Step 1: Understanding the Concept:
Symmetry is a fundamental criterion in animal classification. It describes how body parts are arranged around a central axis.
- Radial Symmetry: Any plane passing through the central axis divides the organism into identical halves (found in Cnidarians).
- Pentamerous Radial Symmetry: A specialized type of radial symmetry where body parts are arranged in fives or multiples of five around a central point.
- Bilateral Symmetry: Only one specific plane divides the body into identical right and left halves (found in most higher animals).
One phylum in the animal kingdom is particularly unique because it changes its symmetry as it grows.
Step 2: Detailed Explanation:
Let's examine the options to see which matches "pentamerous radial symmetry":
- Ctenophores (Option A): These are "Comb Jellies." They show biradial symmetry (radial symmetry with some bilateral elements), but never pentamerous.
- Sea Anemones (Option C): These belong to Cnidaria. They have radial symmetry, but it is usually hexamerous (six-part) or octamerous (eight-part).
- Echinoderms (Options B and D): This phylum (which includes starfish, sea urchins, and brittle stars) is the only group famous for pentamerous radial symmetry. However, we must distinguish between the life stages:
1. Larvae (BIPINNARIA/PLUTEUS): Surprisingly, the larval stages of echinoderms are bilaterally symmetrical. This is evidence that they evolved from bilateral ancestors.
2. Adults: During metamorphosis, the bilateral larva settles on the ocean floor and transforms into a radial adult. In most species, the body is divided into five segments (like the five arms of a starfish). This is the "Pentamerous Radial Symmetry."
This shift is an adaptation to a slow-moving or sessile life on the sea floor, allowing the animal to sense the environment equally from all directions.
Step 3: Final Answer:
Pentamerous radial symmetry is the hallmark of adult Echinoderms. The larval stages are bilateral.
Thus, option (D) is the correct answer.