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Groups Of Mollusks

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Jul 20th, 2012
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Mollusks are divided into several classes according to characteristics of the foot and the shell. The three major classes of mollusks are gastropods, bivalves, and cephalopods.

Gastropods Members of the class Gastropoda, or gastropods (GAS-truh-pahdz), include pond snails, land slugs, sea butterflies, sea hares, limpets, and nudibranchs (NOO-duh-branks). Gastropods are shell-less or single- shelled mollusks that move by using a muscular foot located on the ventral side.

Many gastropods have a single shell that protects their bodies. When threatened, they can pull completely into their shells. Some snails are also protected by a hard disk on the foot that forms a solid “door” when they withdraw.

Land slugs and nudibranchs have no shell but protect themselves in other ways. Most land slugs spend daylight hours hiding under rocks and logs, hidden from birds and other potential predators. Some sea hares, when threatened, can squirt ink into the surrounding water, producing a “smoke screen” that confuses predators.

Some nudibranchs have chemicals in their bodies that taste bad or are poisonous. Many nudibranchs are able to recycle the nematocysts from cnidarians they eat, using them to sting predators. These “booby-trapped” nudibranchs are usually brightly colored. The bright coloring serves as a warning to potential predators.

Bivalves Members of the class Bivalvia have two shells that are held together by one or two powerful muscles. Common bivalves include clams, oysters, mussels, and scallops. Most bivalves stay in one place for much of the time. Clams burrow in mud or sand, whereas mussels use sticky threads to attach themselves to rocks. Scallops are the least sedentary bivalves and can move around rapidly by flapping their shells when threatened.

Currents created by cilia on the gills circulate water through the body cavities of bivalves. Once water is inside the body, filter-feeding bivalves use mucus and cilia on their gills to trap food particles in the water. Some bivalves feed on material deposited in sand or mud. They use long, muscular extensions of tissue that surround the mouth to collect food particles from the surrounding sediments. The indigestible sand or mud particles are expelled from the mantle cavity.

Cephalopods Cephalopods (SEF-uh-luh-pahdz)—members of the class Cephalopoda—are the most active of the mollusks. This class includes octopi, squids, cuttlefishes, and nautiluses. Cephalopods are typically soft-bodied mollusks in which the head is attached to a single foot. The foot is divided into tentacles or arms. Cephalopods have eight or more tentacles equipped with sucking disks that grab and hold prey. Nautiluses have many more tentacles than other cephalopods—in some cases up to 90! Their tentacles lack suckers but have a sticky, mucuslike covering.

As with some of the gastropods, most modern cephalopods have only small internal shells or no shells at all. The only present-day cephalopods with external shells are nautiluses. These animals can control their depth in the water by regulating the amount of gas in their shells. Nautiluses look much like ammonites, an extinct group of cephalopods that dominated the seas more than 500 million years ago.

Cuttlefishes have small shells inside their bodies. These are the cuttlebones given to pet birds to condition their beaks. A squid’s internal shell has evolved into a thin supporting rod known as a pen. Octopi have lost their shells completely.

Cephalopods also have numerous complex sense organs that help them distinguish shapes by sight and texture by touch. The eyes of many cephalopods are as complex as those of some vertebrates, such as fishes and humans. Cephalopod eyes can be large—the size of a dinner plate in some species—and can distinguish objects as small as 0.5 centimeters from a meter away, allowing squids to locate a wide variety of prey. Though cephalopod eyes may look something like vertebrate eyes from the outside, their internal structures are quite different.

Reference : Prentice Hall Biology

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