Mollusks are soft bodied animals with an internal or external shell. The name of the phylum is derived from Latin word molluscus meaning soft.

Phylum characteristics

Life Processes


Phylum Significance


Classes (The type of shell and foot are used to group mollusks into classes.)


Table 1 Classes of the Phylum Mollusk
Scientific Name Pronunciation Common Name # Shells Foot # of Species
Polyplacophore [Amphineura] chitons 8-plates ~650
Gastropoda GAS-troh-pahdz univalves (snails, slugs) 1 or none stomach foot ~90,000
Bivalvia [Pelecapoda] bivalves 2 hatchet foot ~8000
Celphalopoda SEHF-uh-loh-pahdz octopus, squids internal head foot ~650
Scaphopoda tusk shells 1 ~350

Polyplacophore (many plates). [Amphineura].

Examples ...chitons. (Green-striped chiton)


Gastropoda (GAS-troh-pahdz)(stomach foot)

Examples: Snails, chonches, abalones, periwinkle, limpet, sea butterflies. Some with no shells...slugs, sea hares and nudibranches. (Angelic tooth snail, Sneda nudibranch, Olive snails, Sea slug Tube snail)


Bivalvia (two shells) [Pelacepoda (hatchet foot)]

Examples: clams, oysters, scallops, mussels, shipworm (teredo), giant clam (Tridacna) large as 1.9m

(Swimming clam, Horse mussel)

Cephalopoda (SEHF-uh-loh-pahdz) (cephalo = head: pod = foot) p. 592

Scaphopoda - tusk shell


The unusual

Mr and Mrs Smith's Life Science


Polyplacophora: Chiton
Chiton.bmp (131254 bytes)

In the Latin language, poly means many. I have a shell made out of eight separate, but overlapping, plates or valves. You may also know me by my common name, the Chitons.  I only live in the ocean and am generally found clinging tightly onto the rocks in the intertidal zone (the area in the ocean where waves wash in and out and where water from the tidal changes wash in and out of). A few of my Class do live in very deep (more than 5,000 meters) ocean places. The members of my Class range in size from 2 millimeters to 40 centimeters (1/16 inch to 16 inches), and there are about 800 living Species represented in my Class.    By having several valves that can be moved separately, we are able to change the shape of our bodies to fit onto the uneven hard rocky places we like to live on. Our broad, fleshy foot holds us to the rocks so tightly that neither the violent ocean waves nor an enemy can dislodge us easily. You would have to take a sharp object, like a knife, to pry us off our rock and if you did succeed in getting me off, I would curl up into a ball. My hard plates would be on the outside of this ball and my soft vulnerable body parts protected on the inside of the ball.

Most of the members in my class are herbivores (we eat plants). In our mouths is a special mechanism known as a radula. This radula is like a ribbon of tiny hard teeth (think of something very rough like a piece of sandpaper or a fingernail file). By licking the rock with this radula, we rasp off pieces of plant material or algae growing there. We then swallow and digest these food particles.

We breathe in oxygen from the water through six to eighty-eight pairs of gills.

Both the male and females of my class generally release their eggs and sperm directly into the water. When they meet, a new baby polyplacophora is born. However some of the females in my class do keep their eggs inside their body (in the mantle cavity). When they draw in water to breathe, they also draw in the sperm and thus fertilize their eggs. They then give live birth to their young, all ready to fend for themselves in the sea!



Monoplacophora.bmp (105390 bytes)

 Prior to the year 1952, scientists thought I was extinct (no longer living). Then while on the Danish "Galathea" expedition, scientists discovered ten living specimens of Neopilina (the fancy Latin name of one of my relatives). A few years later a second living specimen of Neopilina was discovered. Today, there are about a dozen known living representatives in my class. The rest of the monoplacophora members are only known by their fossil records.

The reason that I am so rarely found is that I live in very deep water, well below 200 to 600 meter (1/3 mile) depth level and that there appear to be only a very few of us living today.

My shell is only about 1.0 to 3.5 centimeters long and is shaped like a cap. It is often very thin and fragile. I crawl around the ocean bottom by means of a large, round, flat foot. Inside my body, my organs are in pairs almost like those of the earthworm; however, I am not segmented like he is. I have several paired gills (one per section) through which I breathe and one heart and kidney per body section.

The females of my Class release their eggs directly into the surrounding water in great clouds. The males also release their sperm into the same water. If these eggs and sperm meet by chance, new baby monoplacophora are born.


Scaphopoda: Tusk Shells

TuskShell.bmp (228150 bytes)

We are often known and called by our more common name, the "Tusk Shells".

I, as do all the members of my Class, live in the marine environment (salt-water habitat) only. We all live in a shell house that we build to protect our very soft body. There are about 350 different species in our Class living in shallow to relatively deep (2,000 meters) water. We range in size from 2 millimeters to about 15 centimeters (1/16th of an inch to 6 inches).

Some of our shells look like an elephant s tusk, while others look like a swollen cucumber. Whichever shape we take, our shells are open a both ends making us look like a fat drinking straw that has swallowed a big lunch. This tube-like shell usually has several heavy ribs running the length of it and it is most often coloured white and brown or white and green.

We tusk shells have a spade-shaped (like a shovel) foot that we use to dig ourselves into the soft muddy or sandy ocean bottom. Here we stay all our lives.

We draw water in through the small tip of our shell that sticks out of the sand or mud where we live.  This water flows into our mantle cavity (remember, the mantle is the organ that builds our shells) where oxygen is just absorbed directly into our blood. We do not have gills for breathing, nor do we have a heart or blood vessels to carry and pump our blood. We are what scientists call "a very primitive and simple organism".

My cousins and I always live head down in the sand or mud. We have many skinny thread-like tentacles that have a tiny sticky pad at their tip. These captacula project from our heads and wiggle through the surrounding mud or sand in search of food. These fine threads, called captacula, latch onto very small food items (called detritus) found in the sand or mud and then they pull them in to our mouth. From there, the food is rasped by the radula (which looks and acts much like a fingernail file) into finer particles and is then digested.

Both the females and males of my Class release their eggs and sperm directly into the surrounding water. If, by chance, the eggs and sperm get together, fertilization occurs and a baby scaphoda is born.



1.   The Prosobranchia:
This is the biggest of the subclasses and contains most of the mollusc seashells that people recognize and pick up along our beaches. Most of the members here are marine inhabitants, or in other words, they live only in salt water. However, many of the members can also live in freshwater or even on land (terrestrial). Every member of this big subclass has a coiled shell. Their soft bodies have a head complete with two eyes located on the tops of two tentacles. These cousins of mine all breath by means of gills. They have a big flat foot, which they use for locomotion and on the back end of this foot is a structure called an operculum, which acts as a trap door. When a prosobranch is threatened by an enemy or he is afraid of getting dried-out (all molluscs must keep moist to stay alive), they pull all their soft, moist body into their shell then they close the operculum behind them just as you close the door to your house.

Most of the seashells you people collect along beaches or lakes are from the members of this subclass. Some of the families in this subclass are known by names such: conchs (Strombidae), whelks (Buccinidae), limpets (Lotiidae), periwinkles (Littorinidae), cones (Conidae), volutes (Volutidae), and cowries (Cypraeidae).


2.The Opisthobranchia:
All of the members of this subclass are marine inhabitants. Most members in this sub-class only have a very small, fragile shell and it is often contained right inside their soft bodies or they may not have a shell at all. Most of these cousins of mine breathe through their gills; however, some absorb oxygen from the water directly through a specialized membrane (something like the thin skin lining the insides of your cheeks) lining their mantle cavity.

Many of these molluscs have very colorful bodies. We know some of the Opisthobranches as: sea hares, sea  butterflies (Thecostoma), sea slugs (saccoglossans and nudibranchs), and canoe (Scaphandridae) and bubble shells (several families).


3. The Pulmonates:
These are my woodland and garden dwelling snail and slug cousins. Most of these molluscs live on land or in freshwater lakes and streams; however a few are marine dwellers. They all breathe by means of pulmonary sacs not gills.

Some have coiled shells, but many do not have a shell at all. One very interesting fact about most pulmonates is, the members of this subclass are both male and female (hermaphrodites) at the same time.

This comes in very handy as these snails are very slow moving and don't like moving around too much. If they had to go looking for a boyfriend or girlfriend, it could take them a very, very long time to have babies. Being both male and female solves this problem. When they come across another snail of their kind, they just exchange sperm packets with each other and then they both go off and lay eggs. Baby molluscs hatch out some time later and they too are born as both boy and girl within the same slug or snail body.



SciBiValve.bmp (322134 bytes)

I am called a bivalve because in the Latin language (the language scientists and doctors use) any word that starts with the letters Bi means two. Everyone in my Class has a two-part shell that is hinged together. These two shell parts are called valves. I open and close these valves by using my strong adductor muscles and ligaments just like you bend your elbow or knee.

I don't have a mouth like you do; I have a siphon (like a short, fat drinking straw that feels like rubber) which I use to pull in water and tiny animals that live in the water, as well as "detritus" (i.e., little particles of organic matter from a variety of sources) . When this water enters my body, my gills take the oxygen out of the water and release my used up air back into the water. My gills also filter out the tiny food particles from the water and pass them on to my stomach where they are digested.

I really don't have much of a head but I and a few of my cousins have many eyes; (see my picture at the beginning of the article) however, most of my cousins do not have any eyes at all. They don't need them because they live firmly attached to solid objects or live deeply buried beneath the sand or muddy bottoms of oceans, lakes or rivers and they don't have to see anything.

Covering my soft body is a thin membrane called the mantle (like a thick piece of skin). The mantle takes lime and calcium out of the water and turns it into my two-piece shell. Some of my cousins have very, very tiny shells - as tiny as half a millimeter (there are 24mm in an inch)!!, while my biggest cousins have shells well over one meter (thirty nine inches) across and may weigh over three hundred and thirty kilograms (seven hundred pounds). Some of us have shells so thin and fragile that just picking them up will crush them while some others are so hard that you could jump up and down on them and they wouldn t break. Some are shiny smooth, while some are just like fingernail files or sandpaper. Some have sharp spines all over them to protect themselves so that fish and other animals won t eat them (example: Spondylidae - the "spiny oysters"). Our shells come in every colour of the rainbow and some of our shells are so pearly on the inside that people often use them to make fancy buttons for their cloths.

Our mantle can do one other thing that is very important to people. If a foreign object such as a piece of sand gets into our soft mantle, it HURTS, so we take the same smooth shelly material that we put on the inside of our shell and cover the offending object up and guess what! We ve just made a pearl. The longer we live, the bigger the pearl gets. If the pearl comes from one of my cousins who have the pearly insides on their shell, these pearls become very valuable; however, most of us just produce a pearl that is not very pretty or useful to man.

Most of our families reproduce by laying millions of eggs into the water surrounding us. The male bivalves then release their sperm into the same water. If the eggs and sperm meet, a new baby bivalve is born. Some of my female cousins hold their eggs in a space called the mantle cavity in their body. The males still spurt their sperm into the water and when she pulls this water in through her siphon, the eggs are fertilized. These are then brooded inside her body until she knows they are big enough to live in the water. She then releases them into the water. All baby bivalves start life as tiny specks, (larval stage of growth) swimming in the water. When these larva become big enough, they start to settle onto their new homes.  When they are still young, yet settled, they are called "spat".

My cousins choose many different kinds of homes to live in or on. Some like to live attached to hard objects such as rocks or man-made objects. Some live all their lives buried beneath the sandy or muddy ocean, lake or stream bottoms. Some actually live inside wood. These cousins of mine used to cause man a lot of trouble when he used to sail in wooden ships. HE ATE HOLES IN THE SHIP and it often SANK. Some of my other cousins are parasites, meaning that they live inside a living host, such as a fish, and survive by eating part of the host.




The internal structure (long oval chalky "bone") of the cuttlefish is used in birdcages as a dietary supplement and for keeping their beaks in good condition.

We are the smartest, (of all the invertebrates) fastest and the most handsome (in my opinion) of all the molluscs. We are a most ancient order and are well represented in fossil records (Ammonites are our most famous ancestors). We live only marine (salt-water) environments and there are around 700 species of cephalopods living today. Some members of my Class are the octopus, squid, cuttlefish and nautilus.

All my cousins and I have large heads and very complex brains. We also have two eyes and use them to see almost as you humans do (Our optical nerve is of particular interest, being amazingly large. It has been used to study how optical nerves work). We can think through fairly complex puzzles (like where you hid some food in a maze) and remember how to solve them faster the next time.

Only the nautilus has a shell and I will show you a picture of this (link). The argonaut has a shellac-like structure but this not a true shell.  It is but a brood case (sort of cradle) to hold the eggs until the young hatch. This case is not attached to the animal and is held in place with one pair of her tentacles.  After the Agonauta (the argonaut genus) mother is through with this egg case, it is left to float on the ocean, and is often picked up by men, who call it a "paper nautilus". Click HERE to view a picture of an Argonaut Egg Case

We have cells in our skin, called chromataphores, which enable us to rapidly change our skin color and pattern whenever we want to. Example: if we get mad, we go a blotchy red color; if afraid we go a very pale almost white color. We can change our skin color and pattern to match our surroundings as well; this is called being able to camouflage ourselves.

If an enemy threatens us, we squirt out a cloud of purple-black ink, which confuses them and we jet away. This ink is also toxic or poisonous to our enemies such as sharks.

We all have arms modified with sucker discs with which we capture, hold and pull our food to our hard parrot-like-beak of a mouth. Octopus have eight of these tentacles. Squid have ten (called decapods) tentacles. The cuttlefish and nautilus have many. If we ever lose one of these tentacles in a fight or due to an injury, we just grow a new one!  The tips of these tentacles are so sensitive that we can tell the difference in size, shape and texture of objects.

I and all my cousins breath through gills, and have three hearts. We are all carnivorous predators (we catch live food) eating such things as other molluscs, fish and other marine invertebrates. We crush our food in our hard bill then rasp off bits with the radula (like a fingernail file) located in our mouths.

Most cephalopods have separate sexes and fertilization is internal. The males produce a "sperm packet" which he places inside the female's body using one of his tentacles. Sometime later, the female then lays eggs. Many cephalopods are good mothers and stay with their eggs until they hatch. They keep clean, fresh water flowing over the eggs and caress them to keep them clean of derbies. The young hatch out as perfect small copies of their parents.


With 70,000 living species (perhaps only a third of the number out there), the mollusks must be counted as among the most successful animals on earth today. Most belong to the first 3 of the 6 classes shown here:

  1. Bivalvia. Two shells encase the body. Includes the clams, mussels, oysters, and scallops.
  2. Gastropoda. Snails and slugs. Snails have a single shell ("univalves') while slugs have none.
  3. Cephalopoda. This marine group includes the various species of octopus, squid, as well as the chambered nautilus. A record 28-foot octopus and 60-foot (18 m) squid make these the largest of all the invertebrates.
  4. Scaphopoda. Marine, filter-feeding "tooth shells".
  5. Amphineura. Includes the chitons, whose shell is made of 8 overlapping plates ("polyplacophora")
  6. Monoplacophora. Until a live specimen was discovered in 1952, these animals were thought to have been extinct for millions of years. It has a single shell (hence the name) and, unlike the other mollusks, is segmented (as are its relatives the annelids).