Phylum Mollusca

Body Structure

mollusk body structure
All mollusks have a flexible body wall, it contains the internal organs which are surrounded by a body cavity. The wall is usually folded to form the mantle, which is at the top of the body and is attached like a tent. The mantle secretes material that forms a shell. The thick mass called the foot, is on the underside of the body and it is stretched out. Mollusks range in size from the 20-meter-long giant squid to the microscopic and less than a millimeter long Aplacophorans.

Obtaining Food

To obtain their food, a mollusk uses its radula, a tongue-like organ that protrudes from the mouth and scratched a belt of teeth back and forth to loosen and snatch up their food. The amount of teeth a mollusk has differs from mollusk to mollusk, as some may have a single tooth, and others may posses hundreds of thousands. An adaptation certain mollusks have is that there is poison contained in their hollow teeth. Some mollusks, the bivalves, are filter feeders. Water rushes in between their two shells and food is filtered out as it is released.


Mollusk reproduction can be simple or very complex. An example of a method of reproduction is that of the freshwater mollusks. The male releases his sperm, which goes with the ocean, until a female mollusks catches onto it, at which time the sperm begins to fertilize. Once fertilized, the egg becomes a swimming form known as a trochophore larva, then develops into a veliger larva. The process then differs as the larva forms into one of the many different classes of mollusk. Some mollusks, like snails, can become hermaphrodites, meaning they can be a female or a male. They do this to allow for more potential mates. Other mollusks, such as the oyster can switch between the sexes.


Mollusks move with their feet. The feet of different kinds of mollusks are for crawling, digging, and catching prey. Mollusks have two ways of movement. The first is jet propulsion in which they siphon water through a hole and shoot it out. The second is clams and bottom dweller mollusks have feet that they move to get around. They also use jet propulsion to escape an enemy.


Mollusks have glands that make mucus, which mollusks use in many important uses such as locomotion, food entrapment and prevention of water loss. They breath through gills called ctenidia. The gills are located in the mantle which is near the body wall of the mollusks. Some mollusks , like the nudibranch have their lungs out side their body for protection. For example, the nudibranch uses it lungs, which are outside its body, to sting its predators away from it. The nudibranch also uses it legs that it has to get around from place to place. Another thing is that it eats the oceans sand and reproduces more of it.

Importance to Humans

Theses animals are very important to humans. They provide us with food, jewelry, tools, and even pets. On the pacific coast of California, native Americans used to pick up, or dive down and scrape off abalone from the sides of the ocean. Now a days, people use abalone to build docks and some ships. Some mollusca are also used for measuring pollution. Leeches (another type of mollusk) since medieval times have been used for a number of medical purposes. The mollusk phyla definitely play a big part in society today. A lot of rare and valuable shells have been prized throughout history.



Gastropods, commonly known as the snail, has many different types of species (60,000-75,000). They live everywhere. They live in gardens, in woodland, in deserts, and on mountains; in small ditches, great rivers and lakes; in estuaries, mudflats, the rocky intertidal, the sandy subtidal, in the abyssal depths of the oceans, and countless other areas. The class gastropods includes the limpets, top shells, periwinkles, slipper shells, snails, slugs, sea hares, abalone, nudibranchs, or sea slug, and sea butterflies. Gastropods are mostly are found in saltwater, but some can be found in freshwater as well. Others also live near moist areas, such as a swamp or maybe a garden. The typical gastropod shell is a three-layers. Some gastropods don’t have shell, such as slugs. This is the largest and most successful class. It contains around 35,000 living species including conches, snails, periwinkles, and sea slugs.

giant clam

Bivalves are mollusks belonging to the Class Bivalvia. They also are called pelecypods. They typically have 2- part shells with both halves being symmetrical along the hinge line. The class has 3,000 species including scallops, clams, oysters, and mussels. They include both marine and fresh water forms. but, some like the mussels could survive out of the water for a short period of time by closing their valves. They feed by siphoning and filtering large particles from the water. Some bivalves such as scallops can swim.


Cephalopods can be found anywhere in the Earth ocean at any and every depth. They cannot live in fresh water. They are widely regarded as the most intelligent of the invertebrates and have well developed skills and big brains. All cephalopods are carnivores. They also have a highly developed eyesight. Some examples of cephalopods are: octopus, cuttlefish, nautilus, and squid. The class cephalopods also contains the largest of all mollusks, the giant squid.