Which one of the following animals shows bilateral symmetry in its body organisation, having left and right halves that are mirror images of each other?

Introduction / Context:
Symmetry is an important concept in animal body organisation and is used in classification. Bilateral symmetry means that the body can be divided into equal left and right halves by a single plane passing through the longitudinal axis. Radial symmetry, on the other hand, allows many planes of division through a central axis. Understanding which groups of animals exhibit which type of symmetry is a common requirement in zoology exams. This question asks you to identify the bilaterally symmetrical animal from the given list.

Given Data / Assumptions:
• The concept under consideration is bilateral symmetry in body organisation.• The options include Asterias (starfish), sea anemone, Nereis, Euchinus (sea urchin), and Aurelia (jellyfish).• We assume typical adult forms of these animals in their natural habitats.• We focus on overall body symmetry rather than minor local irregularities.

Concept / Approach:
Bilateral symmetry is typical of more advanced animal phyla such as Annelida, Arthropoda, and Chordata. Nereis is a polychaete annelid worm with a distinct head, segmented body, and left and right sides that form mirror images. In contrast, Asterias (a starfish) and Euchinus (a sea urchin) are echinoderms that show radial symmetry in their adult stage. Sea anemones and Aurelia are cnidarians that also possess radial symmetry, with body parts arranged around a central oral axis. Therefore, Nereis is the only bilaterally symmetrical animal in this list.

Step-by-Step Solution:
Step 1: Recall that bilateral symmetry means a single plane can divide the body into equal left and right halves.Step 2: Identify Nereis as a segmented annelid worm with a definite head end, tail end, dorsal side, and ventral side.Step 3: Recognise that this arrangement naturally gives bilateral symmetry, as one side mirrors the other.Step 4: Consider Asterias and Euchinus, both echinoderms that show pentaradial or radial symmetry in adults, with body parts radiating from a central point.Step 5: Consider sea anemone and Aurelia, both cnidarians whose tentacles and body parts are arranged radially around a central mouth.Step 6: Conclude that Nereis is the only bilaterally symmetrical animal among the options.

Verification / Alternative check:
Standard zoology texts depict Nereis with a clear bilateral outline and internal organisation that mirrors from left to right. Echinoderms such as starfish and sea urchins are described as having radial symmetry in adulthood, even though some larval stages may be bilateral. Sea anemones and jellyfish are classic examples of radially symmetrical animals, often used to illustrate this type of symmetry in teaching. These descriptions from different animal groups confirm that Nereis is bilaterally symmetrical, while the others are radially symmetrical as adults.

Why Other Options Are Wrong:
Asterias (starfish) has arms arranged around a central disc and can be divided into similar parts by several planes, which is radial, not bilateral, symmetry. Sea anemone has tentacles and body arranged around a central oral cavity, again radial. Euchinus (sea urchin) is a globular echinoderm with spines arranged around a central body, showing pentaradial symmetry. Aurelia, a jellyfish, has a bell shape with tentacles around the margin and is radially symmetrical. None of these animals meets the requirement of a single plane dividing the body into mirror image left and right halves.

Common Pitfalls:
Students sometimes associate bilateral symmetry only with vertebrates and forget that many invertebrates, such as annelids and arthropods, are also bilaterally symmetrical. Another pitfall is to overemphasise larval forms in echinoderms, which can be bilateral, while the question usually refers to adult body organisation. To avoid these errors, remember that annelids like Nereis are good examples of bilateral symmetry, while echinoderms and cnidarians typically illustrate radial symmetry in adult forms.

Final Answer:
The animal with bilateral symmetry in its body organisation is Nereis, a segmented marine worm with bilateral symmetry.