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In this article we will discuss about the structure and locomotion of obelia.
Obelia is a marine colony of hydroid organisms. It is a moss like animal found on submerged rocks or sea weeds, in shallow water off the sea coast.
It may be regarded as a Hydra which has undergone repeated budding but the buds have not separated from the parent. The supposed budding must be visualised to have occurred in regular order, producing polyps upon a common axis called hydrocaulus. The entire colony is whitish-brown in colour. It is fastened to the substratum by a root-like base, called hydrorhiza. The polyps are of two kinds: hydranths and blastostyles.
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The hydranths are more numerous and constitute the feeding zooids. Each of them resembles a Hydra in appearance, having about twenty solid tentacles. A hydranth, is encased and protected in a transparent vase-like hydro-theca.
It captures minute prey by using its tentacles which contain batteries of nematocysts. The coelenteron of the hydranth communicates with the cavity of the hydrocaulus and thus the absorbed food is distributed to the entire colony.
Blastostyles are the reproductive zooids which appear only when the colony has reached a mature size. Each is covered by a transparent gonotheca, thus forming a cylindrical gonangium which has no mouth or tentacles. A gonangium occurs in the axil between the hydrocaulus and the axis of a hydranth. On the axis of the blastostyle are formed lateral buds which ultimately develop into the medusde.
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The hydrocaulus is invested in a horny, transparent case or perisarc which is continuous with the hydro-theca and gonotheca. The soft parts of the hydrocaulus is known as the coenosarc. The minute structure of the body wall conforms to that of the Hydra, and the various cell-types found in the latter are also present in Obelia in a slightly modified form.
The medusae are budded off from the axis of the blastostyle. They escape into the sea through an opening in the gonotheca. A medusa is popularly known as the jelly-fish. Each medusa is a minute umbrella-shaped body bearing numerous tentacles arranged round its margin or velum. Its convex surface is known as the ex- umbrella and the concave surface as the sub-umbrella.
The short handle-like structure hanging down from the centre of the sub- umbrella is the manubrium. The tip of the manubrium is perforated by the mouth which leads through a gullet into the coelenteron of the medusa. Four radial canals radiate from the middle of the coelenteron to join a circular ring canal, situated round the margin of the umbrella.
There are two nerve rings around the velum of the medusa connected with a nervous network within the body. A gelatinous mesogloea fills the space between the ectoderm covering the outer surface and the endoderm lining the coelenteron.
Gonads develop on the sub-umbrella, whence ova and sperms are released into the water. The sexes are separate, so that medusa may either produce sperms or ova, but not both. There are four gonads, one upon each radial canal.
Locomotion is effected by contraction of the sub-umbrellar surface, so that water is driven out, and the organism is propelled forwards with its ex-umbrellar surface in front, or float and feed in the sea. When mature the medusa reproduces sexually. Fertilisation takes place in water.
The fertilised ovum, by repeated cleavage, produces a hollow blastula, the cavity of which becomes obliterated by migration of cells from the bordering wall to form a solid mass of endoderm. The outer layer of the blastula may now be considered as the ectoderm. Later on a split appears in the middle of the endoderm which represents the coelenteron.
The latter communicates by a mouth with the exterior. Instead of secreting a horny case, as in Hydra, the ectoderm becomes ciliated and a planula larva is formed, which freely swims about in water.
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Subsequently, the planula settles down, fixes to a substratum, develops tentacles at the free end, and is thus converted into a polyp. The polyp produces buds in a regular and orderly manner and the result is a moss-like colony of Obelia bearing the two kinds of hydroid zooids: hydranths and blastostyles.
The medusa is a means for dispersal. It would appear, at first sight, that a medusa is entirely different from a hydranth, not only in form but also in minute structure. This idea is erroneous. The basic structure of the two is essentially the same and we can easily derive one from the other.
The manubrium corresponds to the hypostome of the hydranth and if in imagination, we lightly pull out a medusa by holding the ex-umbrella in one hand and the manubrium on the other, the medusa is converted into a polyp.
There is a distinct alternation of generations in the life- history of Obelia. The fixed colony represents the diploid or asexual generation, reproducing by budding. The medusae derived from the polyp are independent units, living apart, and feeding themselves.
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They represent the haploid or sexual generation, reproducing by gametes. Product of fertilisation is the free swimming planula larva, which subsequently settles down to form the fixed asexual colony. This phenomenon is known as metagenesis.
An Obelia colony exhibits polymorphism. It includes three types of individuals:
(1) Hydranths or feeding polyps;
(2) Blastostyles or reproductive polyps, reproducing asexually by lateral budding;
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(3) Medusae, reproducing sexually by germ cells and serving as means for dispersal.
