The relationship between metagenesis and polymorphism has raised the question—is metagenesis an outcome of polymorphism or has it lead to the emergence of polymorphism?
A discussion on the existence of different grades of metagenesis in cnidarians will help us to find a logical answer.
Gradations of Metagenesis:
Four different forms of life-histories are seen in the cnidarians which exhibit a trend towards complexity.
These different grades are:
Monomorphic form:
Typical representative is Hydra. Freshwater forms lack free larval stage but the marine forms do possess it.
Polyp → Egg → Polyp
Dimorphic form:
Typical representative is Aurelia.
Trimorphic form:
Typical representative is Obelia.
Polymorphic form:
The classical example is the Siphonophora.
Examination of these grades has given rise to two plausible answers. One view holds that ancestral cnidarians were polyps and the medusae developed secondarily from it. The polyp passed the function of sexual reproduction to the medusa which established the cycle of metagenesis.
The alternate answer is that the original cnidarians were medusae and that polyp is a larval state. The life-history of the Trachymedusae, a primitive group of cnidarian, lends support to this contention.
In this group the life-history is represented as:
Here actinula stage fixes itself like a polyp and asexually produces medusoid form. This evidence together with the nature of development of a siphonophore colony, also strongly endorses the second view.