Chapter 27, Problem 36A

Summary Introduction

To explain:

Development of additional arm in echinoderms

Introduction:

Echinoderms are marine animals with spiny endoskeletons, water-vascular systems, and tube feet; they have radial symmetry as adults. Echinoderms are deuterostomes. The approximately 6000 living species of echinoderms are marine animals and include sea urchins, sea stars, and sand dollars, sea cucumbers, brittle stars, sea lilies and feather stars, and sea daisies.

Explanation of Solution

Most adult echinoderms have arms with a five-part body plan that radiate from a central disc. The number of arms however can vary. Echinoderms are without brain or head. Rather, the nervous system consists of a central nerve ring with branches running in each of the arms. Even though echinoderms are capable of complex patterns of response, each arm works more or less independently. Many species may regenerate a new arm, including sea stars, if a portion of an arm is damaged. In some sea star species, a complete animal will regenerate from an arm attached to the central disc section. A full sea star, however, cannot regenerate from an arm alone.

Conclusion

Five arms are usually present which are not sharply marked off from the central axis. These considered the characteristics of Echinoderms.