A number of small buds—the medusae buds develop on the walls of the blastostyle. 7. 3. The asexual generation is dependent on, and is alternated by the sexual generation.
13. Then examine the Obelia medusa w.m. A square mouth is present, at the apex of the manubrium. A circlet of about twenty-four tentacles are present around the hypos tame. The polyps can alternate between reproductive phases and feeding phases for up to twenty-five years (2). i. These are a few in number and restricted to the basal end of the hydrocaulus. The proximal end gradually narrows down and a disc appears for attachment. Description: The Obelia dichotoma medusa is a small hydromedusa with the bell diameter greater than its height. 5. The medusae are sexual reproductive zooids or gonozooids possessing gonads. What capsule do these cells contain? 1. 2. Get an Obelia medusa slide - there are two small specimens on each slide. 2. The whole canal system is lined by endoderm cells, continued, at the lip of the mouth, into the surface ectoderm. 14, 20.15) 6. Since sexes are separate, these are either testes or ovaries. The tentacles do not contain prominent rings of nematocysts. Umbrella-like reproductive zooids bearing gonads, enclosed in a gonotheca. From the edge of bell numerous small solid tentacles hang downwards. These are radial canals, opening into a ring canal or circular canal running in the margin of the bell. 5. 7. 3. 3. slide preparation to identify both feeding and reproductive polyps. are found only in the epidermis and germ cells arise in the gastrodermis but
Each branch consists of a granular coenosarc made of two cell layers enclosing the coelenteron and surrounded by a thin transparent horny perisarc. By invagination the blastula is converted into an oval, ciliated planula larva.
14. How do these cells help cnidarians feed? 6. The medusae with a velum are known as craspedote and those without a velum are acraspedote.
There are several species ofObelia and any can be used for this exercise. They are confined to manubrium and tentacles and a few on the bell margin. 4. 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. A hydro-medusa (Fig.20.14) is a solitary and free-swimming modified zooid. 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.
Examine the Obelia hydroid colony w.m. The Medusa is umbrella shaped. It has no ocelli. More than 16 unbranched tentacles, all about the same size, are distributed evenly along the margin of the bell. Barrel-shaped and responsible for the nutrition of the colony. by repeated budding of the hydrula. 6. 1. 2004. 2. The gonads mature after the medusae escape from the gonotheca. 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. Gonads may be on the manubrium or on the radial canals.
The medusa stage of the jellyfish reproduce sexually. The perisarc enclosing the polyp is termed hydro theca. The hydromedusa is craspedote. 2.
The mouth leads to a small, central, rounded or quadrangular chamber known as gastric cavity or stomach. The manubrium contains a mouth as its tip, … From gastric cavity four gastro dermal canals radiate to the margins of the bell. The pigments are accumulated excretory products. 20. Our mission is to provide an online platform to help students to discuss anything and everything about Zoology. 2. 3. Cnidarians - Obelia Medusa Stage.
5. 11. slide preparation. The zooid is barrel-shaped, partially enclosed by a cup-shaped hydro theca, a continuation of the perisarc. a. Polyps or gastro zooids (vegetative zooids). 20.12, 20.16). Identify some nematocysts and draw them. The tentacles have swollen bases of accumulated interstitial cells, called vesicles or bulbs. Subsequently, they migrate to the gonads to lie between the mesoglea and the subumbrellar ectoderm and undergo maturation. Each medusa bears only four gonads situated on the subumbrellar surface, one on the middle of the course of each radial canal. 1. Invertebrate Zoology, A functional evolutionary approach, 7 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. b. Blastostyles. 3. A statocyst is a tiny, circular, closed vesicle lined with ectoderm and filled with a fluid containing calcareous granules called otoliths. 7.
Before publishing your Notes on this site, please read the following pages:Zoology Notes | Exclusive Notes on Zoology for Students 8. When the gonads are ripe, ectodermal covering ruptures and the germ cells are shed in water. From your observations draw a detailed labeled diagram that illustrates the life cycle of Obelia. 1. These are medusae, tiny jellyfish (larger jellyfish may or may not be a medusa stage). 5. The medusae die soon after liberating the gametes. The cleavage is holoblastic and a blastula is formed (Fig. Medusae buds are unisexual and free- living at maturity. 12. The tentacles and the hypostome bear cnidocytes. Once the ciliated larvae develop from the egg, they settle on or near the sea floor and develop into benthic polyps. The gonads mature after the medusae escape from the gonotheca. The tentacles are solid with a core of endoderm cells surrounded by a layer of ectodermal cells. Gametes, even if gastrodermal, are released directly to the surrounding water, never, in
Club- shaped zooids, bearing the medusae buds. Near the bulbs the ectoderm has pigment granules and nerve cells called ocelli.
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