Chapter 13: Mollusks, Worms, Arthropods, Echinoderms

Classification of Mollusks
The first thing scientists look at when they classify mollusks
is whether or not the animal has a shell. Mollusks that have
shells are then classified by the kind of shell and kind of foot
that they have. The three most common groups of mollusks are
gastropods, bivalves, and cephalopods.
Gastropods The largest group of mollusks, the gastropods,
includes snails, conchs like the one in Figure 2, abalones, whelks,
sea slugs, and garden slugs, also shown in Figure 2. Conchs are
sometimes called univalves. Except for slugs, which have no shell,
gastropods have a single shell. Many have a pair of tentacles with
eyes at the tips. Gastropods use a radula (RA juh luh)—
a tonguelike organ with rows of teeth—to obtain food. The
radula works like a file to scrape and tear food materials. That’s
why snails are helpful to have in an aquarium—they scrape the
algae off the walls and keep the tank clean.
How do gastropods get food?
Slugs and many snails are adapted to life on land. They move
by rhythmic contractions of the muscular foot. Glands in the
foot secrete a layer of mucus on which they slide. Slugs and
snails are most active at night or on cloudy days when they can
avoid the hot Sun. Slugs do not have shells but are protected by
a layer of mucus instead, so they must live in moist places. Slugs
and land snails damage plants as they eat leaves and stems.
Conch
Garden
slugs
Figure 2 Conchs, sometimes
called marine snails, have a single
shell covering their internal
organs. Garden slugs are mollusks
without a shell.
Identify the mollusk group that
both conchs and garden slugs
belong to.
SECTION 1 Mollusks 361
(l)Jeff Rotman Photography, (r)James H. Robinson/Animals Animals