Interesting Facts

  • There are over 160,000 species of mollusks known, of which about 128,000 are living and 35,000 are fossil records.
  • Mollusks can be found almost EVERYWHERE! They live from the deepest ocean floor up to the intertidal zone. They can also be found in freshwater and everywhere on land.
  • Cephalopods are shy and avoid humans.
  • Cephalopods can move backwards quickly and move slowly in any direction.
  • Cephalopods don't respond to sound but to very low frequencies Many land snails can lift 10 times their own weight up a vertical surface.
  • Helix aspersa Linneus (common garden snail) travels about 2 feet (0.6 m) in....3 minutes.


Bivalve - mollusk that lives within a shell made of two sections that are hinged together
Cecum - part of the digestvie tract, a pouch connected to the large intestine and the ileum
Cephalopod - marine mollusk whose head is attached to its foot, which is divided into tentacles
Chromatophores - pigment that contains light-reflecting cells, aids in camolouge
Gastropod - mollusk that moves by mean of a broad, muscular foot located on its ventral side; usually has one-piece shell for protection
Hectocoylus - tentacles of a male that is used for fertilization of the female's eggs
Hemocyanin - a blue respiratory pigment in the plasma
Hemoglobin - pigment that carrys oxygen in red blood cells and gives a red coloring
Hyponome - muscular funnel where water is drawn for locomotion
Foot - muscular structure in mollusks that usually contains the mouth and other feeding structures
Gill - filamentous respiratory structure in an aquatic animal
Mantle - thin, delicate layer of tissue that covers most of a mollusk's body and secretes the shell when one is present
Mollusks - soft-bodied invertebrate animal that is characterized by an internal or external shell,a foot, a mantle, and visceral mass; members of phylum Mollusca
Open Circulatory System - system in which blood does not always travel inside blood vessels
Nephridia - simple tube-shaped excretory organ used to remove ammonia from the blood and release it from the body
Radula - in some mollusks, layer of flexible skin with hundreds of tiny teeth used for feeding
Shell - structure in mollusks made by glands in the mantle that secrete calcium carbonate
Visceral Mass - structure in mollusks that contains the internal organs

Comparison between phyla


  • Mollusca is very similiar to the Annelids due to their common ancestor
  • Mollusca and Annelids both removes metabollic wastes through nephridia
  • Blood are pumped by heart in both Mollusca and Annelids
  • Mollusks and Cnidarians are both marine animals
  • Mollusks (Cephalopods and Gastropods) feeds on food including detritus and some Annelids also feed on detritus
  • Molluks, Annelids, Platyhelminthes and Nematoda may be hermiphrodites under desperate conditions when they need to reproduce


  • Mollusca have one heart while Annelids have five hearts
  • Mollusca can have open circulatory system but Nematoda and Platyhelminthes have closed circulatory system
  • Mollusks are marine creatures while Nematoda and Platyhelminthes are land creatures
  • Mollusks have a more complex system then any other phyla
  • Mollusks have shells, either externally or internally, but phylum such as Cnidarians, Annelids, Nematoda and Platyhelminthes do not



Bivalves, meaning "hatchet-foot", is also called Pelecypoda that includes animals such as clams, mussels, scallops, and oysters. Approximately 8,000 living species of bivalves live mostly in marine; however, many of them live in fresh water as well. They are found in almost every marine environment, from the intertidal zone to the deepest marine habitats.


Bivalves are filter-feeder that use their gills to extract organic particles from the water; thus, there is no need for a muscular head-foot or radula for locomotion and grinding respectively. Cilia, covering the gills and labial palps, filter food from the water and intake the food particles toward the mouth. Particles that are too large are flushed along with the water forced out by the closing of the valves.


Most bivalves have a single pair of large gills used for respiration. Enlarged gills form lamellae or folds with cilia to increase the surface area for gas exchange between the water and the blood vessels. The cilia create currents over the gills moving water through the incurrent opening into the mantle cavity. Water is also move through the siphon to the gills before the exchange, and finally exiting through suprabranchial chamber and excurrent opening.


Food particles are carried to the esophagus and down into the stomach after entering the mouth. Cilia and a mucoid food string move the food against the gastric shield, which is an abrasive structure. As the food is being ground up, enzymes are released. The stomach further breaks down the food. The indigestible particles are taken to the intestine and excreted through anus. Partially digested food goes to digestive gland for more intense digestion.

Circulation/Internal Transport:

They have an open circulatory system which means that blood is pumped by a heart, but not all blood are contained in blood vessels. Oxygen and nutrients are carried by the blood to all parts of the body.


The solid wastes are removed as feces through the anus.The metabollic waste materials are removed by nephridia. Nephridia are invertebrate organs which have function similar to kidneys.


Most of pelecypods are dioecioius, which means that individuals are either male or female. Most species simply just shed their eggs and sperms directly into the sea all at the same time. A free swimming larvae is developed after the sperm and egg fertilized.

Adaptation/Response to Environment:

Bivalves have two shells that are hinged together at the back and held together by one or two powerful muscles. They also secretes a layer called "Mother of Pearl". Most bivalves are sessile; however, some, such as scallops, can move around rapidly by flapping their shells when threatened. Some of them will close up their bivalves and dig into mud. Mussels will secrete sticky threads to attach themselves to rocks. They have simply nervous system since they have inactive lives burrowing in mud or sand.


Bivalves have a highly muscular organ called the foot, which muscle fibres run in all direction. They use it for digging and anchoring. This is an advantage because it helps to anchor the organism to the ground. Its disadvantage is that it's hard to move freely and quickly.

Internal Structure of a Clam


Class Cephalopoda

Cephalopods are marine animals and it can be found in oceans all around the world. Some of this species live in caves or rocky areas. Smaller individuals dig dens in sand-shell substrates. All cephalopods have internal shells that have chambers connected by siphuncle. They also have a large brain formed by nerve ganglia and protected by cartilaginous cranium. All cephalopods have flexible tentacles. Cuttlefish and squids generally have two long tentacles. Octopus have eight and nautilus can have up to 90 tentacles! These organisms can range in size from a few centimeters up to more than 20 meters long. They are also known as head-foot because a cephalopod's head is connected with its foot. Some examples of exphalopods are octopus, squids, cuttlefish, and nautiluses.

Feeding :

All cephalopods are carnivores. They are predators with incredible senses to help them detect food. Octopus uses their acute vision to seek a prey. They grab on to the prey with their suckers, engulf the organism with its tentacles and draw the food into them. Some octopus and cuttlefish bite their food into smaller bite-size pieces before digesting it. They would inject their prey with saliva that paralyzes. Diet of bottom-dwelling octopus includes mollusks, crustaceans, and polychaete worms. Diet of open-ocean octopus and squid are fish, prawns, and cephalopods. Furthermore, squid catches its prey by shooting out its tentacles towards their prey and draw the prey inside. Cuttlefish and squids have long, sticky tentacles with suckers that aid them in capturing prey. Others would dangle their long arms down onto a school of fish or prawns and catches the food that goes through the arms. In addition, squid that lacks tentacles have suckers with hooks to lunge at their prey. Feeding patterns of cephalopods vary across different species. Like snails and clams, cephalopods also have a radula used for feeding purposes.


Like a typical aquatic animal, Cephalopods uses gills to breath and to filter food. Cephalopods use hemocyanin rather than hemoglobin to transport oxygen throughout its body. Because these animals use hemocyanin, their blood is colorless when deoxygenated and changes blue when it gets in contact with air.

Digestion :

Digestion in cephalopods is rapid. It starts with a strong beak which contains the radula. The radula is covered with teeth for digging and scraping food. There are two salivary glands (one of them is poisonous) that secrete alpha and beta cephalotoxins that aids them in digestion. Food enters the esophagus and goes through the crop where food can be stored until ready for digestion. The food then goes inside the stomach where food is mashed by digestive substances from the salivary glands, liver, and pancreas. Food is absorbed in the liver, pancreas and cecum. The intestine supplies the stomach and path to the anus with mucous.

Circulation/Internal Transport :

Cephalopods are the only mollusks with a closed circulatory system with three hearts. This is because open circulatory system is not efficient enough for octopi and squids to move quickly. Their coelom is like a bag that surrounds their hearts. The first two hearts, gill hearts (branchial hearts) moves blood through the gill's capillaries whereas, the reminding heart, systemic heart, provides the rest of the body with oxygenated blood.

Excretion :

Wastes are excreted through nephridia. They excrete in the form of urine which contains ammonium and feces.

Reproduction :

Cephalopods are gonochoric which means their sexes are separate. Female possess a single oviduct, whereas, the male produces spermatophore. These are passed from male to the female's genital pore by using specialized arms which is the male's gonoduct. In some species, this specialized arm may tip off into the female's mantle cavity. The arm is called hectocoylus arm. However, in other species that doesn’t have hectocotylus, the male's reproductive organ (penis) is long and strong enough to transfer their spermatophores directly into the female. The eggs are then fertilized in the female as they leave the oviduct. As a result they lay a batch of fertilized eggs and are released into open water. Most cephalopod are semelparous, they die after they lay their eggs. This is different in Nautiloidea, this class produces a few large eggs in a batch and live afterwards. Cephalopods may mate by their color changes, body movements or both. They are spiral cleavage and protostomes.

Response to Environment :

Cephalopods contains special pigments cells called chromatophores enables the organism to change color and patterns precisely and quickly to match a new environment. This can help the cephalopod camouflage and communicate when danger or emotion is detected. However, this is not the case for Nautilus. Furthermore, these organisms can defend themselves with the ability to release a large amount of "smoke screen" of ink when they sense danger. For cephalopods, they also have photophores as organs that produce light and make them bioluminescence. These help them to startle predators. Cephalopod can also tell the difference between brightness, size, shape, and horizontal or vertical orientation of things. When they try to camouflage themselves, they use their chromatophores from the background they see. In addition, they can match the correct color of a background from cells called iridophores and leucophores. These cells reflect light from the surrounding. Moreover, Cephalopods have a well developed nervous system as well as complex sensory organs and brain. They can recall memories and learn because they possess ganglia that can control their thinking. <= CLICK HERE: FASCINATING CLIP ON HOW OCTOPUS DEFENDS ITSELF

Movement :

This class of animals uses jet propulsion as a primary method to move. First, they draw in oxygenated water into their mantle cavity to the gills where the circular muscles contract around the mantle cavity and expels the water out of the tube like siphon or the hyponome. The Cephalopod can then aim their siphon in different directions and forces a jet of water that can propel them backwards. Some cephalopods can move by crawling along the ocean floor or sea bed with their arms. Squids and cuttlefish can move a short distance by moving a flap of their muscles around the mantle cavity.

Anatomy of Squid:



Gastropods are the most important class of the phylum Mollusca as they make up about 80% of the mollusk species. They can also been seen as the most 'successful' class, since they inhibit almost all the habitats.
Some examples of this class include snails, slugs, nudibranch, sea butterflies, and etc.


There are many types of feeding for gastropods. For the marine gastropods, there are carnivores, herbivores, and detritus feeders. For herbivores, the radula is used to scrape the algae off rocks and twigs in the water or to eat buds, roots, flowers and plants on land. Carnivores use radula to drill hole in the shells of other animals and then extend the radula into the shell to tear up and swallow the soft tissues.


The marine gastropods breath through their gills that's inside their mantle cavities, but the land snails and slugs breathe through a special mantle cavity that is lined with many blood vessels. The surface has to be moist so the oxygen can enter the cells. Therefore most land snails and slugs live in moist places to avoid losing water to dry air.


After food has been taken in by the radula, ciliary currents or muscle contractions will transport food through the digestive tract to the stomach where digestion takes place. The food will then be broken down by the enzymes secreted by the salivary and digestive glands.

Circulation/Internal Transport:

Gastropods have an open circulatory system in which blood is pumped by a heart. Since not all blood will be contained in blood vessels, some will travel through the body tissues in spaces called sinuses, which will lead to the gill (where oxygen and carbon dioxide are exchanged) and back to heart. The nutrients obtained by digestion will be carried throughout the body by blood.


Undigested food will become feces that are eliminated through the anus, and nitrogen-containing waste, ammonia that resulted from the cellular metabolism will be removed from the body fluid through the tube-like nephridia.

Adaptation/Response to environment:
Most of the gastropods have a one piece shell, called protoconch, that is spiraled or coiled and usually opens at the right hand side. They all have a definite head with 2 or 4 sensory tentacles. When they feel threatened, they will curl up into their shells for protection. Some will even have an operculum, a hard disk that’s located on their foot, to help them close their opening, the mouth. However, for other species, like slugs and nudibranchs, that don’t have shells, they use other methods for protection. Slugs usually hide under logs and rocks in broad daylight to avoid predators. Sea butterflies will swim quickly away from danger. Sea hares use their special ink sac to squirt ink into the water and confuse the predator while they get away. Nudibranch collects the nematocysts from the cnidarians they ate and will release them when being threatened. Their bright coloring also keeps the predators away as they warn the predator of their poison.


The word gastropoda means stomach foot. It's quiet accurate, since most of the gastropods move with their broad and muscular foot located on their ventral side. The advantage of this type of movement is that they can be attached firmly to the surface using their muscular foot. However, the disadvantage shows in their slow movements, making it harder for them to escape predators.

Gastropods' reproduction varies, but most of them have separate sexes and reproduce sexually. External fertilization occurs for most of the marine gastropods, while a few have internal fertilization. Some gastropods are hermaphrodites and some are even protandric hermaphrodites (male first and then become female as they age).

Anatomy of Snail


The squid before the dissection.

After being cut open.

The white stuff on the top are the sperms produced by this male squid.
You can also see the black ink sac.

A female squid...indicated by the yellow ova.