Like polyp, the medusa is radially symmetrical. The tentacles are longer than hypostome, tapering and filiform. 2. The cleavage is holoblastic and a blastula is formed (Fig. It has long, granular epitheliomuscular cells, their muscle processes point outwards and are circular. A hydro-medusa (Fig.20.14) is a solitary and free-swimming modified zooid. Antonyms for planula. ONLINE BIOLOGY LESSONS IN BOTANY,ZOOLOGY,ZOOGEOGRAPHY, PHYSIOLOGY, AND MEDICINE 1. 6. 8. The outer, exumbrellar surface of the medusa is homologous with the base of the polyp providing attachment with the parental colony. 14, 20.15). The bud grows larger and its coenosarc becomes like a vesicle which is attached to the blastostyle by a narrow stalk. 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. 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. 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. The body and manubrium of the polyp enclose a spacious enteric cavity or gastro vascular cavity. In this article we will discuss about the life cycle of obelia, explained with the help of suitable diagrams. 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. The muscular system of medusa is somewhat more specialised than in the polyp. Obelia is a permanently fixed colony but the planula larvae it produces are free swim­ming. The muscles coordinate the snake-like swimming movements of the medusa. 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. In obelia colony which type of polyp is formed first? At the growing ends of the main branches are immature club-shaped polyps. 20.16). 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. Fibres also interconnect the two rings. But, in Obelia the condition is somewhat different and, therefore, objections were raised to use the term alternation of generations for it. STUDY. 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 basal swellings of tentacles are called vesicles or bulbs, nematocysts are formed continuously in the bulbs from where they migrate to the tentacles. 4. 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. The tentacles of polyps are solid with no enteron. Each medusa bears only four gonads situated on the subumbrellar surface, one on the middle of the course of each radial canal. Club- shaped zooids, bearing the medusae buds. Share Your PDF File The hydrula sends out lateral buds and, by a repetition of this process, it is converted into a complex obelia colony. From gastric cavity four gastro dermal canals radiate to the margins of the bell. 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. ... and the sperm and egg produced,fertilized to produce the zygote immature larva called Planula. The edge of the bell is produced inwards as a thin fold called velum. 3. Near the bulbs the ectoderm has pig­ment granules and nerve cells called ocelli. The nervous system of medusa is more highly specialised than that of the polyp. 8. There are now two layers of ectoderm outside the bell rudiment and one layer on the inner side. The food consists of living animals or bodies of animals. What are synonyms for planula? Algae in Koi Ponds, Learn this lesson & have a clear water garden for life! This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. So, the correct answer is 'Parenchymula'. 5. The apex of the hypostome bears a terminal aperture called mouth which is capable of great dilation and contraction. The life history may be represented as male and female gametes → zygote → planula larva → hydrula → colony → sexual medusae → gametes → zygote and so on. 1. The apex of the umbrella of medusa corresponds with the base of a hydranth. By invagination the blastula is con­verted into an oval, ciliated planula larva. 6. 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. 7. The medusae are dioecious, they have either four testes or four ovaries in the sub-umbrella just below the radial canals. 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. 7. 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. Sub-radii are of no importance in this particular form. Otolith produces a stimulus on thin sensory processes of the sensory tells of ectodermal lining which are transmitted to the muscles. or gonozooids. Seen here is a picture of the medusa stage in the Obelia life cycle. This will also help you to draw the structure and diagram of obelia. The ocelli are typically located on the side of the tentacular bulbs. 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. Furthermore, the muscular system is best developed around the bell margin and sub umbrella surface where the fibres form a radial and circular system. Our mission is to provide an online platform to help students to discuss anything and everything about Zoology. 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. Digestion is extracellular as well as intracellular in both the cases. 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. Well-developed musculature with regu­larly arranged circular, longitudinal and ra­dial tracts is present. They have a single- layered core of vacuolated endoderm cells with thick walls inside a layer of ectoderm. 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 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 cavity of the blastostyle pushes the coenosarc out to form a small protuberance or bud. Ultimately, the fertilized egg gives rise to a zygote, a blastula, and then to a ciliated, swimming larva known as a planula. Animals, Biology, Diversity, Obelia, Phylum Cnidaria, Zoology. 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 shows polymorphism in which the polyps are for feeding the colony, blastostyles for budding and medusae for disseminating gametes. 13. Soon the solid endoderm splits and develops a cavity, the enteron. 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. The medusa is circular and tiny umbrella-like in shape. The planktonic planula metamorphoses into a benthic polyp (hydroid), which buds off further polyps and forms a colony. The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. Polyps or hydranths (nutritive zooids); 2. The pigments are accumulated excretory products. The mode of sexual reproduction provides wide dispersal of the species due to its free swimming habit. 7. 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 endoderm cells have no muscle processes, they are ciliated epithelial cells, they are digestive. The nervous system shows differentiation into two nerve rings constituting the central nervous system and nerve nets forming the peripheral nervous system. 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