Phylum Mollusca

Introduction and Defining Characteristics
Mollusca is a Phylum within the Kingdom Animalia. Mollusks are named after the latin word “molluscus” meaning soft body. Most mollusks have muscular feet, whereas others have lost this feature in evolution. Mollusks also have a visceral mass and a mantle.
The 50,000 different species of mollusks can be found in a variety of regions, from the terrestrial mountaintops to the deep sea. They have soft bodies with a "head" and a "foot" region that is covered by a hard exoskeleton. (AM source 11)
Basic Anatomy:
We just learned that mollusks have a muscular foot, visceral mass, and a mantle, but what does this mean?
The muscular foot, like a snail’s tail, is used for locomotion. Different classes of mollusks use their feet in different ways, but the nerve chords running through the foot are consistent in most.
Mollusks have a unique visceral mass, which holds their organs and coiled digestive tract. The visceral mass makes up most of the mollusk, housing, in particular, the stomach, intestine, heart, gonads, coelomn, anus, and gills, if present.
The mantle is a layer of tissue folds that drapes over the mantle. It usually “secretes a shell (if one is present)” (656). Some mollusk have a mantle cavity, which is an extension of the mantle.The mantle cavity generally contains the gills or ctenidia, a water current, generated by beating cillia, enters the mantle cavity at the sides, passes over the gills and departs centrally, i.e. the outward bound current runs out between the two inward bound currents. (RW)
Most mollusks have a hard exterior shell made of calcium carbonate. Some mollusks have lost their shells in evolution or now have interior shells.

The mantle cavity is also responsible for holding the gills as well as having the reproductive, digestive, and excretory systems discharge into this space that is between the mantel and the visceral mass. (CP source 10)

Basic Anatomy of a Mollusk

The mantle is actually a tissue that is a “part of the epidermis, or skin, of the mollusk”. The mantle often give rise to mollusks’ shells, unless the organism does not have a shell, in which case the mantle forms an “outermost skin-like body covering,” but it contacts the shell only at its outer edges; the areas where the mantle does not come in direct contact with the shell is fluid-filled and acts as a protective layer for the body of the mollusk. (MR; Sources 16 and 17)

In shell-bearing organisms, the mantle secretes a substance that eventually develops into a three-layered shell. The names of the outermost and innermost layers are the periostracum layer and nacreous or prismatic layer, respectively. A name for the middle layer is not given. The outer layer is made out of solid calcium carbonate, known also as limestone; the middle layer contains “a strengthening sheet of more calcium carbonate crystals”; and the innermost layer is a multicolored layer that is made of a number of different organic chemicals. If you have ever seen the shell of an abalone, you have seen the colorful nacreous layer, which is also the layer responsible for the formation of pearls. (MR; Sources 16 and 17)

This is a picture of the inside of an abalone shell. More specifically, the picture shows the inner nacreous or prismatic layer of mollusks' three-layered shells and illustrates the colorful nature of the innermost layer of shell-bearing mollusks. (MR; Source 18)
This is a picture of the inside of an abalone shell. More specifically, the picture shows the inner nacreous or prismatic layer of mollusks' three-layered shells and illustrates the colorful nature of the innermost layer of shell-bearing mollusks. (MR; Source 18)

Most mollusks live in the ocean, yet some live in fresh water. Slugs and land snails have adapted to be able to live on land. Mollusks can be found in almost any part of the ocean, from the intertidal zone (which is essentially where the ocean meets the land) all the way out to deepest parts of the ocean. It doesn't matter what type of intertidal zone it is, mollusks can live on rocky shores, mud flats, sandy beaches, or coral reefs. Different types of mollusks can live on the different types of intertidal zones. (MM-source 4)

Mollusks are a part of almost every ecosystem in the world. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea. They range in size from 20-meter-long giant squids to microscopic alacophorans, a millimeter or less in length, that live between sand grains. (VN)

Major Types of Mollusks:
Class Polypacophora (Chitons):
Chitons are marine mollusks that have shells with eight plates. They use their muscular foot to move and have reduced heads.
The muscular foot is also used like a suction cup to anchor the Chiton to rocks. This is used to prevent the Chiton from being pulled away into the water by the tide. (JS 8)

Class Gastropoda (slugs and snails):
Gastropods occupy marine, fresh water, and land habitats. They are classified due to their asymmetric body; think of a snail’s coiled shell. All Gastropods undergo the Torsion Process, an occurrence during embryonic development that creates the asymmetrical body and rotation of mantle. This is why many snails have coiled shells. After the torsion process, the anus and mantle cavity are above the mouth. Can you imagine how we’d look if our bodies were situated as such?! Scientists speculate this distribution helps the gastropod center its heavy shell over its body.
This was not in the textbook, but it was so cute, I needed to add it! Liza

(Not in the textbook, but they were so cute, i was forced to add them- liza)

Class Bivalvia (clams, mussels, scallops, oysters, and more):
These mollusks live in fresh and salt water. They have a flat shell with two halves, hinged at the mid dorsal line. Abductor muscles draw the halves of the shell together to protect the animal. Like Chitons, they have reduced heads. They often use their foot to anchor themselves to a rock or deck.

Class Cephalopoda (Squid, Octopi, Chambered Nauriluses):
Cephlapods are the most unique type of mollusk, when compared to the other classes. They are marine animals, with a head surrounded by tentacles. These tentacles can have suction cups on them. Cephalopod’s shell may be internal, external, or absent. Cephalopods may or may not have radula. Their mantle has a siphon, which intakes water and pushes it out, moving the cephalopod with a jet propelled movement. The siphon steers the animal. The reason cephalopods must be so fast is that they’re carnivores and must catch prey. Cephalopods poison their victims to immobilize them. They also have a closed circulation system, nervous system, and a complex brain, unlike the other classes of mollusk.
Cephalopods (AM)
Cephalopods (AM)

Class Monoplacophora (Neoplina, etc)
This class has bilateral symmetry, a wide and flat foot, and one dome-shaped shell. This mollusk class also typically has three-six pairs of gills in shallow mantle cavity. These emerged at the same time as gastropods and live on the seabed. The Monophlacophora are segmented and their organs are duplicated in each segment. They possess a single ventricle and two auricles for circulating blood per body segment, and their nervous system is similar to the polyphlacophora. There haven't been many studies done on this class as they are found deeper in the ocean, away from more developed predators. (JP- Source 2)

The anatomy of monoplacophora (PS

Transport of Materials:
Most mollusks have a radula, which is located by the mouth and used for feeding. The radula has backward facing teeth, which work “like a backhoe” to scrape food back and forth, feeding the mollusk.
Mollusks of class Polyplacophora use their radula to graze and cut and ingest algae.
Most Gastropods use their radula to graze on algae and plants.
Bivalia have gills for feeding and gas exchange. Most are suspension feeders, meaning they trap food in mucus covering their gills, and the cilia bring the food to the mouth.
Cephalopods, being carnivores, have developed beak-like mouths to bite their prey, rather than use the radula.
Mollusks have excratory organs called nephrida. The nephrida removes waste from the mollusks circulatory fluid, hemolymph.

Acquiring and Digesting Food, and Excretion:
The anterior of a mollusc features what's known as a buccal cavity, the cavity between the jaw and the cheeks. This cavity contains radula, which features teeth supported by an odontophore (a muscular structure). With this adapatation, molluscs are able to feed.

After a mollusk consumes food, it goes into the digestive gland and then to the intestine. Wastes exits the body afterwards through the anus. (MB)

Mollusks have metanephridia, which filter nitrogenous wastes out of the blood and into the nephrostome, a ciliated funnel. Waste goes through this funnel and into the mantle cavity. From there, it passes through the anus. (Matt B - Source 6)

There is no standard of reproduction practices among mollusks, although many have sex organs with their anus. Clams and oysters are r-selected species, with a Type III survivorship curve, meaning they release many eggs into the water (lots of offspring), although few live to the maximum life span.
Many go through a larval stage, particularly the trochophor larval stage.

Trochophore are small, translucent, free-swimming larva that are characteristic of marine annelids and most mollusks. In some mollusks the trochophore develops into a second stage called the veliger before becoming an adult. The veliger has large, ciliated lobes or velum, which form from the ciliary ring or prototroch (characteristic of the trochophore stage). These velum are useful in swimming, gas exchange, and feeding.They are lost or resorbed when the mollusk metamorphoses into an adult. During the veliger stage the mollusk also begins to develop a foot and shell. (IL- source 12)

Environmental Adaptations:
Polyplacophora, which dwell on shores an ocean bottoms, have developed their muscular foot to help them cling to rocks and docks. Their foot, therefore, is like a suction cup.
Gastropods which have adapted to land lost their gills, but have adapted lungs in their mantle linings. Torsion ensures balance of the gastropod shell, centralizing it. Since torsion allows the shell to be so large, gastropods can retreat into their shells when predators come. Carnivorous gastropods have adapted their radula to be able to tear through tissue of their prey.
Bivalia have adapted mucus for suspension feeding.
Cephalopods adapted from shelled mollusks called amnonites, which are extinct. Amnonites were carnivorous, but the shell has been lost, allowing the cephalopods to move faster with their jet propelling system. They’ve adapted their siphon to be used for motion and steering. Cephalopods have also developed brains, as thinking is necessary for stalking prey. Experiments have shown that octopi are extremely smart.
Some mollusk have very complex nervous systems. For example the giant squid has adapted to catch transparent prey by means of polarized vision. (AM)

Osmotic Regulation:
Like most marine invertebrates, Mollusks (ie. gastropods) are isosmotic. This means that the blood of mollusks has the same osmotic pressure as the external marine environment. When placed in brackish water (semi salty environment), mollusks decrease their blood concentration to equal that of the environment. Fresh water species on the other hand, maintain a low blood concentration levels. (ZJ- source 13)

For protection, mollusks use their shells to protect their soft bodies inside the shell. (CM)
Cephalopods defend themselves through the use of their tentacles and their beak which makes up the jaw. In some species the suction cups of the tentacles have adapted with spikes surrounding them. These spikes can then be used to maim and cut attacking predators as well as hold onto potential prey. Many species of gastropods, such as slugs, do not develop a protective shell and are forced to find unique defense alternatives. Mostly found only on sea slugs, the skin is brightly colored and in some species have poisonous appendages to fight against other organisms . (MS 15)
Review Questions
1. What are the contents of the Visceral mass?(RK)
2. How do some mollusks protect themselves from danger? (AM)
3. How do most mollusk reproduce? What type of species does this make them? (RJ)
4. Why do many snails have coiled shells (describe the process)? In what way does this process benefit a mollusk? (SP)

Sources Cited
Campbell, Neil A., James B. Reece, Evelyn Dahlgren, Donna Kalal, and Karl Miyajima. Biology, Campbell, Reece, Sixth Edition. San Francisco, CA: B. Cummings, 2002. Print.
Campbell, Neil A., James B. Reece, Evelyn Dahlgren, Donna Kalal, and Karl Miyajima. Biology, Campbell, Reece, Sixth Edition. San Francisco, CA: B. Cummings, 2002. Print.


3. (TB)

4. "The mollusca." 25 Oct. 2011 (MM)

5. (MB)

6, (Matt B)
7. (AM)

8. Campbell, Neil A., James B. Reece, Evelyn Dahlgren, Donna Kalal, and Karl Miyajima. Biology, Campbell, Reece, Sixth Edition. San Francisco, CA: B. Cummings, 2002. Print. (JS)

9. Bunje, Paul. "The Mollusca Sea Slugs, Squid, Snails, and Scallops." The Mollusca Sea Slugs, Squid, Snails, and Scallops. Unversity of California Museum of Paleontology. Web. 6 Nov. 2011. <>. (VN)

10. "Phylum Mollusca." Infusion. Web. 06 Nov. 2011. <>. (CP).
11. "The Mollusca." Web. 30 Oct. 2011.
12. "Veliger (mollusk Larva), Trochophore (larva)." Britannica Online Encyclopedia. Web. 13 Nov. 2011. <>. (IL)

13.Barnes, Robert. Invertebrate Zoology. 2nd. Philadelphia, PA: W.B. Saunders Company, 1969. Print.
14. (RW)
"The Wonders of the Seas: Mollusks." Oceanic Research Group. Web. 13 Nov. 2011. <>.
16. "Mollusca." Animal Sciences. Ed. Allan B. Cobb. New York: Macmillan Reference USA, 2010. Gale Science In Context. Web. 28 Nov. 2011.
17. "Mollusk." Biology. Ed. Richard Robinson. New York: Macmillan Reference USA, 2009. Gale Science In Context. Web. 28 Nov. 2011.
18. Foltyn, Linda. Abalone Shell. N.d. Healing Properties of Abalone Shell and the Ocean, White Buffalo Beads and Stones, n.d. Web. 28 Nov. 2011. <>.