Q.4. Give salient features of life-history of Obelia and add a note on metagenesis. (2016)
Other Related Questions -
Q. Write short note on metagenesis. (2017)
Q. Write short note on Metagenesis in Obelia. (2019)
Ans. Alternation of generations may be defined as a phenomenon whereby, in the life-history of an organism, a diploid asexual phase and a haploid sexual phase regularly alternates with each other.
In Obelia, life-cycle includes two clearly defined phases : a fixed polypoid phase (hydroid colony) and a pelagic medusoid phase. Hydroid colony has no gonads and reproduces by asexual budding to give rise to medusae. On the other hand, medusae reproduce exclusively by sexual method (ova and sperms) to give rise to new hydroid colonies. This fact apparently seems to have given rise to the idea of alternation of generations, also called metagenesis, in coelenterates, in which an asexual polypoid generation appears to alternate regularly with a sexual medusoid generation.
But, in Obelia, medusoid phase does not represent a true haploid sexual generation, because: (1) Medusa arises from blastostyle (diploid) by a process of asexual budding. It implies that medusa too is a diploid zooid. (ii) Sex cells do not originate in medusa, but in the epidermis of blastostyle, from where they migrate into gonads of medusa. These facts show that medusa does not represent a sexual generation. It is simply a free-swimming diploid zooid specialized for dispersal of gametes of the sedentary hydroid colony. In fact, the so-called sexual generation in Obelia is indistinct and represented by haploid gametes only.
Thus, it is clearly impossible to differentiate between sexual and asexual generations in Obelia. Asexual hydroid colony and sexual medusa merely represent different phases or zooids, an example of polymorphism, and belong to a single diploid generation, so that a true alternation of generations can not be said to occur in Ohelia.
In Coelenterates (obelia), a regular alternation between fixed asexual hydroid and free- swimming medusoid phases, both of which are diploid, has been termed metagenesis.
Q.5. Write short note on morphology of Aurelia. (2007)
Other Related Questions -
Q. Write short note on structure of Aurelia. (2011)
Q. Write short note on Aurelia. (2015)
Ans. Morphology of Aurelia: -
In general Aurelia is like a large version of the medusa of obelia.It is easily recognized by its soft bell or umbrella shaped body with four red or purple horse shoe shaped gonads on its upper surface and four long and narrow oral lobes hanging downwards from lower surface.Its circular body measures about 90 mm in diameter and presents a convex aboral and a concave oral surface.
Colour: -
Body is perfectly transparent and bluish white in colour.The reddish or pinkish gonads are clearly visible from the body surface.
Manubrium, Mouth and Oral Arms: -
From the center of subumbrellar or convex surface hangs down a very short and inconspicuous maubrium. At its free distal end is a square mouth from each corner of which hangs down a long tapering much frilled and delicate process the oral arm.Each of the four oral arms has a ventral ciliated groove leading into mouth and its edges are armed with nematocytes.The radii along which angles of mouth and oral arms lie are referred to as perradii.Midway between two adjacent perradii is an interradii and between each peradius and its adjacent interradius on either side is an adaradius.
Subgenital Pits: -
On each interradius a little distance from mouth the subumbrellar surface bears a circular aperture .It leads into a small shallow cavity the subgenital pit lying immediately beneath a gonad and of uncertain function.
Gonads: -
Just above each subgenital pit within umbrella is a horse shoe shaped and frilled gonad red or purple in colour.Free arms of all the four gonads are directed towards the centre of umbrella.There is no connection between gonads and subgenital pits.
Lappets and Tentaculocysts: -
The circular margin of umbrella or bell is broken into 8 lobes by 8 notches 4 of which are perradial and the other 4 interradial.In eachnotch there are two delicate leaf like processes called the marginal lappets .Between lappets lies a small sensory organ the tentaculocyst or rhopalium.
Marginal Tentacles: -
Between notches the free edge of umbrella is beset closely with a row of numerous small delicate and hollow threads or marginal tentacles.The tentacles bear batteries of stinging cells or nematocysts.
Velarium: -
Margin of subumbrellar surface bearing lappets forms a thin and flexible flaps called velarium or pseudovelum.It differs from true velum of Obelia in having gastrodermal canals running into it.Such a medusa with a pseudovelum is called acraspedote medusa while a medusa with a true velum is called craspedote medusa.
Q.6. Write short note on Ephyra larva. (2017)
Ans. In autumn and winter, scyphistoma undergoes a remarkable process of budding or transverse fission of oral end, called strobilation. Distally, body develops a series of ring-like transverse contrictions of furrows which gradually deepens so that the organism resembles a pile of minute saucers or discs, placed one above the other. At this stage, scyphistoma with a segmented body is called a strobila and each of the segments is called an ephyra larva. About a dozen ephyrae are formed in a single strobilation. the basal segmented part of scyhphitoma grows new tentacles and continues to live as a polyp or hydratuba.It may live for several years, feeding, growing and multiplying by budding in summers, but producing ephyrae by strobilations in winters.
Q.7. Write shorte note on Coral reef and its importance. (2005, 09, 12, 14)
Ans. Coral Reefs: -
Coral reefs are aragonite structures produced by living organisms, found in marine waters with little to no nutrients in the water. High nutrient levels such as those found in runoff from agricultural areas can harm the reef by encouraging the growth of algae.
In most reefs, the predominant organisms are colonial cnidarians that secrete an exoskeleton of calcium carbonate. The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life.
Formations: -
Coral reefs can take a variety of forms, defined in following:
Fringing Reef: -
A reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
Barrier Reef: -
A reef separated from a mainland or island shore by a deep lagoon
Patch Reef: -
An isolated, often circular reef, usually within a lagoon or embayment.
Apron Reef: -
A short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
Bank Reef: -
A linear or semi-circular shaped-outline, larger than a patch reef.
Ribbon Reef: -
A long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
Atoll Reef: -
A more or less circular or continuous barrier reef extending all the way around a lagoon without a central island;
Table Reef: -
An isolated reef, approaching an atoll type, but without a lagoon.
