External features:
Obelia colony is delicate semitransparent and whitish in colour, found attached by means of horizontal thread-like roots, called hydrorhizae.
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A number of vertical branches with size 2 to 3 cm arise from hydrorhiza, called hydrocauli. Obelia colony is good example of polymorphism because it bears different kind of life forms which differ both in structure and function. Polymorphism is the peculiar character of coelenterates.
In colony each hydrocaulis gives out alternate branches that terminate in a nutritive zooid, the polyp or hydranth.
Image Source: wearetimpanogos.org
In the axils of older polyps there are cylindrical reproductive zooids, the blastostyles or gonangia. In this stage colony is dimorphic but when blastostyle bears medusae, it becomes trimorphic.
Coenosarcs:
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Coenosarc is a cellular wall that forms the main frame of the colony and encloses a canal, the gastro-vascular cavity.
This coenosarc consists of two layers, an outer epidermis and an inner gastrodermis, with a gelatinous mesogloea in between layers.
Perisarc:
Perisarc is a non-living chitinous layer that surrounds coenosarc externally. It is tough, transparent and yellowish or brown in colour.
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It is secreted by epidermis of the coenosarc to protect and to support the colony as an exoskeleton.
At the base of each zooid perisarc bears annular constrictions called perisarcal annuli. Below the base of each lateral branch there is single annulus.
These annular constrictions allow the swaying movements within the colony under the influence of water current.
A. Polyp or hydranth:
Polyp is a nutritive zooid and is also called gastrozooid or trophozooid. It is a yellowish, cylindrical or conical hollow sac with radial symmetry, that very much resembles with a miniature hydra.
At its free end (distal end) a conical elevation, the manubrium or hypostome is formed. Hypostome opens outside by mouth which is contractile.
From the base of hypostome a circle of 30 filiform tentacles longer than hypostome arises. Around the polyp perisarc makes a loose, cup-like transparent protective sheath by dilation which is called Hydrotheca while, at the base of polyp it forms a ring-like horizontal shelf that helps polyp to be on its place.
B. Blastostyle:
Blastostyes are club shaped bodies that found in the axils of older hydranth and are less numerous than hydranths.
Its free distal ends are closed and devoid of mouth and tentacles. The perisarc covering blastostyle forms a loose, transparent, vase like capsule, the gonotheca.
Blaststyle is an asexual zooid and produces medusae or gonophore by asexual budding. Matured medusae detach from the blastostyle and escape into the water through an aperture, the gonopore.