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Mollusks, Arthropods, and Echinoderms
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Section Summary
Mollusks
Guide for Reading
■ What are the main characteristics of mollusks?
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What are the major groups of mollusks and how do they differ?
Mollusks are invertebrates with soft, unsegmented bodies that are often
protected by a hard outer shell. In addition to a soft body often covered by
a shell, a mollusk has a thin layer of tissue called a mantle that covers its
internal organs, and an organ called a foot. Mollusks have bilateral
symmetry and a digestive system with two openings.
Most species of mollusks have an open circulatory system, in which the
blood is not always inside blood vessels. Most mollusks that live in water
have gills, organs that allow oxygen from the water to enter the body.
Biologists classify mollusks into groups based on physical characteristics
such as the presence of a shell, the type of shell, the type of foot, and the type
of nervous system.
Gastropods are the largest group of mollusks, and include snails and
slugs. Gastropods have a single external shell or no shell at all. Gastropods
can live nearly everywhere on Earth. Some gastropods are herbivores. Still
others are carnivores. All have an organ called a radula (plural radulae), a
flexible ribbon of tiny teeth. Gastropods usually creep along on a broad foot.
Clams, oysters, scallops, and mussels are bivalves. Bivalves are
mollusks that have two shells held together by hinges and strong muscles.
Bivalves live in all kinds of watery environments. Bivalves do not have
radulas. Most bivalves are filter feeders; they strain their food from the
water. Most bivalves are omnivores, animals that eat both plants and
animals. As adults, most bivalves stay in one place or move slowly. Bivalves
cover sand or grit lodged in their shells with a protective coat, which
eventually forms a pearl.
Octopuses, cuttlefish, nautiluses, and squids are cephalopods. A
cephalopod is an ocean-dwelling mollusk whose foot is adapted to form
tentacles around its mouth. Cephalopods are carnivores and use a radula to
scrape the flesh of their prey. Cephalopods have large eyes and excellent
vision. They have the most complex nervous system of any invertebrate.
Cephalopods swim by jet propulsion.
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