Mollusca
- The molluscs or mollusks (pronounced /ˈmɒləsks/), compose a large phylum of invertebrate animals, Mollusca. Around 85,000 extantspecies of molluscs are recognized. Molluscs are the largest marinephylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10taxonomic classes, of which two are entirely extinct. Cephalopodmolluscs, such as squid, cuttlefish and octopus, are among the mostneurologically advanced of all invertebrates—and either the giant squidor the colossal squid is the largest known invertebrate species. Thegastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total.
- Molluscs have such a varied range of body structures, it is difficult to find defining characteristics to apply to all modern groups. The two most universal features are a mantle with a significant cavity used for breathing and excretion, and the structure of the nervous system. As a result of this wide diversity, many textbooks base their descriptions on a hypothetical "generalized mollusc". This has a single, "limpet-like"shell on top, which is made of proteins and chitin reinforced withcalcium 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 is very small, and the main body cavity is a hemocoel through which bloodcirculates; 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 inbivalves. 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.
- A striking feature of molluscs is the use of the same organ for multiple functions. For example, the heart and nephridia ("kidneys") are important parts of the reproductive system, as well as the circulatory and excretory systems; in bivalves, the gillsboth "breathe" and produce a water current in the mantle cavity, which is important for excretion and reproduction.
- Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestralLophotrochozoa 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, but with a risk of food poisoning from toxins that accumulate in molluscs under certain conditions, and 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 a 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.
Definition
- The two most universal features of the body structure of molluscs are a mantle with a significant cavity used forbreathing and excretion, and the organization of the nervous system. The most abundant metallic element in molluscs is calcium.[5]Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies (defining characteristics) to apply to all modern groups.[6] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.[7] The following are present in all modern molluscs:[8][9]
- The dorsal part of the body wall is a mantle (or pallium) which secretes calcareous spicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity.
- The anus and genitals open into the mantle cavity.
- There are two pairs of main nerve cords.[9]
Other characteristics that commonly appear in textbooks have significant exceptions:Class Characteristic[8] Aplacophora[10] Polyplacophora[11] Monoplacophora[12] Gastropoda[13] Cephalopoda[14] Bivalvia[15] Scaphopoda[16] Radula, a rasping "tongue" withchitinous teeth Absent in 20% ofNeomeniomorpha Yes Yes Yes Yes No Internal, cannot extend beyond body Broad, muscular foot Reduced or absent Yes Yes Yes Modified into arms Yes Small, only at "front" end Dorsal concentration of internal organs (visceral mass) Not obvious Yes Yes Yes Yes Yes Yes Large digestivececa No ceca in some aplacophora Yes Yes Yes Yes Yes No Large complexmetanephridia("kidneys") None Yes Yes Yes Yes Yes Small, simple
