Cnidaria: Metagenesis | Zoology Optional Notes for UPSC PDF Download

Metagenesis and polymorphism, both captivating biological phenomena, have generated an intriguing debate within the Phylum Cnidaria regarding their interconnection. The scientific community is actively exploring whether metagenesis results from polymorphism or if it has, in fact, catalyzed the development of polymorphism. To gain a deeper understanding of this intricate relationship, it is essential to conduct a thorough analysis of the various stages of metagenesis in cnidarians.

Gradations of Metagenesis

Cnidarians, a wide-ranging assembly of aquatic organisms, exhibit diverse life-history patterns that provide valuable perspectives on the dynamic relationship between metagenesis and polymorphism. These variations encompass four distinct life-history forms within cnidarians, each demonstrating a gradual evolution toward heightened 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.