nacre The iridescent inner shell layer produced by some bivalves, some gastropods, and
some cephalopods; also known as mother of pearl.
pearl The hard, round object produced within the mantle of a living shelled mollusk.
radula A molluscan feeding structure, composed mostly of chitin.
Points to Consider
- Many mollusks demonstrate bilateral symmetry. How do you think this differs from
the radial symmetry evident in echinoderms, in the next lesson? - As we have seen, some species of mollusks live in the deep ocean around hot hydrother-
mal vents. In the next lesson we will learn that many echinoderms also live in the deep
sea. What adaptations do you think both groups might have for living in such a unique
environment? - Mollusks have an exoskeleton, which is primarily external and composed of calcium
carbonate. As a result many of these are preserved in the fossil record. How do you
think this compares to the type of skeleton that an echinoderm has and to its fossil
record?
12.2 Lesson 12.2: Echinoderms
Lesson Objectives
- Discuss the traits of echinoderms.
- List the types of echinoderms.
- Explain the roles echinoderms play.
Check Your Understanding
- What is meant by body symmetry?
- What is radial symmetry?
- What is bilateral symmetry?
Introduction
We’re all familiar with starfish (Figure12.10), and also maybe sea urchins (Figure12.11)
andsanddollars(Figure12.12). Theradialsymmetryiswhathitsusrightaway,asymmetry
in which the body is arranged in five parts around a central axis. Much of the perceived