MORPHOLOGY OF Obelia
Colony structure
Obelia colony consists of a horizontally branching root-like stolon or hdrorhiza
from which emerge many vertical branching stems called hydrocaulus.
Branches emerging from the stem terminate either in nutritive zooids, called
the polyps or hydranths or in reproductive zooids called blastostyles or
gonangia, which produce medusa. Thus, Obelia colony is trimorphic, exhibiting
two types of zooids attached to the sedentary colony and a free swimming
zooid or medusa.
The colony is made of an inner, tubular and living tissue, the coenosarc, which
encloses a canal, called the gastrovascular cavity, the walls of which consist of
two layers, an outer epidermis and an inner gasrodermis and a gelatinous
mesogloea in between. Coenosarc is externally surrounded by a yellowish,
non-living chitinous layer, called perisarc that provide protection to the colony
and serves as an exoskeleton.
The body wall
The body wall of the colony is made up of two layers, the outer epidermis and
inner gastrodermis enclosing a thin, transparent, non-cellular gelatinous layer,
mesogloea in between. Epidermis carries stinging cells or nematocysts, which
are especially abundant on tentacles. A nerve-net composed of branched
nerve cells is present on each side of mesogloea Gastrodermis consists of large
nutritive-muscle cells and narrower gland cells that secrete digestive enzymes
and help in digestion, which is both extracellular and intracellular.
Zooids types in Obelia colony: There are three types of zooids in Obelia colony
as follows:
1. Polyp or Hydranth or gastro zooid; (vegetative zooids): These zooids are
responsible for the nutrition of the colony. Its distal end is produced into a
conical elevation called the manubrium or hypostome, in the middle of whic is
located the mouth. From the base of hypostome arises a circle of up to 30
filiform tentacles containing nematocysts. Perisarc around hydranth dilates to
form a loose cup-like, transparent protective sheath, the hydrothecai which
the polyp withdraws when in danger.
2. Blastostyle or Gonangium: When the colony has reached full development,
it produces special club-shaped bodies called blastostyles or blastozooids or
gonozooid these are a few in number and restricted to the basal end of the
hydrocaulus. The blastostyle is long, cylindrical and devoid of mouth and
tentacles and enclosed in a transparent gonotheca.
The perisarc, covering the blastostyle, forms a loose, transparent, vase-like
capsule called the gonotheca.
The blastostyle, by lateral asexual budding, produces sexual zooids called
medusa or gonophores. Fully formed medusae detach from blastostyle and
escape through an aperture called the gonopore. Gonotheca, together with
blastostvle and medusae is referred to as Gonangium.
Medusa or Nectophore: Large number of medusae is produced by budding on
blastostyle. They detach from gonangium and escape to the sea water through its
opening called gonopore and freely swim and feed in water. A fully grown Obelia
medusa is a radially symmetrical tiny umbrella, measuring 1 or 2 mm in diameter, the
concave surface of which is described as sub-umbrella and the outer convex surface
as ex-umbrella. The subumbrellar surface has four radial canals and a circular canal
on the margin. A mature medusa bears four gonads, one in the middle of each radial
canal. From the centre of the sub-umbrellar surface projects down a hollow process,
the manubrium that bears a four-sided mouth in the centre leading into a
gastrovascular cavity or stomach.
From gastric cavity four gastro dermal canals radiate to the margins of the bell. These
are radial canals, opening into a ring canal or circular canal running in the margin o
the bell.
Circular edge of umbrella is produced inwards into a fold called velum, which does
not carry any canal in it. Margin of the umbrella bears numerous short tentacles with
nematocysts. Nervous system in medusa is a diffused network of neurons. From the
middle of the radial canal four gonads project. Since sexes are separate, these are
either testes or ovaries. Medusa floats in water and is drifted by water currents with
manubrium hanging downward and tentacles swaying freely. It also swims actively
by muscular contractions and velum. Medusa is carnivorus and feeds on planktons.
Marginal sense organs, statocysts or lithocysts are eight in number, attached at
regular intervals to the bases oftentacles on the subumbrellar surface for providing a
sense of equilibrium to the animal.
