Jointless

DIGESTIVE GLAND

INTESTINE

-Digestive tract

The body of most mollusks is soft, extremely flexible, and without joints, yet has a large and very hard shell. Most mollusks live in the ocean, but they are also found in lakes and land environments. All modern mollusks have bilateral symmetry, one cephalopod foot with sensory organs and locomotion, a visceral mass, and a covering, called the mantel, that secretes the shell. Mollusks also have a very peculiar mouth structure called a radula.

BROWN GARDEN SNAIL

Helix aspersa

Gastropods

These mollusks are characterized by their large ventral foot, whose wavelike motions are used to move from place to place. The group comprises snails and slugs, and they can live on land, in the ocean, and in fresh water. When these animals have a shell, it is a single spiral-shaped piece, and the extreme flexibility of the rest of the body allows the gastropod to draw itself up completely within the shell. Gastropods have eyes and one or two pairs of tentacles on their head.

PROSOBRANCHIA

This mollusk subclass mainly includes marine animals. Some have mother-of-pearl on the inside of their shell, whereas others have a substance similar to porcelain.

SALIVARY GLAND

ESOPHAGUS 's.

LUNGED

Snails, land slugs, and freshwater slugs have lungs, and their lung sacs allow them to breathe oxygen in the atmosphere.

OPISTHOBRANCHIA

are sea slugs, which are characterized by having a very small shell or no shell at all.

FEMALE SEXUAL ORGAN

SEA ANGEL

Candida sp.

BENDING OF THE SNAIL

In snails, bending is a very special phenomenon that moves the cavity of the mantle from the rear toward the front of the body. The visceral organs rotate 180 degrees, andlhe digestive tube and the nervous connections cross in a figure eight.

-Nervous system

Otter shell

MOLLUSK SPECIES; AS

GREEN MUSSEL Perna viridis

NAUTILUS

Nautilus sp.

Cockle

Tellin

SCALLOP Pectenjacobaeus

Cockle

Tellin

Razor clam

Otter shell

Under the Sand

Many mollusks live buried under the sand in order to hide from predators and the effects of waves, wind, and sudden changes in temperature.

PROTOBRANCHIA

This class includes bivalves with a split lower foot, called a sole. Bivalves use their gills only to breathe. This subclass includes small bivalves 0.5 inch (13 mm) wide, called nutclams (Nucula nitidosa).

COLEOIDEA

Cephalopods of this class have a very small internal shell, or none at all, and only two gills. Except for the nautilus, this class includes all cephalopods alive today-octopus, cuttlefish, and squid.

Bivalves

Mollusks with a shell divided into two halves. The two parts of the shell are joined by an elastic ligament that opens the shell, adductor muscles that close the shell, and the umbo, a system of ridges that helps the shell shut together. Almost all bivalves feed on microorganisms. Some bury themselves in the wet sand, digging small tunnels that let in water and food. The tunnels can be from a fraction of an inch long to over a yard long.

SCALLOP Pectenjacobaeus

LAMELLIBRANCHIATA

include most bivalves. They use gills to breathe and to feed. They have no differentiated head, eyes, or extremities. They can grow up to 5 inches (13 cm) long, and they rest on the ocean floor.

100,000

THE NUMBER OF LIVING

GREEN MUSSEL Perna viridis

MOLLUSK SPECIES; AS

Cephalopods

Cuttlefish, octopus, squid, and nautilus are called cephalopods because their extremities, or tentacles, are attached directly to their heads. These predators are adapted to life in the oceans, and they have quite complex nervous, sensory, and motion systems. Their tentacles surround their mouths, which have a radula and a powerful beak. Cephalopods can be 0.4 inch (1 cm) long to several yards long.

NAUTILOIDEA

This subclass populated the oceans of the Paleozoic and Mesozoic periods, but today only one genus-Nautilus-survives. A nautilus has an outer shell, four gills, and ten tentacles. Its shell is made from calcium, is spiral in shape, and is divided into chambers.

NAUTILUS

Nautilus sp.

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