Classification of mollusca

Phylum Mollusca: Features and Classification

Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. Many taxa remain poorly studied. 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. Cephalopod molluscs, such as squidcuttlefishand octopusesare among the most neurologically advanced of all invertebrates —and either the giant squid or the colossal squid is the largest known invertebrate species. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretionthe presence of a radula except for bivalvesand 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 carbonateand is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomatesthe 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 cordsor three in bivalves. The brainin species that have one, encircles the esophagus. Most molluscs have eyesand all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilizationbut more complex variations occur. Nearly all produce eggsfrom which may emerge trochophore larvaemore complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion. Good evidence exists for the appearance of gastropods, cephalopodsand bivalves in the Cambrian period, — However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. Molluscs have been and still are an important food source for anatomically modern humans. A risk of food poisoning exists from toxins that can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearlsmother of pearlTyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies. Mollusc species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems. The words mollusc and mollusk are both derived from the French mollusquewhich originated from the Latin molluscusfrom mollissoft.

Classification of Mollusca

Mollusks are soft-bodied invertebrates of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebratesmollusks are one of the most diverse groups in the animal kingdom, with nearlypossibly as many asdescribed species. This is a list of notable mollusks, ordered alphabetically by class. Also included are extinct mollusks. See also index fossil. List of mollusks. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Introduction class Aplacophora clams, mussels, oysters, and scallops class Bivalvia octopi, squid, cuttlefish, and nautili class Cephalopoda snails and slugs class Gastropoda class Monoplacophora class Polyplacophora class Scaphopoda extinct mollusks. See Article History. Elysia chlorotica. Learn More in these related Britannica articles:. Molluskany soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly ,…. Invertebrateany animal that lacks a vertebral column, or backbone, in contrast to the cartilaginous or bony vertebrates. More than 90 percent of all living animal species are invertebrates. Worldwide in distribution, they include animals as diverse as sea stars, sea urchins, earthworms, sponges, jellyfish, lobsters, crabs, insects, spiders, snails,…. Insectclass Insecta or Hexapodaany member of the largest class of the phylum Arthropoda, which is itself the largest of the animal phyla. Insects have segmented bodies, jointed legs, and external skeletons exoskeletons. Insects are distinguished from other arthropods by their body, which is divided into three major regions:…. History at your fingertips. Sign up here to see what happened On This Dayevery day in your inbox! Email address. By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Article History.

28.3F: Classification of Phylum Mollusca

Classification can be great fun and not as utterly boring as it sometimes turns out to be. Enjoy it and then you would even be able to score well in an exam. Seriously- no joke being played on you. Do you ask? Because it never is static, the taxonomy and the names are every now and then changing. Not really. It is an indication of the continuous nature of learning that is going on as we try to understand the magnificent life on this planet. Just to remind you that all organisms are classified following a hierarchy which is given below. Each hierarchical group is called taxon, ie Phylum is a taxon, class is a taxon etc etc. This is an oversimplified version of classification. But as a beginner it would suffice if you are able to remember the 7 hierarchical categories given above. The other minute details are important if and when you chose to study something scientifically. In case you have problems remembering the 7 groups in sequence, then you could perhaps remember this sentence, that I remember reading in a biology text book during my school days:. No doubt you would know that all molluscs come under the Phylum Mollusca. They are the second largest group of living organisms and are represented by a humongous number of species, more than aHow many global warming situations and climate changes they must have been witness to! But they have adapted and evolved and now face another challenge with the onset of the latest round of global warming and climate change. Will they survive? To some extent perhaps they will, but their fate lies in the hands of us human beings! Well to get on with classification. When it comes to sorting and grouping them into categories we can see one long journey of changes from the days of Aristotle to the present. Aristotle who lived from B. C classified Molluscs broadly into Malachia present day cephalopods and Ostracodermata. Then some of these terms underwent changes. It did not stop there. Classification of molluscs underwent several changes in its hierarchial categories down the years — In with Linnaeus, in with Cuvier, in by Dumeril and from through Jean Lamarck.

Molluscs: Definition, Features and Classification

In this article we will discuss about Phylum Mollusca:- 1. Definition of Phylum Mollusca 2. Habit and Habitat of Phylum Mollusca 3. Taxonomic Retrospect 4. Characteristic Features 5. Classification 6. Number and Size 7. Mantle and Mantle Cavity 8. Excretory System 9. Reproductive System The members of this group have adapted either on land or in water. Bilaterally symmetrical, coelomate protostomia with spiral cleavage and soft un-segmented body, covered by a thick muscular fold, the mantle which secrets a single or many shells, and a radula in the oesophagus except Bivalvia, and ventral muscular foot for locomotion. Molluscs are noted for their adaptive modifications to different modes of life. They are distributed in almost all the parts of the earth and exhibit variety of forms. Besides their abundance in space, they have left behind a continuous palaeontological records since Cambrian period. Usually the snails and slugs lead a land life. Few Bivalves and snails inhabit freshwater or brackish water. They usually live in the sea shores or in shallow water. Some Molluscs are pelagic and a few are recorded to sink down to the depth of about 35, feet. Most of the Molluscs are nocturnal. Greatest number of Molluscs are free- living forms. As regards locomotory power, they range from very slow moving to fast swimming forms. They exhibit variety of modifications. By the modification of foot, they can creep, leap, burrow, float or swim. Molluscs are mostly herbivorous and live on available vegetable by scraping with the radular apparatus. Most of the Bivalves live on micro-organisms, but the larval forms for some time lead ectoparasitic life in the gills of fishes. All Cephalopods of predaceous forms live on small fishes and Crustaceans. The Gastropods have different food habits. They are mostly vegetable feeders and some are predaceous. Amongst the Gastropods, the members of the order—Pyramidellacea lead semiparasitic life, whereas that of endoconchidae includes all endoparasitic forms living in the body of Holothurians. Cuvier threw the modern light on molluscan taxonomy.

Phylum Mollusca: General Characteristics and Classification

Phylum Mollusca is a very diverse 85, species group of mostly marine species, with a dramatic variety of form. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia excretory organs. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in Today, scientists have identified nearly two dozen extant species. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Chitons live worldwide, in cold water, warm water, and the tropics. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. The majority are filter feeders and have no head or radula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces. The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell. Shells may be planospiral like a garden hose wound upcommonly seen in garden snails, or conispiral like a spiral staircasecommonly seen in marine conches. The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling. Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia singluar: ctenidium as well as a pair of nephridia singular: nephridium.

Kingdom Animalia: Phylum Mollusca

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