This document defines and describes several key anatomical features and concepts related to mollusks, including the trochophore larval stage, foot, mantle, shell, visceral mass, radula, siphon, and open circular blood system. It also outlines the basic body plans of mollusks in general, as well as the distinguishing characteristics of gastropods, bivalves, and cephalopods.

1. 27-4 Mollusks

2. trochophore
• Free-swimming larval stage of an aquatic
mollusk

3. foot
• Muscular part of a mollusk

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

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

6. Visceral mass
• Area beneath the mantle of a mollusk that
contains the internal organs

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

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

9. Open circular system
• System in which blood is not always contained
within a network of blood vessels

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

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

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

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

14. 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.