Mollusks Print Friendly and PDF

Mollusks are a diverse group of invertebrate animals that belong to the phylum Mollusca, which includes creatures like snails, slugs, clams, oysters, octopuses, and squids.


Characteristics of Mollusks

Mollusks display a diverse range of characteristics, but some common features define this phylum:

  1. Soft body: Mollusks typically have a soft, unsegmented body that is often covered by a mantle, which secretes a protective shell in many species.

  2. Shell: Many mollusks possess a hard outer shell made of calcium carbonate. However, not all mollusks have shells. For instance, squids and octopuses lack external shells.

  3. Mantle: This is a specialized tissue responsible for secreting the shell in shell-bearing mollusks. It also plays a role in respiration and can sometimes produce coloration or aid in movement.

  4. Radula: Most mollusks have a unique feeding organ called a radula, which is like a rough, toothed ribbon used for scraping or cutting food.

  5. Bilateral symmetry: Mollusks generally exhibit bilateral symmetry, meaning their bodies can be divided into two equal halves along a central plane.

  6. Foot: Many mollusks have a muscular foot used for locomotion. This foot can vary greatly in shape and function depending on the species.

  7. Complete digestive system: They possess a mouth, digestive tract, and anus, allowing for the ingestion, digestion, and excretion of food.

  8. Open circulatory system: Most mollusks have an open circulatory system where blood, pumped by the heart, flows through vessels and sinuses, bathing the organs directly.

  9. Variety of habitats: Mollusks are incredibly adaptable and can be found in diverse environments, including oceans, freshwater habitats, and terrestrial ecosystems.

  10. Sexual reproduction: Most mollusks have separate sexes, but some species can also be hermaphroditic (having both male and female reproductive organs).

Types of Mollusks

  1. Gastropoda: This class includes snails and slugs. Gastropods are characterized by a single, coiled shell (in most species) and a muscular foot used for locomotion. They have a head with sensory tentacles and a radula used for feeding.


  2. Bivalvia: Clams, oysters, mussels, and scallops belong to this class. Bivalves possess two hinged shells that enclose the body. They typically have a hatchet-shaped foot for burrowing into sediment or anchoring themselves.

  3. Cephalopoda: Squids, octopuses, cuttlefish, and nautiluses are part of this class. Cephalopods are known for their complex nervous systems, well-developed eyes, and tentacles. Many have the ability to change color and texture for camouflage.

  4. Polyplacophora: Also known as chitons, these mollusks have flattened bodies covered by eight shell plates. They inhabit rocky intertidal areas and use a large, muscular foot to cling to surfaces.


Student Resources

Dissection of the Squid

Virtual Squid Dissection

Mollusk Reading Guide (CK 12)