Chapter 27 Mollusks and Segmented Worms

Chapter 27.1 Mollusks

What is a Mollusk?

• Slugs, snails and animals that once lived in the ocean or on the beach are all mollusks

• These organisms belong to the phylum mollusca• Animals in this phylum range from a slow moving

slug to a jet propelled squid• Most species live in the ocean • Some ocean mollusks live firmly to attached to

things and others swim freely

27.1 Mollusks

• The Mollusca phylum is the second largest phylum of animals next to insects and their relatives.

• 3 Classes of Mollusks – Snails, slugs, their shell less relatives and other

one shelled animals as a limpet make up the largest class of mollusks.

– Oysters, clams and scallops have 2 hinged shells– Predatory squid and octopuses are mollusks that

do not have an outer shell.

27.1 Mollusks• Some mollusks have shells and others are

adapted to life without a hard covering.• All mollusks have bilateral symmetry, a coelom

and 2 body openings, a muscular foot for movement and 2 openings.– The mantle is the thin membrane that surrounds the

internal organs of the mollusks, the mantle secretes the shell.

• Although mollusks look different from one another on the outside they share many internal similarities

27.1 Mollusks

27.1 Mollusks

How Mollusks Obtain Food • Many mollusks use a rasping structure called a

radula to obtain food. – A radula is located within the mouth of a mollusk,

it is a tongue like organ with rows of teeth.• It is used to drill, scrape, grate or cut food.

27.1 Mollusks

27.1 Mollusks

Reproduction in Mollusks• Most mollusks reproduce sexually, sperm and egg are

released into the water at the same time where fertilization occurs.

• Many gastropods that live on land are hermaphrodites (have the ability to produce both sperm and eggs)

• Commonly found in slow moving animals because it increases the likelihood of fertilization.

• All mollusks share similar developmental patterns, the larval stages of mollusks are similar

27.1 Mollusks

Nervous Control in Mollusks• Mollusks have simple nervous systems that

including a brain and associated nerves that coordinate their movements and behavior.

• Most mollusks have paired eyes that range from simple to complex.

27.1 Mollusks

27.1 Mollusks

Circulation in Mollusks• Mollusks have a well developed circulatory

system that includes a 3 chambered heart that pumps blood through an open circulatory system.– Open Circulatory System – blood moves through

vessels and into open spaces around the body organs• Some Mollusks have a closed circulatory system – Closed Circulatory System – blood moves through the

body enclosed entirely in a series of blood vessels. (provides an efficient means of gas exchange within the body)

27.1 Mollusks

27.1 Mollusks

Respiration in Mollusks• Most mollusks have respiratory structures

called gills.– Gills are specialized parts of the mantle that

consist of filamentous projections that contain a rich supply of blood for the transport gasses.

27.1 Mollusks

27.1 Mollusks

Excretion in Mollusks • Mollusks are the oldest known animals that

have evolved excretory structures called Nephridia.– Nephridia are organs that remove metabolic

wastes from an animals body.– Mollusks have 1 or 2 that collects waste from the

coelom• Waste are discharged into the mantle cavity and

expelled from the body by the pumping of gills

27.1 Mollusks

27.1 Mollusks

Diversity of Mollusks • Within the large phylum there are seven

classes. – Three of the classes which include the most

commonand well known species:• Gastropoda• Bivalvia • Cephalopoda

27.1 Mollusks

Gastropods(Shelled – Snails, Abolones, Conches,

Periwinkles, Whelks, Limpets, Cowries, Cones / Slugs – no shell)

27.1 Mollusks

Gastropods (One shelled mollusks)• Largest class of mollusks • Stomach footed mollusks • Most species have a shell, some like the slug

have no shell• Most shelled gastropods can be found in all

types of environments and can be plant eaters, predators or parasites.

27.1 Mollusks

Bivalves(Clams, Oysters, Scallops)

27.1 Mollusks

Bivalves (Two shelled mollusks)• Most are marine but few live in freshwater• Occur in a range of sizes • No distinct head or radula • Bivalves are filter feeders

27.1

Cephalopods (Octopus, Squid, Cuttlefish, Chambered Nautilus)

27.1 Mollusks

Cephalopods (Head footed Mollusks)• Only cephalopod with a shell is the chambered

nautilus. Some have a reduced internal shell.• Considered to have the most complex structures.• Foot has evolved into tentacles with suckers

hooks or adhesive structures• Swim or walk for their food over the ocean floor

for their food.• Like bivalves cephalopods have siphons that

expel water.