biology 27.4

27-4 Mollusks

trochophore

• Free-swimming larval stage of an aquatic mollusk

foot

• Muscular part of a mollusk

mantle

• Thin layer of tissue that covers most of a mollusk’s body

Shell

• Structure in mollusks made by glands in the mantle that secrete calcium carbonate

Visceral mass

• Area beneath the mantle of a mollusk that contains the internal organs

radula

• Tongue- shaped structure used for feeding by snails and slugs

siphon

• Tubelike structure through which water enters and leaves a mollusk’s body

Open circular system

• System in which blood is not always contained within a network of blood vessels

Key Concept

• Mollusks are soft- bodied animals that usually have an internal or external shell.

Key Concept

• The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass

Key Concept

• Gastropods are shell-less or single-shelled mollusks that move by using a muscular foot located on the ventral side.

Key Concept

• Members of the class Bivalvia have two shells that are held together by one or two powerful muscles.

Key Concept

• Cephalopods are typically soft-bodied mollusks in which the head is attached to a single foot. The foot is divided into tentacles or arms.