Obelia is a tree-like branched, marine colonial cnidarian, growing on the surface of sea-weeds, rocks and pilings along the sea-coasts. Colonies appear like whitish or light-brown branched threads, about 30 mm. to several cms in height. The colony arises by budding from a single hydra-like individual, the buds fail to separate and after repeated budding there results a tree-like growth permanently fixed to some object and consisting of numerous members joined to the main stem. The members are known as zooids.
If a colony of Obelia is examined under low power of microscope, it is found to be made up of a root-like horizontal part resembling a basal stem, called hydrorhiza which is attached to the substratum. From hydrorhiza grows an upright stem called hydrocaulus (Plural-hydrocauli). Hydrocaulus and hydrorhiza consist of two distinct layers.
a) Perisarc: It is a transparent, tough outer layer, yellow in colour, of homy consistency. It is not composed of cells but is a cuticular secretion of the ectodermal cells.
b) Coenosarc: Fibrous processes connect the perisarc with coenosarc. It is an inner, granular layer. It is hollow and its tubular cavity is continued into cavities of polyps, thus, forming a part of gastrovascular cavity. Coenosarc consist of an outer layer, the ectoderm and an inner layer, the endoderm.
From the upright stem, called hydrocaulus. several side-branches arise. These side-branches are ringed and their end is expanded into a hydra-like structure Known as hydranth or polyp. Hydranth is a kind of zooid.
Towards the proximal region of the colony are found cylindrical bodies known as blastostyles. These give rise to several small lateral offshoots, called medusa buds.
Thus, Obelia colony is trimorphic, having
(a) Polyp or hydranth (nutritive zooids)
(b) Blastostyle (reproductive zooids) and
(a) Hydranth or Polyp
These arc the nutritive zooids or gastrozooids of the colony. They are specialized for capture, ingestion and digestion of food. Each hydranth is closely similar in structure to that of hydra. It has a sac-like body. The basal end is not closed but connected by a hollow stalk with the hydrocaulus. The body wall composed of ectoderm, mesogloea, and endoderm encloses a simple enteron that opens to outside by the mouth situated at the free end of the hydranth. Mouth, as in hydra, is situated at the apex of the hypostome. The ectoderm of the hydranth is thin. The nematoeysts are present only on the tentacles. The middle structureless layer, the mesogloea, has nerve net present on its both sides.
The endoderm cells are similar to those of hydra. The nutritive-muscular cells possess flagella at their inner ends. The gland cells are large and with granular inclusions. The tubular perisarc surrounding the coenosarc extends around the hydranth to form its conical protective covering known as hydrotheca.
After the hydrocaulus has reached its full development it gives rise to special zooids called blastostyles. They are fewer in number as compared to hydranths in the colony.
The mouth and tentacles are absent in these simplified zooids. They cannot feed hence their enteron is reduced in size. The distal closed end usually forms a flattened disc. The perisarc extends over the blastostyle to form a cylindrical or vase-like transparent gonotheca.
The blastostyles are the reproductive zooids as they reproduce asexually to give rise to numerous lateral buds called medusa buds or gonophores. These buds develop into third type of zooids of the colony called medusae. When they are fully formed they are set free and swim away from the colony by escaping through the ruptured distal end or by opening of the lid of the gonothcca.
These are small, transparent, solitary, free swimming saucer-shaped or bell-shaped zooids. They measure about 6 mm in diameter. These are the reproductive zooids which produce the sex cells.
The inner concave side of the body is known as sub-umbrella and outer convex as exumbrella. A short, hollow, quadrangular projection, the manubrium, hangs down from the middle of the subumbrella surface. This structure together with the disc or bell-shaped body of the medusa gives it an umbrella-like appearance. The mouth, a square or four sided opening, is situated at the tip of the manubrium.
The mouth leads through the cavity of the manubrium into a small gastral cavity or enteric cavity or stomach situated in the central part of the main body of the medusa.
From this cavity radiate four narrow radial canals situated at equal distances from each other. These canals run outwards toward the edge of the umbrella and open into a circular canal running around the edge of the umbrella. This system of canal enables the food to be taken in at the mouth and manubrium and digested in the stomach to be distributed through them to the entire medusa.
The edge of the medusa gives off on its inner side a very narrow, rudimentary fold or shelf called the velum. The margin of the umbrella gives off short tentacles which are sixteen in number in the newly born medusa but are numerous in the adult. Ectoderm covers both the surface of the umbrella and the outer surface of the manubrium. The endoderm lines the cavity of the manubrium, ‘stomach’, radial canals and circular canals which together represent the enteron. Endoderm does not extend into the velum.
The primary function of the medusa is sexual reproduction. Obelia is dioecious as each medusa has reproductive organs of only one sex. There is no difference between the male and female medusae. The gonads (testis or ovaries) are four in number and lie on the sub-umbrella, below the radial canals, in the form of knobs. Thus gonads are per-radial in position. They are situated almost at equal distances between the manubrium and the velum. The gonads which arise as diverticula of the radial canals have the same structure as the body wall of medusa. The sex cells start developing very early when the medusa is being formed. These cells originate in the ectoderm of the manubrium, migrate to the endoderm and finally make their way to the gonads. They lie between the ectoderm of the sub-umbrella and the mesogloea. The sperm and ova when fully formed are set free in water by rupture of the outer wall of the gonad. Sometimes the flagellated sperms swim about in water and fertilize the ova present in female medusae. The fertilization takes place in water.
The medusa performs two important functions for the colony, that of reproduction and dispersal of the species.
The fertilized egg undergoes cleavage which is equal and holoblastic (complete). The blastula is a hollow boll consisting of single layer of cells enclosing the blastocoel. This cavity gets completely filled up with cells budded from the wall of the blastula. The embryo is now called stereogastrula or solid gastrula. The embryo is set free from the egg membrane as a free-swimming larva called the planula. The larva swims about for some time and brings about wide distribution of the species. A cavity soon appears in the endoderm cell mass which becomes the enteron.
After the free-swimming life the larva loses its cilia and settles down on the bottom of the sea, gets attached to the substratum by its broader end and undergoes metamorphosis.
The attached or proximal end widens into a disc of attachment. A short distance from the free or proximal end a dilatation is formed. From this portion tentacles arise in a circle as short buds. The narrow portion beyond their origin becomes the hypostome. Soon an aperture, the mouth, is formed at the end of the hypostome. The young hydranth closely resembles a simple polyp like hydra and is called hydrula which undergoes repeated asexual budding to gives rise to complex obelia colony.
Alternation of Generation and Metagenesis
The study of life-cycle of Obelia reveals that there is a regular alternation of colonial hydroid phase with solitary medusoid phase. The hydroid is fixed and reproduces asexsually by budding. The blastostyles of the colony give rise to medusae by asexual budding. The medusae are solitary and free-swimming. They possess gonads but never give rise to other medusae but to the eggs and sperms. The egg of medusae after fertilization develop into hydroid colony. This phenomenon has been called alternation of generation where the organism exists in two distinct forms, which alternate regularly in the life cycle. One generation possesses the power to reproduce the next generation by asexual reproduction and this generation in turn gives rise to the next generation by sexual mode of reproduction.
The medusae can be considered as a kind of zooid modified for free swimming existence. They help in dispersal of gametes. It is doubtful whether the gametes produced by the medusae actually arise from the ectoderm of blastostyle from which they migrate into the medusa. Hence it is impossible to clearly differentiate between the asexual and sexual generation in Obelia. The term metagenesis has been used to describe this peculiar life cycle of Obelia. It means that sexual generation has been postponed. The medusae are not produced directly from the egg but arise by asexual budding from the hydroid phase. It can be also considered as particular example of polymorphism. The hydranths of the colony are feeding zooids, the blastostyles are asexually budding zooids producing the third motile type of zooids the medusae which help in dispersal of thus sedentary form.