Jackson Murphy


There are over 150,000 known species, including snails, slugs, squids, octopuses, oysters, and clams, in the phylum Mollusca. Most are marine (live in the ocean), with the exception of those that live in fresh water and snails and slugs that live on land. Mollusks (derived from the latin word molluscus, meaning soft) are generally soft-bodied animals with a hard shell made of calcium carbonate, with the exception of slugs, squid, and octopuses.

Some Different Classes of Mollusks

As you read more about mollusks, you will see that there is a wide array of body structures a

nd functions differing from class to class in the phylum. Use the list below as a reference while learning about the different aspects of mollusks.

  • Class Polyplacoda: This class includes different species of chitons, a marine animal that are ovular with a shell consisting of eight dorsal plates.
  • Class Gastropoda: There are over 40,000 species of gastropods which include snails and slugs that live in the ocean, in fresh water, and on land. They are most often differentiated by the spiral coiled shell which houses the body of the organism. (LD)
  • Class Bivalvia: This class includes different species of oysters, mussels, clams, and scallops. There are two halves to a bivalve shell, that are hinged and can open and close with the use of adductor muscles.
  • Class Cephalopoda: Cephalopods include squids, octopuses, an

d nautiluses. Squids have reduced and internal shells, while many species of octopuses don't have any shell at all. The only cephalopods with external shells are chambered nautiluses. Cephalopods are the most active of the molluscs, and some squids can move so quickly that their swimming speeds rival those of fishes. Although there are relatively few species of living cephalopods, they can be found in a great variety of habitats throughout all the world's oceans. (SW)
  • Class Monoplacophora: Commonly called called "Gastroverms", this class was thought to be extinct until the 1950's when scientists discovered twelve different species of Monoplacophora. These organisms are segmented and are shaped like limpets. They are found deep in the ocean where they are safe from most predators. (Jake Schwartz)
  • Class Aplacophora: Also known as Solenogasters, Aplacophora includes small mollusks that resemble worms. These molluscks live symbiotically with another class of mollucks: cnidaria. They usually inhabit deep waters and lack eyes and tentacles. Though they do not have a shell, they do have scales. (JAF)

external image mollusca.gif(APS)

Diagnostic Characteristics

The mollusk body has 3 main parts:
  • a muscular foot used for movement
  • a visceral mass that contains most of its internal organs
  • a mantle, which is a fold of tissue that drapes over the visceral mass and secretes a shell (if present). Many mollusks have a mantle cavity, a water filled chamber which holds the gills, excretory pores, and anus of the animal.
  • a radula (a tongue-like structure)
  • gills for respiration

  • bilateral symmetry
  • true coelom (body cavity) (LS)
  • [Many marine mollusks live on various substrates, including rocky shores, coral reefs, mud flats, sandy beaches. Gastropods and Chitons usually live on hard substrates such as these, while bivalves (has a shell which consists of two hinged valves) prefer soft substrates. They burrow into the substrate. Some exceptions of bivalves that do not live on soft substrates, however, include Tridacna gigas, which lives on coral reefs, and mussels and oysters which cling to hard substrates. Also, some microscopic gastropods may live between sand grains. (DG)]

Basic body plan of a mollusk
Basic body plan of a mollusk

Mollusca anatomy. (AS)
Mollusca anatomy. (AS)

Acquisition and Digestion of Food

Mollusks come in all types: herbivores, carnivores, detritus feeders, omnivores, filter feeders, and parasites. To accomplish this variety of feeding styles, mollusks have a very systematic way of feeding (KNS). Several mollusks feed using an organ called the radula that extends from the mouth to scrape up food like a backhoe with its backward bent teeth. The visceral mass holds a long, coiled digestive tract. Clams, oysters, and other bivalves filter feed by trapping food particles in a mucus that covers the gills as water is passes i

n and out of what are called incurrent and excurrent siphons. The food particles are then moved by cilia (hair-like structures) and an elongated flap called the palp to the mouth. Squids and octopuses of the class Cephalopada are the most aggressive feeders. They use beak-like jaws in the center of an arrangement of tentacles to bite into their prey, injecting poison to paralyze it.[The adoption of different feeding habits appears to have had a profound influence on molluscan evolution. The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. Based on our current understanding of relationships, the earliest molluscs grazed on encrusting animals and detritus. Such feeding may have been selective or indiscriminate and will have encompassed algal, diatom, or cyanobacterial films and mats, or encrusting colonial animals. RS]

Anatomy of a clam
Anatomy of a clam
Sensing the Environment

Most mollusks have nerve cor

ds along the muscular foot, which allow them to feel the surrounding area. The cords extend from a nerve ring around the esophagus. Several gastropods have eyes at the tips of antennae and scallops have many eyes that look out between the two halves of its shell. Squids and octopuses have complex brains and well developed nervous systems and sensory organs, including eyes. Squids, octopi, and snails share the trait of having good eyes, though snails do not have the same level of nervous system that octopi and squid share. (AZ) Squids have a thick nerve fiber (the axon) which sends signals to the muscles in the mantle. The squid's axon is called the "giant axon" because other species have smaller axons. (KTD)


Most mollusks use their muscular foot as a mean of transportation. Gastropod

s move about at a very slow pace by making a rippling motion with the foot. Because most bivalves are filter-feeders, they are mainly sedentary (stay in one place). However, clams can use the muscular foot to pull themselves into the sand or mud, and scallops can use it to dig and anchor into the sea floor. Scallops can also flap their shells to hop along the bottom of the ocean. Cephalopods are the most mobile of the mollusks, which is useful in seeking prey. A squid moves about by sucking water into its mantle cavity, and shooting it out of an excurrent siphon, which propels it across the water. It uses the siphon to steer by pointing it in different directions. Octopuses live and hunt on the sea floor, instead of swimming across the open ocean like squids do. The muscular foot of a cephalopod (which actually means "head foot") has evolved into this muscular siphon, the tentacles, and head.

Gastropod Anatomy


Most mollusks, with the exception of terrestrial snails and slugs, contain gills in the mantle cavity for means of gas exchange. Instead of gills, the lining of the mantle cavity in terrestrial gastropods works as a lung as it exchanges respiratory gases with the air. Mollusks have an open circulatory system. This means that the blood does not circulate entirely within vessels but is collected from gills, pumped through the heart, and released directly into spaces in the tissues from which it returns to the gills and then to the heart. Such a blood-filled space is known as a hemocoel ("blood cavity"). (JJF)


Most mollusks have an open circulatory system in the visceral mass in which a fluid called hemolymph bathes the organs as it is pumped through a dorsal heart through arteries into body spaces called sinuses. [The sinuses make the body cavity of a mollusk a hemocoel, meaning there are spaces between each of the organs in which the hemolymph flow. Because there is no separation of blood and interstial fluid (aka extracellular fluid), the hemolymph transports oxygen through hemocyanin, a pigment that turns blue when oxygenated. (Humans use hemoglobin for this function.) (BY)] Cephalopods are the only mollusks with a closed circulatory system, meaning that their circulatory fluid is kept in vessels and separate from other fluids in the body.

Circulatory system of a mullusca (Ali Kirsch)

Removal of Metabolic Waste
The excretory organs, known as nephridia, of most mollusks are located in the mantle cavity and remove metabolic wastes from the hemolymph. Digested waste is secreted out of the anus.

Self Protection

All mollusks, with the exception of slugs, squids, and octopuses, are protected by a hard shell made out of calcium carbonate. The structures of these shells vary among the different classes of mollusks. Chitons of the class Polyplacophora have a shell consisting of eight plates. Clams, scallops, oysters, and other bivalves can close their hinged shell for protection using their adductor muscles. When threatened, most gastropods can retreat into their spiraled shells for defense. Many squids and octopuses can secrete a jet of ink to distract their predators, allowing for quick escape. Slugs can protect themselves from predators by using defensive coloring. Some slugs camouflage to look like things in their environments and some are very brightly colored to warn creatures that they are poisonous. The nudibranch group contains slugs that are camflauging and those that are brightly colored. (Walker K.)
external image Ab_mollusca_01.jpg
Above is a picture of a mollusk shell (CP)

Osmotic Balance

Many marine mollusks are osmoconformers, meaning that their total solute concentration is the same, or isoosmotic, as the water surrounding them. Terrestrial snails have waxy layers on their shells to reduce water loss.

Temperature Balance

Marine mollusks are mainly thermoconformers, meaning that their internal temperatures are one to two degrees Celsius within the surrounding temperature. Therefore, they have very little control over their body temperature.

Review Questions:
1. Name three different classes of mollusks and list their major characteristics. [Rabya S]
2. How is the circulatory system of mollusks unique to that of other organisms? (SF)
3. What is the muscular foot used for? What allows it to sense its surrounding environment? (JL)

Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin Cummings, 2002.
"Defensive colour in sea slugs - Australian Museum." Australian Museum - nature, culture, discover - Australian Museum. Web. 26 Oct. 2009. <>.(Walker K.)
(Jake Schwartz) (KTD)[Rabya S.] (KTD) [BY] [BY] (JAF) (Alyssa Zisk)
(APS) (Liz Daley) (Ali Kirsch) (KNS) (Sara Waugh) Schwartz) (JJF)