Understanding Metagenesis in Organisms

The question asks to identify an organism that exhibits metagenesis. Metagenesis is a biological phenomenon where an organism's life cycle involves an alternation between different body forms or generations, specifically in cnidarians, between the polyp and medusa stages. The polyp stage is typically sessile and reproduces asexually, while the medusa stage is usually free-swimming and reproduces sexually.

Analysing the Options

Let's look at the given options:

1. Euspongia
2. Obelia
3. Sycon
4. Spongilla

Why Sponges (Euspongia, Sycon, Spongilla) do not show Metagenesis

Euspongia, Sycon, and Spongilla are all types of sponges (Phylum Porifera). Sponges are simple multicellular organisms. Their life cycles can be quite different from cnidarians. They do not have the distinct polyp and medusa body forms that characterize metagenesis as seen in many cnidarians. Their reproduction involves both asexual methods (like budding or gemmule formation) and sexual reproduction, but this does not involve the clear alternation between sessile polyp and free-swimming medusa generations.

Understanding Metagenesis in Obelia

Obelia belongs to the Phylum Cnidaria, Class Hydrozoa. A key characteristic of many hydrozoans, including Obelia, is the presence of both polyp and medusa stages in their life cycle. The life cycle of Obelia demonstrates metagenesis:

• The sessile colony form is the polyp stage, which reproduces asexually by budding.
• These buds develop into free-swimming medusae.
• The medusae are the sexual stage, producing gametes (eggs and sperm).
• Fertilization occurs, forming a zygote.
• The zygote develops into a larva (planula), which settles and forms a new polyp colony.

This clear alternation between the asexually reproducing polyp generation and the sexually reproducing medusa generation is precisely what defines metagenesis in cnidarians.

Conclusion on Metagenesis

Based on the life cycles of the organisms listed, Obelia is the organism that exhibits metagenesis due to the regular alternation between its polyp and medusa forms.

Revision Table: Key Terms

Additional Information on Metagenesis and Cnidarian Life Cycles

Metagenesis is often confused with alternation of generations seen in plants. While both involve alternating forms, the specific body forms (polyp/medusa) and reproductive methods (asexual budding vs. sexual gametes) are characteristic of cnidarians like Obelia. Not all cnidarians exhibit metagenesis; some groups are exclusively polyps (like corals and sea anemones), while others are exclusively medusae (like some jellyfish). Hydrozoans, like Obelia, are the classic examples demonstrating this type of life cycle alternation.