Chapter 27, Problem 6A

Summary Introduction

Introduction: Approximately 6000 life forms of echinoderms are aquatic animals which include coral stars, sea urchins, sand dollars, brittle stars, sea cucumbers, sea lilies, and sea daisies.

Answer to Problem 6A

Correct answer:

The correct answer is option (D) arthropods.

Explanation of Solution

Explanation/justification for the correct answer:

Option (D) is arthropods. Arthropods are grouped into classes based on common characteristics, such as body segment form, appendages and mouthparts. The nomenclature of arthropods is still questioned by taxonomists. Arthropods have joint-appended exoskeletons that allow them to move in diverse ways.So,the correct answer is option (D).

Explanation for incorrect answer:

Option (A) is a pattern of development. The echinoderm was the first group of animals to have endoskeletons in evolutionary history.So, this is an incorrect answer.

Option (B) is mouth develops from somewhere on the gastrula away from the opening. The anus of a deuterostome grows from the gastrula bottom opening and mouth is developed from elsewhere. Only echinoderms and the chordates that formed after echinoderms have developed in this manner. So, this is an incorrect answer.

Option (C) is echinoderms. Echinoderms are deuterostomes which represent a significant transition in animal phylogeny. Echinoderms are aquatic mammals and are the only species to have a deuterosome development and an endoskeleton in evolutionary history. So, this is an incorrect answer.