Phylum: Molluska
Introduction:
Molluska is the common name for members of a phylum of soft-bodied animals (Latin mollus,"soft"), usually with a hard external shell.
Familiar mollusks include the clam, oyster, snail, slug, octopus, and squid.
The mollusk phylum is the second largest in the animal kingdom, after the arthropods.
Earlier estimates of the number of mollusk species sometimes exceeded 100,000, but more recently this figure has been reduced to fewer than 50,000; the new estimates are incorporated here.
Mollusks are highly successful in terms of ecology and adaptation, with representatives in virtually all habitats, but they are most diverse in the sea.
Among them are some advanced animals, such as the octopus and squid.
Giant squid are also the largest invertebrates, weighing up to 1980 lbs.
Most mollusks, however, are about 0.4 to 8 in. long, and some are barely visible to the unaided eye
Ecology & importance:
Mollusks are abundant and hence important in food chains in many habitats.
Most bivalves filter suspended materials from the water.
Many gastropods (snails, slugs) are carnivorous, most of them preying on slow-moving or attached animals.
Cephalopods (octopus, squid) are active predators on larger animals such as crabs.
Numerous mollusks are important food sources for humans.
Some gastropods (snails, slugs) damage crops, and others harbor disease-causing parasites.
Some important or interesting mollusks:
Oysters:
Oyster is the common name for any of several different species of marine bivalve mollusks.
Several of the more than 50 living species of oysters are edible.
Oysters attach themselves to rocks or lie on the sea bottom.
They are unable to move but are often dislodged from their resting place by waves.
The shell of the oyster is irregularly oval in shape.
It consists of a left and right valve joined together at the narrow anterior end by an elastic ligament that acts as a hinge.
Attached to both valves is a strong muscle called the adductor, which keeps the shell tightly closed.
Oysters can snap their shells closed with lightning speed and with the power of a vise.
To open a live oyster, you must insert a knife blade between the shells and sever the adductor muscle, then remove the meat. This is called "shucking."
When the adductor relaxes, the elastic ligament pulls the valves apart.
The left valve, upon which the oyster rests, is deeper and thicker than the right one.
Except for the dark, pigmented areas where the shell is connected to the adductor, the inner surfaces of the valves are white.
Two folds of fleshy membrane, called the mantle, cover the oyster's soft body and line the inside of the shell.
The mantle secretes the organic and inorganic substances that make up the shell.
At the anterior end of the body, between two pairs of thin lips, or palps, is an opening that constitutes the mouth of the oyster.
Two pairs of sickle-shaped respiratory organs, the gills, are covered with hairlike structures called cilia.
A short gullet connects the mouth to the stomach.
The body also contains the digestive, reproductive, circulatory, excretory, and nervous systems.
The oyster feeds on microorganisms that are brought into the shell with the current produced by the movement of the cilia and sorted out by the labial palps before they reach the mouth.
Oyster reproduction:
Oysters have varying methods of reproduction.
The European oyster and the Olympia oyster of the American Pacific Coast are hermaphrodites-that is, their reproductive organs contain both eggs and sperm.
The eggs are fertilized within the body and are retained in the gills until shell-bearing larvae are formed.
In the American bluepoint oyster of the Atlantic Coast, the sexes are separate.
Females discharge millions of eggs into the water, where fertilization occurs.
The fertilized oyster larvae, or spat, float for many weeks until they settle on a hard surface and begin to develop a shell.
Spat looks like grayish dirt.
When it settles onto a shell, rock or piling to begin development it is called spatfall.
Oysters switch their sex during their lifetime.
All oysters begin life as males and become females after a couple of years.
It even appears they can detect sexual imbalances among them and switch back to become males again as needed.
Distribution:
Oysters are found throughout the world.
They usually form large beds, which extend in warm waters from the tidal zone to a depth of up to 100 ft.
Beds of American bluepoint oysters are found along the eastern coast of the continent.
Chesapeake Bay is the largest oyster-producing body of water in the world, although many of its oyster beds have been depleted through overfishing or pollution.
Large beds of edible oysters also exist in Japan and Australia.
The native American Olympia oyster, which is much smaller and has a thinner shell than other edible species, is found on the west coast of North America.
Pearls:
When a foreign body gets lodged in the oyster shell, the oyster begins to build concentric layers of onion-like material, thereby giving birth to a "pearl."
Certain tropical species of oysters produce pearls of iridescent luster that are commercially valuable.
Pearls come in many different shapes and hues, but the most valuable are the large, perfectly round and flawless, black pearls.
Rarely is a pearl of any value found in North American waters.
History of pearls as jewelry:
Long known as the "Queen of Gems," pearls possess a history and allure far beyond what today's wearer may recognize.
Throughout much of recorded history, a natural pearl necklace comprised of matched spheres was a treasure of almost incomparable value, in fact the most expensive jewelry in the world.
Now we see pearls almost as accessories, relatively inexpensive decorations to accompany more costly gemstones.
Before the creation of cultured pearls in the early 1900s, natural pearls were so rare and expensive that they were reserved almost exclusively for the noble and very rich.
A jewelry item that today's working women might take for granted, a 16-inch strand of perhaps 50 pearls, often costs between $500 and $5,000.
At the height of the Roman Empire, when pearl fever reached its peak, the historian Suetonius wrote that the Roman general Vitellius financed an entire military campaign by selling just one of his mother's pearl earrings.
Rome's pearl craze reached its zenith during the first century B.C. Roman women upholstered couches with pearls and sewed so many into their gowns that they actually walked on their pearl-encrusted hems.
Pearls, in fact, played the pivotal role at the most celebrated banquet in literature.
To convince Rome that Egypt possessed a heritage and wealth that put it above conquest, Cleopatra wagered Marc Antony she could give the most expensive dinner in history.
The Roman reclined as the queen sat with an empty plate and a goblet of wine (or vinegar).
She crushed one large pearl of a pair of earrings, dissolved it in the liquid, then drank it down.
Astonished, Antony declined his dinner -- the matching pearl -- and admitted she had won
The Arabs have shown the greatest love for pearls.
The depth of their affection for pearls is enshrined in the Koran, especially within its description of Paradise.
The Koran reads: "The stones are pearls ... the fruits of the trees are pearls and emeralds; and each person admitted to the delights of the celestial kingdom is provided with a tent of pearls ... and emeralds; is crowned with pearls of incomparable lustre, and is attended by beautiful maidens resembling hidden pearls."
Oyster culture is practiced in many countries.
Kokichi Mikimoto, the son of a noodle maker, had a dream and a hard-working wife, Ume.
Together they set about to do what no one else had done -- entice oysters to produce round pearls on demand.
Mikimoto did not know that government biologist Tokichi Nishikawa and carpenter Tatsuhei Mise had each independently discovered the secret of pearl culturing -- inserting a piece of oyster epithelial membrane (the lip of mantle tissue) with a nucleus of shell or metal into an oyster's body or mantle causes the tissue to form a pearl sack. That sack then secretes nacre to coat the nucleus, thus creating a pearl.
Mise received a 1907 patent for his grafting needle. When Nishikawa applied for a patent for nucleating, he realized that he and Mise had discovered the same thing.
In a compromise, the pair signed an agreement uniting their common discovery as the Mise-Nishikawa method, which remains the heart of pearl culturing.
Mikimoto had received an 1896 patent for producing hemispherical pearls, or mabes, and a 1908 patent for culturing in mantle tissue.
But he could not use the Mise-Nishikawa method without invalidating his own patents.
So he altered the patent application to cover a technique to make round pearls in mantle tissue, which was granted in 1916.
With this technicality, Mikimoto began an unprecedented expansion, buying rights to the Mise-Niskikawa method and eclipsing those originators of cultured pearls, leaving their names only for history books.
Largely by trial and error over a number of years, Mikimoto did contribute one crucial discovery.
Whereas Nishikawa nucleated with silver and gold beads, Mikimoto experimented with everything from glass to lead to clay to wood.
He found he had the highest success rates when he inserted round nuclei cut from U.S. mussel shells.
Although some countries continue to test other nuclei, U.S. mussel shells have been the basis for virtually all cultured saltwater pearls for 90 years.
Even though third with his patents and his secrets, Mikimoto revolutionized pearling.
Ever the flamboyant showman and promoter, he badgered jewelers and governments to accept his cultured products as pearls.
His workers created massive pearl structures, which he displayed at every major international exposition.
By mastering the techniques, Mikimoto, then hundreds of other Japanese firms, made pearls available to virtually everyone in the world.
What's Killing the Oysters?
Japan's akoya oysters have been preëminent among cultured pearls ever since Kokichi Mikimoto first established the industry in the early 20th century.
But since 1994, something has been killing off the akoyas of Ago Bay, the heart of Japan's cultured-pearl business, and elsewhere in the country.
Experts attributed the initial oyster deaths in 1994 to "red tide," a bloom of microscopic, toxin-producing animals in the ocean that proved deadly to the oysters.
Yet the oyster deaths continued long after the 1994 tide had dissipated; in fact, they increased.
In 1996, the most recent year for which figures are available, 150 million akoya oysters perished.
In the wake of the die-off, pearl production has taken a beating.
This has helped open the door for other kinds of pearls, such as Chinese freshwater and South Seas saltwater pearls, with a consequent loss of market share for the long-reigning akoyas.
The Japanese have just had to accept a much smaller market share than they've been accustomed to.
Even after several years of scientific investigation, the specific cause of the disease remains a mystery.
The illness first makes itself known when the abductor muscle, which holds the two parts of the oyster shell together, turns a reddish-brown.
Ultimately, eight out of ten affected oysters die from the affliction, which so far has only affected akoya oysters.
Even isolating apparently healthy oysters has had no effect: the disease seems to ferret them out.
Government and company researchers both within and outside of Japan have struggled to identify a specific pathogenic source, but without success.
Scientists and others have suggested everything from inbreeding to climate change as possible causes.
The most prevalent notion is that the culprit is a virus.
Many remain skeptical of the virus theory, however, and point to pollution as the cause.
When Kokichi Mikimoto first began growing oysters in Ago Bay early in the 20th century, the bay was close to pristine, with few people living around it and no industry.
Today, however, the heavily populated bay faces a continuous assault from industrial and automobile pollution, pesticides and herbicides, household detergents and other chemicals, even raw sewage.
Others feel the oyster farmers themselves might be to blame.
"The Japanese have always tended to place too many oysters too close together," wrote Andy Müller in the December 1996/January 1997 issue of Pearl World. "In earlier days (when sea conditions were better), most oysters still had a fair chance to survive in their densely packed state. But today's equation of high pollution, plus high shell density, just doesn't work out. It takes just a little change in either condition to trigger high mortality in oysters . . . even in a species as hardy as the akoya."
Some authorities take issue with the idea that pollution is to blame for such reasons as.
Akoya oysters share the same waters with other Japanese aquaculture products, including shrimp, other oysters, and blowfish, which have not been affected.
Also, 20 percent of those oysters that get the disease survive and go on to produce a decent pearl.
Will Japan become a nation of pearl processors and marketers rather than producers? Only time will tell.
Danger of eating raw oysters:
Source of information
U. S. Food and Drug Administration
Center for Food Safety and Applied Nutrition
FDA Prime Connection
This information is meant for certain individuals who are at a higher risk for infections from a bacterium of public health significance in our coastal waters.
The bacterium, Vibrio has probably always been a part of our marine environment, but increasing scientific investigations and medical knowledge have raised concerns for its occurrence in coastal waters and certain raw foods.
Vibrio vulnificus is a bacterium that can be found in warm coastal waters most common about the Gulf of Mexico, but it also has been found in water samples from both the Atlantic and Pacific coasts.
It occurs naturally, rather than as a result of pollution.
Thus Vibrio vulnificus is often present in clean waters, including those that are approved for the harvest of oysters and clams.
Little else is known about the organism, but reports of illnesses and infections associated with this bacterium are most prevalent during warm months of the year, primarily April through October, leading scientists to believe there might be a correlation between the bacterium's presence and seawater temperatures.
Concern for Vibrio vulnificus exists because certain people who eat raw oysters or expose open wounds to warm seawater can develop a severe and potentially fatal infection.
Most people's immune systems are able to ward off these infections.
Certain conditions do put some individuals in a "high risk" category.
People in the high risk category must consider diet modifications and general changes in life style in order to maintain good health.
Conditions for "high risk" category
Liver disease including cirrhosis.
Chronic alcohol use
Cancer (especially if taking anti-cancer drugs or radiation treatment)
Lymphoma, leukemia, AIDS, Hodgkin's disease
Diabetes mellitus
Chronic kidney disease
Inflammatory bowel disease
Any person receiving immunosuppressive drugs
Steroid dependency.
Medicines or other conditions that reduce stomach acid
How can people in this "high risk" category avoid Vibrio vulnificus?
Vibrio vulnificus infections are either transmitted to humans through open wounds in contact with seawater or through consumption of raw oysters.
Studies have shown that Vibrio vulnificus is most likely to be present during warm months (April-October).
Avoid exposure of recent or healing wounds, cuts, punctures, burns, etc. to warm seawater.
When swimming or wading, temporarily cover the wound with a water tight wrap.
The Vibrio vulnificus lives naturally in warm seawater, can enter a person's wound and, in some cases, extend to the bloodstream and cause a potentially fatal illness.
The highly invasive nature of this bacterium is cause for special concern.
Consumers in high risk categories should avoid consumption of raw oysters.
Oysters are filter feeding animals that can concentrate
Vibrio bacteria from the water into their system.
This concern exists for any raw oysters regardless of harvest from approved or questionable waters.
When eating shellfish, particularly oysters, be sure that they are properly
and thoroughly cooked.
Thorough cooking kills the Vibrio bacteria and markedly
reduces the risk of becoming ill.
Steaming to open the oyster shells or blanching the shellfish does not always provide enough heat to kill all the bacteria.
Additional heating is necessary to impart a noticeable cooked
appearance.
Avoid cross-contamination by placing cooked shellfish in the original
container used for raw shellfish, or storing raw and cooked shellfish in the same area.
What are the chances for an infection in people not in a high risk category?
Rare! Most healthy individuals are not troubled by Vibrio vulnificus infections
from water or food.
Also, extensive federal and state regulatory programs monitor the production and marketing of raw shellfish to assure product safety.
Thus, the Vibrio vulnificus problem is primarily restricted to individuals in the risk
categories.
These individuals should restrict consumption of raw shellfish.
The cephalopods (Octopus, Squid, chambered nautilus)
The cephalopods are unusual mollusks because most lack a hard shell.
The chambered nautilus is the only type that has a complete shell.
The squid has a small shell that is located inside of the body, rather than outside.
The octopus, on the other hand, has no shell at all.
All of the cephalopods are marine animals and all are carnivores.
While cephalopods appear to be very different from their mollusk relatives, they all have the same basic body plan: mantle, internal organs, gills, foot.
In the cephalopods, instead of one muscular foot, there are at least eight arms projecting from the head region, accounting for the name Ahead-footed.@
Cephalopods have unique abilities which most mollusks lack. They are able to change the color of their skin so that it blends in with the surroundings (discussed below).
They are also known for their ability to squirt a potential predator with "ink" and escape backwards by forcing water from a siphon near the head. This is called jet propulsion. (Discussed below).
Cephalopod eyes have received a lot of attention because they are much more developed than other invertebrate eyes.
They are more similar to vertebrate eyes because they have a cornea, lens, and retina.
Cephalopods can see images, an ability that is not found in other mollusks or any other invertebrates.
These animals reach the largest size of any invertebrate.
The giant squid, Architeuthis, which lives in the North Atlantic, may reach 56 ft. in length and weigh three tons.
This length includes the tentacles, which may measure 20 ft.
In an animal this size, the eye is 1 ft. across.
The Anti-social Octopus:
Octopuses are perhaps the most familiar of all cephalopods.
There are about 200 known species.
They range in size from a few inches to 2 ft.
In general, the larger species are found in colder, northern waters.
The common octopus, Octopus vulgaris, is found in all oceans and grows to 3 ft. in length.
Most live in shallow coastal waters, inside dens or small caves on the ocean bottom.
They live alone and do not travel in schools.
It never knows its parents (discussed below).
Newborn common octopuses, flealike creatures the size of rice grains, spend their first weeks as ocean plankton, drifting at the surface.
After gaining weight, they drop to the bottom, where they spend most of their lives hiding watchfully in dens, which can be rocky crevices, abandoned shells, or holes scooped in the sand.
Some will even block the entrance to the den with rocks to keep intruders out!
If no natural caves are available, octopuses will gladly live inside of an old car tire, clay pot or glass jar.
The mantle of the octopus looks like a wrinkled leather sack.
In front of the eyes are eight arms.
The arms may have as many as 240 suction cups on the underside, in two rows.
These suckers make it almost impossible to remove an octopus once it has attached to an object.
If an arm is bitten off by a predator, it will grow back over time.
When the octopus spreads its arms out, and floats down to the ocean bottom, it almost looks like a parachute.
In the center of the arms is the mouth and beak.
The beak, which is used for feeding, looks very much like a parrot's beak.
It is used to crush the shells of crabs and lobsters, which are the octopus' favorite food.
Once the beak has pierced the prey, the salivary glands of all octopuses secrete a chemical that helps disable its prey and breakdown its muscle tissue.
In at least one species, Australia's blue-ringed octopus, the secretion contains a neurotoxin that constitutes the deadliest venom known in nature, capable of killing an adult human in minutes.
However, these octopuses do not bite humans unless handled or disturbed.
The poison paralyzes the prey and softens the meat so that the octopus can suck all of the flesh into its small mouth.
In this way, it devours everything but the shell.
Often, a diver can locate an octopus den by looking for a pile of empty shells that have been tossed out after feeding.
Anyone who attempts to keep an octopus in captivity will find out the hard way that it is an escape artist.
Because it does not have a shell, it can squeeze into and out of very small openings.
Even if an aquarium is covered the octopus needs only a tiny space to squeeze through.
Without a shell, how does an octopus defend itself against such enemies as sharks & moray eels:
If attacked, the first thing that an octopus can do is change colors.
It can make the color of its body match the surroundings.
It has special color cells all over its body called chromatophores which expand and contract to control the pigments inside.
Therefore, the mantle may appear red-brown, yellow-orange, blue-green, or even white.
The texture of the mantle can be changed as well.
It can be camouflaged to look like rock, coral or seaweed.
If these tactics do not work, and there is no shelter available, the octopus can release a cloud of black ink.
The ink does two things that will help it to escape.
First, it hides the animal.
A hungry moray eel may not see the octopus in a patch of murky water.
Before the ink clears, it can shoot water out of its siphon and make a quick getaway.
The octopus can only swim rapidly over very short distances.
The second thing that the ink does is to temporarily destroy the eel's sense of smell. Be the time the moray can smell again, the octopus is safe inside its lair!
The octopus usually remains inside of its lair unless it is searching for food.
Octopus reproduction:
Octopuses as a group act like leisurely, almost lazy animals.
The reason is chemical: Octopus blood is a poor carrier of oxygen.
As a result, an octopus tires easily.
To stay alive, it relies on a system involving three hearts and permanently high blood pressure.
This helps explain why even sex is a sluggish activity in many octopus species.
With little or no foreplay, without even raising its pulse, a male will extend a specialized arm and insert it into the female's mantle cavity.
A sperm packet then slides slowly down a narrow groove in the arm and enters the female's oviduct.
"Yes, it can be pretty blasé," says John Forsythe, a research scientist at the National Resource Center for Cephalopods in Galveston, Texas. "Sometimes the female will continue foraging."
Yet there is one time in its short life when it does not look for food at all.
In the common octopus, Octopus vulgaris, the female is so devoted to the care of her eggs that she does not eat at all after they have been shed.
After mating, a female may lay as many as 45,000 eggs, which she attaches to the roof of her den.
She will take care of the eggs during the development period which lasts 1-2 months.
She uses her arms to remove any particles that might settle down on the eggs and even squirts water from a siphon if necessary.
This is known as brood care.
She never leaves the eggs, not even to look for food.
Unfortunately, soon after the eggs hatch and release hundreds of tiny octopuses, the mother will die.
The octopus has a very advanced nervous system:
The central nervous system of the octopus is among the largest and most complex in the invertebrate world, rivaling that of many vertebrates, including birds and fish.
How intelligent that nervous system makes the octopus is still a matter of scientific debate, however.
Over the years, scientists have tested octopus intelligence by teaching captive specimens to slither through simple mazes and to tell squares from crosses.
Octopuses even learn to unscrew lids to get at food.
The most dramatic evidence for octopus intelligence came in 1992.
Researchers in Naples, Italy used conventional means--food as a carrot, mild electric shock as the stick--to train a group of captive common octopuses to grab a red ball instead of a white one.
The scientists then let untrained animals watch from adjoining tanks as their experienced companions reached for red balls over and over.
Thereafter, most of the watchers, when offered a choice, pounced on red balls.
In fact, they learned to do so more quickly than had the original group.
The octopuses, according to the researchers, were doing something invertebrate had never been known to do before: learning by watching.
Criticism of this 1992 experiment:
Critics since then have weighed in with a list of complaints about the experiment.
Controls were sloppy: the researchers themselves concede that untrained octopuses at the outset already preferred red balls by more than three to one.
Other critics wrote that octopuses typically "are reluctant to attack novel stimuli." Having watched trained octopuses repeatedly snatch the red ball, the untrained animals may simply have gotten used to watching that ball and so were more apt to pounce on it themselves.
The Swift Squid
Whereas octopuses are known for their intelligence, squids are known for their speed and agility.
They swim, usually backwards, using water jets.
They have been known to move as fast as 23 mph.
There are 375 species of squids, with the largest being the giant Architeuthis.
The North American squid, Loligo pealei, however, is only 8-20 in. long.
Squids live from 1 1/2-3 years, depending on the species.
The North American squid prefers shallow water, although many species inhabit deeper, offshore waters.
The squids, unlike their octopus relatives, are not solitary creatures.
They swim in schools and will frequently follow the schools of fish on which they feed.
Their cigar-shaped bodies are well suited for swimming.
Two fins help to stabilize or balance the body when swimming, and the internal shell, called a pen, gives support to the muscular mantle.
Squids can cruise along at constant speeds or dart about in quick jerky movements.
Some have been know to jet 12 ft. out of the water and land on boat decks.
Squids have ten arms, two of which are called tentacles.
The tentacles are longer than the arms and have flattened ends like a spatula.
Suckers are on the underside of all the arms and only on the flattened ends of the tentacles.
The tentacles are used for feeding.
The suckers, which may have hooks, help capture small fish and shrimp, which are quickly torn apart by the beak.
The quick moving squids are eaten by a variety of predators.
The smaller species are food for sea otters, sea birds, large fish and humans.
For protection against predators, squid use the same methods as other cephalopods: camouflage coloring, ink clouds and speed.
Their pale bodies can become almost transparent if necessary or take on the coloring of nearby rocks or seaweeds.
The ink cloud emerges in the shape of the squid, thus forming a false target for the predator.
Also, some deep sea squids have bacteria in the ink which make it glow in the dark.
They may also have special light organs which not only frighten away predators but also help to attract food or a mate in the permanent darkness of the deep sea.
Predators of squid:
The giant squid are eaten by sperm whales.
When a dead whale is examined, it is not unusual for a huge squid to be found inside its stomach.
People, on the other hand, concentrate on the smaller varieties, like the North American squid, for food.
Japan alone may catch 650,000 tons.
In many countries, squid may also be used as fish bait.
Most of the squid are caught during breeding season, when they gather by the millions to spawn.
From November to April, the waters off the coast of California become dense with millions and millions of the Pacific squid, Loligo opalescens.
Squid reproduction:
After ma ting, the females produce 10-50 egg strings, each containing hundreds of eggs.
These are attached to the ocean floor.
Many females may attach their egg strings at the same site, forming a "mop."
The eggs are left to develop without any care and will hatch about 10 days later.
The adult squid will then leave the spawning ground and die soon afterward.
The gastropods - snails & slugs - the Abelly-footed@ animals
Snails:
Snail are of 50,000 marine, freshwater, and terrestrial species.
They have been able to use their singular means of locomotion in a wide range of water and land habitats, from the depths and shorelines of oceans to all bodies of fresh water, and from tropical areas to mountains and deserts.
Snails move by means of a wavelike series of muscular contractions along the bottom of the foot.
This motion is often aided by cilia and, in land snails, by a track of laid-down slime.
Snails feed mainly on algae and decaying matter and are important members of the food web, being a source of food to fish and waterfowl.
A snail browses by means of a radula, a ribbonlike tongue often containing many thousands of denticles, or teeth, that are projected from the mouth opening and drawn along rocks or leaves.
Some carnivorous snails have radulae that bore holes through the shells of other mollusks to reach the soft flesh.
Many species of snails are hermaphroditic and capable of self-fertilization.
Snails have prominent tentacles on which, in many species, the eyes are often located.
Size: Many snails are as small as 0.04 in. long; others, such as conchs and the African land snail, are as long as 8 in.
The spiral shell into which the snail withdraws serves mainly as protection against predators and from drying out.
Land snails are particularly well adapted to changes in moisture; some desert species are able to remain sealed within their thick shells for two or more years.
Land snail species of more moist habitats usually have thinner shells.
Escargots, the snails of French cuisine, come from the cultivated land snail.
Slugs:
A slug is a terrestrial gastropod mollusk, related to the snail, but with the shell represented by an internal horny plate overlying the respiratory cavity.
Slugs are vegetation eaters and often ascend trees in search of food, then let themselves down by means of a mucous thread spun from a gland opening on the anterior edge of the foot.
They may do extensive damage to cultivated plants and are particularly damaging in greenhouses and truck farms.
The great gray slug, sometimes 4 in. long, is a European species, introduced into and now common in eastern North America.
A native American slug common in the United States is a small species, less than 1 in.
Putting salt on a slug destroys it in keeping with formation of a hypertonic solution (discuss details).
Other ways to eliminate slugs around homes: black plastic around garden, containers of beer buried in the ground (discuss details).