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In this article we will discuss about Aurelia:- 1. Structure of Aurelia 2. Sense Organs of Aurelia 3. Histology 4. Life History.
Structure of Aurelia:
Adult Aurelia represents medusoid stage. It has the typical form of a medusa (Fig. 12.18). But the concave and convex sides of the umbrella are more pronounced. The edge of the umbrella is provided with a number of small tentacles.
The outline of the umbrella is not smooth but is curved into eight equidistant notches within which sense organs are located. Each notch is provided with a pair of marginal lappets.
Velarium:
The true velum as seen in Obelia medusa is absent. The edge of the umbrella of the Aurelia is very delicate and flexible due to submbrellar extension called the velarium, which is analogous to the velum of Obelia and contains endodermal cavity.
Mouth:
Mouth is distinctly four-cornered and is borne at the tip of a short manubrium (Fig. 12.19B). The corners of the mouth are drawn out into four lobes, called oral arms, which are beset with batteries of nematocysts. Each oral arm is infringed with small lobules at its margin and possesses a median groove.
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Subgenital pits:
On the subumbrellar surface, between the manubrium and the bell margin and along the inter-radial planes of the body, there are four small depressions, called subgenital pits. Though they are placed immediately beneath the gonads, yet the sub-genital pits have no connection with them.
Coelenteron:
Mouth leads into a short tube (gullet) which opens into a centrally located stomach. Stomach is produced into four inter-radial gastric pouches extending up to the half-way between the centre and the edge of the umbrella. Numerous small gastric tentacles are present on the floor of the gastric pouches.
The gastric tentacles are composed of endoderm with a central core of mesoglea. Numerous nematocysts are also present in the gastric tentacles are composed of endoderm with a central core of mesoglea. Numerous nematocysts are also present in the gastric tentacles.
The function of these structures is to kill the prey if they are taken in the stomach alive. From the periphery of the stomach as well as from the gastric pouches, sixteen radial canals originate, of which four are per-radials, four inter-radials and eight ad-radials (Fig. 12.19C).
Both per- radials and inter-radials are branched but the ad-radials are un-branched. All the radial canals ultimately open into a ring or circular canal situated at the margin of the body.
Gonads:
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A medusa is unisexual and the gonads are exclusively endodermal in origin. Gonads are four in number, coloured and horse-shoe shaped bodies, situated at the floor of the gastric pouches.
Sense Organs of Aurelia:
The sense organs are known as tentaculocysts or rhopalia, which are eight in number (Fig. 12.20). Each tentaculocyst is a modified tentacle. It is kept hidden by marginal lappets and by a hood-like process on the outer side. Each tentaculocyst is a hollow tube having connection with the ring canal. At the tip of this tube there is an aggregation of calcareous particles (statocyst) derived from the endoderm.
The ectoderm on the outer side of it gives rise to an ocellus. Another endodermal ocellus associated with the statocyst is present. Two cavities are present in a tentaculocyst, one in the exumbrellar side and the other is internally placed. These cavities are designated as the olfactory pits and are lined by sensory epithelium. Tentaculucysts regulate and coordinate the movement of the umbrella.
Histology of Aurelia:
Histology of the scyphozoan medusa has the same organisation as that of hydrozoan medusa. The main difference lies in the structure of the mesoglea which contains fibres and loose amoeboid cells. The muscular system is well developed.
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The broad circular muscle band on the subumbrellar surface is very strongly built and is known as coronal muscle. Coronal muscle helps the animal to swim. The nervous system has the general arrangement of the hydrozoari medusa but the marginal nerve ring is either lacking or feebly developed.
Life-History of Aurelia:
The sexes are separate. The unisexual medusae do not exhibit any external difference in their appearance. The male medusa produces many sperms which pass out into the surrounding water through the mouth.
The sperms swim in water and find their way into the enteron of the female to fertilize the eggs produced in her ovaries. The fertilization is thus internal. The fertilized egg or zygote, through repeated divisions, attains a stage, called morula. Morula becomes spherical and fluid accumulates in the inner cavity. The outer wall of this stage (blastula) is made up of one layer of cells (Fig. 12.21).
An invagination from one end obliterates the cavity of the blastula and a new cavity is formed within the layer of cells. The new cavity opens to the exterior through a small opening. This stage is called gastrula. The gastrula transforms into a hollow two-layered planula larva.
The transformation of zygote into the Planula larva in Aurelia is peculiar. The zygotes emerge to lodge themselves on the oral arms of the female medusa. Each developing embryo forms deep spherical pit in the endoderm of gastrodermis in which each embryo develops into a ciliated Planula larva. The transformation of a zygote into a planula larva takes two days in Aurelia.
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 planula larva after a very brief period of free existence, fastens itself to the sea bottom by one pole and loses its cilia and metamorphoses into a stage called hydratuba.
Structure of Hydratuba and Scyphistoma. These two stages represent the hydroid phase of Aurelia. After fixation of the planula larva, a definite mouth is formed at the free end and it is now called the hydratuba stage. The mouth becomes square and its edge becomes raised into a short manuburium.
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The aboral portion becomes narrowed into a distinct peduncle. Around the mouth four per- radial, four inter-radial and eight ad-radial tentacles are developed. Eventually the hydratuba possesses sixteen tentacles. Endoderm forms four longitudinal inter-radial gastric ridges or taenioles.
The ectoderm between the mouth and tentacles invaginates to form four inter-radial depressions known as the septal funnels or infundibula. The hydratuba remains unchanged for a long time, feeds and produces horizontal branches from which new hydratuba buds off. Finally, the hydratuba becomes segmented by transverse furrows. The hydratuba in this segmented state is known as Scyphistoma.
Formation and structure of ephyra larva:
The scyphistoma bears a series of transverse constrictions. These constrictions gradually become deeper and as a result the body appears segmented. These segments are ultimately detached from the body column of the scyphistoma and develop into small saucer-shaped individuals or Ephyrae.
Each ephyra larva is characterised by the possession of eight long bifid arms. The concavity at the tip of the arm contains the developing tentaculocyst which is its sense organ. The centre of the sub-umbrella contains a very short and inconspicuous manubrium. The corners of the mouth are prolonged into four long and frilly oral arms.
The original gastric ridges develop into gastric filaments. The original coelenteron shows branching. The region between the arms was initially deeply notched but grows more rapidly and ultimately joins with the tip of the bifid arms to establish regular outer margin. On further metamorphosis, ephyra develops into a full-grown Aurelia.
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The process of strobilation of the hydratuba depends largely upon extrinsic factors, particularly on food supply. Sometimes the whole hydratuba may transform into a single adult medusa and under certain conditions ephyra may become again hydratuba.
Metagenesis in Aurelia:
The phenomenon of metagenesis is not so well-marked in the life-history of Aurelia (Fig. 12.22). Medusoid generation covers the major and prominent part in its life. The hydroid generation is only represented by the hydratuba and scyphistoma stages. The hydroid generation as such is quite insignificant and may be assigned to be an elaboration of the larval condition.
