• The phenomenon of occurrence of an individual in two or more distinct morphological and functional forms.
• It occurs in the same species of an individual.
• It is also known as genetic polymorphism. For example different individuals of a species may remain separate as represented by various castes in termites, ants and Cuban snails.
• Polymorphism is an important feature of phylum coelenteratea.
• Each individual member of Coelenterates is known as Zooid and they often units to form a colony which acts as a single unit (individual).

Polymorphic form in coelenterata: two types of zooids

Coelenterates which may be single or colonial, they exists in two forms- polyps and medusa

1. Polyp:

• In Hydrozoa, polyps has a tubular body with a mouth surrounded by tentacles at one end. Other end is blind and usually attached to pedal disc to the substratum
• Polyps are generally sessile
• They reproduce asexually

2. Medusa:

• In Hydrozoa, medusa has a bowl or umbrella shaped body with marginal tentacles and mouth centrally located on a projection called manubrium of the lower concave surface.
• Medusas are generally motile
• They reproduce sexually

 Importance of polymorphism:

• It is essential for division of labor among the individual zooids.
• Different functions are assigned to different forms. For examples; polyps are concerned with feeding, protection and asexual reproduction whereas medusa are concerned with sexual reproduction.

Patterns of polymorphism in Hydrozoa:

Degree of polymorphism varies greatly in different group of Hydrozoa.

1. Dimorphic:

• It is simplest and commonest pattern of polymorphism
• They have only two types of zooids (gastrozooids and gonozooids) and the phenomenon is known as dimorphism.
• Exhibits by many Hydrozoan colonies
• Examples: Obelia , Tubularia

2. Trimorphic:

• They have three types of zooids- gastrozooids, gonozooids and dactylzooids.
• Gastrozooids is responsible for feeding and asexual reproduction
• Similarly, gonozooids is responsible for sexual reproduction
• Dactylzooids is functionally non-feeding form and is responsible for defense.

3. Polymorphic

• Some coelenterate have more than three forms called polymorphism
• In Hydractinia, five polymorphic form or zooids exists- gastrozooids, spiral dactylzooids, tentaculozooids, skeletozooids and gonozooids.
• Gastrozooids: responsible for feeding
• Spiral dactylzooids: responsible for protection
• Tentaculozooids: responsible for sensory impulse
• Skeletozooids: responsible for spiny projections
• Gonozooids: responsible for sexual reproduction

Polymorphism in Coelenterata

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