A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers. The shells are empty because the animal has died and the soft parts have been eaten by another animal or have rotten out. The term seashell usually refers to the exoskeleton of an invertebrate (an animal without a backbone). Most shells that are found on beaches are the shells of marine mollusk, partly because many of these shells endure better than other seashells. Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crab and brachiapods. Marine annelid worms in the family Serpulidae create shells which are tubes made of calcium carbonate that are cemented onto other surfaces. The shells of sea urchins are called tests, and the moulted shells of crabs and lobsters are called exuviae. While most seashells are external, some cephalopods have internal shells.Seashells have been used by humans for many different purposes throughout history and pre-history. When the word "seashells" is used to refer only to the shells of marine mollusks then studying seashells is part of cochology. Conchologists or serious collectors who have a scientific bias are in general careful not to disturb living populations and habitats: even though they may collect a few live animals, most responsible collectors do not often over-collect or otherwise disturb ecosystems. When studying the whole molluscan animal is included as well as studying the shell, then the study is known as malacology ; a person who studies mollusks is known as a malacologist. Seashells are commonly found in beach drift, which is natural detritus deposited along strandlines on beaches by the waves and the tides. Shells are very often washed up onto a beach empty and clean, the animal having already died, and the soft parts having rotted away or having been eaten by either predators or scavengers. Empty seashells are often picked up by beachcombers. However, the majority of seashells which are offered for sale commercially have been collected alive (often in bulk) and then killed and cleaned, specifically for the commercial trade. This type of large-scale exploitation can sometimes have a strong negative impact on local ecosystems, and sometimes can significantly reduce the distribution of rare species. Bivalves are often the most common seashells that wash up on large sandy beaches or in sheltered lagoons. They can sometimes be extremely numerous. Very often the two valves become separated. There are more than 15,000 species of bivalves that live in both marine and freshwater. Examples of bivalves are clams, scallops, mussels, and oysters. The majority of bivalves consist of two identical shells that are held together by a flexible hinge. Inside the shells holds the animal’s body. Bivalves that do not have two shells either have one shell or they lack a shell all together. The shells are made of calcium bicarbonate and are secreted by the mantle. Bivalves, also known as pelecypods, are mostly filter feeders; they draw in water through their gills which then traps tiny food particles. Some bivalves have eyes and even an open circulatory system. Bivalves are used all over the world as types of food and as a way of getting pearls. But in the water, the larvae of some freshwater mussels can be dangerous to fish and can even bore through wood.