The nervous system of medusa is more highly specialised than that of the polyp. Obelia. Metagenesis-in life cycle of Obeliasp. 11. Pulsation will continue in the bell as long as any portion of the ring is intact. 8. Log in Sign up. The two layers of ectoderm which enclose the bell rudiment from outside now break leaving a marginal and circular shelf called velum. 1. Obelia life cycle. Thus, it is rather difficult to distinguish between sexual and asexual generations. The vertical branches towards the base are further branched and all the branches end in zooids. What is the significance of transpiration? The planula after a short free-swimming existence settles on some solid object by its broader end. Which part of the male reproductive system store the sperm? Formation of scyphistoma: The ciliated planula larva after a short free-swimming existence attaches itself to a substratum by its aboral end. 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. 2. 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 size is large, often called jelly fish, and is formed by metamorphosis of ephyra, which is produced by strobilation of scyphistoma. Such an alternation of generations between two diploid phases is known as metagenesis. The cavity of the vesicle is continuous with the enteron of blastostyle. 12. The tentacles have swollen bases due to the accumulation of interstitial cells which are practically absent from other regions. 8. The nervous system shows differentiation into two nerve rings constituting the central nervous system and nerve nets forming the peripheral nervous system. Obelia longissima is a colonial species of hydrozoan in the order Leptomedusae. 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. 3. 2. However, the parent medusae die soon after liberating their respective gametes. Therefore, a true alternation of generations is always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant. The blastosyle is long, cylindrical and devoid of mouth and tentacles and enclosed in a transparent gonotheca, modified perisarce (Figs. But, in Obelia the condition is somewhat different and, therefore, objections were raised to use the term alternation of generations for it. The mouth leads into an enteric cavity or gastric cavity in the manubrium. The cavity would remain only along four meridional areas, the radial canals and as a circular area the circular canal close to the edge of the bell. The term alternation of generations means that the individual exists in two distinct forms, which alternate each other regularly in the life history. Is this an example of necrosis or apoptosis? 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. The main stalky body of the colony is composed of a coenosarc, which is covered by a protective perisarc. 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. 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 animal may then respond by muscular contractions to bring itself back into a horizontal position. The enteron has a fluid and its lining is flagellated. Statocysts are located between the tentacles or associated with the tentacular bulb at the tentacle base. B. Planula is the larva of Obelia which is a coelenterate. The gonads mature after the medusae escape from the gonotheca. 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 result of this to be a medusa-like body with a double wall to the entire bell, the narrow space between the two layers containing a prolongation of coelenteron and being lined with gastro dermis. In the endoderm layer are nerve cells and club-shaped gland cells which produce digestive enzymes. The larva now closely resembles a simple polyp and is called hydrula. Specimen study of hydra, obelia, aurelia, sea-anemone, gargomia, scyphytoms - Duration: 17:25. its outer layer of ectoderm cells becomes ciliated, and now it is called planula. The tentacles are longer than hypostome, tapering and filiform. The zooid is barrel-shaped, partially enclosed by a cup-shaped hydro theca, a continuation of the perisarc. A square mouth is present, at the apex of the manubrium. 4. In this way a medusa is derived completely from a polyp (Fig. The entire colony is covered by a tough, yellow chitin secreted by the ectoderm, this covering is known as perisarc. This stage of a scyphozoan polyp is known as scyphistoma larva, and it is about 1.25 cm in length. The marginal sense organs present at the bases of 8 tentacles are of special advantage to the free swimming habit of the medusa. The gonangium (Gr., gonos = seed; angeion = vessel) (Fig. The hydrorhiza and hydrocaulus are hollow tubes. Individual hydranthan in the mature colony is known as a gastrozooid, which has a medusa bud. The settlement sites may be selected because of a suitable surface film on the substrate. The planula larva has an elongated and ovoid appearance. Like polyp, the medusa is radially symmetrical. It includes three types of individuals: 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. Answer (3) Obelia (sea fur) belong to class hydrozoa, phylum cnidaria. STUDY. It is carnivorous and feeds upon aquatic crustaceans, nematodes and other worms. The outer layer of the body is composed of ciliated ectodermal cells and the inner layer is made of endodermal cells. It is the slide of obelia, which is a marine, colonial and arborescent animal. The lower ring is the centre of rhythmic pulsations, i.e., it contains the pacemakers. ... Planula. Near the bulbs the ectoderm has pigment granules and nerve cells called ocelli. 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. Ciliated free swimming larva of obelia. 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. So, the correct answer is 'Parenchymula'. After that, the medusa stage sexually reproduces through the production of sperms and eggs. Obelia is a permanently fixed colony but the planula larvae it produces are free swimming. 6. Tentacles develop around the mouth. 5. A. Tadpole is the larval stage of frogs. Each polyp is very much like a miniature Hydra. 4. 1. The tentacles have swollen bases of accumulated interstitial cells, called vesicles or bulbs. 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. Life cycle that exhibits asexual, then sexual, then asexual forms. It is found attached to … Thus, an asexual polypoid generation alternates with a sexual medusoid generation. The gelatinous mesoglea form the main bulk of the body and contains certain non-cellular fibres. Towards the base of the hydrocaulus in the axils of the polyps, are reproductive polyps called blastostyles. 6. c. Medusae buds. TOS4. There are now two layers of ectoderm outside the bell rudiment and one layer on the inner side. Obelia has a worldwide distribution except the high-arctic and Antarctic seas. The mode of sexual reproduction provides wide dispersal of the species due to its free swimming habit. The tentacles of polyps are solid with no enteron. Velum is characteristic of hydrozoan medusae but it is insignificant in Obelia. 5. Alternation of Generations and Metagenesis of Obelia: The best answers are voted up and rise to the top. The perisarc enclosing the polyp is termed hydro theca. Other hydranths are specialized for defence. The epidermis resembles the epithelium of higher Metazoa forming a thin, protective and sensitive layer of small cells. The endoderm lines the enteric cavity and the radial and circular canals. 3. After the attachment the cilia of the larva are lost and a mouth opens at the oral end. They are strictly homologous or typically similar structures. The planula swims with cilia to attach to a substrate and later developed to mature polyp. At the growing ends of the main branches are immature club-shaped polyps. 3. The medusae die soon after liberating the gametes. A cylindrical projection, the manubrium, hangs from the middle of the subumbrellar surface. 18. This will also help you to draw the structure and diagram of obelia. The ocelli consist of patches of pigment and photoreceptor cells organised either within a flat disc or a pit. The ectoderm covers the bell on all sides, its epitheliomuscular cells are produced into muscle processes which run longitudinally in the manubrium and tentacles. Metamorphosis of the larvae takes place and new colonies are formed. These are animals which belong to the class of Hydrozoa and Phylum Cnidaria and consist of various species. 1. Below the hypostome is the stomach region of the polyp. The manubrium acquires a mouth, marginal tentacles are formed, the stalk breaks and its aperture closes up, thus, a medusa is formed which is set free, it escapes from the gonotheca, later its gonads mature. Ch-09 Life Sciences, Botany, Zoology, Bio-Science 3,387 views Each branch consists of a granular coenosarc made of two cell layers enclosing the coelenteron and surrounded by a thin transparent horny perisarc. 8. Barrel-shaped and responsible for the nutrition of the colony. Some cells migrate into blastocoele, eventually filling it completely to form a solid gastrula known as stereo gastrula. But the medusae buds, some of the zooids of the colony, develop gonads and, from their fertilized egg, new Obelia colony arises. When fertilized, an egg develops into a simple non-feeding planula larva. The life history may be represented as male and female gametes → zygote → planula larva → hydrula → colony → sexual medusae → gametes → zygote and so on. TOS4. The statocysts act as organs of equilibrium. The distal end expands and by developing a manubrium and a circlet of tentacles, it turns to a hydrula or simple polyp. Share Your Word File
The gastrula elongates and. 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. Mesogloea is a thin jelly-like substance with no structure or cells. The basal swellings of tentacles are called vesicles or bulbs, nematocysts are formed continuously in the bulbs from where they migrate to the tentacles. 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. 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. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed. PLAY. Answer Now and help others. Soon, a cavity called enteron develops in the solid endodermal cell mass by the process of delamination and the planula becomes a two-layered larva having an outer ciliated ectodermal cells and an inner layer of endodermal cells. 6. The endoderm cells have no muscle processes, they are ciliated epithelial cells, they are digestive. D. Parenchymula. ONLINE BIOLOGY LESSONS IN BOTANY,ZOOLOGY,ZOOGEOGRAPHY, PHYSIOLOGY, AND MEDICINE It is a branched, fixed colony (Fig 20.12). Development and formation of scyphistoma from planula larva: The planula will metamorphose into a small polyp or hydra tuba which … The presence of the four radial canals distinguishes the four principal radii or per-radii. At the distal end a conical projection the hypostome or- manubrium, bearing a mouth is present (20.13). Fibres also interconnect the two rings. 5. Before sharing your knowledge on this site, please read the following pages: 1. The medusae are modified zooids capable of free swimming existence and moreover they are not produced directly from a zygote but are budded off from the blastostyle. 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. One individual possesses the power to reproduce the other by asexual reproduction, which again by sexual reproduction gives rise to the next generation. The colony of Obelia has many polyps (Gr., polypus – many-footed) or hydranths (Gr., hydra = water serpent; anthos = flower) or gastro zooids. 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 Obelia colony is sexless, bears no gonads and develops by asexual process, i.e. The swimming movement of the medusa is dependent base on these pulsations and is largely vertical in lamella direction. A gonad has an outer ectoderm and inner endoderm with mesogloea between the two layers. Halfway between any two per-radii a radius of the second order or inter-radius may be taken. The bell margin is richly supplied with sensory cells and also contains two types of true sense organs, viz., light sensitive ocelli and statocysts. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. Each polyp has a stem and a terminal head called a hydranth. 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. These are radial canals, opening into a ring canal or circular canal running in the margin of the bell. Share Your PDF File
In most hydrozoan medusae, the velum grows and becomes prominent, but in Obelia it decreases and becomes insignificant. 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 gastro dermal cells lack contractile extensions, and the muscular system is, thus, restricted to the epidermal layer. 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 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. The mouth leads to a small, central, rounded or quadrangular chamber known as gastric cavity or stomach. Both are diploblastic with outer epidermis (ectodermal) and inner gastro dermis (endodermal). By invagination the blastula is converted into an oval, ciliated planula larva. 6. C. Placula. The cleavage is holoblastic and a blastula is formed (Fig. The gonad has a small diverticulum of the radial canal. Why do lymph nodes often swell and become tender or even painful when you are sick? 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. The gonads mature after the medusae escape from the gonotheca. The planula consists of an outer layer of ciliated ectoderm and an inner mass of endoderm cells enclosing a … 2 words related to planula: hydroid, hydrozoan. 3. The apex of the hypostome bears a terminal aperture called mouth which is capable of great dilation and contraction. 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. A hydro-medusa (Fig.20.14) is a solitary and free-swimming modified zooid. Search. Seen here is a picture of the medusa stage in the Obelia life cycle. Polyp is the nutritive zooid of the colony. The proximal end gradually narrows down and a disc appears for attachment. 7. The radial canals mark out four principal radii. After the eggs of the female jellyfish are fertilized by the male's sperm, they undergo the embryonic development typical of all animals. 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. 1. At the base of the polyp, it forms ring-like horizontal shelf at which the polyp rests. Welcome to BiologyDiscussion! Between the two ectoderm layers of the bell is a thin sheet of endoderm lamella except where the enteron lies. 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. 3. The muscles coordinate the snake-like swimming movements of the medusa. Planula Larvae . The medusae with a velum are known as craspedote and those without a velum are acraspedote. The endoderm of tentacles has cubical, vacuolated cells with thick walls. The edge of the bell is produced inwards as a thin fold called velum. 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. Nutritive polyp. One medusa bears either four testes or four ovaries close to the four radial canals. The nerve rings connect with fibres innervating the tentacles, the musculature, and the sense organs. The medusa produces eggs and sperm. The life history may be represented as male and female gametes → zygote → planula larva → hydrula → colony → sexual medusae → gametes → zygote and so on. From hydrorhiza arises a vertical branching stem about 2.5 cm long which is known as a hydrocaulus. Rhythmical contractions of the hydranths cause a current which distributes food obtained by some polyps to those parts of the colony where feeding is not taking place. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. The tentacles are solid, each containing a core of vacuolated endodermal cells covered by ectoderm. 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 body and manubrium of the polyp enclose a spacious enteric cavity or gastro vascular cavity. The asexual generation is dependent on, and is alternated by the sexual generation. Polyps or hydranths (nutritive zooids); 2. 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. Umbrella-like reproductive zooids bearing gonads, enclosed in a gonotheca. Digestive enzymes are secreted from the endoderm of bulbs. The polyp colony reproduces asexually. Planula. 11. The larvae are ciliated and free-swimming. They are confined to manubrium and tentacles and a few on the bell margin. The medusae are sexual reproductive zooids or gonozooids possessing gonads. Submerged substratum by one end, eventually filling it completely to form a single-layered blastula with a velum acraspedote..., nematodes and other worms four gastro dermal canals radiate to the four principal radii or per-radii since sexes separate! 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