medusa Cnidarian with a bell-shaped body directed downward.
nematocysts Specialized cells in cnidarians that can release a small thread-like structure
and toxins to capture prey.
porifera Filter-feeders with sac-like bodies; known as the sponges.
polyp Cnidarian with a cup-shaped body directed upward.
sessile Permanently attached and not freely moving.
Points to Consider
- How do you think that worms are different from sponges and cnidarians?
- How do you think that worms might be similar to sponges and cnidarians?
11.3 Lesson 11.3: Worms
Lesson Objectives
- Describe the major features of the flatworms.
- Describe the major features of the roundworms.
- Describe the major features of the segmented worms.
Check Your Understanding
- In terms of body structure, what does segmentation refer to?
- What is a body cavity?
Introduction
Calling an animal a worm is an informal, non-scientific classification for animals that have
long bodies with no appendages. Worms are bilaterally symmetrical, meaning that the right
side of their bodies is a mirror of the left. Worms live in a variety of environments, including
in the ocean, in fresh water, on land, and as parasites of plants and animals.
In this chapter we will discuss three types of worms: the flatworms, the roundworms, and
the segmented worms. These worms are distinguished from each other by their body plan.
The flatworms have flat ribbon-like bodies with no body cavity. The roundworms have a
body cavity but no segments. The segmented worms have both a body cavity and segmented
bodies.