![helix fossil helix fossil](https://i.ytimg.com/vi/N1jYl3EHv_Y/maxresdefault.jpg)
The generalized mollusc has two paired nerve cords, or three in bivalves. 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 main body cavity is a hemocoel through which blood circulates as such, their circulatory systems are mainly open. Although molluscs are coelomates, the coelom tends to be small. The underside of the animal consists of a single muscular "foot". 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. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below).
![helix fossil helix fossil](http://www.shutupandtakemyyen.com/wp-content/uploads/2015/05/Dome-And-Helix-Pokemon-Fossils.jpg)
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. The gastropods ( snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species. 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 phylum is typically divided into 7 or 8 taxonomic classes, of which two are entirely extinct. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. Numerous molluscs also live in freshwater and terrestrial habitats. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms.
![helix fossil helix fossil](https://i.pinimg.com/736x/57/b5/95/57b595f65e3a625802da465b4cb0adaa--geek-jewelry-fossils.jpg)
The proportion of undescribed species is very high. The number of fossil species is estimated between 60,000 and 100,000 additional species. Around 85,000 extant species of molluscs are recognized. Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks ( / ˈ m ɒ l ə s k/). Cornu aspersum (formerly Helix aspersa) – a common land snail