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The mantle is an important part of the body of a mollusk. It forms the outer wall of the mollusk's body. The mantle encloses the mollusk's visceral mass, which is its internal organs, including the heart, stomach, intestines, and gonads. The mantle is muscular, and many species have modified it to use for siphoning water for feeding and propulsion.
In mollusks that have shells, such as clams, mussels, and snails, the mantle is what secretes calcium carbonate and a matrix to form the mollusk's shell. In mollusks that lack shells, such as the slug, the mantle is completely visible. In some mollusks with shells, you can see the mantle extending from under the shell. This leads to its name, which means cloak or robe. The Latin word for mantle is pallium, and you may see that used in some texts. In some mollusks, such as the giant clam, the mantle can be very colorful. It can be used for communication.
The Mantle Margin and Siphons
In many types of mollusks, the edges of the mantle extend beyond the shell and are called the mantle margin. They can form flaps. In some species, they have been adapted to use as a siphon. In species of squid, octopus, and clams the mantle has been modified as a siphon, and it is used to direct water flow for several purposes.
Gastropods draw water into the siphon and over the gill for respiration and to search for food with chemoreceptors inside it. The paired siphons of some bivalves draw water in and expel it, using this action for respiration, filter feeding, excreting wastes, and reproduction.
Cephalopods such as the octopus and squid have a siphon called the hyponome that they use to expel a jet of water to propel themselves. In some bivalves, it forms a foot that they use for digging.
The Mantle Cavity
A double fold of the mantle creates the mantle skirt and the mantle cavity inside it. Here you find the gills, anus, olfactory organ, and genital pore. This cavity allows water or air to circulate through the mollusk, bringing with it nutrients and oxygen, and it can be expelled to carry away wastes or provide propulsion. The mantle cavity is also used as a brood chamber by some species. Often it serves multiple purposes.
Mantle Secreting the Shell
The mantle secretes, repairs, and maintains the shell of those mollusks that have shells. The epithelial layer of the mantle secretes a matrix on which calcium carbonate crystals grow. The calcium comes from the environment via water and food, and the epithelium concentrates it and adds it to the extrapallial space where the shell forms. Damage to the mantle can interfere with shell formation.
One irritation that can lead to forming a pearl is caused by a piece of the mollusk's mantle that becomes trapped. The mollusk then secretes layers of aragonite and conchiolin to wall off this irritation and a pearl is formed.