page_title: Chiton
section_title: Shell
hierarchical_section_title: Chiton / Morphology / Shell
caption_attribution_description: English: Disarticulated chiton plates of Chiton tuberculatus from the beachdrift on the island of Nevis, West Indies. The head plates are at the top, the tail plates are at the bottom, and the various intermediate plates are in between.
context_page_description: Chitons are marine molluscs of varying size in the class Polyplacophora, formerly known as Amphineura. About 940 extant and 430 fossil species are recognized.
  They are also sometimes known as sea cradles or "coat-of-mail shells", or more formally as loricates, polyplacophorans, and occasionally as polyplacophores.
  Chitons have a shell composed of eight separate shell plates or valves. These plates overlap slightly at the front and back edges, and yet articulate well with one another. Because of this, the shell provides protection at the same time as permitting the chiton to flex upward when needed for locomotion over uneven surfaces, and even allows the animal to curl up into a ball when dislodged from rocks. The shell plates are encircled by a skirt known as a girdle.
context_section_description: All chitons bear a protective dorsal shell that is divided into eight articulating aragonite valves embedded in the tough muscular girdle that surrounds the chiton's body. Compared with the single or two-piece shells of other molluscs, this arrangement allows chitons to roll into a protective ball when dislodged and to cling tightly to irregular surfaces. In some species the valves are reduced or covered by the girdle tissue. The valves are variously colored, patterned, smooth, or sculptured. 
  The most anterior plate is crescent-shaped, and is known as the cephalic plate (sometimes called a "head plate", despite the absence of a complete head). The most posterior plate is known as the anal plate (sometimes called the "tail plate", although chitons do not have tails.)
  The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange, called the articulamentum. This inner layer may also be produced laterally in the form of notched insertion plates. These function as an attachment of the valve plates to the soft body. A similar series of insertion plates may be attached to the convex anterior border of the cephalic plate or the convex posterior border of the anal plate.
  The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle.
  After a chiton dies, the individual valves which make up the eight-part shell come apart because the girdle is no longer holding them together, and then the plates sometimes wash up in beach drift. The individual shell plates from a chiton are sometimes known as "butterfly shells" due to their shape.
caption: Loose valves or plates of Chiton tuberculatus from the beachdrift on Nevis, West Indies, head plates at the top, tail plates at the bottom
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "300", "Image YResolution": "300", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "GIMP 2.6.1", "Image DateTime": "2009:01:08 19:00:09", "Image ExifOffset": "146", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "714", "EXIF ExifImageLength": "1731"}