Velum is characteristic of hydrozoan medusae but it is insignificant in Obelia. The medusae are dioecious, they have either four testes or four ovaries in the sub-umbrella just below the radial canals. The nerve rings connect with fibres innervating the tentacles, the musculature, and the sense organs. Halfway between any two per-radii a radius of the second order or inter-radius may be taken. The blastostyle has no mouth and no tentacles, but ends distally into a flattened disc. a. Polyps or gastro zooids (vegetative zoo­ids). Synonyms for planula in Free Thesaurus. Planula. Between the two ectoderm layers of the bell is a thin sheet of endoderm lamella except where the enteron lies. However, the features of similarity between them are listed below: 2. Log in Sign up. The ocelli are typically located on the side of the tentacular bulbs. 4. The planula swims with cilia to attach to a substrate and later developed to mature polyp. 3. 7. 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. Planula Larva. Some of the horizontal branches anchoring the colony on some support are called Hydrorhiza while other branches are vertical and known as Hydrocaulus. The hydranths are feeding polyps, they feed by capturing minute animals and larvae. 10. From such a form the actual condition of things found in the medusa would be produced by the continuous cavity in the bell being for most part obliterated by the growing together of its walls so as to form the endodermal lamella. 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. 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 tentacles have swollen bases due to the accumulation of interstitial cells which are practically absent from other regions. 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 free end forms a manubrium with a mouth and a circlet of tentacles. Zygote formed after fertilization, immediately undergoes cleavage. But the medusae buds, some of the zooids of the colony, develop gonads and, from their fer­tilized egg, new Obelia colony arises. 2. 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. Animals, Biology, Diversity, Obelia, Phylum Cnidaria, Zoology. Mouth is absent. Each polyp is very much like a miniature Hydra. 2. 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. Therefore, a true alternation of generations is always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant. 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. A square mouth is present, at the apex of the manubrium. These are radial canals, opening into a ring canal or circular canal running in the margin of the bell. The tentacles have swollen bases of accumulated interstitial cells, called vesicles or bulbs. Formation of scyphistoma: The ciliated planula larva after a short free-swimming existence attaches itself to a substratum by its aboral end. 2. 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. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. Answer (3) Obelia (sea fur) belong to class hydrozoa, phylum cnidaria. However, the parent medusae die soon after liberating their respective gametes. 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. The zygote undergoes complete or holoblastic and equal cleavage to form a single-layered blastula with a blastocoele. Planula finally grows into a simple hydrula stage which by repeated bud­ding, produces obelia colony. The planktonic planula metamorphoses into a benthic polyp (hydroid), which buds off further polyps and forms a colony. Which part of the male reproductive system store the sperm? In the endoderm layer are nerve cells and club-shaped gland cells which produce digestive enzymes. The gonotheca opens at its distal end by a gonopore, through which the medusae escape. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed. 3. 7. Some of the epitheliomuscular cells of the velum have their contractile extensions oriented to form a powerful circular band of fibres which are striated. Thus, it is rather difficult to distinguish between sexual and asexual generations. The life history may be represented as male and female gametes → zygote → planula larva → hydrula → colony → sexual medusae → gametes → zygote and so on. 8. One medusa bears either four testes or four ovaries close to the four radial canals. 2. The velum has a double layer of ectoderm and the thick mesogloea in the middle, it has no endoderm. Between the ectoderm and endoderm is thick mesogloea forming the bulk of the bell of the medusa, manubrium and tentacles. This is a special mode of asexual reproduction. The larvae are ciliated and free-swim­ming. From the middle of the radial canal four gonads project. Our mission is to provide an online platform to help students to discuss anything and everything about Zoology. The Obelia is a trimorphic colony, that is, having three kinds of zooids which are as follows: 1. The cavity of the bell rudiment assumes the shape of the sub-umbrella, and a manubrium is formed in the centre. In the ectoderm layer are very few interstitial cells, some branching nerve cells and cnidoblasts with nematocysts. Nematocysts are confined to the manubrium and tentacles, there may be some on the bell margin. Nutritive polyp. 20. 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