Question

What is body symmetry? Describe how the body symmetry of the adult sea urchin evolved or changed from the symmetry of the pluteus larval stage.

Answer 1

Answer:

The arrangement of the parts of the body around a central axis is known as symmetry. It refers to a balanced distribution of the duplicate parts of the body or shapes within the organism's body. There are three kinds of symmetry. These are asymmetry, radial, and bilateral symmetry.  

A sea urchin demonstrates a characteristic pattern at the time of development. They go through a bilateral planktonic larval stage and after some weeks, this larvae go through metamorphosis to produce a form of an adult body exhibiting radial symmetry. The sea urchin transforms into a pluteus larva by producing hollow cell balls, which produces the gut and mouth with internal skeleton. This larva then utilizes its cilia to engulf food and gathers nutrients for the next phase of development. The sea urchin shows radial symmetry at the time of metamorphosis in a bottom-dwelling. This phase is known as indirect development.  

Some species of sea urchin could load their eggs with adequate nutrients so that they can skip the stage of the larva, and goes directly into the adult stage, this is known as direct development. They exhibit a bilateral symmetry at the larval stage and radial symmetry at the adult stage.