Mollusk

---

  Mollusks are soft bodies animal that are gifted with nature to build protective calcium carbonate shell out side there body.A mollusks or mollusc is any member of the large phylum Mollusca of invertebrate animals. they can be rage in size from very tiny to largest up to 3 ft.  since ancient times it's always delight all of us from Their bright colors, curious shapes, and amazing patterns. And also this make them among the most fascinating of nature's creations. The universal appeal (Sea, Fresh water & Land) of shells is due to their tactile qualities as much as their inner & outer beauty. While some shells are fragile and easily broken, many are solid and pleasant to hold.   Around 85,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 species.It's a largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and Land. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, 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 in terms of classified species, and account for 80% of the total.  

 Exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 541 to 485.4 million years ago. 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. 

 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 things, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc". 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; 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. 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. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. 

 Molluscs live throughout the world, from the Arctic and Antarctic oceans to the equatorial regions. Also, some species have adaptations to survive in the Marine or Fresh water and some others on land.For their part, land snails are among the most widely distributed invertebrates around the world. They can be found on five continents and even in the sub-Antarctic region, where temperatures are very low & also In desert. Out of that, some Common Habitat are as shown bellow. 

 Molluscs have been and still are an important food source for anatomically modern humans. However there is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions, 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 pearls, mother of pearl, Tyrian 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 for 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. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. 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.