Occurrence in the same species of more than one type of individuals, which differ in form and function.
Ensures an effective division of labour between the individuals.
In coelenterates, the different individuals are united in the form of a colony.
| Polyps | Medusa |
| Tubular body with a mouth surrounded by tentacles at one end. Other end is blind and attached to a substratum. Generally sessile. Feeding, protection and asexual reproduction. Velum is absent. No circular and radial canals. No sense organs. No gonads. Mesogloea poorly developed. Tentacles generally 24. | Bowl or umbrella shaped body. Tentacles hanging from lower concave surface. Generally motile. Sexual reproduction. Velum is present around the margin of the umbrella. 4 radial and 1 circular canal. Sense organs called Statocysts present at the margins of the tentacles. 4 gonads on the radial canals. Mesogloea enormously developed. 16 tentacles in young Medusa, numerous in older Medusa. |
| Dimorphic | Simplest and most common. e.g. Obelia, Tubularia. Gastrozooids (Hydranths): Concerned with feeding. Gonazooids (Blastostyles): Asexual budding forming sexual medusa. |
| Trimorphic | e.g. Plumularia Gastrozooids, Gonazooids, Dactylozooids. Dactylozooids: Non feeding and defensive Polyps bearing Nematocysts. |
| Polymorphic | Gastrozooids Gonazooids Dactylozooids Skeletozooids: Spiny projections of Chitin. Tentaculozooids: Sensory cells. Extreme specialisation in Siphonophora: Modification of Polyps: Gastrozooid: Feeding polyp with a mouth and long tentacle. Dactylozooid: Protective polyp without mouth and usually with a long basal tentacle. Gonozooid: Reproductive Polyp which produces sexual medusa or gonophores. Modification of Medusae: Nectophore or Nectocalyx: Swimming zooid with muscular bell without manubrium or tentacles. Pneumatophore: Bladder like Medusa filled with a secreted gas. Phyllozooid: Usually leaf like and studded with Nematocysts. Gonophore: May be either male or female. |
Metagenesis:
Diploid asexual phase and a haploid sexual phase regularly alternate with each other.
Haploid gametophytes produce haploid ova and sperms. After fertilization, they give rise to a new diploid saprophyte. This completes one life cycle.
Alternative phases in Cnidaria: Polyp and Medusa.
Polyp and Medusa alternate successively where the polyp reproduces asexually to form a large number of medusa, each medusa reproduce sexually by the union of eggs and sperms to form zygote.
The zygote grows into larva, which fix itself to a substrate and finally form a new polyp.