Which of the following organisms has both a free-swimming medusa stage and a…

Concept

Cnidarians can occur in two basic body forms: polyp and medusa. A polyp is usually sessile and attached to a substrate, whereas a medusa is free-swimming. In many cnidarians, these two forms occur at different stages of the life cycle.

Application

Jellyfish belong to Cnidaria. Their life cycle includes an attached polyp phase and a swimming medusa phase, so the description in the stem matches jellyfish.

1. A ciliated planula larva settles on a suitable surface.

2. It develops into a sessile polyp attached to the substrate.

3. The polyp produces juvenile medusae, which grow into free-swimming adults.

Contrast

• Phytoplankton are photosynthetic aquatic drifters; the name describes an ecological plankton group.

• Zooplankton are heterotrophic aquatic drifters; the name also describes an ecological group rather than one fixed animal life cycle.

• Fungi grow mainly as hyphae or yeasts and do not have polyp and medusa body forms.

Cross-check

The sequence from an attached polyp to a free-swimming medusa independently confirms the cnidarian life-cycle pattern described in the question.

Therefore, jellyfish are the matching example.