Chapter 24.2, Problem 5MI

Summary Introduction

To analyze:

The evolutionary sequence by completing the given sentences.

Introduction:

The change in characteristics of a species over various generations is termed as the evolution. It is based on the process of natural selection. The theory of evolution says that all species are related to each other and their characteristics change over time.

Explanation of Solution

Asymmetrical animals do not have any symmetry in their body plans like sponges. Sponges are the only known animals with asymmetric body plan. As the time passed, some species like hydras, starfish, and other animals started to have radial symmetry. This symmetry helped these organisms to prey on other organisms from any direction. At last, the bilateral symmetry came into existence. In this the head was at the anterior portion and tail was at the posterior portion.

So, the sentences are completed as shown below:

The earliest animals had asymmetrical body plans, as do their modern descendants, such as sponges.

Later, sea stars, hydras, and other animals appeared with radial symmetry. They were able to detect and capture prey coming from any direction.

The last body plan to develop was bilateral symmetry with a head at the anterior end of the body and a tail at the posterior end of the body.