Chapter 27, Problem 4A

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 4A

Correct answer:

The correct answer is option (B) sea lily.

Explanation of Solution

Explanation/justification for the correct answer:

Option (B) sea lily. 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. Thus, sea lily is immobile for part of its life. So, the correct answer is option (B).

Explanation for incorrect answer:

Option (A) sea cucumber. Sea cucumbers are not like other echinoderms. They look like animated cucumbers that crawl across the sea floor. Thus, sea cucumber is not immobile for the part of its life. So, this is an incorrect answer.

Option(C) brittle star. 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. Thus, brittle star is not immobile for the part of its life. So, this is an incorrect answer.

Option (D) sea urchin. Sea urchins burrow into rocky surroundings. Sea urchins absence arms, but their test represents the five-part pattern of arms in starfish and starfish. Thus, sea urchin is not immobile for the part of its life. So, this is an incorrect answer.