Chapter 27, Problem 9A

Summary Introduction

Introduction: 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.

Answer to Problem 9A

Correct answer:

The correct answer is option (A) sea cucumber

Explanation of Solution

Explanation/justification for the correct answer:

Option (A) sea cucumber. Sea cucumbers, in the shape of respiratory plants, are the only echinoderms to have respiratory organ. These multi-branched tubes flow into the anus for absorption of oxygen in seawater. Even the respiratory tree functions in excretion by eliminating cell waste. So, the correct answer is option (A).

Explanation for incorrect answer:

Option (B) sea stars. Many sea star species have solid arms grouped around a middle disc. Sea stars can be observed in deeper water near the coast as the tide recedes, and in tide pools. They can be found in communities that stick to rocks with their tubular feet. So, this is an incorrect answer.

Option(C) sea lilies and feather stars. Fossil records indicate that sea lilies and feather stars are the most primitive of echinoderms and common before the development of other echinoderms. Sea lilies have a floral body at the tip of a long stalk while feather stars' long-branched bodies radiate outward from a central field. So, this is an incorrect answer.

Option (D) sea urchins and sand dollars. Sand dollars, though sea urchins burrow into rocky areas, can be found in deeper water burrowing into the sand. These echinoderms also have a small body in a tough endoskeleton, a test that looks like a shell. So, this is an incorrect answer.