Biology Of Mollusks

Want to know more about the biology of mollusks? This phylum includes snails, claims and others!

Mollusks include about 100,000 species of clams, oysters, abalones, limpets, snails, slugs, squids, octopuses, and an array of other diverse species. In molluscan evolution, the body wall has stayed flexible and its musculature has become well developed.

In chitons, limpets, and snails, the shell is a protective shield from all but the most persistent, clever, and forceful predators. The saying that is attributed to moving slow is "snail's pace". This reflects on the shell on a snail making his movement slow. There are other molluscan lines, such as squids and octopuses that have become adapted for speed rather than for protection. As evolution has occurred through the years the shell of these predators has been reduced to a tiny internal structure (cuttlebone) or has disappeared entirely.

A mantle is located on the dorsal surface and is a flap formed in the body wall. The mantle has become a conelike envelope surrounding internal organs. A mantle is located on the dorsal surface and is a flap formed in the body wall. In several molluscan lines, mantle cells secrete substances that, together with mineral includions, eventually form a rock-hard shell. The mantle is highly muscularized and can be contracted with great force. When water moves into the mantle cavity the cavity will expand. These contractions then force water out in a powerful stream that propels the body in the opposite direction. For example, the squid is the magnificent swimmer of the invertebrate world. It is freed from the inertia of a shell and is equipped with a jet-propoulsive mantle.

Squid and octopi represent the peak of complexity in the invertebrate world because of their nervous systems. The complexity and size brains of these animals approach those of mammals. Their brain includes a cortical region where information is stored as in mammals.

In some of the species this information is processed and used in modifying behavior thus some have the capacity for learned behavior. These animals have acute vision and refined motor control.

One of the biggest classes of the mollusks is the freshwater clam. The clam is a complex animal as it has muscles and a skeleton in the form of its shell. It also has an intestine (digestive system), gills (respiratory system), heart (circulatory system), nervous system, and an excretory system. The clam has the protection of a double shell with the two parts hinging together and opening like a book. A piece of muscular flesh can be pushed out through the shell opening and is called the foot. As the clam sticks the foot into the mud it is able to pull along. Loose flaps of flesh close off the gap between the outer edges of the open clam shell. Two tubes called siphons on one end lead to the space inside the shell. Water is able to pass into the animal through one siphon and out the other.



The clam stays still almost half buried on the bottom as it does not move around much. It is very much like a sponge as it is a filter feeder that gets many small bits of food from the water that passes through its siphons. It has two gills that hang down on each side of the foot with many little holes in them like a sieve. This gives them a great deal of surface area for absorbing oxygen from the water. These gills are also food-catching organs as food particles in the water will stick to them. The food is pushed along by cilia from the gills to the mouth. Cilia are simply very short flagella. The space with the gills and foot in it is called the mantle cavity because it is surrounded by the mantle.

The freshwater clam hatches eggs in the mantle cavity. They develop into tiny larvae smaller than the head of a pin. When the shadow of a passing fish falls upon the female clam, she clamps her shell shut shooting water containing larvae through her siphon. The larvae become parasites as they will clamp on to the gills or fins of the fish and ride for a while. Then they drop to the bottom and grow up. This hitchhiking on fish spreads clams through our freshwater streams and lakes.

There are other important classes of mollusks that have the same body organization as the clam. Their parts are arranged in a different way that adapts them to other ways of living. The snail class is one of these mollusk groups. A single twisted shell forms on the mantle on snails.

This shell contains the vital organs, such as the intestines and heart. Not all snails have gills as some have a lung instead. The part that looks to us as the snail's body is mostly its foot with a head having tentacles on it. These tentacles allow the snail to feel and to taste. On some snails there are eyes on the ends. Snails have a tongue-like structure covered with tiny sharp teeth used for scraping off bits of food from the surface when crawling. Some snails, called drills, cut holes through thick oyster shells and eat the flesh inside. The snail will put in the foot when attacked.

The octopus class is a very important class of mollusks. They have the same parts as other mollusks but they are arranged differently. The squid has no shell on the outside and the mantle covers part of the body. Gills are contained inside the mantle. There is a head area with a group of nerve centers that form a brain. On the end of the foot there are eight short and two long tentacles. These ten tentacles have a firm grip as they contain suction cups. In the center of the ring of tentacles is the mouth with a sharp beak to bite off chunks small enough to swallow. There is one big eye on each side of the head much like in humans. This animal can see shapes.

The octopus has a body somewhat like a squid but it is short and rounded. It does not have the two long tentacles as the squid. It has the shape of being short and rounded so that it is adapted to living on the ocean bottom. Water is sent shooting through its siphons when it is in a hurry.

It can move very swiftly in the sea. The jet of water enables the octopus or squid to streak away like a rocket. This is a form of jet propulsion. The members of this class have a defense system as they shoot out an inky substance that clouds the water. They can remain hidden in this cloud and either leave by swimming away or attack their prey. Most of the octopi in the Atlantic waters are never more than four feet across measuring from the tip of one tentacle to the other. In the Pacific waters they are usually about thirty feet across. Smaller octopi and squids are eaten by people especially in Japan and Europe. They are used for fish bait also.

The mollusks are a very successful phylum. The mollusks long ago might have all looked a lot alike, almost like a flat-shelled snail. Due to living in different conditions, changes were brought to better adapt for their individual way of life. For an example the octopus class is mostly a hunter and needs a different shell than perhaps the snail. A snail is an animal that sits and waits for food to come to him. Snails are in an in-between class of clams and octopus in their life style. They do not need to move around to find food as it comes to them. The octopi need to find their own food.

Man will eat squids, snails, octopi, oysters, scallops and clams. The shells of mollusks are used in many different ways. Beads and other decorations are made from piece and from whole shells. They can be used to make buttons. Any mollusks that have a shell can make a pearl. The pearl forms around some foreign object, usually a parasitic worm, which gets inside the flesh of the mollusk. The worm passes through the mantle and some of its shell-forming cells are dragged deep into the flesh of the mollusk. Then these cells multiply and keep on producing shell coating the parasite with the shell material resulting in a pearl.

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