Rachel Kornetsky

Well-known Classes
sea-shell-mollusk-01.gif (10 SV)
Polyplacophora (chitons)
Gastropoda (snails, slugs)
Bivalvia (clams, mussels, scallops, oysters)
Cephalopoda (squids, octopuses, chambered nautiluses)

Chitons have oval shapes, and their shells are divided into eight dorsal (having to do with the back) plates. The eight plates are surrounded by a mantle called the girdle. They live in marine habitats and are often found attached to rocks. Chitins use extracellular digestion (the digestion of food outside the organism's body). When they reproduce, the eggs are fertilized either externally in the seawater (meaning that the female and male release their gametes into the water to fuse together) or in the female's mantle cavity. A chiton has a subradular chemosensory organ, which is used to "taste" the surroundings (3) [HZ] .

Gastropods have a spiral shell. A distinctive feature of gastropods is the process of torsion. When a gastropod is still growing as an embryo (the growing organism after fertilization), an asymmetrical muscle contracts and rotates the visceral mass to a maximum of 180 degrees, placing the anus and mantle cavity above the head when the organism becomes an adult. A possible advantage of this is that the weight of visceral mass and the shell is balanced upon the center of the body (1) [HZ].

Bivalvias has two shells, and are the most well known type of mollusk, just because of their prominence in human's everyday life. Over 80% of the bivalvia class is marine, ocean habitants, while the rest can be found in fresh water. Bivalves do not use a radula to feed, they filter organic particles straight from the water through their gills. The gills are the bivalves have a mucous coating, which traps the particle of food while water passes through the body. (6, GR) One site indicates, however, that the word Bivalva, "Bi" meaning "two" and "valva" meaning "shells", is a misnomer, for there is only a single shell with with left and right lobes. Many people think that one half of a clam shell is the top and the other is the bottom, but in fact, these animals are laterally flattened, meaning there is a shell on the left hand side and the right hand side. (15) (SS)

Cephalopods have distinguished themselves as the most intelligent, the most mobile, and largest of all mollusks. This group is extremely diversified, consisting of over six hundred different organisms, includings squids, octopuses, cuttlefish, and chambered nautiluses. These organisms display remarkable diversity in their size and lifestyle, with specializations for predation, movement, communication, and self-protection. These invertebrates have deveoped suckered tentacles, camera-like eyes similar to those of humans and other mammals, color-changing skin, and complex learning behavior. A large part of this class is made up of fossils found from nearly 500 million years ago. Cephalopods are also the most discussed of all mollusks in history and literature. (7 Nangia)

Basic Body Plan of Mollusks

All Mollusks have the same fundamental body plan containing three distinct body zones. There is the head-food zone that contains the sensory and motor neurons. The second zone is the visceral mass that contains organs involved in digestion, excretion, and reproduction. The third zone is called the mantle, specialized tissue that hangs over and enforces the visceral mass and that secretes the shell. The mantel cavity is a space between the mantle and the visceral mass that houses the gills. (8) (BMB)

Diagnostic Characteristics
Mollusks are soft bodied organisms that are marine animals. Some well-known mollusks include snails, slugs, oysters, clams, octopuses and squids. Mollusks are known for their external shells made of calcium carbonate, although some mollusks such as slugs, squids, and octopuses have reduced internal shells or no longer have a shell but did so at some point in its evolution. The body of a mollusk includes a head-foot which is usually used for movement, a visceral mass which contains most of the internal organs, and a mantle which is a fold of tissue that drapes over the visceral mass and is responsible for the secretion of a shell.The head-foot contains the sensory and motor organs(8 VM). The organs contained in the visceral mass include the organs of digestion, excretion, and reproduction(8 VM). The specialized tissue of the mantle allows for the shell's secretion(8 VM). Mollusks all contain a soft body that in encased in a hard calcium containing shell, although some forms, such as slugs, do not have a hard shell. Mollusks are known for containing coelom, fluid filled cavity that develops within a layer of the embryo. This functions as a place where internal organs can be suspended by the membrane that invests the intestines. (8) (BMB)

mollusca.gif (NK)

Acquiring and Digesting food, Metabolic Waste Removal
Most mollusks have a mouth region with a radula. The radula is a strap-like organ that scrapes up food. It consists of a belt of backward-curved teeth that extend from the mouth and slide back and forth. Once the food is broken down in the radula it enters the digestive tract. The digestive tract consists of the esophagus, stomach, intestine and anus. Organs called nephridia remove metabolic wastes from the digestive tract. Mollusks like other aquatic organisms secrete ammonia as waste. Different classes have certain variations of this digestive system and consume in different ways. Polyplacophora ingest mostly algae. They are grazers and use their radulas to cut and eat their food. Most Gastropoda are grazers as well and use their radulas to consume algae and plant material. However, there are some Gastropoda that are predators and have a modified radula that can penetrate the shells of their prey and tear through tough animal tissue. Bivalves do not have radulas, but instead intake food through use of their gills. Their gills are coated with mucus that trap food particles which are then transmitted to the mouth. Bivalves are known as suspension feeders (strain matter and food from water) and are mostly sedentary (stay in one place). Cephalopods are carnivores and use beaklike jaws to bite prey. They also inject poison into their prey to immobilize them. In cephalopods, the mouth is located at the center of several tentacles. They consume crabs and other organisms that live on the see floor.

Sensing the Environment
Mollusks have a nervous system that consists of a nerve ring around the esophagus which nerve cords extend from. Some mollusks have eyes. Gastropods often have a distinct head with eyes at the ends of tentacles. Bivalves have many eyes peering out from the two halves of the hinged shell. Cephalopods have the most developed nervous systems of the mollusks and some even have a complex brain and other sense organs. One of these organs is a Cephalopods eyes. They see and focus on images almost as well as humans do a major difference between the two is that Cephalopods are almost all color blind.(SJB)(9) The complexity of the nervous system is demonstrated through the octopus, which is thought to be the most intelligent of all invertebrates. The octopus's mental capacity is very similar to that of a domestic cat. Giant squid have the largest eyes in the animal kingdom (the size of plates). Squid locate/capture their transparent prey through specialized polarization vision (12) (AR) Chemoreceptors are very important in detection of the environment as well. Chemoreceptors are cells that allow organisms to receive chemical messages. (RJS) Gastropods have chemosensory cells in the skin all over their bodies. Also, around the mouth, lips, and lip feelers, there are specialized taste receptors. Feelers on the head specialize in smell. To detect food, mollusks have an osphradial sensory organ, which analyzes the chemical composition of their surroundings.(13) (AR).


Most mollusks have two separate sexes, however some gastropods are hermaphrodites (an organism having both male and female reproductive organs). The gonads (ovaries or testes) are located in the visceral mass. The first stage of reproduction for freshwater mollusks is fertilization, where the sperm are released into a water current until they are collected by a female to fertilize her egg (4 SES). Mollusks often take the form of ciliated larva called trochophore for the first part of their life cycle. Afterward, the larva turns into a free-swimming veliger larvae (2) (DP). Freshwater mollusks do not go through these two stages, but rather enter a larval stage where they are called a glochidium (2) (DP). It can take from one to ten months for the glochidium to fully develop inside the female, and then they are released into the water (4 SES). The glochidium attaches itself to fish or other objects that can withstand the current (2) (DP). Some female mollusks have adapted large mantles that resemble some sort of fish food (4 SES). This way, the glochidia have a better chance of finding a fish to attach to (4 SES). Once these larvae have found their host fish, they act like tiny parasites by feeding off the host's nutrient-rich cells for about one to four weeks (4 SES). After this time period, the glochidia are considered juvenile mussels (4 SES). They detach from the fish, and they begin to filter feed via their radula (4 SES).

Most mollusks have a muscular foot that is used in their movement. In the Polyplacophora class, the foot can act like a suction cup anchoring them to rocks, etc. Gastropods move by a rippling motion of the foot. Bivalves have a hatchet-shaped foot that they use for digging or anchoring. However, they move very little and mostly stay stationary or attach themselves to rocks, docks, boats, and shells of other animals by secreting strong threads from sessile mussels. Some bivalves can move along the seafloor by flapping their shells open and closed. The foot of Cephalopods has evolved to become parts of a muscular siphon (tube-like protrusion from the mantle), tentacles, and the head of the organism. To move, Cephalopods draw water into their mantle cavity and then fire it out in a jet stream though a siphon which propels the organism forward. Cephalopods can steer by adjusting the siphon in different directions.

The presence of a discrete respiratory system has led to an improved capacity for oxygen uptake and distribution in mollusks(8 VM). Mollusks use gills (outfoldings an organism’s body surface) for respiration which are housed in the mantle cavity for most mollusks. As water passes over the gills, gas is exchanged through diffusion. Some Gastropods lack gills, but instead the lining of the mantle cavity exchanges gases with its surroundings and acts as a lung. The shells of gastropods help keep a thin lining of water on the mantle cavity that allows oxygen in the air to dissolve and be absorbed while the organism is on land. In bivalves, the water enters the mantle through an incurrent siphon, passes over the gills, exchanges gas (and also in this case food), and then exits through an excurrent siphon.

Most mollusks have an open circulatory system (the organs are bathed in a circulatory fluid, includes blood, called hemolymph). The heart pumps the hemolymph into sinuses where the materials are exchanged with the cells. The hemolymph returns to the heart through valves that open and close called ostia. Most Mollusks have a dorsal (pertaining to the back) heart that is two or three chambered. Cephalopods are the only mollusks that do not have an open circulatory system. They have a closed system circulatory system (blood circulates through vessels that are distinct from other bodily fluids).

Self Protection

Many Mollusks have hard, protective shells made out of calcium carbonate that protects their soft bodies. Mollusk's feet can often act like a suction cup to anchor them to rocks, etc. so that they are not swept away by the tide and to make it more difficult for predators to remove mollusk's from their rocky shelters (ER). Some mollusks in the class Gastropoda and Cephalopoda can inject poison that immobilizes other organisms. Cephalopoda can dart about relatively quickly. Also, Cephalopoda can change color to camouflage into its environment. This helps because preadators are less likely to see them. It also helps when they are hunting because their prey will not be able to see them coming. (JAC) [5]

Osmotic Balance
Many Mollusks are osmoconformers which means that they do not actively adjust their internal osmolarity. Its internal osmolarity is the same as that of its environment and there is no tendency to gain or lose water. However, some types of mollusks are also osmoregulators which means that they actively regulate the osmotic pressure of the fluids that surround them. A particular type of mollusk that is an "osmoregulator" is the Bivavle Mollusk. The Bivalve Mollusks regulate the osmotic balance of intracellular fluids, not extracellular fluids, and they do this by making sure the salinity of amino acids stays within ideal regulation. (Jesse Carmen){14} However, mollusks who can live in water and on land have other mechanisms to maintain an osmotic balance.

*osmotic balance is the diffusion of water in and out of cells to maintain a balance with the total solute concentration

Temperature Balance
Mollusks are ectotherms (organisms who use environmental energy and behavioral adaptations to regulate its body temperature). Ectotherms have very slow metabolic rates, so they do not produce enough heat to affect their body temperatures. Mollusks are also thermoconformers which means that they have little control over their body temperatures. Like other Ectotherms, mollusks may be able to adjust to new temperatures through acclimatization (physiological changes) at the cellular level.

1) Mollusks have three different regions that make up their body, first zone, second zone, and third zone. Describe what makes up each of the zones. (HS)
2) Describe and explain the two different circulation systems Mollusks can have. (YS)
3) What are the ways by which mollusks can sense their enviroments? (KN)
4) What are the difference in shells of the chitons, gastropods, and the bivalvias? (DJ)
5) What does it mean if a gastropod is a hermaphrodite? (BL)

Evolutionary Relationships Between Seven Classes
external image mollusc_classes.jpg

Works Cited

Campbell, Neil, and Jane Reece. Biology. Sixth ed. Ed. Beth Wilbur. Boston: Benjamin Cummings, 2002. 656-658, 872-873,887-888.
Kellogg, Derek, and Daphne G. Fautin. "Class Bivalvia." Animal Diversity Web. University of Michigan Museum of Zoology. 2 Dec. 2008 <>.
3.) Campbell, A. and D. Fautin. 2001. "Polyplacophora." Animal Diversity Web. 6 December, 2008 <>.
4.) "Reproduction." 5 Nov. 2008. Marietta College Biology Department. 7 Dec. 2008. <>
5.) Wood, James, and Kelsie Jackson. "Why Cephalopods Change Color." 20 May 2004. The Cephalopod Page. 7 Dec. 2008 <>.
6.) "Snails and their Relatives." The Wonders of the Seas: Mollusks. 5 June 2007. Oceanic Research Group. 7 Dec. 2008 <>.
7.) Vendetti, Jann. "The Cephelopoda: Squids, octopuses, nautilus, and ammonites." The Cephalpoda. 2006. University of California Museum of Paleontology. 7 Dec. 2008 <>.
8.)7 Dec. 2008 <>
10) "All the Sea." 10 Dec. 2008 <>
12.) "Phylum Mollusca." 13 Dec 2008. <>.
13.) 'Structural organization of receptor elements and organs of the land molluskPomatia." Neuroscience and Behavioral Physiology. 13 Dec 2008. <>.
14.) "Adaptations and Responses to Physiological Conditions." 15 December 2008. <>.
15) Hebert, Paul D. "Mollusca." The Encyclopedia of Earth. 11 Oct. 2007. Boston University. 17 Dec. 2008

Edited by: Hanna Z., Ethan Richman, Sarah Schwarzschild, Josh Czik, Grace Rehnquist, Meru Nangia, Vonai Moyo, Sam Blatchford, Brittany Marcus-Blank, Hilary Stepansky, Arielle Reiter, Kevin Nayer, NK, Jesse Carmen, SS, Becca Levenson