Slide Preparation Of Coelenterate (Obelia Colony & Obelia Medusa)

Slide Preparation Of Coelenterate (Obelia Colony & Obelia Medusa)


Obelia is colonial, mainly sedentary hydrozoan zoophyte attached to the seaweed, hills and rocks.It is mostly found in shallow water and also up to approximately 250 ft.deep. There are currently 120 known species, with more to be discovered

Method for slide preparation of coelenterate

Coelenterates are first narcotized in water mixed with menthol crystal or Magnesium sulphate. After decanting the narcotizing liquid, fix the animal by adding drop by drop formol.(commercial preparation).These are then preserved in 70 %alcohol or 5%formalin solution. For making permanent mount ,keep the material in 70%alcohol,then stain in borax carmine, if overstain, destain with acid alcohol. Dehydrate in 70%, 90% and 100% alcohol. Clear in xylol or benzene and finally mount on Canada balsam. Then study under the microscope, draw the diagram, label them and note down the characteristic features.

Classification of obelia

  • Phylum :- Coelenterata – Tissue grade, diploblastic and acoelomate.
  • Class :- Hydrozoa – Hydroids: medusa with velum.
  • Order :- Hydroidea – Polypoid generation well developed
  • Sub order :- Calyptoblastea – Hydranths have hydrotheca and gonophores with gonotheca.
  • Genus :- Obelia

Habit and habitat

  • Obelia is colonial, marine, sedentary hydrozoan zoophyte, attached to seaweeds, shells and rocks


  • Its range is from the Arctic region to the Gulf of Mexico and the Pacific coast, and from Southern California to Oregon. it is found in shallow watter and also upto approximately 250 feet deep.


Slide Preparation Of Coelenterate (Obelia Colony & Obelia Medusa)


  • It is a dimorphic colony in the form of small seaweed filaments, measuring several cm in height. The filaments may be horizontal and vertical. The colony consists of several parts.
  • Hydrorhiza: It is basal or horizontal portion called as stolon or rhizostome, which is meant for attachment to substratum. Hydrorhiza gives vertical branches called hydrocaulus.
  • Hydrocaulus gives alternate branches that terminate into individual zooids called as polyps and medusa.
  • Coenosarc: Stems and zooids are made of a living hollow, cellular tube called as coenosarcs. It is made up or ectoderm, endoderm and mesogloea.
  • Stems and zooids are made up of following components :
    • Outer protective tough, transparent non cellular covering called as perisarc
    • Mesogloea
    • Inner living hollow cellular tube called coenosarcs.
  • Zooids consist of polyp and medusa.
  • Medusa grows at the base of polyp-bearing branches and is enclosed in blastostyles. Medusa is composed of upper exumbrellar and lowr sub-umbrellar surfaces, manubrium and gonads. Free medusa occurs in the life cycle. It is a reproductive zooid.
  • Polyp is a bell-shaped cup made up of lower cub-shaped hydrotheca and upper hypostome. Hypostome is a feeding zooid having circlet of 24 nematocyst bearing tentacles.
  • Growth of the colony is sympodial, i.e., each new hydranth arises as bud from the stem, just proximal to the next youngest polyp.
  • It reproduces asexually and sexually.


  • The colony has alternate branches of polyps, blastostyles and all above features, hence it is Obelia.

Obelia : Medusa

Slide Preparation Of Coelenterate (Obelia Colony & Obelia Medusa)


  • Medusa is a modified zooid for sexual reproduction.
  • It is a solitary free-swimming zooid, originating from blastostyles.
  • Medusa is umbrella-like and has convex exumbrellar and concave sub-umbrellar surfaces with well defined radial symmetry.
  • Umbrellar edge contains radially symmetrical tentacles.
  • Base of fully grown tentacle is thickened to tentacular bulb which contains a number of stinging cells.
  • In the four radial positions each tentacular bulb contains two otocysts, which are hollow and balancing organs containing calcareous otoliths.
  • Manubrium hangs from the centre of sub-umbrella, having mouth.
  • Mouth communicates with 4 radial canals which join with circular canal lining umbrellar margin which all around contains velum.
  • Beneath the radial canals are gonads lying in Sub-umbrellar ectoderm.


  • Since the mount has circular tentaculated body, 4-radial rounded gonads and all above features, hence it is obelia medusa a very favourite slide-spot

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