Chapter 17: Invertebrate Animals

Figure 16 More than 8,000 species of marine worms exist.
Some polychaetes, like this fireworm,
move around in search
of food.
Polychaetes, like this sea mouse,
have long bristles that look like
hair.
Body Types Some marine worms are filter feeders. They
either burrow into the mud or build their own tube cases and
use their featherlike bristles to filter food from the water. Some
marine worms move around eating plants or decaying material.
Other marine worms are predators or parasites. The many different
lifestyles of marine worms explain why there are so many
different body types.
Although annelids do not look complex, they are more complex
than sponges and cnidarians. In the next section, you will
learn how they compare to the most complex invertebrates.
Some polychaetes, like this tubeworm,
cannot move around in
search of food. Instead, they use
their featherlike bristles to filter
food from the water.
Summary
Mollusks
•
Mollusks are soft-bodied invertebrates that
have a mantle, a large, muscular foot, and
usually have a shell.
•
Types of Mollusks
Mollusks are separated into three groups—
gastropods, bivalves, and cephalopods.
•
Segmented Worms
Repeating body segments give segmented
•
worms flexibility.
Segmented worms have a coelom, or internal
body cavity, that separates the internal
organs from the body wall.
Self Check
1. Explain what gills are used for.
2. Describe how an earthworm feeds and digests its food.
3. Identify which type of circulatory system that a
cephalopod develops.
4. Think Critically Why would it be beneficial to a leech to
be able to store blood for months and release it slowly
5. Communicate Choose a mollusk or annelid and
write about it in your Science Journal. Describe its
appearance, how it gets food, where it lives, and
other interesting facts.
red.msscience.com/self_check_quiz
SECTION 3 Mollusks and Segmented Worms 511
(l)Chris McLaughlin/Animals Animals, (c)Nancy Sefton, (r)A. Flowers & L. Newman/Photo Researchers