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contentconclusionMOLLUSCADefinition Molluscs and mollusks are words derived from the French mollusque, originated from the Latin molluscus, from mollis which means soft.

Malacology is the scientific study of molluscs.

The most general characteristics of molluscs is that they are unsegmented and bilaterally symmetrical.

The following characteristics are present in modern molluscs:the dorsal part of the body wall is a mantle (pallium) which secretes calcareous spicules, plates or shells. the anus and genitals open into the mantle cavity. there are two pairs of main nerve cords.

Fig. 1 Dorsal view of a chiton with a spike bearing mantle.MOLLUSCAis one of the most diverse groups of animals on the planet. It includes familiar organisms like snails, octopuses, squid, scallops, oysters and chitons..is the largest marine phylum, comprising about 23% of all named marine organisms. Numerous mollusks also live in freshwater and terrestrial habits.also includes some lesser known groups like the monoplacophorans, a group once thought to be extinct for million of years until one was found in 1952 in the deep ocean of the coast of Costa Rica.they are a clade of organisms that all have soft bodies which typically have a head and a foot region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.they have been important to humans throughout history as a source of food, jewelry, tools and even pets. their shells are considered quite beautiful and valuable.they can also be nuisance, such as the garden snail; and they make up a major component of fouling communities both on ducks and on the bulls of ships.they also have a very long and rich fossil record (more than 550 mya), making them one of the most common types of organism used by paleontologists to study the history of life.4MorphologyMOLLUSCABody Parts and FunctionsCharacteristicsDigestion and ExcretionNervous System and Sensory CapabilityRespiration and CirculationLocomotionMOLLUSCADiversityIn 2001, Haszprunar estimated about 93,000 named species which include 23% of all named marine organisms. In 2009, Chapman estimated the number of described living species at 85,000. Molluscs have more varied forms than any other animal phylum. The majority of the species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. They are extremely diverse in tropical and temperate regions but can be found at all latitudes. Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the number of non-marine molluscs vary widely. However, in 2004 the IUCN (International Union for the Conservation of Nature and Natural Resources) Red List of threatened Species included nearly 2,000 endangered non-marine molluscs.

Fig. 2 About 80% of all known mollusc species are gastropods (snails and slugs), including the cowry(a sea snail) pictured here.MOLLUSCAClass: CephalopodaGreek word kephalpoda means head-feet.Characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot, a muscular hydrostat, into the form arms or tentacles.became dominant around Ordovician period.The class contains two extant subclassesColeoidea mollusk shell has been internalized or is absent; includes the octopus, squid and cuttlefishNautiloidea the shell remains; this subclass includes the nautilus.Two important extinct taxa areAmmonoidea the ammonitesBelemnoidea the belemnites.Fishing industry name this class as inkfish, referring to many cephalopods ability to squirk ink,Teuthology branch malacology. It is the sudy of cephalopods.

Fig. 12 Cuttlefish (sepia) seizing a shrimp with the use of its tentaclesMOLLUSCALife History and EcologyIn 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 lead litter can exceed more than 500 individuals in four liters or litter,Marine molluscs occur on a large variety of substrates including rocky shores, coral reefs, mud flats and sandy beaches.Gastropods and Chitons are characteristic of these hard substrates and Bivalves are commonly associated with softer substrates where they burrow into the sediment.Tridacna Gigas are the largest living bivalve. It lives on coral reefs and many bivalves (e.g., mussels and oysters) attach themselves to hard substrates.Some microscopic gastropods live interstitially between sand grains.

Fig. 3 Many marine molluscs emerge from their eggs as planktonic trocophore larvae, however Sinistral Pond Snails (Physella sp) emerge from their eggs as young snails the whitish, jellybean-shaped organisms are ostracodes (crustacenans).

Fig 4 Tridacna GigasMOLLUSCAClass: BivalviaThe approximately 7, 500 living species of bivalves include each common animals as clams, oysters, scallops and mussels.Derive their name from the two parts, or valves, into which the shell is divided. One or two large, well-developed adductor muscles are used to close shell swiftly and tightly in times of dangerAbundant in both salt and fresh water , most adult bivalves are sedentary, herbivorous filter feeders, using currents set up by cilia on their gills to bring in food particles, usually microscopic algae.The bivalves have sensory cells for discrimination of touch, chemical changes, and light. The scallop has quite complex eyes; a single individual may have a hundred or more eyes located among the tentacles on the fringe of the mantle.

MOLLUSCAClass: Aplacophorais an informal group of smalll, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world.They are cylindrical and worm-like, and most very small, being no longer than 5 cm (2 in), some species, however, can reach a length of 3ocm(12 in)They mainly burrow into the substrate in water regions deeper than 20m (66 ft). They are typically either carnivores or detritivores.This class was once classified as sea cucumbers in the echinoderms. In 1987, they were officially recognized as molluscs and given their own class. This class is polyphletic , and consists of two clades: the Solenogastres and Caudofoveata,

Fig.7 Interior view of right valve showing the muscel scars.

Most bivalves poses large gills for the purposes of respiration and filtering out of small food partiles

Fig. 8 Bivalve with left valve and rmantle removed.

Fig. 9 Section through the viscual mass showing the internal organs.Because of this diet and feeding, they lack radula. They poses one or two pairs of gills (ctenida) or branchia for respiration.

Fig. 13 Dorsal view of a squid (loligo) in swimming position. The tentacles and arms are held together and functions as a rudder.

Fig.14 Sagital section of nautilus.

Fig. 15 Nautilus: the only shelled cephalopodMost of cephalopods are active and predatory swimmers pressing jaws and radula. They poses eyes as complex as those of humans, and a greater capacity of learning than any other invertebrates.

Fig. 15 Air-breathing land gastropod Helix pomatia, the Roman snail

Fig. 16 Gastropod anatomy

Fig. 17 Sea slug Prosobranchia they have a spiral shaped shell, well developed head that poses tentacles, radula, and a large flat foot for motion. The primitive members are herbivores (rasp seaweeds and micro algae). The advanced forms are predators (poses a long proboscis and cylindrical siphon). Opisthobranchia(sea slugs, sea hares, nudibranches etc.) have forsaken their gills and shells. It has been speculated that their ancestors were sand-burrowers, for whom these would have been a hindrance.

Drawing of the shell of Pilina unguis. Head region is on the left.

Ventral view of the (fossil) shell of Tryblidium reticulatum Lindstrm, 1880. There are visible muscular attachment scars. Head region is on the upper part of the drawing. The shell region is up to 43mm.

Dorsal view of the shell of Tryblidium reticulatum.

Dorsal view of a chiton with a spike nearing mantle.

Vental view of a chiton with a spike bearing mantle.Respiration occurs through 6-80 pairs of gills in a groove around the foot. Chitons are herbivores that have strongly toothed radulae. They are chiefly found in shallow coastal waters.

Chiton Olivaceous: West Indian Ocean

The shell and mantle are slenderly tubular, slightly curved (shaped like an elephant tusk) and open at both ends. conical foot protrudes from the larger ventral end of the shell and is used for burrowing. Delicate ciliated contractile tentacles are found around the mouth to capture food. The large mantle cavity serves for respiration.

Afossil Dentaliumshell from thePliocene of CyprusThe aragonitic shells of scaphopods are conical and curved in a planispiral way, and usually whitish in color.the shell resembles a miniature elephants tusk.The shells are hollow and open at both ends; the opening at the larger end is the main or anterior aperture of the shell. The smaller opening is known as the apical aperture.Some are minute, most are 4-6 cm (1.6 -2.4 in)long; some species reach 15cm (5.9 in) in length.

Body Parts and Functions

There are three distinct body zones: head-foot - contains both the sensory and motor organsvisceral mass - contains the well-developed organs of digestion, excretion, and reproductionMantle- a specialized tissue formed from folds of the dorsal body wall, that hangs over and enfolds the visceral mass and that secretes the shell.Themantle cavity, a space between the mantle and the visceral mass, houses the gills; the digestive, excretory, and reproductive systems discharge into it. Radula - a toothed tongue, composed primarily of chitin. It serves both to scrape off algae and other food materials and to convey them backward to the digestive tract. In some species, it is also used in combat.

CharacteristicsMOLLUSCANervous System and Sensory CapabilityMollusks have a relatively complex nervous system, which varies from species to species reaching the height of complexity at the octopus. The octopus is thought to be among the most intelligent of all invertebrates, with a mental capacity likened to that of a domestic cat. Sensory ability in some mollusks (notably the cephalopods) is considerable, with a variety of organ systems, as well as large, complex eyes. The eyes of the giant squid are the largest in the animal kingdom, approaching the size of dinner plates. It has recently been demonstrated that squid can successfully locate and capture transparent prey in the water by means of a specialized polarization vision.

CharacteristicsMOLLUSCARespiration and Circulation Mollusks (excluding cephalopods)have anopencirculatory system, the blood does not circulate entirely within vessels but is collected from thegills, pumped through the heart, and released directly into spaces in the tissues from which it returns to the gills and then to the heart. Hemocoel("blood cavity"). In mollusks, the hemocoel has largely replaced the coelom, which is reduced to a small area around the heart and to the cavities of the organs of reproduction and excretion. Cephalopods have vigorous activities that require the cells to be supplied with large quantities of oxygen and food molecules. They have aclosed circulatory systemof continuous vessels and accessory hearts that propel blood into the gills.

CharacteristicsMOLLUSCALocomotion Herbivorous forms are commonly gliders, moving on waves of muscular contraction. Carnivorous forms have achieved more advanced forms of locomotion. Cephalopods swim actively by a type of jet propulsion, in which water is rapidly expelled from the mantle cavity via the siphon. The Cuttlefish and the Sea Hares rely upon undulating lateral fins for highly maneuverable locomotion. In the bivalves the foot has developed into a tool for burrowing, which can be remarkably rapid for example in the common Razor Shells.The three major classes range from:largely sedentary or sessile filter-feeding animals, such as clams and oyster (class Bivalvia),through aquatic and terrestrial snails and slugs (class Gastropoda)to the predatory cuttlefish, squids, and octopuses (class Cephalopoda).CharacteristicsMOLLUSCA1. Body is short and partially or wholy enclosed by a fleshy outgrowth of the body wall called the mantle. Between the mantle and the visceral mass is a mantle cavity containing components of several systems (secondarily lost in a few groups)

2. A shell (if present) is secreted by the mantle and consists of one, two or eight parts. the head and the ventral muscular foot are closely allied (the foot being variously modified for burrowing, crawling, swimming, or food capture).

3. The digestive canals are complete and intricate with ciliary canals for the sorting of particles. The mouth with a rudula bearing traverse rows of minute chitinous teeth to rasp food , except inBivalvia. The anus opening in the mantle cavity. A large digestive gland and often salivary glands are present.

4. The circulatory system is open, except inCephalopodaand usually includes a dorsal heart with one or two atria and one ventricle. An anterior aorta and other vessels and many blood spaces (hemocoels) exist in the tissues.

MOLLUSCACharacteristics5. Respiration occurs via one to many uniquely structured ctenidia (gills) in the mantle cavity (secondarily lost in some), by the mantle cavity, or by the mantle.

6. Excretion by kidneys (nephridia), one or two or six pairs, or only a single one. They usually connect to the pericardial cavity and they exit in the mantle cavity. The coelom is reduced to the cavities of the nephridia, gonads and pericardium.

7. The nervous system is typically a circumesophageal nerve ring with multiple pairs of ganglia and two pairs of nerve cords (one pair innervating the foot and another the visceral mass).

8. The sexes are usually separate(some are monoecious, a few are protandric). Gonads add up to four, two or one, all with ducts. Fertilization occurs externally or internally. Egg cleavage determinate, spiral, unequal and total (meroblastic inCephalopoda). Trochophores and veliger larvae form, or a parasitic stage occurs(Unionidae), or the development is direct (Plumonata,Cephalopoda).

9. Unsegmented (exceptMonoplasophora). Symmetry bilateral or asymmetrical.

EvolutionMOLLUSCAFossil Record

The Mollusca include some of the oldest metazoans known. LatePrecambrianrocks ofsouthern Australiaand theWhite Sea regionin northern Russia contain bilaterally symmetrical, benthic animals with a univalved shell (Kimberella),resembles those of molluscs. The earliest unequivocal molluscs are helcionelloid molluscs that date from LateVendianrocks. In the EarlyCambriantheCoeloscleritophoraare also present (gastropods, bivalves, monoplacophorans, and rostroconchs) Cephalopods are first found in the Middle Cambrian. Polyplacophorans in the Late Cambrian. Scaphopoda in the MiddleOrdovician. Late Vendian-Early Cambrian taxa bear little resemblance to the Cambrian-Ordovician taxa (most of which remain extant today).On the left isInoceramussp., a bivalve from the Cretaceous of Alameda County, CA. At right isTurritella andersoni, a gastropod from the Eocene of Ventura County, CA.EvolutionMOLLUSCAPhylogeny Thephylogeny (evolutionary "family tree") is a controversial subject. Molluscs are generally regarded members of theLophotrochozoa , a group defined by havingtrochophore, larvae and, in the case of livingLophophorata, a feeding structure called alophophore. The other members of the Lophotrochozoa are the annelidworms and seven marinephyla. The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into acuticle. An analysis in 2009 that used bothmorphologicalandmolecular phylogeneticscomparisons concluded that the molluscs are not monophyletic. A 2010 analysis similarly concluded that the molluscs are notmonophyletic, this time suggesting that solenogastres are more closely related to the non-molluscan taxa used as anoutgroupthan to other molluscs.

Relations with HumansMOLLUSCAUses by Humans Bivalves such asclamsand mussels, have been an important food source since at least the advent of anatomically modern humansand this has often resulted in over-fishing. bivalvesand some gastropodswhose shells are lined withnacreare valuable. The best natural pearls are produced bypearl oystersPinctadamargaritiferaandPinctada mertensi, which live in thetropicalandsub-tropicalwaters of thePacific Ocean. Natural pearls form when a small foreign object gets stuck between themantleand shell. Mollusc shells were used as a kind ofmoneyin several pre-industrial societies. When used for commercial transactions they functioned as commoditymoney.Relations with HumansMOLLUSCAThreats to HumansSTINGS AND BITES When handled alive, a few species of molluscs can sting or bite and with some species, this can present a serious risk to the human handling the animal. All species ofcone snailsare venomous and can sting when handled (carnivorousgastropods that feed on marine invertebrates ). Their venom is based on a huge array oftoxins, some fast-acting and others slower but deadlierthey can afford to do this because their toxins require less time and energy to be produced compared with those of snakes or spiders PESTSSchistosomiasis (also known as bilharzia, bilharziosis or snail fever) is "second only to malaria as the most devastating parasitic disease in tropical countries. The parasite itself is not a mollusc, but all the species have freshwater snails asintermediate hosts. snails andslugs, can be serious crop pests and when introduced into new environments can unbalance localecosystems.THE END