Metagenesis refers to
(a) Presence of different morphile forms
(b) Alternation of generations between asexual and sexual phases of an organism
(c) Occurrence of drastic changes in form during post embryonic development
(d) Presence of a segmented body and parthenogenetic mode of reproduction

Options

A. Presence of different morphile forms
B. Alternation of generations between asexual and sexual phases of an organism
C. Occurrence of drastic changes in form during post embryonic development
D. Presence of a segmented body and parthenogenetic mode of reproduction

Answer

To solve the question regarding the definition of metagenesis, we will analyze each option provided and determine which one correctly describes the term.

### Step 1: Understand the term "Metagenesis"
Metagenesis is a biological term that refers to a specific reproductive strategy seen in certain organisms, particularly in the phylum Cnidaria.

### Step 2: Analyze the options
- **Option (a): Presence of different morphic forms**
- This option suggests that metagenesis involves different physical forms of an organism. While organisms can exhibit different forms, this does not specifically define metagenesis.

- **Option (b): Alternation of generations between asexual and sexual phases of an organism**
- This option describes the alternation between two reproductive strategies: asexual reproduction (often seen in the polyp stage) and sexual reproduction (often seen in the medusa stage). This is a key characteristic of metagenesis, especially in organisms like cnidarians.

- **Option (c): Occurrence of drastic changes in form during post-embryonic development**
- This option refers to metamorphosis or developmental changes, which is not synonymous with metagenesis.

- **Option (d): Presence of a segmented body and parthenogenetic mode of reproduction**
- This option describes characteristics that are not related to metagenesis. Segmentation and parthenogenesis are not defining features of metagenesis.

### Step 3: Identify the correct answer
Based on the analysis, **Option (b)** is the correct answer as it accurately describes metagenesis as the alternation of generations between asexual and sexual phases of an organism.

### Final Answer
**(b) Alternation of generations between asexual and sexual phases of an organism**

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