Habit and Habitat of Obelia:
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. Obelia is cosmopolitan in distribution, forming a whitish or light-brown plant-like fur in the sea; hence, the common name sea-fur is assigned to it.
Obelia Colony – A Gross Structure:
Each colony of Obelia consists of a horizontal thread-like root called hydrorhiza which is attached to a weed or any substratum. From hydrorhiza arises a vertical branching stem about 2.5 cm long which is known as a hydrocaulus. The hydrorhiza and hydrocaulus are hollow tubes.
The hydrocaulus bears zooids or polyps on either side in a cymose formation. At the growing ends of the main branches are immature club-shaped polyps. Each polyp has a stem and a terminal head called a hydranth. The hydranths are feeding polyps, they feed by capturing minute animals and larvae. Towards the base of the hydrocaulus in the axils of the polyps, are reproductive polyps called blastostyles.
The polyps, their tubular connections and blastostyles are made of ectoderm, mesogloea and endoderm, these layers are together called coenosarc and its cavity is an enteron which is continuous and common to all the members, through the enteron digested food is distributed in solution.
The entire colony is covered by a tough, yellow chitin secreted by the ectoderm, this covering is known as perisarc. 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.
The hydrotheca has a shelf across the base which supports the hydranth, and the hydranth can contract and withdraw into the hydrotheca.
The perisarc around a blastostyle is a gonotheca, the blastostyle and gonotheca are together called a gonangium. The perisarc is an exoskeleton, at first it is continuous with the coenosarc but on growing thick, it separates and is joined to the coenosarc only at intervals by minute projections, at these places it gets ringed which allows bending.
The Obelia is a trimorphic colony, that is, having three kinds of zooids which are as follows:
1. Polyps or hydranths (nutritive zooids);
2. Gonangia or blastostyles (budding zooids);
3. Medusae, (sexual zooids).
In fact, to start with Obelia is a monomorphic form having polyp only but later due to the development of blastostyle it becomes a dimorphic colony and finally medusae bud over the blastostyle in a mature colony, then it becomes a trimorphic colony.
Polyp or Hydranth:
The colony of Obelia has many polyps (Gr., polypus – many-footed) or hydranths (Gr., hydra = water serpent; anthos = flower) or gastro zooids. Each polyp is very much like a miniature Hydra. It has a cylindrical body attached to the axis of the hydrocaulus by its proximal end and free at its distal end. It is covered by a cup-shaped hydro theca.
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 hypostome is surrounded by a circle of numerous (about 24) tentacles. The tentacles are longer than hypostome, tapering and filiform.
The apex of the hypostome bears a terminal aperture called mouth which is capable of great dilation and contraction.
Below the hypostome is the stomach region of the polyp. The body and manubrium of the polyp enclose a spacious enteric cavity or gastro vascular cavity. The polyp is protected in hydro theca, which is prolongation of the perisarc. At the base of the polyp, it forms ring-like horizontal shelf at which the polyp rests.
Histology of Polyp or Hydranth:
The polyps have an outer layer of ectoderm and an inner layer of endoderm, between them is a thin, transparent mesogloea; all these layers constitute the coenosarc which is soft and tubular, the continuous cavity is the enteron or gastro vascular cavity.
The enteron has a fluid and its lining is flagellated. Rhythmical contractions of the hydranths cause a current which distributes food obtained by some polyps to those parts of the colony where feeding is not taking place. The tentacles of polyps are solid with no enteron. They have a single- layered core of vacuolated endoderm cells with thick walls inside a layer of ectoderm.
It consists of long, conical columnar epitheliomuscular cells, their inner ends are produced into muscular processes which run longitudinally. In the ectoderm layer are very few interstitial cells, some branching nerve cells and cnidoblasts with nematocysts. The nematocysts are abundant on the tentacles and manubrium only.
The cnidoblasts are found in the basal part of the hydranth and in the coenosarc. They form nematocyst and migrate actively to reach their final positions. Obelia has only one kind of nematocysts called basitrichous isorhizas in which the capsule is oval, butt is absent, the thread is open at the tip and has spines on its base.
It has long, granular epitheliomuscular cells, their muscle processes point outwards and are circular. Endoderm cells have flagella which produce a current in the enteron. They can also form pseudopodia for engulfing food. The endoderm of tentacles has cubical, vacuolated cells with thick walls.
In the endoderm layer are nerve cells and club-shaped gland cells which produce digestive enzymes. Mesogloea is a thin jelly-like substance with no structure or cells. On each side of the mesogloea is a nerve net composed of nerve cells and their fibres, the two nerve nets are inter-connected.
Polyp is the nutritive zooid of the colony. It is carnivorous and feeds upon aquatic crustaceans, nematodes and other worms. Tentacles help in catching and conveying the prey to the mouth. Digestive juice is secreted in the gland cells of gastro dermis and the process of digestion is extracellular as well as intracellular.
The gonangium (Gr., gonos = seed; angeion = vessel) (Fig. 32.3) is club-shaped, cylindrical form. It is covered by a transparent gonotheca and contains an axis or blastostyle on which lateral buds form that develop into medusae or gonophores. The blastostyle has no mouth and no tentacles, but ends distally into a flattened disc.
The gonotheca opens at its distal end by a gonopore, through which the medusae escape. Gonotheca, blastostyle and the gonophores together form a gonangium.
When the temperature of the water exceeds 20°C, the buds which would normally form gonangia in the colony break free from the colony and settle down; a stolon arises from the lower end of the bud which produces a new colony of Obelia asexually. This is a special mode of asexual reproduction.
The medusa is a modified zooid produced as a hollow bud from the coenosarc of the blastostyle in spring and summer. Medusa swims freely on the surface water.
Structure of Medusa:
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 medusa is circular and tiny umbrella-like in shape. The convex outer surface is known as the ex-umbrella and the concave inner surface is the sub-umbrella. 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 mouth leads into an enteric cavity or gastric cavity in the manubrium. From the enteric cavity, arise four radial canals which are delicate ciliated tubes, they run to the margin of the bell to join a ciliated circular canal running near the margin.
The enteric cavity and the canals represent the enteron which distributes food. Projecting from the middle of the radial canals are four gonads, since sexes are separate they are either four testes or four ovaries,they are patches of modified sub-umbrellar ectoderm.
The gonads mature after the medusae escape from the gonotheca. The edge of the bell is produced inwards as a thin fold called velum.
Velum is characteristic of hydrozoan medusae but it is insignificant in Obelia. The medusae with a velum are called craspedote, and those with no velum are acraspedote (Scyphozoa). From the edge of the bell numerous small solid tentacles hang downwards. The tentacles have swollen bases due to the accumulation of interstitial cells which are practically absent from other regions.
The basal swellings of tentacles are called vesicles or bulbs, nematocysts are formed continuously in the bulbs from where they migrate to the tentacles.
Digestive enzymes are secreted from the endoderm of bulbs. Near the bulbs the ectoderm has pigment granules and nerve cells, they are often called ocelli and it is claimed that the ocelli are sensory to light, but more probably there are no ocelli, the pigment granules are accumulated excretory matter.
Above the bulb of every tentacle is a tiny fluid-filled vesicle. Nematocysts are confined to the manubrium and tentacles, there may be some on the bell margin. There are eight marginal sense organs called statocysts or lithocysts lying at regular intervals, being attached on the sub-umbrella side to the bulbs of eight tentacles, they are developed in response to a locomotory habit.
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.
The lining has some sensory cells with thin sensory processes on which the otoliths produce a stimulus which is transmitted by nerves to muscles; the muscles coordinate the snake-like swimming movements of the medusa, and should the medusa become tilted, the muscles contract to right the position of medusa bell, thus, statocysts are balancing organs.
Histology of Medusa:
The ectoderm covers the bell on all sides, its epitheliomuscular cells are produced into muscle processes which run longitudinally in the manubrium and tentacles. 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.
The muscle processes form a striated circular muscle and some radial muscles in the sub-umbrella, they bring about locomotory movements. The ectoderm of the ex-umbrella is devoid of musculature.
The endoderm lines the enteric cavity and the radial and circular canals. The endoderm cells have no muscle processes, they are ciliated epithelial cells, they are digestive. Between the two ectoderm layers of the bell is a thin sheet of endoderm lamella except where the enteron lies.
The endoderm lamella is formed by the fusion of upper and lower layers of endoderm, the fusion having occurred at all places except in the region of the enteron. Between the ectoderm and endoderm is thick mesogloea forming the bulk of the bell of the medusa, manubrium and tentacles. The velum has a double layer of ectoderm and the thick mesogloea in the middle, it has no endoderm.
Radial symmetry of Medusa:
Like polyp, the medusa is radially symmetrical.
The presence of the four radial canals distinguishes the four principal radii or per-radii. Halfway between any two per-radii a radius of the second order or inter-radius may be taken. Halfway between any per-radius and inter-radius on either side a radius of third order, or ad-radius, and halfway between any ad-radius and the adjacent per- or inter-radius, a radius of fourth order or sub-radius.
Thus, there are four per-radii, four inter-radii, eight ad-radii and sixteen sub-radii. 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. Sub-radii are of no importance in this particular form.
Development of Medusa:
The blastostyle produces medusae by budding in large numbers. The cavity of the blastostyle pushes the coenosarc out to form a small protuberance or bud. The bud grows larger and its coenosarc becomes like a vesicle which is attached to the blastostyle by a narrow stalk. The cavity of the vesicle is continuous with the enteron of blastostyle.
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. There are now two layers of ectoderm outside the bell rudiment and one layer on the inner side. The cavity of the bell rudiment assumes the shape of the sub-umbrella, and a manubrium is formed in the centre.
The two layers of ectoderm which enclose the bell rudiment from outside now break leaving a marginal and circular shelf called velum.
In most hydrozoan medusae, the velum grows and becomes prominent, but in Obelia it decreases and becomes insignificant. The manubrium acquires a mouth, marginal tentacles are formed, the stalk breaks and its aperture closes up, thus, a medusa is formed which is set free, it escapes from the gonotheca, later its gonads mature.
Medusae are carnivorous and the processes of their nutrition are essentially the same as in the polyp. The food consists of living animals or bodies of animals.
Digestion is both extracellular and intracellular. Extracellular digestion occurs in the main part of the gastro vascular cavity and is purely proteolytic. 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.
The digested food is distributed to the entire medusa through the system of radial and circular canals.
The muscular system of medusa is somewhat more specialised than in the polyp. The gastro dermal cells lack contractile extensions, and the muscular system is, thus, restricted to the epidermal layer. Furthermore, the muscular system is best developed around the bell margin and sub umbrella surface where the fibres form a radial and circular system.
Some of the epitheliomuscular cells of the velum have their contractile extensions oriented to form a powerful circular band of fibres which are striated. The contractions of the muscular system, particularly of circular fibres produce pulsation of the bell.
The swimming movement of the medusa is dependent base on these pulsations and is largely vertical in lamella direction. Horizontal movement is dependent upon water currents.
The nervous system of medusa is more highly specialised than that of the polyp. 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.
The nerve rings connect with fibres innervating the tentacles, the musculature, and the sense organs. Fibres also interconnect the two rings. The lower ring is the centre of rhythmic pulsations, i.e., it contains the pacemakers. Pulsation will continue in the bell as long as any portion of the ring is intact. It is with the lower ring that the statocysts are connected.
The bell margin is richly supplied with sensory cells and also contains two types of true sense organs, viz., light sensitive ocelli and statocysts. The ocelli consist of patches of pigment and photoreceptor cells organised either within a flat disc or a pit. The ocelli are typically located on the side of the tentacular bulbs.
Statocysts are located between the tentacles or associated with the tentacular bulb at the tentacle base. They may be either in the form of pits or closed vesicles but in both cases, the walls contain sensory cells with bristles projecting into the lumen. Attached to the bristles are from a few to many calcareous concretions known as statoliths.
The statocysts act as organs of equilibrium. When the bell tilts, the statoliths respond to the pull of gravity and stimulate the sensory bristles to which they are attached. The animal may then respond by muscular contractions to bring itself back into a horizontal position.
The medusae are sexual or reproductive zooids possessing gonads. The medusae are dioecious, they have either four testes or four ovaries in the sub-umbrella just below the radial canals. A gonad has an outer ectoderm and inner endoderm with mesogloea between the two layers. The gonad has a small diverticulum of the radial canal.
The germ cells of Obelia do not arise in the gonads, they arise from interstitial cells of the ectoderm of the blastostyle where they may be seen in various stages of maturation, then they migrate into the medusa, then through the radial canals they take up their position in the ectoderm of the gonads. When germ cells mature, the gonads rupture and spermatozoa and ova are discharged externally into water.
Life History of Obelia:
Fertilisation usually takes place in open sea water where the gametes are set free. Sometimes, the sperms are carried into the female medusae with water currents and there they fertilize the eggs in situ. However, the parent medusae die soon after liberating their respective gametes.
The zygote undergoes complete or holoblastic and equal cleavage to form a single-layered blastula with a blastocoele. Some cells migrate into blastocoele, eventually filling it completely to form a solid gastrula known as stereo gastrula. Its outer cell layer becomes the ectoderm and inner cell mass the endoderm.
The gastrula elongates and. its outer layer of ectoderm cells becomes ciliated, and now it is called planula. Soon, a cavity called enteron develops in the solid endodermal cell mass by the process of delamination and the planula becomes a two-layered larva having an outer ciliated ectodermal cells and an inner layer of endodermal cells.
The planula after a short free-swimming existence settles on some solid object by its broader end. The free end forms a manubrium with a mouth and a circlet of tentacles. Thus, a simple polyp or hydrula is formed which grows a hydrorhiza from its base, from which an Obelia colony is formed by budding.
The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. The life history may be represented as male and female gametes → zygote → planula larva → hydrula → colony → sexual medusae → gametes → zygote and so on.
Alternation of Generations and Metagenesis of Obelia:
It is clearly evident from the life history of Obelia that there is an alternation of polypoid and medusoid generations.
The polypoid generation is asexual and produces polyps and blastostyles by asexual budding. The blastostyle also produces medusae by asexual budding. The medusae do not produce medusae but they give rise to gametes, which after fertilisation develop into a polypoid colony from which medusae are produced again by budding.
Thus, an asexual polypoid generation alternates with a sexual medusoid generation. This phenomenon is known as alternation of generations, till recently. The term alternation of generations means that the individual exists in two distinct forms, which alternate each other regularly in the life history.
One individual possesses the power to reproduce the other by asexual reproduction, which again by sexual reproduction gives rise to the next generation. Therefore, a true alternation of generations is always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant.
But, in Obelia the condition is somewhat different and, therefore, objections were raised to use the term alternation of generations for it. Because, in Obelia, there are no true two generations to alternate each other. 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 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. Thus, it is rather difficult to distinguish between sexual and asexual generations. Hence the term metagenesis is used to replace the term alternation of generations in Obelia.
Thus, in the life history of Obelia, there is a regular alternation between fixed polypoid and free-swimming medusoid phases, both of them being diploid.
Such an alternation of generations between two diploid phases is known as metagenesis. Although, the phenomenon of metagenesis is also reported in other groups of animals but it is well represented by polymorphic hydrozoan like Obelia. Obelia shows polymorphism in which the polyps are for feeding the colony, blastostyles for budding and medusae for disseminating gametes.
Advancement of Medusa over Polyp:
Medusa exhibits many features of advancement over polyp, few of them are as follows:
1. The epidermis resembles the epithelium of higher Metazoa forming a thin, protective and sensitive layer of small cells.
2. 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.
3. The nervous system shows differentiation into two nerve rings constituting the central nervous system and nerve nets forming the peripheral nervous system.
4. The marginal sense organs present at the bases of 8 tentacles are of special advantage to the free swimming habit of the medusa.
5. The mode of sexual reproduction provides wide dispersal of the species due to its free swimming habit.
Similarities between Polyp and Medusa:
Striking as is the difference between polyp and medusa. They are strictly homologous or typically similar structures. Both of them are formed on the same pattern.
However, the features of similarity between them are listed below:
1. Both are radially symmetrical.
2. Both are diploblastic with outer epidermis (ectodermal) and inner gastro dermis (endodermal).
3. The mouth is homologous in both the cases; the mouth situated on the hypostome in polyp is homologous with the mouth situated on the manubrium of the medusa. Anus is absent in both the cases.
4. The stomach, radial canals and circular canal of medusa are homologous with the gastro-vascular cavity of the polyp. All these are lined by gastro dermis and serve the purpose of digestion and distribution of digested food.
5. Both are carnivorous; the food is captured and ingested with the help of tentacles.
6. Digestion is extracellular as well as intracellular in both the cases.
7. The outer, exumbrellar surface of the medusa is homologous with the base of the polyp providing attachment with the parental colony.
Derivation of Medusa from Polyp:
Striking as is the difference between polyp and a medusa, they are strictly homologous structures, and the more complex medusa is readily derived from the simpler polyp-form. The apex of the umbrella of medusa corresponds with the base of a hydranth. The mouth and manubrium are also homologous structures.
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. The result of this to be a medusa-like body with a double wall to the entire bell, the narrow space between the two layers containing a prolongation of coelenteron and being lined with gastro dermis.
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.
The cavity would remain only along four meridional areas, the radial canals and as a circular area the circular canal close to the edge of the bell. In this way a medusa is derived completely from a polyp (Fig. 32.12).