Regardless of class, mollusks typically have three regions: a head, a foot, and a visceral mass. The variations in the body plan of each class of mollusk are the very reasons they are classified the way they are. Click through the slides below to learn about the different parts of a mollusk and learn a bit about the variations between the classes.
Head
A mollusk has a head that contains the brain. In some species, particularly the bivalves, the head isn’t clearly seen. In cephalopods--like the octopus shown--the head is easily distinguishable. They have relatively large and complex brains that can solve problems.
Foot
Most mollusks have a single foot (or wide, flat muscle) that is used for movement. This foot assists them in burrowing in the sand or mud (as many shelled mollusks do) or sliding along the ground (like slugs and snails do). In mollusks related to the squid, this foot is divided into tentacles that are used for feeding and for movement.
Visceral Mass
The visceral mass is made up of the digestive, reproductive, excretory, and respiratory systems. It contains all of the mollusk’s organs, including a heart and kidneys. The digestive system includes a mouth, a digestive tract, and an anus. Mollusks do not have lungs; they breath through gills if they live in water or a membrane that acts like a lung if they live on land.
A gastropod’s visceral mass is always contained within the shell.
From the visceral mass comes the mantle.
Mantle
The mantle is a layer of skin that surrounds the soft body tissue. In clams, oysters, mussels, and snails, the mantle produces their hard shells. In squids and octopuses, the mantle is soft and helps the animal move through the water. Slugs don’t appear to have shells, but there is a soft one just below the surface of their skin.
Radula
Most mollusks also have a tongue-like organ called a radula that is used for scraping food from rocks or other objects. Cephalopods and gastropods have them, but not bivalves. Bivalves, such as clams, oysters, and mussels, filter food particles from the water with their gills.
Reflection
Question
What makes mollusks unique from other phyla?
Mollusks have three special structures: the head, the foot, and the visceral mass.
