what does mollusca mean
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Mollusca
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (/ˈmɒləsk/). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.
Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. The phylum is typically divided into 7 or 8 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species.
Foot
The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a “hypothetical ancestral mollusc” (see image below). This has a single, “limpet-like” shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular “foot”. Although molluscs are coelomates, the coelom tends to be small.
The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The “generalized” mollusc’s feeding system consists of a rasping “tongue”, the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered “hairs” called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus.
Eyes
Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.
History
According to fossil records, the first mollusks appeared on earth some 600 million years ago. After another 100 million years, primitive forms of six of the seven currently recognized classes of mollusks existed. It took another 100 million years before mollusks first appeared in and near freshwater lakes and streams. It is only within the last 100 million years that mollusks have fully evolved to their present states, populating diverse land, freshwater and marine habitats worldwide.
Common Attributes
Species of mollusk vary widely; however, most mollusks have a few basic features in common with one another. All mollusks have soft, unsegmented bodies. Most have a single muscular foot used for moving from place to place or digging into the sand or mud of their habitat. In the cephalopod class of mollusk, tentacles take the place of a foot. A mantle that secrets an exoskeleton or shell further distinguishes mollusks from other animals.
Gastropods
At least 60,000 species make up the largest class of mollusks, the gastropods. These mollusks populate land, freshwater and marine environments. Snails and slugs are common examples of gastropods. Land gastropods have lungs, while their marine counterparts breathe through gills. Most gastropods feed on plant matter, and many land snails and slugs are a nuisance to farmers and gardeners.
Cephalopods
The 600 to 650 species of cephalopod inhabit the sea, including squid, octopuses, and cuttlefish. Most lack the shell of other mollusca, but have tentacles that propel them through the water using suction. All cephalopods are scavengers and predators. Their advanced nervous system makes them the most intelligent of all invertebrates. Cephalopods also are the largest mollusks — a giant squid can be as much as 65 feet long.
Other Marine Classes
Five other classes of mollusks live solely in the ocean; most on the ocean floor. The 900 species of chitons, or polyplacophora, are distinguished by a shell of eight plates and graze on algae. Around 250 species of worm-like aplacophora live in and on the ocean floor. These mollusks feed on detritus, though some are predators. Scaphopoda mollusks, known as “tusk shells” for the shape of their tubular shells, are also detritus feeders.
Bivalves inhabit both oceanic and freshwater habitats worldwide. These filter feeders can grow to over four feet in length. Monoplacophora mollusks are the fewest in number, and mostly known from fossil records. A few species still exist today, consuming algae and microorganisms on the ocean floor.
Importance to Humans
Mollusks have been important to humans throughout history. Their shells have used for tools and as jewelry, many of which considered beautiful and high in value. Some ancient groups used mollusk shells as a form of currency. Sepia, an artist’s dye, is made from the ink of the cuttlefish, a cephalopod. Mollusks have long been a staple of the human diet worldwide, particularly gastropods and cephalopods such as clams, mussels, scallops, oysters and squid.
Morphology
Despite their amazing diversity, all molluscs share some unique characteristics that define their body plan. The body has a head, a foot and a visceral mass. This all covered with a mantle (also known as a pallium) that typically secretes the shell. In some groups, like slugs and octopuses, the mantle secondarily lost, while in others, it used for other activities, such as respiration.
The buccal cavity, at the anterior of the mollusc, contains a radula (lost in bivalves) — a ribbon of teeth supported by an odontophore, a muscular structure. The radula generally used for feeding. The ventral foot used in locomotion. This foot propels the mollusc by utilizing muscular waves and/or cilia in combination with mucus.
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Typically, at least in the more primitive members of each group, there are one or more pairs of gills (called ctenidia) which lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. The pallial cavity typically contains a pair of sensory osphradia (for smelling) and is the space into which the kidneys, gonads, and anus open.
Coelomate
Molluscs are coelomate, although the coelom reduced and represented by the kidneys, gonads, and pericardium, the main body cavity which surrounds the heart.
Life history and ecology
Molluscs occur in almost every habitat found on Earth, where they are often the most conspicuous organisms. While most found in the marine environment, extending from the intertidal to the deepest oceans, several major gastropod clades live predominantly in freshwater or terrestrial habitats. Remarkably, one study found around 3000 species within a single locality at a coral reef in New Caledonia. In terrestrial communities, gastropods can achieve reasonably high diversity and abundance: as many as 60-70 species may coexist in a single habitat and abundance in leaf litter can exceed more than 500 individuals in four liters of litter.
General features
Size range and diversity of structure
Typical molluscan features have substantially altered, or even lost, in many subgroups. Among the cephalopods the giant squids (Architeuthis), the largest living invertebrates, attain a body length of eight metres (more than 26 feet); with the tentacle arms extended, the total length reaches to 22 metres. Other cephalopods exceed a length of one metre. Many of the remaining molluscan classes show a large variation in size: among bivalves the giant clam (Tridacna) ranges up to 135 centimetres (four feet) and the pen shell (Pinna) from 40 to 80 centimetres.
Among gastropods the sea hares (Aplysia) grow from 40 to 100 centimetres and the Australian trumpet, or baler (Syrinx), up to 60 centimetres; among placophores the gumshoe, or gumboot chiton (Cryptochiton), achieves a length up to 30 to 43 centimetres; and, among solenogasters, Epimenia reaches a length of 15 to 30 centimetres. Finally, gastropods of the family Entoconchidae, which are parasitic in echinoderm sea cucumbers, may reach a size of almost 1.3 metres. In contrast, there are also minute members, less than one millimetre (0.04 inch) in size, among the solenogasters and gastropods.
Importance to humans
Mollusks are of general importance within food chains and as members of ecosystems. Certain species are of direct or indirect commercial and even medical importance to humans. Many gastropod species, for example, are necessary intermediate hosts for parasitic flatworms (class Trematoda, phylum Platyhelminthes), such as the species that cause schistosomiasis in humans. Most bivalves contribute to the organic turnover in the intertidal (littoral) zones of marine and fresh water because, as filter feeders, they filter up to 40 litres (10 gallons) of water per hour.
This filtering activity, however, may also seriously interfere with the various populations of invertebrate larvae (plankton) found suspended and free-swimming in the water. One species, the zebra mussel (Dreissena polymorpha), is regarded as a particularly harmful exotic invader. Carried from Europe in ship ballast water, zebra mussels taken to the Great Lakes in 1986. To date, they have caused millions of dollars in commercial damage by clogging the water pipes of power plants and cooling systems. They are driving many native freshwater bivalve species to extinction.
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