Its D...Metagenesis means alternation of generations between asexual and sexual forms...ex:- polyp n Medusa forms seen in obelia(sea fur) that belongs to cnidaria .....

Metagenesis refers to the alternation of generations between asexual and sexual phases of an organism. It is commonly observed in many aquatic animals, particularly in cnidarians (such as jellyfish and sea anemones) and certain invertebrates (such as some species of flatworms).

A. Occurrence of a drastic change in form during post embryonic development:
Metagenesis involves the occurrence of a drastic change in form during post-embryonic development. This means that the organism goes through a series of distinct morphological stages in its life cycle. For example, in cnidarians, the medusa stage (sexual phase) is characterized by a bell-shaped body with tentacles, while the polyp stage (asexual phase) is characterized by a tubular body with a mouth surrounded by tentacles.

B. Presence of a segmented body and parthenogenetic mode of reproduction:
Metagenesis does not necessarily involve the presence of a segmented body or parthenogenetic mode of reproduction. Segmentation refers to the division of the body into repeated units, which is more commonly observed in organisms such as annelids (segmented worms) and arthropods (such as insects and crustaceans). Parthenogenesis refers to the reproduction without fertilization, where offspring develop from unfertilized eggs. While some organisms may exhibit both segmentation and parthenogenesis, these features are not essential for metagenesis.

C. Presence of different morphic forms:
Metagenesis does involve the presence of different morphic forms. As mentioned earlier, organisms exhibiting metagenesis undergo a series of distinct morphological stages in their life cycle. These stages may differ in body shape, size, mobility, and reproductive strategies. The transition from one form to another is usually triggered by environmental cues or internal factors.

D. Alternation of generation between asexual and sexual phase of an organism:
The fundamental characteristic of metagenesis is the alternation of generations between an asexual phase (usually referred to as the "asexual generation" or "asexual phase") and a sexual phase (usually referred to as the "sexual generation" or "sexual phase"). The asexual phase typically reproduces asexually, producing offspring that are genetically identical to the parent. In contrast, the sexual phase reproduces sexually, involving the fusion of gametes (sperm and egg) to produce genetically diverse offspring. This alternation between asexual and sexual reproduction allows for increased genetic diversity and adaptation to changing environments.

In conclusion, metagenesis refers to the alternation of generations between asexual and sexual phases of an organism. It involves a drastic change in form during post-embryonic development and the presence of different morphic forms in the life cycle. While segmentation and parthenogenesis are not essential features of metagenesis, the alternation between asexual and sexual phases is a defining characteristic of this phenomenon.