The phylum Mollusca is the largest and most diverse phylum of animals next to arthropods. Mollusks can be found in nearly every ecosystem on earth, from high, barren mountains to grassy plains, lakes, rivers and in all seas and oceans. There are nearly 100,000 species of mollusks identified today, with new species being encountered and named every year as new discoveries are made in ocean depths and tropical rainforests.
The body of a mollusk is generally composed of the shell and the fleshy, living part. The fleshy parts of a mollusk can be further divided into the foot and the visceral mass. The foot is a distinctive molluscan feature, adapted in a variety of ways for locomotion. The visceral mass includes the organs for digestion, circulation, reproduction, and respiration. The visceral mass also includes two external flaps of tissue called the mantle, which secretes the calcareous shell and encloses a mantle cavity. The fluid in the mantle cavity, which in aquatic mollusks is continually replaced with water from the outside, carries away excess water, ions and wastes, and helps circulate nutrients and oxygen. Another structure unique to mollusks, found in most groups except bivalves and a few others is the radula. In most forms the radula is a rasping organ near the mouth variously modified for special feeding techniques.
These two structures – the mantle and radula – are found in Mollusca and nowhere else in the animal kingdom.
Most taxonomists now recognize eight classes of mollusks, based mainly on differences in the foot and shell. These differences are usually quite apparent, making it easy to identify on sight the class to which a mollusk belongs. Only the following four main classes will be discussed in this guide:
This largest and most successful class of mollusks contains 35,000 living species including snails, periwinkles, conches, whelks, limpets and sea slugs.
Members of this class such as clams, cockles, mussels, oysters and scallops, are an important food source for humans as well as for gastropods, fish, and shore birds.
The most highly organized group of mollusks includes squids, octopuses, cuttlefish, and nautiluses.
Members of this class are commonly referred to as chitons. These rock-clinging marine mollusks are abundant on rocky coasts throughout most of the world.
Interaction with humans and other animals
Mollusks are important to humans as well as other animals as food. Some shells are a major source of calcium for some birds. The consumption of mollusks goes back centuries. Indeed, humans found a way to use oysters to increase the food supply indirectly: The crushed shells attract micro-organisms that kill the nematodes that are agricultural pests. Mollusks also nourish humans culturally. Rare and beautiful shells have been prized throughout history and many are still extremely valuable to collectors. In some early cultures mollusk shells served as money. Humans are undoubtedly more harmful to mollusks than the reverse.
Because bivalves are filter feeders, they tend to accumulate pollutants and in many places they are collected and analyzed as a means for monitoring water pollution.
Pollution effects on bivalve mollusks are particularly apparent in coastal waters, though ocean dumping can contaminate offshore stalks as well. Coastal pollution has in recent decades become a significant and growing problem, calling for greater understanding of the effects of man’s activities on habitats and increased awareness of this aspect of environmental degradation.
Conchs, Slugs, and Snails: Class Gastropoda
More than one third of all mollusk species are gastropods. This largest molluscan class includes a diversity of forms that can be divided into three subclasses: Prosobranchia, Opisthobranchia and Pulmonata.
Most gastropods have a single spiral shell, but many are shell-less. The fleshy body of gastropods has clearly defined regions: head, foot, mantle and visceral mass. The head includes sense organs (tentacles and eyes) and a mouth that is armed with a radula and sometimes jaws. The mantle often includes the tubular respiratory siphon. But there are deviations from all these standard features, since gastropods have undergone a certain amount of evolutionary experimentation. One of the peculiarities of gastropod organization is the way the digestive tract doubles back on itself, with the mouth and anus at the same end. This is due to torsion, one of the few features found in all living gastropods and in no other group.
Clams, Mussels, and Oysters: Class Bivalvia
The distinctive members of the class Bivalvia are not likely to be confused with any other marine animals. They include the familiar clams, mussels, oysters and other animals with shells consisting of two laterally poised symmetrical valves. Normally the valves are held open by an elastic ligament near the hinge; when threatened the bivalve can clam up using its
adductor muscles. The soft body of a bivalve consists of a visceral mass, a hatchet shaped foot, sided by a pair of gills and a mantle lobe next to each shell that secrete the shell. The foot is a distinctive feature of a bivalve, located toward the front of the body. Usually they are fused below to enclose a mantle cavity, leaving an opening for the foot. The foot is located toward the front of the body. The foot is well developed in clams and other strong burrowers but almost non existent in scallops and oysters. The rear part of the bivalve is defined by a pair of tubes or siphons. One of these siphons carries water in while the other pumps water out; in the process, the bivalve receives oxygen and food items and eliminates wastes. Bivalves are thus filter feeders.
Diagram of a Scallop
Squids, Cuttlefish and Octopuses: Class Cephalopoda
Members of the class Cephalopoda are among the most highly evolved invertebrates. Unlike most mollusks, cephalopods have a closed circulatory system capable of sustaining high metabolic levels. They are active, fast moving and seemingly intelligent. Cephalopods are named for the foot, which is integrated into the head in the form of arms, tentacles or a funnel. They expel water through the funnel for locomotion, and they use their tentacles and arms mainly during predation. When the animal is threatened, an ink sac ejects its contents through the siphon, providing a smoke screen for escape.
Cephalopods are divided into two quite different subclasses that differ in the number and form of the tentacles. Subclass Coleoidea comprises squid, cuttlefish and octopuses, all of which have eight arms with suckers lining the inner surfaces.
Cephalopods have a phenomenal ability to change color, and they do so according to mood and the need for camouflage. Out of water, squids are soft and flaccid, a sorry transformation from their grace and alertness in the water.
Chitons: Class Polyplacophora
Members of class Polyplacophora are commonly referred to as chitons, because of their dorsal shells consisting of eight overlapping plates. Chitons clamp themselves tightly to rocks and pilings by a wide sole-shaped foot from which it is very difficult to dislodge them undamaged. Chitons possess a head of primitive type which has neither tentacles nor eyes. The foot is flat and motion is achieved by waves of contraction passing forward. The Polyplacophora show bilateral symmetry. The mouth and anus lie at opposite ends of the body, the visceral mass in between, covered by a girdle loaded with spicules that unite to form plate-like valves.
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