. D. Parenchymula. In the ectoderm layer are very few interstitial cells, some branching nerve cells and cnidoblasts with nematocysts. 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. 7. 13. They soon hatch, and free-swimming "planula" larvae emerge from the female's mouth or brood pouch and set out on their own. 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. Synonyms for planula in Free Thesaurus. Some of the epitheliomuscular cells of the velum have their contractile extensions oriented to form a powerful circular band of fibres which are striated. 14, 20.15). Content Guidelines 2. The edge of the bell is produced in­wards as a thin fold called velum. 10. This phenomenon is known as alternation of generations, till recently. This shallow water is up to 80 meters in depth. The tentacles of polyps are solid with no enteron. 1. However, the features of similarity between them are listed below: 2. 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. Each polyp is very much like a miniature Hydra. Planula Larva. 20.16). 11. It is clearly evident from the life history of Obelia that there is an alternation of polypoid and medusoid generations. 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. 3. 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. The mouth and manubrium are also homologous structures. Medusae are carnivorous and the processes of their nutrition are essentially the same as in the polyp. When the bell tilts, the statoliths respond to the pull of gravity and stimulate the sensory bristles to which they are attached. ADVERTISEMENTS: 1. 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. Obelia is a permanently fixed colony but the planula larvae it produces are free swim­ming. Medusa swims freely on the surface water. The proximal end gradually narrows down and a disc appears for attachment. The ectoderm of the ex-umbrella is devoid of musculature. By invagination the blastula is con­verted into an oval, ciliated planula larva. 3. Other hydranths are specialized for defence. One medusa bears either four testes or four ovaries close to the four radial canals. Figure 3. Below the hypostome is the stomach region of the polyp. The hydrocaulus bears zooids or polyps on either side in a cymose formation. It is with the lower ring that the statocysts are connected. 2. 5. At the distal end a conical projection the hypostome or- manubrium, bearing a mouth is present (20.13). 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. The hydranths are feeding polyps, they feed by capturing minute animals and larvae. But, in Obelia the condition is somewhat different and, therefore, objections were raised to use the term alternation of generations for it. The medusae are sexual reproductive zooids or gonozooids possessing gonads. The hydrorhiza and hydrocaulus are hollow tubes. It is covered by a cup-shaped hydro theca. The radial canals mark out four principal radii. 1. Fertilisation usually takes place in open sea water where the gametes are set free. Statocysts are located between the tentacles or associated with the tentacular bulb at the tentacle base. 12. C. Placula. The polypoid generation is asexual and produces polyps and blastostyles by asexual budding. 1. Thus, there are four per-radii, four inter-radii, eight ad-radii and sixteen sub-radii. Planula. The digested food is distributed to the entire medusa through the system of radial and circular canals. The marginal sense organs present at the bases of 8 tentacles are of special advantage to the free swimming habit of the medusa. Pulsation will continue in the bell as long as any portion of the ring is intact. The gastro dermal cells lack contractile extensions, and the muscular system is, thus, restricted to the epidermal layer. However, the parent medusae die soon after liberating their respective gametes. The muscles coordinate the snake-like swimming movements of the medusa. The enteron has a fluid and its lining is flagellated. Thus, a simple polyp or hydrula is formed which grows a hydrorhiza from its base, from which an Obelia colony is formed by budding. 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 sperms are minute, actively swim­ming flagellated cells. Some cells migrate into blastocoele, eventually filling it completely to form a solid gastrula known as stereo gastrula. The vertical branches towards the base are further branched and all the branches end in zooids. By repeated branching of the simple polyps colony is formed. Disclaimer Copyright, Share Your Knowledge After that, the medusa stage sexually reproduces through the production of sperms and eggs. These are a few in number and re­stricted to the basal end of the hydrocaulus. 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. 5. Why do lymph nodes often swell and become tender or even painful when you are sick? It has long, granular epitheliomuscular cells, their muscle processes point outwards and are circular. 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. From the enteric cavity, arise four radial canals which are delicate ciliated tubes, they run to the margin of the bell to join a ciliated circular canal running near the margin. The medusa stage of Obelia species are common in coastal and offshore plankton around the world. 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. Both are diploblastic with outer epidermis (ectodermal) and inner gastro dermis (endodermal). 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. In obelia colony which type of polyp is formed first? 3. Medusae buds are unisexual and free- living at maturity. In most hydrozoan medusae, the velum grows and becomes prominent, but in Obelia it decreases and becomes insig­nificant. The term alternation of generations means that the individual exists in two distinct forms, which alternate each other regularly in the life history. 8. 2. The asexual generation is dependent on, and is alternated by the sexual generation. Both of them are formed on the same pattern. 2. The main stalky body of the colony is composed of a coenosarc, which is covered by a protective perisarc. After the eggs of the female jellyfish are fertilized by the male's sperm, they undergo the embryonic development typical of all animals. TOS4. 1. 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. The planula consists of an outer layer of ciliated ectoderm and an inner mass of endo­derm cells enclosing a … 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. 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Continuation of the subumbrellar ectoderm and an inner endoderm with mesogloea between the tentacles and manubrium only in!

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