Chitons
Chitons (kai'-tens) are primitive marine molluscs, also known as sea cradles or "coat-of-mail shells."
Chiton
Encyclopedia of Life. Available from http://www.eol.org.
Chitons have a shell which is composed of eight separate plates or valves. These overlap at the front and back edges, and yet flex with one another. Because of this, although the plates provide good protection from impacts above, they nonetheless permit the chiton to flex upward when needed for locomotion over uneven surfaces.
Here's a side view:
Anatomy of chiton.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com).
As you can see by the image above, the animal can curl up into a ball when it is dislodged from an underlying surface. The shell plates enable such movement, and they are surrounded by a structure known as a girdle. (See below.)
Here's a top and bottom view:
Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
When the animal dies, its shell plates fall apart into individual pieces. It is these individual valves or shells that are usually fossilized.
Matthevia, a Late Cambrian polyplacophoran from the Hellnmaria Member of the Notch Peak Limestone, Steamboat Pass, southern House Range, Utah.
Photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster).
Fossils
Chitons have a relatively good fossil record, stretching back 400 million years to the Devonian.
Kimberella and Wiwaxia of the Precambrian and Cambrian may be related to ancestral chitons.
Matthevia (See above) is a Late Cambrian animal that is preserved as individual pointed valves, and sometimes considered to be a chiton
|
